Advances in carbon quantum dot technology for food safety: preparation, modification, applications and mechanisms

Functionalized carbon quantum dots (CQDs) show great potential for application in the field of food safety. CQDs have attracted widespread attention in this regard due to the wide range of sources of raw materials for their synthesis, and their good biocompatibility and stable fluorescence. This paper analyses the properties of CQDs and compares with those of conventional semiconductor quantum dots (SCQDs). It analyses the similarities and differences between hydrothermal carbonization, pyrolysis and microwave-assisted synthesis of CQDs, and reviews the principles and methods of functionalization of CQDs through surface modification and doping. Finally, it discusses the applications of functionalized CQDs in food safety, such as detection and sensing, bio-inhibition and photocatalytic degradation, and the mechanisms of detection.


Introduction
Carbon quantum dots (CQDs) are zero-dimensional fluorescent nanomaterials with the advantages of good biocompatibility, resistance to photobleaching and simple synthesis methods [1] .A wide range of materials that can be used to synthesize CQDs is wide, as anything containing carbon, such as large carbon skeleton (e.g., carbon target), organic molecule and biomass materials, can be used as a raw material.Biomass materials are available from a wide range of sources, including natural biomass (food waste, straw, crab shells, etc), biomass fractions (cellulose, lignin, etc) and micro-molecules derived from biomass (citric acid, glucose, etc) [2].At present, the commonly used preparation methods include hydrothermal method, microwave-assisted method, pyrolysis method, etc.The prepared CQDs usually have low fluorescence quantum yield and single function.Modification of the functionalization of CQDs can improve their luminescence and give them the ability to bind directly or indirectly to various substances.CQDs have been used in the detection of heavy metal ions [3], pesticides and antibiotics [4], photocatalytic decontamination [5] and other applications.
With regard to sensing in foods, CQDs can be combined with biology and immunology to achieve improved detection levels.Not only can CQDs be used for the detection of heavy metal ions, but they can also be used in combination with antibodies, nucleic acid aptamers and other nanomaterials for the qualitative/quantitative detection of specific organic substances or bacteria.This makes them suitable candidates for new means and methods for rapid detection in biosensing applications.The current challenges for the development of CQDs in the food safety are to effectively improve fluorescence quantum yield, to increase the use of agricultural and food waste as raw materials for their preparation and to develop new detection applications with regard to foods.
In this work, we firstly review the properties, preparation methods and functionalization pathways of CQDs, mainly focuses on the application of CQDs in the field of food safety, and then discusses the detection principles of different types of CQDs as well as the influencing factors of the detection and processing of toxic and hazardous substances.Finally, we analyze the development prospects and challenges of CQDs in the current stage (figure 1).

Structure and characteristics of CQDs
Carbon quantum dots (CQDs) are zero-dimensional carbon nanomaterials with sizes ranging from 1 to 10 nm [6].It has attractive properties similar to semiconductor quantum dots (SCQDs), such as good electrical conductivity, low toxicity, high chemical stability, strong photoluminescence and unique optical properties [7][8][9].Conventional SCQDs have a core-shell structure (an outer shell wrapped around a central core) that can be doped with other elements, and the surface of the core-shell can be modified to improve the luminescence [2,10].CQDs are mainly composed of carbon, hydrogen, and oxygen, with a higher content of carbon.The edges of CQDs are covered with many oxygen-containing groups, and the carbon nucleus is mainly sp 2 /sp 3 carbon.Currently, there are reports on structures such as diamond like, graphite like oxide, and amorphous carbon [11].It endows CQDs with stability and chemical inertness.The mechanism of the photoluminescence (PL) of CQDs is not known, but the emission wavelength can be modified by modifying the synthesis parameters, including size, shape, composition, internal structure and surface modification [12].The structures of SCQDs and CQDs are shown in figure 2.
Traditionally, CQDs were synthesized using non-renewable resources such as coal, petroleum coke, citric acid, acetic acid and octadecenoic acid [13].However, in recent years, it has become possible to prepare CQDs using biomass waste (agricultural waste, food waste, etc), which means that the synthesis of CQDs has unique and environmentally friendly [14] properties.The extraction of carbon sources from biomass waste is achieved by hydrothermal carbonization (HTC) or pyrolysis processes, which do not require the use of solvents that are difficult to decompose and thus facilitate large-scale production.The surface of the prepared CQDs is covered with hydroxyl, carboxyl, amino and carbonyl groups [15].The presence of these groups enables the CQDs to be easily dispersed in water and able to interact with antibodies [16], proteins [17,18], nucleic acids [19,20] etc through intermolecular forces.At the same time, CQDs have a broad and strong visible near-infrared emission spectrum, which can be improved both optically and electronically by metallic or non-metallic elemental doping of the CQDs [21].
The superior physicochemical properties of CQDs make them stronger than SCQDs and this enables a wide range of applications in bioimaging, biosensing, photocatalysis, drug delivery etc [22][23][24].

