A Fluorescent Covalent Organic Framework NH₃ Sensor with Femtomolar Limit-of-Detection by Facilitated Cis-to-Trans...

Fluorescence sensing materials, especially for simple and robust chemical sensors that do not require the use of fragile enzymes while preserving excellent sensitivity and selectivity parallel to bioprobes, have been the focus of research due to their extreme convenience. However, it still remains challenging to detect analytes at femtomolar concentration levels using nonenzymatic chemical fluorescence materials because, so far, they have been limited to stoichiometric ligand-to-analyte response mechanisms. Herein, we report a completely novel fluorescence enhancement sensing approach in which a very small number of targeted analytes can trigger a domino cis-to-trans isomerization of a partially emitting covalent organic framework (COF), leading to fluorescence expansion throughout the material. The as-designed and synthesized TAPA-BTD-COF emits blue light when suspended in acetonitrile and shows a rare "turn-on" response to NH₃ with an unparalleled limit of detection (LOD) of 95.2 fM compared to other chemical sensors. Such high sensitivity equals per NH₃ molecule triggering 12.1 million COF emitting units following 20% fluorescence enhancement. The sensor exhibits excellent selectivity, showing no response to amines, imines, ammoniums, and other nucleophiles. Our results offer novel insights, suggesting that as long as emitting COF materials are set up in advance along domino effect, the sensitivity of fluorescent sensors can be increased by orders of magnitude.