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Cathodic electrogenerated chemiluminescence of aromatic Tb(III) chelates at polystyrene-graphite composite electrodes
Abstract
Tb(III) chelates exhibit intense hot electron-induced electrogenerated chemiluminescence during cathodic polarization of metal/polystyrene-graphite (M/PG) electrodes in fully aqueous solutions. The M/PG working electrode provides a sensitive means for the determination of aromatic Tb(III) chelates at nanomolar concentration levels with a linear log-log calibration curve spanning more than five orders of magnitude. The charge transport and other properties of these novel electrodes were studied by electrochemiluminescence measurements and cyclic voltammetry. The present composite electrodes can by utilized both under pulse polarization and DC polarization unlike oxide-coated metal electrodes which do not tolerate cathodic DC polarization. The present cost-effective electrodes could be utilized e.g. in immunoassays where polystyrene is extensively used as a solid phase for various bioaffinity assays by using electrochemiluminescent Tb(III) chelates or e.g. Ru(bpy)3²⁺as labels.
... Solvated electrons, and hydrated electrons in water, are the chemist's perfect reducing agent in many ways. Recently, we made efforts to develop low-cost replacements [6,7] for chemically quite non-resistant oxide-coated aluminium electrodes [8][9][10] and typically a bit too expensive oxide-coated silicon electrodes [7,8] for hot electron injection into fully aqueous electrolyte solutions [1][2][3][4][5]. ...
... Our recent developments, composite electrodes, have been so far made by using polymer materials, such as polystyrene [6,7] and ethyl cellulose [35] as a matrix that can easily be dissolved into common organic solvents and can then immediately be doped with suitable conducting particles by simply mixing and sonicating with ultrasound. The doped polymer is finally spin-coated upon a conductive substrate such as on a metal or a strongly doped semiconductor disc. ...
... The doped polymer is finally spin-coated upon a conductive substrate such as on a metal or a strongly doped semiconductor disc. Thus, the final structure of C/CPDP (conductor/ conducting particle doped polymer) composite electrode has been created [6,7,35]. In these composite electrodes, the final electron injection to solution is typically occurring through an ultrathin polymer layer naturally formed on top of the conducting particles during manufacture process [7]. ...
This paper presents a simple and inexpensive method to fabricate chemically and mechanically resistant hot electron-emitting composite electrodes on reusable substrates. In this study, the hot electron emitting composite electrodes were manufactured by doping a polymer, nylon 6,6, with few different brands of carbon particles (graphite, carbon black) and by coating metal substrates with the aforementioned composite ink layers with different carbon-polymer mass fractions. The optimal mass fractions in these composite layers allowed to fabricate composite electrodes that can inject hot electrons into aqueous electrolyte solutions and clearly generate hot electron-induced electrochemiluminescence (HECL). An aromatic terbium (III) chelate was used as a probe that is known not to be excited on the basis of traditional electrochemistry but to be efficiently electrically excited in the presence of hydrated electrons and during injection of hot electrons into aqueous solution. Thus, the presence of hot, pre-hydrated or hydrated electrons at the close vicinity of the composite electrode surface were monitored by HECL. The study shows that the extreme pH conditions could not damage the present composite electrodes. These low-cost, simplified and robust composite electrodes thus demonstrate that they can be used in HECL bioaffinity assays and other applications of hot electron electrochemistry.
... Ta/Ta 2 O 5 /Pt) [23] and lately novel conductor particlesdoped composite electrodes. [7,13,24,25] Recently studied disposable composite electrodes based on polymers doped with conductor particles provide certain advantages when compared to more traditional insulating film-coated electrode materials by having more stable and reproducible HECL output in a wider pH range, and most importantly, without considerable background emission. [26] Our research group have studied e.g. ...
... [26] Our research group have studied e.g. polyetherimide-carbon black-based electrodes [7], polystyrene-graphite-composite/metal electrodes [24] and polyvinyl butyral-carbon black/metal composite electrodes [25] and presently the use the polystyrene-based composite materials as a solid substrate for an immunoassay [13]. In these previous studies carbon black was the most usable and low-cost conductor particle material. ...
... In these previous studies carbon black was the most usable and low-cost conductor particle material. The main advantage of these novel electrode types are their good tolerance in changes of composite layer thickness and composition of the film [7,13,24,25]. ...
