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Spatial confinement of electrochemical reactions at solid/liquid interfaces is a mature area of research, and a central theme from cell biology to analytical chemistry. Monitoring or manipulating the kinetics of a charge transfer reaction in 2D is generally achieved using scanning electrochemical microscopy or multielectrode arrays, techniques that rely on moving physical probes or on a network of electrical connections. This tutorial is introducing concepts and instruments to confine faradaic electrochemical reactions in 2D without resorting to the mechanical movement of a probe, and with the simple design of one semiconducting electrode, one electrical lead and a single-channel potentiostat. We provide a theoretical background of semiconductor electrochemistry, and describe the use of localised visible light stimuli on photoconductor/liquid and semiconductor/liquid interfaces to address electrical conductivity - hence chemical reactivity - only at one specific site defined by the experimentalist. This enables shifting of the tenet of one electrode/one wire towards one wire/many electrodes. We discuss the applications of this emerging platform in the context of surface chemistry patterning, redox imaging, chemical and biological sensing, generating chemical gradients, electrocatalysis, nanotechnology and cell biology.

Two different interfaces prepared via UV-hydrosilylation of undecylenic acid and 1,8-nonadiyne on silicon(111) have been explored to develop a robust electrochemical DNA sensor. Electrodes modified with undecylenic acid were found to stably immobilise DNA but could not resist the growth of insulating oxides, whereas 1,8-nonadiyne modified electrodes satisfy both requirements.

Organic coatings on electrodes that limit biofouling by proteins but are of sufficiently low impedance to still allow Faradaic electrochemistry to proceed at the underlying electrode are described for the first time. These organic coatings formed using simple aryl diazonium salts present a zwitterionic surface and exhibit good electrochemical stability. The layers represent a low impedance alternative to the oligo (ethylene glycol) (OEG)-based anti-biofouling coatings and are expected to find applications in electrochemical biosensors and implantable electrodes. Two different zwitterionic layers grafted to glassy carbon surfaces are presented and compared to a number of better-known surfaces, including OEG-based phenyl-layer-grafted glassy carbon surfaces and OEG alkanethiol SAMs coated on gold, to allow the performance of these new layers to be compared to the body of work on other anti-biofouling surfaces. The results suggest that phenyl-based zwitterionic coatings are as effective as the OEG SAMs at resisting the nonspecific adsorption of bovine serum albumin and cytochrome c, as representative anionic and cationic proteins at physiological pH, whereas the impedance of the zwitterionic phenyl layers are two orders of magnitude lower than OEG SAMs.

The efficient transfer of electrons between enzymes and electrodes is important for understanding the intrinsic redox properties as well as for developing protein-based biosensors and bioelectronic devices. One strategy to achieve efficient electron transfer to proteins is to build up the electrode inside the protein so that it is close to the redox-active center of the protein. To achieve this requires exceedingly small electrodes. Carbon nanotubes, which are as small as 1 nm in diameter, have the potential to be such electrodes. This chapter outlines recent research toward this goal via the self-assembly of vertically aligned single-walled carbon nanotubes on electrode surfaces followed by the subsequent attachment of proteins to the free ends of the tubes.

Herein is explored a dual optical and electrical cell based biosensor that can provide insights into cellular events. The fabrication steps along with the electrical characterization of the microelectrodes is described. Initial experiments show that the ability of indium tin oxide (ITO) to detect biological cells at the electrode‐cell layer interface mainly depends on the size of sensing area. Following this, the impact of conductivity was also explored to assess the real time impedance signal during the adhesion of a non‐confluent layer of the cells on different substrates. Electrodes with a higher conductivity than ITO gave much higher senstivity of impedance changes which allowed the dynamics of subtle cellular morphology changes to be monitored at densities far lower than a confluent layer of cells. Finally, the capability of ITO and gold microelectrode as a functional readout for G protein couple receptor activation were determined. These set of analyses highlighted the challenges and opportunities of ITO substrate as a dual cell‐based assay for quantitative analysis of subtle changes in cell morphology. [ABSTRACT FROM AUTHOR]

