**3. QDs in food packaging**

Food packaging is an essential component of the food supply cycle that acts as a shield or barrier against contamination, the external environment, and mechanical damage during transport, and its main purpose is to ensure the quality, health, integrity, and safety of the product. In addition to maintaining product quality, packaging systems also help reduce waste [52]. The use of nanoparticles in food packaging is a new technology that has received much attention due to its many benefits. Nanoparticles as fillers in packaging materials increase product shelf life, reduce the growth of microorganisms, improve mechanical properties, and block gases and UV light. These packages are a good alternative to nonbiodegradable plastic packaging materials due to their high-performance nanostructures and low weight.

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


#### **Table 4.**

*Application of QDs in the detection of insecticide and antibiotic residues in food.*

Food packaging is classified into four groups based on the application of nanoparticles: improved, active, smart, and bio-based packaging [53].

#### **3.1 Improved packaging**

The main purpose of this type of packaging is to use nanomaterials applied inside polymeric materials to improve the mechanical properties of the packaging, UV barrier, and the effect on permeability to water vapor and oxygen [54].

#### *3.1.1 Improving the mechanical properties of packaging*

Nanoparticles are the potential candidates for the synthesis of polymer nanocomposites due to their small size, high surface-to-volume ratio, high strength, and extremely high surface activity. Polymers reinforced with nanoparticles are very suitable for various applications such as food packaging due to high strength, low weight, and low cost [55]. In one study, CQDs were used to modify the polyvinyl alcohol matrix and cellulose nanofibers. The results showed that the addition of carbon dots to the matrix increases the tensile modulus and tensile strength of the composite due to the formation of hydrogen bonds, van der Waals force, and other chemical bonds

between carbon surfactants, polyvinyl alcohol, and cellulose nanofibers. Electron microscopy images also showed that the carbon-modified film had a denser refractive index than the control film, indicating better composition and bonding in the film with carbon dots [56]. Yu and colleagues synthesized a film based on carbon dots and carboxymethylcellulose (CMC). The tensile strength of the synthesized film was 55% higher than that of CMC film, which indicates that the addition of carbon dots improves the mechanical properties of the film, which is attributed to the strong interaction between the carbon dots and the film [57].

### *3.1.2 UV barrier*

Many nanomaterials can block UV light due to the combination of scattering and absorption effects, so they are used as a light blocker in packaging materials and protect light-sensitive foods. Research has shown that carbon dots are very effective in absorbing UV light. Although the mechanism of blocking UV light at carbon dots is not yet fully understood, some attribute this feature to functional groups on the surface of carbon dots. Transition electrons are transferred from n to π\* or π to π\*, which are mainly supplied by different functional groups, which in turn depend on the precursor used and the synthesis method. In films with carbon dots, intramolecular proton transfer in the excited state by O ... H ... O, O ... H ... N tunnels and abundant conjugate structures provide optical stability and film-blocking properties against UV light [58]. Polyvinyl alcohol is a semicrystalline polymer that is widely used in packaging due to its advantages, such as linear and strong structure, nontoxicity, biocompatibility, and thermal stability. One of its disadvantages is the passage of light, which limits its use in the packaging of light-sensitive products. CQDs were synthesized in one study and combined with the polyvinyl alcohol polymer drying method. The results showed that the ratio of UV light transmission is inversely proportional to the amount of CQDs, so it can be concluded that CQDs improve the inhibitory properties of the polymer against UV light [56].

#### *3.1.3 Impact on gas and water vapor permeability*

One of the important features of food packaging materials is permeability to oxygen and water vapor because they affect chemical reactions and microbial growth and thus food safety and quality. The addition of carbon dots to polymer films affects the permeability to oxygen and water vapor, the extent of which depends on the molecular and physicochemical properties of the carbon dots and polymer. Amphiphilic carbon dots are suitable for controlling the wettability and permeability of the polymer layer. Doping carbon dots and radical polymerization can be used to prevent phase separation of carbon dots in polymer networks and thus improve film permeability. The permeability of canola protein films increases after the addition of carbon dots with hydrophilic groups on their surfaces. Also, chitosan films with improved water absorption properties have been made using different types and amounts of carbon dots [58].

