The impact of plasma inhomogeneity on the sp² content of thin film (∼micron) boron doped diamond (BDD) electrodes, grown using microwave chemical vapour deposition (MW-CVD) under different methane (CH₄) concentrations (1% and 5%), is investigated. The sp²surface content (critical for interpreting electrochemical data) is comparatively assessed using a variety of electrochemical measurements: capacitance; solvent window analysis and quinone surface coverage. For all growths, distinctive regions containing appreciably differing amounts of sp² carbon are identified, across the wafer. For example, on the 1% CH₄ wafer, some areas exhibit electrochemical signatures indicative of high quality, minimal sp² content BDD, whereas others show regions comprising significant sp² carbon. Note Raman microscopy was unable to identify these variations. On the 5% CH₄ wafer, no region was found to contain minimal levels of sp² carbon. Changes in sp² content across the BDD films indicates spatial variations in parameters such as temperature, methane and atomic hydrogen concentrations during growth, in this case linked directly to the use of a commonly employed multi-moded (overmoded) chamber for MW-CVD BDD synthesis, operated under low power density conditions. Varying sp² levels can have significant impact on the resulting electrochemical behaviour of the BDD.

The kinetics of crystal growth of gypsum is determined by measuring the 3D time-evolution of isolated microcrystals (∼10 μm characteristic dimension) by in situ AFM. By coupling such measurements to a well-posed diffusion model, the importance of mass transport to the overall rate can be elucidated readily. Indeed, because microscale interfaces that act as source or sink sites are characterized by intrinsically high diffusion rates, it is possible to study crystal growth free from mass transport effects in many instances. In the present study, a particular focus is to elucidate how the ratio of Ca²⁺ to SO₄^2– ions at constant supersaturation influences the rate of growth at the major crystal faces of gypsum. It is found that growth at the {100} and {001} faces, in particular, is highly sensitive to solution stoichiometry, resulting in needle-like crystals forming in Ca²⁺-rich solutions and plate-like crystals forming in SO₄^2–-rich solutions. The maximum growth rate occurs with a stoichiometric solution of Ca²⁺:SO₄^2– The much slower growing basal {010} face also shows a stoichiometry dependence and growth is found to occur at step sites in growth hillocks. Importantly, overall growth rates derived by measuring the volumetric expansion of microcrystals by 3D in situ AFM are in reasonable agreement with previous bulk studies on suspensions. This study is a further illustration that the study of individual microcrystals is a powerful approach for resolving face-specific kinetics and in providing a link between microscopic observations and macroscopic rates in bulk systems. In this study it has further been possible to link face-specific kinetics to the resulting crystal morphology.

Fourier-transformed large amplitude alternating current voltammetry (FTACV) provides a sensitive analytical tool for the discrimination of electrode reactions that are complicated by surface heterogeneity. In this paper, it is shown how the FTACV response at a dual-electrode system comprised of different electrode materials having different heterogeneous charge transfer (k⁰₁ and k⁰₂) can be resolved into its individual electrode kinetics components without prior knowledge of the electrode size ratio (θ₁:θ₂). This is possible when one process is reversible and the other is quasi-reversible; achievable by careful selection of the FTACV frequency. The applicability of the FTACV method over a wide range of electrode kinetic values and size ratios is considered for conditions under which numerical simulations based on a 1D diffusion model are adequate to describe the mass transport problem.

Quantitative studies of electron transfer processes at electrode/electrolyte interfaces, originally developed for homogeneous liquid mercury or metallic electrodes, are difficult to adapt to the spatially heterogeneous nanostructured electrode materials that are now commonly used in modern electrochemistry. In this study, the impact of surface heterogeneity on Fourier-transformed alternating current voltammetry (FTACV) has been investigated theoretically under the simplest possible conditions where no overlap of diffusion layers occurs and where numerical simulations based on a 1D diffusion model are sufficient to describe the mass transport problem. Experimental data that meet these requirements can be obtained with the aqueous [Ru(NH₃)₆]^3+/2+ redox process at a dual-electrode system comprised of electrically coupled but well-separated glassy carbon (GC) and boron-doped diamond (BDD) electrodes. Simulated and experimental FTACV data obtained with this electrode configuration, and where distinctly different heterogeneous charge transfer rate constants (k⁰ values) apply at the individual GC and BDD electrode surfaces, are in excellent agreement. Principally, because of the far greater dependence of the AC current magnitude on k⁰, it is straightforward with the FTACV method to resolve electrochemical heterogeneities that are ∼1–2 orders of magnitude apart, as applies in the [Ru(NH₃)₆]^3+/2+ dual-electrode configuration experiments, without prior knowledge of the individual kinetic parameters (k⁰₁ and k⁰₂) or the electrode size ratio (θ₁:θ₂). In direct current voltammetry, a difference in k⁰ of >3 orders of magnitude is required to make this distinction.

A multifunctional dual-channel scanning probe nanopipet that enables simultaneous scanning ion conductance microscopy (SICM) and scanning electrochemical microscopy (SECM) measurements is demonstrated to have powerful new capabilities for spatially mapping the uptake of molecules of interest at living cells. One barrel of the probe is filled with electrolyte and the molecules of interest and is open to the bulk solution for both topographical feedback and local delivery to a target interface, while a solid carbon electrode in the other barrel measures the local concentration and flux of the delivered molecules. This setup allows differentiation in molecular uptake rate across several regions of single cells with individual measurements at nanoscale resolution. Further, operating in a “hopping mode”, where the probe is translated toward the interface (cell) at each point allows self-referencing to be employed, in which the carbon electrode response is calibrated at each and every pixel in bulk for comparison to the measurement near the surface. This is particularly important for measurements in living systems where an electrode response may change over time. Finite element method (FEM) modeling places the technique on a quantitative footing to allow the response of the carbon electrode and local delivery rates to be quantified. The technique is extremely versatile, with the local delivery of molecules highly tunable via control of the SICM bias to promote or restrict migration from the pipet orifice. It is expected to have a myriad of applications from drug delivery to screening catalysts.

A vast range of interfacial systems exhibit charge heterogeneities on the nanoscale. These differences in local surface charge density are challenging to visualize, but recent work has shown the scanning ion conductance microscope (SICM) to be a very promising tool to spatially resolve and map surface charge and topography via a hopping potential sweep technique with a single nanopipette probe, with harmonic modulation of a bias applied between quasi-reference counter electrodes in the nanopipette and bulk solution, coupled with lock-in detection. Although powerful, this is a relatively slow process, with limitations on resolution and the size of the images that can be collected. Herein, we demonstrate a new scanning routine for mapping surface charge and topography with SICM, which increases the data acquisition rate by an order of magnitude and with the potential for further gains. Furthermore, the method is simplified, eliminating the need for bias modulation lock-in detection, by utilizing a potential-pulse, chronoamperometric approach, with self-referencing calibration of the response at each pixel in the image. We demonstrate the application of this new method to both a model substrate and living PC-12 cells under physiological (high ionic strength) conditions, where charge mapping is most challenging (small Debye length). This work contributes significantly to the emergence of SICM as a multifunctional technique for simultaneously probing interfacial structure and function with nanometer resolution.