L-Tryptophan Adsorbed on Au and Ag Nanostructured Substrates: A SERS Study

The objective of this study was to determine the most stable conformation of L-tryptophan (L-Tryp) on gold and silver nanoparticles. In addition, this work explored how these parameters were affected by analyte concentration, nanoparticle size, and pH. The purpose was to establish whether L-Tryp molecules interact with the nanoparticles through the carboxylate end, the amino group end, or both. This research has brought diverse applications in biophysics and medical diagnostics, potentially opening new avenues in these fields. Moreover, it may enrich the disciplines of chemistry and nanotechnology by offering innovative approaches for future research. These findings represent a significant advancement in understanding the interactions between L-Tryp and nanoparticles, making a meaningful contribution to biophysics and medical diagnostics. Surface-Enhanced Raman Scattering (SERS) spectra of L-Tryp in the 200–3500 cm⁻¹ spectral range were obtained using a 785 nm laser for excitation. Gold and silver nanoparticles were synthesized using the citrate reduction method. The experimental procedure involved the use of electrolytes (such as NaCl) for colloid activation, which resulted in very high SERS signals. Modification of nanoparticle surface charge was achieved by adjusting the pH of Au and Ag colloidal suspensions between 2 and 11. The SERS spectra indicate that small-sized nanoparticles require high concentrations of L-Tryp to achieve high sensitivity, whereas larger nanoparticles perform effectively at lower concentrations. The pronounced enhancement of stretching vibrations in the COO⁻ group in the SERS spectra strongly suggests that the carboxylate group attaches to silver nanoparticles (AgNPs). Conversely, for gold nanoparticle (AuNP), a new band at approximately 2136 cm⁻¹ was observed, indicating that the amino group of L-Tryp interacts with Au in its neutral form. These analyses were complemented with theoretical modeling, employing the Density Functional Theory (DFT) running under Gaussian™ to study molecular models in which L-Tryp interacted with the AgNPs and AuNPs substrates in neutral, cationic, and anionic forms.