Interfacial Redox-Driven Crystallization on MXene Enables Ultrasensitive Hg²⁺ Detection

Mercury contamination in water poses a serious threat to public health and ecosystems, demanding rapid and ultrasensitive detection at trace levels. We present, for the first time, an MXene-integrated fiber-optic Fabry-Pérot interferometer (MXene-FFPI) that demonstrates a mechanism-guided sensing platform for ultrasensitive Hg²⁺ detection at trace concentrations. High-quality mono- and bilayer Ti₃C₂T_x MXene nanosheets were synthesized via a minimally intensive layer delamination method, serving as efficient optical transducers and signal amplifiers. The sensing principle is based on interfacial redox-driven crystallization, in which Hg²⁺ ions are captured by MXene and converted into crystalline Hg₂Cl₂ nanoclusters, translating chemical interactions into amplified optical signals through modulation of the refractive index of the intracavity medium. Owing to the strong adsorption affinity and catalytic activity of MXene, chemical events are directly transduced into measurable interferometric responses. The MXene-FFPI exhibits an ultrahigh sensitivity of 5 pm/nM and a trace-level limit of detection (LOD) of 0.2 nM (0.04 ppb), more than 2 orders of magnitude lower than the WHO guideline limit for mercury in drinking water. This work establishes an MXene-interferometer nanophotonic architecture that converts interfacial reactions into amplified signals, providing a promising platform for ultrasensitive environmental monitoring and biomedical sensing.