No full-text available
To read the full-text of this research,
you can request a copy directly from the author.
Ultramicroelectrodes: Design, Fabrication, and Characterization
Abstract
Ultramicroelectrodes (UMEs) have led to unprecedented advances in electrochemical studies since their introduction to electroanalytical chemistry about twenty five years ago. During this time, several UME geometries have been reported of which disk, ring, ring-disk, hemispherical, spherical, and etched (finite cone) UMEs are the most commonly used. In this review, the design and fabrication procedures for each are described. Issues related to UME electrode surface treatment and characterization are also addressed.
... sizes produce currents measured between μA and lower mA for analytes whose 53 concentration is close to 1 mM under transient (time-dependent) conditions (Bond 54 1994; Anon 2020). 55 Bard and colleagues define four categories of electrodes in terms of their sizes: 56 (1) normal, conventional, or macroelectrodes (dimensions of millimeters, centime-57 ters, or meters), (2) microelectrodes (MEs) (dimensions between 25 μm and 1 mm), 58 (3) ultramicroelectrodes (UMEs) (dimensions between 10 nm and 25 μm), and 59 finally nanoelectrodes or nanodes (dimensions smaller than 10 nm) (Bard et al. such as disk, sphere, hemisphere, or cylinder forms, or width for a band or ring UME 63 (Zoski 2002). Although there are different lengths mentioned to define UMEs, 64 generally, 20 μm or 25 μm is determined (Xie et al. 2005). ...
... These are (1) point electrodes, which 134 are concentrated and distributed in the solution in a spherical form; (2) line elec-135 trodes, which generally take cylinder form in concentration and distribution; and 136 (3) plane electrodes, which are composed of point and line electrodes on a planar 137 insulator, specifically taking a disk form (Aoki 1993). 138 Zoski made a similar shape-based classification: (1) disk UMEs, (2) hemispherical 139 and spherical UMEs, (3) inlaid ring UMEs, (4) ring-disk UMEs, and (5) finite 140 conical (etched) UMEs (Zoski 2002). On the other hand, Kaifer and Gómez-Kaifer's 141 classification was more comprehensive: (1) disk UMEs, (2) sphere UMEs, (3) wire 142 UMEs, (4) ring UMEs, (5) band UMEs, (6) array UMEs, and (7) interdigitated array 143 UMEs (Kaifer and Gómez-Kaifer 1999). ...
... 156 More recently, SECM has been utilized to evaluate the size and shape of the 157 designed UMEs by bringing the UME tip closer to electrically insulating or con-158 ductive substrates. This is the diffusion-limited current that has been measured by 159 using a redox mediator at the surface of the tip of the UME which has a distance from 160 the substrate (Zoski 2002). ...
An ultramicroelectrode is an electrode whose characteristic dimensions are smaller than 25 μm. They have been used in electrochemistry for the last 30 years and can be used in studying extremely small sample volumes. Due to these advantages, ultramicroelectrodes have revolutionized electrochemical applications and methodologies. They have also transformed the time/space accessibility of experimentation. Nanomaterials are materials whose dimensions are at the nanometer scale (1–100 nm) or assembled at this range. When compared to conventional materials the structure and properties of nanomaterials are essentially changed. They show chemical, physical, and electronic properties that are not seen in other materials or even materials from which the nanoparticles were prepared. Nanomaterials are used for electrode modification, also including ultramicroelectrodes at electrochemical sensing systems; among these nanomaterials, one can count carbon nanoparticles, noble metal nanoparticles, metal oxide nanoparticles, and bimetallic nanoparticles. This chapter examines the role of nanoparticles in designing ultramicroelectrodes and their applications. The concepts, geometries, fabrication, and advantages of ultramicroelectrodes are defined and discussed. Finally, the applications of nanoparticle-modified electrodes are presented in such a way as to display the advantages of modification as well as the ultra-small size of the electrodes.
... UMEs can have a variety of different geometries, with each shape having different characteristics (Mirkin et al. 1992). Possible UME electrode geometries are disc, conical, hemispherical, ring or combinations such as ring-disc (Lee et al. 1991b;Zoski 2002). Detailed information on the different UME geometries can be found in Bard and Faulkner (2000) as well as in Mirkin et al. 1992. ...
... The capillary is then sealed around the wire by placing it in a heated coil and polishing it to expose the electroactive surface. Detailed information about the fabrication can be found in Ballesteros Katemann and Schuhmann (2002), Danis et al. (2015) and Zoski (2002). ...
... They were able to produce UMEs with small RG values between 2.5 and 3.6 and high reproducibility of the UME geometries. Moreover, the fabrication time for these UMEs was less than one hour (Ballesteros Katemann and Schuhmann 2002;Zoski 2002). ...
Over the last 30 years, scanning electrochemical
microscopy (SECM) has become a fundamental technique
in corrosion research. With its high spatial resolution and
its ability to study local electrochemistry, it contributes
essentially to the understanding of corrosion processes. By
using selective micro- and nano-sensors, concentration
profiles of different corrosion relevant species, from protons
to metal ions, can be established. This review provides
a comprehensive overview about SECM based techniques
and discusses various types of microsensors, including
materials selection and preparation techniques, and it
provides extensive tables on redox-couples for specific
corrosion research applications.
... CF sensors-based show chemical stability, a wide potential window, a significant increase in the electroactive area, electronic conductivity, and excellent electron transfer kinetics 11,17 . Moreover, the use of CF for the construction of small-size sensors can provide not only the advantages of UMEs 18,19 but also has the features of carbon materials (low cost and environmentally friendly). ...
... UMEs are unprecedented tools in electroanalytical chemistry, since they offer several advantages that make them feasible and useful devices for different determination, even in severe experimental conditions 18,19 . It is possible to perform electrochemical measurements in low volumes, non-aqueous solutions, and the absence of support electrolytes using the simplest electrode arrangement. ...
... CVs were smoothed if necessary. So, it is possible to choose the electrolyte-free aqueous medium for further studies 18,19 . For the present work, this result is significant, since it allows working for direct determination of the analyte in a real sample matrix, which usually presents a different and unknown concentration of electrolytes. ...
This work describes the construction and evaluation of carbon fiber ultramicroelectrodes (CF-UME) in the voltammetric estimation of the antioxidant capacity of wine and grape samples based on caffeic acid (HCAF) oxidation. For this, lab-made CF-UMEs were constructed using an arrangement of six carbon fibers (7 µm diameters individual) assembled in a glass capillary, and caffeic acid (HCAF) was used as a standard solution. By the most straightforward 2-electrodes cell arrangement (CF-UME as working electrode and Ag/AgCl as a reference/auxiliary electrode), voltammetric measurements of a 1.0 mmol L-1 HCAF solution were done in the absence of supporting electrolyte. A sigmoidal voltammetric profile was observed in CF-UME caused by a more effective mass transport by radial diffusion, which leads to a rapid formation of the diffusion layer. Reproducibility studies for different 6-fibers electrodes manually constructed in different batches showed an RSD of less than 5%. For the same electrode surface, a variation of 2.7% was observed. Under optimized conditions, a linear relationship between anodic peak current and HCAF concentration from 3.0 to 500 µmol L-1 with a sensitivity of 12 μA L mol-1 was reached. The limits of detection (LOD) and quantification (LOQ) were calculated as 0.41 and 1.26 μmol L-1, respectively. The proposed electrochemical method was applied in the estimation of the antioxidant capacity in three different wine samples as well as in green and red grapes. Concordant and satisfactory results by comparison with a proper method were obtained, which suggests that the proposed sensor can be successfully applied for direct analysis of wine and grape samples by estimation of HCAF content.
... Since the pioneering work of Fleischmann et al. [1] and Wightman [2], each year has seen intense and increased construction and application of microelectrodes as working electrodes in a variety of methods [3,4,5,6,7,8], partly because of the innumerable advantages of these electrodes and, more especially, because of the numerous possibilities of their use in improving the responses obtained. ...
... Microelectrodes have already been used to study reactions and electrode processes in low-conductivity solvents without any support electrolyte, thus minimizing costs and manipulation of samples. Study of rapid electron-transfer processes and chemistry-coupled reactions has been possible with these electrodes [3]. Microelectrodes can also be used in a variety of experiments such as ''in vivo'' analysis [9], in which conventional electrodes are known to be impossible. ...
This paper reports the use of laboratory-prepared gold microelectrodes and square-wave voltammetry for analytical determination of low concentrations of the pesticide dichlorvos in pure and natural water samples. After optimization of the experimental and voltammetric conditions, the best voltammetric responses—current intensity and voltammetric profile—were obtained in 0.1 mol L−1 NaClO4 with f=100 s−1, a=50 mV, and ΔE
s=2 mV. The observed detection and quantification limits in pure water were 7.8 and 26.0 μg L−1, respectively. The reproducibility and repeatability of the method were also determined; the results were 1.4% (n=5) and 1.2% (n=10), respectively. Possible interfering effects were evaluated in natural water samples collected at different points with different levels of contamination from agricultural, domestic, or industrial waste from an urban stream. Results showed that the detection and quantification limits increased as a function of the quantity of organic matter present in the samples. Nonetheless, the values observed for these method characteristics were below the maximum value allowed by the Brazilian code for organophosphorus pesticides in water samples. Recovery curves constructed using the standard addition method were shown to be satisfactory compared with those obtained from high-performance liquid chromatography, confirming the suitability of the method for analysis of natural water samples. Finally, when the method was used to determine dichlorvos in spiked cows’ milk samples, satisfactory recovery and relative standard deviations were obtained.
... For a reversible process, the potential difference corresponding to 3/4 and 1/4 of the steady-state current for LSV curves (E3/4-E1/4) depends on the number of electrons exchanged, n. At 25 °C, this value should be 0.0564/n V [22,23]. The values obtained for both steps of the electrode reaction are 0.055 and 0.086 V, respectively. ...
... For a reversible process, the potential difference corresponding to 3/4 and 1/4 of the steady-state current for LSV curves (E 3/4 -E 1/4 ) depends on the number of electrons exchanged, n. At 25 • C, this value should be 0.0564/n V [22,23]. The values obtained for both steps of the electrode reaction are 0.055 and 0.086 V, respectively. ...
In this study, a carbon fiber microelectrode (CF) was applied for the investigation of the electrochemical behavior of the natural antioxidant, apocynin (APO). Given the limited solubility of APO in water, a mixture of anhydrous acetic acid (AcH) with 20%, v/v acetonitrile (AN) and 0.1 mol L−1 sodium acetate (AcNa) was used. The electrochemical properties of APO were examined through linear sweep voltammetry (LSV), differential pulse voltammetry (DPV), and cyclic voltammetry (CV). The anodic oxidation of APO, which is the basis of the method used, proved to be diffusion-controlled and proceeded with a two-electron and one proton exchange. Both radicals and radical cations, arising from the first and second step of electrode reactions, respectively, underwent subsequent chemical transformations to yield more stable final products (EqCiEiCi mechanism). Using optimized DPV conditions, the anodic peak current of APO at a potential of 0.925 V vs. Ag/AgCl showed a good linear response within the concentration range of 2.7 × 10−6–2.6 × 10−4 mol L−1. The detection and quantification limits were determined as 8.9 × 10−7 and 2.7 × 10−6 mol L−1, respectively. The developed DPV method enabled the successful determination of APO in herbal extracts and in dietary supplements. It should be noted that this is the first method to be used for voltammetric determination of APO.
