Amino Acids‐Enabled Fast‐Restore of Triplet‐Triplet Annihilation Upconversion Luminescence for Background‐Free Sensing of Herbicides

Abstract Overuse of herbicides poses a serious threat to ecosystems and human health; thus, the accurate determination of herbicide residue is very meaningful. Thanks to the advantage of no background fluorescence interference, the upconversion luminescence allows for reliable analysis of target molecules in complicated samples. Here, through screening of 20 natural amino acids, it is discovered that the photooxidation of methionine exhibited the fastest recovery rate of triplet‐triplet annihilation upconversion (TTA‐UC) luminescence via oxygen consumption, which is 400‐fold faster compared to the well‐known photooxidation of oleic acid. Furthermore, oxygen‐resistant, small‐size, red‐to‐blue TTA‐UC nanoparticles with a record upconversion efficiency (7.2%, normalized to 100%) are prepared using hydrophobic butyl methionine as an oxygen scavenger. Surface negatively charged TTA‐UC nanoparticles are able to selectively enrich positively charged paraquat on their surface. Accordingly, a photoinduced electron transfer process occurred between the triplet excited state of the photosensitizer and the electron‐deficient paraquat, quenching the upconversion luminescence. Relying on this principle, TTA‐UC‐based paraquat sensing is achieved with a fast response (less than 1 s), high selectivity, and a low limit of detection (1.54 µg mL −1 ). Further, the TTA‐UC nanoparticles are utilized to implement paraquat analysis in lake water without sample pretreatment.