Reversible Aggregation of Au Nanoparticles Controlled by Excited State Proton Transfer and Excited State Hydroxide 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 local concentration of protons due to their effect on the protonation state of the carboxyl groups, and accordingly the electrostatic interactions between the NPs. We can achieve the reversibility of this aggregation process by controlling both proton capture as well as proton release by the Gold NPs. It has been shown that this can be achieved by a photoisomerization process. In our study, we use excited state proton transfer (ESPT) and excited state hydroxide transfer (ESHT) mechanisms for the remote protonation or deprotonation of the carboxyl groups on the Gold NPs surface. ESPT and ESHT 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, we present the use of photoacids and photobases to control the concentration of protons in the solution. We focus on 8-Hydroxypyrene-1,3,6-trisulfonic acid and Malachite Green Carbinol base, while using gold NPs of 4 nm in diameter. We use UV-Vis spectroscopy to follow proton or hydroxide release and DLS to follow the reversible aggregation of Gold NPs, while exciting the sample in the UV by an external LED source.