... In the case of nanoelectrodes, their size in at least two dimensions is substantially below 1 μm [15]. The lower limit in downscaling the critical dimension is typically set to 10 nm, where the electrode size approaches the thickness of the double layer or molecule dimensions and originates a peculiar electrochemical behavior that deviates from theory [14,16]. The small size of micro-and nanometric probes is the basement for their peculiar electrochemical features, including (i) reduced RC constant (cell time constant), which greatly influences the time resolution and allows measurements of fast processes; (ii) reduced iR drop, thus allowing measurements with simplified instrumentation (from three to two-electrode setup) in scarcely conducting media, such as non-polar solvents or real matrices, and in the absence of supporting electrolyte; (iii) improved signal to noise ratio (enhanced Faradaic over non-Faradaic current ratio) and increased diffusional mass transport rate; (iv) relative insensitivity to convection thanks to the high mass transport rates, thus allowing in-flow electrochemical measurements; and (v) high spatial resolution and minimal invasiveness, thus causing negligible system perturbation [14,[17][18][19][20]. ...
... Thereafter, the electrode tip is exposed by mechanically polishing or chemically etching of the insulator. Technical aspects and recent advancements in carbon nanoelectrode fabrication, including the flameetching of carbon microfibers, chemical vapor deposition (CVD) of a carbonaceous layer inside of pulled quartz capillaries with methane gas as a precursor, and pyrolysis of a propane/butane gas mixture have extensively been reviewed elsewhere [15,16,21]. On the other hand, the chip-like miniaturized structure has evidently benefited from the rapid evolution of microelectronics and the integrated circuit industry and relies on thin-/thick-film techniques employed in bottom-up manufacturing technologies including physical vapor deposition, photolithography, electron-beam lithography, and focused ion beam (FIB) milling. ...
Electrode miniaturization has profoundly revolutionized the field of electrochemical sensing, opening up unprecedented opportunities for probing biological events with a high spatial and temporal resolution, integrating electrochemical systems with microfluidics, and designing arrays for multiplexed sensing. Several technological issues posed by the desire for downsizing have been addressed so far, leading to micrometric and nanometric sensing systems with different degrees of maturity. However, there is still an endless margin for researchers to improve current strategies and cope with demanding sensing fields, such as lab-on-a-chip devices and multi-array sensors, brain chemistry, and cell monitoring. In this review, we present current trends in the design of micro-/nano-electrochemical sensors and cutting-edge applications reported in the last 10 years. Micro- and nanosensors are divided into four categories depending on the transduction mechanism, e.g., amperometric, impedimetric, potentiometric, and transistor-based, to best guide the reader through the different detection strategies and highlight major advancements as well as still unaddressed demands in electrochemical sensing.
Graphical Abstract
... 8 The history of microelectrode development and their advantages over traditional bulk electrodes have been described in comprehensive reviews . [9][10][11][12][13][14] The earliest reported electrochemical measurements conducted with the microelectrodes in the 1960s were not in the traditional electroanalysis or electrosynthesis fields, but in neuroscience for analyzing brain activities. In a 1960 paper Wolbarsht et al. reported a method to make microelectrodes, "which requires little skill or equipment and is much less time consuming." ...
... 22 Typical geometries of microelectrodes are shown in Fig. 1b: disk, hemispherical, band, cylindrical, array, and interdigitated microelectrodes. 5,10,14 In chronoamperometry, modified forms of the Cottrell equation have been derived for various shapes of microelectrodes to account for the size and shape effects. 10 For disk microelectrodes, the modified Cottrell equation is: ...
Crystallization is at the heart of many industrial processes in pharmaceuticals, dyes and pigments, microelectronics, and emerging wearable sensors. This paper reviews nucleation and early-stage crystal growth activated by an electrical pulse at microelectrodes and nanoelectrodes. We review thermodynamic and kinetic theories of electrochemistry developed around microelectrodes. We describe various methods to make microelectrodes and nanoelectrodes. Fundamental understanding is still needed for predicting and controlling nucleation and early-stage crystal growth. Using nanoelectrodes, nucleation and growth kinetics can be studied on one nucleation site at a time. In contrast, on macroelectrodes, nanoparticles are nucleated at random sites and at different times. This gives rise to overlapping growth zones resulting in inhomogeneous particle deposition and growth. The random size and density distributions prevent electrodeposition from being widely adopted as a manufacturing tool for making nanodevices. We describe advances in electrodeposition of metal nanoparticles and organic charge-transfer complexes on micro/nanoelectrodes. We anticipate increased interests in applying electrochemistry for making nanodevices particularly nanosensors and nanosensor arrays. These electrochemically fabricated nanosensor arrays will in turn fulfill the promise of nanoelectrodes as the most advanced analytical tools for medical diagnostics, environmental monitoring, and renewable energy.
... To gain microscopic understanding of electrodeposition, the ultramicroelectrode technique has been developed. [131][132][133][134][135][136][137][138][139][140][141] Ultramicroelectrodes, by definition, are electrodes with dimeter smaller than 25 μm. With innovative experimental setups and advanced fabrication tools, submicron and nanoelectrodes of smaller than 500 nm are also made possible. ...
Electrodeposition is used at the industrial scale to make coatings, membranes, and composites. With better understanding of the nanoscale phenomena associated with the early stage of the process, electrodeposition has potential to be adopted by manufacturers of energy storage devices, advanced electrode materials, fuel cells, carbon dioxide capturing technologies, and advanced sensing electronics. The ability to conduct precise electrochemical measurements using cyclic voltammetry, chronoamperometry, and chronopotentiometry in addition to control of precursor composition and concentration makes electrocrystallization an attractive method to investigate nucleation and early-stage crystal growth. In this article, we review recent findings of nucleation and crystal growth behaviors at the nanoscale, paying close attention to those that deviate from the classical theories in various electrodeposition systems. The review affirms electrodeposition as a valuable method both for gaining new insights into nucleation and crystallization on surfaces and as a low-cost scalable technology for the manufacturing of advanced materials and devices.
... The defining feature of microelectrodes is their minute size, which is on par with the thickness of the diffusion layer. The following advantages can be attributed to this configuration of working electrodes [38][39][40][41]: (i) low capacity of the electrical double layer; (ii) due to the occurrence of spherical diffusion at the microelectrode surface, it is possible to undertake the measurements from unmixed solutions that offer the prospect of the streamlined measurements and the capacity of conducting the measurements in field conditions; (iii) the signal-to-noise ratio is more favorable when working with microelectrodes as compared to the conventionally sized electrodes; (iv) the possibility of analyzing samples of a very small volume, from solutions with a low concentration of a supporting electrolyte and solutions of organic solvents are further advantages of using microelectrodes. On the other hand, the limited surface area of the microelectrodes results in the recording of low currents that are susceptible to interference. ...
This article presents for the first time a new working electrode with a long service life— the bismuth-plated array of carbon composite microelectrodes for the simple, fast and sensitive determination of quercetin by adsorptive stripping voltammetry. The main experimental conditions were selected. The calibration graph was linear from 1 × 10−9 to 2 × 10−8 mol L−1 with an accumulation time of 60 s. The detection limit was equal to 4.8 × 10−10 mol L−1. The relative standard deviation for 2 × 10−8 mol L−1 of quercetin was 4.4% (n = 7). Possible interference effects resulting from the presence of other organic and surface active compounds and interfering ions were studied. The developed procedure was successfully applied to determine quercetin in pharmaceutical preparations and the spiked urine samples.
... The device was placed on a dampening plate inside a Faraday cage. A 25 μm diameter platinum ultra microelectrode with a ratio between the total tip radius and the radius of the Pt disk of 4, was fabricated following a procedure described in detail by Zoski, 19 and used as a working electrode. On the bottom of the polytetrafluoro-ethylene electrochemical cell, the properly prepared S-PtE was mounted and the cell was filled with 5 mL of supporting electrolyte. ...
This communication introduces a quick and easy method to modify a typical disk electrode’s surface geometry. The method involves masking fragments of the flat surface of the working electrode, by applying a specific thin layer of chemically stable insulating material, leaving unvarnished, electrochemically active surface. Desired shapes are achieved by using a properly laser-engraved stainless-steel plate and a stamper to transfer the profiled varnish from the steel plate to the surface of the electrode. Three shapes - microdisk, microband, and ring electrodes - were applied to a platinum disk electrode, validated through optical and scanning electrochemical microscopies, and cyclic voltammetry.
... An SECM probe (generally referred to as an SECM tip) is a critical component that defines the spatial resolution of the measurement. While a substantial number of reports have focused on the design and fabrication of various types of probes [31][32][33], diskshaped micro(nano)electrodes are generally the most common geometry ( figure 1(b)). Other important accessories of an SECM include EC cells, vibration isolation stages, optical microscopes, and complementary spectrometers [22]. ...
Research interests in two-dimensional (2D) materials have seen exponential growth owing to their unique and fascinating properties. The highly exposed lattice planes coupled with tunable electronic states of 2D materials have created manifold opportunities in the design of new platforms for energy conversion and sensing applications. Still, challenges in understanding the electrochemical (EC) characteristics of these materials arise from the complexity of both intrinsic and extrinsic heterogeneities that can obscure structure–activity correlations. Scanning EC probe microscopic investigations offer unique benefits in disclosing local EC reactivities at the nanoscale level that are otherwise inaccessible with macroscale methods. This review summarizes recent progress in applying techniques of scanning EC microscopy (SECM) and scanning EC cell microscopy (SECCM) to obtain distinctive insights into the fundamentals of 2D electrodes. We showcase the capabilities of EC microscopies in addressing the roles of defects, thickness, environments, strain, phase, stacking, and many other aspects in the heterogeneous electron transfer, ion transport, electrocatalysis, and photoelectrochemistry of representative 2D materials and their derivatives. Perspectives for the advantages, challenges, and future opportunities of scanning EC probe microscopy investigation of 2D structures are discussed.
