New Electrochemical Methods - Analytical Chemistry (ACS Publications)

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REVIEW pubs.acs.org/ac

New Electrochemical Methods Christopher Batchelor-McAuley, Edmund J. F. Dickinson, Neil V. Rees, Kathryn E. Toghill, and Richard G. Compton* Department of Chemistry, Physical & Theoretical Chemistry Laboratory, Oxford University, South Parks Road, Oxford, OX1 3QZ, United Kingdom

’ CONTENTS Electrochemical Scanning Probe Microscopy Atomic Force Microscopy (AFM) Scanning Electrochemical Microscopy (SECM) Combined Techniques: AFM-SECM Scanning Ion Conductance Microscopy (SICM) Combined with SECM Scanning Kelvin Probe (SKP) Combined with SECM New Theoretical Methods Weakly Supported Voltammetry Mechanistic Analysis via Weakly Supported Voltammetry Influence of Convection of Mass Transport Theories of Electron and Proton Transfer Nanoelectrochemistry and Double Layer Structure Development of New Electrochemical Pulse Procedures Stochastic Methods in Voltammetry Stochastic Experiments in Confined Volumes Nanofluidic Experiments Stochastic Theory in Electrochemistry Nanoparticle Electrochemistry Bioelectrochemistry Nanopores and Single Molecule Detection Single-Cell Electrochemistry Outlook Author Information Biographies References

liquids,4
10 glucose sensing,11
13 screen-printed electrodes,14 graphene,15
18 wired enzyme electrodes,19 and bismuth-based electrodes,20 among many others. The demand for ever more selective and sensitive procedures is large but if work is to fulfill its potential then an increased understanding of the fundamentals influencing the observed electrochemical responses is imperative. To this end, the first section of this review focuses on the recent developments in electrochemical scanning probe microscopy. Such procedures give the experimentalist detailed information regarding the topography and reactivity of the electrodes surfaces being used. Beyond surface chemistry, another highly influential area is the mass transport of a species to the electrochemical interface. Experimentally, the use of high levels of supporting electrolyte is in certain cases prohibitive; for example, the introduction of large quantities of added salt is often inappropriate for work with biological compounds. Subsequently, the application of electrochemical methods to systems containing low concentrations of supporting electrolyte is challenging. Furthermore, if we are to gain quantitative information from such experiments, we require the use of advanced theoretical models. The second section of this article focuses on new theoretical methods, specifically in regards to mass transport and the theory of heterogeneous electron transfer. Away from classical electrochemical studies involving the measurement of bulk properties from large ensembles, there is an increasing impetus toward single-molecule techniques. The third section of this review aims to give an overview of the progress in studying diffusional single-molecule electrochemical systems. Because of the small numbers of molecules involved, the use of continuum theories for the modeling of such systems is invalid; consequently this review looks at both newly developed experimental and theoretical methods. Apart from single-molecule work, stochastic analysis may also play a significant role in nanoparticle experiments. The increasing use of nanoparticles within research but also for industry means there is a significant demand for quick and effective methods by which nanoparticles may be characterized. The fourth section of this article provides a brief overview on the newly developed method of nanoparticle detection from electrode collisions. The final section of this review looks toward the field of bioelectrochemistry, in which we highlight the significant

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his review is predominantly focused upon articles dating from 2009 to 2011. The authors have attempted to highlight areas of recent and significant development, with the aim of providing the reader with a clear and concise view of the current state-of-the-art in areas ranging from fundamental electrochemistry to applied methods for biological analysis. Specific areas of interest include recent developments in surface analysis techniques, single-molecule and nanoparticle stochastic analysis, and nanopore technology. The use of electrochemistry for analytical procedures is vast, with a number of reviews providing an in-depth look at specific areas, including DNA voltammetric analysis,1
3 ionic r 2011 American Chemical Society

Special Issue: Fundamental and Applied Reviews in Analytical Chemistry Published: October 21, 2011 669

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developments being made in both the fields of nanopore technology and the electrochemical analysis of single cells. Nanopore techniques have advanced majorly in the last 2 years, such that in 2009 the first example of an electrochemical system able to sequence short lengths of DNA was demonstrated. Large obstacles still must be overcome before such methods become commercially viable, with current work looking toward improving both stability and reproducibility. Figure 1. EC-AFM cell for the MFP-3D AMF, for dynamic electrochemical experiments and simultaneous AFM. The arrangement shows a boron doped diamond (BDD) working electrode interchangeable stub (central), graphite rod counter electrode (left) and leakless Ag/AgCl reference electrode (left,bleow). Reprinted with permission from ref 6, with thanks to Asylum Research, Santa Barbara, CA.

