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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.

Indium tin oxide (ITO) glass substrates are widely used in electrochemical devices and as substrates for in situ spectro-electrochemical experiments owing to their interesting optical, electrical and electrochemical properties. In this work, we found ITO surfaces to undergo substantial changes in their properties on potentiodynamic stresses in the acidic pH range. The surface changes occurring during cathodic cycling of ITO were characterized by redox couples in cyclic voltammetry, and the resultant surfaces examined by scanning electron microscopy (SEM), atomic force microscopy (AFM), optical transmittance measurements, X-ray diffraction (XRD) and the compositional changes by X-ray fluorescence spectroscopy (XRF). The studies of electrical, optical and structural properties of ITO during cathodic cycling in the acidic pH range clearly point to a set of conditions that help retaining the stability of ITO substrates intact for electrochemical applications.

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.

A poly(3,4-ethylenedioxythiophene) (PEDOT) modified glassy carbon electrode (GCE) was used to determine uric acid in the presence of ascorbic acid at physiological pH facilitating a peak potential separation of ascorbic acid and uric acid oxidation (ca. 365 mV), which is the largest value reported so far in the literature. Also, an analytical protocol involving differential pulse voltammetry has been developed using a microchip electrode for the determination of uric acid in the concentration range of 1 to 20 mM in presence of excess of ascorbic acid.

The effect of surfactant matrices on the voltammetry of an antimicrobial agent, triclosan (TCN) and its release into the aqueous continuous phase for voltammetric detection have been studied in this paper. As the surfactant concentration (wt%) was increased, the peak current was found to decrease in the order: Tween-80 > Tween-60 >>> sodium laurylsulphate. The results of these experiments are found to correlate with the published data obtained using microcalorimetry. Attempts have been made to overcome the trapping effect of surfactant matrices by designing a solution composition based on hydrotrope-driven disruption of micellar encapsulants for complete release of TCN into the analyte solution for voltammetric detection. The hydrotropes employed in this study are sodium p -toluenesulphonate and sodium benzenesulphonate. The experiments intended to show the effect of hydrotropes on the ‘pre-adsorbed’ triclosan-surfactant “pseudo-complex” proved that hydrotrope-driven disruption of micelles takes place even on the electrode surface. Thus, immersion of the surfactant-covered electrode into a hydrotrope solution can “strip” the antimicrobial agent off the surfactant. The results of these proof-of-concept experiments were applied to the analysis of low concentrations (0.1–0.8 mM) in product formulations containing TCN. A linear relationship was found between TCN concentration and current magnitude with a correlation coefficient of 0.97, the sensitivity of the measurement towards the TCN oxidation being 12.8 μA mM −1 .

Detection of dopamine (DA) in the presence of excess of ascorbic acid (AA) has been demonstrated using a conducting polymer matrix, poly (3,4-ethylenedioxythiophene) (PEDOT) film in neutral buffer (PBS 7.4) solution. The PEDOT film was deposited on a glassy carbon electrode by electropolymerization of EDOT from acetonitrile solution. Atomic force microscopy studies revealed that the electrodeposited film was found to be approximately 100 nm thick with a roughness factor of 2.6 nm. Voltammetric studies have shown catalytic oxidation of DA and AA on PEDOT modified electrode and can afford a peak potential separation of ∼0.2 V. It is speculated that the cationic PEDOT film interacts with the negatively charged ascorbate anion through favorable electrostatic interaction, which results in pre-concentration at a less anodic value. The positively charged DA tends to interact with the hydrophobic regions of PEDOT film through hydrophobic–hydrophobic interaction thus resulting in favorable adsorption on the polymer matrix. Further enhancement in sensitivity to micro molar level oxidation current for DA/AA oxidation was achieved by square wave voltammetry (SWV) which can detect DA at its low concentration of 1 μM in the presence of 1000 times higher concentration of AA (1 mM). Thus the PEDOT modified electrode exhibited a stable and sensitive response to DA in the presence of AA interference.

Self-assembled monolayers (SAMs) of thiols with carboxylic acid terminal groups were formed on gold substrates. The electron transfer characteristics of redox species on the above SAM-modified electrodes were studied in acid and neutral media with the help of voltammetry under two different conditions: (1) solution phase electron transfer and (2) bridge mediated electron transfer. Two redox systems, viz., [Fe(CN)6]4-/3− and Ru[(NH3)6]2+/3+ were chosen for the solution phase study. Investigations of bridge mediated electron transfer were carried out by functionalising the SAM with redox moieties and then studying their redox behaviour. For this study, ferrocene carboxylic acid and 1,4-diamino anthraquinone were used and they were linked to carboxylic acid terminated thiols by covalent linkage. The voltammetric results with mercaptoundecanoic acid SAM demonstrate the difference in behaviour between solution phase and bridge mediated electron transfer processes.

