Synthesis of pH sensitive gold nanoparticles for potential application in radiosensitization

Synthesis of gold nanoparticles (GNPs) for in-vivo applications involves reduction of chloroauric acid by a biologically important molecule which itself undergoes redox reaction during physiological processes in human body. But the GNPs often prepared by this method are not stable enough. In order to stabilize these particles, surfactants are used which may or may not be compatible for in-vivo applications. Is there any other way to stabilize these particles in solution? In this work, the answer to this question is explored and a detailed study of the mechanism of the formation of GNPs is done to understand the basis of stabilization of nanoparticles without using a stabilizer. Chloroauric acid is reduced by L-tryptophan (Trp) in buffered medium and the formation mechanism is studied both visually and by UV-vis spectroscopy. The pH dependent structure of Trp was found to play an important role not only in the formation of stable GNPs but also in the stabilization of redispersed nanoparticles. pH sensitive property of the synthesized GNPs was utilized to make the GNPs accumulate at polar-non-polar liquid-liquid interface similar to hypoxic tumor tissue environment. Mechanistic study of the formation of GNPs by gas chromatography throws light on intermediate decarboxylation process. On the other hand, fluorescence study gives information about the interaction of Trp with GNPs.