Plasmonically Generated Tryptophan Radical Anion on Gold Nanoparticles Investigated by Combined Surface-Enhanced Raman Scattering and Density Functional Theory Calculations

Abstract

Surface-enhanced Raman scattering (SERS) offers increases in chemical sensitivity associated with the Raman signal from molecules interacting with plasmonic nanoparticles and with use for diverse applications. This signal enhancement is the result of a combination of the enhanced electromagnetic field generated at the surface of the metal nanostructures and possible chemical enhancements specific to the molecule being detected. These chemical effects can alter the chemical identity of the analyte and manifest as differences in the SERS spectrum relative to the spontaneous Raman spectrum. In this work, we examine changes in the vibrational spectrum resulting from the previously hypothesized formation of a radical anion of the amino acid tryptophan on gold (Au) nanoparticles. Density functional theory calculations are used to model changes in the vibrational frequencies attendant to the tryptophan radical anion and correlated with the experimentally observed vibrational modes in the SERS spectrum of tryptophan (Trp) on Au nanoparticles. The calculated vibrational frequencies are in close agreement with the experimental data. N-Acetyltryptophanamide (NATA) is shown to have the same trends in the calculated and experimental vibrations, indicating that the captured electron is localized to the indole ring structure. Changes in the SERS spectrum with pH are readily explained by the calculations, further indicating that the observed signal originates from the ground-state radical anion in these experiments. This evidence for the capture of a hot electron by tryptophan and the resulting changes in the vibrational spectrum are important for molecular understanding of proteins and other molecules as detected by SERS.

Investigation of SERS Frequency Fluctuations Relevant to Sensing and Catalysis

“…In Figure S9, we have identified spectral examples of fluctuation data that correlate to previously simulated Raman spectra of radical cation and radical anion formation of MBA . Other reports of radical formation through plasmonic hot-carrier formation support the possibility of this type of hot-carrier species. − In addition to radical formation, other transient species can be identified as products of photodamage from hot carriers. The formation of thiophenol from the plasmonically induced conversion of MBA has been reported to result in the formation of peaks at 998, 1022, and 1572 cm –1 , which are features we also observe with increasing laser power .…”

“…Spectral fluctuations can alter the observed frequencies and intensities through transient interactions with hot carriers. , Previous SERS reports of bovine serum albumin (BSA) with gold nanoparticles have acknowledged spectral intensity and frequency fluctuations, which they attribute to varying conformations of BSA on the nanoparticles that result in different amino acid residues being enhanced . However, our group, as well as others, have previously identified the formation of radical anion species resulting from hot-carrier transfer to analytes of interest, including tryptophan and 4–4′-bipyridine molecules, that result in temporal spectral and intensity fluctuations. − Density functional theory (DFT) calculations are commonly used in support of the formation of radical species to simulate the Raman spectrum of the observed intermediates. − TERS experiments also report similar temporal signal fluctuations resulting from transient species chemically formed from hot carriers. Previous reports have monitored a hot carrier induced transformation of 4-nitrobenzene thiol (4-NBT) conversion to 4-NBT thiolate, which resulted in temporal fluctuations in the TERS spectra .…”

Characterization of three different benzimidazolium ligands and their organo-selenium complexes by using density functional theory and Raman spectroscopy

“… 19 ) and GaussView 06. 20 DFT measurements are based at B3LYP (Becke's three parameter exchange functional B3 combined with LeeYang–Parr correlation functional LYP). 21 …”

Surface Enhanced Raman Spectroscopy Combined With Multivariate Analysis for Fingerprinting Clinically Similar, Fibromyalgia and Long-COVID Syndromes

“…Gold nanoparticles (AuNPs) were synthesized using the citrate reduction method [27,38]. Prior to use, glassware and stir bars were immersed in a sulfuric acid solution of Alnochromix TM (Alconox laboratories) overnight and then rinsed with boiling ultrapure water.…”