The possibility of using cellulose derivative films as (i) insulating material on metal electrodes or (ii) in composite electrode films on metal to produce hot electron-induced electrochemiluminescence (HECL) was studied. It was shown that the luminophores known to produce HECL at thin insulating film coated cathodes (e.g. Si/SiO2 and Al2O3 electrodes) produced HECL with the present novel electrodes. In the case of composite films consisting of cellulose material doped with conducting carbon particles, the optimal cellulose/carbon black ratios were investigated by measuring the time-resolved HECL (TR-HECL) of an aromatic Tb(III) chelate. In addition to Tb(III) chelate, other well-known labels, fluorescein and Ru(bpy)3²⁺ chelate, were demonstrated to produce strong HECL at the present composite electrodes, which are more environmental friendly in disposable assay cartridges as the plastic-based composites we have studied previously. Thus, it is now possible on the present basis to manufacture biodegradable paper-based assay cartridges with HECL detection of labels at biodegradable electrodes. It was shown that the present composite films are stable over wide pH range, and also time-resolved detection of Ru(bpy)3²⁺ chelate is possible although its luminescence lifetime if quite short. The calibration curves were measured for presently used aromatic Tb(III) chelate and for Ru(bpy)3²⁺. The detection limit (s/n = 3) was 2 · 10⁻¹⁰ M for the Tb(III)-chelate and 4 · 10⁻⁹ M for Ru(bpy)3²⁺ in time-resolved detection mode. The relative standard deviation for Tb(III)-L1 (n = 5) emission at 10⁻⁵ M concentration was 2%. Wide linear range and low detection limits suggests that cellulose based composite electrodes can be used in HECL bioaffinity assays which was finally demonstrated here by an immunometric immunoassay.
... R þ hn ðRadiative transitionÞ (4) There are several reports about ECL of Eu(III) and Tb(III) complexes in an aqueous medium. [21][22][23][24][25][26][27][28][29][30][31][32] Lis et. al., reported ECL on Tb(III)-doped Al/Al 2 O 3 electrode, [25] and ECL of Eu(III) complexes with Kegging polyoxomatalates [26] and coumarine-3carboxylic acid. ...
... al., performed ECL measurements for Eu(III) and Tb(III) complexes using Al/Al 2 O 3 and polystyrenegraphite composite electrodes. [21,24,29,31] Some of the ECL measurements needed high applied voltage (e. g., 40 V), and the mechanism is probably similar to that of the electroluminescence (the excited states are formed by recombination of a hole and electron). Some analytical applications using ECL of Tb(III) complexes in an aqueous solution were also reported. ...
We report the electrochemical properties, and electrogenerated chemiluminescence (ECL) of eight different organometallic complexes of Eu(III) and Tb(III), Eu(dbm)3phen, Eu(dbm)3bath, Eu(fod)3phen, Eu(tta)3phen, Tb(acac)3phen, Tb(acac)3bath, Tb(fod)3phen, and Tb(tmhd)3bpy in acetonitrile, where phen, bath, and bpy are 1, 10‐phenanthroline, bathophenanthroline, 2,2′‐bipyridyl, respectively, and the anionic ligands, acac⁻, dbm⁻, fod⁻, and tta⁻ are the enolate forms of acetylacetone, dibenzoylmethanate, 6,6,7,7,8,8,8‐heptafluoro‐2,2‐dimethyl‐3,5‐octanedione, and thenoyltrifluoroacetone, respectively. The electronic states of the neutral and some charged states of the complexes were clarified with density functional theory calculations. The ECL spectra were recorded by using S2O8²⁻ as a coreactant, and for all complexes, sharp ECL spectra, which are attributable to the f‐f transitions, were observed. On the potential‐dependent ECL spectra, broad ECLs, which were originated from the ligands, were also detected. Based on the ECL behavior and the electronic states, the mechanisms of the intraconfigurational ECL were discussed.
... Insulating polymer material surrounds the carbon black particles and enables electrons to be transported through the thin composite layer with very low IR-loss, and finally hot electrons to be emitted at the electrode/solution interface. The main advantage of these novel electrodes over traditional insulating thin-film covered electrodes is their insensitivity to composite layer thickness and composition of the film [9,15,21,22]. Compared to thin insulating film-coated electrode materials, such as aluminum or silicon, composite electrodes are found to have a more stable and reproducible HECL output in a wider pH range, and most importantly, without considerable background emission [15,23,24]. ...
Novel hot electron-emitting working electrodes and conventional counter electrodes were created by screen printing. Thus, low-cost disposable electrode chips for bioaffinity assays were produced to replace our older expensive electrode chips manufactured by manufacturing techniques of electronics from silicon or on glass chips. The present chips were created by printing as follows: (i) silver lines provided the electronic contacts, counter electrode and the bottom of the working electrode and counter electrode, (ii) the composite layer was printed on appropriate parts of the silver layer, and (iii) finally a hydrophobic ring was added to produce the electrochemical cell boundaries. The applicability of these electrode chips in bioaffinity assays was demonstrated by an immunoassay of human C-reactive protein (i) using Tb(III) chelate label displaying long-lived hot electron-induced electrochemiluminescence (HECL) and (ii) now for the first time fluorescein isothiocyanate (FITC) was utilized as an a low-cost organic label displaying a short-lived HECL in a real-world bioaffinity assay.
