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Catalytic Co and Fe porphyrin/Fe3O4 nanoparticles assembled on gold by carbon disulfide
... Yang et al. first reported iron-based-nanoparticle−decorated Co 9 S 8 in situ grown on an rGO surface (Fe 3 O 4 @Co 9 S 8 /rGO), which exhibited good durability and excellent OER electrochemical activity with a small overpotential of 0.34 V at the current density of 10 mA cm −2 (Figure 40). 345 386,702 have been used to maximize the electroactive surface area of catalysts and improve their catalytic activity and durability. Among these, graphene, a two-dimensional single-layer sheet of hexagonal carbon, has emerged as a new-generation catalyst support because of its excellent electrical conductivity, high surface area, good chemical and environmental stability, and strong adhesion to catalyst particles. ...
There is considerable research interest in thesynthesis and characterization of organic-based nanoparticlesbecause of the wide range of nanomaterials that can be designedwith useful functional properties. With emerging research in thesynthesis of organic nanoparticles comprised of porphyrins andphthalocyanines, there is a challenge to prepare multifunctionalnanoparticles with the desired properties, size, and compositionthat are stable in environments that are exposed to stresses of pH,oxidation, or heat. This review describes innovative strategies thathave been reported for synthesizing nanoparticles of porphyrinsand phthalocyanines for applications. The tunability of thestructure of porphyrinoid molecules through synthetic chemistryprovides an opportunity to design a diverse range of multifunc-tional nanoparticles with desired properties, e.g., optical, magnetic, electronic, chemical. Porphyrins and pthalocyanines aremacrocyclic compounds which have a rigid aromatic structure, are thermally stable, and have useful photophysical, optical, andelectronic properties. Various elements can be inserted within the center of the macrocycles of porphyrinoids, which are held in placeby the four nitrogen atoms within the rings. The inserted elements can impart tunable properties to molecular assemblies.Approaches for preparing nanoparticles of porphyrins and composite nanoparticles have been reported that are based on strategieswith covalent and/or noncovalent binding chemistries. Methods reported for synthesizing porphyrinoid nanoparticles encompassprotocols with solution self-assembly, ultrasonication, mixed solvents, core−shell strategies, laser ablation, host−guest solvents,reprecipitation, and microwave heating. Encapsulating nanoparticles with porphyrins in a core−shell design can impart usefulfunctionalities that are a combination of properties from the inner core material and the chemistry of the outer coating.Nanoparticles of porphyrins and porphyrin-related compounds have been developed commercially for technology in areas such as inbiomedicine, photodynamic therapy, catalysis, electronics, and sensors.
This chapter provides an overview on the basics of ferrites. It will start with an introduction to the classification of energy storage and ferrite with properties. Ferrites are very important materials having electrical and magnetic properties. These ferrites are used in various applications; mainly this book gives the detail use of spinel ferrites in energy storage application. This chapter gives the overview of the basics of ferrites.
Electrocatalysis is an effective way of production of hydrogen. However, the use of platinum as an electrode does not make it cost-effective, and therefore, there is a need of exploring a new material that shows catalytic performance equivalent to the platinum. Studies have been reflected that the ferrite is a promising material for electrocatalysis. Therefore, in this chapter, the use of ferrite as an electrode in the catalysis process is discussed. The effect of different parameters such as morphology of ferrite nanoparticles, surface area, and active sites are explained. Furthermore, the effect of graphene and graphene oxide (GO) as a conducting loading base is elaborated. Reduced ferrites loaded on GO have been proved to be more effective toward electron transfer mechanism at electrode. It is found that to understand the electron transfer rate and active surface area of ferrite, it is necessary to do electrochemical impedance studies and electrochemical double-layer studies. At the loading percentage of 20% that is equal to platinum loading, metal-substituted reduced ferrite loaded on GO is considered to be an effective alternative to the platinum in lowering the overpotential.
Ferrites are proved themselves as the precious materials for energy storage and water splitting applications such as, supercapacitors, electrocatalysts, photocatalysts, etc. Among the various affecting factors on the performance of the ferrites for aforesaid applications, synthesis methods are one of the most important factors. The ferrites, either spinel or inverse spinel, can be prepared using number of synthesis techniques such as, hydrothermal method, sol-gel auto-combustion method, co-precipitation method, spray pyrolysis method, etc. One can optimize the various properties of the ferrites viz. grain size, crystallinity, porosity, etc by following proper protocol during synthesis. Various physical and chemical synthesis techniques with their optimization conditions and protocols are discussed in this chapter briefly.
The ferrites, with wide range of applications in energy storage such as, supercapacitor, batteries and water splitting, have some shortcomings, primarily conductivity and surface area which are most important and result decisive properties of the electrode materials. The conductivity and surface area of the various ferrite electrode materials can be enhanced with the help of various high conducting materials such as, polymers, graphene oxide, reduced graphene oxide and carbonaceous materials which can improve the electrochemical performance of the ferrite/ferrite-based electrode materials. The graphene oxide, reduced graphene oxide and carbonaceous materials not only enhance the electrochemical performance but also the specific capacitance, cyclic stability and ion diffusion rate of the electrode material. The cost-effective ternary ferrites with composition with these materials are coming forward as trustworthy electrode materials with excellent specific capacitance, long discharge time and good conductivity. In this chapter, deficiencies with resolution and the future perspectives of the ferrite electrode materials have been discussed.
In this work, we propose a simple and effective approach to build nanostructured immunosensor platforms. The one-step strategy relies on i) the in situ formation of dithiocarbamates from the reaction between carbon disulfide and amine groups, present on protein A, ii) their attachment to gold nanoparticles (AuNPs), and iii) the linkage of the modified AuNPs to the electrode surface, which depends on the strong interaction between gold substrates and sulfur moieties. AuNPs and protein A have been used to increase the surface coverage of Immunoglobulin G (IgG) and promote the oriented immobilization of the antibodies on the immunosensing interface. The modified gold surfaces with biomolecules were thoroughly characterized by a combination of techniques: UV–vis spectrophotometry, conventional ellipsometry and atomic force microscopy. The immunosensor performance was assessed in real-time, by surface plasmon resonance and by the highly sensitive total internal reflection imaging ellipsometry, through the specific biorecognition of anti-IgG by the immobilized IgG molecules. We demonstrate that the presence of AuNPs improves the sensitivity of the anti-IgG specific detection, whereas the presence of co-adsorbed CS2 is responsible for blocking the undesired protein nonspecific adsorption to the gold substrate. Overall, we report a simple and innovative one-step method, to chemically modify gold surfaces with protein A and AuNPs, able to specifically detect antigen/antibody interactions with capability of preventing nonspecific adsorption.