Mixed self-assembled monolayer of thiols were formed on the electrode surface using aromatic and aliphatic thiols and the modified interface were used to demonstrate charge trapping behavior at the modified interface. The electrode surface was modified by two schemes. In Scheme 1, electrode surface modified with methylene blue (MB) shows charge trapping behavior when contacts the electrolyte containing potassium ferricyanide. In Scheme 2, a redox-active bilayer is constructed using hexamineruthenium (II) chloride and methylene blue. This redox-active bilayer assembly exhibits unidirectional flow of current.

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 this paper, the electrochemical current rectification phenomenon exhibited at an electrochemical interface constituted by a glassy carbon electrode covered with a bilayer of polymer films is discussed. The authors have shown that Methylene Blue (MB) redox species can be confined to a very thin insulating polymer film formed from orthophenylene diamine. The poly(opd) film exhibited excellent blocking properties to redox molecules in solution. On the other hand, the insulating poly(opd) film trapped with MB could mediate electron transfer between the redox molecules in solution and the electrode. Further, a second polymeric layer (Nafion film) trapped with ferrocene redox species was formed as the outer layer over the inner poly (opd) film containing MB. This bilayer-modified electrode, due to the significant difference in the redox potentials of the MB and ferrocene species immobilized in the inner and outer layers, respectively, exhibits unidirectional current flow and the results of the voltammetric investigations on the modified electrodes are described in this communication.

In this work, we examine the electrocatalytic activity of electrodeposited Platinum (Pt)-Nickel (Ni) alloy layers on an inert substrate electrode for methanol oxidation reaction. Analyses using energy-dispersive fluorescent X-ray analysis and powder X-ray diffractometry confirm alloying of Pt with Ni in a range of compositions. Steady-state polarisation measurements in 0.5 M methanol+0.5 M H2SO4 solutions clearly show that the onset of electro-oxidation shifts to less anodic potential values (approximately 160 mV), while also exhibiting current enhancements up to ~15 times the currents obtained for the pure Pt electrodeposit. A linear relationship between the cyclic voltammetric peak (oxidation) current and [MeOH] is observed at a scan rate of 50 mVs−1, thus indicating reduced influence of adsorbed CO (COads) surface poison. A critical composition, Pt (92%)/Ni (8%) [denoted Pt-Ni(3) alloy] is found to exhibit maximum electrocatalytic activity, beyond which the activity drops, whereas pure Ni does not catalyse the reaction. While the promotion of electro-oxidation is understood to be largely due to the alloy catalyst, surface redox species of Ni oxide formed during the electro-oxidation process may also contribute to the oxygenation of COads, thereby enhancing the oxidation current. Plausible mechanisms of methanol oxidation on Pt/ transition metal alloy electrocatalysts are discussed in terms of electron transfer (in the alloy) and the role of Ni oxide species.

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.

A phenomenon analogous to current rectification occurring in a diode could be demonstrated at the electrode/electrolyte interfaces constructed using a mixed monolayer configuration formed by the self assembly of decane thiol and thioctic acid. Two types of configuration have been described. In the first type (Configuration-1), diode like electron transfer is shown to occur in a ferrocene monocarboxylic acid functionalized mixed monolayer, in contact with the electrolyte containing potassium ferrocyanide. In the second type (Configuration-2), the functionalized mixed monolayer consists of two redox species ferrocene dicarboxylic acid and 1,4-diaminoanthraquinone linked through covalent linkage. This bilayer represents an electrochemical analogue of a diode and current rectification is observed.

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

In this work, conducting polymer incorporated gold nanoparticles are synthesized through a sequence of chemical and electrochemical routes. These materials are characterized using TEM and FE-SEM techniques. The advantages of these films are demonstrated for sensing biologically important compounds such as dopamine and uric acid in presence of excess ascorbic acid. The PEDOT matrix is recognized to be responsible for the peak separation (selectivity) while also favouring catalytic oxidation of the above compounds and the nanometer-sized gold particles allow nanomolar sensing of DA and UA (sensitivity). Thus, it is possible to detect nanomolar levels of DA and UA in presence of excess of AA.

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.

Hexacyanoferrate modification of gold electrode has been achieved by immobilizing the species in a self-assembled monolayer of 3,3‘-thiodipropionic acid anchored to the gold surface. The surface coverage of hexacyanoferrate is estimated from the voltammetric data point to be about one monolayer, and the modified electrode exhibits excellent surface redox behavior. Further, it shows good electrocatalytic influence on the oxidation of hydrazine and ascorbic acid.

