High spin Fe³⁺-related bonding strength and electron transfer for sensitive and stable SERS detection

The intrinsic electronic states of transition metal-containing SERS substrates, especially the effect of spin state on the detection sensitivity, still remain unknown. Herein, we propose a simple co-precipitation approach to form trimetallic MIL-101(FeNiTi) with high-spin (HS) Fe³⁺ as a result of geometric distortion of the octahedral symmetry. Using methylene blue as a demonstration, the trimetallic MIL-101(FeNiTi) shows a high enhancement factor (EF) of 6.1 × 10⁶, a low detection limit of 10^-9 M and excellent detection stability after long-term preservation. X-ray absorption fine structure and photoelectron spectra demonstrate that coupling between high-spin Fe³⁺ and aliovalent transition metals Ni²⁺ and Ti⁴⁺ with different filling degree of 3d e_g-orbitals results in electron delocalization. The DFT calculation suggests that MIL-101(FeNiTi) with high-spin Fe³⁺ favors molecular adsorption and the charge transfer from the molecule to MIL-101(FeNiTi) is promoted, benefitting from the enhanced electron delocalization, which both contribute to the distinguished SERS performance of MIL-101(FeNiTi). This finding provides in-depth mechanistic understanding of the effect of the spin state of transition metals on mediating SERS activity, which is expected to efficiently promote the development of SERS platforms based on non-noble metals.