DataSheet1_Conformational Selectivity of Merocyanine on Nanostructured Silver Films: Surface Enhanced Resonance Raman Scattering (SERRS) and Density Functional Theoretical (DFT) Study.PDF

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.

在本研究中，采用拉曼光谱、表面增强拉曼散射（SERS）以及表面增强共振拉曼散射（SERRS）技术，对亚甲氰基的结构和振动特性进行了深入分析。借助密度泛函理论（DFT）计算，拉曼、SERS和SERRS研究明确揭示了亚甲氰基质子化形式（MCH+）在固体和乙腈溶液中的顺反异构体普遍存在。通过激发依赖性SERS、浓度依赖性SERRS以及DFT研究，探讨了吸附在纳米结构银涂层薄膜（SCFs）上的亚甲氰基的键合特性。识别了参与表面吸附过程的亚甲氰基异构体。在亚甲氰基乙腈溶液拉曼光谱中观察到的位于1538 cm−1（乙烯基C=C伸缩）和1133 cm−1（吡啶基C-N伸缩）的显著标记带，在纳米结构SCFs上分别红移至1540和1126 cm−1。标记带的位移与选择性异构体的优先结合或苯环和醌环形式的共振平衡变化有关。激发波长依赖性SERS光谱表明，除了电磁增强的主要贡献外，化学（共振）效应导致1540 cm−1带的增强。浓度依赖性SERRS研究表明，当使用1 μM浓度的亚甲氰基功能化的纳米结构SCFs时，表现出最大增强，表明单分子层覆盖。对于较低的亚甲氰基浓度，SERRS信号强度降低，而峰值位置未发生任何变化。因此，SERRS研究揭示了使用纳米结构SCFs对亚甲氰基进行亚纳摩尔（0.1 nM）传感的能力，对于MC浓度为0.1 nM的1126 cm−1和1540 cm−1拉曼带，其分析增强因子（AEF）约为1010。在本研究中，SERRS与DFT的结合明确证实了在纳米结构银表面，顺-MCH+占主导地位，而顺-MCH+则通过苯氧基环氧原子与表面进行专一性结合，贡献较小。利用纳米结构表面进行亚甲氰基几何异构体的构象表面选择性，可以进一步探索其在能源高效和经济分离几何异构体方面的应用。