A simple one-step methodology was explored to prepare enzyme-modified nanostructured electrodes for the development of biosensing interfaces. Magnetite type nanoparticles conjugated with Laccase were immobilized on gold surfaces. This approach relies on the reaction between carbon disulfide and amine groups of biomolecules to form dithiocarbamate (DTC) moieties, as well as on the strong affinity between sulfur species and metals. Special emphasis was given to demonstrate DTC formation in aqueous solution and further attachment to iron oxide nanoparticles and to gold electrodes. UV-visible spectroscopy confirmed the functionalization of nanoparticles by DTC using a model secondary amine (N-hexylmethylamine). The direct attachment of modified iron oxide nanoparticles (with ca. 20 or 40 nm mean sizes) to gold electrodes was investigated using the hormone epinephrine, with well-known electrochemical properties. A high amount of immobilized epinephrine and a facilitated redox conversion was observed for modified electrodes containing iron oxide nanoparticles. The success of this simple and robust method was confirmed by X-ray photoelectronic spectroscopy. Finally, the catalytic activity of modified gold with iron oxide nanoparticles and Laccase was evaluated toward 2,2'-azino-bis (3-ethylbenzothiazoline-6-sulphonic acid diammonium salt (ABTS). Chronoamperometric studies revealed a significant catalytic activity of immobilized Laccase in the presence of the nanoparticles, in particular for the largest ones (40 nm), with a sensitivity for ABTS oxidation of 100 mA M⁻¹ cm- 2.
This study examines the oxygen reduction reaction (ORR) in a homogeneous catalyst system, comparing between the outer-sphere and inner-sphere electron-transfer mechanisms. The rate constants are measured using aqueous trifluoromethane sulfonic acid (TFMSA) and water-soluble M * meso-tetra (pyridyl) porphine chloride complexes [M * TMPyP, M * = Fe(III), Co(III), Mn(III) and Cu(II)] at given pH and molar ratio of metal complexes to oxygen. An outer-sphere model consistent with Marcus theory explains that an outer-sphere electron transfer mechanism occurs in the activation-control region. However, higher rate constants than predicted suggests that a possible reaction pathway is a quasi-redox mechanism associated with the formation of an intermediate bond between M ' TMPyP [M ' = Fe(II), Co(II), Mn(II) and Cu(I)] with O 2 followed by proton-activated decomposition. An increase in the catalyst turnover frequency was also observed upon addition of imidazole base, indicating the role of protonation is crucial to the ORR mechanism. The results are encouraging for replacement of platinum with non-noble metal-polymer complex systems for oxygen reduction in that the reorganization barrier for reaction pathway significantly decreases. The positive effect of proton activation on the catalytic activity of the homogeneous redox catalysts is of considerable interest for future studies. In a three-dimensional, molecular catalysis model, the predicted results using the measured reaction rate suggest that the non-noble metal catalysts can be used for practical electrochemical cell designs.
The effects of different redox mediators on the oxygen reduction reaction (ORR) catalyzed by an iron porphyrin complex, iron(III) meso-tetra(N-methyl-4-pyridyl)porphine chloride [FeIIITMPyP], in 0.1 M triflic acid were investigated by cyclic voltammetry (CV) and spectroelectrochemistry in conjunction with density functional theory (DFT) calculations. The formal potentials of the FeIIITMPyP catalyst and the redox mediators, as well as the half-wave potentials for the ORR, were determined by CV in the absence and presence of oxygen in acidic solutions. UV/Vis spectroscopic and spectroelectrochemical studies confirmed that only the 2,2′-azino-bis(3-ethylbenzothiazioline-6-sulfonic acid)diammonium salt (C18H24N6O6S4) showed effective interactions with FeIIITMPyP during the ORR. DFT calculations suggested strong interaction between FeIIITMPyP and the C18H24N6O6S4 redox mediator. The redox mediator caused lengthening of the dioxygen iron bond, which thus suggested easier dioxygen reduction. Consistent results were observed in electrochemical impedance spectroscopic measurements for which the electron-transfer kinetics were also evaluated.
Abstract Surface functionalized magnetic iron oxide nanoparticles (NPs) are a kind of novel functional materials, which have been widely used in the biotechnology and catalysis. This review focuses on the recent development and various strategies in preparation, structure, and magnetic properties of naked and surface functionalized iron oxide NPs and their corresponding application briefly. In order to implement the practical application, the particles must have combined properties of high magnetic saturation, stability, biocompatibility, and interactive functions at the surface. Moreover, the surface of iron oxide NPs could be modified by organic materials or inorganic materials, such as polymers, biomolecules, silica, metals, etc. The problems and major challenges, along with the directions for the synthesis and surface functionalization of iron oxide NPs, are considered. Finally, some future trends and prospective in these research areas are also discussed.
Work performed with gold catalysts before about 1980 is briefly reviewed, and early indications of the importance of using very small particles to obtain good activity are noted. The apparent contrast between silver and gold in catalysing carbon monoxide oxidation was anticipated by studies in matrix isolation chemistry. The unexpected and in some ways unique properties of gold are attributable to the operation of a relativistic effect which stabilises the 6s(2) electron pair. Essential requirements for high oxidation activity include: small particle size, use of 'reactive' support, and a preparative method that achieves the desired size of particle in intimate contact with the support. Surface atoms on such small particles behave more like individual atoms, and this together with awareness of the relativistic factor may help to explain why gold can be such an effective catalyst.
The synthesis of a new carbonyl ruthenium(II) deuteroporphyrin-IX lipoic acid deriva-
tive (Ru(PLip)(CO)) (4) and its self-assembly on Au(111) surfaces were accomplished. Ru(PLip)(CO) 4
was prepared by reduction of the ester groups of carbonyl-ruthenium(II) deuteroporphyrindimethylester
1 and further esterification with D,L-a-lipoic acid 3. The self-assembly of Ru(PLip)(CO) 4 was con-
firmed by X-ray photoelectron spectroscopy (XPS) and cyclic voltammetry (CV). The S 2p XP spectrum
of SAM formed by Ru(PLip)(CO) showed the S2p 3/2 peak at 162.4 eV which corresponds to thiolated
species bounded to gold. The influence of the interaction of porphyrin moieties on SAM stability was
studied. The reductive desorption voltammogram of Ru(PLip)(CO) 4 self-assembled on gold showed
an intense reduction peak at -1.02 V while when pyridine was coordinated to the central ruthenium(II)
the reductive desorption peak was shifted to -0.85 V. The capacity of the modified Ru(PLip)(CO) gold
electrode to detect nitric oxide (NO) was investigated by cyclic voltammetry. An irreversible reduction
peak which increased with time on NO exposure was registered at -0.69 V indicating NO coordination
to ruthenium(II).
Effects of axial coordination of the metal center on oxygen reduction catalyzed by iron tetraphenylporphyrin have been studied using spectroelectrochemical and rotating ring disk electrode techniques. Among numerous ligands studied, the strongest complexation of the metal center was observed for imidazole and some substituted imidazole ligands. Around 300 mV positive shift of the oxygen reduction potential in 0.5 M H2SO4 was observed when the catalyst support (high surface area carbon Vulcan XC72) was impregnated with polyvinylimidazole. A smaller 200 mV shift of the reduction potential was recorded when graphene modified with 1-(3-aminopropyl)imidazole was used as the catalyst support instead of Vulcan. In both cases, the catalyst selectivity for four-electron reduction was also substantially improved. The effects of iron complexation on oxygen reduction kinetics and mechanism result from both a positive shift of the potential of the first porphyrin reduction and an increase of the electron density on the iron center through the so-called “push effect”. The observed phenomena are discussed in relation to heat-treated Fe/N/C catalysts for oxygen reduction.
