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Proton transfer at solid/liquid interfaces is a fundamental step in many complex biological and electrocatalytic processes. Previous model studies using electrodes modified with self-assembled monolayers (SAMs) of carboxylic acid-terminated alkanethiols have demonstrated that interfacial proton transfer is controlled by the local electrochemical microenvironment. The thermodynamic driving force for electrochemically driven protonation/deprotonation of acid/base SAMs is governed by a combination of the electric potential at the SAM/solvent interface, the pKaof the acid group, and the solution pH. Here, we develop a kinetic model to describe electric potential-driven protonation/deprotonation as a two-step process. This comprises a reversible proton transfer step at the SAM/electrolyte interface (i.e., (de)protonation) and a proton transport step describing the motion of protons as they traverse the diffuse electrical double layer to and from the solution bulk. The kinetics of the transport step are investigated using finite element simulations, providing numerical estimates for the transport rate constants under combined diffusional and migrational transport modes. Using the dependence of these rate constants on the electric potential at the SAM/electrolyte interface, we define situations where the overall rate expression is limited by either (de)protonation, proton transport, or a combination of both. From this analysis, we determine a lower limit for the acid group pKaof ˜3, above which proton transfer at the plane of acid dissociation is generally the rate-determining step. The electric potential-driven proton transfer/transport kinetic model developed herein provides a general approach to treat electric potential-driven coupled ion transfer and transport phenomena, with potential applications including proton-coupled electron transfer processes, ion intercalation in alkali metal batteries, and ion transport across biological membranes.

This work demonstrates the use of an sp2-bonded carbon microspot boron doped diamond (BDD) electrode for voltammetric measurement of both pH and analyte concentration in a pH-dependent speciation process. In particular, the electrode was employed for the voltammetric detection of pH and hypochlorite (OCl–) in unbuffered, aerated solutions over the pH range 4–10. Knowledge of both pH and [OCl–] is essential for determination of free chlorine concentration. The whole surface of the microspot BDD electrode was found active toward the voltammetric oxidation of OCl–, with OCl–showing a characteristic response at +1.5 V vs SCE. In contrast, it was only the surface integrated quinones (Q) in sp2-bonded carbon regions of the BDD electrode that were responsible for the voltammetric pH signal. A Nernstian response for pH (gradient = 63 ± 1 mV pH–1) was determined from proton coupled electron transfer at the BDD-Q electrode, over the potential range −0.4–0.5 V vs SCE. By measuring both OCl–and pH voltammetrically, over the pH range 4–10, the OCl–oxidative current was found to correlate extremely well with the predicted pH-dependent [OCl–] speciation profile.

The α-[S2W18O62]4–/5–and α-[S2W18O62]5–/6–polyoxometalate (POM) reduction processes at boron-doped diamond (BDD) disk electrodes in acetonitrile (0.50 M [n-Bu4N][PF6]) have been investigated using large-amplitude Fourier transformed alternating current (FTAC) voltammetry. The origins of subtle differences in experimental data and simulated data modeled assuming the Butler–Volmer relationship with ion-paring implicitly included, planar diffusion and a uniform surface are considered. Parameters estimated are the apparent heterogeneous electron-transfer kinetics (kapp0′), apparent formal reversible potential (Eapp0′), and apparent charge transfer coefficient (αapp′). The electrode kinetic parameters, in contrast to theoretical predictions of the model employed, are dependent on frequency, POM concentration, and the data analysis method. Reasons for nonconformance to the model include the adsorption of POM on graphite-like sp2-bonded carbon impurities, limitations in the availability of charge carriers in BDD, and the method of incorporating ion pairing when slow electrode kinetics apply. Masking of the sp2carbon-rich (edge) region of the BDD disk provided FTAC voltammetric data that complied much more closely with the simulated data. Nevertheless, data analysis still produces a concentration dependence in the estimated kapp0′values, which is considered in terms of the assumption of an infinite number of charge carriers. Conclusions derived from this study are likely to be generally applicable to electrode kinetic investigations at BDD electrodes.

Square wave voltammetry (SWV) is commonly used in electroanalytical applications to enhance analyte faradaic signals and minimize nonfaradaic processes. However, little attention is given as to how best use SWV to minimize faradaic interference signals that arise from redox species present in solution that have redox potentials that convolute with that of the analyte. In conventional SWV, a series of current–time (i–t) transients are collected, and iis averaged over a specified window of each transient (potentiostat dependent). This average iis reported against the electrode potential, E. As the i–tresponse is governed by the type of electron transfer reaction under investigation, we show how by collecting all i–tdata and through judicious choice of the current averaging window, it is possible to enhance the analyte response while at the same time reducing the interferent signal. We look at three different electron transfer reactions, fast electron transfer outer sphere, metal electrodeposition/stripping, and surface-confined proton-coupled electron transfer (PCET) and demonstrate different i–tbehaviors in SWV, visually aided by the use of 3D i–t–Eplots. In the case of PCET quinone-based voltammetric sensing of pH in the presence of a heavy metal (here Cu2+), we show that the use of a much earlier current averaging window (2–10% of the i–tresponse) results in the pH signal being clearly distinguished from that of the overlapping heavy metal.