... Electrochemistry at the nanometer regime has drawn much attention for the characterization of the synthesized nanomaterials [1,2], the better electrocatalysts/active nanomaterials in electrochemical energy conversion/storage [3][4][5][6], and the electrochemical sensors for various analytes [7,8]. Compared to the conventional macroelectrodes, electrochemistry using nanoelectrodes has tremendous advantages such as enhanced masstransfer rates, smaller RC constants, enhanced faradaic to capacitive current ratio and negligible influences from solution resistance [9][10][11]. The understanding of electrochemical properties under the nanoscale domain was mainly conducted using nanoelectrode ensembles (NEEs), which do not provide information on the molecular level but the averaged properties. ...
p>An unconventional fabrication technique of nanoelectrode was developed using atomic force microscopy (AFM) and hydrogel. Until now, the precise control of electroactive area down to a few nm2 has always been an obstacle, which limits the wide application of nanoelectrodes. Here, the nanometer-sized contact between the boron-doped diamond (BDD) as conductive AFM tip and the agarose hydrogel as solid electrolyte was well governed by the feedback amplitude of oscillation in the non-contact mode of AFM. Consequently, this low-cost and feasible approach gives rise to new possibilities for the fabrication of nanoelectrodes. The electroactive area controlled by the set point of AFM was investigated by cyclic voltammetry (CV) of the ferrocenmethanol (FcMeOH) combined with quasi-solid agarose hydrogel as an electrolyte. Single copper (Cu) nanoparticle was deposited at the apex of the AFM tip using this platform whose electrocatalytic activity for nitrate reduction was then investigated by CV and Field Emission-Scanning Electron Microscopy (FE-SEM), respectively.</p
... Progress in nanoelectrode research is, as one might expect, directly correlated to progress in fabrication and characterization of these nanostructures. Electrode geometry and that of the insulating protective cover surrounding the electrode are issues as important as actual electrode size, since they determine the mass transport of electroanalytes [14,15] and thereby the proper interpretation of currents and current-potential curves. ...
Increasing sensitivity and selectivity, as well as, long-term durability of electrochemical sensors are the reasons for designing active layers on electrodes. Significant advances in this field originate from the chemical approach to nanotechnology, involving bottom-up synthetic pathways to generate nanostructured materials on electrode surfaces. In this work randomly nanoarrayed electrodes were fabricated and electrochemically characterized.This was achieved by depositing gold nanoparticles, AuNPs on bare glassy carbon, from 0.1 mmol/L KAuCl 4 in H 2 SO 4 using chronoamperometry followed by surface passivation through reduction of in situ prepared nitrophenyl diazonium cation. To increase the number of nucleated metal nanoparticles self assembly monolayers (SAMs) of 2-mercaptoetha nol (2-ME) was used during three deposition steps. The nitrophenyl grafted film was characterized by cyclic voltammetry and has shown a significant blocking property towards Fe(CN) 6-3 probe. The nanoholes were produced by stripping the deposited gold nanoparticles (AuNPs) and their response to common probes such as hydroquinone and ruthenium hexamine chloride were studied at different scan rates in comparison with the signal obtained at bare glassy carbon. The improvement in selectivity is atributed to controlling the charge of insulation layer made from nitrophenyl film.-118 Determination of dopamine hydrochloride, DA and Chlorpromazine hydrochloride, CPZ was made with their respective limit of detection 7.4× 10-8 mol/L and 8.5 mol/L measured over concentration ranges of 9.9 ×10-7 mol/L-3.4 × 10-5 mol/L and 7.96 × 10-6 mol/L-2 ×10-4 mol/L, respectively. The limits of detection for DA, CPZ and ascorbic acid, AA, on unmodified (bare) GC electrode were 1.02 × 10-7 mol/L, 1.05 ×10-7 mol/L, and 1.1 ×10-7 mol/L, respectively. The developed method can be applied in selective determination of cationic drugs where anionic species interfere.
... The electrochemical behavior of 3D printed microelectrodes is studied by cyclic voltammetry (CV), which is a classical method for evaluating the quality of electrodes [23] . First, the Pt, Au, Ag and C microelectrodes are cycled (30 scans) in 0.1 mol/L H 2 SO 4 , and the Cu and W microelectrodes are cycled (4 scans) in PBS buffer solution (pH = 7.4). ...
Electrode fabrication is one of the basic practices for electrochemists. Especially, microelectrodes are generally known as “hand-made” and their fabrication is often like an art. In this work, we report a new protocol for fabricating microelectrodes and multi-electrode probes based on recently matured 3D Fused Deposition Modeling (FDM) printing technique. The general concept is to print half of the insulating body in PETG, insert the (etched) metal or carbon wire(s) in the channel(s), and resume printing to complete the whole electrode. The printed electrodes are then sealed by heating and mechanically polished before use. The process requires only low-cost non-specialized facilities that can be easily equipped even in teaching laboratories. Single microelectrodes of Pt, C, Au, Ag, W and Cu with diameter below 5 µm are fabricated and examined by cyclic voltammetry and scanning electron microscopy. Furthermore, a multi-electrode probe consisting of W, Cu, Ag (oxidized to Ag/AgCl) and Pt is also printed and demonstrated for pH (potentiometric) and H2O2 (amperometric) sensing applications.
... Since a nanoparticle is the ultimate case of an ultramicroelectrode, it is appropriate to discuss some of the properties of nanoparticles employing the equations developed for microelectrodes, for calculating the increased rate of diffusion towards an isolated nanoparticle and the corresponding decrease in solution resistance. As discussed by Sequeira et al. [109][110][111], for a single nanoparticle, assumed to be spherical, the limiting current is given by [112,113]: ...
This study examines how the several major industries, associated with a carbon artifact production, essentially belong to one, closely knit family. The common parents are the geological fossils called petroleum and coal. The study also reviews the major developments in carbon nanotechnology and electrocatalysis over the last 30 years or so. In this context, the development of various carbon materials with size, dopants, shape, and structure designed to achieve high catalytic electroactivity is reported, and among them recent carbon electrodes with many important features are presented together with their relevant applications in chemical technology, neurochemical monitoring, electrode kinetics, direct carbon fuel cells, lithium ion batteries, electrochemical capacitors, and supercapattery.
... Fortunately, by that time, the alternatives to mercury were far more exciting than some decades before [21]. Ultramicroelectrodes [22], rotating disk electrodes [23], composite solid electrodes [24] and new and promising solid materials like boron-doped diamond (BDD) [25] were especially versatile and offered far more possibilities of modification than some years before. Many relevant characteristics of mercury such us the wide cathodic range and the ability to accumulate reduced metal ions could be acceptably reproduced with bismuth and antimony films coated on solid electrodes (typically made of glassy carbon) [26][27][28]. ...
A personal mini-review is presented on the history of electroanalysis and on their present achievements and future challenges. The manuscript is written from the subjective view of two generations of electroanalytical chemists that have witnessed for many years the evolution of this discipline.
The single‐entity electrochemistry (SEE) of electrocatalytic platinum (Pt) single nanoparticles (NPs) on a less electrocatalytic silver (Ag) ultramicroelectrode (UME) surface was investigated using the electrocatalytic amplification method. Two characteristic types of current responses—current staircases and blips (or spikes)—were observed during single NP collision experiments, depending on the applied potential at the Ag UME. Notably, at applied potentials of 0.13 and 0.17 V, the Ag UME becomes passive due to the formation of a delicate oxide layer, resulting in a highly stable background current. This leads to an enhanced signal‐to‐noise (S/N) ratio, attributed to the low background current, when using Ag UME compared to commonly used UMEs such as Au, C, Ni, and Hg for the SEE of Pt NPs. The exceptionally low background current can provide a significant advantage for detailed observation of SEE signals and further mechanistic studies based on the current response.
This review will investigate the impact of electrochemical characterization method design choices on intrinsic catalyst activity measurements by predominantly using the oxygen reduction reaction (ORR) on supported catalysts as a model reaction. The wider use of hydrogen for transportation or electrical grid stabilization requires improvements in proton exchange membrane fuel cell (PEMFC) performance. One of the areas for improvement is the (ORR) catalyst efficiency and durability. Research and development of the traditional platinum-based catalysts have commonly been performed using rotating disk electrodes (RDE), rotating ring disk electrodes (RRDE), and membrane electrode assemblies (MEAs). However, the mass transport conditions of RDE and RRDE limit their usefulness in characterizing supported catalysts at high current densities, and MEA characterizations can be complex, lengthy, and costly. Ultramicroelectrode with a catalyst-filled cavity addresses some of these problems, but with limited success. Due to the properties discussed in this review, the recent floating electrode (FE) and the gas diffusion electrode (GDE) methods offer additional capabilities in the electrochemical characterization process. With the FE technique, the intrinsic activity of catalysts for ORR can be investigated, leading to a better understanding of the ORR mechanism through more reliable experimental data from application-relevant high-mass transport conditions. The GDEs are helpful bridging tools between RDE and MEA experiments, simplifying the fuel cell and electrolyzer manufacturing and operating optimization process.
As biomarkers of cancer, the accurate and sensitive detection of microRNAs is of great significance. Therefore, we proposed a surface-enhanced Raman scattering (SERS)/electrochemical (EC) dual-mode nanosensor for sensitively detecting miRNA-141. The nanosensor uses Au@Ag nanowires as a novel SERS/EC sensing platform, which has the advantages of good biocompatibility, fast response, and high sensitivity. The dual-mode nanosensor can not only effectively overcome the problem of insufficient reliability of single signal, but also realize the amplification and stable output of the detection signal, to ensure the reliability and repeatability of miRNA detection. With this sensing strategy, the target miRNA-141 can be detected over a wide linear range (100 fM to 50 nM) (LOD of 18.4 fM for SERS and 16.0 fM for electrochemical methods). In addition, the process shows good selectivity and can distinguish miRNA-141 from other interfering miRNAs. The actual analysis of human serum samples also proves that our strategy has good reliability, repeatability, and has broad application prospects in the field of analysis and detection.
Scanning electrochemical microscopy (SECM) enables the high‐resolution imaging and measurement of electrochemical, chemical, and biological reactions at various interfaces. The power of SECM originates from the versatility based on the highly accurate and precise positioning of a micrometer‐ or nanometer‐sized ultramicroelectrode tip near the surface of substrates including nanomaterials and biological cells. The current response of the SECM tip (and that of the substrate when needed) is monitored to quantitatively probe the dynamics of a targeted reaction in situ and in real‐time. This article is concerned about the fundamental principle and recent progress of SECM for both imaging and nonimaging applications. Major progress was made through the development of specific instrumentation and protocols to transition nanoscale SECM into a reliable and powerful scientific tool. Nanoscale SECM can image and characterize single molecules and single nanostructures, quantitatively address the kinetics of otherwise unmeasurably fast reactions, and dynamically resolve a complicated reaction into multiple steps. Further technological advancement will find the wide and routine applications of nanoscale SECM. By contrast, microscale SECM is broadly used as an established method in various research areas including biology, corrosion, energy, and materials science to gain deeper insights into the mechanisms of important processes such as electrocatalysis and cellular activity and signaling. The ever‐lasting development of SECM during the last three decades manifested that the fundamental concept of SECM far exceeded and preceded our research needs and technological capabilities, which have not matched yet to fully realize the high potentials of this already remarkable methodology.
A nanoelectrode with a controllable area was developed using commercial atomic force microscopy and a hydrogel. Although tremendous advantages of small electrodes from micrometer scale down to nanometer scale have been previously reported for a wide range of applications, precise and high-throughput fabrication remains an obstacle. In this work, the set-point feedback current in a modified scanning ionic conductance microscopy system controlled the formation of electrodes with a nanometer-sized area by contact between the boron-doped diamond (BDD) tip and the agarose hydrogel. The modulation of the electroactive area of the BDD-coated nanoelectrode in the hydrogel was successively investigated by the finite element method and cyclic voltammetry with the use of a redox-contained hydrogel. Moreover, this nanoelectrode enables the simultaneous imaging of both the topography and electrochemical activity of a polymeric microparticle embedded in a hydrogel.
