A Metal-COF-Based SERS Platform for PPb-Level Toluene Sensing With Tailored Metal Selectivity

Ultrasensitive detection of nonpolar toxic VOCs like toluene remains a great challenge for environmental monitoring. This work presents a rationally designed SERS platform based on metallized covalent organic frameworks (M-COFs, M = Fe, Ni, Cu) obtained by a mild metal-modulation strategy that enables metal‑tunable COF crystallinity and porosity. M-COFs were one-pot synthesized from PyTA and PDA under 70 °C and atmospheric pressure. Spectroscopic characterizations confirm atomic dispersion of M²⁺ with four-coordinate M─N/O bonding in the frameworks. Particularly, Fe─COF exhibits superior crystallinity and an enhanced BET surface area of 2078 m² g^-1 with a pore size of 1.87 nm, whereas Ni‑ and Cu─COF show reduced structural order. The Au/Fe─COF SERS platform achieves a toluene detection limit of 0.1 ppb with an enhancement factor of 1.5 × 10⁶, outperforming Ni─COF (0.5 ppb), Cu─COF (1 ppm), and metal-free COF (10 ppb). DFT calculations verify Fe─COF 's ultrahigh toluene adsorption affinity with an adsorption energy of -34.17 kcal mol^-1 and its higher crystallinity compared to its counterparts. This platform also shows excellent anti-interference and enables specific toluene detection in complex gas mixtures. The M-COF-based SERS system offers a practical solution for ultra-trace nonpolar VOC monitoring.