Synthesis of CQDs
CQDs preparation methods fall into two categories: top-down methods, in which CQDs are exfoliated from macroscopic matter, and bottom-up methods, in which CQDs are synthesized from small molecules of matter [25].Since 2004, when Xu et al [26] accidentally discovered CQD particles when using the arc discharge method for the preparation of single-walled carbon nanotubes, the methods for preparing CQDs have developed significantly (table 1).
Hydrothermal carbonization (HTC) and pyrolysis processes are convenient for large-scale production because they do not require the use of complicated solvents, and can even be undertaken without the use of solvents.The use of these methods and the microwave technique for the preparation of CQDs is described below.

Hydrothermal carbonization (HTC)
Hydrothermal carbonization is an effective way to convert biomass into CQDs.The method involves mixing biomass with solvent (water) in a specific proportions under a controlled temperature and pressure for a specific time [38].The reaction is carried out hydrothermally by controlling the temperature to a specific value within the range 120 °C-280 °C, up to 375 °C, for a specific time between 0.5 and 24 h, at the self-produced pressure of the reaction process (2-6 mPa) [39][40][41][42].The use of HTC is attracting attention due to its mild reaction conditions and the fact that the it is an economical and environmentally friendly means of producing CQDs.The reaction takes place in the subcritical water state, where the biomass undergoes reactions such as hydrolysis, dehydration, decarboxylation and polymerization to form small molecules of carbon monomers, which are then repolymerized by pyrolysis to form CQDs. The HTC reaction is affected by the reaction temperature, time and additives [43].Use of a higher temperature leads to decreased of yield and increased pore size, acidity and aromaticity.When the temperature is below 250 ℃, the specific surface area (SSA) increases with the increase of temperature [44].The relationship between SSA and temperature is less well studied when temperatures above 250 ℃.But according to the study of Wang et al [45], when the temperature is above 250 ℃, the SSA decreases with the increase of temperature.But the decrease in heavy metal migration and ecological risk.A longer reaction time has no significant effect on the yield, but otherwise has the same effects as those produced by increasing the temperature.The selection of the biomass and additives used has a huge impact on the quality of the CQDs produced, such as SSA, production rate, porosity and so on.The equipment used for the HTC procedure is shown in figure 3(a).

Pyrolysis
Pyrolysis is another effective method for the large-scale production of CQDs.The equipment used for the pyrolysis and the schematic diagram of principle are shown in figure 3(b).Pyrolysis is the carbonization of biomass materials in a gaseous medium (air, oxygen, argon, nitrogen and other gases) at high temperature (600 °C-1200 °C) without the addition of any solvent.And several chemical reagents such as Sodium hydroxide (NaOH), Lithium chloride (LiCl) and Zinc chloride (ZnCl 2 ) can also be used to assist in the reaction to prepare CQDs with a porous structure and a high specific surface area [46,47].Arc discharge method Graphite rod CQDs were prepared using graphite rods as the anode and cathode in a low-voltage arc chamber.The crude product was treated with concentrated nitric acid to enhance its solubility in water, and extracted with NaOH and gel electrophoresis to obtain the final product.

∼1 nm
The fluorescence performance of the product is good, but the quantum yield is low and the purification process is complicated.
Electrochemical oxidation process

Carbon
Multiwalled carbon nanotubes were prepared by electrochemical oxidation.
-Uniform size CQDs with stable properties are obtained, with strong repeatability.
Detection of heavy metal ions and erythromycin [28,29] Chemical oxidation process Carbon-based -5-8 nm A variety of carbon-containing molecules or polymers can be used as carbon-based precursors. - Laser ablation Precursor The laser beam illuminates the carbon target, which spalls the carbon nanoparticles.

∼7.5 nm
The synthetic source material is rich, but the yield is not high.

