**4. Water purification**

The development of industry and agriculture leads to the release of various pollutants and their mixing with underground and surface water, which is a threat to human health and the environment. On the other hand, due to the growth of the population in the world, the demand for water will increase in the coming years. Therefore, it seems necessary to purify polluted water for various uses, including drinking, agriculture, and industrial use [76]. Water pollutants include heavy metals, inorganic and organic pollutants, especially dyes, polycyclic aromatic hydrocarbons, pesticides, and pharmaceuticals. Most of the mentioned pollutants exist in very low concentrations, but their risks to humans and living organisms are very

#### *Applications of Quantum Dots in the Food Industry DOI: http://dx.doi.org/10.5772/intechopen.107190*

high because they are very persistent in normal environments and water treatment systems. Organic compounds are important pollutants that come out of manufacturing plants. These organic pollutants lead to the reduction of dissolved oxygen and endanger human health. Heavy metals such as lead, nickel, cadmium, essential oil, copper, manganese, chromium, and cobalt are very harmful to human health even in low concentrations, and their frequent consumption can damage the liver and brain and cause cancer [77]. The most important water purification methods are filtration, crystallization, sedimentation, gravity separation, flotation, coagulation, ionic oxidation, solvent extraction, evaporation, distillation, reverse osmosis, ion exchange, electrodialysis, electrolysis, absorption, centrifugal and membrane fluid separation, neutralization and remineralization, reduction, and oxidation. None of the mentioned methods are completely effective for providing safe drinking water. Nanotechnology has provided new solutions for water purification. Nanomaterials are suitable for adsorption, catalysis, and sensing applications due to properties such as high aspect ratio, reactivity, adjustable pore volume, electrostatic, hydrophilic, and hydrophobic interactions. Nanomaterials are widely used in various water treatment methods, including membranes in filtration, adsorbents, and photocatalysts for pollutant degradation and detection. Recently, much research has been conducted on the potential application of QDs in water treatment due to their unique properties. Of course, it should be noted that nanomaterials used for drinking water purification must also be environmentally friendly and nontoxic, because unsafe particles in contact with the human body cause severe damage to vital organs and their dimensional properties aggravate biological damage slowly [78]. There is a lot of research on the use of CQDs in water treatment. In general, the use of CQDs in water purification can be investigated from two aspects.

#### **4.1 CQDs as membranes**

Nanofiltration membranes have good potential for water purification. One disadvantage of the traditional water filtration methodology is that soluble salts and a few soluble minerals and organic substances cannot be removed. Nanotechnology provides new solutions for water purification. This is done using nanoporous polymers, nanomembranes, etc., which usually have pore sizes between 1 and 50 nm and separate most bacterium and harmful substances. Desalinization also belongs to the current methodology. The general structure of the membrane consists of many layers. In a simple membrane structure, CQDs are uniformly distributed between the dense upper layer and the porous substrate. The addition of CQDs to thin-film nanocomposite (TFN) membranes will increase membrane efficiency, water flux, power density, and water purity. Also, because of the electrostatic repulsions between the deposits and also the membrane surface, a major improvement in antifouling properties happens within the designated layer. Efficient and large surfaces and large intermediate spaces with several functional groups are the most reasons for increasing fluxes. The presence of specific hydrophilic groups in CQDs reduces the nonspecific absorption and therefore will increase the selectivity within the needed absorption of pollutants. These membranes also are utilized in reverse osmosis because of enhanced water permeability, high permeation flux, and antifouling capability. By incorporating CQDs on the membrane surfaces, all processes like desalination performance, porosity, permeability, hydrophilicity, selectivity are increased [79].

