Surface-enhanced Raman Scattering Mechanism Research Articles

Mechanism of Surface-Enhanced Raman Scattering Based on 3D Graphene-TiO2 Nanocomposites and Application to Real-Time Monitoring of Telomerase Activity in Differentiation of Stem Cells.

With a burst development of new nanomaterials for plasmon-free surface-enhanced Raman scattering (SERS), the understanding of chemical mechanism (CM) and further applications have become more and more attractive. Herein, a novel SERS platform was specially designed through electrochemical deposition of graphene onto TiO2 nanoarrays (EG-TiO2). The developed EG-TiO2 nanocomposite SERS platform possessed remarkable Raman activity using copper phthalocyanine (CuPc) as a probe molecule. X-ray photoelectron spectroscopy measurement revealed that the chemical bond Ti-O-C was formed at the interface between graphene and TiO2 in EG-TiO2 nanocomposites. Both experimental and theoretical results demonstrated that the obvious Raman enhancement was attributed to TiO2-induced Fermi level shift of graphene, resulting in effective charge transfer between EG-TiO2 nanocomposites and molecules. Taking advantage of a marked Raman response of the CuPc molecule on the EG-TiO2 nanocomposite surface as well as specific recognition of CuPc toward multiple telomeric G-quadruplex, EG-TiO2 nanocomposites were tactfully employed as the SERS substrate for selective and ultrasensitive determination of telomerase activity, with a low detection limit down to 2.07 × 10-16 IU. Interestingly, the self-cleaning characteristic of EG-TiO2 nanocomposites under visible light irradiation successfully provided a recycling ability for this plasmon-free EG-TiO2 substrate. The present SERS biosensor with high analytical performance, such as high selectivity and sensitivity, has been further explored to determine telomerase activity in stem cells as well as to count the cell numbers. More importantly, using this useful tool, it was discovered that telomerase activity plays an important role in the proliferation and differentiation from human mesenchymal stem cells to neural stem cells. This work has not only established an approach for gaining fundamental insights into the chemical mechanism (CM) of Raman enhancement but also has opened a new way in the investigation of long-term dynamics of stem cell differentiation and clinical drug screening.

Metal–Semiconductor Contacts Induce the Charge-Transfer Mechanism of Surface-Enhanced Raman Scattering

A model metal–semiconductor–molecule–metal assembly has been designed for probing the charge-transfer (CT) mechanism of surface-enhanced Raman scattering (SERS). We measured the SERS of ZnO–PATP–Ag, Au–ZnO–PATP–Ag, and Cu–ZnO–PATP–Ag assemblies at excitation wavelengths of 514.5, 785, and 1064 nm. Our results demonstrate that the metal–semiconductor contact can alter the charge distribution through p-aminothiophenol (PATP) molecules. This is attributed to the chemical SERS enhancement mechanism with additional electrical transport properties within these assemblies. These inhibit the CT from the metal to the molecule, resulting in the different degrees to which CT contributes to the overall SERS enhancement of PATP.

Selection Rules of the Charge Transfer Mechanism of Surface-Enhanced Raman Scattering: The Effect of the Adsorption on the Relative Intensities of Pyrimidine Bonded to Silver Nanoclusters

Surface-enhanced Raman spectra (SERS) of pyrimidine recorded on a silver electrode have been analyzed on the basis of a resonant Raman (RR) process involving photoexcited charge transfer (CT) states of the metal-adsorbate surface complex. The main feature of the SERS of benzene and azine derivatives is the enhancement of the totally symmetric ring stretching mode 8a due to Franck-Condon contributions related to the CT transition. Although this behavior is observed in the SERS of pyrimidine, its spectrum is also characterized by the strong enhancement of the nontotally symmetric mode 8b. This peculiar feature can be explained only by the redistribution of the Franck-Condon factors between the 8ab pair of vibrations originated by the descent in symmetry occurring when pyrimidine is bonded to silver nanoclusters. This conclusion is a new evidence of the main role of the RR-CT enhancement mechanism in the SERS of aromatic molecules and shows once again the usefulness of the methodology developed by our group in order to analyze these complex spectra.

Role of the Electrode Potential in the Charge-Transfer Mechanism of Surface-Enhanced Raman Scattering

A general model concerning the role of the electrode potential on the charge-transfer enhancement mechanism of SERS is proposed, justifying the existence of a parameter ‚ which reduces the effectiveness of the electrode potential in order to shift the excited charge-transfer level. This explains the observed deviation of ‚ from the unity, and allows for relating the intensity/electrode-potential (SERS -CT) profiles of similar adsorbates with the respective electron affinities. The SERS of pyridine, pyrazine and methylpyrazines have been recorded on silver, being found that the electrode potential in the maximum of the SERS-CT profiles shifts toward more negative values as the number of methylsubstituents attached to the pyrazine ring increases. This result has been explained on the basis of the relative stability of the radical anions given that a correlation between those shifts and the calculated ab initio vertical transition energies between the neutral molecules and their respective radical anions can be established. This confirms the participation of a resonant charge-transfer mechanism in the SERS as well as the usefulness of this technique to gain insight into the electronic properties of the doublet states of the here studied molecules.