The electrodeposition mechanism of gold nanoparticles (NPs) on pristine single walled carbon nanotubes (SWNTs) at high driving forces has been elucidated using the microcapillary electrochemical method. Here, a small capillary (internal diameter ∼50−100 μm) filled with a gold plating solution, and positioned so that the capillary meniscus makes contact with a two-dimensional SWNT random network, was used to record current−time transients. Nucleation and growth transients were observed in which the current increased with time to a maximum value beyond which the current decreased (planar diffusion regime). With increased driving force, the current maximum shifted dramatically to increasingly shorter times. Atomic force microscopy (AFM) analysis indicated that this was not due to significant differences in NP growth rates, but rather to increased densities of NPs formed at more cathodic potentials. Detailed microscopic analysis showed that the size of the NPs initially increased with deposition time and the particle surface coverage was constant. However, at the highest driving forces the NP density decreased with deposition time and AFM revealed the presence of both larger and smaller particles at long times. This was attributed to electrochemically induced Ostwald ripening, whereby larger particles grow at the expense of smaller ones. As NP nucleation and growth on SWNT two dimensional network electrodes is highly directional and enforced in particular locations, it is inappropriate to analyze electrochemical data using conventional models. There is thus a need to complement chronoamperometric measurements with high resolution microscopy to fully interpret nucleation on complex electrode surfaces.

The electrochemical properties of boron-doped diamond (BDD) electrodes are strongly influenced by the boron doping level and the presence of sp2carbon impurities. In this study, the impact of highly localized sp2carbon concentrated at the edge of a BDD electrode, arising from laser cutting during fabrication and exposed during electrode polishing, on the resulting overall electrode kinetics is identified. Fourier transformed large-amplitude alternating current (FTAC) voltammetric data for the usually ideal Fc0/+(Fc = ferrocene) process in the highly viscous ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate show relatively poor agreement with simulations based on a uniformly active electrode surface using the Butler–Volmer formalism for the electrode kinetics. In this ionic liquid medium, the impact of heterogeneity on the macroscopic electrode activity is enhanced under the conditions of slow mass transport, where sites of disparate activities are spatially decoupled on the voltammetric time scale. Physically blocking this edge region leads to a response that is much more consistent with a uniform electrode and substantially improves the agreement between the FTAC voltammetric experiment and simulation (standard heterogeneous electron-transfer rate constant, k0= 0.0015 cm s–1). To complement the macroscopic measurements, local voltammetric measurements with an electrochemical droplet cell show directly that the sp2carbon found in the edge region is able to support much faster electron-transfer kinetics for the Fc0/+process than the sp3BDD surface. Overall, this study demonstrates that caution should be taken in reporting electrode kinetic data obtained at BDD or any other heterogeneous electrode materials. Macroscopic electrode kinetic characterization in traditional electrochemical media such as acetonitrile is blind to such heterogeneities in the activity. However, tuning the diffusional time scale through the use of FTAC voltammetry in viscous ionic liquids provides a powerful approach for detecting the spatially nonuniform electrode activity.

Boron doped diamond (BDD), given the robustness of the material, is becoming an electrode of choice for applications which require long-term electrochemical monitoring of analytes in aqueous environments. However, despite the extensive work in this area, there are no studies which directly assess the biofilm formation (biofouling) capabilities of the material, which is an essential consideration because biofouling often causes deterioration in the sensor performance. Pseudomonas aeruginosais one of the most prevalent bacterial pathogens linked to water-related diseases, with a strong capacity for forming biofilms on surfaces that are exposed to aquatic environments. In this study, we comparatively evaluate the biofouling capabilities of oxygen-terminated (O-)BDD against materials commonly employed as either the packaging or sensing element in water quality sensors, with an aim to identify factors which control biofilm formation on BDD. We assess the monospecies biofilm formation of P. aeruginosain two different growth media, Luria–Bertani, a high nutrient source and drinking water, a low nutrient source, at two different temperatures (20 and 37 °C). Multispecies biofilm formation is also investigated. The performance of O-BDD, when tested against all other materials, promotes the lowest extent of P. aeruginosamonospecies biofilm formation, even with corrections made for total surface area (roughness). Importantly, O-BDD shows the lowest water contact angle of all materials tested, that is, greatest hydrophilicity, strongly suggesting that for these bacterial species, the factors controlling the hydrophilicity of the surface are important in reducing bacterial adhesion. This was further proven by keeping the surface topography fixed and changing surface termination to hydrogen (H-), to produce a strongly hydrophobic surface. A noticeable increase in biofilm formation was found. Doping with boron also results in changes in hydrophobicity/hydrophilicity compared to the undoped counterpart, which in turn affects the bacterial growth. For practical electrochemical sensing applications in aquatic environments, this study highlights the extremely beneficial effects of employing smooth, O-terminated (hydrophilic) BDD electrodes.

Square wave voltammetry (SWV) is most commonly used to enhance electroanalytical current signals of a redox analyte of interest. The SWV is typically recorded in a potential region where both non-faradaic and faradaic currents are collected; however, only data in the faradaic region are analyzed. In this article, we show how by collecting the full current–time (i–t) data, arising from the SWV potential pulse sequence, and analyzing in the region of the potential scan where non-faradaic currents arise, further analytical information can be collected, over short time periods (typically seconds). Importantly, we show how information on solution resistance, R(and solution conductivity), and double layer capacitance, C, can be extracted from this data, without the need to conduct further electrochemical experiments, such as electrochemical impedance spectroscopy or independent solution conductivity measurements. Knowledge of the latter is important for measurements in real world samples, where the electrolyte concentration is unknown. We show how SWV measurements of electrode capacitance with repeat SWV scans can also be used to determine whether a changing faradaic SWV signal is due to electrode fouling or changing analyte concentration. Finally, knowledge of both parameters (RC) is essential for optimization of the SWV parameters, such that the faradaic SWV signal is truly free from non-faradaic contributions.

