# Nitrite: From Application to Detection and Development
## 3 Optical-Based Detection Methods
### Detection Methods Based on Colorimetric Methods
Furthermore, the method based on the reaction of nitrite with 60 mM HCl to produce free radicals, which further oxidizes ABTS (50 \(\upmu\)M) to form a water-soluble blue-green product that can be measured at a maximum absorption wavelength of 412.5 nm in only 2 min [46]. In addition, nitrite ions were detected using a polyethylene glycol (PEG) hydrogel overlaid with glass fiber membrane strips, and a colorimetric Griess assay was carried out on a hydrogel modified with N-(1-naphthyl) ethylenediamine by dropping asolution of nitrite and sulfanilamide. The colorimetric reaction (Figure 5c) can be completed in a few seconds and is suitable in clinical samples due to its excellent specificity [47]. The use of different strategies for the simultaneous detection of nitrite is essential to the reliability and accuracy of the experiment. A novel red carbon dot (r-CD) fluorescent probe (Figure 5e) was prepared for the dual-mode detection of nitrite, which allows both convenient colorimetric analysis and accurate fluorescence detection [48]. Similarly, a novel colorimetric fluorescent probe, ND-1, based on the photogenerated electron transfer effect (PET), was strategically constructed using naphthalimide as the fluorescent moiety and o-phenylenediamine as the specific recognition site for NO\({}_{2}\)\({}^{-}\)[49]. Furthermore, a  
Figure 5: (**a**) Conventional Griess strategy for the detection of nitrite. (**b**) Improved hydrogel-based Griess detection approach. (**c**) The preparation of N-(1-naphthyl) ethylenediamine (1-Nap) preloaded PEG-hydrogel on a glass fiber strip and nitrite ions detection accompanied by sulfanilamide (SA). (**d**) Microfluidic colorimetric paper–chip device fabrication process. (**e**) Schematic diagram of probes based on red emitting carbon dots for the colorimetric and fluorometric detection of nitrite. colorimetric-fluorescent bimodal sensing probe with a unique photophysical process was designed [50]. Similarly, a bimodal proportional colorimetric/fluorescent method was developed for the high-performance determination of nitrite in complex matrices [51], and the synthesis and evaluation of an azulene-based nitrite chemodosimeter revealed that azulene probes can also detect the presence of nitrite [52]. The fluorescent colorimetric probe P-NO\({}_{2}\)\({}^{-}\) has been used for detection in the real environment [53]. In addition, the Cu-MOF-modified exfoliated graphite paper (EGP) was used as a signal reporter gene for visual and electrochemical dual-mode sensing of nitrite [54]. On the one hand, the dual-mode combined detection of colorimetric and fluorescence improves the accuracy of the detection and enhances the flexibility of the application; on the other hand, the combination of colorimetric and chemical sensors enables sensitive detection in real time. Therefore, it is of great significance that multimodal detection permits the simultaneous detection and verification of multiple entities. To further improve detection rate and accuracy, in recent years, more and more studies have been conducted to combine the smartphone with the colorimetric method. Generally speaking, the principal method of detection is the utilization of a mobile phone's camera or another identification tool to ascertain the color shade of the reagent subsequent to the color development reaction, and the processing of this data by a specific application or software on the mobile phone. Then, the processed data are the corresponding nitrite level. Therefore, this allows for the ultra-sensitive multimodal detection of nitrite. Compared with single-signal detection, the multimode sensing system can achieve self-verification and obtain more reliable detection results [55]. The plasma-enhanced oxidase-like activity provides an intelligent method for achieving high sensitivity [56]. Similarly, the dual-channel colorimetric sensor has made some progress while detecting nitrite.