Thermal transformations of manufactured nanomaterials as a proposed proxy for ageing†
Ageing is an important part of a manufactured nanomaterial's life cycle and can be considered as a transformation over time. It is particularly relevant to nanomaterials (NMs) because they are more reactive than their bulk counterparts and therefore are likely to undergo more significant or faster transformations with time. The conditions upon exposure of a NM to the environment, e.g. temperature, humidity and redox, will all individually affect ageing, as well as time. In experimental simulations, time has to be substituted by a proxy that makes timescales more realistic. Thermal ageing accelerates the normal ageing processes of NMs and thus elevated temperatures can be used to simulate prolonged ageing, allowing access to information on the long-term effects of NM ageing within a shorter time. Similar approaches are utilised in experimental simulation of protein fibrillation, for example, where processes that naturally occur over decades are accelerated to days or hours. In this work, time and temperature dependent studies were carried out on a fully characterised library of laboratory synthesised comparable polyvinylpyrrolidone (PVP) capped NMs (with core compositions of ceria, copper oxide and zinc oxide) and a commercially available uncoated cerium dioxide NM, to assess their transformations. Specifically, physical and chemical changes were studied on NMs exposed to various temperatures (25, 45, 65 and 80 °C) for a period of 4 weeks. The size, zeta potential, agglomeration/aggregation and valence state of the NMs were studied through dynamic light scattering (DLS), zeta potential, ultra-violet visible light spectroscopy (UV-VIS), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS), as a function of time. Results generally show a decrease in NM stability with increasing temperature and time. Changes in the NM size and core oxidation state were noted with increasing temperature/time. These changes varied depending on the NM core composition. Additionally the PVP capping, despite stabilising the NM dispersion, still allowed the NM core to be influenced by external factors, thus indicating likely ageing-related reduction in efficiency, though to a lesser extent than the uncapped particles. Overall the experiments recorded a complex picture of transformations as a function of time/temperature highlighting the complexity of NMs' ageing.