Optically Responsive Gold Nanorod-Polypeptide Assemblies
H-C. Huang, P. Koria, S.M. Parker, L. Selby, Z. Megeed, K. Rege
Arizona State University, US
Keywords: photoresponsive, gold nanorods, elastin-like polypeptides, near infrared, surface plasmon, nanoassemblies
Abstract:Novel optically responsive nanoassemblies have promising applications in molecular switching devices, sensors, drug delivery systems, and biomedical imaging. In particular, the interfacing of proteins / polypeptides with nanoparticles can result in the generation of novel functional nanomaterials for biological / biomedical applications. Gold nanorods demonstrate a tunable photothermal response to near infrared (NIR) light as a function of nanoparticle aspect ratio and have been investigated as potential diagnostics, therapeutic systems, imaging agents, and sensors. The ability to convert incident light energy to heat energy due to surface plasmon resonance activity makes gold nanorods attractive candidates for modulating polypeptide (or protein) structure / phase transition using optical methods (i.e. near infrared light). While the ability to induce irreversible structural change in proteins can play a role in therapeutic applications, the ability to reversibly control protein structure lends flexibility for a variety of applications including site-specific drug delivery, biosensors, and switching. We report the generation of gold nanorod (GNR)-elastin-like polypeptide (ELP) nanoassemblies whose optical response can be manipulated based on exposure to near infrared (NIR) light. ELPs exhibit a thermally induced phase transition at the inverse transition temperature, characterized by reversible intramolecular contraction and intermolecular coacervation. The thermal transition behavior of ELPs has been exploited in a number of applications including, biomolecule purification, drug delivery, sensors, and tissue engineering. Cysteine-containing ELPs were self-assembled on gold nanorods mediated by gold-thiol bonds, leading to the generation of GNR-ELP nanoassemblies. Exposure of GNR-ELP assemblies to near-infrared (NIR) light resulted in the heating of gold nanorods due to surface plasmon resonance. Heat transfer from the gold nanorods resulted in an increase in temperature of the self-assembled ELP above its transition temperature (Tt), which led to a phase transition and aggregation of the GNR-ELP assemblies. This phase transition was detected using an optical readout (increase in optical density); no change in optical behavior was observed in case of either ELP alone or GNR alone. The optical response was reproducible and reversible across a number of cycles following exposure to or removal of the laser excitation. Our results indicate that polypeptides may be interfaced with gold nanorods resulting in optically responsive nanoasssemblies. Such ability to control nanoscale assembly and nanomaterial properties by optical manipulation can be exploited in the development of novel sensors, drug delivery systems, functional molecular / nanoscale devices, and imaging agents.