# Nitrite: From Application to Detection and Development
## 3 Optical-Based Detection Methods
### Methods Based on Fluorescence Detection
Furthermore, a carbon quantum dot based on a primary aryl amine such as resorcinol was constructed and prepared by an efficient one-pot hydrothermal route using isophthalamide [71]. Additionally, acenapthene [1,2-d]imidazole derivatives were used as proportional colorimetric probes [72]. Similarly, the Rh6G@MOF-5 nanoprobe provides insights into the fabrication of low-cost and conveniently operated MOF sensors in analyte detection [73]. The combination with nanomaterials has improved the performance of fluorescence detection. The fluorescence of gold nanoparticles (AuNPs) is strongly burst by Mb, for the sulfhydryl or amino structure on the surface of Mb can be bonded to the surface of AuNPs through the formation of Au-S or Au-N bonds. Furthermore, Mb readily reacts with nitrite to form azo products in acidic media. Then, with the increase in nitrite concentration, the fluorescence of Mb-AuNPs was gradually restored, the turn-on fluorescence sensing of nitrite was achieved. This provides a feasible solution for the rapid response [74] (Figure 6b). In addition, tannic-acid-protected fluorescent copper nanoclusters (TA-Cu NCs) [75] have also been used as good nanomaterials. Multifunctional and multimodal detection through the development of new detection modes has also been a development. Researchers have developed a smartphone-integrated paper sensor based on manganese-doped silicon quantum dots (Mn-Si QDs) for the visual dual-mode multicolor/multi fluorescence determination of nitrite [76]. A lot of studies have been carried out on bifunctional fluorescent probes, and 2-(1H-benzimidazol-2-yl) aniline (BMA) has been used as a simple bifunctional probe for the samples in different environments. The response of BMA to nitrite under acidic conditions has been highly selective and sensitive. Most importantly, BMA has been successfully used for detection in foods, and good recoveries were obtained by adding standard sodium nitrite [77]. Novel fluorescence sensors have also been hotspots for development in recent years. Similar to colorimetric methods, fluorescence methods are often combined with smartphones, and many advanced technologies are gradually being applied to fluorescence detection as research work advances. Researchers have constructed a new fluorescence sensor that combines a power source, light source, filter, sample cell, and smartphone. The sensor has been used to detect nitrite in cured meat and pickles, and it is interesting that the results have been found to be consistent with the results of fluorescence and spectrophotometer methods [78]. In other words, the sensor has the advantages of both fluorescence and spectrophotometer methods. Similarly, based on a self-programmed python program (Figure 6c,d), researchers also prepared a portable fluorescent device based on smartphones, whose good sensing performance provides a good fluorescence platform [79]. The platform has chosen NSGQD (nitrogen-sulfur co-doped graphene quantum dots) for the quantification of nitrite, which is coupled with a smartphone using color detection by its software [80]. Hydrogels are three-dimensional networks of polymers that can hold a large amount of water. In modern assays, the use of fluorescent probes/hydrogels [81] in combination with smartphone technology allows for immediate and simultaneous ultra-sensitive detection of nitrite. In a word, the use of fluorescent probes/hydrogels has improved the performances of traditional fluorescent methods. In addition, a new detection strategy using IR780 dye as a NIR (near-infrared) probe has been invented by combining NIR probe technology with smartphone imaging. The new system is capable of detecting NO\({}_{2}\)\({}^{-}\) ions in water resources and complex food matrices with high sensitivity and selectivity [82]. Fluorescent probes have become one of the most important methods for the detection of nitrite in small-volume systems due to their simplicity of preparation, stability, sensitivity and suitability [74].