The electrochemistry of metal nanoparticles
Due to the tiny dimensions of nanoparticles, their behaviour is dominated by their surface. This gives them properties such as enhanced strength and chemical reactivity, which are key to groundbreaking developments in fuel cells, electronics and biotechnology. Researchers initiated the EU-funded project SNISEB (Single nanoparticle impact studies: The direct observation of electrochemical behaviour at the nanoscale) to study the behaviour of ensembles of silver nanoparticles. Using nano-impacts, they also probed the electrochemistry of individual nanoparticles. Nano-impacts is a new technique developed within the last 20 years that relies on recordings of particles impacting on an electrode. The nanoparticle-electrode impact is recorded as an electrochemical signal generated by the redox reaction occurring on the nanoparticle. In the SNISEB experiments, silver-capped gold nanoparticles diffused as a result of Brownian motion and hit the carbon microelectrode held at a suitable oxidising potential. The silver of nanoparticles was oxidised to silver ions, generating a current spike that was observed in the chronoamperogram recorded. The silver oxidation charges associated with these spikes were then used to determine the sizes of the silver shells. Researchers compared the results with electron microscopy measurements, establishing the accuracy of the bimetallic nanoparticle size calculations. An innovative aspect of the approach adopted was the presence of gold cores remaining on the electrode that could be assessed after dissolution of the silver shell. Specifically, a decrease in the silver oxidation charge due to particle loss from the electrode could be distinguished from silver dissolution. Before the end of SNISEB, a series of nano-impact experiments were carried out on gold nanorods, demonstrating the applicability of this powerful technique on non-spherical nanoparticles. Lastly, the team explored the electrochemical behaviour of gold-silver alloy nanoparticles. SNISEB research covered various aspects of the electrochemical behaviour of nanoparticles with different compositions, shapes and sizes. Understanding their physicochemical properties is necessary to predict the conditions under which nanoparticles react and release ions to the surrounding environment. In particular, silver nanoparticles are extensively utilised in the fabrication of materials that inhibit the growth of bacteria and fungi. While their biocidal properties are beneficial for a broad range of applications, they may become toxic through the release of silver ions.
Keywords
Electrochemistry, metal nanoparticles, SNISEB, nano-impacts, gold-silver alloy