Thiols are widely utilized to functionalize metal nanoparticles, including ubiquitous citrate-stabilized gold nanoparticles (AuNPs), for fundamental studies and biomedical applications. For more than two decades, citrate-to-thiol ligand exchange has been used to introduce functionality to AuNPs in the 5-100 nm size regime. Contrary to conventional assumptions about the completion of ligand exchange processes and formation of a uniform self-assembled monolayer (SAM) on the NP surface, coadsorption of thiols with preadsorbed citrates as a mixed layer on AuNPs is demonstrated. Hydrogen bonding between carboxyl moieties primarily is attributed to the strong adsorption of citrate, leading to the formation of a stabilized network that is challenging to displace. In these studies, adsorbed citrates, probed by Fourier transform infrared and X-ray photoelectron spectroscopy (XPS) analyses, remain on the surface following thiol addition to the AuNPs, whereas acetoacetate anions are desorbed. XPS quantitative analysis indicates that the surface density of alkyl and aryl thiolates for AuNPs with an average diameter of ∼40 nm is 50-65% of the value of a close-packed SAM on Au(111). We present a detailed citrate/thiolate coadsorption model that describes this final mixed surface composition. Intermolecular interactions between weakly coordinated oxyanions, such as polyprotic carboxylic acids, can lead to enhanced stability of the metal-ligand interactions, and this needs to be considered in the surface modification of metal nanoparticles by thiols or other anchor groups.
Unpyrolyzed, non noble metal catalysts for Oxygen Reduction Reaction (ORR), denoted MeOx–CoP/C, were obtained using a two-step procedure. The procedure consisted of a synthesis of carbon-supported transition metal (Me═Co, or Ni, or Fe) nanoparticles, followed by adsorption of cobalt porphyrin (CoP). TEM and XPS analyses confirm the formation of nanoparticles and the presence of transition metal oxides. Rotating disk electrode measurements showed that the as-synthesized materials exhibit catalytic ORR activity in acidic medium toward oxygen reduction, which is higher than that of cobalt porphyrin on carbon. This reveals that the metal oxide nanoparticles enhance the activity of the metalloporphyrin without being electroactive themselves. The catalytic activity follows the sequence: CoOx–CoP/C > NiOx–CoP/C > FeOx–CoP/C, showing the influence of nature of the transition metal on the enhancing effect. The presence of a cobalt center incorporated in the macrocycle was found to be essential to the oxygen reduction reaction, appearing thus to be the catalytic active site of the reaction. Our data suggest the ORR occurs at a single active site.
Iron oxide nanoparticles with mean diameter ranging from 7 to 20 nm were synthesized using two routes: the precipitation method in controlled atmosphere and a reduction–precipitation method under air, in some cases followed by a hydrothermal treatment. The smallest nanoparticles were obtained by the reduction–precipitation method.In order to establish the composition of the iron oxide nanoparticles and its relation with size, the morphological, structural and magnetic properties of the prepared samples were investigated using X-ray diffraction, transmission electron microscopy, Mössbauer spectroscopy and SQUID magnetometry. The results allow to conclude that the nanoparticles can be essentially described as Fe3−xO4, x decreasing with the particle size increase. The composition and magnetic behavior of the synthesized iron oxide nanoparticles are directly related with their size. The overall results are compatible with a core@shell structure model, where a magnetite core is surrounded by an oxidized magnetite layer (labeled as maghemite), the magnetite core dimension depending on the average particle size.
The electrochemical behaviors of self-assembled substituted porphyrins (SH-terminated, abbreviated as H2TPPO(CH2)nSH, n = 3, 12) on a gold electrode were investigated using the steady-state scanning electrochemical microscopy (SECM). The different electron-transfer (ET) kinetics, including the bimolecular ET between the porphyrin self-assembled monolayers (SAMs) and the redox mediator [K3Fe(CN)6], the tunneling ET between the underlying gold electrode and [K3Fe(CN)6], and pinholes or defects, were clearly distinguishable. The SECM strategy was developed to deal with the two types of porphyrin SAMs. First, a model using alkanethiols [(CH2)nSH, n = 3, 12] as the functional template was proposed to change the conformation of porphyrin SAMs in a unit area of the electrode. Second, the porphyrin SAMs were directly prepared by inserting a metal (cobalt) into the center of the porphyrin ring. The results show the distinct effect of the presence of alkanethiols on the kinetics of the different-chain length porphyrins. In addition, the rate constants of the bimolecular ET significantly increased after the insertion of cobalt. The results are in agreement with the density functional theory (DFT).
The oxygen reduction reaction (ORR) was studied on carbon dispersed Pt and Pt-Co alloyed nanocatalysts with high contents of Co in H2SO4 and H2SO4/CH3OH solutions. The characterization techniques considered were transmission electron microscopy (TEM), X-ray diffraction (XRD) and in situ X-ray absorption near edge structure (XANES). The electrochemical activity for the ORR was evaluated from steady state polarization measurements, which were carried out in an ultra thin layer rotating disk electrode. The results showed that with the increase of Co content, the nanoparticle size distributions become sharper and the mean particle diameters become smaller. XRD indicated low degree of alloy formation but significant phase segregation of Co was observed only for Pt-Co/C 1:3 and 1:5 (Pt:Co atomic ratios). The electrochemical measurements indicated that the four-electrons mechanism is mainly followed for the ORR on all materials and the electrocatalytic activities per gram of Pt is higher for the catalysts with higher Co contents. This was explained based on the XANES results which evidenced a decrease of the coverage of oxygenated Pt adsorbates due to the presence of Co. In the methanol-containing electrolyte, the Pt-Co/C 1:5 catalyst showed the highest performance. This was attributed to its low activity for the methanol oxidation due to the smaller probability for presenting three Pt neighboring Pt active sites.
Taking advantage of the spontaneous deposition of noble metals on polymers containing sulphur, the inclusion of gold and platinum in poly(3-methylthiophene) and poly(3,4-ehylenedioxythiophene) (PEDOTh) layers, achieved by immersion of the polymer into the metal nanoparticles suspension, is reported in the present work.Platinum and gold nanoparticles (NPs), with diameters between 3 and 17nm, have been prepared from colloidal methods (citrate or borohydride reduction in the presence of citrate capping agent) and characterized by transmission electron microscopy, ultraviolet–visible spectrophotometry and X-ray diffraction (XRD). The electropolymerization was carried out under potentiostatic and potentiodynamic conditions, imparting distinct morphologies, as revealed by atomic force microscopy. After polymer films immersion in the colloidal solutions, evidence of the NPs confinement and distribution was provided by XRD analysis and scanning electron microscopy. For thin layers, the quantity of attached metal NPs could be estimated from quartz crystal microbalance data collected throughout the films immersion.The influence of the polymer type and morphology, NPs nature, size and incorporated amount on the electrocatalytic activity of the so-prepared modified electrodes towards the hydrazine oxidation, in phosphate buffer solution, has been investigated by cyclic voltammetry. The results clearly show the superior properties of potentiodynamically prepared PEDOTh films attaching very small (3nm) freshly prepared Pt-NPs.
New metalloporphyrinoid complexes functionalized with two pyrrole groups and bearing two phosphonate residues have been synthesized starting from readily accessible deuterioporphyrin derivatives. The functional pyrrole groups allow the formation of metalloporphyrinoid films on electrodes by oxidative electropolymerization. On the other hand, the phosphonate functionalities could be used to immobilize the metalloporphyrinoids on polycrystalline titanium dioxide electrodes. Electrodes modified by immobilization of the metalloporphyrinoid complexes have been shown to be useful for electrocatalysis and as electrochemical sensors.
