Framework Response-Activated SERS Sensing: A Guest-Induced "Gate-Opening" Strategy for Ultrasensitive Sub-Ppt...

Precise in situ monitoring of trace formaldehyde (CH₂O), a high-risk indoor volatile organic compound (VOC), is critical for public health. However, direct surface-enhanced Raman scattering detection remains a formidable challenge due to the negligible Raman scattering cross-section of CH₂O and weak adsorption affinity on noble metals. Herein, an ordered Au@ZIF-8 core-shell heterostructure array was developed to achieve efficient "capture-and-enhance" synergistic sensing. Beyond passive physical enrichment, a unique "guest-induced framework response" mechanism was elucidated through ¹³C isotope labeling spectroscopy and density functional theory (DFT) simulations. Atomistic calculations reveal that highly electrophilic CH₂O preferentially binds to deprotonated defect sites within the framework, inducing a localized conformational torsion of the imidazole linkers and a substantial narrowing of the bandgap. This minor structural torsion relaxes spatial symmetry constraints, thereby activating and enhancing specific host vibrational modes that serve as robust Raman signal reporters. By optimizing the array configuration and shell thickness, the localized electromagnetic field was tailored to compensate for the intrinsic scattering deficiencies of CH₂O. The resulting sensor achieves an ultra-low limit of detection (LOD) of 0.16 ppt and exhibits excellent linearity (R² ≥ 0.99) from 0.01 ppb to 100 ppm. Furthermore, the cooperative synergistic effects of kinetic molecular sieving and localized host-guest chemical chemisorption within the ZIF-8 matrix grant the sensor robust resistance to complex interferences such as ethanol and toluene. This mechanism-driven strategy, leveraging the flexible response of metal-organic frameworks, establishes a new paradigm for the molecular-level identification of trace VOC gases.