On the other hand, CQDs can increase water resistance, which is attributed to the reduction of layer porosity and the abundance of carboxyl groups at the surface of carbon dots, which result in a reduction in free radicals due to the reaction between hydroxyl and carboxyl. The hydroxyl functional group leads to a decrease in water absorption of the film. In one study, CQDs were used to improve the properties of polyvinyl alcohol/nanocellulose layers. By increasing the amount of carbon dots from 0.1 to 4 ml, the water absorption of the film decreased from 119.6 to 42%, which

indicates the improvement of the film barrier property compared with moisture using CQDs [59].

#### **3.2 Active packaging**

Food spoilage causes environmental pollution, devastating impact on human health, economic losses, and increased treatment costs; therefore, technology development is needed to reduce food waste and improve food security. One possible way to reduce food waste and spoilage is to develop active ingredients for active packaging to increase product shelf life. Active packaging includes coatings with antimicrobial and antioxidant properties. These agents can be included in conventional nondegradable packaging or used in combination with biodegradable components [54].

#### *3.2.1 Antimicrobial effect*

Increasing resistance of microorganisms to antibiotics and other disinfectants has led food industry researchers to look for alternative antimicrobial approaches [51 m]. In one study, sulfur quantum dots (SQDs) were used to prepare food packaging film. The films showed strong antioxidant and antibacterial activity against bacterial food pathogens (*Escherichia. coli* and *Listeria monocytogenes*) and fungi (*Aspergillus niger* and *Penicillium chrysogenum*). When the film was used as a bread wrapping test, the film prevented mold growth for 14 days [60].

CQDs have shown great potential in killing and inhibiting bacteria, fungi, viruses, and drug-resistant species. The mechanisms of this property include the adhesion of CQDs to the bacterial surface, destruction of the bacterial membrane or cell wall, induction of oxidative stress through RNA/DNA degradation, and oxidative induction of proteins and other intracellular biological molecules. The most important killer effect of CQDs on microorganisms is due to the production of reactive oxygen species, which leads to the production of hydroxyl free radicals and single oxygen, which destroys vital biomolecules in the cell, and inactivates intracellular proteins, mitochondrial dysfunction, and lipid peroxidation, and eventually, the cell dies [61]. Ezzati et al. used sulfur-modified CQDs to modify the gelatin/pectin film in food packaging. The resulting film showed strong antimicrobial properties against foodborne pathogens such as *E. coli* and *L. monocytogenes* [62]. In another study, nitrogen-doped CQDs were synthesized and used to modify cellulose nanofibers. The obtained film showed high antibacterial and antifungal activity due to the production of reactive oxygen species. The prepared film was used for packaging tangerine and strawberry fruits. The results showed that the growth of fungi on the fruit surface was effectively inhibited, and their shelf life was increased by 10 and 2 days, respectively, indicating its good potential for use in active packaging [63].

Fan et al. used a combination of CQDs and chitosan to inhibit microorganisms in the packaging of freshly cut cucumbers. CQDs established a strong hydrogen bond with chitosan. The results showed that the diameter of the inhibitory zone against *E. coli* and *Staphylococcus aureus* improved with the increasing concentration of CQDs. In addition, the total number of bacteria, molds, and yeasts decreased during storage [64].

