Aromatic Amino Acid Monolayers Sandwiched between Gold and Silver: A Combined Tip-Enhanced Raman and Theoretical Approach

Tip-enhanced Raman scattering (TERS) measurements and theoretical calculations of the aromatic amino acids phenylalanine, tyrosine, and tryptophan sandwiched between gold and silver layers are presented and discussed. Atomically flat micrometer-sized gold nanoplates were chosen as substrates and covered with a monolayer of the respective amino acid. On the basis of spectral data, the carboxylate and amino groups were determined to be the preferential moieties attached to the metals in the cavity. The experimental observation of benzene rings oriented parallel to the gold surface and the influence of the silver tip are discussed and compared with theoretical calculations. Finally, it is shown that TERS allows a clear distinction between phenylalanine and tryptophan moieties from a mixture on the substrates; hence, demonstrating the selectivity of the method on the nanometer scale without further need of any labeling techniques.