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The main focus of this review is to illustrate the amenability of self-assembled monolayers (SAMs) for functionalisation with different receptors, catalytic materials, biomolecules, enzymes, antigen-antibody, etc for various applications. The review discusses initially about the preparation and characterization of SAMs and tailoring of SAMs by incorporation of suitable recognition elements. A description of how the molecular recognition is achieved through forces like electrostatic, covalent and host-guest interactions is included in the review.

In this work we demonstrate the preparation of highly catalytically active Pt formed by the galvanic replacement of the copper adlayer on Au substrates, modified by the self-assembly of fourth generation amine terminated PAMAM dendrimer (G4NH2). The copper adlayer was formed on the dendrimer-modified gold substrate by chemical preconcentration of copper ions followed by electrochemical reduction. The Pt overlayer was characterized by SEM, XPS and by cyclic voltammetry. The catalytic efficiency of the modified film thus prepared through soft route was evaluated by the electro catalytic oxidation of methanol using cyclic voltammetry, chronoamperometry and AC impedance techniques. This work also demonstrates that the copper adlayer formed on the dendrimer-modified electrode can undergo galvanic replacement by nobler metals like Au and Ag, besides Pt. An elegant soft route involving a new three-step protocol to build the concentration of active Pt on the Au surface has been developed. Concentration of the same metal (Pt) or two different metals (Pt–Au) can be built at the interface in a stepwise manner at ambient temperature.

Palladium (Pd) incorporated poly (3,4-ethylenedioxythiophene) (PEDOT) films were synthesized through an electrochemical route and characterized using scanning electron microscopy (SEM) and atomic force microscopy (AFM). The electrochemical study showed catalytic oxidation of dopamine (DA) with optimum loading of Pd. DA and uric acid (UA) were detected using differential pulse voltammetry (DPV). In the presence of ascorbic acid (AA), DA-AA showed peak potential separation of 0.19 V while 0.32 V between UA-AA on Pd-incorporated PEDOT. These peak separations are large enough for sensing DA and UA in the presence of AA. DA and UA exhibited linear calibration plots and the minimum detection limits are 0.5 and 7 µM respectively. On Pd-PEDOT, the reversibility of DA oxidation was found to increase compared to bare glassy carbon electrode (GCE) and PEDOT modified GCE. Fouling effects were also found to be minimal making Pd-PEDOT composite suitable for electroanalysis.

The direct electron transfer exhibited by the yeast cells, Hansenula anomala has been demonstrated using the electrochemical technique cyclic voltammetry by immobilizing the microorganisms by two different methods viz., physical adsorption and covalent linkage. The analysis of redox enzymes present in the outer membrane of the microorganisms has been carried out in this work. This paper demonstrates that yeast cells with redox enzymes present in their outer membrane are capable of communicating directly with the electrode surface and contribute to current generation in a mediatorless biofuel cells. The efficiency of current generation has been evaluated using three anode materials.

Corrosion of Iron in Sulphuric AcidAn Accelerated Tafel Study

The layer-by-layer assembly of silver nanoclusters with 3,3'-dithiodipropionic acid (DTDPA) as spacers was prepared through self-assembly on a gold foil and has been characterized by cyclic voltammetric and AFM techniques. The DTDPA molecules acting as spacers between the layers of silver serve as molecular interconnects for the four layers prepared in this work. The organization of layers was found to decrease with an increase in the number of layers. The layer-by-layer assembly of silver clusters motivated us to prepare silver nanowires stabilized by the bifunctional molecules DTDPA through template synthesis using cellulose nitrate membranes. The nanostructures formed by this method were characterized by SEM, TEM, AFM, FTIR, CV, and photoluminescence studies. It is observed that the DTDPA molecules, instead of forming molecular interconnects, protect the structures by self-assembling themselves along the edges of the nanostructures. The concept of self-assembly protecting the nanostructures is demonstrated in this work.

Investigations are carried out with Clostridium sp. which is isolated from sugar industry effluents, as a biocatalyst for current generation in a microbial fuel cell. Two different configurations of the cell are evaluated. In both cells, the anode compartment consists of suspended cells of Clostridium sp. in a nutrient broth in a phosphate buffer. In the first configuration, the cathode compartment consisted of ferricyanide solution (0.05 M). In the second configuration, the cathode compartment consisted of cells of Thiobacillus ferrooxidans suspended in a nutrient broth. The second configuration represents a complete microbial cell with the anodic and cathodic reactions driven by micro-organisms. The performance characteristics of the two configurations are evaluated with two different anode materials, graphite and graphite felt.

