We report the significant effect of intermolecular hydrogen bonding (H-bonding) on surface-enhanced Raman scattering (SERS) spectra in which the vibrational frequencies and intensities of some characteristic peaks of p-mercaptobenzoic acid (MBA) change with varying concentrations of aniline. These changes can be attributed to modifications in the electronic structure of the MBA molecule and the conjugation of the system under the influence of H-bonding. Of remarkable note is that the nontotally symmetric (b 2 ) mode of MBA is dramatically enhanced, which can be considered as a manifestation of the charge-transfer (CT) transition process in the system. By comparing SERS spectra obtained under normal and basic conditions, the effect caused by H-bonding can be further understood. These results manifest that the CT resonance between MBA and Ag NPs through Herzberg−Teller contributions can be promoted by H-bonding. The current work may, therefore, be instructive for studying the influence of H-bonding on the electronic structure of molecules in a system via SERS technique.

Efficient and generic enantioselective discrimination of various chiral alcohols is achieved by using surface-enhanced Raman scattering (SERS) spectroscopy through charge-transfer (CT) contributions. The relative intensities of the peaks in the SERS spectra of a chiral selector are strongly dependent on the chirality of its surroundings. This highly distinct spectral discrepancy may be due to the tendency of chiral isomers to form intermolecular hydrogen-bonding complexes with the chiral selector in different molecular orientations, resulting in different CT states and SERS intensities of the adsorbates in the system. This study opens a new avenue leading to the development of novel enantiosensing strategies. A particular advantage of this approach is that it is label-free and does not employ any chiral reagents, including chiral light.

With the explosive development of analysis and detection techniques based on surface-enhanced Raman scattering (SERS), the further understanding and exploitation of the chemical mechanism becomes particularly important. We investigated the charge transfer (CT) effect on SERS in a semiconductor−molecule−metal system constructed with Ag NPs, 4-mercaptobenzoic acid (MBA) molecule, and atomic level TiO 2 . To ensure more ordering, the system is constructed by a layer-by-layer self-assembly method. After introducing TiO 2 , we found that the relative band intensity of some peaks displayed a distinct difference, which is attributed to the Herzberg−Teller contribution that occurs via CT. We also proposed a possible mechanism responsible for the selective enhancement observed in the SERS spectra of the Ag NPs/MBA/ TiO 2 system. This work will not only provide much deeper insight into the CT mechanism in SERS but also help in the development of a method to construct metal−semiconductor-based SERS substrates. ■ INTRODUCTIONSince it was discovered on a rough silver electrode in 1974, the surface-enhanced Raman scattering (SERS) technique has gotten more and more attention because of its high sensitivity, high selectivity, and nondestructive trace detection. 1 Nowadays, SERS is being widely applied to many areas, such as ultrasensitive detection, biological analysis, biomedical application, and medical diagnosis. 2−5 As is known to all, there are two primary mechanisms that account for SERS: electromagnetic mechanism (EM) and chemical enhancement mechanism (CM). 6−8 EM requires the coupling of metallic nanoparticles and incident radiation, 9 whereas CM contains a charge transfer (CT) process between substrate and absorbate. 10−13 When molecules are adsorbed on the metal substrate, the Fermi energy level of metal nanoparticles and the HOMO and LUMO energy levels of adsorbed molecules interact with each other and shift. If the energy of incident laser matches with charge transfer transition energy, resonance electron transition occurs between the Fermi level of the substrate and the molecular orbital of the adsorbate. Then the molecular polarization can be changed, which produces the SERS effect. The interaction between molecules and substrate forms a new excited charge transfer state. 14,15 Both mechanisms contribute to the Raman enhancement. EM contributes more to the enhancement of SERS signal; however, CT also plays an important role. Many researchers have investigated CT in SERS with various materials. For most semiconductor materials, the surface plasmon resonant frequency is located in the infrared region. Thus, when semiconductor materials are applied as SERS substrate, the CT mechanism can be the dominant contribution to the surface-enhanced Raman signal on semiconductor substrate, which provides an extensive space for studying CT mechanism.As a wide-band gap semiconductor, TiO 2 is increasingly concerned in the SERS field nowadays. TiO 2 as an active substrate widens the application field for SERS in various areas,...

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Efficient and generic enantioselective discrimination of various chiral alcohols is achieved by using surfaceenhanced Raman scattering (SERS) spectroscopy through charge-transfer (CT) contributions. The relative intensities of the peaks in the SERS spectra of a chiral selector are strongly dependent on the chirality of its surroundings. This highly distinct spectral discrepancy may be due to the tendency of chiral isomers to form intermolecular hydrogen-bonding complexes with the chiral selector in different molecular orientations, resulting in different CT states and SERS intensities of the adsorbates in the system. This study opens a new avenue leading to the development of novel enantiosensing strategies. A particular advantage of this approach is that it is label-free and does not employ any chiral reagents, including chiral light.