’ ELECTROCHEMICAL SCANNING PROBE MICROSCOPY Electrode modification, whether intentional or not, is a fundamental aspect of electrochemistry, as nanoscale and microscale changes to electrode surfaces have direct consequences on the electroactivity of the interface under investigation. As interest into nanomaterials continues to increase, improving the observation of electrochemical processes at these scales is an ever present challenge. One of the main methods of characterizing electrodes at the nanoscale and in situ is to use scanning probe microscopy (SPM) techniques. Electrochemical SPM methods are not necessarily recent advancements in terms of new electrochemical methods, as investigation via in situ SPM and electrochemical modification of single crystal surfaces21
23 was underway almost as soon as atomic force microscopy (AFM) and scanning tunnelling microscopy (STM) were invented in 1986.24 However, the development of electrochemical AFM cells (ECAFM) and STM cells (EC-STM) has made slow progress, particularly with respect to commercially viable products with high adaptability to a range of applications. The following will discuss very recent developments in potential-controlled and electrochemical SPM. Atomic Force Microscopy (AFM). Since the turn of the century, various AFM companies have introduced an electrochemical AFM cell into their range of microscope accessories, each with a varying degree of ability. Electrochemical AFM is not a new concept, yet with constant improvement in instrumentation, AFM capability, and understanding of EC-AFM requirements, modern day EC-AFM cells are providing a whole new dynamic to microscopic characterization of electrochemical processes and surface modification. In a recent publication by Valtiner et al.,25 the authors report an improved electrochemical AFM cell designed to address the main issues faced when performing EC-AFM using commercial AFMs. In their report the authors discuss in detail the aspects of EC-AFM cell design that are necessary to achieve dynamic, potential-controlled modification of the working electrode substrate without invalidating the results observed. Early 2011 saw the release of a new EC-AFM cell (Figure 1) from Asylum Research Ltd.,26 a modified liquid cell with underlying circuitry that allowed for dynamic electrochemistry to proceed at the central working electrode with respect to interchangeable reference and counter electrodes. The cell was designed with a view to satisfying the needs of a wide range of users, and correspondingly it features a highly diverse working electrode platform, allowing for irregular pieces of electrode material to be imaged as well as controlled deposition investigations at boron doped diamond and glassy carbon electrode stubs of specific geometry (3 mm or 5 mm discs). During its design and development, the cell was applied to a number of investigations of metal deposition, and recent publications have used the cell to demonstrate the nucleation and growth of bismuth27,28 in real

