Heteropolyacids: An Ultrasensitive Ionic Volume-Enhanced Raman Scattering Platform

Surface-enhanced Raman scattering (SERS) is regarded as the most direct and powerful tool to identify chemical fingerprints. However, current SERS substrate materials still face some critical challenges, including low molecular utilization efficiency and low selectivity. Herein, a novel oxygen vacancy heteropolyacid─H₁₀Fe₃Mo₂₁O₅₁ (HFMO)─is developed as a high-performance volume-enhanced Raman scattering (VERS)-active platform. Due to its merit of water solubility, HFMO forms a special coordination bond with the probe molecule at the molecular level, which allows its enhancing ability to be comparable to that of noble metals. An enhancement factor of 1.26 × 10⁹ and a very low detection limit of 10^-13 M for rhodamine 6G were obtained. A robust O-N coordination bond was formed between the anion of HFMO and the probe molecule, resulting in a special electron transfer path (Mo-O-N) with high selectivity, which is verified using X-ray photoelectron spectroscopy analysis and density functional theory calculations. That is to say, the proposed HFMO platform has excellent VERS enhancing effect, specifically for the molecules containing the imino group (e.g., methyl blue, detection limit: 10^-11 M), offering the merits of high reproducibility and uniformity, high-temperature resistance, long-time laser irradiation, and strong acid resistance. Such an initial effort on the ionic type VERS platform may enable the further development of highly sensitive, highly selective, and water-soluble VERS technology.