Calcium carbonate (CaCO3) is one of the most well-studied and abundant natural materials on Earth. Crystallisation of CaCO3 is often observed to proceed via an amorphous calcium carbonate (ACC) phase, as a precursor to more stable crystalline polymorphs such as vaterite and calcite. Despite its importance, the kinetics of ACC formation have proved difficult to study, in part due to rapid precipitation at moderate supersaturations, and the instability of ACC with respect to all other polymorphs. However, ACC can be stabilised under confinement conditions, such as those provided by a nanopipette. This paper demonstrates electrochemical mixing of a Ca2+ salt (CaCl2) and a HCO3− salt (NaHCO3) in a nanopipette to repeatedly and reversibly precipitate nanoparticles of ACC under confined conditions, as confirmed by scanning transmission electron microscopy (STEM). Measuring the current as a function of applied potential across the end of the nanopipette and time provides millisecond-resolved measurements of the induction time for ACC precipitation. We demonstrate that under conditions of electrochemical mixing, ACC precipitation is extremely fast, and highly pH sensitive with an apparent third order dependence on CO32− concentration. Furthermore, the rate is very similar for the equivalent CO32− concentrations in D2O, suggesting that neither ion dehydration nor HCO3− deprotonation represent significant energetic barriers to the formation of ACC. Finite element method simulations of the electrochemical mixing process enable the supersaturation to be estimated for all conditions and accurately predict the location of precipitation.

Single walled carbon nanotube (SWNT) network electrodes, in which a planar arrangement of SWNTs on an inert surface serve as a working electrode for voltammetry, offer considerable attributes for electroanalysis. Here, the effect of SWNT network density on the trace voltammetric analysis of a water-soluble ferrocene derivative (FcCOOH) is investigated in the presence of polyethylene glycol (PEG) or albumin, species that can foul (block) an electrode via adsorption. Fc-based analytes typically find use in redox labelling or redox shuttling, point-of-care electrochemical detection devices. Comparison is made between SWNT electrodes, grown by catalyzed chemical vapour deposition at three different surface coverages, 5, ~20 and ~30 μmSWNT μm-2, and commercial screen-printed carbon electrodes (SPCEs). In the presence of PEG (8% 2 K), for cyclic voltammetry, the lowest detectable concentration decreases as SWNT network density decreases. However, when employing differential pulse voltammetry, all three networks show a 1 nM FcCOOH limit of detection, three orders of magnitude smaller than achievable with SPCEs. This is attributed to the low capacitance of the SWNTs and absence of amorphous carbon structures which can contribute a pseudo-capacitive response. For both polyethylene glycol (PEG) and albumin (4%), repeat cycling shows the higher density SWNT network electrodes (≥20 μmSWNT μm-2) are far less susceptible to electrode fouling. Toward practical devices, a three-electrode chip, similar in design to that used in SPCEs, but using high density SWNT network electrodes, is also demonstrated to have impressive detection sensitivity for FcCOOH (nM level) in PEG solutions. The simplicity and practicality of the design widens the potential applications of these ultra-sensitive diagnostic tools based on planar network SWNTs.

Thin film electrodes, produced by coating a conductive support with a thin layer (nm to μm) of active material, retain the unique properties of nanomaterials (e.g., activity, surface area, conductivity etc.) while being economically scalable, making them highly desirable as electrocatalysts. Despite the ever-increasing methods of thin film deposition (e.g., wet chemical synthesis, electrodeposition, chemical vapor deposition etc.), there is a lack of understanding on the nanoscale electrochemical activity of these materials in relation to structure/composition, particularly for those that lack long-range order (i.e., amorphous thin film materials). In this work, scanning electrochemical cell microscopy (SECCM) is deployed in tandem with complementary, co-located compositional/structural analysis to understand the microscopic factors governing the electrochemical activity of amorphous molybdenum sulfide (a-MoSx) thin films, a promising class of hydrogen evolution reaction (HER) catalyst. The a-MoSx thin films, produced under ambient conditions by electrodeposition, possess spatially-heterogeneous electrocatalytic activity on the tens-of-micron scale, which is not attributable to microscopic variations in elemental composition or chemical structure (i.e., Mo and/or S bonding environments), shown through co-located, local energy dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) analysis. A new SECCM protocol is implemented to directly correlate electrochemical activity to the electrochemical surface area (ECSA) in a single measurement, revealing that the spatially-heterogeneous HER response of a-MoSx is predominantly attributable to variations in the nanoscale roughness/porosity of the thin film. As microscopic composition, structure and porosity (ECSA) are all critical factors dictating the functional properties of nanostructured materials in electrocatalysis and beyond (e.g., battery materials, electrochemical sensors etc.), this work further cements SECCM as a premier tool for structure−function studies in (electro)materials science.