While quinone proton coupled electron transfer (PCET) under buffered conditions is well understood, the situation is more complicated in unbuffered aqueous solutions. With a view to producing a quinone-based voltammetric pH electrode that can function universally in both buffered and unbuffered solutions by following a two-electron (2e–)/two-proton (2H+) Nernstian pathway over a wide pH range, the voltammetric response of strongly electronically coupled surface-bound quinones, directly integrated into a boron-doped diamond (BDD) electrode, is investigated. A laser ablation process enables integration of quinones into the BDD electrode surface with a high pKa1(first protonation state) and with controllable, very low surface coverages (as low as 2 orders of magnitude below monolayer coverage). Under buffered conditions, one wave results for all pH values, and the 2e–/2H+pathway is followed across the entire pH range. The measured ET rate constant values, from Laviron analysis, are also high, indicative of fast ET pathways. Under unbuffered conditions, one wave is again observed for all pH values; however, deviations from the buffered 2e–/2H+behavior are seen in the neutral region (pH 6–8). While 2e–/2H+transfer is maintained at all times, we attribute the observed deviation to local pH changes caused by the consumption and generation of protons at the electrode surface during the redox electrochemistry of the quinone. The associated proton fluxes generated at such sparse surface coverages are thought to be sufficiently high enough to prevent ET from occurring exclusively via a proton-independent route. By reducing surface coverage (down to ∼4 × 10–12mol cm–2; the limit of our laser ablation process) local pH changes can be reduced but are not eradicated completely. By moving to a pulsed voltammetric technique, where for each potential step protons consumed at the electrode are immediately replaced, it is possible, provided the surface coverage is low enough, to obtain a Nernstian 2e–/2H+response across a wide pH range in unbuffered solution.

In electrodeposition the key challenge is to obtain better control over nanostructure morphology. Currently, a lack of understanding exists concerning the initial stages of nucleation and growth, which ultimately impact the physicochemical properties of the resulting entities. Using identical location scanning transmission electron microscopy (STEM), with boron-doped diamond (BDD) serving as both an electron-transparent TEM substrate and electrode, we follow this process, from the formation of an individual metal atom through to a crystalline metal nanoparticle, under potential pulsed conditions. In doing so, we reveal the importance of electrochemically driven atom transport, atom cluster formation, cluster progression to a nanoparticle, and the mechanism by which neighboring particles interact during growth. Such information will help formulate improved nucleation and growth models and promote wider uptake of electrodeposited structures in a wide range of societally important applications. This type of measurement is possible in the TEM because the BDD possesses inherent stability, has an extremely high thermal conductivity, is electron beam transparent, is free from contamination, and is robust enough for multiple deposition and imaging cycles. Moreover, the platform can be operated under conditions such that we have confidence that the dynamic atom events we image are truly due to electrochemically driven deposition and no other factors, such as electron-beam-induced movement.

The production of crystalline lead oxide (PbO) structures, directly on the surface of an electrode in (nitrate) solution, via electrochemical deposition of lead ions (Pb2+), is unequivocally demonstrated and the formation mechanism elucidated. Boron doped diamond electrodes are used as the deposition platform. We show the effect of electrode potential, deposition time, presence of oxygen, and temperature on the formation process. At room temperature, under both deoxygenated and aerated conditions, high-resolution microscopy reveals a predominant nanoparticle (NP) morphology. In contrast, under laser-heated conditions, both NPs and half-hexagon shaped “plates” result. Transmission electron microscopy reveals these “plates” to be crystalline β-PbO. Plate prominence, under heated conditions, increases as the driving potential and deposition time is increased. By deoxygenating the solution and applying a deposition potential such that hydroxide ion (OH–) formation is negligible, only NPs are observed, which, from cyclic voltammetry data, are confirmed to be elemental Pb. We thus propose that Pb NPs and OH–play a crucial role in the PbO formation process. Electrodeposited Pb NPs catalyze OH–generation from either oxygen or nitrate reduction (oxygen reduction occurs at a less negative applied potential than nitrate reduction) driving the formation of lead hydroxide (Pb(OH)2) via a precipitation route. The Pb(OH)2subsequently dehydrates to PbO, a process significantly accelerated by temperature. Hence, by controlling temperature, potential, and solution conditions, cathodic electrodeposition of Pb2+can lead to the preferential formation of PbO crystalline structures on the electrode surface.

The use of two different electrodeposition approaches to form nickel hydroxide, Ni(OH)2, nanoparticles (NPs) of different crystallographic orientations on single-walled carbon nanotubes is demonstrated via: (i) the electrochemical generation of OH–(∼mM), in the presence of Ni2+, resulting in the formation of disordered α-phase Ni(OH)2NPs by precipitation (direct approach); (ii) the electrodeposition of Ni NPs that are converted to Ni(OH)2through potential cycling in alkaline media to form the more thermodynamically favorable, ordered β-phase Ni(OH)2NPs (indirect approach). NPs produced by the direct approach exhibit remarkable electrocatalytic activity toward both methanol and ethanol oxidation, with excellent specific activities (SAs) of ∼2.8 kA g–1for 0.5 M methanol and ∼3.7 kA g–1for 0.5 M ethanol. In contrast, NPs produced by the indirect approach show SA values about an order of magnitude lower. This study demonstrates the capability of electrochemistry for the tailored synthesis of Ni(OH)2nanostructures for electrocatalytic applications and a powerful, but simple, combinatorial approach for quick activity screening.

