Construction of portable hydrogel sensors with self-ratio optical bimodal detection and smartphone imaging functionality for visual quantitative nitrite detection

Excessive use of nitrite in food production and preservation causes blood and digestive system diseases, underscoring the need for rapid and accurate quantitative detection of nitrite. In this study, we developed a novel visual method for nitrite detection based on a near-infrared (NIR)-excited ratiometric fluorescent hydrogel sensor. In this sensor, NIR-excited upconversion nanoparticles (UCNPs) and basic fuchsin (BF) served as energy donors and acceptors, respectively, collectively forming an integrated ratiometric nanofluorescent probe. The upconversion optical probe was further embedded into a three-dimensional (3D) porous polyacrylamide (PAM) hydrogel to fabricate a smart hydrogel nanosensor that exhibited specific optical and colorimetric responses to nitrite. The hydrogel nanosensor demonstrated a high loading capacity for upconversion probes and provided a liquid-phase detection environment, thus significantly enhancing both fluorescence signal stability and sensing sensitivity. The NIR-excitation of the UCNPs prevented background fluorescence interference from the hydrogel and real samples. BF acted as a specific recognition unit for nitrite ions. The addition of nitrite altered the green-to-red intensity (G/R) ratio of the hydrogel sensor, facilitating real-time quantitative nitrite detection. Furthermore, the hydrogel sensor was integrated with a smartphone to create a portable platform for practical visual nitrite detection. The detection limits for nitrite using the probe and hydrogel sensor were extremely low (60.7 and 72 nM, respectively). The proposed novel UCNP-based hydrogel nanosensor provides a new sensing approach for detecting harmful substances in food, showing considerable potential to advance the applicability of portable fluorescence sensing devices in food safety monitoring.