With the development of modern industrial technology, the residual heavy metal ions in environment-related matrices poses a serious threat to environmental and public health, because they could accumulate in human body and affect human health through the exposure of food chain. On account of this, it is of great significance to develop an efficient environmental surveillance and traceability assay toward heavy metal ions for thus establishing an efficient warning-early mechanism to safeguard environmental and public health. In this case, as an emerging technology, electrochemiluminescence (ECL)-based sensors have exhibited the promising potentials in various heavy metal ions monitoring with high sensitivity, near zero background signal, wide detection range, and low cost. To better promote the development of this field, we here summarized the recent progresses of the innovative ECL sensors for quantitative monitoring the residual levels of heavy metal ions (e.g., Hg²⁺, Pb²⁺, Cd²⁺, Cu²⁺, Ni²⁺, Co²⁺, Ag⁺, Cr³⁺, Cr⁶⁺, Fe³⁺, & their coexistence) in environment-related matrices, profiled the inevitable challenges, and discussed the future perspectives. We expected that this timely and systematic review might offer the significant insights to develop the ECL technique-driven novel management systems, commercial point-of-care sensors, and efficient warning-early mechanisms for monitoring and controlling the residual pollutions of heavy metal ions in environment-related matrices, thus ensuring environmental and public health.
For hot electron-induced electrochemiluminescence (HECL), developing facile and controllable insulating layer on the electrode surface is key to the effective electron tunneling. Herein, a polystyrene modified glassy carbon electrode based the spin-coating was proposed for HECL. The constructed interface was characterized by atomic force microscopy (AFM). And HECL performance was investigated in details using the system of Ru(bpy)3²⁺ and S2O8²⁻. The as-prepared film shows the stable and reproducible HECL performance and the HECL mechanism was discussed using ECL spectra and hydrated electron quenchers. Finally, on the basis of the inhibition of SO4•-, the present polystyrene film was applied to the detection of rutin in the range of 0.1-1.3 μM with a detection limit of 90 nM (S/N = 3).
Electrochemistry of hot electrons in fully aqueous solutions at tetrahedral amorphous carbon thin film electrodes is discussed. The generation of these highly reducing chemical species was confirmed by normal pulse voltammetry and several electrochemiluminescent systems. Electron transfer into pre‐existing solvent cavities was observed at approximately ‐2.65 V vs. Ag/AgCl (sat.). Electrogenerated hot electrons were utilized as chemiluminescent mediators in heterogeneous sandwich immunoassay of Serum Amyloid A. The calibration curve was linear over four orders of magnitude and the detection limit was 85 ng L‐1 that demonstrates the efficiency of hot electron generation at this electrode material.
The development of the hot electron‐induced cathodic electrochemiluminescence (HECL) often accompanied with the advance in the electrode materials and instruments. In this work, dimethyl silicone oil was employed as the coating layer for initiating the HECL. The insulating layer was obtained by simple drop casting and subsequent polishing, which can withstand high voltage and meet the requirement of HECL. With the as‐prepared interface, the quantitative detection of puerarin in the aqueous solution was achieved based on the quenching effect on the SO4·‐.
Carbon composite electrodes consisting of graphite and a non-conductive binder are widely used due to their low cost and ease of fabrication but often suffer from slow electrode kinetics and high capacitance relative to traditional electrode materials such as gold and glassy carbon. Modifiers including metallic nanoparticles and carbon nanotubes are used to enhance electrode performance, but this complicates electrode preparation, increases cost, and limits reusability. Our group recently reported a new type of carbon composite electrodes called thermoplastic electrodes (TPEs) that use common thermoplastics mixed with graphite in a solvent processing system that give enhanced electrochemical performance. The polymers reported to date include polymethylmethacrylate, cyclic olefine copolymer, polycaprolactone (PCL), and blends of these materials. Polystyrene (PS) is an inexpensive and commonly used thermoplastic substrate, however, there are only several reports incorporating PS as a binder in carbon composite electrodes. Here, we report the first PS TPEs and show their unique electrochemical behavior relative to electrodes made with other binders. These unmodified composite materials surpass the performance of traditional electrode materials and show similar behavior to nanomaterial-modified electrodes. The capacitance and conductivity were measured for PS and PS-polycaprolactone (PCL) blends using PCL TPEs as a reference. Even small additions of PS to PCL electrodes substantially lowered capacitance and increased conductivity. The electrochemistry of PS and PS-PCL TPEs was investigated using cyclic voltammetry with various common and biologically relevant redox probes. The effect of differing molecular weight and the use of waste expanded polystyrene (EPS) were explored as well, and all PS binder TPEs were found to exhibit similar redox behavior. Optical profilometry (OP) and scanning electron microscopy (SEM) were used to examine morphological characteristics. OP showed lower surface roughness for PS electrodes than for PCL ones, explaining the lower capacitance. SEM data suggests the source of this enhanced redox behavior stems from the edge-plane rich PS TPE surfaces.
