Formation of Biodegradable Gold Nanoclusters with High-NIR Absorbance for Biomedical Imaging

Formation of Biodegradable Gold Nanoclusters with High-NIR Absorbance for Biomedical Imaging

 

Mengdi Sun, Ehsan Moaseri, Behzad Chang, Alexis Cepeda, Lindsay Johnson, Thomas Truskett, Keith Johnston

Abstract:

Biodegradable gold nanoclusters have attracted significant attention as optical contrast agents in biomedical applications including photo-acoustic imaging. Herein, we report a novel assembly method to assemble gold nanoclusters from 5 nm primary gold nanospheres. The nanoclusters exhibit high near-infrared (NIR) absorbance and have the capability to fully dissociate back to primary particles, which would enable efficient renal clearance. The equilibrium and kinetic aspects of nanocluster formation are manipulated via controlling colloidal interactions, concentration pathways, and quenching with a weakly adsorbed polymer stabilizer. The charge on the primary ~5 nm citrate-coated gold nanospheres is tailored through place exchange reactions with a variety of biocompatible ligands including positively charged lysine and neutral cysteine. During nanocluster growth, a biodegradable polymer, PLA(1k)-b-PEG(10k)-b-PLA(1k) provides passivation and steric stabilization. In some cases controlled solvent evaporation results in further screening of particle charge which mediates the formation of clusters between 20-40 nm in diameter depending on gold, ligand, and polymer concentration pathways. The electrostatic repulsion is further decreased through addition of salts or changes in solution pH to further adjust the morphology. The adsorption of the polymer onto the cluster surface effectively quenches the particles in a nanocluster state, as characterized with TEM and dynamic light scattering. In response to pH 5 environments within endosomes, the polymer stabilizer degrades and the clusters dissociate back to primary constituents, which are potentially small enough for efficient renal clearance.