Binding of Glutamic Acid to Silver and Gold Nanoparticles Investigated by Surface-Enhanced Raman Spectroscopy

Glutamate is the most important excitatory neurotransmitter, which is relevant for the study of several diseases such as amyotrophic lateral sclerosis and Alzheimer. It is the form L-glutamic acid (Glu) takes at physiologically relevant pHs. The surface-enhanced Raman spectra of Glu obtained at pH values ranging from 3.3 to 12 are collected in the presence of silver and gold colloids and on solid substrates. The observed bands are compared with the positions of calculated normal modes for free neutral glutamic acid, glutamic acid monohydrate, glutamic acid bound to gold and silver atoms, and sodium glutamate. Although gold atoms prefer to bind to the NH₂ group as compared to carbonyl groups, silver atoms prefer binding to hydroxyl groups more than binding to the amino group. SERS spectra of glutamic acid solutions with a pH value of 12, in which both carboxylic groups are deprotonated, indicate a complexation of the glutamic acid dianion with the sodium cation, which was introduced into the solution to adjust the pH value. Further research towards an optimal substrate is needed.