Biodegradable near-infrared plasmonic nanoclusters for biomedical applications

Nanoparticles such as gold and silver with plasmonic resonances in the near-infrared (NIR) optical region, where soft tissue is the most transparent, are of great interest in biomedical applications. A major roadblock in translation of inorganic nanoparticles to clinical practice for systemic targeting of disease is their nonbiodegradable nature. In addition, gold nanoparticles that absorb in the NIR are typically greater than 50 nm, which is above the threshold size of 5.5 nm required for effective excretion from the body. Here we describe a new class of biodegradable gold nanoparticles with plasmon resonances in the NIR region. The synthesis is based on controlled assembly of very small (less than 5 nm) primary gold particles into nanoclusters with sub-100 nm overall diameter and an intense NIR absorbance. The assembly is mediated by biodegradable polymers, polyethylene glycol (PEG) and polylactic acid (PLA) copolymer, and small capping ligands on the constituent nanoparticles. Nanoclusters deaggregate into sub-5nm primary gold particles upon biodegradation of the polymer binder in live cells over one week, as shown by dark-field reflectance and hyperspectral imaging.