Bimetallic silver‐copper electrocatalysts are promising materials for electrochemical CO2 reduction reaction (CO2RR) to fuels and multi‐carbon molecules. Here, we combine Ag core/porous Cu shell particles, which entrap reaction intermediates and thus facilitate the formation of C2+ products at low overpotentials, with gas diffusion electrodes (GDE). Mass transport plays a crucial role in the product selectivity in CO2RR. Conventional H‐cell configurations suffer from limited CO2 diffusion to the reaction zone, thus decreasing the rate of the CO2RR. In contrast, in the case of GDE‐based cells, the CO2RR takes place under enhanced mass transport conditions. Hence, investigation of the Ag core/porous Cu shell particles at the same potentials under different mass transport regimes reveals: (i) a variation of product distribution including C3 products, and (ii) a significant change in the local OH‐ activity under operation. [ABSTRACT FROM AUTHOR]

The direct quantification of programmed death-ligand 1 (PD-L1) as a biomarker for cancer diagnosis, prognosis and treatment efficacy is an unmet clinical need. Herein, we demonstrate the first report of rapid, ultrasensitive and selective electrochemical detection of PD-L1 directly in undiluted whole blood using modified gold-coated magnetic nanoparticles as "dispersible electrodes" with an ultralow detection limit of 15 attomolar and a response time of only 15 minutes. [ABSTRACT FROM AUTHOR]

The reaction of hydrogen‐terminated Si(100) with 1,8‐nonadiyne was previously shown to afford robust surfaces where oxidation of the substrate is not evident. Here, the experimental conditions required for reacting hydrogen‐terminated Si(100) with α,ω‐diynes of different lengths is explored via thermal hydrosilylation of 1,6‐heptadiyne, 1,8‐nonadiyne, 1,10‐undecadiyne and 1,15‐hexadecadiyne. X‐ray photoelectron spectroscopy indicated monolayers were successfully formed with oxide levels below the XPS detection limit for molecules with lengths varying from 7 to 11 carbons. It was observed that the apparent rate of electron transfer (ket) was affected not only by the SAM thickness, but also by hopping between ferrocene moieties where the presence of tiny oxidative defects act as electron transfer hotspots. Electron transfer on monolayer protected silicon: SAM thickness plays a major influence on electron transfer kinetics. However, on silicon, the process is also affected by the presence of SiOx hotspots and by the surface coverage of tethered redox species. The consequence is SAMs with a higher‐coverage of redox species on 9 or 11 carbons length displaying faster ket than SAMs with a lower‐coverage of redox species on 7 carbons length. [ABSTRACT FROM AUTHOR]

Gold-coated magnetic nanoparticles (Au@MNPs) have attracted significant interest in electrochemistry in recent years. This is especially the case with their application as dispersible electrodes where modified Au@MNPs are dispersed into a solution, selectively bind to the analyte of interest and are then brought to an electrode via application of a magnetic field for measurement. This paper characterizes four types of Au@MNPs with different sizes, shapes, and method of synthesis as dispersible electrodes. The Au@MNPs were characterized by transmission electron microscopy and X-ray photoelectron spectroscopy and scanning electron microscopy. In addition, the electrochemical behaviour of Au@MNPs was investigated using cyclic voltammetry. The four sorts of Au@MNPs were evaluated with regards to the three main features required in the dispersible electrodes approach, well-defined morphology, well-defined electrochemistry and fast response to a magnetic field. The Cubic-Au@MNPs, which presents the simplest synthetic route, showed the best electrochemical stability and performance, responding quickly to a magnet and had a well defined shape. [ABSTRACT FROM AUTHOR]