1-Hexyne monomers were potentiostatically electropolymerized upon confinement in 1D channels of a surface-mounted metal-organic framework Cu(BDC) (SURMOF-2). A layer-by-layer deposition method allowed for SURMOF depostition on substrates with prepatterned electrodes, making it possible to characterize electrical conductivity in situ, i.e., without having to delaminate the conductive polymer thin film. Successful polymerization was evidenced by mass spectroscopy, and the electrical measurements demonstrated an increase of the electrical conductivity of the MOF material by 8 orders of magnitude. Extensive DFT calculations revealed that the final conductivity is limited by electron hopping between the conductive oligomers.
Hot electron-induced electrochemiluminescence (HECL) of calcein and calcein-Tb(III) complex was generated at oxide-covered aluminum electrode during cathodic pulse polarization. The excitation of calcein as a molecule or as a ligand is based on subsequent one-electron oxidation and reduction steps by oxidizing radicals and solvated electrons. During HECL excitation of calcein-Tb(III) solution a reaction product of the calcein is formed that also enables the photoexcitation of Tb(III) via the formed ligand derivative by ligand-sensitized mechanism. The determination of low concentrations of calcein with aluminum electrodes was complicated by a relatively strong background electroluminescence originating from the Al2O3-layer. Polyvinyl butyral-carbon black composite electrodes coated on brass were fabricated to solve this problem and a fifty-fold lower background emission was obtained for these novel composite electrodes in comparison to that of oxide-covered 99.9% pure aluminum electrodes. The obtained detection limits were 3.2·10⁻¹⁰ M for calcein and 6.4·10⁻⁹ M for calcein-Tb(III) at the present composite electrodes. These species could potentially be utilized as electrochemiluminescent labels in bioaffinity assays.
Solvated electrons are typically generated by radiolysis or photoionization of solutes. While plasmas containing free electrons have been brought into contact with liquids in studies dating back centuries, there has been little evidence that electrons are solvated by this approach. Here we report direct measurements of solvated electrons generated by an atmospheric-pressure plasma in contact with the surface of an aqueous solution. The electrons are measured by their optical absorbance using a total internal reflection geometry. The measured absorption spectrum is unexpectedly blue shifted, which is potentially due to the intense electric field in the interfacial Debye layer. We estimate an average penetration depth of 2.5±1.0 nm, indicating that the electrons fully solvate before reacting through second-order recombination. Reactions with various electron scavengers including H(+), NO2(-), NO3(-) and H2O2 show that the kinetics are similar, but not identical, to those for solvated electrons formed in bulk water by radiolysis.
Voltammograms for electrodes containing nanostructured carbon of various morphology (single-walled carbon nanotubes, filament, columnar structures) are obtained in neutral aqueous electrolytic solutions. Experimental proofs for the existence of injection of solvated electrons into electrolytic solutions at moderate cathodic potentials are presented for all the electrodes. It is established that this effect is connected with the presence of atomically sharp areas on the electrode surfaces. It is assumed that the reason for the appearance of solvated electrons is the autoelectron emission at the interface between the conducting surface of the carbon material and the electrolytic solution. By studying the nitrate anion reduction it is shown
that the reduction over-voltage of stable compounds may be lowered by substituting a fast homogeneous reaction of solvated electrons with the initial substance for the hindered heterogeneous stage of the first electron transfer.
Hydrophobic polystyrene is the most common material for solid phase immunoassay. Proteins are immobilized on polystyrene by passive adsorption, which often causes considerable denaturation. Biological macromolecules were found to better retain their functional activity when immobilized on hydrophilic materials. Polyacrylamide is a common material for solid-phase carriers of biological macromolecules, including immunoreagents used in affinity chromatography. New macroformats for immunoassay modified with activated polyacrylamide derivatives seem to be promising.
New polymeric matrices for immunoassay in the form of 0.63-cm balls which contain hydrazide functional groups on hydrophilic polymer spacer arms at their surface shell are synthesized by modification of aldehyde-containing polystyrene balls with hydrazide derivatives of poly(meth)acrylic acid. The beads contain up to 0.31 micromol/cm2 active hydrazide groups accessible for covalent reaction with periodate-oxidized antibodies. The matrices obtained allow carrying out the oriented antibody immobilization, which increases the functional activity of immunosorbents.
An efficient site-directed antibody immobilization on a macrosupport is realized. The polymer hydrophilic spacer arms are the most convenient and effective tools for oriented antibody coupling with molded materials. The suggested scheme can be used for the modification of any other solid supports containing electrophilic groups reacting with hydrazides.
Tris(2,2′-bipyridine)ruthenium(II) chelate exhibits strong electrogenerated chemiluminescence during cathodic high-voltage pulse-polarization of pointed Pt electrode in aqueous solutions. The present method is based on a field emission or other type of tunnel emission of hot electrons into an aqueous electrolyte solution. The method allows the detection of tris(2,2′-bipyridine)ruthenium(II) and its derivatives below nanomolar concentration levels and yields linear log-log calibration plots spanning several orders of magnitude of concentration.