#### *3.2.2 Antioxidant properties*

ROSs, including single oxygen and free radicals (hydrogen peroxide, hydroxyl, and superoxide), play an important role in food spoilage, chemical degradation,

polymers, and the destruction of biological structures. Therefore, the presence of antioxidant compounds or free radical scavengers in food and food packaging is essential for good health. Recently, a range of carbon nanoparticles such as nitrogendoped carbon dots, selenium**-**doped carbon dots, and nitrogen/sulfur-doped carbon dots have been turned into antioxidants to remove ROS. The antioxidant properties of carbon dots are attributed to electron transfer, unpaired electrons due to surface defects, hydrogen donor behavior, doping element, and surface functional groups. The carbon dots synthesized in the NaOH electrolysis medium have more active oxygen-containing groups such as carbonyl and hydroxyl, which can be used as hydrogen donors to scavenge free radicals, resulting in higher antioxidant capacity. In addition, recent studies have shown that carboxyl and amine groups directly or indirectly through the hydrogen atom transfer (HAT) mechanism cause the removal of ROS and thus increase the antioxidant activity of CQDs. In this mechanism, hydrogen or electron is transferred from carboxyl and amine groups to active species such as ROS and deactivates them [65].

#### **3.3 Smart packaging**

The purpose of this type of packaging is to monitor the condition of the food or the surrounding environment. In this technology, a visual indicator gives the supplier or customer information about information such as product freshness, packaging leakage, storage temperature throughout the production chain, and corruption. Nanoparticles are used as reactive particles in packaging materials to inform the status of the package. In other words, they are nanosensors that respond to environmental changes (such as temperature, humidity, oxygen level, and microbial contamination). When nanosensors are integrated into food packaging, they can indicate specific chemical compositions and product freshness [66].

#### *3.3.1 Product freshness indicator*

This is done using intelligent detection technology and labels that can measure changes in environmental conditions inside the package. Among the many methods, smart labels have received much attention due to their advantages, such as accurate results, high sensitivity, and ease of use. The color change of the smart label attached to the packaging container indicates the freshness of the product. By monitoring the color change of the indicator, it can be seen that the food is unsuitable for consumption. For example, in marking the freshness of meat packaging, the basic principle is that the marker on the label is sensitive to volatile nitrogen compounds and amines in the packaging environment or changes in ambient pH caused by such components. When such sensitive changes are detected, they can be converted to response values, which are usually color changes that can be detected by the naked eye, so that the freshness of the meat is detected in real time [65]. Cauchy and colleagues used soy protein isolate carbon dots in anthocyanin-based smart films to monitor the freshness of packaged pork. When the meat samples were stored at 25°C, the activity of microorganisms increased the amount of volatile nitrogen inside the package, followed by a change in pH. As the shelf life increased, the fresh pork deteriorated, and the color of the detector changed from purple to green [67].

#### *3.3.2 Oxygen indicator*

At first, the need for oxygen markers in food packaging may not seem necessary until we realize that the main cause of further food spoilage is oxygen. Much of this spoilage is indirect because oxygen allows a myriad of food spoilage aerobic microorganisms to grow. Oxygen can also cause direct spoilage of many foods through enzyme-catalyzed reactions, including browning fruits and vegetables, ascorbic acid degradation, oxidation of a wide range of flavors, and nonenzymatic reactions such as lipid oxidation. During storage, changes in oxygen levels in the packaging are an important indicator of food freshness and product respiration rate. Given the key role of oxygen in food spoilage, it is not surprising that most foods are packaged in a predominantly oxygen-free environment under a modified atmosphere. The main problem with the modified atmosphere is the lack of a simple, inexpensive oxygen indicator that assures the consumer that the package is safe and that oxygen input is insignificant. At present, this level of quality assurance is not possible in modified atmospheric foods that rely solely on a routine sampling of the packaging line. Typically, one packet out of every 300–400 packets is taken out of line for testing by a technician and checked using an expensive analytical system such as FT-IR or GC, and if sufficient packages are found that are not sealed enough, all 300–400 previous food packages are considered unsafe and are destroyed or repackaged. This unfavorable situation has led to special attention to cheap, reliable, and simple oxygen markers for food packaging [68].

