Charge-Transfer Enhanced SERS on MoO2 Nanoparticles with Ultrahigh Sensitivity and Exceptional Environmental Robustness.

Surface-enhanced Raman spectroscopy (SERS) has traditionally relied on noble metals, yet their prohibitive cost, easy oxidation, and poor environmental stability severely restrict scalable practical applications. Herein, the monodisperse MoO2 nanoparticles were synthesized via a facile hydrothermal method, and their SERS performance was systematically evaluated in comparison with MoS2 and MoO3. The as-prepared MoO2 exhibits superior SERS sensitivity toward both rhodamine 6G (R6G) and O-safranin (SO). For the SO molecule, MoO2 delivers an ultralow detection limit down to 10-10 M and a remarkable enhancement factor as high as 3.52 × 107. Quantitative analysis reveals that the degree of charge transfer (ρCT) exceeds 0.5 for SO adsorbed on MoO2, indicating that interfacial charge transfer (CT) dominates the SERS enhancement, as further confirmed by transient absorption spectroscopy and density functional theory (DFT) calculations. DFT calculations further reveal a larger CT quantity of 0.78 e and richer bidirectional CT pathways in the MoO2/SO system, leading to stronger chemical enhancement than that of MoO2/R6G system. Benefiting from robust intrinsic metallic and electronic properties, the MoO2 substrate maintains stable SERS performance under harsh environments including acid-alkali corrosion, thermal annealing, and physiological saline immersion, without obvious degradation in sensitivity and detection capability. This work clarifies the critical role of interfacial band alignment and CT efficiency in MoO2 SERS systems and establishes MoO2 as a high-performance substrate that combines ultrahigh sensitivity, exceptional stability, and exceptional environmental endurance, which offers a solid foundation for its practical sensing applications in complex environments.