Ruthenium(II) tris-(2,2′-bipyridine) chelate exhibits strong electrogenerated chemiluminescence during cathodic pulse polarization of oxide-covered aluminum electrodes in aqueous solutions. The present method is based on a tunnel emission of hot electrons into an aqueous electrolyte solution. The method allows the detection of ruthenium(II) tris-(2,2′-bipyridine) and its derivatives below nanomolar concentration levels and yields linear log-log calibration plots spanning several orders of magnitude of concentration. This method allows simultaneous excitation of derivatives of ruthenium(II) tris-(2,2′-bipyridine) and Tb(III)-chelates. The former label compounds have a luminescence lifetime of the order of microseconds, while the latter compounds generally have a luminescence lifetime of around 2 ms. Thus, the combined use of these labels easily provides the basis for two-parameter bioaffinity assays by either using wavelength or time discrimination or their combination.
Alkali metals can react explosively with water and it is textbook knowledge that this vigorous behaviour results from heat release, steam formation and ignition of the hydrogen gas that is produced. Here we suggest that the initial process enabling the alkali metal explosion in water is, however, of a completely different nature. High-speed camera imaging of liquid drops of a sodium/potassium alloy in water reveals submillisecond formation of metal spikes that protrude from the surface of the drop. Molecular dynamics simulations demonstrate that on immersion in water there is an almost immediate release of electrons from the metal surface. The system thus quickly reaches the Rayleigh instability limit, which leads to a 'coulomb explosion' of the alkali metal drop. Consequently, a new metal surface in contact with water is formed, which explains why the reaction does not become self-quenched by its products, but can rather lead to explosive behaviour.
Kinetic data for the radicals H⋅ and ⋅OH in aqueous solution,and the corresponding radical anions, ⋅O− and eaq−, have been critically pulse radiolysis, flash photolysis and other methods. Rate constants for over 3500 reaction are tabulated, including reaction with molecules, ions and other radicals derived from inorganic and organic solutes.
A study of monochloroacetic acid as a scavenger in glow-discharge electrolysis in aqueous solutions suggests that there is a primary yield of 3.3 mol mol−1 of hydrated electrons under conditions where the primary yield of OH radicals is 6.5 mol mol−1. The speed of formation of molecular hydrogen from hydrated electrons or hydrogen atoms in glow-discharge electrolysis is such that other reactions of these species are usually unimportant.
Light emission from aluminium oxide during cathodic pulse-polarisation of oxide-covered aluminium in aqueous solution was observed to be strongly enhanced in the presence of peroxydisulfate ions. The spectrum of the light emission had a broad maximum between 400 and 450 nm being attributed to F-centre luminescence of aluminium oxide. The mechanism of the luminescence is associated with the two-step reduction of peroxydisulfate anions near the oxide-covered cathode where the first one-electronic reduction step occurs either (i) by tunnel-emission generated hydrated electrons or (ii) by trickling down the surface states to the energy level of peroxydisulfate ion or (iii) by direct heterogeneous electron transfer from the bottom of the aluminium oxide conduction band to peroxydisulfate ions during strong downward band bending induced by cathodic pulse-polarisation. The second step occurs by electrons from F- or F
+
-centres at the oxide/electrolyte interface. Transitions of aluminium oxide conduction band electrons to fill the sulfate radical-emptied electron trapping sites (oxygen vacancies) produces analogous F- and F
+
-centre luminescence to that occurring during photoluminescence and thermoluminescence of aluminium oxide. No enhancement of light emission was observed in the presence of hydrogen peroxide which is also reduced in a two-step process at oxide-covered aluminium electrode. This can be explained by the fact that the energy level of hydroxyl radical under the present conditions lies ca. 1 eV above, whereas the energy level of sulfate radical lies somewhat below the colour-centre sub-band of aluminium oxide. Therefore, the sulfate radical is a sufficiently strong oxidant but the hydroxyl radical is too weak an oxidant to abstract electrons from F- and F
+
-centres.
Hot electron injection into aqueous electrolyte solutions
from metal/insulator/metal/electrolyte and
metal/insulator/electrolyte tunnel junctions is considered
and the possibility of an electrochemical generation of
hydrated electrons is discussed. The hot electron-induced UV
electrochemiluminescence of (9-fluorenyl)methanol was used
to demonstrate the presence of highly energetic transient
species in aqueous solution at several
metal/insulator/electrolyte hot electron tunnel emitters.
These transient species cannot be produced electrochemically
in fully aqueous solutions at any active metal electrodes. A
detailed mechanism for the present electrochemiluminescence
is suggested.
Generation of hydrated electrons at an oxide-covered aluminium electrode during its cathodic pulse-polarization in an aqueous electrolyte may well be the primary step of the electrogenerated luminescence of aromatic Tb(III) chelates. In the presence of appropriate precursors, hydrated electrons produce secondary oxidizing radicals, thus resulting in an energetic electrode/electrolyte interface which initiates the radiative 5D4 → 7Fj transitions of chelated terbium(III); the aromatic moiety of a Tb(III) chelate is excited by its consecutive interactions with hydrated electron and the secondary oxidizing radical followed by an intramolecular transfer of this excitation energy to the 5D4 level of coordinated terbium(III).
