Reversible Aggregation of Au Nanoparticles Controlled by Excited State Proton Transfer Processes

The ability to control reversible aggregation of Gold nanoparticles (NPs) is important for various biomedical applications. Aggregation of Gold NPs functionalized with carboxyl groups can be tuned by concentration of protons due to the effect of pH change on the protonation state of the carboxyl groups. Reversibility of the pH-controlled aggregation requires external stimulus to affect pH of the solution. It has been shown that this can be achieved by a photoisomerization process. In our study, we use excited state proton transfer (ESPT) mechanism for the remote induction of pH drop. ESPT can take place upon the excitation of photoacids and photobases, aromatic molecules that display properties of weak acid/base in their ground electronic state, but exhibit great pKa drop or increase, respectively, in their excited state. Herein, I present the use of photoacids and photobases in organic or aqueous solutions to control the transient pH of the solution. We focus mainly on naphthol-based photoacids and quinoline-based photobases, while using gold NPs of 3±0.5 nm in diameter. We use UV-Vis spectroscopy to follow the plasmon peak of the gold NPs and their aggregation, while exciting the sample in the UV by an external LED source. One of our future directions is to exploit excited state hydroxide transfer (ESHT) properties of a group of hydroxide-containing aromatic molecules that we synthesize in our research group.