Aptamer-gated luminescence enhancement and red-shift in a MOF: a ratiometric and color-tonality visual biosensor for pathogenic E. coli detection.

An advanced aptasensing platform is introduced that integrates the luminescence properties of zirconium-based metal–organic frameworks (Zr-MOFs) with the molecular recognition capacity of aptamer. The integration of a specific Escherichia coli (E. coli)-targeting aptamer with the Zr-MOF induces a remarkable dual emission response: a significant enhancement in intensity accompanied by a pronounced red-shift from 470 nm to 530 nm, forming a highly fluorescent aptamer@Zr-MOF composite. This transformation serves as the foundational “On” state in a rationally designed “Off-On-Off” sensing cycle. The initial Zr-MOF emission is quenched (“Off”), then enhanced and red-shifted by aptamer binding (“On”), and finally quenched again upon target recognition as E. coli selectively binds the aptamer, displacing it from the MOF surface and restoring the “Off” state. This ratiometric response enables highly sensitive and selective quantification of E. coli. The platform was adapted for both instrumental analysis and instrument-free, smartphone-based visual detection. The smartphone method achieved an exceptionally low detection limit of 1.6 CFU/mL with a linear range from 10 to 1 × 106 CFU/mL. The aptasensor demonstrated high selectivity against competing bacteria and was successfully applied to detect E. coli in spiked lettuce and wastewater samples, with recoveries between 94.5% and 98.0%. This strategy provides a powerful, portable tool for on-site pathogen monitoring.