A novel Co(II) porphyrin lipoic acid derivative was synthesized starting from the commerically available red blood pigment hemin. The disulfide functionalities of the lipoic acid moieties allowed its immobilization on gold by a self-assembly method. The Co(II) porphyrin self-assembled monolayer (SAMs) on gold (111) surfaces were characterized electrochemically through monolayer reductive desorption and evaluation of the redox properties of the immobilized molecules in organic medium, and by scanning tunneling microscopy (STM). It was found that after assembly the Co(II) porphyrin is electroactive exhibiting the typical redox processes observed for its precursor without the appended lipoic acid in solution. A coverage of 2.7 x 10(-10) mol. cm(-2) has been estimated assuming that four electrons (one per each sulfur atom) are involved in the process. The porphyrin-modified gold electrodes exhibit catalytic acitivity demonstrated towards the reduction of molecular oxygen in acidic solution.
Multilayers film of nanostructured gold nanoparticles (AuNPs) has been fabricated based on the layer-by-layer (LBL) technique using a self-assembled monolayer of 5,15-di-[p-(6-mercaptohexyl)-phenyl]-10,20-diphenylporphyrin (trans-PPS2). AuNPs act as physical cross-link points in the multilayers. Electrochemical impedance spectroscopy (EIS) and scanning electrochemical microscopy (SECM) are applied to study the formation of the organic–inorganic multilayers film and have determined the electrochemical parameters, i.e., the heterogeneous electron transfer rate constant (Ket). The observed phenomena indicate that the electron transfer (ET) process is affected by material properties and the molecular structure of self-assembled monolayers (SAMs). Using the high sensitivity of ET of ferricyanide to the modification of the gold surface with multilayers film, we select this reaction as a probe to study the different modification stages at this modified electrode. ET is retarded on the trans-PPS2 alternative deposition of layers on the electrode surface and is accelerated on the AuNPs’ layers. SECM images are used to collect surface information in the course of the successive modification process. SECM images obtained from bare and different modification stages show very high resolution with different topographies.
Gold nanoparticles with narrow and controlled size distributions have been synthesized chemically and deposited onto a carbon support. Using the resulting gold on carbon (Au/C) catalysts, Au particle size effects on the kinetics of the oxygen reduction reaction (ORR) were analyzed in acidic media (0.5M H2SO4). From rotating ring-disk electrode (RRDE) voltammetric studies, it was found that, for bulk gold, the number of electrons, n, involved in the ORR was nearly constant at potentials above −0.2V. On the contrary, for the catalysts with diameters less than 10–15nm, the value of n increased as the potential became more negative, and the highest value of n was obtained when the size of Au particles was less than 3nm. Those results showed that further reduction of H2O2 or direct 4-electron reduction of O2 proceeded at relatively low overpotential on extremely small gold clusters.
A comparative study of the electrode kinetics of oxygen reduction of platinum in perchloric, phosphoric, sulfuric, trifluoromethanesulfonic acids (all at pH = 0) and in potassium hydroxide (pH = 14) was made at 25°C using rotatating ring-disc electrode techniques. The platinum electrode was first characterised in these electrolytes using the cyclic voltammetric method. The results showed that in the potential region from 0.8 to 0.6 V/rhe, the kinetics of oxygen reduction in these electrolytes decreases in the order KOH > H2SO4 ∼ CF3SO3H > H3PO4 > HClO4. This order of activity is reflected in the effects of the electrolytes, in respect to specific adsorption of anions, on the platinum oxide formation reaction. The role of anion adsorption is also apparent in the dependence of the rate constant for oxygen reduction to water or to hydrogen peroxide and of hydrogen peroxide reduction to water on potential. The superior behavior of oxygen reduction in KOH is due to minimal adsorption of the OH− ion. The more complex adsorption behavior of the oxyanions in the investigated acid electrolytes than that of simple anions like the halide ions presents difficulties in drawing detailed correlations between oxygen reduction kinetics and adsorption behavior of oxyanions of platinum.
The cathodic reduction of oxygen has been investigated at a gold nanoparticles-electrodeposited gold electrode in 0.5 M H2SO4 solution. Two well-defined reduction peaks were observed at +50 and −250 mV vs. Ag/AgCl/KCl (sat.). Those two peaks indicated a 2-step 4-electron reduction pathway of O2 in this strong acidic medium. The former peak was ascribable to the 2-electron reduction of O2 to H2O2, while the latter was assigned to the reduction of H2O2 to H2O. The observed electrocatalysis for the reduction of O2 is attributable to the extraordinary catalytic activity of the gold nanoparticles over the bulk gold electrode, at which the 2-electron reduction peak of O2 to H2O2 was observed at −200 mV.
This work describes a simple methodology to bio-functionalize flat Au(1 1 1) electrodes through the one-step reaction between gold nanoparticles (AuNPs), carbon disulfide and a secondary amine (epinephrine) and an aminoacid (tryptophan). The process relies on the in situ dithiocarbamate formation between carbon disulphide and amine groups and also on the strong linkage between sulfur and gold. The redox behavior of modified gold with epinephrine or tryptophan, prepared from both ethanolic and aqueous solutions confirms their covalent immobilization and reveals a significant increase of their amount on the electrode due to the presence of AuNPs. Electrochemical reductive desorption in basic solution provided qualitative information on the amount of sulfur linked the gold surface and complements the redox studies. The co-immobilization of an enzyme (glucose oxidase, GOx) and gold nanoparticles, using carbon disulfide has been also tested. The presence of GOx on modified Au(1 1 1) electrodes has been confirmed by electrochemical detection of the catalytic oxidation of glucose in the presence of a redox mediator and through the evaluation of H2O2 reduction, formed during the catalytic reaction. XPS analysis, topographic and phase imaging by atomic force microscopy (AFM) further confirmed surface modification by AuNPs and also their functionalization. The successful one-step amine adsorption, in the presence of AuNPs, from aqueous solutions reveals the potential of this method in the construction of nanostructured biosensing interfaces.
We have prepared self-assembled monolayers (SAMs) of 4-aminothiophenol (4-ATP) and 1-(4-mercaptophenyl)-2,6-diphenyl-4-(4-pyridyl)pyridinium tetrafluoroborate (MDPP) functionalized with iron phthalocyanine (FePc) and copper phthalocyanine (CuPc) adsorbed on gold (111) electrodes. The catalytic activity of these SAMs/MPc was examined for the reduction of O2 in aqueous solutions and compared to that of bare gold and with gold coated directly with preadsorbed MPc molecules. Scanning tunneling microscopy (STM) studies confirm the functionalization of the 4-ATP by MPc. STM images reveal that iron phthalocyanine molecules are chemically anchored to 4-aminothiophenol organic monolayers, probably having an “umbrella” type orientation with regards to the surface. The electrocatalytic studies carried out with Au/4-ATP/FePc and Au/MDPP/FePc electrodes show that the O2 reduction takes place by the transfer of 4-electron to give water in contrast to a 2-electron transfer process observed for the bare gold. The modified electrode obtained by simple adsorption of FePc directly to the Au(111) surface still promotes the 4-electron reduction process, but it shows a lower activity than the electrodes involving SAMs with FePc molecules positioned at the outmost portion of the self-assembled monolayers. The activity of the electrodes increases as follow: Au < Au/FePc < Au/4-ATP/FePc < Au/MDPP/FePc with the highest activity when FePc molecules are more separated from the Au surface. In contrast, the less active CuPc shows almost the same activity in all three configurations. Theoretical calculations suggest the importance of the back-bonding into the adduct formation, showing the relevance of the supporting gold surface on the electron-transfer process mediated by anchoring ligands.
