# Nitrite: From Application to Detection and Development
## 3 Optical-Based Detection Methods
### Methods Based on Fluorescence Detection  
Fluorescence detection is a widely used assay for nitrite detection. Its working principle is mainly based on the energy level jump mechanism of the absorption and fluorescence spectra of material molecules, when the material molecules absorb a certain wavelength of light. They can instantly emit light longer than the wavelength of excitation light, i.e., fluorescence. In recent years, although fluorescence spectroscopy analysis is used as a good detection method with its high selectivity and sensitivity, there are some shortcomings in its application. For example, only a few compounds can produce fluorescence, and most molecules cannot emit fluorescence. To solve this problem, researchers usually introduce derivable functional groups to convert non-fluorescent substances into fluorescent ones. Fluorescent probes are molecules with special fluorescent properties that respond sensitively to changes in the environment in which they are found. Such molecules have characteristic fluorescent properties in the ultraviolet-visible-near-infrared region. For this reason, fluorescent probes are also known as fluorescent chemical sensors or \"molecular devices\". Overall, due to their high sensitivity and wide range of applications, fluorescent probes have become an indispensable tool in modern scientific research. The presence of fluorescent probes is widely used for their excellent specificity, and nitrite in water samples and sausages can be determined using the fluorescent probe colorimetric method [62]. Detailed information is shown in (Figure 6a) and described below. In recent years, much attention has been paid to modifying the recognition groups in fluorescent moieties and modulating the energy transfer process for efficient analyte detection. Therefore, it is of great importance to design specific functional probes through the precise modulation of the binding sites of fluorescent moieties. For example, scientists have chosen 2-(2-amino-4-carboxyphenyl)benzothiazole (ortho-BT) to design a probe for the detection of nitrite by precisely modulating the benzothiazole binding site. Only the fluorophore bond in the orthogonal position of the aniline portion of the fluorophore can specifically recognize nitrite [63]. The materials required for fluorescent probes have increasingly become a key direction of research; therefore, many materials have been developed as fluorescent probes. For example, a series of explorations have been made on amphiphilic carbon dots, and an amphiphilic carbon dot has been prepared from white berries [64]. Meanwhile, ternary ZnCdS quantum dots have been synthesized by aqueous synthesis using air-stabilized  
Figure 6: (**a**) Schematic illustration of the fluorescent and visual detection of nitrite through the IFE of BPS-Fe\({}^{2+}\) complex and g-CNQDs. (**b**) Fluorescence sensing measurement of nitrite by Mb-AuNPs. Schematic diagram of (**c**) working mechanism and (**d**) application of the portable sensing platform BDP-OPD for NO\({}_{2}\). compounds [65]. CdS quantum dots capped with polyethyleneimine [66] have also been used in fluorescent probes. Similarly, a new near-infrared fluorescent probe was designed and synthesized using the principle of \"covalent assembly\", which showed excellent selectivity and high sensitivity to nitrite [67]. In addition, fluorescent silicon quantum dots were designed and synthesized. Additionally, fluorescent silicon quantum dots (Si QDs) have excellent water dispersibility, photostability and salt resistance [68]. The highly fluorescent carbon dots (R-CDs) synthesized from sodium ascorbate and polyethyleneimine at room temperature are energy-saving, environmentally friendly and easy to put online [69]. Researchers have continued innovating and a new type of Fe-CDs (Fe-doped carbon dots) has been prepared by MDES (metal deep eutectic solvent) with high sensitivity and selectivity [70].