Modification of electrode surfaces with bilayers of organic/inorganic materials is shown to exhibit a variety of functional characteristics. The use of conducting polymers for bilayer construction has recently been explored by various groups. Gold surface has been modified with bilayers of polyaniline and polypyrrole, and the photoelectrochemical and electrochemical characteristics of such bilayer electrodes have been investigated and are reported in this communication.

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.

Simultaneous deposition of lead and thallium UPD s on polycrystalline silver has been investigated under cyclic voltammetric conditions. Voltammetric data, resulting from systematic variation of potential sweep rates and concentration of metal ions, are presented and discussed. The experimental results for composite lead and thallium UPD s, obtained from mixed as well as single metal UPD s, obtained from single salt solutions under comparable conditions, have been analyzed in terms of the empirical models proposed for the growth mechanism based on adsorption/nucleation phenomena.

The formation and characterization of self-assembled monolayers of organosulfur compounds like alkanethiols and dialkyl (di)sulfides on metal surfaces such as gold are areas of current research interest. The presence of an aromatic ring in a thiol molecule can enhance the binding between Au and the thiol, resulting in the formation of compact and impervious self-assembled monolayers. The formation of a monolayer of 2-mercaptobenzothiazole (MBT), containing an aromatic group with a fused thiazole ring but no long alkyl chain, is achieved on a gold electrode surface. Voltammetric investigations of ferro/ferricyanide and ferrous/ferric redox systems carried out on this Au|MBT electrode are reported. Further, the possibility of using such an Au|MBT electrode to distinguish between inner and outer sphere electron transfer reactions is indicated.

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.

Formation of self assembled monolayers on gold surface by thiols and disulphides is a well known phenomenon and extensive research work has been carried out in this area with envisaged applications in the area of sensors, molecular electronics, lithography, device fabrication using bottom-up approach, etc. Recently, it has been established that thiophene molecules can self assemble on gold surface due to Au-S interactions. 3-(3-thienyl) acrylic acid, a bifunctional ligand is used in this work to form self-assembled monolayers on planar gold surfaces (two dimensional assemblies) and to prepare monolayer protected gold nano clusters (three-dimensional assemblies). The electron transfer blocking properties of the two-dimensional monolayers were evaluated by using standard redox probes like ferrocyanide anions and Ruthenium hexamine cations. The functionalisation of the two-dimensional and three-dimensional assemblies has been carried out with ferrocene carboxylic acid and the functionalised monolayers were characterized by Cyclic voltammetry. The formation of thienyl acrylic acid protected nanoclusters has been verified by TEM and surface plasmon resonance absorption. It has been observed that when thiophene based ligands are used as stabilizers for the formation of metal nanoparticles, they tend to aggregate as a result of pi-pi interactions between adjacent thiophene ligands. In this case it is found that aggregation is prevented. The substituent at the thiophene ring hinders pi-pi interactions. The quantised nature of electrochemical charging of these nanoparticles has been demonstrated by differential pulse voltammetry (DPV), which exhibit peak like features (coulomb's staircase). This work also explores the possibility of using 3-(3-thienyl) acrylic acid as building blocks or spacers on planar and colloidal gold surfaces for potential applications in the field of sensors and devices.

In this work, conducting polymer impregnated gold nanoparticles are synthesized through a sequence of chemical and electrochemical routes. The nanocomposite film is characterized using UV-vis, FTIR spectroscopy, and SEM techniques to study the formation of oxidized PEDOT and Au0. The advantages of these films are demonstrated for sensing biologically important compounds such as dopamine and uric acid in presence of excess ascorbic acid, one of the major interferants in the detection of DA and UA (mimicking the physiological conditions), with superior selectivity and sensitivity when compared to the polymer film alone. Simultaneous determination is realized at 115 mV and 246 mV for DA and UA, respectively. The PEDOT matrix is recognized to be responsible for the peak separation (selectivity) while also favouring catalytic oxidation of the above compounds and the nanometer-sized gold particles allow nanomolar sensing of DA and UA (sensitivity). Thus, it is possible to detect nanomolar levels of DA and UA in presence of excess of AA. The combined effect of Au nanoparticles and the PEDOT matrix is rationalized that the Aunano surrounded by a "hydrophobic sheath (PEDOT)" tending to reside within these hydrophobic regions of PEDOT, thus favouring the selectivity and sensitivity of the DA/UA detection. This new generation of nanocomposites is expected to enhance the value of electroanalytical techniques, as it is possible to tune their properties suiting the analytical needs.

A gold electrode surface has been modified with a monolayer of 3,3'-thiodipropionic acid (TPA) by self-assembly. The pH-sensitive TPA monolayer is further derivatized to form a monolayer of nickel hexacyanoferrate. This electrode shows molecular discrimination between potassium and sodium ions and exhibits a near-Nernstian response, with a slope of 55 mV/decade to potassium ions in the range from 10 -2 to 1 M.

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.