**2. QDs in food analysis**

The development of modern agriculture and the food processing industry can lead to chemical and biological food contamination. Food analysis is essential for quality assessment as well as safety control. Common methods of analysis include spectroscopy, immunoassay, culture and colony counting, chromatography, nuclear magnetic resonance, and electrochemical methods. The advantages of these methods include reproducibility, sensitivity, and high selectivity. However, these methods require expensive equipment, trained personnel, and long separations [6]. So, finding faster, more accurate, more sensitive, and cheaper methods of analysis has always been the focus of researchers and food manufacturers. In this regard, fluorescence sensors based on QDs can play a key role in ensuring food quality and safety [7]. Various strategies have been developed to improve the performance and selectivity of sensors based on QDs. The following are the most common techniques for food analysis:

a.**QD/CQD@MIP:** New and effective analytical methods in nanomaterial-based food samples include QDs and CQD. One effective approach to increase the

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

> selectivity of these nanomaterials is their combination with a molecularly imprinted polymer (MIP). Combining the unique optical properties of QD/CQD with a high selection of MIPs is a significant advantage of QD/CQD@MIP. In most cases, the adsorption process in MIP proportionally reduces the intensity of QD/CQD fluorescence, making it valuable for quantitative analysis, so it has promising potential to build sensors with high stability, sensitivity, and selectivity response. Preparation of QD/CQD@MIP conjugates consists of two main steps. In the first step, fluorescent QD/CQD are synthesized as nuclei. Then, the prepared fluorescent nucleus is used directly for QD/CQD@MIP synthesis. In some cases, the surface modification process is performed during or after the synthesis of fluorescent dots. The QD/CQD modification process provides appropriate functional groups or compatibility with synthetic medium for the better embedding of fluorescent nanoparticles in the MIP. Methods used to prepare the nucleus for the synthesis of QD/CQD@MIPs include silylation, treatment with mercaptoperboxylic acids, acrylic acids, oleic acid, polyethylene glycol, polyethylene imine, and L-cysteine. The second step is the MIP printing process, which often uses three polymerization techniques, including the sol-gel method, free radical polymerization, and reverse microemulsion method [3].

b.**MNPs@QDs:** Food pollutants are one of the main factors jeopardizing food safety, and with biological accumulation, they are considered as a serious threat to human health. Currently, there are several instrumental analytical techniques for identifying different food contaminants, which, due to having disadvantages such as the need for trained people, complex pretreatment process, and long time, make them unsuitable for quick and on-site detection. MNPs@QDs nanocomposites have good application potential for the analysis of food parameters. Magnetic nanoparticles (MNPs) have a strong magnetic response and are quickly recovered under an external magnetic field, so they have good potential for easy control of adsorption and release of target analytes at low concentrations in complex food samples. MNPs are environmentally friendly because they can be recycled multiple times. In addition to taking advantage of the magnetic properties of MNP, magnetic fluorescent QDs nanocomposites (MNPs@QDs) have QDs-induced fluorescence properties that can rapidly enrich target analytes and separate complex food matrices under a magnetic field. These nanocomposites are also capable of quantitative analysis of analytes, thus greatly simplifying the pretreatment process, which prevents analyte wastage, reduces detection time, and increases detection efficiency [8].

## **2.1 Detection of food additives**

Additives are widely used in food processing to improve flavor and shelf life and enhance nutritional value. Additives have always been abused and are one of the potential causes of food safety problems. It is necessary to develop a rapid and reliable diagnostic method to strengthen the monitoring of food additives [9]. **Table 1** provides an overview of recent studies on using QDs to detect food additives.

#### **2.2 Detection of pathogen**

One of the main causes of death in the world is infectious diseases caused by food. Infection caused by food pathogens causes great financial losses to the industry due to delays in product distribution and market recall, in addition to threatening consumer


**Table 1.**

*Application of QDs in the detection of food additives.*

health. A common method for detecting pathogens is culturing a specific species and then examining its biochemical and immunological properties, which is very time consuming [19]. For this reason, fluorescence nanosensors based on QDs, which are faster in detecting pathogens, can be effective. The research done in this field is presented in **Table 2**.