In this study we have modified indium tin oxide (ITO) by electrografting phenylphosphorylcholine (PPC) and evaluated its protein-resistance performance using electrochemical impedance spectroscopy. During electrografting, significant amounts of PPC gets physically adsorbed on the surface, which cannot be removed by simply rinsing with water or buffer, and as a consequence the electrochemical response of such electrode is quite unstable. We have developed a simple protocol to remove the physically adsorbed PPC such that a stable interface is produced. It was found that such a stable surface, with appropriate amount of PPC, can dramatically reduce nonspecific adsorption of protein, giving us a platform of performing both electrochemical and spectroscopic studies in biological fluids. [ABSTRACT FROM AUTHOR]

Biofouling on surfaces can deleteriously affect the function of electrochemical sensor used in a biological fluid. The demand is thus to develop a sensor operatable in complex matrices. Herein, we concisely review the technologies developed to date to minimize fouling of electrochemical interfaces by blood-based fluids with an emphasis on affinity biosensing. The methods toward this sensor fabrication that are discussed here include approaches where; chemically modified surfaces with antifouling layers such as ethylene oxides, zwitterionics and some alternative antifouling agents; physical forces such as fluidic systems; electrode surface topography, prevent biofouling, as well as recent approaches where biofouling does not interfere with the response (aptasensors). Furthermore, the important emerging field of implantable biosensors, originated from such fabrication procedures is discussed. [ABSTRACT FROM AUTHOR]

Multiple electrodes, combined with a chemometric strategy to calibrate the measurement response, have been used for the determination of an analyte across a broader dynamic range than is possible with a single electrode. The model system used for the detection of copper comprised electrodes modified with a self-assembled monolayer. The electrodes were modified with the copper-complexing species (3-mercaptopropionic acid, thioctic acid, and the peptides cysteine and Gly-Gly-His) and copper was determined over concentrations ranging from nanomolar to millimolar using voltammetric analysis. We have demonstrated that by combining the calibration functions from the four electrodes a better estimate (i.e. with smaller variance) of the concentration of the analyte is obtained. Measurement uncertainty is expressed for independently prepared electrodes, which allows the possibility of commercial production and factory calibration. The principles of using multiple electrodes modified with recognition elements with different affinities for the target analyte to extend the dynamic range of sensors is a general one that could be applied to other analytes. [ABSTRACT FROM AUTHOR]

The label-free amperometric detection of a rabbit IgG antigen by an anti-rabbit IgG antibody is achieved by observing the electrochemistry at a glassy carbon electrode modified with antibody entrapped in an electrodeposited polypyrrole membrane. In a flow injection apparatus the electrode is pulsed between -0.2 and +0.4 V versus Ag/AgCl. The pulsing of the electrode switches the polypyrrole membrane between the oxidised and reduced states. When antigen is injected into the flow stream a change in current is observed at the electrode despite the antigen or antibody being redox inactive at the potentials employed. It is proposed that this current is due to a change in the flux of ions into and out of the polypyrrole matrix during a pulse when the poly-anionic antigen is present. The immunoreaction was reversible because the 200 ms pulse at each potential was too short to allow secondary bonding forces (hydrogen bonding and hydrophobic forces) which are responsible for the strength of the antibody–antigen complex to be established. The consequence of the reversibility of the antigen–antibody binding is a low apparent affinity constant but an easily regenerated recognition interface. [Copyright &y& Elsevier]

Herein, we introduce the concept of utilizing conductive gold-coated magnetic nanoparticles as ‘dispersible electrodes’, which serve as the active element in the selective capture and direct electro-analytical quantification of analytes. This concept reduces response times and decreases detection limits by bringing the sensor to the analyte rather than the conventional paradigm of the analyte finding the sensor. [ABSTRACT FROM AUTHOR]