The ability to make electrochemical measurements under controlled fluid flow has many applications in electrochemical analysis, especially at trace levels. With a move to rapid, on-line, small volume measurements and a need to improve measurement performance, there is a requirement to manufacture more complex fluidic systems with controlled internal 3D geometries at low cost. Both precision machining and lithography typically fail to meet these needs. In this article we describe the role of additive manufacturing (AM) for the fabrication of (micro)fluidic electrochemical flow systems. We show how AM has advanced device manufacture in these areas compared to the more traditional approaches. Different AM methods are also discussed with respect to their capabilities and advantages. A selection of current AM electrochemical flow cell applications are highlighted and future perspectives discussed.

Heterogeneous electron transfer rate constants (k0values) at a boron-doped diamond (BDD) electrode (10–2cm s–1range) that are more than an order of magnitude smaller than found at metal or glassy carbon (GC) electrodes (≥2.0 cm s–1) are reported for the outer-sphere [Ru(NH3)6]3+/2+, [α-SiW12O40]4–/5–, and [α-SiW12O40]5–/6–redox couples in aqueous electrolyte media. The k0values and other parameters relevant to electrode kinetics were derived from analysis of the higher-order harmonic components available in Fourier transformed large amplitude alternating current voltammetry at BDD, GC, and platinum (Pt) or gold (Au) electrodes for the reduction of highly positively charged [Ru(NH3)6]3+and highly negatively charged Keggin-type silicon tungstate ([α-SiW12O40]4–and [α-SiW12O40]5–). The slower electron transfer kinetics at the BDD electrode, relative to the other electrode materials, are discussed in terms of the double layer, density of states, and other factors.

High resolution electrochemical imaging methods provide opportunities to study localized phenomena on electrode surfaces. Here, we review recent advances in scanning electrochemical microscopy (SECM) to study materials involved in (electrocatalytic) energy-related applications. In particular, we discuss SECM as a powerful screening technique and also advances in novel techniques based on micro- and nanopipets, such as the scanning micropipet contact method and scanning electrochemical cell microscopy and their use in energy-related research.

Individual nanowires (NWs) and native single-walled carbon nanotubes (SWNTs) can be readily used as well-defined nanoscale electrodes (NSEs) for voltammetric analysis. Here, the simple photolithography-free fabrication of submillimeter long Au, Pt, and Pd NWs, with sub-100 nm heights, by templated electrodeposition onto ultralong flow-aligned SWNTs is demonstrated. Both individual Au NWs and SWNTs are employed as NSEs for electron-transfer (ET) kinetic quantification, using cyclic voltammetry (CV), in conjunction with a microcapillary-based electrochemical method. A small capillary with internal diameter in the range 30–70 μm, filled with solution containing a redox-active mediator (FcTMA+((trimethylammonium)methylferrocene), Fe(CN)64–, or hydrazine) is positioned above the NSE, so that the solution meniscus completes an electrochemical cell. A 3D finite-element model, faithfully reproducing the experimental geometry, is used to both analyze the experimental CVs and derive the rate of heterogeneous ET, using Butler–Volmer kinetics. For a 70 nm height Au NW, intrinsic rate constants, k0, up to ca. 1 cm s–1can be resolved. Using the same experimental configuration the electrochemistry of individual SWNTs can also be accessed. For FcTMA+/2+electrolysis the simulated ET kinetic parameters yield very fast ET kinetics (k0> 2 ± 1 cm s–1). Some deviation between the experimental voltammetry and the idealized model is noted, suggesting that double-layer effects may influence ET at the nanoscale.

Double take: Double‐barrel carbon nanoprobes with integrated distance control for simultaneous nanoscale electrochemical and ion conductance microscopy can be fabricated with a wide range of probe sizes in less than two minutes. The nanoprobes allow simultaneous noncontact topographical (left image) and electrochemical imaging (right) of living neurons, as well as localized K+delivery and simultaneous neurotransmitter detection.

We demonstrate, for the first time, the electrosynthesis of uniformly dispersed nickel hydroxide nanoparticles (NPs) on polycrystalline boron-doped diamond (pBDD). This has been achieved by electrogenerating OH−at the pBDD surface in the presence of Ni2+to create local conditions near the electrode where highly supersaturated (relative saturation ratio > 105) nickel hydroxide solutions are generated for short periods of time (approximately seconds). This results in the deposition of nickel hydroxide NPs directly on the electrode surface, as confirmed by X-ray photoelectron spectroscopy. The NPs have a reasonably homogeneous size distribution and are deposited uniformly across the heterogeneous pBDD surface. We show that by simply increasing the electrogeneration time and, hence, increasing both the local concentration of OH−and extent of the precipitation reaction, it is possible to increase the size of the NPs. For example, after 1 s, NPs with dimensions of 12 ± 3 nm form, whereas after 15 s, NPs of size ∼39 ± 9 nm result. Longer times result in larger particles, which form aggregated structures. The effect of nickel hydroxide NP size on electrocatalytic activity was investigated by measuring the steady-state current for the oxidation of glucose in alkaline media. For NPs ≥ 25 nm in size, glucose oxidation is close to diffusion-controlled. However, for the smallest NPs produced (∼12 nm) the currents passed suggest kinetic limitations. For glucose at an effective surface coverage of nickel hydroxide of ∼20 nmol cm−2, equivalent to 15 ng of nickel hydroxide, this functionalized electrode showed a sensitivity of 330 μA mM−1cm−2and a limit of detection of 400 nM. The latter represents one of the lowest limits of detection for glucose for nickel hydroxide-based electrodes. The electrocatalytic oxidation properties of this electrode toward methanol and ethanol was also found to be very efficient, yielding very high density currents of ∼1010 A g−1for 0.5 M ethanol and 990 A g−1for 0.47 M methanol.

