**1. Introduction**

Quantum dots (QDs), which are spherical particles with a size of <10 nm, are classified based on the materials used into semiconductor quantum dots and carbon-based quantum dots [1]. Semiconductor quantum dots are three-dimensional nanoparticles composed of inorganic elements like heavy metals and non-heavy materials (such as cadmium, selenide, and zinc sulfide). Semiconductor quantum dots are prepared by a bottom-up approach. They have a spherical shape, crystal structure, and size of fewer than 6 nm, which have quantum mechanical properties due to their small size. Size-dependent photoluminescence, excitation-independent photoluminescence, long life, and good photostability are specific features of QDs. Quantum dots are highly stable against photobleaching and can be considered an alternative to fluorescent index dyes and proteins. Semiconductor quantum dots are

one of the most popular nanoparticles of choice for fluorescent labels, detection, labeling, and tracking [1]. One of the disadvantages of semiconductor quantum dots based on heavy metals is their toxicity, which leads to environmental and health concerns. Graphene quantum dots (GQDs) have been considered in various research fields, including the food industry, due to their lower toxicity and environmental friendliness. The small size of GQDs and their high oxygen content lead to reduced toxicity. The GQDs refer to a 0D graphene plate with a size of less than 10 nm. GQDs have good potential for practical applications due to their specific characteristics, such as excellent luminescence, functional groups, and surface defects. They are widely used as fluorescent probes for several analytes because of their significant properties such as photobleach resistance, low cytotoxicity, good biocompatibility, stable photoluminescence, and high water solubility. Photoluminescence is the most interesting feature of GQD, which can be easily changed by managing its size, surface performance, and chemical doping. In addition, GQDs contain carboxyl, carbonyl, epoxy, and hydroxyl groups that can bind to various biological molecules such as proteins, antibodies, enzymes, and so on [2].

Carbon quantum dots (CQDs) were introduced as environmentally safe alternative nanomaterials that can provide satisfactory optical properties, good biocompatibility, and nontoxicity [3]. CQDs were first discovered in 2004 by Zhu et al. during the purification of single-walled carbon nanotubes. The size of these nanoparticles is less than 10 nm. Like common semiconductor quantum dots, their major advantage over organic dyes is their high optical stability. Also, unlike other QD, CQDs usually have low toxicity and good biocompatibility [4]. In terms of chemical structure, carbon atoms (sp2 and sp3 ) are located as nuclei in amorphous shells consisting of functional groups. Many hydroxyl and carboxyl functional groups are present on the surface of carbon dots, which cause good solubility in water and ease of functionalization by different species. The amount of oxygen in CQDs varies from 5 to 50 wt%, depending on the synthesis method [5].

There is a lot of research on the use of quantum dots in various fields, but few articles report the use of quantum dots in food science. The purpose of this chapter is to review the application of quantum dots in the food industry.