Sub-10 nm nanoparticles (NPs) of M(II)-substituted magnetite MxFe3-xO4 (MxFe1-xO•Fe2O3) (M = Mn, Fe, Co, Cu) were synthesized and studied as electrocatalysts for oxygen reduction reaction (ORR) in 0.1 M KOH solution. Loaded on commercial carbon support, these MxFe3-xO4 NPs showed the M(II)-dependent ORR catalytic activities with MnxFe3-xO4 being the most active followed by CoxFe3-xO4, CuxFe3-xO4, and Fe3O4. The ORR activity of the MnxFe3-xO4 was further tuned by controlling x and MnFe2O4 NPs were found to be as efficient as the commercial Pt in catalyzing ORR. The MnFe2O4 NPs represent a new class of highly efficient non-Pt catalyst for ORR in alkaline media.
Size, shape, composition and crystalline structures of noble metal nanoparticles are the key parameters in determining their electrocatalytic performances. Here, we report on a robust chemical-tethering approach to immobilize gold nanoparticles onto transparent indium tin oxide (ITO) glass electrode surface to systematically investigate their size- and shape-dependent electrocatalysis towards methanol oxidation reaction (MOR) and oxygen reduction reaction (ORR). Monodisperse 20 nm nanospheres (NS20s), 45 nm nanospheres (NS45s) and 20 × 63 nm nanorods (NRs) were synthesized, which could be chemically tethered to ITO surface forming submonolayers without any aggregation. These nanoparticle-modified ITO electrodes exhibited strong electrocatalytic activities towards MOR and ORR but their mass activities were highly dependent on particle sizes and shapes. For particle with similar shapes, size determined the mass activities: smaller particle size led to greater catalytic current density per unit mass due its greater surface-to-volume ratio (NS20s > NS45s). For particles with comparable sizes, shapes or crystalline structures governed selectivity of electrocatalytic reactions: NS45 exhibited a higher mass current density in MOR than that for NRs due to its dominant (111) facets exposed; whereas NRs exhibited a higher mass current density in ORR due to its dominant (100) facets exposed.
Carbon disulfide (CS2) can spontaneously react with amine groups to form dithiocarbamates on gold surface, providing a possibility to immobilize some compounds with primary or secondary amine groups in one-step. Using this principle, an immunosensor interface prepared for immunoglobulin G (IgG) sensing surface towards anti-IgG has been fabricated for the first time by simply immersing gold slides into a mixed aqueous solution of CS2 and protein A, followed by incubation in immunoglobulin G solution. The reaction between CS2 and protein A has been followed by UV-Vis spectroscopy whereas cyclic voltammetry have been employed in the characterization of the modified gold surface with CS2 and protein A, both methods indicate that protein A immobilization is implemented by CS2. Conventional ellipsometry, atomic force microscopy (AFM) as well as surface plasmon resonance (SPR) have been used to evaluate the specific binding of protein A with IgG and IgG with anti-IgG, revealing that IgG is specifically captured to form the biosensing interface, maintaining its bio-activity. Compared to direct adsorption of IgG on gold surface, IgG sensing surface constructed by CS2 and protein A is far more sensitive to capture anti-IgG as its target molecule. In addition, the modified surface is proved to have good capability to inhibit non-specific adsorption, as supported by control experiments using lysozyme and BSA. To conclude, antibody immobilization using this one-step method has a potential as a simple and convenient surface modification approach for immunosensor development.
Non-noble metal catalysts for O2 reduction were prepared by dispersing iron(II) phthalocyanine, cobalt(II) tetra-tert-butylphthalocyanine, cobalt(II) 2,3,7,8,12,13,17,18-octaethyl-porphine, and cobalt(II) 5,10,15,20-tetrakis(4-tert-butylphenyl)-porphyrine on carbon nanotubes (CNTs) used as high surface area support. Different types of CNTs (SWCNTs, DWCNTs and MWCNTs) were investigated as an effective substitute for commonly used carbon black in carbon-supported phthalocyanines and porphyrins. The oxygen reduction reaction (ORR) activity of those CNT-supported catalysts in alkaline and acidic solutions was studied. The results show that: (i) all catalytic systems including MWCNTs are more efficient for O2 reduction than those with SWCNTs and DWCNTs, (ii) the oxidative chemical treatment of the CNTs increases the electrocatalytic performance of the corresponding CNT-supported catalysts, (iii) similarly to Vulcan-supported catalysts, iron(II) phthalocyanine gives the best electroactivity among the investigated CNT-supported materials and (iv) finally, the MWCNT-supported iron(II) phthalocyanine catalyst chemically treated in oxidative conditions shows an ORR catalytic activity comparable to a commonly used Pt/C catalyst with similar current densities and a very low overpotential (60 mV).
The electroreduction of O2 at graphite electrodes on which the complex of Co(III) with the macrocyclic ligand C-meso-5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane (hmc) is adsorbed proceeds in two steps that are well separated in potential. The first step consumes two electrons per molecule of adsorbed (hmc)Co3+ to yield adsorbed (hmc)CoOOH2+. The second step involves the reduction of the adsorbed (hmc)CoOOH2+ to (hmc)Co2+ + H2O2 followed by the reduction of O2 to H2O2 catalyzed by the adsorbed (hmc)Co3+/2+ complexes. A mechanistic scheme that accommodates the observed electrochemical responses is presented and tested by comparison with responses calculated by digital simulation methods. Differences between the electrochemical responses obtained with dissolved or adsorbed (hmc)Co3+ are exposed and discussed. In addition, the behavior of the adsorbed (hmc)Co3+/2+ system as it acts to electrocatalyze the reduction of O2 is compared and contrasted with that exhibited by analogous adsorbed cobalt porphyrin electrocatalysts.
Following a seeding growth approach, gold nanoparticles of diameters 5−40 nm were prepared with 10−15% standard deviation in diameter from 3.5 ± 0.7 nm gold particle seeds. Particle size can be controlled by varying the ratio of seed to metal salt, and thus any size in the range 5−40 nm can be prepared. The method can also be scaled up to produce 10−100 mg of gold nanoparticles.
Disulfides, with a systematic series of alkyl spacers containing porphyrins at both terminals, were prepared to investigate the effect of the spacer length on the structure and photoelectrochemical properties of self-assembled monolayers (SAMs) of the porphyrins on a gold electrode. The structure of the SAMs was studied using ultraviolet (UV)−visible absorption spectroscopy in transmission mode, cyclic voltammetry, UV−visible ellipsometry, and fluorescence spectroscopy. These measurements showed that as the length of the spacers increases, the SAMs tend to form highly ordered structures on the gold electrodes. In addition, the structures of the monolayers vary depending on the even and odd number of the methylene spacers (n). From these measurements a porphyrin dimer model is proposed in that the two porphyrins take J-aggregate-like partially stacked structures in the monolayers. Photoelectrochemical studies were carried out in argon-saturated Na2SO4 aqueous solution containing methyl viologen as an electron carrier using the modified gold working electrode, a platinum wire counter electrode, and a Ag/AgCl reference electrode. The quantum yield increases in a zigzag fashion with an increase in the spacer length up to n = 6 and then starts decreasing slightly as the chain lengths become longer. A plausible explanation for the photocurrent trend comes from the following points: (i) there are two competitive deactivation pathways for the excited singlet state of the porphyrin dimer, i.e., the quenching by the electrode via energy transfer and by the electron carrier via electron transfer, (ii) the porphyrin aggregation enhances the rate of nonradiative pathway in the excited state, and (iii) the electron transfer rate from the gold electrode to the resulting porphyrin cation radical decreases with an increase of the spacer lengths. These results will provide basic information for the construction of molecular assembly with photoactive function on surface.