An important open question on the electrochemistry of single-walled carbon nanotube (SWNT) electrodes concerns the sites at which electron transfer (ET) occurs. This issue is addressed herein for the case of a simple outer sphere redox couple, (ferrocenymethyl)trimethylammonium (FcTMA+). Using relatively sparse networks (<1% surface coverage) of electrically connected SWNTs, coupled to a scanning electrochemical microscopy (SECM) substrate generation−tip collection setup, we show that high rates of mass transport can be generated to SWNTs, allowing kinetic effects to be observed in the voltammetric waveshape. By developing a numerical model that faithfully represents all aspects of the experimental geometry, highly accurate kinetic data can be obtained. Assuming that all SWNTs are active, a minimum average ET rate constant k0SWNT> 1.0 ± 0.6 cm s−1is assigned, which is of similar size to other electrode materials and suggests that the sidewall of SWNTs has considerable ET activity.

It has been well documented that single-crystal stable-temperature (ST)-cut quartz substrates can horizontally align single-walled carbon nanotubes (SWNTs) along the xdirection ([100]) during catalyzed chemical vapor deposition. It has been suggested that alignment is due to either surface-guided growth along step edges and/or lattice directions, although recent atomic force microscopy (AFM) evidence suggests the latter is more appropriate [Liu, et al. J. Am. Chem. Soc.2008, 130, 5428]. In light of this we explore the growth mechanism on single-crystal quartz in more detail, focusing particularly on the crystallography of the alignment surface and the effects of annealing treatments. X-ray diffraction (XRD) studies enable us to identify, for the first time, the crystallographic plane of ST-cut quartz as (011̅1), with a surface normal lying close to [121], which is 42.5° from the yaxis ([010]). For this plane, formulation of both crystallographic orientation and polar diagrams, the latter which reveals the magnitude of the longitudinal piezoelectric effect, indicate a high degree of correlation between the preferential SWNT growth direction and both the atomic arrangement and piezoelectric effect, strongly supporting a crystal plane driven alignment process. This is further supported by the absence of steps on the surface, observed topographically using AFM. Further evidence for crystal plane controlled alignment comes from field emission scanning electron microscopy (FE-SEM) and AFM observations of bending of the SWNTs away from the preferred alignment direction at defined angles. These angles are in agreement with the Si−O−Si atomic bond angles in quartz. Additionally, surfaces which are shown by XRD to have either minimal distortion in the lattice or surface misorientation caused by, e.g., tilts, cracks, etc., are those that are found to align the SWNTs best. Improvement in surface crystallinity can be achieved by annealing in air at high temperature; optimal annealing conditions of 950 °C in air for 30 mins were determined, in agreement with XRD data and FE-SEM images.

In this paper we demonstrate that the nucleation density of single-walled carbon nanotubes (SWNTs), formed by thermal catalytic chemical vapor deposition, strongly depends on the grain size of Al underlayers covered with a native oxide (Al/Al2O3). By varying the substrate temperature during Al sputter deposition it was possible to investigate the effect of Al grain size on growth without inducing changes in the underlayer thickness, surface chemistry, or any other growth parameter. The resulting SWNT growth structures ranged from low-density 2D nanotube networks that lay across the surface of the substrate to high density 3D nucleation which gave rise to vertical “forest” growth. The height of the SWNT “forest” was observed to increase with increasing Al deposition temperature as follows, 200 > 100 > 60 > 20 °C on Si/Al but in the order 100 > 200 > 60 > 20 °C on SiO2/Al substrates for fixed growth conditions. The differences in the SWNT growth trends on Si and SiO2substrates are believed to be due to the existence of an optimal Al/Al2O3underlayer grain size for the formation of active catalytic nanoparticles, with larger Al/Al2O3grains forming on SiO2than Si at a fixed substrate temperature. Numerous surface analysis techniques including AFM, XPS, FESEM, TEM, and Raman spectroscopy have been employed to ascertain that the observed changes in nanotube growth for this system are related primarily to changes in underlayer morphology.

Rotating ring disk electrodes (RRDEs) are a powerful and versatile tool for mechanistically investigating electrochemical reactions at electrode surfaces, particularly in the area of electroanalysis and catalysis. Despite their importance, only limited electrode materials (typically glassy carbon, platinum, and gold) and combinations thereof are available commercially. In this work, we present a method employing three-dimensional (3D) printing in conjunction with machined brass components to produce housing, which can accommodate any electrode material in, e.g., pressed powdered pellet, wafer, rod, foil, or vapor deposited onto a conductive substrate form. In this way, the range and usability of RRDEs is extended. This custom do-it-yourself (DIY) approach to fabricating RRDEs also enables RRDEs to be produced at a significant fraction of the cost of commercial RRDEs. To illustrate the versatility of our approach, coplanar boron-doped diamond (BDD) RRDEs are fabricated for the first time using the approach described. Experimental collection efficiencies for the redox couple FcTMA+/FcTMA2+are found to be very close to those predicted theoretically. BDD electrodes serve as an ideal electrocatalyst support due to their low background currents, wide solvent potential window in aqueous solution, and chemical and electrochemical stability in acid and alkali solutions. The BDD RRDE configuration is employed to investigate the importance of surface-incorporated nondiamond carbon in BDD on hydrogen peroxide generation viathe oxygen reduction reaction in acid solutions.

