# Nitrite: From Application to Detection and Development
## 3 Optical-Based Detection Methods  
### Spectrophotometry  
The most common spectrophotometric method used for the detection of nitrite is the Griess method, which is the reference method. Firstly, the appropriate aromatic amines are diazotized, and then the azo reaction is carried out to form pigment chromophores, and the absorbance of the reaction products is detected by the relevant instruments. Then it can indirectly determine the content of nitrite. In many cases, the violet-red product formed by naphthalene ethylenediamine hydrochloride under weak acid conditions has the maximum absorbance at 540 nm [24]. With this principle, it can detect most of the nitrite. However, some improvements need to be made to obtain higher accuracy and sensitivity. At present, spectrophotometry has become one of the internationally accepted methods for the detection of nitrite, whose advantages include simplicity, rapidity, accuracy, high sensitivity and low cost. However, the method has some shortcomings, such as the  
Figure 3: EcoSens Aquamonitrix smart wastewater quality monitoring system. need to use a large number of reagents and steps such as pre-treatment, as well as the problem of cross-reactivity. Therefore, future research should focus on how to improve the selectivity of the spectrophotometric method, reduce the detection limit and simplify the operation process. To address these issues, researchers have begun to explore a number of new technologies and approaches, for example, the use of fluorescent probes to improve detection sensitivity and selectivity; the combination of other technical methods, such as mass spectrometry and nuclear magnetic resonance, to achieve more accurate and comprehensive detection; and the development of new types of reagents. These innovations will help to further improve the performance and application scope of spectrophotometry and provide more reliable technical support for food safety. While using the Griess reaction as a reference method for spectrophotometry, the principle of its detection has been optimized, and a fluorescent sensor has been designed that also works by detecting colored azo compounds, in which the nitrite content is indirectly measured through the detection of shades of brown. Its use of 3-acetoacetyl-7-diethylaminoocumarin and 3-aminophenol (Figure 4a) for the highly selective detection of nitrites is a good improvement strategy for the simplification of the Griess reagent assay [25]. Detailed information has been shown in Figure 4 and is described below. Figure 4: (**a**) Illustration of IFE-based fluorescent sensor for nitrite using ADC. (**b**) The preparation of paper-based analytical device for the coloration development in nitrite determination. (**c**) Preparation of the proposed method for NO\({}_{2}\)\({}^{-}\) detection using both spectrophotometric and smartphone-based analyses. Under acidic conditions, fenugreek is also detectable as a dye [26], which is not a new way. In addition, for the determination of nitrate concentration in natural water, using premixed vanadium and Griess reagents is fast and efficient [27]. A stable optical nano sensor was fabricated by immobilizing 1-naphthylamine on mesoporous silica nanospheres. The determination of nitrite is based on the reaction of nitrite with the optical sensor and the formation of diazonium salts, which are then coupled to form a stable brown azo dye for simple, sensitive and rapid detection. Additionally, a time-saving and high-recovery method for the spectrophotometric analysis of nitrite based on the diazonium coupling reaction is promising [28]. In the Griess reagent test, N-(1-naphthalenyl) ethylenediamine dihydrochloride is a substance with a certain degree of toxicity, while the exposure of the detector may be harmful to the organism or the environment. Therefore, researchers have been looking for a more environmentally friendly reagent, which is very critical and of strategic significance for sustainable development.