Pure 1,10-decanedithiol (C10-SH) and mixed (1-decanethiol:1,10-decanedithiol) self-assembled monolayers (SAMs) prepared from ethanolic solution on Au(111) surfaces have been used in order to investigate the effect of the SAM organization and the availability of free −SH groups at the SAM/solution interface on the development of layer-by-layer architectures containing SAMs and gold nanoparticles (Au-NPs). The SAM modified electrodes have been electrochemically characterized by cyclic voltammetry in alkaline medium (reductive desorption) and in the presence of an electroactive species, Fe(CN)63−, in KNO3 solution, enabling the evaluation of the stability and organization of the SAMs. Enhanced stability, organization, and hindrance to the electron transfer were found for the mixed SAMs with increasing thiol content, when compared with the pure dithiol SAM. The mixed SAM prepared from solution containing the thiol to dithiol proportion of (50:1) and pure C10-SH SAMs have been selected for further modification; the electrochemical quartz crystal microbalance (EQCM) enables the detection of different amount of citrate stabilized Au-NPs attachment to the selected SAMs modified electrodes due to distinct availability of free −SH groups at the SAM/solution interface and the electrochemical characterization of the layer-by-layer assemblies (based on pure C10-SH and mixed SAMs) showed that the electron transfer (ET) properties of the such architectures strongly depend on the nature of the base SAM and amount of immobilized Au-NPs. Atomic force microscopy (AFM) morphological characterization of the C10-SH SAM upon layer-by-layer modification was performed ex situ in air.
The interfacial bonding properties of succinic acid, myristic acid, and succinic anhydride molecules with a set of differently pretreated zinc samples have been investigated using Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS). The molecules were selected to model the typically used carboxylic-based adhesives, coatings, and self-assembling monolayers. The differently pretreated oxidized zinc surfaces were found to be capable of adsorbing the monomers, resulting in the formation of carboxylates. The interactions between the three types of monomers and the zinc samples resulted in the formation of a bridging bidentate coordination state on all samples, implying the contribution of the same functional groups in the adsorption mechanism. Moreover, relative FTIR peak intensities were utilized to evaluate the molecular structures upon adsorption. It was found that a sufficiently positive potential promoted an interaction between the adsorbates depending on the applied experimental parameters. The interactions resulted in a structural rearrangement of the molecules and formation of different end groups. The structural variations are expected to represent different interfacial characteristics of the polymer coatings.
Oxygen reduction and related processes are studied at nanostructured Pt electrodes assembled from polyacrylate-capped Pt nanoparticles (d = 2.5 ± 0.6 nm) in poly(diallyldimethylammonium)chloride (PDDA) on indium tin oxide or glassy carbon with varying nanoparticle surface coverage. The nanoparticle density was varied laterally by varying the dipping time of PDDA-modified electrodes in the nanoparticles solution, or vertically with the number of nanoparticle/polyelectrolyte (bi)layers following a layer-by-layer assembly. TEM images revealed submonolayer coverage in one bilayer at 60 min dipping with a fractal distribution, and a significant surface coverage at four bilayers with evidence of multilayer assembly. Cyclic voltammetry in oxygen-containing electrolytes showed the assemblies to be electroactive for oxygen and hydrogen peroxide reduction, with a pH-dependent oxygen reduction peak shifting by −50 mV/pH unit. OH adsorption was found to be less favored occurring at more positive potential at the nanostructured electrode compared to polycrystalline Pt, while the oxide reduction peak was negatively shifted at the former electrode, in agreement with reports of increased oxophilicity with decreased particle size. The oxygen reduction peak potential shifted positively upon increasing Pt nanoparticles coverage, consistent with the catalytic activity of Pt for oxygen reduction. The active surface area of Pt nanoparticles was measured electrochemically from the charge of hydrogen underpotential deposition at the assemblies in H2SO4, and the diffusion-limited peak current for oxygen reduction measured per real Pt surface area is reported to decrease with increasing catalyst loading, as a result of reaching a limiting effective diffusion field.
Two- and three-dimensional Au nanoparticle/[tetrakis(N-methylpyridyl)porphyrinato]cobalt (CoTMPyP) nanostructured materials were prepared by “bottom-up” self-assembly. The electrocatalytic and plasmonic properties of the Au nanoparticle/CoTMPyP self-assembled nanostructured materials (abbreviated as Au/CoTMPyP SANMs) are tunable by controlled self-assembly of the Au nanoparticles and CoTMPyP on indium tin oxide (ITO) electrode. The electrocatalytic activity of the Au/CoTMPyP SANMs can be tuned in two ways. One way is that citrate-stabilized Au nanoparticles are positioned first on ITO surface with tunable number density, and then positively charged CoTMPyP ions are planted selectively on these gold sites. The other way is that Au nanoparticles and CoTMPyP are deposited by virtue of layer-by-layer assembly, which can also tune the amount of the as-deposited electrocatalysts. FE-SEM studies showed that three-dimensional SANMs grow in the lateral expansion mode, and thermal annealing resulted in both surface diffusion of nanoparticles and atomic rearrangement to generate larger gold nanostructures with predominant (111) facets. The annealed SANMs show a strongly enhancing gold surface plasmonic resonance band in comparison with unannealed SANMs. The changes in gold plasmonic properties correlate with annealing-induced structural changes of the SANMs.
The mechanisms by which the reduction of dioxygen at graphite electrodes is catalyzed by cofacial dicobalt and related porphyrins adsorbed on the electrode surface have been scrutinized. The products of the reduction, the electrode potential where the reduction proceeds, and the mechanistic role of protons were among the topics examined. For the best catalyst it was possible to relate the electrochemical response of the adsorbed porphyrin to the potential where the catalyzed reduction of dioxygen proceeds. The electrocatalytic behavior of several new heterobinuclear cofacial porphyrins is reported as well as that of a new cofacial dicobalt porphyrin in which the bridge connecting the two porphyrin rings consists of only three atoms. A comparison of the behavior of the various catalysts has led to a more detailed proposal for the mechanisms by which they operate in catalyzing the electroreduction of dioxygen.
Photosensitizer/electron acceptor molecular cross-linked Au-nanoparticle arrays are assembled on indium-doped tin oxide (ITO) electrodes by a layer-by-layer deposition process. A Ru(II)−tris-(2,2‘-bipyridine)-cyclobis(paraquat-p-phenylene) catenane (1) or Zn(II)-protoporphyrin IX−bis(N-methyl-N‘-undecanoate-4,4‘-bipyridinium) (2) are used as molecular cross-linkers for the generation of Au-nanoparticle (13 ± 1 nm) arrays of a controlled number of layers. The Au-nanoparticle arrays are characterized by absorbance spectroscopy and by electrochemical means. The electrodes functionalized with 1- or 2-cross-linked Au-nanoparticle arrays are used in photoelectrochemical experiments. The resulting action spectra of the photocurrents follow the absorbance spectra of the respective chromophores. Mechanistic studies indicate that the photocurrents originate from intramolecular electron-transfer quenching of the photoexcited state of the photosensitizer by the electron acceptor units, leading to the formation of intermediate redox species. The oxidized photoproduct oxidizes the sacrificial electron donor, Na2EDTA, whereas the reduced bipyridinium radical cations transfer the electrons to the bulk electrode support.