The voltammetric response of electrodes coated with a redox-active monolayer is computed by finite element simulations based on a generalized model that couples the Butler–Volmer, Nernst–Planck, and Poisson equations. This model represents the most complete treatment of the voltammetric response of a redox film to date and is made accessible to the experimentalist via the use of finite element modeling and a COMSOL-generated report. The model yields a full description of the electric potential and charge distributions across the monolayer and bulk solution, including the potential distribution associated with ohmic resistance. In this way, it is possible to properly account for electrostatic effects at the molecular film/electrolyte interface, which are present due to the changing charge states of the redox head groups as they undergo electron transfer, under both equilibrium and nonequilibrium conditions. Specifically, our numerical simulations significantly extend previous theoretical predictions by including the effects of finite electron-transfer rates (k0) and electrolyte conductivity. Distortion of the voltammetric wave due to ohmic potential drop is shown to be a function of electrolyte concentration and scan rate, in agreement with experimental observations. The commonly used Laviron analysis for the determination of k0fails to account for ohmic drop effects, which may be non-negligible at high scan rates. This model provides a more accurate alternative for k0determination at all scan rates. The electric potential and charge distributions across an electrochemically inactive monolayer and electrolyte solution are also simulated as a function of applied potential and are found to agree with the Gouy-Chapman-Stern theory.

Electrochemical ozone production (EOP) from water is an attractive, green technology for disinfection. Boron doped diamond (BDD) electrodes, grown by chemical vapor deposition (CVD), have been widely adopted for EOP due to their wide anodic window in water and excellent chemical and electrochemical stability. High pressure high temperature (HPHT) synthesis, an alternative growth technique used predominantly for the high-volume synthesis of nitrogen doped diamond microparticles, has been seldom employed for the production of conductive BDD electrodes. In this paper, we demonstrate, for the first time, the use of BDD electrodes fabricated from HPHT conductive BDD microparticles for EOP. The BDD microparticles are first compacted to produce freestanding solid electrodes and then laser micromachined to produce a perforated electrode. The compacted HPHT BDD microparticle electrodes are shown to exhibit high EOP, producing 2.23 ± 0.07 mg L−1of ozone per ampere of current, at consistent levels for a continuous 20 h period with no drop off in performance. The HPHT electrodes also achieve a reasonable current efficiency of 23%, at a current density of 770 mA cm−2.

Laser scanning confocal microscopy (LSCM) coupled to a constant volume flow-pressure measuring system is introduced as a new technique for the quantitative measurement of fluid flow across porous materials. Such processes are ubiquitous from the life sciences to materials science and the methodology herein could find widespread application. The methodology has been applied to the detection of fluid flow through human dentine, in-vitro, and in the assessment of occlusion actives. Dentine is a calcareous material sandwiched between the pulp and enamel in the tooth structure that contains tubules which traverse dentine in the pulp to enamel direction. The tubules become patent during enamel erosion or gum recession, leading to dentinal hypersensitivity. Understanding the nature of fluid flow is important, as a pressure gradient exists across dentine in-vivoand this has implications for the development of suitable treatments. The methodology described herein firstly allows a ready assessment of the general efficacy of treatments viahydraulic permeability measurements. Second, LSCM images allow the nature of the flow process and the mode of action of the treatments to be revealed at high spatial resolution. For the particular case of dentine, we demonstrate how the method allows candidate treatments to be compared and assessed.

This review highlights numerous and wide ranging biophysical and biochemical applications of scanning electrochemical microscopy (SECM). SECM instrumentation and theoretical modelling, necessary for experimental interpretation, are outlined, followed by a detailed discussion of the diverse applications of this technique. These include the measurement of flow through membranes, the determination of kinetic parameters of reactions, the investigation of the permeability of small molecules in tissues and monitoring biological processes, such as the production of oxygen or nitric oxide by cells. The significant impact of micro-electrochemical techniques on our understanding of basic physicochemical processes at biologically relevant interfaces is also considered. Studies reviewed include transport across and within bilayers and monolayers. Recent advances in SECM include the combination of SECM with other techniques, such as atomic force microscopy and optical microscopy. These developments are highlighted, along with prospects for the future.

This paper describes in detail the use of electron beam lithography (EBL) to successfully batch microfabricate combined scanning electrochemical-atomic force microscopy (SECM-AFM) probes. At present, the process produces sixty probes at a time, on a 1/4 of a three-inch wafer. Using EBL, gold triangular-shaped electrodes can be defined at the tip apex, with plasma enhanced chemical vapor deposited silicon nitride serving as an effective insulating layer, at a thickness of 75 nm. The key features of the fabrication technique and the critical steps are discussed. The capability of these probes for SECM-AFM imaging in both tapping and constant distance mode is illustrated with dual topographical–electrochemical scans over an array of closely-spaced 1 μm diameter Pt disc electrodes, held at a suitable potential to generate an electroactive species at a transport-limited rate. As highlighted herein, understanding diffusion to heterogeneous electrode surfaces, including array electrodes, is currently topical and we present preliminary data highlighting the use of SECM-AFM as a valuable tool for the investigation of diffusion and reactivity at high spatial resolution.