time and at intervals, nickel29 and very recently antimony.30 Almost all these systems were then applied to laboratory based analytical investigations, with the same electrode, demonstrating the ease of transfer from lab-based experiments to microscopic characterization in situ and under potential control. AFM imaging in liquid is a complicated process, not only in terms of cell arrangement and well sealed and interchangeable electrodes, but also in terms of image acquisition. AFM is known to routinely achieve atomic resolution images in high vacuum conditions31 and even in air can achieve very high-resolution nanoscale images. In situ AFM is well known to be less well resolved, however, owing to inhibited cantilever resonance and a vast increase in noise. Recently a new EC-AFM technique has been developed which involves independent control of the AFM tip and working electrode substrate, with an advantage in high spatial resolution at low loading forces. The method is frequency modulated and is therefore named EC-FM-AFM.32 Frequency modulation AFM (FM-AFM) measures tip
sample interaction very sensitively by detecting changes in the resonant frequency of the oscillating cantilever. Tip
sample interactions affect the cantilever resonance, and therefore by analyzing these changes, the nature of the substrate can be ascertained and further control of the cantilever and AFM unit can be achieved allowing for atomic resolution images in air and even in liquid.33 In liquid, the motion of the cantilever is greatly reduced, and as such it is essential to have a clear understanding of the cantilever dynamics in the medium, and its influence on the force gradients experienced by the cantilever. In recent publications by Umeda and Fukui,32,34 the authors describe a new electrochemical FM-AFM made in-house that allowed for the study of electroactive ferrocenylundecanthiol (FcC11H22SH) islands embedded in a self-assembled monolayer (SAM) of n-decanthiol32 and a shorter matrix of n-heptanthiol34 on a Au(111) substrate. The modified substrate was studied as a model system to observe a high-resolution change in the structure of the molecular islands depending on the oxidation state. Under potential control and in situ, the technique allowed for the highly sensitive determination of a 0.44 nm height increase in the ferrocene based islands from the SAM surface on oxidation of the terminal ferrocene groups.32 Not only was the topography of the substrate accurately ascertained at very high resolution, but the frequency modulation measurement of energy dissipation, associated with maintaining constant vibrational amplitude at the cantilever, distinctively changed on oxidation of the Fc groups. Furthermore the ECFM-AFM was able to provide microscopic information regarding the electrical double layer at the electrode surface due to the 670

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is even finding application in local corrosion studies, observing nanoscale degradation of protective coatings applied to metals.58 SECM is conventionally a very small scale scanning probe process, limited by the use of an ultramicroelectrode (UME) to observe macroscale surface activity and topography. SECM can be highly affected by the nature of the substrate or surface under investigation, as the steady-state current observed at the tip during the scan is dependent on the distance of the probe from the surface. Furthermore, the tip is often very fragile at dimensions of just a few hundreds of nanometers and is fabricated of glass; therefore probe
substrate crashes are a regular occurrence in SECM analysis. Consequently surfaces which are curved, tilted, or corrugated are difficult to image without observing artifacts owing to variation in the steady-state current or crashing the delicate tip.45 To avoid regular tip crashes, etc., it is necessary to scan small areas very slowly, so as to allow the piezo to responsed to height variation and in good time. The time frame for SECM is therefore very long, with an area of just a few square millimeters taking over 10 h to fully scan, leading to the possibility of substrate fouling and aging, solvent evaporation, and the possibility of irreversible chemical reaction in solution. In recent work by Cort
es-Salazar and co-workers,45,59 a new SECM probe that allows for the analysis large sample areas as well as complicated and highly variable topography has been reported. The novel approach to SECM operates in contact mode, unlike most SECM which travels just above the surface to avoid breaking the fragile tip. By operating in contact mode the distance, d, between the substrate and tip remains constant and therefore free from artifacts. To achieve this, the researchers have microfabricated a microelectrode probe from polyethylene terephthalate (PET). A microchannel is etched into the soft polymeric material and filled with a conductive carbon ink. The microchannel is sealed by further coating with a polymer and then cut to expose a cross-section of the conductive ink before mechanical polishing. The resulting soft stylus SECM probe is flexible and durable and, in addition to achieving constant d by operating in contact mode, also overcomes the problem of probe
sample crashes.59 SECM is well established for small surface areas of micrometers, but there is a desire to greatly increase the feasible working area to centimeter squared surfaces. As mentioned previously, conventional SECM may take over 10 h to scan areas larger than a few square millimeters,37 but further advances in the Girault group are set to vastly speed up SECM processes.60 Using the soft stylus technique, the researchers have developed tips with an array of eight individual channels, such that multiple scans may be achieved in a single traverse of the surface. Consequently the SECM can scan a surface in minutes rather than hours, allowing a high throughput of samples as well as imaging centimeter squared surfaces in relatively short times.60 Furthermore, the durability of the array probes is such that they can be reused as regeneration of the electrode surface is readily achieved by simply cutting the exposed surface. A number of research groups have been investigating a means to analyze the dry surface or highly localized electrochemical experiments without immersing possibly sensitive samples in solution.60
65 Using the same microfabrication technology described above, the Girault group have presented a fountain pen probe,61 consisting of the soft stylus ink based probe developed earlier,59 but coupled with a microfluidic channel to introduce a mediator containing electrolyte and a second reference/counter electrode positioned opposite the working electrode carbon

