Reversible assembly of metal nanoparticles induced by penicillamine. Dynamic formation of SERS hot spots

We report a systematic study of the surface modification of gold and silver nanoparticles with DL-penicillamine (PEN) and N-acetyl-DL-penicillamine (NAP), motivated by the possibility of inducing pH-controlled reversible nanoparticle assembly. The interaction of PEN and NAP with the metal nanoparticle surface was studied by isothermal titration calorimetry (ITC). The results indicate that equilibrium is reached with the formation of a submonolayer corresponding to ca. 40% and 64% of total surface coverage for PEN and NAP, respectively. Both PEN and NAP modified nanoparticles could be reversibly aggregated at acidic pH due to the protonation of the carboxylic groups, leading to a decrease in their stability by electrostatic interactions and the advent of hydrogen bonding interactions which promote interparticle linkage. The process was monitored by UV-Vis spectroscopy, transmission electron microscopy (TEM) and surface enhanced Raman scattering (SERS) spectroscopy. Interestingly, the SERS characterization demonstrated the pH-controlled formation of hot-spots.