PCN222(Fe)@NU-1000 based MOF-on-MOF dual-signal sensing strategy for sensitive detection of phoxim

Phoxim is a broad-spectrum organophosphorus pesticide widely used in the world; however, its neurotoxicity and potential risk pose a significant threat to human health and ecosystems. To address this issue, based on the same metal clusters and similar topologies, a MOF-on-MOF composite strategy, i.e., PCN222(Fe)@NU-1000, was developed for rapid evaluation of phoxim via ratiometric fluorescence and colorimetric dual-signal output. Herein, the inner core, PCN222(Fe), imparts peroxidase-like catalytic activity enabling colorimetric detection based on the 3,3′,5,5′-tetramethylbenzidine (TMB) and the solution decolourisation caused by acetylcholinesterase (AChE) products, while the outer shell, NU-1000, provides a strong fluorescence emission at 461 nm. This core-shell structure also facilitates fluorescence resonance energy transfer (FRET), whereby phoxim selectively quenches the fluorescence of NU-1000 without affecting PCN222(Fe), enabling ratiometric fluorescence sensing. This integrated dual-signal system-comprising ratiometric fluorescence and enzyme-mimetic colorimetry-ensures signal cross-verification and improved detection accuracy. The fluorescence mode exhibits a quantification range of 5.0 ng/mL to 4.0 μg/mL with a detection limit of 4.0 ng/mL, while the colorimetric mode shows a linear range from 2.5 ng/mL to 25.0 ng/mL and a detection limit of 1.76 ng/mL. Furthermore, the method accurately evaluates phoxim levels in fruits and vegetables such as apples and celery, achieving recoveries ranging from 95 % to 115 % and a relative standard deviation (RSD) of less than 9 %. This MOF-on-MOF dual-signal sensing strategy is designed to be simple, user-friendly, and highly reliable, providing significant value for the development of rapid and sensitive approaches in food and environmental science.