A hybrid sp2-sp3 electrochemical sensor comprising patterned regions of nondiamond-carbon (sp2) in a boron doped diamond (sp3) matrix is described for the simultaneous voltammetric detection of dissolved oxygen (DO) and pH in buffered aqueous solutions. Using a laser micropatterning process it is possible to write mechanically robust regions of sp2 carbon into a BDD electrode. These regions both promote the electrocatalytic reduction of oxygen and facilitate the proton coupled electron transfer of quinone groups, integrated into the surface of the sp2carbon. In this way, in one voltammetric sweep (time of measurement ∼4 s) it is possible to determine both the DO concentration and solution pH. By varying the sp2 pattern the response can be optimized toward both analytes. Using a closely spaced sp2 microspot array, a linear response toward DO, across the range 0.0 to 8.0 mg L–1 (0.0 to 0.25 mM; sensitivity = −8.77 × 10–8 A L mg–1, R2 = 0.9991) and pH range 4–10 (sensitivity = 59.7 mV pH–1, R2 = 0.9983) is demonstrated. The electrode is also capable of measuring both DO concentration and pH in the more complex buffered environment of blood. Finally, we show how the peak position for ORR is independent of pH, and thus via measurement of the difference in ORR and pH peak position, internal referencing is possible. Such electrodes show great promise for use in applications ranging from biomedical sensing to water analysis.

Scanning ion conductance microscopy (SICM) is becoming a powerful multifunctional tool for probing and analyzing surfaces and interfaces. This work outlines methodology for the quantitative controlled delivery of ionic redox-active molecules from a nanopipette to a substrate electrode, with a high degree of spatial and temporal precision. Through control of the SICM bias applied between a quasi-reference counter electrode (QRCE) in the SICM nanopipette probe and a similar electrode in bulk solution, it is shown that ionic redox species can be held inside the nanopipette, and then pulse-delivered to a defined region of a substrate positioned beneath the nanopipette. A self-referencing hopping mode imaging protocol is implemented, where reagent is released in bulk solution (reference measurement) and near the substrate surface at each pixel in an image, with the tip and substrate currents measured throughout. Analysis of the tip and substrate current data provides an improved understanding of mass transport and nanoscale delivery in SICM and a new means of synchronously mapping electrode reactivity, surface topography, and charge. Experiments on Ru(NH3)63+ reduction to Ru(NH3)62+ and dopamine oxidation in aqueous solution at a carbon fiber ultramicroelectrode (UME), used as the substrate, illustrate these aspects. Finite element method (FEM) modeling provides quantitative understanding of molecular delivery in SICM. The approach outlined constitutes a new methodology for electrode mapping and provides improved insights on the use of SICM for controlled delivery to interfaces generally.

The development of tools that can probe corrosion related phenomena at the (sub)microscale is recognized to be increasingly important in order to understand the surface structural factors (grain orientation, inclusions etc.) that control the (electro)chemical stability (corrosion susceptibility, pitting, passivity etc.) of metal surfaces. Herein we consider the application of scanning electrochemical cell microscopy (SECCM), a relatively new member of the electrochemical droplet cell (EDC) family, for corrosion research and demonstrate the power of this technique for resolving structure and activity at the (sub)microscale. Hundreds of spatially-resolved (2 μm droplet size) potentiodynamic polarization experiments have been carried out on the several hours timescale and correlated to complementary structural information from electron backscatter diffraction(EBSD) and energy dispersive x-ray spectroscopy (EDS) in order to determine the effect of grain orientation and inclusions on electrochemical processes at low carbon steel in neutral solution (10 mM KNO3). Through this approach, it has been shown unequivocally that for the low index planes, anodic currents in the passive region (an indicator of corrosion susceptibility) are greatest on (101) planes compared to (100) and (111) planes. Furthermore, individual sub-micron MnS inclusions have been probed and shown to undergo active dissolution followed by rapid repassivation. This study demonstrates the high versatility of SECCM and the considerable potential of this technique for addressing structure-activity problems in corrosion and electromaterials science.

A scanning electrochemical droplet cell technique has been employed to screen the intrinsic electrocatalytic hydrogen evolution reaction (HER) activity of hexagonal boron nitride (h-BN) nanosheets supported on different metal substrates (Cu and Au). Local (spatially-resolved) voltammetry and Tafel analysis reveal that electronic interaction with the underlying metal substrate plays a significant role in modulating the electrocatalytic activity of h-BN, with Au-supported h-BN exhibiting significantly enhanced HER charge-transfer kinetics (exchange current is ca. two orders of magnitude larger) compared to Cu-supported h-BN, making the former material the superior support in a catalytic sense.

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.

Review Article as part of the Fundamental and Applied Reviews in Analytical Chemistry 2019 special issue.