A new type of anthracene-bridged dimeric porphyrin substituted with one or two cobalt centers has been tested as a catalyst for the electroreduction of dioxygen. Both the dicobalt and monocobalt derivatives provide four-electron-reduction pathways. Both derivatives also catalyze the reduction of hydrogen peroxide but at lower rates. Kinetic analysis shows that hydrogen peroxide is not an intermediate along the pathway for the four-electron reduction of dioxygen.
Carbon nanotube (CNT) compositions were prepared by covalently grafting a Co(II) porphyrin to functionalized multiwalled carbon nanotubes (MWCNTs) via zwitterionic functionalization of the CNT sidewalls followed by a SN2 substitution reaction. The MWCNT−Co−porphyrin compositions, mixed with Nafion, displayed excellent catalytic performance for oxygen reduction in acidic media (pH range, 0.0−5.0) at room temperature. With low catalyst loading, the oxygen reduction rates achieved are more than 1 order of magnitude higher than previously reported values for similar Co−porphyrin catalysts. These results demonstrate the advantages of systems of MWCNTs covalently linked to electrocatalytic molecules. The electrodes are easily fabricated by a drop-casting vacuum drying procedure. Rotating disk electrode (RDE) and rotating ring disk electrode (RRDE) measurements revealed the mechanism to be a direct four-proton and four-electron reduction of oxygen to water. These results demonstrate that new MWCNT electrocatalytic systems are potential substitutes for platinum or other metal-based cathode materials in proton conducting membrane fuel cells.
The electrochemical reduction of oxygen has been studied on gold nanoparticle/multi-walled carbon nanotubes (GNP/MWCNT) modified glassy carbon (GC) electrodes in 0.5 M H2SO4 solution using the rotating disk electrode (RDE) technique. The oxygen reduction behaviour of GNP/MWCNT electrodes was compared with that of a bulk gold electrode. Electrochemical studies indicate that the GNP/MWCNT catalyst shows a remarkable electrocatalytic activity towards O2 reduction in acid media. The RDE results reveal that the reduction of oxygen proceeds by a two-electron pathway at low overpotentials.
Adsorption of carboxylic acids from liquid solutions on gold was investigated by electrochemical experiments including voltammetric measurements and quartz crystal microgravimetry. The adsorption was found to occur at gold surfaces held at relatively high electrical potentials, and the adsorption step was associated with anodic currents. From the experimental evidences, we propose that the carboxylic acid molecules adsorb on a gold surface through an anodic reaction that is analogous to the previously studied adsorption reaction of organic sulfur compounds forming self-assembled monolayers. The potentials at which the carboxylic acids adsorb to appreciable extents were much higher than the potentials at which thiols start to adsorb. In all of the adsorption reactions of thiols, disulfides, and carboxylic acids, oxidation of the metal surface, assisted by the adsorbate-metal interaction, appears to be a common requirement for the adsorption.
Small molecules or analytes present at low concentrations are difficult to detect directly using conventional surface plasmon resonance (SPR) techniques because only small changes in the refractive index of the medium are typically induced by the binding of these analytes. Here, we present an amplification technique using core-shell Fe(3)O(4)@Au magnetic nanoparticles (MNPs) for an SPR bioassay. To evaluate this amplification effect, a novel SPR sensor based on a sandwich immunoassay was developed to detect α-fetoprotein (AFP) by immobilizing a primary AFP antibody (Ab(1)) on the surface of a 3-mercapto-1-propanesulfonate/chitosan-ferrocene/Au NP (MPS/CS-Fc/Au NP) film employing Fe(3)O(4)@Au-AFP secondary antibody conjugates (Fe(3)O(4)@Au-Ab(2)) as the amplification reagent. The stepwise fabrication of the biosensor was characterized using UV-vis spectroscopy, electrochemical impedance spectroscopy, and cyclic voltammetry. A calibration curve of Fe(3)O(4)@Au-Ab(2) conjugates amplification for AFP detection was obtained to yield a correlation in the range of 1.0-200.0 ng mL(-1) with a detection limit of 0.65 ng mL(-1), and a significant increase in sensitivity was therefore afforded through the use of Fe(3)O(4)@Au-Ab(2) conjugates as an amplifier. This magnetic separation and amplification strategy has great potential for the detection of other biomolecules of interest with low interference and high sensitivity by changing the antibody label used in the Fe(3)O(4)@Au-antibody conjugates.
Dithiocarbamates have been regarded as alternative anchor groups to thiols on gold surfaces, and claimed to be formed in situ through the reaction between secondary amines and carbon disulphide. In this paper, we further exploit this methodology for a convenient one step biomolecule immobilisation onto gold surfaces. First, the reactivity between CS2 and electroactive compounds containing amines, primary (dopamine), secondary (epinephrine), and an amino acid (tryptophan) has been investigated by electrochemical methods. Cyclic voltammetric characterisation of the modified electrodes confirmed the immobilisation of all the target compounds, allowing the estimation of their surface concentration. The best result was obtained with epinephrine, a secondary amine, for which a typical quasi-reversible behaviour of surface confined electroactive species could be clearly depicted. Electrochemical reductive desorption studies enabled to infer on the extent of the reaction and on the relative stability of the generated monolayers. Bio-functionalisation studies have been accomplished through the reaction of CS2 with glucose oxidase in aqueous medium, and the catalytic activity of the immobilised enzyme was evaluated towards glucose, by electrochemical methods in the presence of a redox mediator. Scanning tunnelling microscopy (STM) and Atomic force microscopy (AFM) were used respectively, to characterize the gold electrodes and Glucose Oxidase coverage and distribution on the modified surfaces.
Multi-walled carbon nanotubes (MWCNTs) modified with iron tetramethoyxphenyl-porphyrin chloride (FeTMPP-Cl) and heat treated are active towards electrocatalytic oxygen reduction in acidic media. The activity slightly depends on the heat treatment temperature (850 < 550 °C) and the amount of porphyrin deposited onto the nanotubes before the heat treatment step. In comparison with as-received MWCNTs no increase in activity has been found with iron phenanthroline or iron acetate impregnated and heat treated MWCNTs. When MWCNTs are pretreated in an oxidation step using HNO3, there is only a slight increase in activity after FeTMPP-Cl modification and heat treatment compared to the not pretreated MWCNTs. The HNO3 treatment itself, however, leads to an increase in activity of the unmodified MWCNTs. TEM-measurements revealed an amorphous layer surrounding the MWCNTs after HNO3 treatment, while XPS showed an increased amount of oxygen functional groups. It is suggested that there are different kinds of active sites at the catalyst surface, the first ones consisting of oxygen functionalities or other entities introduced by the HNO3 treatment, and the second ones containing nitrogen (and probably iron) introduced via the porphyrin. Pyridine-type nitrogen has been found by XPS after heat treatment at both temperatures, indicating that the active sites are already formed at 550 °C.
