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Nano-Impacts: the chemistry of single nanoparticles

Final Report Summary - NANOIMPACTS (Nano-Impacts: the chemistry of single nanoparticles)

Nano-impacts has realised the study of individual manmade nano-particles leading to an analytical methodology which can identify the chemical nature of the particles, measure their concentration in solution, quantify their size and porosity and establish their state of agglomeration. In particular the new approach reports the number of atoms or molecules in the particles of interest so providing a more direct measure of ‘size’ than is possible with other solution based techniques such as dynamic light scattering or nanoparticle tracking analysis. Furthermore, much smaller particles can be detected than is typically possible for these techniques – notably silver nanoparticles as small as 5nm and as large as 50nm have been quantitatively assessed. Measurements are made directly in the solution phase. Diverse materials have been interrogated both as part of the methodology verification and validation as for study in their own right. These include nano-particles of silver, gold, nickel, magnetite, organic molecules such as indigo, polymeric species (such as poly-vinylcarbazole), and conducting salts as well as core-shell structures such as gold core-silver shell particles, nano-rods, nano-droplets and non-conducting particles such as of alumina.

In parallel with the analytical measurements fundamental insights into the chemistry of nanoparticles and their reactions has been revealed for diverse systems. Thus the catalytic behaviour of platinum nanoparticle aggregates has been shown to reflect their intrinsic porosity with internal surfaces of the particles being active and in contact with the solution phase in which they are suspended. In the case of nanoparticles used in lithium ion battery systems (such as LiMn2O4) the step controlling the rate of charging of individual nanoparticles has been shown to be ion transfer across the particle-solution interface. The catalytic behaviour of nano-droplets carrying catalysts such as vitamin B12 and the suggested four electron reduction of oxygen to water – a process at the heart of fuel cell technology – conclusively proved. In the case of non-conducting particles such as of Al2O3, which is widely used as an adsorbing material, the uptake of electroactive species such as quinones has been studied yielding not only the extent of adsorption (surface coverage) but the rates of charge diffusion across the particle surface. Of generic fundamental interest has been the work on nanoparticle agglomeration where we have proposed a model for the extent of particle agglomeration in solution based on the entropy of mixing created by the formation of dimers, trimers, ... etc., from an initial population of monomeric species. In this way some degree of agglomeration is always expected in a solution even when the particles mildly repel one another – the thermodynamics, via entropy promotes ‘diversity’ in the solution phase by creating a mixture of different agglomerates.

More generally the project – via papers published from it - has stimulated a jump of electrochemistry away from the study of molecules or simple nanoparticles into what has become known as ‘single entity electrochemistry’. This work within the project has shown how single enzymes, bacteria, viruses and red blood cells can all be studied – both analytically and fundamentally – via impact electrochemistry. This is transforming the vision and activity of electrochemists worldwide. Of particular fundamental interest is our electrochemical work showing how the catalytic activity of single enzymes fluctuates over time rather than showing a steady behaviour. Thus the enzyme (for example catalase) has periods of high intense activity interspersed with dormant phase.

Last our nano-toxicology studies explain why nanoparticles of silver might be toxic whereas macro-sized (bulk) silver is not. Surprisingly this is shown to result from the increased diffusion rate away from smaller particles which in vivo can lead to the formation of toxic hydrogen peroxide rather than benign water when the silver is oxidised by dissolved oxygen.