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Herein, we employ Ag@TiO2 core-shell nanoparticles for surface-enhanced Raman scattering (SERS) investigations of TiO2-N719 dye interfaces. In situ electrochemical SERS investigations of the Ag@TiO2-N719 interaction are systematically carried out under a series of electrode-potential controls. By comparing the potential dependence of resonant and pre-resonant SERS spectra recorded with different laser excitations, bidentate carboxylate linkage is considered to be involved in N719 adsorption on TiO2. Meanwhile, SCN ligand shows obvious interactions with TiO2, and their role in the adsorption and orientation of N719 on TiO2 should not be underestimated. The in situ SERS spectra of Ag@TiO2 show a clear bell-shaped intensity-potential relation for the major bands of N719. A molecule-to-TiO2 charge-transfer resonance is tentatively attributed to account for such a phenomenon. Under the influence of such a charge-transfer resonance, valuable information about the N719-TiO2 interaction as well as the intramolecular deformation of N719 is obtained.

In this paper, we present a nanoparticle-enhanced Raman spectroscopic study of TiO 2 -N719 (bis(tetrabutylammonium) [cis-di(thiocyanato)-bis (2, 2-bipyridyl-4-carboxylate-4-carboxylic acid)-ruthenium(II)])) interface by employing Ag 2 @TiO 2 dimeric core-shell nanoparticles, where the TiO 2 shell serves as an active semiconductor surface. The Ag 2 @TiO 2 nanoparticles are synthesized by connecting two Ag nanoparticles with thiol-modified complementary DNAs, followed by coating a 2 nm thin layer of TiO 2 on the Ag 2 dimeric nanoparticles. The Ag 2 @TiO 2 dimeric core-shell nanoparticles show red-shift of plasmon resonance frequency compared with Ag@TiO 2 core-shell nanoparticles, and provide significant SERS enhancement over the Ag@TiO 2 nanoparticles, which allows for the investigation of SERS of TiO 2 -N719 interface under off-resonance condition using low energy 785 nm laser excitation. Fine potential dependent Raman measurements show that thiocyano and carboxyl groups of the N719 molecule can adsorb on TiO 2 shell competitively: Along with negative potential excursion, the thiocyano is replaced by carboxyl, which is accompanied by conformation change of the molecule involving orientation change of the bipyridine ring. Furthermore, the potential of maximum intensity of the inter-ring vibration band shifts towards more positive value with laser excitation wavelength from 638 to 785 nm, which verifies the photon-driven charge transfer mechanism from the HOMO of N719 to the conduction bands of TiO 2 . Three-dimensional finite-difference time-domain (3D-FDTD) simulations are performed to evaluate the electromagnetic (EM) enhancement from the Ag 2 @TiO 2 dimeric core-shell nanoparticles. The present work demonstrates that the Ag 2 @TiO 2 dimeric core-shell structures provide significant plasmon resonance, which can be used for Raman investigations of interfaces formed with the semiconductor shell layers of the nanoparticles.

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