Abstract: The modification of gold electrodes with 4-carboxyphenyl diazonium salts to form stable layers for sensing applications is reported. Electrochemical reduction of 4-carboxyphenyl diazonium salts on gold electrodes yielded more stable layers than alkanethiol self-assembled monolayers in terms of extremes of electrode potential, sonication and with time. The application of the 4-carboxyphenyl modified electrodes for electrochemical sensing, which typically requires short chain alkanethiols on gold electrodes, is demonstrated via the covalent attachment of oligopeptides for the selective detection of Cu2+, Cd2+ and Pb2+. The diazonium salt/peptide modified gold electrodes not only had greater stability but also performed with lowest detected concentration to alkanethiol/peptide modified electrodes and with far greater sensitivity than the metal ion sensors when diazonium salt/peptide modified similar glassy carbon electrodes were employed. [Copyright &y& Elsevier]

Abstract: An electrochemical sensor for the detection of lead ions is described which is made by modifying a gold electrode substrate with self-assembled monolayers (SAMs) of 3-mercaptopropionic acid (MPA) or thioctic acid (TA) followed by covalent attachment of a lead binding peptide, human angiotensin I. Cyclic voltammetry of MPA–angiotensin modified gold electrodes complexed with lead displayed voltammograms with prominent lead peaks at E 0′, −0.29V. A detection limit of 1nM was achieved using Osteryoung square wave voltammetry. However, the electrodes were not stable over repeated electrochemical cycles due to partial electrochemical desorption of the SAM. The TA–angiotensin modified gold electrode showed greater stability and were able to be regenerated several times. Using Osteryoung square wave voltammetry for TA–angiotensin modified electrodes, lead concentrations down to 1.9nM were detected. Although the detection limit of the TA–angiotensin modified electrode is higher than achieved with MPA–angiotensin, it is still well below Australian drinking water guidelines. Studies of interference effects on the Pb2+ current showed Hg2+ as a significant interferent but only at levels significantly greater than those found in natural waters. [Copyright &y& Elsevier]

The photoreduction of graphene oxide (GO) with titanium dioxide (TiO 2 ) was exploited to fabricate a UV sensor for real time monitoring of sun exposure. The sensor was fabricated by simultaneous deposition of GO sheets and TiO 2 nanoparticles onto interdigitated electrodes using an AC electrophoresis deposition method. Changes in the resistance of the GO sheets decorated with TiO 2 nanoparticles during repeated cycles of exposure to UV were measured to understand the sensitivity of this sensor to UV radiation. Current-time traces revealed that the fabricated UV sensor retains a memory of each cycle of UV exposure; regardless of whether the sensor is exposed to UV for one long cycle or several short cycles. This memory of the extent of UV exposure is a biomimetic approach, analogous to the response of the skin to sun, and means the sensor requires no power except when the data is read from the sensor. [ABSTRACT FROM AUTHOR]

Electrochemical DNA biosensors composed of a redox marker modified nucleic acid probe tethered to a solid electrode is a common experimental construct for detecting DNA and RNA targets, proteins, inorganic ions, and even small molecules. This class of biosensors generally relies on the binding-induced conformational changes in the distance of the redox marker relative to the electrode surface such that the charge transfer is altered. The conventional design is to attach the redox species to the distal end of a surface-bound nucleic acid strand. Here we show the impact of the position of the redox marker, whether on the distal or proximal end of the DNA monolayer, on the DNA interface electrochemistry. Somewhat unexpectedly, greater currents were obtained when the redox molecules were located on the distal end of the surface-bound DNA monolayer, notionally furthest away from the electrode, compared with currents when the redox species were located on the proximal end, close to the electrode. Our results suggest that a limitation in ion accessibility is the reason why smaller currents were obtained for the redox markers located at the bottom of the DNA monolayer. This understanding shows that to allow the quantification of the amount of redox labeled target DNA strand that hybridizes to probe DNA immobilized on the electrode surface, the redox species must be on the distal end of the surface-bound duplex. [ABSTRACT FROM AUTHOR]