For the first time a solid state lead–tin microelectrode (diameter ϕ 25 µm) was utilized for U(VI) ion determination by adsorptive stripping voltammetry. The described sensor is characterized by high durability, reusability and eco-friendly features, as the need for using lead and tin ions for metal film preplating has been eliminated, and consequently, the amount of toxic waste has been limited. The advantages of the developed procedure resulted also from the utilization of a microelectrode as a working electrode, because a restricted amount of metals is needed for its construction. Moreover, field analysis is possible to perform thanks to the fact that measurements can be carried out from unmixed solutions. The analytical procedure was optimized. The proposed procedure is characterized by two orders of magnitude linear dynamic range of U(VI) determination from 1 × 10−9 to 1 × 10−7 mol L−1 (120 s of accumulation). The detection limit was calculated to be 3.9 × 10−10 mol L−1 (accumulation time of 120 s). RSD% calculated from seven subsequent U(VI) determinations at a concentration of 2 × 10−8 mol L−1 was 3.5%. The correctness of the analytical procedure was confirmed by analyzing a natural certified reference material.
Electrochemical measurements using an agarose hydrogel as a solid electrolyte and ferrocyanide as a redox probe were conducted to analyze transport properties and natural convection effects. The mass transport properties and diffusion coefficients of ferrocyanide were studied using various macroelectrodes and ultramicroelectrodes via cyclic voltammetry. The experimental results confirmed that the mass transfer behavior in agarose was similar to that in solution. The good linearity of the square root of the scan-rate-dependent peak current demonstrated that diffusion is dominant during mass transfer in agarose hydrogel owing to a reduction in other mass transport effects (i.e., migration and convection). Furthermore, chronoamperometry (CA) was performed to estimate the effects of natural convection in the solution and agarose hydrogel. CA curves and plots of current as a function of the inverse square root of time yielded irregular and irreproducible responses in the solution for relatively long-term electrochemistry. However, in the agarose hydrogel, the CA response was more regular and reproducible for > 300 s because of reduced natural convection, based on the Cottrell’s theory.
With the advancement of technology in nanoelectrode fabrication, the fabrication of a nanopore electrode or a polishable solid nanoelectrode has become a routine. These tools make it possible to detect the electrochemistry in nanoscale. In this article, we concentrate on the application of an individual nanoelectrode in nanoelectrochemistry research. Progress of nanoelectrochemistry research in areas, such as detection of reaction kinetics and mechanisms in nanoscale, measurement of local reactions in a cell, exploration of the electrochemical properties of a single nanoparticle (NP) or single molecule, mapping a substrate for high resolution imaging, etc., have been reviewed.
A simple and universal strategy for fabricating flexible 25 μm platinum (Pt) disk ultramicroelectrodes (UMEs) was proposed, where a pulled borosilicate glass micropipette acted as a mold for shaping the flexible tip with flexible epoxy resin. The whole preparation procedure was highly efficient, enabling 10 or more probes to be manually fabricated within 10 h. Intriguingly, this technique permits an adjustable RG ratio, tip length, and stiffness, which could be tuned according to varying experimental demands. Besides, the electroactive area of the probe could be exposed and made renewable with a thin blade, allowing its reuse in multiple experiments. The flexibility characterization was then employed to optimize the resin/hardener mass ratio of epoxy resin and the tip position during HF etching in the fabrication process, suggesting that more hardener, a larger RG value, or a longer tip length obtained stronger deformation resistance. Subsequently, the as-prepared probe was examined by optical microscopy, cyclic voltammetry, and SECM approach curves. The results demonstrated the probe possessed good geometry with a small RG ratio of less than 3 and exceptional electrochemical properties, and its insulating sheath remained undeformed after blade cutting. Owing to the tip's flexibility, it could be operated in contactless mode with an extremely low working distance and even in contact mode scanning to achieve high spatial resolution and high sensitivity while guaranteeing that the tip and samples would suffer minimal damage if the tip crashed. Finally, the flexible probe was successfully employed in three scanning scenarios where tilted and 3D structured PDMS microchips, a latent fingerprint deposited on the stiff copper sheet, and soft egg white were included. In all, the flexible probe encompasses the advantages of traditional disk UMEs and circumvents their principal drawbacks of tip crash and causing sample scratches, which is thus more compatible with large specimens of 3D structured, stiff, or even soft topography.
Single-particle collisions have made many achievements in basic research, but challenges still exist due to their low collision frequency and selectivity in complex samples. In this work, we developed an "on-off-on" strategy based on Pt nanoparticles (PtNPs) that catalyze N2H4 collision signals on the surface of carbon ultramicroelectrodes and established a new method for the detection of miRNA21 with high selectivity and sensitivity. PtNPs catalyze the reduction of N2H4 on the surface of carbon ultramicroelectrodes to generate a stepped collision signal, which is in the "on" state. The single-stranded DNA paired with miRNA21 is coupled with PtNPs to form the complex DNA/PtNPs. Because PtNPs are covered by DNA, the electrocatalytic collision of N2H4 oxidation is inhibited. At this time, the signal is in the "off" state. When miRNA21 is added, the strong complementary pairing between miRNA21 and DNA destroys the electrostatic adsorption of DNA/PtNP conjugates and restores the electrocatalytic performance of PtNPs, and the signal is in the "on" state again. Based on this, a new method for detecting miRNA21 was established. It provides a new way for small-molecule sensing and has a wide range of applications in electroanalysis, electrocatalysis, and biosensing.
Thrombin plays a central role in hemostasis and its imbalances in coagulation can lead to various pathologies. It is of clinical significance to develop a fast and accurate method for the quantitative detection of thrombin. Electrochemical aptasensors have the capability of combining the specific selectivity from aptamers with the extraordinary sensitivity from electrochemical techniques and thus have attracted considerable attention for the trace-level detection of thrombin. Nanomaterials and nanostructures can further enhance the performance of thrombin aptasensors to achieve high sensitivity, selectivity, and antifouling functions. In highlighting these material merits and their impacts on sensor performance, this paper reviews the most recent advances in label-free electrochemical aptasensors for thrombin detection, with an emphasis on nanomaterials and nanostructures utilized in sensor design and fabrication. The performance, advantages, and limitations of those aptasensors are summarized and compared according to their material structures and compositions.
In this work, we present the synthesis of amphiphilic diblock copolymer poly(ethylene oxide-block-methylmethacrylate) (PEO-b-PMMA) using atom transfer radical polymerization and demonstrate its application for the fabrication of recessed nanodisk-array electrode (RNE) as a platform for protein detection. For the fabrication of thin-film, PEO-b-PMMA was spin-coated on the silicon wafer and the phase separation was achieved by a unique direct immersion annealing technique followed by washing with the water/methanol solution to remove the PEO phase. The microphase separation and comprehensive porosity characterizations were done using AFM and electron microscopy. Later, the thin film was loaded on the surface of the glassy carbon electrode by wedge transfer technique and the RNE property of the mesoporous thin film was confirmed using cyclic voltammetry (CV). For the protein sensing application, the thin film-coated electrode was biotinylated using EDC-NHS chemistry, followed by the blocking of the non-specific sites using bovine serum albumin (BSA). The reduction in the CV current and the increase in the channel resistance in EIS at each stage of fabrication confirmed the successful immobilization. For the streptavidin detection, the RNE displayed an ultra-low detection limit of 0.15 fg/ml with enhanced selectivity which confirms the potential of PEO-b-PMMA based RNE as an ultrasensitive immunosensor platform for any biomarker detection.
Scanning electrochemical microscopy (SECM) is a technique that employs a microelectrode as a tip to get electrochemical information with high spatial resolution. The current measured at the tip corresponds to electrochemical processes taking place in a micrometric region surrounding the tip and a specific substrate. SECM has been used to characterize and image various electrochemical systems such as polymers, nanomaterials, electrode surfaces, and liquid–liquid interfaces. Owing to new technologies to fabricate tips in the nanometer dimension (nanoelectrodes), SECM has evolved as a fundamental tool for investigating molecular-level chemical reactions in biological systems. Herein, we shall give an overview of the applications of the technique in multidisciplinary areas ranging from biological research to materials science, energy-related topics, catalysis, and corrosion.
Ultramicroelectrodes (UMEs) have demonstrated their utility in different applications, ranging from probing chemistry to high-resolution electrochemical imaging. Conical UMEs with the apex in the nanometer range are of special interest because their geometrical features may allow the study of single/few nanoparticles, single entities, or electrochemical reactions occurring in the inner structures of living cells which are difficult to access with other types of UMEs. However, there is a lack of experimental studies with individual unshielded conical electrodes aiming at quantifying the impact of the geometry and dimensions on their electrochemical response. In this work, W / WO2 conical UMEs with aspect ratios ranging from 6.6 to 22 and apexes with nm-size dimensions were prepared by electrochemical etching of tungsten wires through an induced dynamic meniscus regime, and in one case followed by focus ion beam milling. The electrodes were characterized by scanning electron microscopy and by cyclic voltammetry in 5 mM [Fe (CN)6]³⁻ and 5 mM [Fe (CN)6]⁴⁻ in 0.5 M KCl as a function of the depth of the UME immersed in the electrolyte solution. Computational fluid dynamics simulations were used to investigate the mass transfer of the electroactive species at the vicinity of the electrodes. Analytical expressions to predict the steady-state current of conical electrodes with aspect ratios from 3 to 22 and radius of curvature below 110 nm were also derived. It was found that the ratio of the electrochemical surface area to the geometric one rapidly increases when the depth of the UME's in solution is lower than 15 µm, in agreement with a rapid increase of the magnitude of the total flux towards the UMEs apex. Both experimental and simulation studies point to the radius of curvature as the most important parameter determining the rate of the oxidation / reduction of the [Fe (CN)6]³⁻/ [Fe (CN)6]⁴⁻ species at non-insulated conical UMEs with high aspect ratio.
Bioelectrochemistry is a rapidly growing field in which electrochemistry is combined with concepts from biochemistry, medicinal chemistry, and analytical chemistry to understand the properties and principles of biomolecule-electrode interactions. The integration of bioelectrochemistry with nanotechnology has led to the development of various types of enhanced biosensing capability. This chapter summarizes the different nanomaterials used in the fabrication of electrochemical biosensors, considering the effect of nanostructuring on biosensing performance. Besides this, different types of nanobiosensors are described, with emphasis on the nanostructure and fabrication, applicable to different biorecognition elements. Lastly, we discuss the combination of electrochemistry with surface plasmon resonance, a recently discovered, powerful technique for studying the binding kinetics of biorecognition events during electrochemical detection.