Voltammograms for electrodes fabricated of nanostructured carbon of various morphology (nanotube paper, columnar and filament structures) in hexamethylphosphoric triamide (HMPA) solution have been obtained and analysed. Intensive dark-blue coloration near cathode surface at potentials as low as E∼−1.2V (s.c.e.) has been observed. An ESR (electron spin resonance) spectrum of this frozen dark-blue solution was recorded. This spectrum coincides with one of free electrons. Experimental proofs of the existence of electron emission into electrolytic solutions at moderate cathodic potentials are present for all electrodes. This effect is established to be connected with the presence of atomically sharp areas on the electrode surfaces.
Double insulating barrier tunnel emission electrodes were fabricated by adding a new pure aluminum layer upon oxidized aluminum electrodes by vacuum evaporation and thermally oxidizing the new aluminum layer in air at room temperature. Resulting Al/Al2O3/Al/Al2O3 electrodes allow the use of various aluminum alloys in the electrode body necessary for hardness or shaping ability of the electrode while obtaining the luminescence properties of pure aluminum oxide. During electrical excitation of luminescent labels by cathodic hot electron injection into aqueous electrolyte solution, the background noise is mainly based on high-field-induced solid-state electroluminescence and F-center luminescence of the outer aluminum oxide film. The more defect states and/or impurity centers the outer oxide film contains, the higher is the background emission intensity. The present electrode fabrication method provides a considerable improvement in signal-to-noise ratio for time-resolved electrochemiluminescence (TR-ECL) measurements when the original native oxide film of the electrode body contains luminescence centers displaying long-lived luminescence. The excellent performance of the present electrodes is demonstrated by extremely low-level detection of Tb(III) chelates, luminol, Pt(II) coproporphyrin and Tb(III) labels in an immunometric immunoassay by time-resolved electrochemiluminescence.
In this paper, the percolation threshold of the carbon black-resin composites is discussed based on the experimental results of the changes in resistivity and relative permittivity for loading carbon black, the electric field dependence of current, and the critical exponent of conductivity. It was found that a formation of infinite clusters is interrupted by a tunneling gap in the volume fraction region of carbon black loading where the change in resistivity is extremely large. It was also found that the critical exponent of conductivity for the universal law of conductivity is satisfied if the percolation threshold is estimated at the volume fraction of carbon black where a nonohmic current behavior changes into an ohmic one. It is concluded that the percolation threshold should be defined at this volume fraction of carbon black.
Observations of the solvated electron in water and the common aliphatic alcohols have been made in the time region ∼ 20–350 psec. Molar concentrations of acid have been used to scavenge eaq− and k(eaq−+Haq+) has been measured as (1.2 ± 0.2) × 1010 liter/mole⋅sec for [Haq+] from 0.5 to 5.0M. The factors limiting the time response of our picosecond pulse radiolysis system are discussed. The time of formation of esol− is estimated to be less than 10 psec for all solvents studied. The yield of solvated electrons in alcohols has been measured to be approximately one‐third of the yield in water at picosecond times.
Hot electron-induced cathodic electrochemiluminescence of the Ru(bpy)(3)(2+)/S(2)O(8)(2-) system was investigated at an oil film-covered carbon paste electrode (CPE) under cathodic pulse polarization for the first time. Compared with other electrodes, the CPE is of lower background, better stability and reproducibility. The method is also applied to the determination of catechol. Under the optimum conditions, the linear correlation between the quenched ECL intensity (ΔI) and the logarithm of catechol concentration (logC(catechol)) was observed over the range of 2.0×10(-10)mol/L-4.0×10(-9)mol/L and 4.0×10(-9)mol/L-4.0×10(-7)mol/L with the limit of detection (LOD) of 2.0×10(-10)mol/L, which is lower than the other reported methods. The proposed method is applied to determine catechol in reservoir water. The mean recoveries of 83.3%-99.0% and the relative standard deviations (RSDs) of 0.8%-2.2% were obtained.
Some of the luminescent aromatic Tb(III) chelates can be excited at oxide-covered magnesium and n-ZnO:Al/MgO composite electrodes by cathodic pulse polarization in aqueous solution. This is based on the cathodic injection of hot electrons into the aqueous electrolyte solution and successive redox reactions in one-electron steps resulting in the excited states of the chelates. Due to the relatively long luminescence lifetime of multidentate Tb(III) chelates, these chelates can be very sensitively detected by time-resolved electrochemiluminescence (TR-ECL) measurements using the present working electrodes. Thus, metal or highly doped semiconductor electrodes coated by a thin MgO film lend themselves as nontransparent or in some cases, even optically transparent working electrodes for generation of free radicals and cathodic ECL in aqueous solution. The results suggest that also other even more efficient optically transparent composite tunnel emission electrodes than the present n-ZnO:Al/MgO electrodes could be constructed by coating n-ZnO:Al glass electrodes with other wide band gap insulating oxides.
