Recent Publications Feed (ADMIN)

Electrochemically controlled nanopipettes are becoming increasingly versatile tools for a diverse range of sequencing, sizing, and imaging applications. Herein, the use of nanopipettes to induce and quantitatively monitor crystallization and dissolution in real time is considered, using CaCO₃ in aqueous solution as an exemplar system. The bias between a quasi-reference counter electrode in a nanopipette and one in a bulk solution is used to mix (or de-mix) two different solutions by ion migration and drive either growth or dissolution, depending on the polarity. Furthermore, Raman spectroscopy can be applied simultaneously to identify polymorphs formed in the nanopipette. The technique is supported with a robust finite element method model that allows the extraction of time-dependent saturation levels and mixing characteristics at the nanoscale. The technique shows great promise as a tool for rapidly screening growth additives and inhibitors, allowing eight different additives to be ranked in order of efficacy for crystal growth rate inhibition.

Single nanoparticle (NP) electrochemical impacts is a rapidly expanding field of fundamental electrochemistry, with applications from electrocatalysis to electroanalysis. These studies, which involve monitoring the electrochemical (usually current–time, I–t) response when a NP from solution impacts with a collector electrode, have the scope to provide considerable information on the properties of individual NPs. Taking the widely studied oxidative dissolution of individual silver nanoparticles (Ag NPs) as an important example, we present measurements with unprecedented noise (< 5 pA) and time resolution (time constant 100 μs) that are highly revealing of Ag NP dissolution dynamics. Whereas Ag NPs of diameter, d = 10 nm are mostly dissolved in a single event (on the timescale of the measurements), a wide variety of complex processes operate for NPs of larger diameter (d ≥ 20 nm). Detailed quantitative analysis of the I–t features, consumed charge, event duration and impact frequency leads to a major conclusion: Ag NPs undergo sequential partial stripping (oxidative dissolution) events, where a fraction of a NP is electrochemically oxidized, followed by the NP drifting away and back to the tunnelling region before the next partial stripping event. As a consequence, analysis of the charge consumed by single events (so-called “impact coulometry”) cannot be used as a general method to determine the size of colloidal NPs. However, a proper analysis of the I–t responses provides highly valuable information on the transient physicochemical interactions between NPs and polarized surfaces.

Physicochemical processes that take place at the oil-water interface of an epoxy-amine emulsion polymerisation system influence the properties and structural morphology of the polymeric microparticles formed. Investigating these processes, such as the transport of monomers across the liquid/liquid interface brings new understanding which can be used to tune polymeric morphology. Two different approaches are used to provide new insights on these processes. Microelectrochemical measurements at expanding droplets (MEMED) is used to measure the transfer of amine from an organic phase comprised of epoxide and amine into an aqueous receptor phase. The rate of amine transfer across the liquid/liquid interface is characterised using MEMED and finite element method modelling and kinetic values are reported. Time lapse microscopy of epoxide droplets held in deionised water or an aqueous amine solution heated to different temperatures is further used to characterise epoxide dissolution into the aqueous phase. Mass-transport of epoxide into the aqueous phase is shown to be temperature-dependent. Epoxide homopolymerisation at the droplet-water interface is found to influence the rate of epoxide droplet dissolution. The rate of the epoxy-amine cure reaction is shown to be faster than the rate of the epoxide homopolymerisation reaction. The combination of methods used here is not limited to emulsion polymerisation and should find application in a myriad of processes at liquid/liquid interfaces.