Thermophilic rearrangement of bio-plasmonic aggregates: morphological and plasmonic related evidences

The peculiar interaction of metallic nanoparticles with the electromagnetic radiation paved the way to design novel nanoarchitectures whose optical properties can be tuned by controlling their structure and the features of the surrounding environment. The research of the last few years heads up to the idea of creating hybrid assemblies made up of metallic nanoparticles and biomolecules with promising applications in the field of nano-medicine and nano-biotechnology, providing a new and powerful tool for innovative diagnosis and therapeutical approaches. We recently developed a bio-plasmonic system based on the colloidal aggregation in solution of anionic gold nanoparticles (AuNPs) mediated by lysozyme. The aggregation is driven by patch-charge interactions [6], induced by the adsorption of the positively charged protein on the AuNPs surface. We demonstrated that the optical properties of the system can be tuned through the clusters morphology, acting on several parameters such as the AuNPs size, the Lysozyme-AuNPs relative molar ratio and the pH of the solution. Proceeding from these, here we would consider also the role of the temperature as a further tool to fine tuning the structural morphology together with the plasmonic properties of the aggregates. In this framework, the thermally enhanced diffusion of the NPs within the clusters can affect aggregate stability and shape, and thereby the own plasmonic profiles. On the other hand, the unfolding of the protein, induced by the increasing temperature and its consequent relaxation on the AuNps surface [7], implies a redistribution of the surface charge, together with an increase of the hydrophobic interactions. Lysozyme unfolding can thus be employed to change the nature of the interaction which holds the aggregates, switching from electrostatic to hydrophobic. As a first step in this direction we undertook a combined study of the temperature effects on the localized surface plasmon resonance and on the size of preformed Lysozyme-NPs aggregates. The plasmonic profile and the related inter-particles plasmonic bands which arise due to the NPs aggregation were monitored by UV-Visible Absorption Spectroscopy at varying the temperature from 20°C to 90°C, while information on the aggregates size has been obtained by Dynamic Light Scattering experiments. The combination of these techniques allowed us to disentangle the two abovementioned aspects, which can interplay in the stability of the clusters, leading to their disaggregation or resulting in the cluster reorganization, depending on the Lysozyme-AuNPs relative molar ratio. It is well known that the localised heating can be also induced by the plasmonic absorption, hence our work sets the foundations to realize a “thermo-plasmonic based annealing”.