Light-activated electrochemistry is a powerful concept, where faradaic electrochemistry at a monolayer protected monolithic silicon electrode can be ‘turned on’ at any location with micron scale resolution simply by illuminating that region with light. The three requirements for light-activated electrochemistry are that i) the silicon must be protected from oxidation with a self-assembled monolayer (SAM), ii) the semiconducting electrode must be biased into depletion at the redox potential of the species of interest and iii) the redox couple must be covalently attached to the electrode. Herein it is shown that light-activated electrochemistry can be performed with solution species if they adsorb to the surface of the SAM. It is shown that redox species such as ferrocene and ruthenium hexamine adsorb onto the 1,8-nonadiyne modified silicon surface and undergo electron transfer upon illumination while potassium ferrocyanide does not. It is proposed this adsorption causes an increase in electronic coupling between the redox species and the silicon electrode such that appreciable electron transfer through the monolayer was achieved. Furthermore, it is demonstrated that light-activated electrochemistry can be used to rapidly switch between macroelectrode- and microelectrode-type voltammetry at the same interface by simple means of modulating the light intensity. This work expands our understanding of light-activated electrochemistry and its application for interrogating discrete regions on an electrode in a highly parallel way. [ABSTRACT FROM AUTHOR]

This work demonstrates the effect of electrostatic interactions on the electroactivity of a persistent organic free radical. This was achieved by chemisorption of molecules of 4-azido-2,2,6,6-tetramethyl-1-piperdinyloxy (4-azido-TEMPO) onto monolayer-modified Si(100) electrodes using a two-step chemical procedure to preserve the open-shell state and hence the electroactivity of the nitroxide radical. Kinetic and thermodynamic parameters for the surface electrochemical reaction are investigated experimentally and analyzed with the aid of electrochemical digital simulations and quantum-chemical calculations of a theoretical model of the tethered TEMPO system. Interactions between the electrolyte anions and the TEMPO grafted on highly doped, i.e., metallic, electrodes can be tuned to predictably manipulate the oxidizing power of surface nitroxide/oxoammonium redox couple, hence showing the practical importance of the electrostatics on the electrolyte side of the radical monolayer. Conversely, for monolayers prepared on the poorly doped electrodes, the electrostatic interactions between the tethered TEMPO units and the semiconductor-side, i.e., space-charge, become dominant and result in drastic kinetic changes to the electroactivity of the radical monolayer as well as electrochemical nonidealities that can be explained as an increase in the self-interaction "a" parameter that leads to the Frumkin isotherm. [ABSTRACT FROM AUTHOR]

Controlling the composition of an interface is very important in tuning the chemical and physical properties of a surface in many applications including biosensors, biomaterials, and chemical catalysis. Frequently, this requires one molecular component to a minor component in a mixed layer. Such subtle control of composition has been difficult to achieve using aryldiazonium salts. Herein, aryldiazonium salts of carboxyphenyl (CP) and phenylphosphorylcholine (PPC), generated in situ from their corresponding anilines, are electrografted to form molecular platform that are available for further functionalization. These two components are chosen because CP provides a convenient functionality for further coupling of biorecognition species while PPC offers resistance to nonspecific adsorption of proteins to the surface. Mixed layers of CP and PPC were prepared by grafting them either simultaneously or consecutively. The latter strategy allows an interface to be developed in a controlled way where one component is at levels of less than 1% of the total layer. [ABSTRACT FROM AUTHOR]

Herein we combined the use of self-assembled monolayer (SAM) modified electrodes and gold coated magnetic nanoparticles (Au@MNPs) to modulate faradaic electrochemistry when nanoparticles are absorbed on and removed from, a passivated electrode using an external magnetic field. Substantial faradaic electrochemistry of potassium ferricyanide in solution is first prevented by modifying gold electrodes with 11-mercaptoundecanoic acid to form a passivating SAM. Restoration of the faradaic electrochemistry is then achieved by introducing Au@MNPs which are brought to the surface using an external magnetic field. The faradaic electrochemistry can again be suppressed by removing Au@MNPs from the SAM using another external magnetic field. [ABSTRACT FROM AUTHOR]

