Conformational Selectivity of Merocyanine on Nanostructured Silver Films: Surface Enhanced Resonance Raman Scattering (SERRS) and Density Functional Theoretical (DFT) Study

Abstract

In this study, detailed structural and vibrational analysis of merocyanine has been investigated using Raman, surface enhanced Raman scattering (SERS) and surface-enhanced resonance Raman scattering (SERRS). The Raman, SERS and SERRS studies aided by density functional theoretical (DFT) calculations clearly established the prevalence of the trans- and cis-conformers of the protonated form of merocyanine (MCH+) in solid and acetonitrile solution. The binding characteristics of merocyanine adsorbed on nanostructured silver-coated films (SCFs) were investigated using excitation-dependent SERS, concentration-dependent SERRS and DFT studies. The conformers of merocyanine involved in the surface adsorption processes were recognized. The prominent marker bands observed at 1538 (ethylenic C=C stretch) and 1133 cm−1 (pyridinium C-N stretch) in the Raman spectrum of merocyanine in acetonitrile shifted to 1540 and 1126 cm−1, respectively on the nanostructured SCFs. The shift in the marker bands is associated with either the preferential binding of selective conformer or change in resonance equilibrium between the benzenoid and quinoid forms. The excitation wavelength dependent SERS spectrum infers that in addition to the major contribution from the electromagnetic enhancement, chemical (resonance) effect leads to the amplification of the 1540 cm−1 band. The concentration-dependent SERRS study showed maximum enhancement for the nanostructured SCFs functionalized with 1 μM concentration of merocyanine, indicative of monolayer coverage. For lower concentrations of merocyanine, the SERRS signal intensity reduced without any alteration in the peak positions. The SERRS study thus, revealed sub-nanomolar (0.1 nM) sensing of merocyanine using nanostructured SCFs with the analytical enhancement factor (AEF) of ∼ 1010 for the 1126 cm−1 and 1540 cm−1 Raman bands for MC concentration of 0.1 nM. In this study, combination of SERRS and DFT have clearly established the predominance of trans-MCH+ on the nanostructured silver surface with minor contribution from cis-MCH+, which remain exclusively bound to the surface via the phenoxyl ring O atom. This conformational surface selectivity of geometrical isomers of merocyanine using nanostructured surfaces can be further explored for energy efficient and economical separation of geometrical isomers.

Insights into Plasmon-Induced Dimerization of Rhodanine–A Surface-Enhanced Raman Scattering Study

“…The UV–visible absorption spectra of the MNPs (AgNP and AuNP) and Rd functionalized MNPs are shown in Figure . The LSPR band, also referred as the bulk-like surface plasmon resonance (BL-SPR) band − of AgNP [Figure A­(a)], shows a single sharp molar extinction maximum at 395 nm. On functionalization of AgNP with Rd, a new band appears at 590 nm in the absorption spectrum [Figure A­(b)] along with decrease in the absorbance of the 395 nm band.…”

“…On functionalization of AgNP with Rd, a new band appears at 590 nm in the absorption spectrum [Figure A­(b)] along with decrease in the absorbance of the 395 nm band. The appearance of the red-shifted band at 590 nm is attributed to the surface-like surface plasmon resonance (SL-SPR) band, − observed due to the aggregation of AgNPs induced by the adsorbate. Aggregation of AgNPs in the presence of the adsorbate has been reported in the literature. , Similar to AgNPs, in the case of AuNPs, the BL-SPR band is observed at 520 nm [Figure B­(a)] and the SL-SPR band is observed at 665 nm [Figure B­(b)] for the Rd functionalized AuNP.…”

Qualitative and quantitative detection and identification of two benzodiazepines based on SERS and convolutional neural network technology

“…[9][10][11] The peak at each Raman frequency shift position in the SERS spectrum corresponds to a vibrational or rotational mode of a molecule. 12,13 Therefore, different Raman peaks jointly represent the structural characteristics of the molecule, which is known as the "fingerprint characteristics" of the Raman spectrum. 14,15 In some previous works, selecting the stronger peak in the SERS spectrum of a substance as the analysis object has become an analytical strategy for SERS trace detection.…”