# Nitrite: From Application to Detection and Development
## 3 Optical-Based Detection Methods
### Detection Methods Based on Colorimetric Methods  
The colorimetric detection of nitrite is a classical method based on a chromogenic reaction that generates colored compounds. The method generally consists of two steps: firstly, the selection of an appropriate chromogenic reagent reacts with the component to be tested to form a colored compound, and then the color depth of the colored compounds is compared and measured. In addition, there are two main types of colorimetric methods: visual colorimetry and photoelectric colorimetry. The former directly derives the amount of the substance to be measured by comparing the color depth of the sample through naked eye observation. The latter uses a spectrophotometer for absorbance measurement, which can more accurately measure the concentration of the substance to be measured. For example, the method based on the principle that the color of the detection system changes from brown to yellow as the concentration of NO2\({}^{-}\) increases has been developed for the simple colorimetric detection [36]. The naked eye can determine the content of the sample, while the measured concentration is not very accurate. Therefore, more and more researchers are studying related optical testing instruments and exploring new methods in order to improve sensitivity. In addition, colorimetric methods generally have low accuracy due to complex interference; therefore, high-purity samples and sensitivity and selectivity assays are necessary to solve the interference problem. At present, the most basic method used in colorimetric assay is still the Griess reagent assay. On the basis of this standard assay, many of related sensors have been derived. These sensors tend to meet the requirements of rapid detection and show good advantages in terms of selectivity. For example, a hydrogel-based chemo sensor (Figure 5a,b) has shown high sensitivity for the detection of nitrite. In addition, the interference testing of this chemo sensor has confirmed its high selectivity for NO2\({}^{-}\) in the presence of environmentally relevant competing ions [37]. The sensor has achieved great success in detection with its good stability and high sensitivity. Thus, many researchers are making a series of preparations to improve its detection performance, and some nanomaterials have been applied in colorimetric detection such as AuNP-CeO2 NP@graphene oxide [38], mesoporous silica [39], and MnO2 NPs [40]. There is no doubt that the application of these nanomaterials is crucial to improve the sensitivity of the detection. Due to the protective effect of PAN (polyacrylamideonitrile) on the reagent, the researchers used PAN-NSS (modified polyacrylamide) [41] as a nitrite color sensor. With this, the validity of the reagent can be extended based on the Griess reagent detection. In addition, it has ensured the stability and long-term validity of the reagent detection to a certain extent. Due to the large amount of reagents required in the Griess reagent assay, the researchers attempted to develop a PAD (paper-based analytical device) [42] colorimetric assay for the determination of nitrite in foodstuffs to reduce the amount of reagents used. Furthermore, it most importantly does not require an instrument to achieve the assay. Similarly, the researchers have proposed a microfluidic colorimetric system consisting of a colorimetric paper device and a micro-analysis cartridge (Figure 5d) for the detection of nitrite concentration in foodstuffs to achieve a low-cost, low-reagent-use assay [43]. In addition, the researchers have utilized a microfluidic colorimetric system using phenanthrene. Additionally, researchers used phenosafranin for the detection of nitrite in food [44]. The use of it as an alternative to Griess reagent is also promising. Despite the high speed of colorimetric methods, many researchers have been trying to shorten the detection time. For example, the rapid colorimetric detection of nitrite based on the interfacial reaction of propylene glycol alginate (PGA) gels can be completed in less than 5 min [45].