Upconversion-based intelligent dual-mode hydrogel nanosensor for visual quantitative detection of formaldehyde

Formaldehyde (FA) is a highly toxic carbonyl compound to both living organisms and the environment, which requires a rapid and sensitive detection method. The cost-effective paper-based fluorescent sensors are robust tools for FA detection while the immobilization of fluorescent probes onto paper substrates results in reduced sensitivity. To overcome these challenges, an intelligent hydrogel nanosensor was prepared by embedding an upconversion optical probe in the 3D porous polyacrylamide hydrogel for the multiplex chroma detection of FA. The hydrogel nanosensor provided a liquid-phase detection environment for the upconversion probe, ensuring the detection sensitivity compared to paper-based sensors. Moreover, the near-infrared excited UCNPs effectively avoided the background fluorescence generated by the hydrogel matrix under UV light, thus enhancing the precision of FA detection. The prepared hydrogel nanosensor exhibited fluorescent and colorimetric dual-mode response for FA, with a limit of detection (LOD) of 10 nM in fluorescent mode and 25 nM in colorimetric mode. Moreover, a portable sensing platform has been further developed to address practical requirements, enabling cost-effective and instant detection. This novel UCNPs-based hydrogel nanosensor provides a convenient and precise sensing strategy for the detection of hazardous substances in food, with the potential to promote the practical application of portable fluorescent sensing devices in food safety monitoring.