Elucidation of HEPES Affinity to and Structure on Gold Nanostars

The zwitterion, N-2-hydroxyethylpiperazine- N'-2-ethanesulfonic acid (HEPES), facilitates the formation and stability of gold nanostars; however, little is known about how this molecule interacts with the metal postsynthesis. Herein, restructuring of gold nanostar morphology is induced upon acidification, an effect that depends on both pH and acid composition as well as on the protonation state of HEPES. Changes in molecular protonation are measured using zeta potential and modeled using DFT. The surface-sensitive technique, surface-enhanced Raman scattering (SERS), reveals that pH variations induce reversible activation of the amine and sulfonate groups in HEPES and that electron redistribution weakens its affinity to the metal thus promoting the adsorption and SERS detection of benzene. By selecting a molecule that does not induce significant desorption of the stabilizing agent, binding energies of benzene to gold are measured even though only weak London dispersion and π-π interactions promote adsorption. All in all, this molecular-level insight is expected to facilitate new applications of these nanostructures in ways that have not been possible to date.