Ultrathin Langmuir–Schaeffer (LS) films were fabricated from blends of regioregular poly(3-hexylthiophene) (P3HT) and highly monodispersed dodecanethiolate-capped gold nanoparticles (Au NPs) mixed in varying weight ratios. The morphology of the ultrathin films was investigated by UV-visible absorption spectroscopy, atomic force microscopy (AFM) and field-emission scanning electron microscopy (FE-SEM). The results of the structural investigations were correlated with the lateral conductivity of the films, with P3HT in its unintentionally doped state, probed by scanning electrochemical microscopy (SECM), which proved to be a very sensitive technique. Control over the P3HT/Au NP ratio led to remarkable changes in the morphology and lateral conductivity of the films. Inclusion of Au NPs into P3HT was found to influence the ordering of P3HT, which ultimately determined the macroscopic charge transport characteristics of the films. Composite films with ca. 33% by weight of Au NPs were found to be the most ordered and exhibited the highest conductivity, substantially higher than P3HT alone. To provide insight into the film formation process, LS composite films comprising equal quantities of P3HT and Au NPs (by weight) were transferred at several surface pressures and investigated by SECM, AFM and FE-SEM.

Scanning electrochemical microscopy (SECM) is shown to be a powerful technique for both the measurement of local solution velocities through human dentine slices, in vitro, and for assessing quantitatively the effect of surface treatments on the flow process. SECM employs a small ultramicroelectrode (micron dimensions) as an imaging probe to provide information on the topography and transport characteristics of dentine, with high spatial resolution. In these studies the dentine sample is a membrane in a two compartment cell, which contains solutions of identical composition, including a redox active mediator (Fe(CN)$\rm{ {_{6}^{4-}})}$. In the absence of an applied pressure, the transport‐limited current response at the probe electrode is due to diffusion of Fe(CN)$\rm{ {_{6}^{4-}}}$to the UME, which depends on the probe to sample separation. Under an applied hydrostatic pressure, hydrodynamic flow across the sample enhances mass transport to the UME. With this methodology it was possible to accurately measure effective fluid velocities, by recording tip currents with and without pressure, and assess the efficacy of potential flow retarding agents for the treatment of dentinal hypersensitivity. For native dentine, the solution velocity was found to vary dramatically with location on the sample. The application of a glycerol monooleate ‐ base paste treatment to the surface of dentine was found to lower local flow velocities significantly. This electroanalytical methodology is simple to implement and is generally applicable to assessing the efficacy and mode of action of a wide variety of potential fluid flow retarding agents.

A new method has been developed for measuring local adsorption rates of metal ions at interfaces based on scanning electrochemical microscopy SECM. The technique is illustrated with the example of Agbinding at Langmuir phospholipid monolayers formed at the waterair interface. Specifically, an inverted 25 μm diameter silver disc ultramicroelectrode UME was positioned in the subphase of a Langmuir trough, close to a dipalmitoyl phosphatidic acid DPPA monolayer, and used to generate AgviaAg electro-oxidation. The method involved measuring the transient current–time response at the UME when the electrode was switched to a potential to electrogenerate Ag. Since the AgAg couple is reversible, the response is highly sensitive to local mass transfer of Agaway from the electrode, which, in turn, is governed by the interaction of Agwith the monolayer. The methodology has been used to determine the influence of surface pressure on the adsorption of Agions at a phospholipid dipalmitoyl phosphatidic acid Langmuir monolayer. It is shown that the capacity for metal ion adsorption at the monolayer increased as the density of surface adsorption sites increased by increasing the surface pressure. A model for mass transport and adsorption in this geometry has been developed to explain and characterise the adsorption process.

Thin composite organic – inorganic membranes are formed at electrode surfaces via a layer-by-layer deposition process employing TiO2 nanoparticles and phytic acid as molecular binder. The mesoporous membranes form uniformly across platinum and glass surfaces and are shown via conducting atomic force microscopy (C-AFM) to behave in dry state as electrically insulating films. Voltammetry at platinum electrodes of 1.5 mm, 100 μm, and 10 μm diameter is used to survey the effect of the composite membrane on electrochemical processes. The electrode diameter as well as concentration and nature of electrolyte have a considerable effect on the electrode process. For the one electron reduction of Ru(NH3)$\rm{ {_{6}^{3+}}}$ in aqueous media adsorption effects, mobility effects, and accumulation or ‘memory’ effects are observed. Similarly, protons or cationic molecules such as dopamine can be accumulated and ‘stored’ in the membrane and then transferred to a clean electrolyte for analysis.

This paper describes the use of confocal laser scanning microscopy CLSM to observe and characterise the fully hydrated glycocalyx of human umbilical vein endothelial cells HUVECs. Viable HUVECs in primary culture were studied at room temperature in HEPES-buffered, phenol red- and serum-free CS-C cell culture medium. A fluorescein isothiocyanate-linked wheat germ agglutinin WGA-FITC 2 μg ml−1, 30 min was used to detect N-acetylneuraminic sialic acid, which is a significant component of the endothelial glycocalyx. Single confocal sections, less than 1.3 μm thick, were collected at intervals of 0.5 μm, scanning through the entire z-axis of a series of cells. Cell-surface associated staining was observed, which enabled the glycocalyx thickness to be deduced as 2.5 ± 0.5 μm. This dimension is significantly greater than that measured by electron microscopy, for glutaraldehyde-fixed cells 0.10 ± 0.04 μm. The specificity of WGA-FITC staining was demonstrated by treatments with several enzymes, known to degrade glycocalyx heparatinase, chondroitinase, hyaluronidase and neuraminidase, of which neuraminidase 1 U ml−1, 30–60 min was the most effective, removing up to 78 ± 2 of WGA-FITC binding to HUVECs. Cell viability was assessed simultaneously with ethidium homodimer-1 staining and confirmed by standard colorimetric 3-4,5dimethylthiazol-2,5diphenyltetrazolium bromide MTT test. CLSM thus provides a useful approach for in situvisualisation and characterisation of the endothelial glycocalyx in viable preparations, revealing a thickness that is an order of magnitude greater than found in ex situmeasurements on fixed cells.

