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

ABSTRACT 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 M 2+ with four‐coordinate M─N/O bonding in the frameworks. Particularly, Fe─COF exhibits superior crystallinity and an enhanced BET surface area of 2078 m 2 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 × 10 6 , 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.