There is a growing interest in devising wet chemical alternatives for physical deposition methods for applications involving thin films, e.g., catalysis. Deposition of platinum on thin gold films is often a problem leading to incomplete coverage and improper adhesion to solid surfaces. Gold substrates often need pre-activation for achieving complete coverage. We demonstrate here that dendrimers with proper functionalities and size work as well-defined nucleating agents and adhesion promoters. This feature is demonstrated using an amine-terminated dendrimer of generation 4.0. This approach allows one to obtain adherent nanoparticulate films of platinum on gold. Unlike other nucleating agents and adhesion promoting compounds, dendrimers have a well-defined ordered structure in terms of their space filling ability. The stability of the films obtained with adsorbed dendrimers is demonstrated using the electrocatalytic reactions of fuels like methanol. The films formed without dendrimers cannot sustain the electro-oxidation currents due to the instability of the films while the films formed with dendrimers can sustain currents for longer duration and for several cycles. The dendrimer-derived Pt films exhibit higher catalytic activity compared to other methods

The behaviour of a poly (aniline boronic acid) (PABA) modified glassy carbon electrode (GCE) for the detection of dopamine (DA) in the presence of excess of ascorbic acid (AA) using cyclic voltammetry (CV) and differential pulse voltammetry (DPV) techniques is investigated. On bare GCE, both DA and AA are oxidized at ~0.16 V, whereas on PABA modified GCE they are oxidized at 0.2 and 0.054 V, respectively. Though PABA favours DA oxidation through ester formation with boronic acid motif, the AA oxidation is also promoted by polyaniline backbone through the involvement of AA in the redox of polyaniline. Since both DA and AA undergo oxidation at closely spaced potentials at a PABA electrode, Nafion®-incorporation into the PABA film was examined for selective determination of DA in the presence of AA. The selectivity was due to accumulation of DA on the electrode surface through ester formation with the boronic acid group and suppression of AA oxidative current through charge discrimination by Nafion.

In a new process for the production of synthetic rutile, aeration rusting is the major step where metallic iron is converted to iron oxide in the presence of catalysts such as NH 4 Cl and air. An electrochemical investigation to understand the anodic dissolution of reduced ilmenite (RI) has been carried out using a reduced ilmenite carbon paste (CP) electrode at 30 °C. Intermediate products with different iron contents were formed during the rusting reaction. Synthetic rutile with a negligible iron content was used for the investigations. The reaction mechanism and the influence of TiO 2 matrix were studied.

Monomolecular level modification of electrode surfaces through a self-assembly approach is, of late, gaining importance in view of its many functional applications in areas such as molecular electronics, molecular recognition, electron transfer studies and electroanalysis. Self-assembled monolayer (SAM) modification of a gold electrode with 2-mercaptobenzimidazole (MBI) has been achieved. On this modified electrode, anodic stripping voltammetric determination of mercury at ppm/sub-ppm level concentrations has been successfully attempted. Pre-concentration, prior to stripping, has been effected through a non-electrolytic process involving chemical interactions between MBI and Hg(II). The results are described and discussed with a plausible scheme. Keywords: 2-Mercaptobenzimidazole, Self-assembled monolayer, Mercury(II), Anodic stripping voltammetry

The monolayers of 2-mercaptobenzothiazole (MBT) on polycrystalline Au and Ag films have been studied by surface-enhanced Raman (SERS) and X-ray photoelectron (XPS) spectroscopies. Whereas MBT adsorbs with its molecular plane flat on Ag, its plane is perpendicular on Au. This difference in adsorbate geometry is manifested as differences in the intensities of certain vibrational peaks in the Raman spectra. Ab initio molecular orbital (MO) calculations suggest that MBT adsorbs on Au in the thione form and on Ag in the thiol form. The monolayers are stable up to a temperature of 473 K, much higher than most of the self-assembled monolayers (SAMs) studied so far. The adsorption geometry does not undergo any significant change when the monolayers are heated, although minor changes are observed in the SER spectra of the Ag monolayer. Raman intensities of all the bands increase with heating, and the enhanced intensity is preserved even after cooling. This is attributed to monolayer-induced coalescence of gold islands leading to the formation of an extended self-assembly. XPS studies confirm chemisorption, although structural details are not manifested. Copyright 1999 Academic Press.

Electrochemical studies on gold electrodes covered with self-assembled monolayers (SAM) of aminoethane thiol (AET), mercaptobenzothiazole (MBT) and octadecyl mercaptan (ODM) have been carried out using cyclic voltammetry. Study of the influence of these monolayers on the double layer capacity of the interfaces involving the Au/SAM electrodes and of the electron transfer kinetics of chosen redox reaction probes, viz., Fe(CN) 6 4−/3− , Fe2+/3+, hydroquinone/quinone(H2 Q/Q) and Cu underpotential deposition, offers a wealth of information that can throw light on the role of SAMs in allowing/moderating/blocking the electron transfer at such interfaces. These details are presented and discussed in the paper.

