A Smartphone-Assisted Ratiometric Fluorescence and Colorimetric Dual-Mode Hydrogel Sensor for On-Site and Visual Detection of Formaldehyde in Indoor Air Phase Based on the Cu-MOFs-Eu 3+ -GMP/OPD Composite Catalysis

Rapid and on-site detection of formaldehyde (FA) is very important for public health. A ratiometric fluorescence and colorimetric dual-mode probe was fabricated for on-site and visual detection of FA based on Cu-MOFs-Eu3+-GMP/OPD composite catalysis. The Cu-MOFs-Eu3+-GMP was prepared by modifying copper–metal organic frameworks (Cu-MOFs) with europium/nucleotide coordination polymer nanoparticles (Eu3+-GMP), which could significantly enhance the water stability and fluorescence quantum yield of Cu-MOFs. The Cu-MOFs-Eu3+-GMP can directly catalyze OPD to yield a fluorescent and color product of 2, 3-diaminophenazine (DAP) via intrinsic oxidase-like activity, while the blue fluorescence of Cu-MOFs-Eu3+-GMP can be quenched by DAP via the inner filter effect (IFE). FA can specifically react with OPD to generate the Schiff base products, which restricts the yield of DAP and causes the recovery of the blue fluorescence and results in the solution fading from yellow to colorless. The dual-mode sensing platform has high sensitivity and excellent selectivity and can also be applied for ratiometric monitoring of FA in living cells. Furthermore, a point-of-care testing (POCT) strategy including a portable smartphone and hydrogel sensor was developed for on-site and visual measurement of FA in the vapor phase. A portable luminescent logic detector for the gradual determination of FA in the vapor phase was also fabricated. This work provides a new strategy for improving selectivity in catalytic systems and a useful approach for on-site and real-time qualitative identification and semiquantitative detection of FA in indoor air.