One of the common methods for detecting oxygen in food packaging is the use of optical sensors, which use fluorophore nanoparticles in a solid matrix. Zhu et al. used a combination of silane-activated carbon dots as oxygen-insensitive fluorophore and ruthenium dihydrochloride as oxygen-sensitive fluorophore to prepare the fibrous membrane. The fibrous membrane prepared by photoluminescence ratiometric imaging is well able to evaluate the amount of oxygen in the packaging of fruits so that the fluorescence of CQDs is at a wavelength of 400 nm and its intensity does not change with changing oxygen concentration, but the intensity of fluorescence emitted by ruthenium at 600 nm depends on the oxygen concentration. They also used a digital camera instead of an expensive optical spectrometer to monitor oxygen concentration. Then, a fiber membrane was used to wrap the grapes, which initially emitted bright blue fluorescence but changed to purple on the third day of storage due to fluorescent respiration. As the respiration process increased, the fluorescent dye turned bright red on days 6 and 7 of storage. Therefore, it can be said that these fibrous membranes have a good potential for assessing oxygen in the packaging of agricultural products [69].

#### **3.4 Biomaterial-based packaging**

Common packaging materials include plastics such as polyvinyl chloride, polystyrene, polyethylene, and polypropylene, which are widely used due to their strength, heat, and moisture resistance. The use of these materials is a threat to the environment due to their nondegradable nature. Therefore, it can be said that biodegradable materials are the best choice for packaging. These coatings and films are monolayers, bilayers, or multilayers of polysaccharides, lipids, and proteins that, due to their sensitive nature, poor mechanical properties, and poor sealing properties, cannot be completely substituted for other materials [70]. Xylan is one

of the hemicelluloses and the second most abundant polysaccharide in plants, which usually has poor mechanical performance due to its relatively low molecular weight. Modifying them with carbon dots improves their filmmaking and mechanical performance. Yang et al. used carbon dots to modify the carboxymethylzylan matrix. The results showed that carbon dots, in addition to creating excellent optical properties, improve the thermal stability and mechanical strength of nanocomposite films, so that by adding 92.1 wt% of carbon dots, due to creating a chemical bond between xylan and carbon dots, the tensile strength and modulus of elasticity increase by 114.3 and 90.7%, respectively. Also, this film can effectively absorb UV light and convert it into blue light. Due to the mentioned advantages, this film has a good capability for food packaging [71].

Schmitz and colleagues synthesized a nanocomposite by combining zein and QDs. The prepared QDs were used as nanofillers to obtain zein-based nanocomposite films that showed good visual appearance, homogeneity, and clarity. The presence of QDs increased hydrophobicity and reduced the water absorption of composite films up to three times compared with pure zein. The presence of QDs in the films led to extensive UV blocking in the absorption spectrum. Antimicrobial assays showed that zinc oxide nanoparticles loaded into zein films were promising antibacterial agents, as growth inhibition of *S. aureus* reached (96.5 ± 4.9) % to 44.8 wt% of ZnO nanoparticles [72]. Grzebieniarz and colleagues synthesized films based on natural resources such as potato starch and chitosan by combining QDs and cadmium sulfide. They performed a storage experiment using poultry meat coated with films produced to assess microbiological quality. The results showed that composites restrict the growth of selected microorganisms in poultry meat [73]. Priyadarshi et al. used sulfur quantum dots (SQDs) as functional fillers for biopolymer films in food packaging. SQDs showed antimicrobial activity with almost negligible toxicity. The addition of SQD to gelatin/agar composite films also increased the UV barrier property without compromising color and clarity. The film showed excellent antioxidant activity, moderate antibacterial effect on *L. monocytogenes*, and inhibitory effect on *E. coli* [74]. Zhang et al. made functional films based on carboxymethylcellulose (CMC) by adding different amounts of CQD. The CQDs were evenly distributed in the polymer matrix to form a very clear UV-blocking film. The addition of CQD increased the tensile strength (up to 27.6%) and elastic modulus (up to 61.5%). The films showed excellent antioxidant and antimicrobial activity. Lemon fruits were coated with film solutions to test the performance of the film. Lemons coated with CMC/CQD film showed an excellent appearance without mold growth even after 21 days of storage [75].
