Band-Engineered MoS ₂ /F ₄ TCNQ Heterojunction for Ultrasensitive SERS Detection of Hg ²⁺ in Water Pollution Monitoring

As human society continues to develop and progress, the hazards posed to human health by emerging pollutants are becoming increasingly serious due to their unique characteristics such as specific toxicity, persistence, and bioaccumulation. It is critical to develop highly sensitive, renewable nonmetallic surface-enhanced Raman scattering (SERS) platforms for environmental monitoring, particularly for the trace detection of mercury ions in water. Here, we utilized a band-tunable F₄TCNQ/MoS₂ heterojunction achieving an ultrasensitive limit of detection (LOD) of 2.77 × 10^-11 M for Hg²⁺ in pure water. The enhancement factor (EF) for the 989 cm^-1 peak was found to be approximately 4.2 × 10⁵, while that for the 722 cm^-1 peak was 2.8 × 10⁵. Such sensitivity arises due to the interfacial charge transfer (CT) and the weak electromagnetic interaction between the heterojunction and Hg²⁺ with the Raman reporter molecule 4-methylpyridine (4-MPy). The substrate exhibits remarkable selectivity against interfering ions (Ca²⁺, Mg²⁺, Pb²⁺, etc.,), along with high reproducibility (RSD < 9.33%), and stability up to one month under environmental conditions. The results indicate that the detection sensitivity is comparable to or even better than that achieved with the conventional SERS substrate to enable specific detection of Hg²⁺. Moreover, the substrate can be reused through photocatalytic degradation showing a 30 min duration for complete removal of Hg²⁺, enabled by the F₄TCNQ/MoS₂ heterostructure. This offers significant advantages for detecting Hg²⁺ in complex environmental samples, such as seawater and soil leachate, while also proving valuable for environmental monitoring and food safety applications.