Interplay of plasmonic and charge transfer effects for ultrasensitive Ag–WO3/TiO2 photonic crystal SERS sensors†

The utilization of hybrid plasmonic metal/semiconductor materials for surface-enhanced Raman scattering (SERS) has emerged as a promising approach towards the development of advanced SERS substrates in terms of sensitivity, uniformity, stability, and reusability, based on the synergy of the powerful electromagnetic mechanism with the chemical amplification and functionality of semiconductor supports. In this work, co-assembled WO₃/TiO₂ inverse opal films were utilized as photonic crystal scaffolds of plasmonic Ag nanoparticles in order to optimally combine plasmonic, charge transfer and slow photon effects for ultrasensitive, recyclable SERS sensing. Compositional and photonic band gap engineering of the Ag-decorated WO₃/TiO₂ photonic crystal substrates provided insight to the interplay of plasmonic enhancement assisted by slow light propagation in the inverse opal structure and charge transfer between the analyte and the heterostructured substrate. Highly sensitive detection of 4-mercaptobenzoic acid as a non-resonant analyte was achieved down to 10⁻¹³ M for the optimal Ag–WO₃/TiO₂ substrate with good uniformity and excellent recyclability due to its enhanced photocatalytic self-cleaning capacity. Comparative performance tests along with photoelectrochemical evaluation showed a significant contribution of cascade electron transfer from plasmonic Ag to the staggered WO₃/TiO₂ heterojunctions and the analyte, providing an additional charge transfer pathway to promote the substrate-to-molecule interaction for the design of efficient and versatile metal/metal oxide SERS platforms.