Recent Publications Feed (ADMIN)

Nanopipettes are becoming extremely versatile and powerful tools in nanoscience for a wide variety of applications from imaging to nanoscale sensing. Herein, the capabilities of nanopipettes to build complex free-standing three-dimensional (3D) nanostructures are demonstrated using a simple double-barrel nanopipette device. Electrochemical control of ionic fluxes enables highly localized delivery of precursor species from one channel and simultaneous (dynamic and responsive) ion conductance probe-to-substrate distance feedback with the other for reliable high-quality patterning. Nanopipettes with 30–50 nm tip opening dimensions of each channel allowed confinement of ionic fluxes for the fabrication of high aspect ratio copper pillar, zigzag, and Γ-like structures, as well as permitted the subsequent topographical mapping of the patterned features with the same nanopipette probe as used for nanostructure engineering. This approach offers versatility and robustness for high-resolution 3D “printing” (writing) and read-out at the nanoscale.

The calcium sulfate crystal system is of considerable fundamental and practical interest, consisting of the three hydrates, gypsum (CaSO₄·2H₂O), bassanite (CaSO₄·0.5H₂O), and anhydrite (CaSO₄). Each has significant applications; however, synthesis of anhydrite via conventional aqueous methods requires elevated temperatures and therefore high energy costs. Herein, we investigate calcium sulfate crystal growth across a nonmiscible aqueous–organic (hexane or dodecane) interface. This process is visualized via in situ optical microscopy, which produces high magnification videos of the crystal growth process. The use of interferometry, Raman spectroscopy, and X-ray diffraction allows the full range of calcium sulfate morphologies and hydrates to be analyzed subsequently in considerable detail. In the case of dodecane, gypsum is the final product, but the use of hexane in an open (evaporating) system results in anhydrite crystals, via gypsum, at room temperature. A dissolution–precipitation mechanism between neighboring microcrystals is responsible for this transformation. This work opens up a simple new crystal synthesis route for controlling and directing crystallization and transformation.

The electro-oxidation of nicotinamide adenine dinucleotide (NADH) is studied at bare surfaces of highly oriented pyrolytic graphite (HOPG) and semi-metallic polycrystalline boron-doped diamond (pBDD). A comparison of these two carbon electrode materials is interesting because they possess broadly similar densities of electronic states that are much lower than most metal electrodes, but graphite has carbon sp²-hybridization, while in diamond the carbon is sp³-hybridised, with resulting major differences in bulk structure and surface termination. Using cyclic voltammetry (CV), it is shown that NADH oxidation is facile at HOPG surfaces but the reaction products tend to strongly adsorb, which causes rapid deactivation of the electrode activity. This is an important factor that needs to be taken into account when assessing HOPG and its intrinsic activity. It is also shown that NADH itself adsorbs at HOPG, a fact that has not been recognized previously, but has implications for understanding the mechanism of the electro-oxidation process. Although pBDD was found to be less susceptible to surface fouling, pBDD is not immune to deterioration of the electrode response, and the reaction showed more sluggish kinetics on this electrode. Scanning electrochemical cell microscopy (SECCM) highlights a significant voltammetric variation in electroactivity between different crystal surface facets that are presented to solution with a pBDD electrode. The electroactivity of different grains correlates with the local dopant level, as visualized by field emission-scanning electron microscopy. SECCM measurements further prove that the basal plane of HOPG has high activity towards NADH electro-oxidation. These new insights on NADH voltammetry are useful for the design of optimal carbon-based electrodes for NADH electroanalysis.