Electrochemical reactions occur at electrode/electrolyte interfaces. Hence, manipulation and design of electrochemical interfaces accompanied by surface modifications have assumed vital importance. Molecular level modification, either at the monolayer or multilayer level of electrode surfaces and leading to functionalization of electrodes, is being actively pursued by researchers. Modification based on the self-assembled monolayer approach has enabled electrodes to acquire molecular recognition and molecular electronic characteristics. Functionalization of electrode surfaces using polymeric materials and enzymes has facilitated electrodes in exhibiting properties like catalysis, molecular recognition, electrochromism and birefringence. The results of such molecular level functionalization studies of electrode surfaces carried out recently in our laboratories are presented in this overview. Besides, some representative results reported from elsewhere are also included.

Simultaneous underpotential deposition of lead and thallium from a tartrate medium on a polycrystalline silver electrode is reported. Results of cyclic voltammetric investigations of the effects of concentration and sweep rate on the codeposition behaviour are discussed. Further investigations of the codeposition behaviour through voltammetric experiments using programmed potential inputs are described and discussed in terms of formation of a two-dimensional alloy. Finally, the validity of the existing models of electrochemical phase formation has been investigated under the codeposition conditions.

This work describes an electrochemical method for the determination of the nitrate and nitrite reductase activities of Rhizobium japonicum. The advantage of the method lies in the use of whole cells for the analysis and we earlier developed this protocol for the assay of NO. The results obtained are comparable to the spectrophotometric Griess assay. As the method is based on electrochemical reduction, the commonly interfering biological components like ascorbic acid, uric acid, dopamine, etc., will not interfere with the analysis. This method can be extended to the fabrication of biosensors for nitrate and nitrite using the same principle.

The oxidation behaviour of methanol on platinum at room temperature in a low concentration of phosphoric acid (≈ 5%) has been studied. Voltammetric behaviour in phosphoric acid medium is compared with that in perchloric acid. The effect of a bismuth underpotential deposition (upd) layer on the oxidation of methanol is also reported.

In this work, gold-incorporated polyethylenedioxythiophene nanocomposite material has been synthesized chemically, employing reverse emulsion polymerization method. Infrared and Raman spectroscopic studies revealed that the polymerization of ethylenedioxythiophene leads to the formation of polymer polyethylenedioxythiophene incorporating gold nanoparticles. Scanning electron microscope studies showed the formation of polymer nanorods of 50–100 nm diameter and the X-ray diffraction analysis clearly indicates the presence of gold nanoparticles of 50 nm in size.

The present work describes, for the first time, in situ electrochemical preparation of dendrimer-encapsulated Cu nanoparticles using a self-assembled monolayer of fourth-generation amine-terminated polyamidoamine (PAMAM) dendrimer as the template. Atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) studies of the modified surface confirmed the presence of Cu nanoparticles entrapped in dendrimer film. Au electrode modified with a monolayer of the dendrimer enables preconcentration and subsequent voltammetric detection of Cu(2+) at picomolar concentrations. Further, Cu nanoparticles in the dendrimer monolayer could be electrochemically derivatised to Cu hexacyanoferrate, which exhibits specific crystal planes, unlike the random distribution of crystal planes in bulk-formed Cu hexacyanoferrate, which is another catalytically active material for sensor applications.

The formation of self-assembled monolayers (SAM) of 5,5'-dithiobis (2-nitrobenzoic acid), DNBA on gold has enabled further derivatization of the electrode surface with functional moieties anchored to the surface bound molecules. A SAM of DNBA was formed on the Au surface. Nickel ions tethered to the SAM-covered Au surface, were subsequently derivatized electrochemically to yield nickel hydroxide overlayers, thereby showing the possibility of preparing ultra-thin films of metal oxides through solution chemistry. The nickel hydroxide surface coverage obtained on bare and SAM-covered electrodes was estimated from voltammetric peaks and it varied from one monolayer to about 300 monolayers. The formation of a monolayer of nickel hydroxide has been achieved for the first time by electrochemical modification. Further, the modified electrodes were subjected to SERS and XPS studies to understand their surface characteristics. Modified electrodes provide a catalytic pathway involving nickel hydroxide for the electro-oxidation of glucose in alkaline solutions.

Results on the simulated η-t transient response of an ‘actively’ corroding system under accelerated Tafel plot (ATP) conditions have revealed the influence of input parameters (Δi, τ) and system parameters (C dl,i corr andb) and explained the observed dependence of kinetic parameters (arrived at on the basis of the intercept-slope method) on τ in certain time domains. New improved methods, which eliminate such dependence and give uniform corrosion rate data over all time domains, are described in the paper.