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The aim of this research project was the study of fundamental electrochemical behaviour at the single nanoparticle level. This represents an important area of research due to the ever-growing interest in the special properties and applications of metallic nanoparticles, where the size, shape and composition of the nanoparticles can lead to such altered behaviour. Electrochemical techniques are particularly well suited to study these aspects, given that they allow the materials to be probed in situ and provide insights into the structure and reactivity of the nanoparticles. The objective of this project was to gain further understanding of the behaviour of ensembles of nanoparticles as well as probing the electrochemistry of individual nanoparticles. This latter aim was achieved through the use of nanoparticle impact experiments, where the Brownian motion driven collision of nanoparticles with an electrode surface allows the quantitative analysis of reactions at individual nanoparticles.

The first project investigated centred on studying the electrochemical behaviour of gold-core silver-shell nanoparticles. Due to the significantly different redox behaviour of the two metals present the response of the silver shell could be readily distinguished from that of the gold core. Importantly this gold core provided a convenient method to study the chemical dissolution of silver in the presence of different electrolytes, as it allowed the results to be corrected for the loss of nanoparticles from the electrode surface. Through this study the dissolution of the silver-shells in the presence of environmentally ubiquitous species (O2, H+ and Cl-) was established. The results clearly showed that the oxidative dissolution of silver is driven by the H+ assisted reduction of O2 to H2O2, however the presence of Cl- stabilised the silver shells by the formation of an insoluble AgCl layer, thus inhibiting the dissolution of the silver shell (B.J. Plowman, K. Tschulik, E. Walport, N. Young, R.G. Compton, Nanoscale, 2015, 7, 12361).

The studies of the core-shell nanoparticles were later extended to the individual nanoparticle level, in order to determine whether the analysis of bimetallic nanoparticles can be achieved through nanoimpacts. After analysing impacts with monometallic gold nanoparticles (to confirm the number of electrons transferred during the oxidation of gold), nanoimpacts were performed using the core-shell nanoparticles. It was found that large oxidative spikes were obtained at high potentials, signifying the complete dissolution of both the silver shell and the gold core, while smaller spikes were present at potentials where only silver oxidation occurs. The charges associated with these spikes were used to determine the sizes of the silver shells and gold cores. Comparison with electron microscopy results showed excellent agreement, establishing the size determination of bimetallic nanoparticles through nanoimpacts (L. R. Holt, B. J. Plowman, N. P. Young, K. Tschulik and R. G. Compton, Angewandte Chemie International Edition, 2016, 55, 397).

Another project that was explored was the electrochemical behaviour of nanoparticle capping agents, which was accomplished by investigating the voltammetry of citrate or cetyl trimethylammonium bromide (CTAB) capped gold nanoparticles. It was found that the presence of bromide (introduced by the CTAB capping agent) results in an oxidation peak that is absent for both citrate capped gold nanoparticles or a bare gold electrode. By comparing these data is apparent that the oxidation peak relates to both the oxidation of bromide to bromine as well as the oxidative dissolution of gold to gold bromide species. These are significant findings as the electrochemical behaviour of bromide/bromine can very easily be mistaken for the formation and reduction of a gold monolayer oxide. This latter feature is frequently used to measure the electrochemically active surface area of gold materials and will therefore be of great value for future electrochemical studies of gold in the presence of bromide containing species. In addition to the voltammetric studies nanoimpact experiments were performed, providing further insight into the mechanism of gold dissolution in the presence of bromide (B. J. Plowman, K. Tschulik, N. P. Young and R. G. Compton, Phys. Chem. Chem. Phys., 2015, 17, 26054).

While nanoimpact experiments are a powerful method to determine the dimensions of quasi-spherical nanoparticles, its extension to non-spherical nanoparticles was unexplored. The electrochemical characterisation of gold nanorods was therefore investigated, so as to determine whether the dimensions of the nanorods (length and diameter) could be obtained by nanoimpacts. This was achieved by determining the volume and surface area of the nanorods through nanoimpacts in KCl or by impacting nitrothiophenol (NTP) modified nanorods, respectively. Through this method we have shown that nanoimpacts may be applied to cases with non-spherical nanoparticles, representing a signficant extension to this powerful technique (B. J. Plowman, N. P. Young, C. Batchelor-McAuley and R. G. Compton, Angewandte Chemie International Edition, 2016, Article in press).

The electrochemical behaviour of gold-silver alloy nanoparticles was also studied, representing a differrent class of bimetallic nanoparticles to the core-shell nanoparticles discussed previously. The alloy nanoparticles showed markedly different behaviour as compared with pure silver or pure gold nanoparticles, with silver oxidation occuring at a significantly higher oxidation potential compared with silver nanoparticles. Importantly upon cycling the response of the alloy nanoparticles were found to change and resemble that of the monometallic nanoparticles, indicating that substantial dealloying of the nanoparticles occurred under these conditions, which is of significant interest for applications involving alloy nanoparticles in electrochemical applications. It was also found that the reduction of silver chloride is greatly enhanced by the presence of gold, either present in the alloy nanoparticles or by the addition of gold nanoparticles to the electrode surface. This signifies that the presence of gold provides nucleation sites on the electrode surface which enhances the reduction of silver chloride (B. J. Plowman, B. Sidhureddy, S. V. Sokolov, N. P. Young, A. Chen and R. G. Compton, ChemElectroChem, 2016, Article in press).

Several other projects were also investigated through collaboration with colleagues within the Compton Electrochemical Group. This includes work on the detection of single influenza viruses, where the viruses were tagged with silver nanoparticles prior to their detection by nanoimpact experiments (L. Sepunaru, B. J. Plowman, S. V. Sokolov, N. P. Young and R. G. Compton, Chemical Science, 2016, 7, 3892). Another project covered the electrochemical detection of non-spherical nanoparticles, where the light-induced reconfiguration of spherical silver nanoparticles to nanotriangular prisms was monitored by nanoimpact experiments. This work is now under preparation for publication (T. R. Bartlett, S. V. Sokolov, B. J. Plowman, N. P. Young and R. G. Compton).

Through the research performed during this fellowship significant progress has been achieved, covering the electrochemical behaviour of metallic nanoparticles with various compositions, shapes and capping agents. Importantly these represent significant advances in the field of nanoimpacts and demonstrate the power of this technique to study the behaviour of nanomaterials at the individual level.