The phenomenon of nanoparticles attached to an electrode passivated by an organic layer allowing efficient electron transfer between redox species in solution and the underlying electrode to be restored has resulted in Chazalviel and Ailongue proposing a theory [Chazalviel, J.-N.; Allongue, p. j. Am. Chem. Soc. 2011, 133, 762-764] to explain this phenomenon The theory suggests that with electrode.organic layer-nanoparticle constructs, high exchange current densities, compared with when the nanopartides are absent, results in the rate of electron transfer being independent of the thickness of the organic layer until a threshold thickness is exceeded. Thereafter, the thicker the organic layer, the slower the rate of electron transfer. Herein we provide the first experimental data to support this theory using a single experimental system that can show the transition from thickness independent electron transfer kinetics to distant dependent kinetics. This was achieved using ethylenediamine electrodeposited on a glassy carbon electrode. Different numbers of deposition cycles were applied in order to fabricate different thicknesses of the organic film. The deposited films showed progressively greater blocking abilities toward ruthenium hexamine, as a redox active probe in solution, as the films got thicker. Electron transfer kinetics of nanoparticle-decorated surfaces showed a change from thickness independent to thickness dependent as the organic layer exceeded an average thickness of 2.0 A. Electrochemical impedance spectroscopy, cyclic voltammetry, scanning electron microscopy, ellipsometry, and atomic force microscopy were used to characterize the fabricated surfaces. [ABSTRACT FROM AUTHOR]

Abstract: The gold-coated magnetic nanoparticles (Au@MNPs) have been proposed to act as “dispersible electrodes” in sensing applications with the dual advantages of reduced detection time and increased sensitivity. As a sensor the Au@MNPs would be modified with ligands to capture the analyte, with exposure of a magnetic field bringing the nanoparticles back to a macroelectrode for electrochemical detection of the amount of analyte. In this paper, the potential of Au@MNPs as dispersible electrodes is assessed via (1) characterization of how they assemble onto a macroelectrode upon application of a magnetic field, (2) their electron transfer behaviour to redox species in solution and redox species attached to the Au@MNPs and (3) how low a detection limit is possible. Visualization with atomic force (AFM), scanning electron (SEM) and light microscopy showed the particles formed a coral-like structure on the electrode. Electron transfer kinetics of the Au@MNPs-modified electrode was shown to be similar to a bare electrode by both cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The electrochemistry of the Os(bpy)2(AMP) modified Au@MNPs enabled an estimation of the smallest number of redox molecules per Au@MNP that could be detected. It was estimated that as little as 0.144±0.007 redox molecules per nanoparticle could be detected with the lowest actual detected number being 0.79±0.11 molecule per nanoparticle. [Copyright &y& Elsevier]

Abstract: Electrodes modified with passivating organic layers have been shown to, here and previously, to exhibit good Faradaic electrochemistry upon attachment of gold nanoparticles (AuNP). Due to their low background capacitances these constructs have good potential in electrochemical sensing. Herein is reported the application of these electrode constructs for impedance based immunosensing. The immunosensor was constructed by modifying a gold electrode with 4-thiophenol (4-TP) passivating layers by diazonium salt chemistry. Subsequently, the attachment of AuNP and then a biotin derivative as a model epitope to detect anti-biotin IgG were carried out. The interfacial properties of the modified electrodes were evaluated in the presence of Fe(CN)6 4−/3− redox couple as a probe by cyclic voltammetry and electrochemical impedance spectroscopy. The impedance change, due to the specific immuno-interaction at the immunosensor surface was utilized to detect anti-biotin IgG. The increase in charge-transfer resistance (R ct) was linearly proportional to the concentration of anti-biotin IgG in the range of 5–500ngmL−1, with a detection limit of 5ngmL−1. [Copyright &y& Elsevier]