Point‐of‐care testing (POCT) is becoming a hot research topic that allows rapid, on‐site, and non‐professional measurements outside the central laboratory. The micro‐fabricated devices prepared by various micro‐machining technologies have shown the advantages of low reagent consumption, high‐throughput samples, and wearability. This review presents the recent progress of electrochemical biosensors based on various micro‐fabricated devices for POCT and the corresponding electrochemical techniques. Signal amplification strategies based on enzyme and nanotechnology are also illustrated for the more sensitive POCT applications of these micro‐fabricated devices. Consequently, the trends and challenges of electrochemical biosensors based on micro‐fabricated devices in POCT diagnosis are discussed.
Heavy metal pollution has become one of the most serious environmental problems in the world, and even a trace amount may cause severe damage to the ecological environment and human health. Therefore, it is essential to develop a fast and reliable on-site detection technique. The conventional methods of detection are capable of effectively carrying out analysis. Nonetheless, concerns such as costly operation and voluminous machinery are raised. Screen-printed electrochemical sensors, also known as screen-printed electrode (SPE), with high sensitivity, short detection time, and excellent adaptability for in-situ measurements, have received tremendous attention recently with the advent of screen-printing technology. Starting with a description of the feasibility of SPE in heavy metal monitoring, this review presented the fundamental principles of SPE sensing, followed by the current development of SPE in terms of advanced design and configurations. The latter part of this review focused on improving the sensitivity and selectivity of SPE through modification in the detection of heavy metals. Different modifying materials (organic, inorganic, composite) and techniques of modification (drop casting, electrochemical deposition) have been scrutinized. Moreover, the applicability of some of these modified SPEs was investigated in real samples.Graphic abstract
Localized electrochemical deposition (LECD) is a promising and economical three-dimensional (3D) microstructure fabrication method. As the current research on LECD mainly focuses on the deposition of copper and nickel, this work investigated the fabrication of silver microcolumns via LECD, using polyethyleneimine (PEI) as an additive. Firstly, the effects of PEI on the surface morphologies and structures of the deposited microcolumns were studied; it was found that adding PEI to the electrolyte could reduce the column surface roughness and promote the formation of uniform and straight columnar structures. Thereafter, the effects of the deposition voltage and initial electrode spacing on the LECD process performance were studied using a 0.08- g/L PEI electrolyte; nanoindentation was performed to determine the hardness and Young's modulus of the silver microcolumns. Furthermore, based on the experimental results, a possible action mechanism for PEI and the formation mechanism of the microcolumns' nodular structures were proposed. This technology was used to manufacture high-density and high-quality interconnects and electrodes for electrochemical biosensors.
The diffusion indicator α described by Haonan and Compton (Journal of Electroanalytical Chemistry, 866 (2020) 114149) is derived for hemispheroidal and ring microelectrodes. The diffusion associated with six electrodes of various geometries (microsphere, microdisc, oblate, prolate, whisker and ring) is studied using this general diffusion indicator parameter. The general parameter α is obtained from theoretical and empirical chronoamperometric current equations for E and EC' reactions in the literature. In the various geometries, the value of the diffusion indicator and its evolution over time demonstrate the contrasting influences of planar and convergent/divergent diffusion.
Glucose oxidase (GOx) is an enzyme frequently used in glucose biosensors. As increased temperatures can enhance the performance of electrochemical sensors, we investigated the impact of temperature pulses on GOx that was drop‐coated on flattened Pt microwires. The wires were heated by an alternating current. The sensitivity towards glucose and the temperature stability of GOx was investigated by amperometry. An up to 22‐fold increase of sensitivity was observed. Spatially resolved enzyme activity changes were investigated via scanning electrochemical microscopy. The application of short (<100 ms) heat pulses was associated with less thermal inactivation of the immobilized GOx than long‐term heating.
Developing novel microelectronic devices for electrochemical measurements and electrochemiluminescence (ECL) study is of great importance. Herein, we fabricated a submicrometer-sized dual carbon electrode (DCE) and investigated its annihilation ECL behavior under steady-state conditions for the first time. The oxidation and reduction of the model luminophore, [Ru(bpy)3]2+, occurred separately at the two sides of the DCE, and the electrogenerated ions then diffused to the gap between the two electrodes to generate the excited-state intermediate [Ru(bpy)3]2+* and ECL emission. Compared with other types of two-electrode systems, the prepared DCE possesses a smaller total size and an ultrasmall interelectrode distance of 60 nm or less, which could result in a shorter diffusion time and an amplified ECL signal without the purification of the solvent and supporting electrolytes. On the basis of the constructed ECL microscopic platform, we successfully obtained a stable and confined ECL signal in the vicinity of the electrode tip. Furthermore, a two-dimensional finite element method simulation of this model system was performed to quantitively analyze the concentration profiles of the electrogenerated species around the tip of the DCE and predict the concentrations of [Ru(bpy)3]2+* with various gap distances. The simulation results also proved that the higher concentrations of [Ru(bpy)3]2+* could be achieved with a smaller distance with a possible amplification factor of 6 (compared with the concentration when the gap distance is greater than 300 nm). This work provides an experimental model for further improvement of ECL efficiency and broadens the availability for annihilation ECL applications in small confined spaces.
This section will provide insight into the most widely used electrodes for bioanalysis, which are carbon fiber microelectrodes. Initially insight will be provided on how to fabricate these electrodes with emphasis on how these have evolved into varying geometries and now are used as multi-barrel or nanotip electrodes. An insight into material properties of the carbon fiber and how this can influence electrochemical measurements will be provided. Additionally, modification of these electrodes for enhanced electrochemical measurement will be discussed.
Nanoporous gold (NPG) has been extensively investigated because of their applications. Here, we report a straightforward method for the preparation of NPG on micrometer‐scale electrodes. Well‐defined NPG structures were formed on Au surfaces by a single potential scan within 100 seconds. This method is applied to Au surfaces with small dimensions regardless of the electrode geometry, whereas it is not applicable to conventional millimeter‐scale electrodes. The effects of electrode sizes and scan rates on NPG formation were systematically examined, and the amperometric glucose detection with 20‐µL sample volumes using an ultramicroelectrode (UME) with NPG surfaces was demonstrated.
In recent years, the environmental pollution in China has become rather serious with the rapid development of economy. Emission of industrial wastewater and overexploitation of metallic minerals strengthens the heavy metal pollution in aqueous environment. For the dissolved heavy metal ions in water are generally colorless and tasteless, it is difficult to be discovered by senses of vision, smell and taste. Since heavy metals of cumulative effect in the body would do a lot of harm to humans, animals and plants, the technique of quick and in-situ heavy metal detection in aqueous environment is of great significance.
In this thesis, electrochemical stripping voltammetry (SV) was selected as an effective method for the in-situ measurements of heavy metal ions in aqueous environment and its principle and quantitative skills are illustrated. We also did researches on the three main development trends which are creative sensing materials, micromation and integration for the sensors focusing on stripping voltammetric detection of heavy metals. Besides that, an automatic monitor and a wireless analysis instrument were developed respectively for the heavy metal detections in the Bohai Sea and the Taihu Lake. The researches were financed by the National High Technology Research Program (863 Program) and the National Key Basic Research Program (973 Program).
The major contents of this thesis are as follows:
1) Heavy metals including zinc, cadmium, lead and copper ions in aqueous sample were determined by the mercury-film electrode using differential pulse stripping voltammetry (DPSV) with the analysis instrument fabricated by our lab. Specific detection methods were applied depending on the backgrounds of the solutions. The accuracy of measuring pure water-background samples was good and the precision achieved 5% while the precision of measuring seawater-background samples was about 15%. The detection limits of all the four elements were in the range of 0.1-0.3µg/L.
2) For the trend of creative sensing materials in developing electrochemical sensors, in-situ coated bismuth electrode was tested for heavy metal detection and the method of detecting pure water-background samples was determined. The method of detecting heavy metal ions in seawater by bismuth electrode was also studied. Attractive characteristics of bismuth electrodes are mercury-free and environmentally friendly.
3) For the trend of micromation in developing electrochemical electrodes, a microband electrode array consisted of 100 microelectrodes was developed. The microband electrodes with inter-electrode spacing of 64µm were 8mm long, 6µm wide and 100nm thick. The microband electrode array with excellent dimensional and voltammetric traits did good performance in the detection of mixed samples of lead and copper ions.
4) For the trend of integration in developing electrochemical electrodes, a photoelectric integrated sensor combined light addressable potentiometric sensor (LAPS) and microelectrode array (MEA) on the same silicon chip was designed and fabricated. It was possible to take respective advantages of LAPS and MEA according to different conditions of the measurements. When LAPS and MEA were applied together to the detection, the linear regression model was built to improve the anti-interference ability.
5) Based on the researches on sensors and combining the techniques of automation and chemical analysis, an automatic heavy metal monitor for seawater was developed. All the parameters of the monitor passed the third party test. The automatic heavy metal monitor experienced a number of sea trials and filled the vacancy of in-situ heavy metal monitor for seawater in China.
6) By integrating techniques of traditional heavy metal detection and wireless communication, a wireless heavy metal analysis instrument was developed. The optimized water path greatly improved the portability. Preliminary test showed that the instrument functioned well and satisfied the requests of the design.
La synthèse de catalyseurs sans métaux nobles est une voie prometteuse pour rendre accessible à l’échelle mondiale les piles à combustible. L’analyse électrochimique de ces matériaux n’est pas aisée que ce soit pour comparer les propriétés électro catalytiques ou pour comprendre le fonctionnement de ces catalyseurs. Ceci provient du fait que la communauté scientifique évalue les performances catalytiques à l’échelle du matériau, donc sur un très grand nombre d’objets dont la réponse est moyennée. Les travaux présentés dans ce mémoire ont mis en place une méthode d’analyse de l’activité électrocatalytique de matériaux sans métaux nobles pour la réduction de l’oxygène en milieu acide par microscopie électrochimique à balayage. Cette approche permet d’étudier aussi bien macroscopiquement que microscopiquement les catalyseurs et d’étudier simultanément plusieurs catalyseurs, ce qui rend plus fiable la comparaison des résultats. Le dispositif présenté dans ce travail a permis de comparer différents catalyseurs avec des compositions proches ainsi que d’étudier l’influence de différentes paramètres sur un catalyseur : le chargement, la surface, la masse déposée et la quantité de Nafion ajoutée. Il a aussi été montré qu’il était possible d’étudier la stabilité des catalyseurs via ce dispositif. Ces différents résultats suggèrent que la méthode mise en place est polyvalente et permettra de nombreuses autres études.
Here, we investigated the sealing quality between the microwire disk and the surrounded glass sheath of platinum disk ultramicroelectrodes (UMEs) using outer-sphere (ferrocene methanol, FcMeOH oxidation) and inner-sphere electrochemical reactions (Hydrogen underpotential deposition, (HUPD) and hydrogen evolution reaction (HER)) by cyclic voltammetry (CV) approach. The tilt aspect in the CV is ascribed to the creeping of the electrolyte solution between the microelectrode wire and the glass sheath, causing iR drop which shows resistive nature in CVs. The resistive nature of CV was discussed by performing HER using both poor and well-sealed disk UMEs. Scan rate dependence double-layer capacitance (Cdl) data confirms the leak between a glass-wire interface in the UMEs. Further, we showed a quantitative treatment for the sealing assessment using analytical expressions. Overall, we show a rapid check procedure of sealing’s quality in fabricating Pt disk UMEs. This simple procedure presented in this work can be used to evaluate the sealing quality of other types of micro/nanoelectrodes during their fabrication.
