Abstract
A Surface-Enhanced Raman Scattering (SERS) spectrum of 4-cyanopyridine (4CNPy) was recorded on silver plasmonic nanoparticles and analyzed by using Density Functional Theory (DFT) calculations. Two simple molecular models of the metal–4CNPy surface complex with a single silver cation or with a neutral dimer (Ag+–4CNPy, Ag2–4CNPy), linked through the two possible interacting sites of 4CNPy (aromatic nitrogen, N, and nitrile group, CN), were considered. The calculated vibrational wavenumbers and intensities of the adsorbate and the isolated species are compared with the experimental Raman and SERS results. The analysis of the DFT predictions and the experimental data indicates that 4CNPy adsorbs preferentially on neutral/charged active sites of the silver nanoparticles through the nitrogen atom of the aromatic ring with a perpendicular orientation.
Highlights
IntroductionSurface-Enhanced Raman Scattering (SERS) is becoming a versatile spectroscopic technique in different scientific fields, such as analytical chemistry, for the detection of analytes/contaminants [1,2] at trace levels, even reaching single-molecule detection [3], biochemistry and forensic medicine, [4,5,6] for the detection of RNA by the recognition of adenine and cytosine using silver nanoparticles, pharmaceutical medicine [7], in the study of new target drugs, and other applications in restoration and artistic heritage [8], archeology [8], food [9,10], etc.; that is, most SERS studies deal with the molecular recognition of vibrational fingerprints of molecules at low concentration
The relative intensity of these four bands is slightly different in these two spectra, recorded at 991 cm−1, the strongest band in the Raman of the solid, while the 2250 cm−1 line dominates the spectrum of the aqueous solution
Surface-Enhanced Raman Scattering (SERS) of 4CNPy shown in Figure 1 corresponds to a monomer species, as expected for this low-concentration spectra [14], given that the dimer or aggregated species are characterized by Raman bands at 1520, 1261, and 970 cm−1, Nwanhoimcahteariraels m201is9s, i9n, 1g21in1 our records
Summary
Surface-Enhanced Raman Scattering (SERS) is becoming a versatile spectroscopic technique in different scientific fields, such as analytical chemistry, for the detection of analytes/contaminants [1,2] at trace levels, even reaching single-molecule detection [3], biochemistry and forensic medicine, [4,5,6] for the detection of RNA by the recognition of adenine and cytosine using silver nanoparticles, pharmaceutical medicine [7], in the study of new target drugs, and other applications in restoration and artistic heritage [8], archeology [8], food [9,10], etc.; that is, most SERS studies deal with the molecular recognition of vibrational fingerprints of molecules at low concentration. Electrochemical SERS experiments of 4CNPy on silver electrodes have been extensively studied [14,15,16,17] under different experimental conditions varying the concentration, the electrode potential, or the type of solvent or electrolyte, because different pyridine-like species can be adsorbed on the surface due to the reduction process of the nitrile group. This yields mixed-up SERS spectra in which appear new bands different to those of the Raman of solid 4CNPy or the aqueous solution, for example at 625, 1580, and 2103 cm−1 [16]. Even these spectra are simpler than those recorded on electrode, and the reduction of the nitrile group shows metal selectivity being observed only on copper surfaces [19]
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