# Nitrite: From Application to Detection and Development
## 4 Biosensors  
In recent years, biosensors have attracted attention in nitrite detection for their high specificity and great real-time performance. The biosensor was first used for glucose detection, which was made by including glucose oxidase in a polyacrylamide colloid and curing it. Then, the colloidal membrane was fixed on the tip of a diaphragm oxygen electrode. It is worth mentioning that this type of sensor belongs to the class of enzyme-based biosensors, which laid the detection foundation for the later generation of various biosensors. Biosensors are usually made with biologically active materials and physicochemical transducers organically. There can be many kinds of applications regarding bioactive materials. Both anthocyanin encapsulated in the CMC/PVA (carboxymethyl cellulose/polyvinyl alcohol) matrix [83] and diastase have great potential as biologically active materials. As another example, scientists have studied HS-\(\beta\)-cyclodextrinin coordination methanobactin/gold nanoparticles as a biologically active material [84] and it has a high sensitivity, which has provided another good idea for us. The main types of biosensors used in the detection of nitrite include the following. (1) Enzyme-based biosensors: specific enzymes are used as biorecognition elements to quantify nitrite by metabolites produced or substrates consumed in enzymatic reactions. (2) Microbe-based biosensors: metabolic pathways within the microbial cells are used to respond to the presence of nitrite. Typically, they have good stability and reusability and are suitable for rapid on-site testing. (3) Nucleic acid-based biosensors. Biosensor technology, although highly specific, has a relatively short lifespan, and its stability is not long-term. Therefore, future biosensors could be improved in terms of lifespan to obtain long-term stability [85]. To address this issue, scientists have made some progress in longevity through the combination of the nitrate reductase (NR) enzyme from the fungus Neurospora crassa and an electrochemical biosensor, which retained more than 70 per cent of its activity over 3 months [86]. In the actual operation of detecting nitrite, the biosensor technology is often cross-used with other technologies to enable multi-mode detection and mutual verification. For example, a novel nitrite biosensor based on carbon quantum dots (PA-CDs), constructed and prepared by an efficient one-pot hydrothermal route using primary aryl amines (PA), is not only a combination of fluorescence detection technology and biosensing technology but also broadens new nano-sensor detection methods [71]. As another example, the combination of biosensor and electrochemical technology is also an improved technique. Hemoglobin (Hb), as a biologically active material, was immobilized on the Co\({}_{3}\)O\({}_{4}\)-CNF/CILE surface. The electrochemical biosensors were prepared using Nafion as a protective film to improve immunity to interference and selectivity and have been used for the quantitative detection of real samples [87]. Similarly, a novel electrochemical biosensor for nitrite was synthesized using sodium carboxymethyl cellulose as a reducing and stabilizing agent in the form of Au-MoS\({}_{2}\) nanoflower-modified nanocomposites. The proposed electrochemical biosensor for nitrite is characterized by a wider linear range, a lower detection limit, and higher sensitivity [88]. In addition to this, the novel cyanide voltametric biosensor based on the inhibitory effect of cyanide on cytochrome c nitrite reductase (ccNiR) is promising in the quantification of nitrite [89]. The combination of a biosensor and a 3D printed sensor has been applied to analyze real biological samples, demonstrated by the amperometry detection of uric acid and nitrite in the linear range of 0.5-250 umol L\({}^{-1}\), with LODs of 0.03 umol L\({}^{-1}\) for nitrite.