Here, a facile, low-cost and recyclable method for the fabrication of electrodes. Organic soluble wax sealing allows one to bypass the complicated laser-pulling and torch-sealing used in conventional fabrication protocols. Electrodes with various electrode wires were successfully fabricated, including platinum (e.g., 200, 100, 76, and 25 μm) and carbon fiber (11 μm). Our electrodes are feasible and convenient to recycle due to the good solubility of wax in acetone. The fabricated electrodes are then characterized by cyclic voltammetry in K3[Fe(NH3)6] and dopamine solutions. The CV curves show well-defined peak and sigmoidal responses, which confirms the successful fabrication of ideal conventional-sized electrodes and microelectrodes.
In this work, we investigated the effect of electrodeposition of polypyrrole (PPy) films on two different self‐assembled monolayers (SAMs) modified gold electrode for the electrochemical construction of ultramicroelectrode (UME). In order to obtain SAM modified surfaces, 4‐mercapto‐1‐butanol and 11‐mercaptoundecanoic acid were used. The effect of these two chains on pyrrole electropolymerization was compared. Electropolymerization of pyrrole on SAM modified Au electrode was carried out by using cyclic voltammetry (CV) and constant potential electrolysis. To investigate the UME formation, the obtained surfaces were tested in Fe(CN)63−/4− redox system. UMEs were characterized using scanning electron microscopy energy‐dispersive X‐ray spectroscopy, attenuated total reflectance fourier transform infrared spectroscopy and electrochemical impedance spectroscopy. The designed UME was applied as an immobilization matrix to entrap a redox protein, Hemoglobin (Hb), as a model. Direct electron transfer between this protein and the fabricated thiol modified PPy based UME was achieved, which is very challenging on bare electrode surfaces. With this study, a simple, low‐cost and reproducible UME production way has been successfully accomplished and Hb modified UME is promising for different bioanalytical applications, for instance; cellular hydrogen peroxide or nitrite sensing.
The construction and characterisation of ring–disk (RD) microelectrodes suitable for use in scanning electrochemical microscopy (SECM) is reported. Such RD electrodes are proposed as probes for novel generator–collector SECM experiments. In this case, the interaction of both the reactants and products with the substrate under investigation can be followed simultaneously from a single approach curve to the substrate. Examples of such approach curves to conducting and insulating substrates are given to demonstrate the potential of this new mode of SECM operation.
Electrodes with electrochemical dimensions as small as 10 angstroms have been fabricated and used for electrochemical studies.
These nanometer-scale electrodes have enabled the measurement of electron-transfer rate constants, khet, that are two orders of magnitude faster than khet values accessible with any other electrochemical method.
The preparation of microelectrodes for use in various electrochemical mapping techniques (e.g., scanning electro‐chemical
microscopy and photoelectrochemical microscopy) is described. A commercial optical fiber (3.7 μm core diameter with
and
diam of cladding and coating, respectively) was stripped of its polymer coating at one end. The stripped core was then etched
in different concentrations of hydrofluoric acid solution until a tip near 1 μm in diameter was achieved. The resulting optical
fiber was coated with gold by de‐sputtering to produce a microelectrode less than 3 μm in total diameter. The optical fiber
microelectrode was then coated with an insulating varnish yielding a final tip size normally less than 5 μm in diam. The final
microelectrode was tested both for leaks in its polymer insulation and for electrical conductivity of the gold coating. The
microelectrode has been used in several studies in our laboratories. Details of the preparation conditions are described here.
A new way to construct small electrodes by the use of metal microbeads is described. Gold beads of 1.5−3.0 μm diameter were used to construct inexpensive and disposable microelectrodes with overall structural diameters of ≤6 μm and electrode diameters of ≤5 μm. The voltammetric response of these electrodes is consistent with existing theory, and the electrodes exhibited the sigmoidal waves expected at ultramicroelectrodes.
: We describe a new ultramicroelectrode fabrication technique wh ich has allowed the preparation of conical and hemispherical platinum-iridium ultramicroelectrodes with radii of 0.5 micro m-10 micro m. These electrodes have been characterized by scanning electron microscopy, cyclic voltammetry and chronoamperometry. The smallest radii electrodes (r < 2 micro m) exhibited diffusional behavior in accord with a hemispherical geometry, while larger radii electrodes exhibited conical diffusion behavior. A first order mathematical approach to diffusion at conical surfaces has been developed to explain the results for these electrodes. Conical, Hemispherical ultramicroelectrodes, Construction, and Characterization.
Methods of construction and the behavior of microdisk and microsphere electrodes having radii down to 1000 and 200 A respectively and of microspheres having widths down to 250 A are discussed. The high steady state rates of mass transport and the low ohmic overpotentials allow the study of systems under simple steady state conditions as well as the study of many novel systems. It is shown that the rates of fast electrode reactions can be readily measured and the kinetics of reactions in solution coupled to electrode reactions are more easily determined than by relaxation methods using planar electrodes. Reactions can be examined in solvents using low or zero concentrations of support electrolyte as well as in low dielectric constant solvents.
The development of extensive research using microelectrodes has produced a literature containing many details which serve as guidelines for successful experimentation. Experimental issues important in working with microelectrodes are fabrication, testing of the resulting electrodes, measurement of small currents, especially in the steady state, and instrumentation for fast experiments (scan rates ≥ 1000 Vs-1 or characteristic times of ≤ 50 μs). Appropriate electrodes and instrumentation are commercially available for types of applications which are routine. The purpose of this chapter is to summarize a discussion held at the NATO ASI on Microelectrodes which dealt with experimental aspects. This discussion is placed in the context of the literature in this field. It is not intended to be comprehensive or a review.
: Electrodes having tip diameters in the few hundred nanometer range have been constructed by coating thin copolymer films onto suitable electrode materials that have been etched to very small dimensions. Desorption of the insulation from the exact tip of the structure is a key element in the construction of electrodes of suitable dimensions for neuronal analysis. Preliminary data obtained in collaboration with Yau Yi Lau of our laboratories shows the utility of these insulated, etched carbon fiber electrodes for analysis of neurotransmitter release from single nerve cells. (js)
Nanometer-sized glass-sealed metal ultramicroelectrodes (UMEs) have been prepared using a laser-based micropipet puller. The tip was exposed to solution either by etching away a small portion of glass insulator or by micropolishing. The characterization of the UMEs was carried out by a combination of steady-state voltammetry, scanning electron microscopy (SEM), and scanning electrochemical microscopy (SECM). The cyclic voltammograms obtained have a regular shape with very small capacitive and resistive background. The effective electrode radii obtained from voltammetry were between 2 and 500 nm. From the SEM micrographs, the shape of polished tips appears to be close to a microdisk, while the geometry of etched electrodes is closer to conical. Accordingly, the SECM current-distance curves (iT-d) obtained with polished electrodes fit well the theory for a disk-shaped tip, while a 20-nm-radius etched electrode was shown to be a fairly sharp cone with a height-to-radius ratio of about 2.5. The experimental data were compared to the theory developed for disk-shaped, conical, and recessed tips to demonstrate suitability of the produced electrodes for quantitative electrochemical experiments. The prospects of steady-state measurements of the rates of fast heterogeneous reactions are discussed. Submicrometer-sized ion selective electrodes (ISEs) were prepared by coating etched Ag tips with silver iodide. The concentration response of such ISEs remained stable and linear after coating of the ISEs with protective Nafion film.
In the 1950s and 1960s fundamental developments in electrochemical methods included voltammetry and low signal techniques. A generation later, the discovery of the unusual properties of ultramicroelectrodes has opened new possibilities of analyzing electrode processes. The changes in mass transport conditions bring about extremely high current densities at ultramicroelectrodes, whereas the currents themselves become very small. This little-noticed phenomenon allows for many electroanalytical applications that are not possible with conventional electrodes, especially experiments in solutions with very low electrolyte concentrations, in nonpolar solvents, in solids, and even in gases. In addition, two factors— changes in the experimental time scale at low scan rates because of the size of the electrode, and insignificant iR effects at very high scan rates—make it possible to study very fast homogeneous and heterogeneous electrode processes.
A new method for high-resolution imaging, near-field scanning optical microscopy (NSOM), has been developed. The concepts governing this method are discussed, and the technical challenges encountered in constructing a working NSOM instrument are described. Two distinct methods are presented for the fabrication of well-characterized, highly reproducible, subwavelength apertures. A sample one-dimensional scan is provided and compared to the scanning electron micrograph of a test pattern. From this comparison, a resolution of > 1,500 A (i.e., approximately lambda/3.6) is determined, which represents a significant step towards our eventual goal of 500 A resolution. Fluorescence has been observed through apertures smaller than 600 A and signal-to-noise calculations show that fluorescent imaging should be feasible. The application of such imaging is then discussed in reference to specific biological problems. The NSOM method employs nonionizing visible radiation and can be used in air or aqueous environments for nondestructive visualization of functioning biological systems with a resolution comparable to that of scanning electron microscopy.
Numerous anodic oxidations of organic compounds occur at platinum electrodes by mechanisms catalyzed by anodic formation of the lower oxide (PtOH). The electrocatalytic response is short-lived because of the transient lifetime of PtOH and because reaction products (i.e., free radicals) are strongly adsorbed to block further surface activity. The adsorbed carbonaceous material is oxidatively desorbed quite efficiently by the application of a large positive pulse to generate a complete surface layer of the higher oxide (PtO). The “clean”, reduced platinum surface is restored quickly by a negative potential pulse and analyte is adsorbed prior to a positive step of potential for electrocatalytic detection. Multistep waveforms at frequencies of about 0.5–2 Hz are described for successful amperometric detection of carbohydrates, amino acids, and sulfur compounds in flow-injection and chromatographic systems. Preliminary results are also given for detection of As(OH)3 and thiourea at a β-PbO2 electrode at a constant potential.
An analysis of transport of charged and uncharged species associated with a steady-state faradaic process at a spherical microelectrode is reported. We examine systems comprising various relative concentrations of a redox species and, if charged, its counterion and an inert electrolyte. Of particular interest is the behavior of these systems when the thickness of the diffuse double layer (characterized by the Debye length, k{sup {minus}1}) and the radius of the electrode (r{sub 0}) are comparable. Transport of each species is assumed to be governed by the Nernst-Planck equation. A generalized solution obtained by using finite-difference simulations demonstrates that significant enhancement or inhibition of the steady-state flux can occur and will depend upon the dimensionless parameter r{sub 0}k, upon the relative values of the applied potential (E{sub app}), the formal redox potential (E{degree}{prime}), and the potential of zero charge (E{sub pzc}), upon the charges and relative concentrations of the species in solution, and upon the distance of closest approach of the reactant to the electrode surface.
