Tuning the optical properties of hybrid bio-plasmonic colloids

The interaction of gold nanoparticles (AuNps) with the electromagnetic radiation is characterized by a resonant absorption in the visible spectral range, the Localized Surface Plasmon Resonance (LSPR). The dependence of the LSPR frequency on the AuNps size and shape, and on the dielectric constant of the environment, provides high versatility in the design of novel systems with the desired optical properties. In particular, hybrid systems made up of AuNps conjugated with biomolecules exploit the synergic interaction between the plasmonic and the biological components, exhibiting enormous potential for nanomedicine and nano-biotechnology applications. In this context, we studied the electrostatic adsorption of a globular protein on anionic AuNps and the colloidal aggregation in solution, subsequently induced by patch-charge interactions. Lysozyme was employed to this aim: its robustness and structural stability within a wide range of environmental conditions make it suitable for our study, while, due to the high isoelectric point (pH 11.35), its positive charge allows to obtain stable aggregates at physiological pH. Furthermore, the employment of a functional protein as mediator of the aggregation let us exploit the specific interaction to the targeting ligand of the enzyme, besides the nonspecific ones which stabilize the aggregates. Our work focused on the plasmonic properties of the system and the possibility to manipulate them, acting on the different degrees of freedom involved in the process [5]. The aggregation was characterized measuring the size and the surface charge of the aggregates by photo-correlation techniques supported by atomic force microscopy, while their LSPR was monitored by UV-Visible absorption spectroscopy. The catalytic activity of lysozyme confined in the clusters was also assayed. We investigated the effects of AuNps size and lysozyme-AuNps relative molar ratio and pointed out the conditions to obtain bio-plasmonicassemblies with the desired finite size (within few hundred nanometers), exhibiting colloidal stability. Correspondingly, it is possible to tune the plasmonic response of the system, which reflects the morphology of the formed clusters. In particular, the LSPR of the system can be selected within a large range of frequencies from visible to near-infrared spectral region, the latter suitable for in vivo applications. Moreover, the role of pH was investigated both as trigger for the lysozyme enzymatic activity and to modulate the protein net charge. Experiments, performed at different pH of the solution and supported by visual molecular dynamics simulations of the correspondent lysozyme charge distribution, showed the possibility to further handle the protein-AuNp interaction and in turn the morphology and the optical properties of the system. Noteworthy, we demonstrated the possibility to switch between aggregation and disaggregation of the formed clusters by changing the pH of the solution.