Voltammograms of electrodes based on nanostructured carbon of different morphology (single-wall carbon nanotubes, carbon nanofilaments,
columnar structures) are taken in hexamethylphosphortriamide solutions. For all listed electrodes, direct experimental proof
of the electron injection to the electrolyte solution at moderate cathodic potentials is obtained. It is found that this phenomenon
is associated with the existence of atomically sharp spots at the electrode surface, which operate as local cathodes for the
electron emission. It is shown that the current-voltage characteristics for the electron injection to the electrolyte solution
differ from those for the field electron emission current at the electrode/vacuum interface.
Cathodic DC polarization of oxide-covered aluminum produces electrogenerated chemiluminescence from hydrated and chelated Tb(III) ions in aqueous electrolyte solutions. At the moment of cathodic voltage onset, a strong cathodic flash is observed, which is attributed to a tunnel emission of hot electrons into the aqueous electrolyte solution and the successive chemical reactions with the luminophores. However, within a few milliseconds the insulating oxide film is damaged and finally dissolved due to (i) indiffusion of protons or alkali metal ions into the thin oxide film, (ii) subsequent hydrogen evolution at the aluminum/oxide interface and (iii) alkalization of the electrode surface induced by hydrogen evolution reaction. When the alkalization of the electrode surface has proceeded sufficiently, chemiluminescence is generated with increasing intensity. Aluminum metal, short-lived Al(II), Al(I) or atomic hydrogen and its conjugated base form, hydrated electron, can act as highly reducing species in addition to the less energetic heterogeneously transferred electrons from the aluminum electrode. Tb(III) added as a hydrated ion in the solution probably luminesces in the form of Tb(OH) 3 or Tb(OH) 4 − by direct redox reactions of the central ion whereas multidentate aromatic ligand chelated Tb(III) probably luminesces by ligand sensitized chemiluminescence mechanism in which ligand is first excited by one-electron redox reactions, which is followed by intramolecular energy transfer to the central ion which finally emits light.
Modern analytical luminescence methods and their recent applications are reviewed with emphasis on the most sensitive methods that can be expected to be useful in future microanalytical systems such as mu-TAS, lab-on-chip, point-of-care (POC) and high throughput screening (HTS) applications. Photoluminescence (PL) is presently the most important group of analytical techniques utilising luminescence. Because of the rapidly increasing popularity of electrochemiluminescence (ECL) and its applications, we have given particular attention to ECL mechanisms and techniques. Due to the present and future importance of capillary electrophoresis (CE) as a separation method, the CE detection methods based on luminescence are also considered in a relatively detailed way. For those researchers, designing novel experiments and assays, experimental set-ups, and apparatus we include web links to the manufacturers of some fairly rare reagents, as well as modem instrument components. (C) 2003 Published by Elsevier B.V.
The primary processes occurring at cathodically polarized oxide-covered aluminum electrode are discussed in detail. It is pointed out that more energetic cathodic processes can be induced in aqueous media at thin insulating film-coated electrodes than at any semiconductor or active metal electrode. It is proposed that tunnel emission of hot electrons with energies well above the level of the conduction band edge of water occur, and the thermalization and solvation of the emitted electrons can result in generation of hydrated electrons. The cathodically pulse-polarized oxide-covered aluminum also generates a strong oxidant (or oxidants) at the oxide/electrolyte interface, and it is proposed that this species is the hydroxyl radical which is generated either by cathodic high field-induced ejection of self-trapped holes as oxygen dianions (i.e. oxide radical ions) into the electrolyte solution, or by the action of anion vacancies and/or F+-centers as the primary oxidants capable of oxidizing hydroxide ions or the hydroxyl groups of the hydroxylated surface on the oxide film. These radicals, hydrated electrons/hydroxyl radicals, can act as mediating reductants/oxidants in reduction/oxidation of solutes. The formation of the primary species is monitored by electrochemiluminophores which cannot be cathodically excited at active metal electrodes in fully aqueous solutions, but which can be chemically excited in aqueous media in the simultaneous presence of highly reducing and highly oxidizing radicals.
Heterogeneous and homogeneous immunoassays of human thyroid stimulating hormone (hTSH) were developed on immunometric basis using aromatic Tb(III) chelates as electrochemiluminescent labels and varied types of disposable oxide-covered aluminum electrodes as the solid phase of the immunoassays. The long luminescence lifetime of the present labels allows the use of time-resolved electrochemiluminescence detection and provide the low detection limits of these labels and, thus, sensitive immunoassays. The primary antibody of immunometric immunoassays was coated upon aluminum oxide surface by physical absorption. In homogeneous immunoassays using 66 μl cell and 15 min incubation time, a linear calibration range of 0.25–324 μU/ml was obtained by applying only a single cathodic excitation pulse in the detection step of the assay.