Communication: Stable nanostructured networks of quantum-sized particles have been prepared (see Figure) using self-assembly techniques. Here, two new techniques are reported for the preparation of non-metallic composite materials comprising nanometer-sized gold particles self assembled into a 3-D network by means of organic dithiols.
Theory, preparation, and applications of microelectrodes and microelectrode arrays are critically reviewed, and future trends in the field are outlined. An operational definition of a microelectrode is also recommended.
Au and carbon fiber ultramicroelectrodes have been fabricated by using an anodic electropolishing method. The radii of them were found to be about 450 nm on Au and 240 nm on carbon electrodes from the calculation based on the steady-state limiting currents.
A new instrumental technique that combines scanning electrochemical microscopy and scanning photoelectrochemical microscopy is described. This technique is capable of monitoring electroassisted and/or photoassisted reactions on semiconductor surfaces either concurrently or sequentially. The instrument uses a probe which consists of an optical fiber coated with gold and isolated from its surrounding environment with a polymer film. Measurements of the electroreduction of Br 2 at the gold ring which had been generated by photo-oxidation of Br - on a 50 A thick TiO 2 film on Ti are discussed
A technique for combining scanning electrochemical microscopy (SECM) and scanning photoelectrochemical microscopy (PEM) is described. The experimental setup is described and the several modes of operation are discussed. The electrochemical characteristics of the probe, collection efficiency, current efficiency, and the coupling efficiency for the two techniques, are examined in several model systems. SPECM was used to identify pitting precursor (PPS) sites on a titanium sample with a 50 Å oxide layer in an acidic KBr solution. It was found that the PPS site could be imaged photoelectrochemically and that the photocurrent is reduced at the PPS.
A method for producing insulated nanometer-sized carbon electrodes is presented. These electrodes are produced using electrochemical etching of carbon fibers followed by deposition of electrophoretic paint. A new deposition approach for insulating the tips, the so-called “inverted deposition” technique, is introduced. This technique allows complete insulation of the whole body of the etched carbon fiber except for the very tip, leaving an electrochemical active area with effective diameters as small as a few nanometers. The process overcomes pinhole formation that can be a problem with the normal electrophoretic paint deposition process. The fabricated electrodes show ideal steady-state voltammetric behavior. The voltammetric response corresponding to the reduction of hexacyanoferrate(III) and hexaammineruthenium(III) is investigated on these small electrodes in the absence and presence of supporting electrolyte. For these two multiple-charged ions the steady-state voltammetric behavior in the absence of supporting electrolyte is found to deviate from expected behavior, especially at very small electrodes.
Simple methods are described for the production of gold nanoparticles with narrow size distributions by reduction of tetrachloroaurate solutions in the presence of thiol-containing organic compounds which self-assemble on the gold surface. Stable solutions of somewhat larger particles can be produced if the thiol is absent. The thiol-derivatized materials are stable in air over long periods and can be handled in much the same way as simple organic compounds.Using dithiols as the derivatizing spacer units, ways have been developed for the preparation of materials in three dimensional form and as thin films attached to a solid substrate. Such materials show conductivities that mimic the behaviour of semiconductors and that depend markedly on the structure of the dithiol used to link the gold particles together. The increase in conductivity with increasing temperature probably involves activated electron hopping from particle to particle. Surfaces treated with a coating of the materials show electroreflectance changes with applied potential that also differ according to the structure of the dithiol spacer. Unusual effects have been observed on heterogeneous electron transfer from electrode surfaces treated with layers of the gold nanoparticles and dithiol spacers.Applications for these nanostructured materials can be envisaged, which range from submicroelectronic devices and circuitry to electrical modification of the reflectance of glass. Such applications will require a multidisciplinary approach with a substantial organic chemical research input.
The shape of steady-state current-distance (iT-d) curves obtained by scanning electrochemical microscopy (SECM) with a submicrometer conical-type tip is substantially different from that obtained with a microdisk tip. A model is proposed to describe the iT-d curves for tips shaped as cones or spherical segments. In each case a family of theoretical working curves, computed for different values of a single adjustable parameter (the height of a cone or a segment), was used to fit the experimental curves obtained with tips approaching a conductive or insulating substrate. A good fit of the experimental and theoretical curves can be obtained only for a very narrow range of the tip shape parameters, giving confidence in the reliability of the proposed method. The shape parameters for two substantially different microelectrodes are evaluated. A 420 nm radius tip was shown to be a fairly sharp cone with a height-to-radius ratio k of 0.8. An 80 nm radius tip can be represented as either a cone or spherical segment, because of the small k = 0.2. The radius values obtained from iT-d curves agree with those estimated from steady-state current values. The cyclic voltammograms (CVs) obtained at these microtip electrodes have a regular shape with very small capacitive and resistive background. New analytical approximations for iT-d curves are also proposed for SECM with a disk-shaped tip over conductive and insulating substrates.
We have developed a fabrication process that allows thin films to be patterned at the end of a sharp tip. Using this method, many different material contrasts can be achieved on the apex of a scanning probe microscope tip. For example, small apertures down to 30 nm have been patterned into an aluminium film covering quartz tips to form scanning near-field optical microscopy probes. Also, small well-defined metal electrodes can be fabricated on the tip apex, which can be used for electrochemical measurements. The tip structuring process is a batch fabrication method and is based on CMOS-compatible technologies, which means that it can be integrated easily into an existing microfabrication process. Furthermore, it allows the patterning of a wide range of tip heights, materials or geometries. Copyright © 1999 John Wiley & Sons Ltd.
The design of a novel photoelectrochemical sensor, the micro-optical ring electrode (MORE), is described. Based on a thin-ring microelectrode and using a fibre-optic light guide as the insulating material interior to the ring, the MORE is capable of delivering light directly to the region of electrochemical measurement and can therefore be used to conduct microelectrochemical studies of systems with complex photochemistry. A novel fabrication procedure is described, involving the coating of commercially available fibre optics (radius 1.25 × 10–4 m) with a 600 nm layer of gold, so allowing exploitation of the electroanalytical advantages peculiar to thin-ring microelectrodes. The dark electrochemistry of the thin-ring microelectrode is characterized by use of cyclic voltammetry and chronoamperometry and found to agree with previously published theoretical results. Preliminary exploration of the photoelectrochemical response of the MORE is reported, achieved via the interrogation of the photoelectrochemically active phenothiazine dye methylene blue (MB+). Photocurrent signals obtained during cyclic voltammetric and chronoamperometric studies of MB+, conducted with the MORE under illuminated conditions and in the absence of any deliberately added reducing agent, are attributed to the formation and subsequent detection of 3MB+ within the diffusion layer of the microring electrode. The data demonstrate that the use of the MORE for the direct electrochemical detection of photogenerated species with lifetimes of < 9 × 10–5 s is possible. The electrochemistry of 3MB+ over the applied potential range from –0.4 to +1.0 V versus SCE is elucidated and discussed in the context of the behaviour of photoexcited MB+ in the presence of the deliberately added reducing agent Fe2+.
The wider availability of microlithographic techniques for the fabrication of electrochemical devices has led to a significant increase in the development of microfabricated arrays of microelectrodes and their use in a wide variety of analytical problems. The major microfabrication steps and the capabilities and limitations of this microsensor technology are reviewed in this article. Several examples are summarized to illustrate the breadth of work with silicon-based microelectrode arrays, with special emphasis on their use for environmental analysis in a range of diverse settings including remote electroanalysis on Mars.
A direct and fast voltammetric method of determination of adenosine, with a novel carbon fiber ultramicroelectrode (CFE), is demonstrated here. To achieve high sensitivity required in biological determinations, before the determination of adenosine, carbon fiber ultramicro electrodes were activated by electrochemical pretreatment. The electrochemical activation process generates a nanostructured carbon fiber surface which is stable. The high stability of the active surface, and the high reproducibility of the activation process, leads to reproducible and sensitive fast scan voltammetric (FSV) determinations of adenosine. Adenosine sensitivity was determined in 7×10–2 M phosphate buffer, pH 7.4, at a scan rate of 500 V s–1. The potential window in the determinations was from –1.0 to 1.5 V (vs. SCE). A high sensitivity of the new method of adenosine determinations was achieved under physiological conditions.
The importance of sensor arrays in environmental, food and clinical analysis is discussed. The possible designs of sensor arrays is shown. The most reliable mathematical models for data processing are presented. The importance of different types of electrochemical sensor arrays in analytical chemistry as well as their performances are shown.
A through-mask electroplating process is described for forming Pt electrode structures with the vertical sidewalls and submicrometer
dimensions desired for dynamic random access memory applications. The plating process used an aqueous KOH-based solution of
“Pt A Salt” (Engelhard) and a sputter-deposited Pt plating base. Plating conditions were first optimized for blanket films
on 200 mm diam wafers by examining the characteristics of the Pt deposits as a function of plating bath temperature and current
density. Electrode features were then formed on similar wafers by plating through a patterned
mask. Integration issues discussed include potential reactions of the dielectric mask with the underlying Pt plating base,
surface modification of the exposed Pt plating base prior to plating, and erosion of the dielectric mask by the plating solution.
© 2001 The Electrochemical Society. All rights reserved.
Due to the complexity of the glassy carbon (GC) | electrolyte interface, several physical, chemical, and electrochemical activation
methods have been developed, mainly to obtain both high activities for heterogeneous charge transfer and reproducibility.
One of the most simple and widely used methods in the pretreatment of GC electrodes is oxidation in aqueous media with different
electrolytes at variable pH. This oxidation can be made through dynamic and/or constant programs of potential. In this work
we studied the kinetics of heterogeneous charge transfer for several couples after different pretreatment procedures: potential
cycling and application of a constant anodic potential. In the dynamic method, the results are correlated with an activation
parameter. Total lack of faradaic current is observed when the electrode surface is oxidized under a constant potential regime,
indicating that the electrode acts as a dielectric. The loss and recovery of electrochemical activity of the GC electrode
occur in a gradual way as a function of the injected oxidation and reduction charges. This fact is very important in systems
where carbon materials are used as electrodes under a continuous oxidation regime, such as high-pressure liquid chromatographic
(HPLC) electrochemical detectors, batteries, capacitors, and for the generation of polymers, by potentiostatic oxidation.
© 2001 The Electrochemical Society. All rights reserved.
We report here on a new, straightforward, and effective method for insulating etched and cut Pt/Ir scanning tunneling microscopy
(STM) tips for in situ electrochemical studies. The coating was formed by electrophoretic painting and subsequent heating. It covered all but the
very end of the tip, which may be attributed to shrinking of the polymer during heating. We have characterized these tips
by voltammetric methods, scanning electron microscopy, and by in situ STM imaging.
THEORY AND INSTRUMENTATION. Imaging. The Tapered Optical Fiber and Other Sensing Elements. NSOM Theory. NSOM Instrumentation. Optical Tunneling Microscopes. PRACTICE. Contrast. Intensity. Polarization. Wavelength. Amplitude and Phase. Time. Plasmons. APPLICATIONS. Surface Chemistry. Biology. Materials Science. Information Storage. Non-Visible Wavelength Instruments. RELATED TECHNIQUES AND CONCLUSION. Related Techniques and Unusual Configurations. Conclusions and Future Directions. Index.