Detection of Cu 2+ [31]
Carbon target After high-temperature calcination, a carbon/silica complex was obtained.
-The particle size is uniform and the purity is high, but the cost of the method is high and the template residue hinders the extraction of CQDs, thus affecting the yield. - Template method Functionalized Rapid carbonization at high temperature ∼1.12 nm CQDs have high purity, low cost and wide range of carbon source Silica spheres and soluble phenolic A solution of the organic precursor is put into a reaction kettle where it undergoes a high-temperature and high-pressure hydrothermal reaction.
-This is a simple and low-cost operation, and the synthesis process is environmentally friendly and the raw materials are easy to obtain.
Detection of heavy metals, bacteria, organic molecules etc [34] Pyrolysis Resin Using microwaves (electromagnetic waves with wavelengths of 1 mm to 1 m), the chemical bond breaking in the carbon-based precursor was rearranged to produce carbon nanoparticles.

2-3 nm
The synthesis time is short, the process is simple and the carbon source material is rich, but the size distribution of the CQDs obtained is uneven.

Heavy metal sensor [35]
Hydrothermal carbonization (HTC) biomass Mixing biomass with solvent (water) in a specific proportion under a controlled temperature and pressure for a specific time.Dispersing biomass precursors in an aqueous solution and synthesizing CQDs at high temperatures and pressures.
∼1 nm HTC is mild reaction conditions and the fact that the it is an economical and environmentally friendly means of producing CQDs.

Detection of diazinon [36]
Microwave Carbon-based CQDs are produced by microwave reaction of carbon-based raw materials.
6nm Good transmission efficiency of the microwaves greatly improves the synthesis rate and enhances the optical properties of the CQDs obtained.However, the preparation of metal-doped CQDs by microwave method has some limitations.

Detection of Ascorbic Acid [37]
Both HTC and pyrolysis require the use of additives or templates to produce CQDs with a high specific surface area and pores.However, as the HTC reaction involves the use of a solvent (water) medium, it is thought to be able to produce CQDs with more functional groups.HTC and pyrolysis are both limited by the use of auxiliary chemicals and low yields, but are economical and environmentally friendly processes.The development of self-activating or in situ activation reaction techniques [48] is the way forward for the HTC and pyrolysis routes.

Microwave
In recent years, microwave assisted method is a technique that has been developed for preparation of CQDs.It is simpler than the HTC and pyrolysis methods [49].Indeed, CQDs can even be easily prepared at home using a microwave oven, glassware and biomass.In the laboratory, microwaves are often used in conjunction with thermoelectric or pyrolysis, in which case the procedures are known as microwave-assisted thermal methods (microwave-assisted heating) [13].Good transmission efficiency of the microwaves greatly improves the synthesis rate and enhances the optical properties of the CQDs obtained.The microwave method is a simple, rapid and environmentally friendly mean of preparing CQDs with oxygen-rich groups on the surface [50].However, considering that the transmission of microwaves can be interfered with by metals, there are limitations on the preparation of metal-doped CQDs using this method.
The preparation methods of carbon quantum dots (CQDs) have been summarized in more articles [51,52].In this paper, the current preparation methods of carbon quantum dots are summarized in table 1.The hydrothermal carbonization, pyrolysis and microwave methods are the most commonly used methods for preparing CQDs.

Functionalization of CQDs
The appropriate functionalization of CQDs is important to achieve the required advantageous properties for their subsequent application [53].There are two methods of functionalization: surface modification and doping.Surface modification is divided into two categories: covalent modification and non-covalent modification [54].

Covalent modification
Covalent modification is achieved by directly modifying the surface of the CQDs with organic molecules to improve and increase functionality, and then improve thire properties [55] (figure 4).Yu et al [56] used covalent coupling to aminate on the surface of CQDs through amidation (N-hydroxysuccinimide (NHS) and 1-(3dimethylaminopropyl)−3-ethylcarbodiimide hydro (EDC) modification, coupled with water-soluble amphotericin B (AmpB)) to obtain a N-CQDs@AmpB composite fluorescent probe for the detection of Candida albicans.Wang et al [57] synthesized CQDs by the microwave method using citric acid and urea.The CQDs then underwent sulfonylation with the reaction products of sulfoxide chloride and 1H-imidazole-4-carboxylic acid to obtain CQDs-imidazole materials for the detection of trace water in anhydrous ethanol.Algarra et al [58] modified the surface of CQDs by esterifying the carboxyl group of mercaptosuccinic acid with the hydroxyl group on the surface of CQDs.The functionalized CQDs were used to detect Ag + , and the obtained detection limit was 385.8 nM and the quantification limit was 1.2 μM.Li et al [59] used cyclodextrin as the carbon source to prepare CQDs with a hydroxyl group rich surface, which polymerized with glycidol at high temperature to produce a CQDs-HPG material that could be used for in vivo imaging of human alveolar basal epithelial cells (A549 cells) in patients with lung cancer.Hou et al [60] prepared a CQD polymer through the silylation reaction of (3-aminopropyl) triethoxysilane (APTES) and Tetraethyl orthosilicate (TEOS).The polymer was used to detect tetracycline in milk, and the detection limit achieved was 5.48 nmol l −1 .