An improved analytical model is developed based on the average interparticle distance (IPD) concept to predict the percolation threshold of conducting polymer composites containing disc-shaped nanoparticles with high aspect ratios. Two different conditions were taken into account in the model in terms of particle distribution, namely two- and three-dimensional random orientations. A 10 nm interparticle distance is adopted as the electrical conducting criterion according to the tunneling mechanism, and the percolation threshold is estimated as a function of geometric shape of the nanoparticle. A parametric study suggests that the thickness and diameter of fillers are important factors that determine the percolation threshold of conducting nanocomposites. The accuracy and the applicability of the present IPD model are verified by comparing with several existing models and experimental data for graphite nanoplatelet reinforced polymer nanocomposites. It is shown that the current model presents much better agreement with experimental results than existing models.
C-reactive protein (CRP) was determined in the concentration range 0.01-10 mg L(-1) using hot electron induced electrochemiluminescence (HECL) with devices combining both working and counter electrodes and sample confinement on a single chip. The sample area on the electrodes was defined by a hydrophobic ring, which enabled dispensing the reagents and the analyte directly on the electrode. Immunoassay of CRP by HECL using integrated electrodes is a good candidate for a high-sensitivity point-of-care CRP-test, because the concentration range is suitable, miniaturisation of the measurement system has been demonstrated and the assay method with integrated electrodes is easy to use. High-sensitivity CRP tests can be used to monitor the current state of cardiovascular disease and also to predict future cardiovascular problems in apparently healthy people.
An improved preparation of antibody-coated polystyrene beads for sandwich enzyme immunoassay of human thyroid-stimulating hormone (TSH) was described. Rabbit anti-TSH IgG was purified by eluting at pH 2.5 from a TSH-Sepharose column, diluted 3 or 9 fold with normal rabbit IgG and used for coating polystyrene beads by physical adsorption. In a sandwich enzyme immunoassay of TSH using rabbit (anti-TSH) Fab'-beta-D-galactosidase conjugate, beta-D-galactosidase activities specifically bound to thus prepared polystyrene beads in the presence of TSH was 2.8-6.3 fold higher than those bound to polystyrene beads coated with anti-TSH IgG before purification. A similar effect was observed when guinea pig anti-pork insulin IgG, rabbit (anti-human IgE) IgG and goat (anti-human IgE) IgG were treated at pH 2.5. This improvement may be based on a conformational change of Fc in IgG molecule which was caused by the treatment at pH 2.5. Other sandwich immunoassays such as fluoro- and radio-immunoassays may also be improved in the same way.
We describe the use of fluorophore-doped nanoparticles as reporters in a recently developed ArcDia TPX bioaffinity assay technique. The ArcDia TPX technique is based on the use of polymer microspheres as solid-phase reaction carrier, fluorescent bioaffinity reagents, and detection of two-photon excited fluorescence. This new assay technique enables multiplexed, separation-free bioaffinity assays from microvolumes with high sensitivity. As a model analyte we chose C-reactive protein (CRP). The assay of CRP was optimized for assessment of CRP baseline levels using a nanoparticulate fluorescent reporter, 75 nm in diameter, and the assay performance was compared to that of CRP assay based on a molecular reporter of the same fluorophore core. The results show that using fluorescent nanoparticles as the reporter provides two orders of magnitude better sensitivity (87 fM) than using the molecular label, while no difference between precision profiles of the different assay types was found. The new assay method was applied for assessment of baseline levels of CRP in sera of apparently healthy individuals.
Oxide-covered aluminium electrodes were used to demonstrate that aromatic compounds, such as the simple derivatives of benzene, can be electrochemically excited at cathodically pulse-polarized conductor/insulator/electrolyte (C/I/E) tunnel junction electrodes (e.g. oxide-covered aluminium electrodes). The primary cathodic process at these electrodes was a tunnel emission of hot electrons into an aqueous electrolyte solution. Fluorescence (FL) and electrochemiluminescence (ECL) spectra were compared and the dependence of the electrochemiluminescence on the concentrations of benzene, toluene, phenol, p-cresol and aniline were measured and detailed mechanisms for the present electrochemiluminescence are proposed.
Strong electrogenerated chemiluminescence (ECL) of fluorescein is generated during cathodic pulse polarization of oxide-covered aluminum electrodes and the resulting decay of emission is so sluggish that time-resolved detection of fluorescein is feasible. The present ECL in aqueous solution is based on the tunnel emission of hot electrons into the aqueous electrolyte solution, which probably results in the generation of hydrated electrons and hydroxyl radicals acting as redox mediators. The successive one-electron redox steps with the primary radicals result in fluorescein in its lowest excited singlet state. The method allows the detection of fluorescein (or its derivatives containing usable linking groups to biomolecules) over several orders of magnitude of concentration with detection limits well below nanomolar concentration level. The detection limits can still be lowered, e.g., by addition of azide or bromide ions as coreactants. The results suggest that the derivatives of fluorescein, such as fluorescein isothiocyanate (FITC), can be detected by time-resolved measurements and thus be efficiently used as electrochemiluminescent labels in bioaffinity assays.