<i>In situ</i> detection of sodium nitrate and sodium nitrite using a cascade Fabry–Perot interferometer based on the photothermal effect

Nitrates and nitrites, as food additives, are extensively used in the curing and preservation of meat and vegetable products. Excessive intake can impair the blood's oxygen-carrying capacity and pose serious health risks, including carcinogenic potential. This paper presents a cascaded fiber-optic Fabry–Pérot (F-P) sensing technique for the simultaneous detection of sodium nitrate and sodium nitrite based on the photothermal properties of the target substances. The proposed sensor unit consists of a cascaded F-P cavity made up of a single-mode optical fiber, a polyurethane film, and a gold film. Experimental results demonstrate that under 222 nm pump light source irradiation, the sensor achieves a sensitivity of 55.342 nm/(mg/L) and a detection limit of 0.316 μg/L, four orders of magnitude lower than traditional high-performance liquid chromatography. With its compact structure and high sensitivity, the sensor enables in situ detection, offering significant potential for applications in food safety monitoring, drug testing, and disease diagnostics without requiring sample pretreatment.