Development and characterization of water-soluble, photo-luminescent gold nanoparticles for bio-imaging

In this dissertation, we explored the synthesis of water-soluble and photoluminescence behavior near infrared emitting (610 nm) gold nanoparticles terminated by mercaptoalkanoic acid and possessing UV range (200~350 nm) excitation. Different effects were monitored as a function of reaction condition including different gold and ligand concentrations, types of ligands, solvents and pH. It is understood that Gold-thiol complexes were formed and developed into nanoparticle-supported complexes. Analyses of the excitation spectra suggests the origin of the photoluminescence to be transitions from the triplet energy state of LMMCT with the electrons transferred from excited orbitals of Au/Au(I) sites of the gold surface. It is also the reason for the enhanced photostability compared with those produced as free molecules via other synthesis methods. The pH dependency of the emission intensity and excitation spectra alteration of the gold nanoparticles was also explored. The emission intensity of the gold nanoparticle showed linear dependency on the pH change in the weak acidic to basic region above the pH 6 with a small peak appearance at pH 4. This trend was accompanied by a distinctive excitation peak wavelength change from 280-290 nm to 250-260 nm at pH 6.. A brush configuration change of the surface ligands was proposed to explain the pH dependency. In the charged and extended form of the carboxylic acid ligands, the accessibility of water to the gold nanoparticles surface is greater than in the uncharged collapsed form. Thus, in the collapsed form, the local hydrophobicity at the gold surface is higher and theCT excitation spectrum shifts to the blue. Its biocompatibility, as suggested by the cytotoxicity test and reactive oxygen species (ROS) generation test, provides broader opportunities for this product to be utilized in biological systems.