assessment of energy dissipation curves. As such it was possible to postulate that the apparent height increase of the islands was due to the formation of an ionic double layer in which the perchlorate ions are tightly bound to the oxidized ferrocene groups.32 Scanning Electrochemical Microscopy (SECM). SECM is a powerful technique that can be employed in the study of a wide range of electrochemical processes. Since its introduction in the late 1980s,35 SECM has been applied to the study of biophysical systems, heterogeneous processes, biological processes, surface reactivity, local corrosion, charge transfer mechanisms, liquid/ liquid interfaces, dissolution processes, and adsorption and desorption. The scope of systems to which the technique may be applied to gain new insight into fundamental processes seems ever increasing. A number of reviews36
45 already exist regarding the achievements of SECM over the past 30 years, yet the focus herein is on developments in the field in the past 3 years. In brief, SECM uses an ultramicroelectrode (UME) probe to scan the surface of interest, and by use of a bipotentiostat, the potential of the probe and the electrode surface are altered relative to one another to induce an electrochemical reaction. As such the chemical reactivity and/or topography of the electrode are determined. Over the past 3 years, a number of new electrochemical approaches have emerged that utilize SECM either in a diagnostic manner or indeed to fabricate very specific electrode surfaces. Surface interrogation SECM46 (SI-SECM) emerged in 2008 and is actively being applied to novel systems such as the in situ quantification of adsorbed hydrogen on platinum electrode surfaces,47 the reaction of bromine with adsorbed carbon monoxide on a platinum surface,48 and kinetic analysis, as the rate of reaction of four different redox mediators with electrogenerated platinum oxides at OCP was explored.47 SI-SECM allows for the quantification of adsorbed species on the surface of an electrode by completely consuming the adsorbed material using a tip generated reactive species. What is unique to this method is that the electrode surface adsorbates formed at a given potential may be explored under open circuit conditions as well as the use of the same electrochemical setup for reactive species generation and detection.46 Specifically the SI-SECM method allows the quantifiable determination of reactive adsorbed intermediates formed on application of a potential to the substrate electrode, independent of their electrochemical reactivity at the substrate and spectroscopic characteristics.46 SECM is now being applied to understanding biofuel cells, exploring catalytic processes occurring at enzyme modified substrates, and mapping their catalytic activity. A recently introduced mode of SECM,49 namely, redox competition SECM (RC-SECM), is being applied in particular, due to the high resolution and accuracy of the method. In this mode of SECM, the tip competes with the sample for the same analyte, such that the current at the tip decreases when the tip is in close proximity to an active area of the substrate. Correspondingly a variation with catalytic activity across the surface is possible, to the extent that the catalytic active site can be resolved. Recent work using this novel approach has allowed researchers to image the activity of the enzyme laccase,50,51 modified carbon nanotubes,51
53 the catalytic activity of Prussian blue toward hydrogen peroxide,54 as well as metalloporphyrins55 and platinum
silver bimetallic nanoparticles56 and their catalytic effect on the oxygen reduction reaction. The platinum
silver nanoparticles were also studied using RC-SECM toward the electrolysis of hydrochloric acid.57 It 671

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track in a parallel microchannel. The probe is therefore able to operate in a feedback mode over a dry surface, providing microliter volumes of mediator solution as it traverses the substrate, delivering low currents over insulating areas, and high, diffusion controlled currents over conductive areas.45,61 The group have further developed this concept and very recently reported a push
pull setup for the microfluidic SECM system.64 In this arrangement the SECM probe consists of a carbon ink working electrode channel, a combined counter and reference electrode, and two other microchannels, one (push) containing a redox mediator spiked electrolyte and the second (pull) is empty but ready to draw up the fluid after analysis. Consequently a constant flow of redox mediator containing electrolyte is passed over the probe tip, allowing SECM to be carried out at dry samples or in microenvironments. In a similar area, Ebejer et al.63 have recently reported a new technique named scanning electrochemical cell microscopy (SECCM) in which a tapered glass theta pipet cell is modified with two nanochannels (