#### *4.1.1 Thin-film nanocomposite (TFN) membranes*

In recent years, many studies are conducted on modifying selected skins by adding CQDs. The CQDs disperse within the aqueous phase and then participate within the surface polymerization process to create TFN membranes. CQD-modified TFN membranes will perform higher than virgin membranes, even with a small addition of CQD in the water phase. The resulting membranes show surface hydrophilicity and better permeability while maintaining solute selectivity, excellent stability, and improved antifouling features. The performance of TFN membranes can be maximized by properly functionalizing CQDs and optimizing their value [80]. In one study, CQDs were used to make new thin-film nanocomposite (TFN) membranes. First, the amino carbon quantum dots (ACQDs) were synthesized through a simple single-pot hydrothermal method and then used in surface polymerization to fabricate the ACQD-TFN membrane. Using ACQDs in membrane led to a water flux increase of 23.2 kg·m-<sup>2</sup> ·h−1 at 70°C during the treatment of a 10 wt% NaCl solution, which was 44% higher performance than thin-film composite membrane without modification [81].

#### *4.1.2 CQD/polymer composite membranes*

In this method, CQDs are added to polymeric dopes to form homogeneous solutions and to form mixed matrices membranes through various spinning methods. The small size of CQDs leads to better dispersion of particles in doped solutions and the formation of membranes with a uniform structure without loss of mechanical strength. One of the challenges of this method is finding a suitable solvent for CQD and polymer. Another challenge is how to control the distribution of CQD in membranes. Therefore, in addition to ensuring the long-term stability of CQDs within the membrane, practical tools for the formation of chemical bonds between CQDs and the polymer matrix must be identified [80].

Colbum's team used an ionic liquid (1-ethyl-3-methylimidazolium acetate) as the common solvent for both CQDs and cellulose to create uniform membrane properties. CQDs are bound with the cellulose domain through hydrogen bond networks, and a stable composite membrane was formed. The presence of CQDs on the surface can also make the membrane negatively charged and more hydrophilic. Also, Field Emission Scanning Electron Microscopy images of the cross section of the membrane indicate that CQDs act as pore formers and help to form membranes with higher permeability [80].

#### *4.1.3 Membranes with CQDs on top of substrates*

In this method, CQDs are coated above the membrane surface with the help of various coating agents such as polydopamine (PDA) and trimethoxy silane (3-aminopropy) (APTMS). One way to stabilize CQDs is to create a covalent bond between the oxygen-containing groups in CQDs and the amine-containing agents on the membrane surface [28]. In research, CQDs were fabricated from citric acid *via* a simple method. Subsequently, they are immobilized onto the polydopamine (PDA) layer grafted on. The carboxylic groups of CQD react with the –NH2 group of polydopamine, resulting in increased resistance of the modified membrane to deposition. The modified membranes possess much enhanced antibacterial activity and anti-biofouling propensity. The continuous PRO operations at 15 bar also confirm

that the CQD-modified membranes exhibit a much higher power density (11.0 vs. 8.8 W/m<sup>2</sup> ) and water recovery after backwash (94 vs. 89%) than the unmodified ones [82].

#### **4.2 CQDs for removal of pollutants**

One of the appropriate strategies for water treatment is photocatalysis. Both sunlight and UV light are employed to destroy pollutants. It is an oxidation process, which is stable and environmentally friendly for water purification. CQDs show excellent photoluminescence. They are excellent fluorescent materials and efficient photocatalysts in UV light. CQDs also are sensitive to visible light, which results in enhanced charge carriers and photocatalytic performance. CQDs transfer electrons at different positions and can reduce the recombination of light-generated charges. All the mentioned properties have made them an appropriate candidate for water purification and removal of organic and inorganic pollutants from water [79].

CQDs can be used to take away organic and inorganic pollutants Cd2+ and Pb2+ ions from wastewater by absorption treatment. N-CQDs were with success incorporated into this treatment. Another adsorbent, polyethyleneimine-functionalized CQDs onto the magnetic materials (MnFe2O4) to provide a nanocomposite (PECQDs/ MnFe2O4), is applied for the removal of uranium [79].