THE ability to observe the optical spectrum of a single molecule can afford insights into the interactions that distinguish one molecular environment from another. Such sensitivity has recently been achieved at liquid-helium temperatures1–5. Here we show that the near-field scanning optical microscope6,7 can be used to obtain the time-dependent emission spectrum of a single molecule in air at room temperature, with a spatial resolution of about 100 nm. We have examined single molecules of l,l'-dioctadecyl-3,3,3',3'-tetra-methylindocarbocyanine (dil) dispersed on polymethylmethacrylate. The spectra of individual molecules exhibit shifts of ±8 nm relative to the average spectrum, and are typically narrower, as is expected for spectral lines broadened inhomogeneously (that is, by a distribution of molecular environments). The spectra also vary in width by up to 8 nm, some being as broad as the far-field many-molecule spectrum. The emission spectra of some individual mol-ecules exhibit time-dependent shifts of up to 10 nm. This variety in spectral position, width, shape and time dependence can be understood within a model of inhomogeneous broadening in which there is a distribution of barrier heights to rearrangement of the molecular environment.
A new approach is described for the quantitative study of the effect of a monolayer on solute transfer rates across an air/water interface. The technique is illustrated through measurements of oxygen transfer across a monolayer of 1-octadecanol as a function of compression. The experimental approach uses an inverted ultramicroelectrode (UME), positioned in the water phase in a Langmuir trough close to the air/water interface to induce the transfer of oxygen from air to water via the reduction (and depletion) of oxygen. The steady-state current response, measured as a function of UME−interface separation at various monolayer compressions, demonstrates that the rate of oxygen transfer is governed primarily by the accessible free area of the interface.
The electrochemical kinetics for the oxidation of ferrocenemethanol (FcCH2OH) over the whole composition range of dimethyl sulfoxide (DMSO)−water solutions of different viscosities (η) containing 50.0 mM (CH3)4NClO4 (TMAP) at a Pt microelectrode was studied using scanning electrochemical microscopy (SECM). The measured diffusion coefficient, DFcCH2OH, as well as the standard rate constant of the heterogeneous electron transfer, k0, as a function of solution composition, showed a minimum at about a DMSO molar fraction (xDMSO) of 0.33, corresponding to the mixture with the maximum solution viscosity. The largest k0 value found, 2.06 ± 0.31 cm s-1 in pure water (electrolyte) medium, was about 15 times larger than that obtained in the solution of xDMSO = 0.33 (0.14 ± 0.02 cm s-1). A good linear correlation between ln k0 and ln η was observed within the solution composition range of 0.10 ≤ xDMSO ≤ 0.60. An excellent linear correlation between ln k0 and ln τL, the longitudinal relaxation time, was also obtained with a slope equal to 1.0 when xDMSO = 0−0.60. Unusually small rate constants found in the solutions of xDMSO ≥ 0.70 were attributed to adsorption effects at the tip and the substrate electrode. The k0 obtained for the present system was generally found to be inversely proportional to the viscosity of the solution and directly proportional to the diffusion coefficient of the electroactive species.
The fabrication and characterization of electrodes constructed from single carbon nanotubes are presented. The sigmoidal voltammetric response of these nanotubular electrodes is characteristic of steady-state radial diffusion. The limiting current of uninsulated electrodes scales linearly with the depth of immersion into electrolyte solutions. However, the walls of nanotubular electrodes can be selectively insulated with a thin layer of polyphenol so that electrochemical activity is limited to the tip region. In this case, the limiting current is essentially independent of immersion depth. These nanotubular electrodes are robust, can be fabricated in high yield, and are of uniform diameter.
By using specially constructed nanometer tips of sharpened Pt-Ir wire in a wax sheath, small numbers of molecules (1−10) can be trapped between the tip and a substrate. Repeated electron transfers of an electroactive molecule as it shuttles by diffusion between tip and substrate produce a current (0.6 pA/molecule) that can be used to detect the trapped molecules. The tip electrode size and shape can be found from the electrode approach curves (current vs tip-to-substrate distance) based on approximate equations and digital simulations. Analysis of the observed fluctuating currents by autocorrelation, spectral density, and probability density functions is also described.
The electrochemical behavior of platinum and gold band electrodes, 0.5 to 1 cm long and 20 to 500 Å wide, is reported. Quasi-steady-state voltammograms are observed at all electrodes for the oxidation of ferrocene (in acetonitrile) and ferrocyanide (in water). Limiting currents obtained at band electrodes of width >200 Å are in quantitative agreement with values predicted by semiinfinite diffusional transport of a soluble redox species to an infinitely long hemicylindrical electrode. For electrodes of width <100 Å, diffusional currents are about an order of magnitude smaller than theoretical predictions. In a parallel set of experiments, the ratio of limiting currents observed for the oxidation of ferrocene (Fc) and decamethylferrocene (DMFc) in nitrobenzene, i(Fc)/i(DMFc), increases significantly as the electrode width is reduced below 100 Å. The results of these experiments are ascribed to size effects that become significant as the electrode width approaches dimensions of the redox active molecule.
The atomic-level structure of ordered Pt(100) electrodes prepared by air-hydrogen flame annealing followed by iodine dosing and solution-phase replacement by CO has been examined by in situ scanning tunneling microscopy. While large (1 × 1) substrate terraces are discernible in the presence of iodine adlayers, subsequent CO adsorption in 0.1 M HClO4 yields dense arrays of smaller, 20-40 Å, (1 × 1) islands. These domains coalesce upon positive alterations in the electrode potential. Parallel in situ infrared spectroscopic measurements shows that these structural changes are accompanied by an attenuation of bridge-bound CO in the close-packed adlayer structure. The bridging CO is speculated to be present at the edges of the (1 × 1) domains, contained within which are predominantly atop-coordinated CO.
The electrochemistry of a poly(vinylferrocene) (PVF) film of ca. 2100 Angstrom was investigated in 1 M aqueous NaClO4 solution with electrochemical and scanning microscopic techniques. The potential-step transients show well-defined maxima that are very similar to those found for the electrochemical switching of electronically conductive polymers, e.g., polypyrrole and polyaniline. We explore here the possibility that the film resistance accompanying changes in the polymer oxidation state is a major factor in determining the shape of these transients. These chronoamperometric curves also allow the determination of an apparent diffusion coefficient. The scanning electrochemical microscope (SECM) has been used to penetrate the PVF film and to obtain directly the thickness of the film immersed in the electrolyte solution. Different shapes of the tip current-distance curves are observed, depending on the oxidation state of the PVF film and its electrochemical treatment history. Mechanisms for the different approach curves are suggested.
The construction of probes with tip diameters of 0.2 to 0.6 μm is described for measuring concentration fluctuations of a dissolved gas in the liquid diffusion boundary layer adjacent to the gas phase. With suitable instrumentation this probe can be used to construct local concentration profiles inside the boundary layer under various liquid velocities. From the data obtained, it is possible to elucidate the mass transfer mechanisms at the gas-liquid interface more clearly than has been possible previously.
Fabrication and characterization of carbon disk ultramicroelectrodes (CDUMEs), embodied in a pulled Teflon capillary, with overall tip dimension of 10 μm in diameter, are described. A CDUME was constructed by inserting a carbon fiber 7 μm in diameter into a commercial Teflon capillary, which was followed by pulling the capillary by means of a microelectrode puller employing appropriate heating and timing, to produce a self-sealing thin Teflon film insulation coating. Then, the so-coated carbon fiber was cut to expose a fresh carbon fiber disk. The proposed one-stage preparation method is fast (5 min), very simple, and inexpensive and eliminates the need for separate embodying, insulation, and sealing steps. It results in CDUMEs exhibiting excellent electrochemical behavior. Scanning electron and optical microscopy, voltammetry, and amperometry were employed to characterize these electrodes. Cyclic voltammograms of ferricyanide in aqueous media and of ferrocene in acetonitrile media displayed low-noise, low-background, sigmoidal responses with virtually no current hysteresis. To check the analytical applicability of these electrodes, a testing with adrenaline was performed by applying the differential pulse voltammetry mode. A linear calibration over the concentration range from 2.5 × 10-6 to 5.0 × 10-4 mol/L in pH 7.2 phosphate buffer solution was obtained with a detection limit of 7.0 × 10-7 mol/L. The proposed CDUMEs with flexible and nonfragile Teflon housing exhibit very low electrical noise and can be reproduced multiple times by simply recutting the tip.
Carbon fibers have been used to fabricate voltammetric electrodes with an active area of approximately 5 multiplied by 10** minus **7 cm**2. The response of these electrodes has been evaluated in aqueous solutions containing K//3Fe(CN)//6 for cyclic voltammetry, differential pulse voltammetry, and chronoamperometry. Because the diameter of these electrodes is smaller than the distance for molecular diffusion on the time scale of these experiments, the current is essentially time independent. The electrodes are shown to be useful for analytical determination of concentration over 3 orders of magnitude. The properties of the electrochemical response for the microelectrodes permit direct residual current correction. In addition, the contribution of homogeneous chemical reactions to the faradaic current is diminished.
A method for the preparation of mercury microvoltammetric electrodes of hemispherical geometry with radii of 2.3-7.3 ..mu..m has been developed. Mercury is electrodeposited from solutions of Hg(I) onto a microvoltammetric platinum disk electrode at a constant potential sufficient to ensure diffusion limited conditions. The radius of the deposited mercury electrode is a function of the square root of the deposition time and was experimentally evaluated by applying the equation for steady-state limiting current at a hemispherical electrode to the reduction of Ru(NHâ)â/sup 3 +/ at the mercury electrode. The mercury microvoltammetric electrode has been employed in several unique applications. Anodic stripping voltammetry with these electrodes can be performed with a quiescent solution during deposition due to the enhanced mass transfer resulting from nonlinear diffusion. The stripping peaks are as narrow as those expected for thin films, and the peak current for the stripping of lead was found to be linear over the concentration range of 7 x 10â»Â¹Â° M to 1 x 10â»â· M (5-min preconcentration interval) and to have higher precision than conventional stripping techniques. Mercury microvoltammetric electrodes also are demonstrated to be of value in fast scan cyclic voltammetry in aqueous solution. A well-defined wave can be obtained for the oxidized form of ascorbic acid at pH 7.0 at a scan rate greater than 1 kV sâ»Â¹. 26 references, 3 figures, 2 tables.
We describe a procedure for the fast and reliable construction of platinum microelectrodes sealed in glass. The procedure involves pulling annealed platinum wire (75 μm) placed inside a borosilicate pipet to give microelectrodes of 1-5 μm in total tip diameter
The coefficient in the steady state current equation for spherical segment microelectrodes has been evaluated experimentally for various values of the height of the spherical segment. For this purpose chronoamperometric reduction of Hg2+2 ion at a silver microdisc electrode in perchloric acid solution has been employed. The diffusion coefficient of Hg2+2 in 0.25 M HC104 at 30 C was determined as .0000112 sq cm/s.