In this work, we report the first spatially-resolved voltammetric measurements of the hydrogen evolution reaction on natural crystals of molybdenite, unequivocally demonstrating enhanced catalytic activity on the edge plane relative to the basal plane.

A versatile in situ multi-microscopy approach to study the dissolution kinetics of single crystals is described, using the loop diuretic drug furosemide as a testbed to demonstrate the utility of the approach. Using optical microscopy and scanning ion-conductance microscopy (SICM) in combination, the dissolution rate of individual crystallographically independent crystal faces can be measured quantitatively while providing a direct visualization of the evolution of crystal morphology in real time in three dimensions. Finite element method (FEM) models using experimental data enables quantitative analysis of dissolution fluxes for individual faces and determination of the limiting process -mass transport or interfacial kinetics -that regulates dissolution. A key feature of the approach is that isolated crystals (typically < 60 µm largest characteristic dimension) in solution during dissolution experience high and well defined diffusion rates. The ability to obtain this quantitative information for individual crystal faces suggests a pathway to understanding crystal dissolution at the molecular level and regulating bioavailability, for example, through manipulation of crystal morphology. AbstractA versatile in situ multi-microscopy approach to study the dissolution kinetics of single crystals is described, using the loop diuretic drug furosemide as a testbed to demonstrate the utility of the approach. Using optical microscopy and scanning ion-conductance microscopy (SICM) in combination, the dissolution rate of individual crystallographically independent crystal faces can be measured quantitatively while providing a direct visualization of the evolution of crystal morphology in real time in three dimensions. Finite element method (FEM) models using experimental data enable quantitative analysis of dissolution fluxes for individual faces and determination of the limiting process -mass transport or interfacial kinetics -that regulates dissolution. A key feature of the approach is that isolated crystals (typically < 60 µm largest characteristic dimension) in solution during dissolution experience high and well defined diffusion rates. The ability to obtain this quantitative information for individual crystal faces suggests a pathway to understanding crystal dissolution at the molecular level and regulating bioavailability, for example, through manipulation of crystal morphology.

The surface charge and topography of human hair are visualized synchronously at the nanoscale using scanning ion conductance microscopy (SICM), a scanning nanopipette probe technique that uses local ion conductance currents to image the physicochemical properties of interfaces. By combining SICM data with finite element method (FEM) simulations that solve for ion transport at the nanopipette under bias, one is able to quantitatively correlate colocated surface charge and topography. The hair samples studied herein, from a 25-year-old Caucasian male with light hair (as an exemplar), reveal that untreated hair, in areas ca. 1 cm from the root, has a fairly uniform negative charge density of ca. −15 mC/cm–2 (in pH 6.8 aqueous solution), with some higher magnitude negative values localized near the boundaries between hair cuticles. Common chemical treatments result in varying degrees of charge heterogeneity. A bleach treatment produces some highly negatively charged localized regions (−80 to −100 mC/cm–2 at pH 6.8), due to hair damage, while a chemical conditioner treatment causes an overall increase in the homogeneity of the surface charge, together with a shift in the surface charge to positive values. Bleached surfaces are temporarily repaired to some extent through the use of a conditioner, as judged by the surface charge values. Finally, SICM is able to detect differences in the surface charge density of hair at different distances from the root (equivalent to hair age). Presently, the assessment of hair surface charge mainly relies on zeta-potential measurements which lack spatial resolution, among other drawbacks. In contrast, SICM enables quantitative surface charge mapping that should be beneficial in deepening understanding of the physicochemical properties of hair and lead to the rational development of new treatments and the assessment of their efficacy at the nanoscale. Given the widespread interest in the surface charge properties of interfaces, this work further demonstrates that SICM should generally become an important characterization tool for surface analytical chemists.