Work performed with gold catalysts before about 1980 is briefly reviewed, and early indications of the importance of using very small particles to obtain good activity are noted. The apparent contrast between silver and gold in catalysing carbon monoxide oxidation was anticipated by studies in matrix isolation chemistry. The unexpected and in some ways unique properties of gold are attributable to the operation of a relativistic effect which stabilises the 6s
2 electron pair. Essential requirements for high oxidation activity include: small particle size, use of ‘reactive’ support, and a preparative method that achieves the desired size of particle in intimate contact with the support. Surface atoms on such small particles behave more like individual atoms, and this together with awareness of the relativistic factor may help to explain why gold can be such an effective catalyst.
After a preliminary survey with the electron microscope of various preparations of colloidal gold, a study was made of the process of nucleation and growth in gold colloids. It was shown that nucleating agents may be identified with reducing agents which form a mixed polymer with chlorauric ion before the reduction to the nucleus takes place. It was also shown that the law of growth is exponential. The average size, the deviation from the average size and the character of the particle size distribution curve are determined by the amount of gold, the nucleation process and the law of growth.
The present work focuses on the surface immobilization by self-assembly of pure and mixed Co-porphyrin (CoPorph-PO3H2) and n-alkane phosphonic acids (n-CnH2n+PO3H2; n =4, 5 and 10) from n-butanol solutions on gold substrates. The stability, amount, and packing of the phosphonic molecules attached to the Au (111) surface were investigated by electrochemical reductive desorption studies, and monolayers' thickness was estimated by ellipsometry. The morphological changes induced by the adsorption of n-decane phosphonic acid on gold were analysed by scanning tunnelling microscopy. The redox behavior of Co-Porph-PO3H2 SAMs was assessed in organic medium and compared that of Co-Porph-CO2CH3 precursor in solution, confirming the self-assembly of the metalloporphyrin molecules. With the purpose of reducing the electrostatic interactions between the porphyrin bulky terminal groups in the SAM, n-C5H11PO3H2 and n-C10H21PO3H2 were used to form mixed monolayers with Co-Porph-PO3H2 on gold. Intermediate electrochemical desorption potentials regarding those values of pure monolayers, as well as an increase of phosphonate surface density compared to that of Co-PorphPO(3)H(2) SAM, confirm the presence of two-component SAMs, which indicates that porphyrin moieties are diluted in the monolayer. The electrocatalytic activity of the immobilized molecules was demonstrated towards the reduction of molecular oxygen, in acidic medium.
Metallophthalocyanines confined on the surface of electrodes are active catalysts for a large variety of electrochemical reactions and electrode surfaces modified by these complexes can be obtained by simple adsorption on graphite and carbon However more stable electrodes can be achieved by coating their surfaces with electropolymerized layers of the complexes that show similar activity than their monomer counterparts In all cases fundamental studies carried out with adsorbed layers of these complexes have shown that the redox potential is a very good reactivity index for predicting the catalytic activity of the complexes Volcano-shaped correlations have been found between the electrocatalytic activity (as log I at constant E) versus the Co(II)/(I) formal potential (E') of Co-macrocyclics for the oxidation of several thiols hydrazine and glucose For the electroreduction of O(2) only linear correlations between the electrocatalytic activity versus the M(III)/M(II) formal potential have been found using Cr Mn Fe and Co phthalocyanines but it is likely that these correlations are incomplete volcano" correlations The volcano correlations strongly suggest that E the formal potential of the complex needs to be in a rather narrow potential window for achieving maximum activity probably corresponding to surface coverages of an M-molecule adduct equal to 0 5 and to standard free energies of adsorption of the reacting molecule on the complex active site equal to zero These results indicate that the catalytic activity of metallophthalocyanines for the oxidation of several molecules can be tuned by manipulating the E' formal potential using proper groups on the macrocyclic ligand This review emphasizes once more that metallophthalocyanines are extremely versatile materials with many applications in electrocatalysis electroanalysis just to mention a few and they provide very good models for testing then-catalytic activity for several reactions Even though the earlier applications of these complexes were focused on providing active materials for electroreduction of O(2) for making active cathodes for fuel cells the main trend in the literature nowadays is to use these complexes for making active electrodes for electrochemical sensors (C) 2010 Elsevier B V All rights reserved
Iron(III) protoporphyrin IX (Fe(III)PP) and iron(III) hematoporphyrin (Fe(III)HP) were selectively and covalently attached to dimercaptoalkane-modified gold electrodes. Reaction of their vinyl or hydroxyethyl groups with the surface-immobilized thiols produced thioether linkages, reminiscent of the heme macrocycle attachment in c-type cytochromes. Cyclic voltammetry revealed reversible electrochemistry of self-assembled monolayers (SAMs) of FePPs and FeHPs on the thiol-modified gold substrates. The surface coverage estimated from the charges transferred corresponds to 30% of a monolayer. The heterogeneous rate constant of electron transfer between the Fe(III)PPs and the gold substrate decreases exponentially with the length of the spacer, demonstrating a value of 1.0 A-1 for the tunneling length coefficient, beta. At pH 8, a linear increase of the formal redox potential (Eo`) with the length of the linker was also observed. This suggests that in the film, there is a close contact between the porphyrins and the alkane SAM: the Eo` is affected by the drop of the electrostatic potential from the electrode to the surface of the alkane SAM, and also that there is a strong ion pairing of the Fe(III)PPs in the film with the anions of the solution. The Eo` of Fe(III)PPs in the SAM shows a strong and complex dependency on the pH of the solution, explained by variations in the coordination of the iron, involving hydroxyl ions, water, and eventually dioxygen molecules. Interactions of the iron with either functional groups present at the surface of the substrate or with the propionate groups attached to the porphyrin ring, do not appear to be involved in the electronproton transfer coupling mechanisms.
A novel iron(III) porphyrin disulphide derivative have been successfully immobilised on gold surfaces by self-assembly. The redox response of the modified electrodes was compared with the obtained for a similar iron porphyrin in solution, confirming the immobilisation of the metalloporphyrin. The gravimetric data obtained by electrochemical quartz crystal microbalance (EQCM) during adsorption allowed an estimation of the electrode coverage, providing further evidence for the formation of the porphyrin SAM. The modified electrodes were also measured by conventional and imaging ellipsometry. The electrocatalytic activity of the two modified electrodes was tested for the reduction of the molecular oxygen.
Metalloporphyrin molecules have a wide range of potential applications in diverse technological areas ranging from electronics to optoelectronics, electrochemistry, photophysics, chemical sensors, and catalysis. In particular, self-assembled monolayers of porphyrin molecules have recently attracted considerable interest. In this work we have studied for the first time the self-assembly of a novel Cu deutero porphyrin functionalized with disulfide moieties using electrochemical techniques, UV-vis absorption spectroscopy, polarization modulation infrared reflection absorption spectroscopy, and photoelectron spectroscopies (XPS and UPS). Experimental results indicate that the molecule adsorbs retaining its molecular integrity without forming molecular aggregates via the formation of Au-S covalent bonds. Furthermore, the monolayer consists of a packed array of molecules adsorbed with the plane of the porphyrin molecule at an angle of around 30° with respect to the surface normal. Interestingly, adsorption induces reduction of the Cu center and its consequent removal from the center of the porphyrin ring resulting in porphyrin demetalation. Our results are important in the design of self-assembled monolayers of metallo porphyrins where not only blocking of the metal center by the functional groups that drive the self-assembly should be considered but also possible adsorption induced demetalation with the consequent loss in the properties imparted by the metal center.