Thin composite organic – inorganic membranes are formed at electrode surfaces via a layer‐by‐layer deposition process employing TiO2nanoparticles and phytic acid as molecular binder. The mesoporous membranes form uniformly across platinum and glass surfaces and are shown via conducting atomic force microscopy (C‐AFM) to behave in dry state as electrically insulating films. Voltammetry at platinum electrodes of 1.5 mm, 100 μm, and 10 μm diameter is used to survey the effect of the composite membrane on electrochemical processes. The electrode diameter as well as concentration and nature of electrolyte have a considerable effect on the electrode process. For the one electron reduction of Ru(NH3)$\rm{ {_{6}^{3+}}}$in aqueous media adsorption effects, mobility effects, and accumulation or ‘memory’ effects are observed. Similarly, protons or cationic molecules such as dopamine can be accumulated and ‘stored’ in the membrane and then transferred to a clean electrolyte for analysis.

A novel form of hydrodynamic modulation voltammetry (HMV) is described, based on the periodic variation of mass transport in a microjet electrode (MJE) system, in combination with phase-sensitive detection techniques. In the configuration developed, a jet of solution is fired from a nozzle that is aligned directly over the surface of a dual disk Pt-Pt ultramicroelectrode (UME). The potential at each electrode is controlled separately. A rotating blade, positioned between the nozzle and the UME probe, is used to periodically interrupt flow to the electrode surface, resulting in modulation of the overall mass transfer rate between two defined extremes. The use of a dual disk UME enables two transport-limited current signals to be recorded simultaneously, one for the analyte of interest, and the other for a ‘reference species’ (oxygen for the studies described herein). The latter current response corresponds to the variation in mass transport rate in the chopped flow (CF) arrangement and is used as the signal for phase sensitive detection of the analyte current. Studies of potassium hexachloroiridate (III) [IrCl$\rm{ {_{6}^{3-}}}$] oxidation in aqueous solution are used to demonstrate the capabilities of the technique. HMV in the CF-MJE arrangement allows quantitative concentration measurements, down to at least 5×10⁻⁷ M.

Pt microwire atomic force microscope (AFM) probes have been employed to characterize the electrical and topographical properties of a anode. The anode consisted of supported Pt metal microparticles in a porous film. Pt microwire probes were found to be more reliable than Pt-coated tips for conductivity mapping in contact mode. Preliminary results revealed a marked spatial heterogeneity in the current-voltage characteristics of the Pt microdeposits, attributed to the nature of the subsurface Pt contact to the underlying substrate. Prospects for using conducting-AFM more widely to characterize the local electrical properties of complex electrode materials are highlighted. (c) 2001 The Electrochemical Society. All rights reserved.

This review considers recent developments in hydrodynamic modulation voltammetry (HMV), and places them into context with earlier work. The concept of hydrodynamically modulating the mass transport rate of species to the surface of an electrode in order to extend detection limits and increase the potential window of a given solvent, is well‐established. Much of the early work in this area, has focused on modulating the rotation speed of a rotating disk electrode or pulsing the solution flow rate through a tubular or channel electrode. Although both methodologies are well‐proven as HMV techniques, the response times and detection limits of these systems have been bound by fairly low modulation frequencies and slow diffusional and hydrodynamic relaxation times. Recent developments, have looked to remove these constraints, by either employing thin layer geometries and/or ultramicroelectrodes (UMEs) in novel hydrodynamic environments. Of particular interest, are the hydrodynamically modulated radial flow microring electrode (RFMRE) and the microjet electrode (MJE), both characterized by extremely high mass transport rates, under steady flow conditions. This results in fast diffusional relaxation times, of the order of tens of microseconds, when mass transfer is modulated, enabling the employment of high modulation frequencies for analysis and reduced detection times.

The use of scanning electrochemical microscopy, a high-resolution chemical imaging technique, to probe the distribution and mobility of solutes in articular cartilage is described. In this application, a mobile ultramicroelectrode is positioned close (∼1μm) to the cartilage sample surface, which has been equilibrated in a bathing solution containing the solute of interest. The solute is electrolyzed at a diffusion-limited rate, and the current response measured as the ultramicroelectrode is scanned across the sample surface. The topography of the samples was determined using Ru(CN)64−, a solute to which the cartilage matrix was impermeable. This revealed a number of pit-like depressions corresponding to the distribution of chondrocytes, which were also observed by atomic force and light microscopy. Subsequent imaging of the same area of the cartilage sample for the diffusion-limited reduction of oxygen indicated enhanced, but heterogeneous, permeability of oxygen across the cartilage surface. In particular, areas of high permeability were observed in the cellular and pericellular regions. This is the first time that inhomogeneities in the permeability of cartilage toward simple solutes, such as oxygen, have been observed on a micrometer scale.

Gold nanoparticles were used as a scaffold to assemble multiple tailor-made europium(iii) complexes yielding water-soluble gold nanoparticles that display red, Euiii, luminescence.

Incorporation of non-luminescent dodecanethiolate-protected gold clusters into regioregular poly3-hexylthiophene films results in a 6-fold increase in the visible photoluminescence PL of the polymer, which arises predominantly from NP-induced structural changes in the composite films.

Polyaniline (PANI)/Pt nanoparticle composites can be prepared by the spontaneous redox reaction of K2PtCl4with PANI, to yield thin films that show electrocatalytic properties in both acidic and neutral aqueous media.

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