#### **2.3 Detection of heavy metals**

The presence of heavy metal ions such as lead, mercury, cadmium, and arsenic in food is attributed to water and soil pollution. Heavy metals cause irreversible changes in protein structure and negatively affect cell function. Excessive consumption can cause side effects such as neurological disorders, kidney damage, and bone damage [27].

In one study, fluorescent nanoprobes were used to determine the multiplicity of Hg2 +, Cu2+, and Ag+ ions. Nanoprobes (CQDs-QDx) were designed by mixing CQDs and multicolor CdTe QDs. CQDs were not sensitive to heavy metal ions. At the same time, CdTe QDs showed a size-dependent fluorescence response to different heavy metal ions, thus creating a ratiometric detection scheme by measuring the fluorescence intensity ratios of CQDs-QDx systems. By evaluating the detection performance, CQDs-QDx (x = 570, 650, and 702) were successfully used to differentiate and quantify Hg2 +, Cu2 +, and Ag+ ions. In addition, they also detected heavy metal ions in actual samples with acceptable results [28]. **Table 3** lists other studies for the detection of heavy metals.

#### **2.4 Detection of insecticide and antibiotic residues**

Insecticides are widely used in agriculture to control weeds and pests and also to improve food production. Excessive use of insecticides can lead to contamination of


#### **Table 2.**

*Application of QDs in the detection of pathogen.*

the target environment and, consequently, food contamination. Negative effects of insecticides on humans include neurotoxicity, endocrine disorders, mutagenicity, and carcinogenicity. Therefore, it is necessary to have a sensitive and selective method for analyzing these compounds [36]. A ratiometric fluorescent sensor was synthesized using CQDs in a study to detect acephate, an organophosphate pesticide. The fluorescent quenching mechanism is related to the inner filter effect (IFE). The limit of detection of this method was 0.052 ppb, which is much lower than the standards for acephate from the EU. The ratiometric fluorescent sensor was successfully evaluated for the detection of OPs in tap water and pear samples [37].

Antibiotics are widely used in agriculture, animal husbandry, aquaculture, and pharmaceutical industries. If left unattended, they can harm human health through the food chain and the environmental cycle and can threaten food quality as a potential hazard. Penicillin, for example, causes allergic skin reactions. Streptomycin damages the kidneys and auditory nerve, and tetracycline causes liver damage and yellowing of the teeth. Therefore, it is necessary to develop antibiotic detection methods that have a lower limit of detection than the legal limit [38]. Tetracycline is an antibacterial compound widely used in food-producing animals to treat various bacterial infections due to its low cost and high practicality. Residues of this antibiotic in food products enter the human body through the food chain and cause allergic reactions, digestive disorders, dizziness, muscle pain, and headaches. On the other hand, excessive use of this antibiotic causes the formation of antibiotic-resistant strains. The common methods of determining the residues of tetracyclines include high-performance liquid chromatography and enzyme-linked immunosorbent assays (ELISA). The disadvantages of these methods include a long time and low sensitivity. Therefore, it seems necessary to develop an extremely sensitive method for determining the content of tetracycline. Wang et al. synthesized tungsten oxide quantum dot for tetracycline detection. The fluorescence of the synthesized QDs is quenched after


#### **Table 3.**

*Application of QDs in the detection of heavy metals.*

the addition of tetracycline due to synergism of IFE, Förster resonance energy transfer (FRET), and photoinduced electron transfer (PET). They successfully tested the sensor to detect tetracycline with a high recovery rate in milk and milk powder [39]. **Table 4** lists some recent research to detect insecticide and antibiotic residues in food.

#### **2.5 Detection of nutritional components**

Numerous studies have been performed to use fluorescence sensors to detect nutrients such as ascorbic acid in fruits and vegetables [49], vitamin B12 [50], retinoic acid [51], and others. One of the common mechanisms in the detection of nutrient components is the fluorescence of QDs covered by metal ions and then the fluorescence recovery by the analyte. In other words, electrostatic interactions between metal ions and the surface groups of QDs cause electron transfer and fluorescence coating. By adding an analyte, fluorescence is restored due to analyte chelation and metal ions. This mechanism has been used to detect glutathione and thiamine in food [9].
