Smart paper-based sensor: A novel bio-enzyme-free dual-mode platform for real-time visual monitoring of organophosphorus pesticides

This study proposed a colorimetric/fluorescent bimodal bio-enzyme-free paper-based sensor for device-free visual detection of organophosphorus (OPs). The Zn-doped Fe-based organic framework (MIL-88B-Fe/Zn) exhibited excellent peroxide-like activity, catalyzing the oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB) in the presence of H₂O₂. The oxidized TMB displayed blue color and quenched the red fluorescence of carbon dots derived from neutral red (NR-CDs) through inner filter effect (IFE). In the presence of OPs, the enzyme-like activity of MIL-88B-Fe/Zn was inhibited due to the collapse of MOF structure, which hindered the chromogenic reaction of TMB, while the fluorescence of NR-CDs was restored by IFE deactivation. TMB and NR-CDs were co-modified on filter paper to construct T-N paper with dual signal responses. As OPs concentration increased, T-N paper changed from blue to red under daylight, and red fluorescence enhanced under UV light. Photographs were captured by smartphone, and standard curves of OPs concentration versus R/B values (colorimetric) and R values (fluorescent) were established for quantitative analysis. Detection limits for glyphosate were 1.04 ng/mL (colorimetric) and 1.22 ng/mL (fluorescent). This sensor was simple to operate, multi-signal self-calibrated, and highly portable, providing a new strategy for rapid on-site detection of OPs residues and the construction of bio-enzyme-free trace analysis platform.