### **5. Protein tracking by QDs**

Proteins are the main components of food and play an important role in nutrition, formation, and maintenance of food structure. Today, the function of protein in food matrices is well known, but it is necessary to study the role of protein in different food matrices. For example, in bread dough, gluten causes the dough to be elastic and viscous and forms a matrix to hold gases in the bread. On the other hand, knowing the function and amount of gluten in the dough is not enough. In the past, gluten function was studied by extracting gluten proteins and studying the behavior of the extracted proteins, but the behavior of the extracted gluten due to its interaction with other proteins may not be similar to that found in the food matrix. Knowing the distribution of proteins leads to a better understanding of their function. One of the effective strategies in this field is labeling proteins with QDs, which leads to a better understanding of their function [83].

Gluten and zein, the most common and consumed proteins, are cereals. The main storage protein of wheat grain is gluten. Gluten is a complex mixture of hundreds of protein components, mainly gliadin and glutenin. Based on solubility in alcohol-water solutions, gluten proteins are divided into soluble gliadin and insoluble glutenins. Both parts play an important role in the rheological properties of dough, but their functions are different. Gliadins contribute to the viscosity and expandability of the dough system, while glutenin is responsible for the strength and elasticity of the dough. In terms of amino acid composition, gluten has high amounts of glutamine and proline and low content of amino acids with charged side groups [84]. The most important storage protein in corn is zein. Zein contains four components (alpha, beta, gamma, and delta) with different peptide chains, molecular sizes, and solubilities. Zein is rich in glutamic acid, leucine, proline, and alanine, but it is exceptional among vegetable proteins in terms of the lack of tryptophan. This protein is considered hydrophobic due to significant amounts of nonpolar amino acids. Also, the high

proportion of nonpolar amino acids and the lack of basic and acidic amino acids lead to a decrease in the solubility of zein [85].

Suzer et al. were the first to use QDs to image and label food structures. The distribution of gluten and zein in the cereal matrix strongly affects the structure and texture. Thus, knowledge of their distribution offers new insights into how distribution affects food structure. In this study, they used nucleus/shell CdSe/ZnS QDs to image the gluten network in flatbread and zein in corn extrudates. A confocal laser scanning microscope (CLSM) was used to observe the structure of QD-labeled cereal proteins and visualize their location in the food matrix. The results showed that QDs could be covalently conjugated with gluten and zein [86]. In one study, QDs were conjugated to gliadin antibodies and used as fluorescent probes to detect gliadin proteins in dough and baked bread samples. CLSM was used to investigate QDs-gliadin antibody conjugates and obtain 3D images of the gliadin distribution in the dough and flatbread matrix. CLSM images showed significant changes in the fluorescence intensity distribution generated by the gliadin-QD conjugate with cooking time. Based on the results obtained from the dough and flatbread samples, they stated that the distribution of gliadin in different layers (top, center, and bottom) is nonuniform and the baking time and location of the layers play an important role in the distribution of flatbread gliadin protein. From the successful binding of QDs to gliadin antibodies, it can be concluded that QDs have good potential as a probe to target protein subunits in food matrices [87].

In the mentioned studies, the antibody-antigen method was used to label the proteins. When the antibodies were conjugated with the QDs, to break the disulfide bonds in the antibodies, and then crosslink the QDs to the new free-SH groups, diethritol (DTT) was used in the antibody. The antigen detection region is strong in antibodies with disulfide bonds. Therefore, the use of DTT may lead to antibody damage. A new method has been proposed for the conjugation of QDs to antibodies in which disulfide bonds are not broken performed with acetylglucosamine. The next step is the integration of azides from modified N-acetylgalactosamine monosaccharides into antibody glycans. The third step is the catalyst-free click conjugation of desferrioxamine-modified dibenzocyclooctynes to the azide-bearing sugars. And the last step is the radiolabeling of modified chelator antibodies with zirconium. In this method, there is no risk of reducing the effectiveness of the antibody protein, because the antibody binding site is not manipulated in any way [83].
