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 synergistic interaction between the plasmonic and the biological components, exhibiting enormous potential for nanomedicine and nanobiotechnology applications.
In this context, we propose a study of the adsorption of a globular protein, lysozyme, on (anionic) AuNPs colloids and of their aggregation subsequently induced by patch-charge interactions.
The general idea of this project is to design a novel system whose plasmonic properties can be manipulated acting on several degrees of freedom. Specifically, we will investigate the effects of AuNPs size, lysozyme-AuNPs relative molar ratio and pH to point out the conditions to obtain bio-plasmonic assemblies with the desired finite size, exhibiting colloidal stability. Furthermore, the possibility to switch between aggregation and disaggregation of the formed clusters by changing the environmental condition such as pH and temperature will be investigated.
The aggregation will be characterized by measuring the size and the surface charge of the aggregates by photo-correlation techniques (Dynamic Light Scattering and Z-potential measurements), while their LSPR will be monitored by UV-Visible absorption spectroscopy.
Lastly, considering that lysozyme performs an antibacterial activity through the lysis of bacterial cellular walls, the functionality of lysozyme confined in the clusters will be also tested by means of an enzyme catalytic assay, probing the possibility to enhance this effect due to the interactions with AuNPs.
The final goal of our project is the development of a bio-plasmonic system based on the colloidal aggregation in solution of anionic gold nanoparticles mediated by lysozyme. The realization of such objective will allow to obtain an innovative system whose optical properties could be tailored 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.
The combined study by means of UV-Visible absorption spectroscopy and Dynamic Light Scattering will enable to relate the plasmonic signal of the aggregates to their mean dimension and surface charge. For aggregates of the proper dimension it will be possible to select the LSPR of the system within a large range of frequencies extending from visible to Near-Infrared (NIR) spectral region, the latter suitable for in vivo applications.
Furthermore, the role of pH will be investigated as trigger to modulate the aggregate size once formed, and hence to switch between aggregation and disaggregation, exploiting the pH dependence of the protein net charge. This opens up the possibility to develop a smart-system able to self-assemble in both neutral and acidic environment, forming NIR-responsive aggregates for photothermal and drug delivery applications.
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. The localised heating, also induced by the plasmonic absorption, will set the foundations to realize a "thermo-plasmonic based annealing".
The catalytic activity of lysozyme confined in the clusters will be also assayed. Once verified that the protein is active even if included in the aggregates, we are planning to exploit the thermo-plasmonic effect to control the activity of the protein. In fact, the plasmon induced local increment of the temperature can be exploited to change the temperature in proximity of the NPs and hence modulate the catalysis product of the lysozyme.
As a further perspective, once that the mechanism involved in the aggregation will be fully characterized, self-assembly of AuNPs would be suitably driven on solid support by combining lithography and surface functionalization techniques to obtain bio-plasmonic substrates suitable for ultrasensitive molecular and cellular recognition. Moreover, the thermo-plasmonic control of the lysozyme enzymatic activity can be accomplished also on the substrate, allowing to obtain a topological modulation of the catalysis product.