Surface-Enhanced Raman Scattering (SERS) Detection of Low Concentrations of Tryptophan Amino Acid in Silver Colloid

Abstract

The surface-enhanced Raman scattering (SERS) spectrum of L-tryptophan has been studied in the concentration range 1.4 × 10(-8) to 5 × 10(-4) M. A borohydride-reduced silver colloid was employed as the nanoparticle enhancing agent and different electrolytes have been tested for activation of the colloid. The optimum conditions have been determined for achieving high sensitivity of detection. The experimental procedure developed, which includes the use of a composite electrolyte (a mixture of NaHCO(3) and NaCl) for colloid activation, results in very high enhancement of the Raman signal (up to 10(8)). This gives the possibility of studying SERS spectra of L-tryptophan at concentrations as low as 10(-8) M, which is several orders of magnitude lower than previously reported in the literature. The observed SERS spectra were very reproducible and were detectable 2 minutes after mixing, reaching maximum strength approximately 15 minutes after mixing. The spectral characteristics were stable over the entire period of observation. We have found that SERS spectra of tryptophan in silver colloid differ in several features at low (below ∼10(-5) M) and at high (above ∼10(-4) M) concentrations. The most important difference is the absence of the peak near 1000 cm(-1) at low concentrations, which is usually assigned to the indole ring breathing mode. The observed spectra allow us to suggest that at low concentrations Trp molecules bind to the surface through the indole ring, which remains flat on the surface. This is in contrast to the previously reported observation of SERS spectra from Trp performed at concentration levels above 10(-5) M.

SERS Microspectroscopy of Biomolecules on Dried Ag Colloidal Drops

“…An example is spectrum (b) in Figure 2. We have to note that our SERS spectra of tryptophan are completely different than that obtained by Aliaga et al [10] from the same SERS system (dried hydroxylamine-reduced Ag colloid/tryptophan drop) but in good agreement with Raman spectra measured from Ag colloidal solutions [11,12] and solid powder [12]. In Figure 2, spectrum (b), only spectral bands that can be surely assigned to tryptophan are marked [12].…”

“…SERS spectra of tryptophan obtained using Ag colloids have been reported in the literature recently [10][11][12], but the results significantly differ from each other. Such discrepancy can be explained by particular experimental conditions (pH, used Ag colloid and its activation, mode of adsorption, and consequently orientation of tryptophan on the surface, etc.).…”

Towards improved quantitative analysis using surface-enhanced Raman scattering incorporating internal isotope labelling

“…These spectra show successful binding of Trp and Trp-d5 to the surface and are similar to other previous studies that investigated unlabelled Trp. 25,52 It is clear that some of the bands are shi ed to lower wavenumbers due to the presence of the heavier isotopes of Trp-d5. Peaks at 1122, 1228 and 1600 cm 1 , assigned to phenyl CH bending vibration, phenol ring breathing and indole ring According to Fig.…”

“…By contrast, recent studies have demonstrated the application of SERS for the detection and quanti cation of Trp and caff eine with high levels of sensitivity and reproducibility. 25,26 Alharbi et al concluded that SERS can be used to identify long-term abuse of illicit materials as well as determine accurate drug dosing for legal therapeutics. Furthermore, Chen and colleagues reported the interaction of caff eine molecules with diff erent colloids at various concen-trations.…”

Use of a micro- to nanochannel for the characterization of surface-enhanced Raman spectroscopy signals from unique functionalized nanoparticles

“…[12][13][14][15][16][17] Silver and gold colloids have also been analyzed using different sizes, preparation methods, and concentrations of analyte or reporter molecules attached to the nanoparticle. [18][19][20][21][22] To increase the Raman scattering event probability further, various mechanical methods have been used to make the nanoparticles aggregate together, which causes strong enhancements to occur. 23 One mechanical aggregation method was that of Hwang et al, in which an optoelectrofluidic device was used to create plasmonic aggregates aligned at the laser beam spot and produce strong enhancements.…”