Non-covalent modification
Non-covalent modification (figure 4) is different from direct modification in that it is more flexible in terms of the methods and raw materials used.Usually, metal particles or other materials, such as proteins, DNA and other polymers, are used to coat the CQDs or to combine them to form composites.In this way, the particle size is increased, the particles are homogeneous and the affinity of the carbon dots for other materials is imparted, thus realizing more efficient functionalization.Chen et al [61] achieved highly selective detection of 2-picolinic acid, a marker of the anthrax organism, with a detection limit of 5 nM, through complexing Tb 3+ with CQDs prepared using ethylene diamine tetraacetic acid.Jin et al [62] used carrot juice to prepare CQDs with a large number of carboxyl and hydroxyl groups on their surface.This gave CQDs that are be negatively charged overall, and can interact electrostatically with positively charged polyethyleneimine and Nile blue to form composites.The CQDs were used to achieve proportional two-photon fluorescence detection of sulfide.Cu 2+ can bind to the amino group of polyethyleneimine, leading to quenching of the fluorescence of the CQDs, while the addition of S 2-restores the fluorescence of the CQDs due to the stronger binding of S 2-to Cu 2+ .Jiang et al [63] synthesized silicon-doped CQDs using APTES and glycerol, and dopamine to modify the surface of the quantum dots through π-π interaction.The modified CQDs were used for the in vivo detection of Ag + in cells, with a detection limit of 2.5 nM in the detection range 5-50 nM.

Doping
Doping is the third way to modulate the fluorescence of CQDs.By doping the internal structure of CQDs with new elements, the CQDs can have stronger fluorescence, better biocompatibility and improved fluorescence stability.Wang et al [64] prepared nitrogen-doped CQDs with citric acid as carbon precursor and glutathione as nitrogen source for the sensitive detection of Cu 2+ , and achieved a detection limit of 0.27 nM with the range 0.20∼200.0μM.Wu et al [65] used citric acid and urea as raw materials to prepare nitrogen-doped CQDs by a hydrothermal method.The CoP-NCQD nanoflowers obtained were used as catalysts in the anodic oxygen evolution reaction to achieve high-efficiency catalysis.Meng et al [66] prepared boron-doped CQDs from citric acid, histidine and borax by the one-step hydrothermal method.The CQDs obtained were sensitive to Hg 2+ , with a detection limit of 2.5 μM.Zhuo et al [67] prepared copper-doped CQDs that were sensitive to hydrogen sulfide, an important intrinsic gas transmitter in mammalian tissue.They used a one-step hydrothermal method with ethylenediamine tetraacetate and copper chloride as raw materials.The CQDs were able to image and sense living cells due to the hydrogen sulfide present, with a detection range of 2∼500 μM and a detection limit of 0.5 μM.Applications of CQD doping are shown in figure 5.
Currently, there are fewer summaries of CQDs surface modification methods, mostly focusing on doping as well as functionalization applications.However, the surface modification methods of CQDs can provide an important basis for the preparation of carbon quantum dots composites and the development of new applications.In this paper, the functionalization methods of CQDs are summarized in detail, especially focusing on the surface modification methods of CQDs.It is hoped that it can provide a little help for the application development of CQDs.

Application and mechanism
Since their discovery, CQDs have been widely used in fluorescence detection due to their superior optical characteristics.After appropriate functionalization, CQDs can be used as probes for the detection of heavy metal ions (Fe 3+ , Cr 6+ , Ag + etc) [68,69], harmful organic compounds and biomolecules, which is of great use in the food sector.

Fluorescent 'on-off'
Oxalic acid is a common substance found in vegetables, especially those in the quinoa, umbelliferae and amaranth families, such as amaranth and spinach [70].Excessive consumption of vegetables containing oxalic acid can lead to kidney stone and urinary stone disease.This is due to the fact that oxalic acid tends to combine with Ca 2+ and Mg 2+ in the body, producing tiny precipitates.The precipitates flow through the body in the blood and are trapped by the kidney or ureter and remain in the kidney or bladder without being excreted, thus forming stones [71].Zhang et al [72] first found that the addition of oxalic acid could make the fluorescence quenching of the CQDs-Cu 2+ system.They therefore used nitric acid to oxidize activated carbon to prepare carbon dots.However, while this method was effective in improving the interference resistance of the modified CQDs to detect oxalic acid, it was not environmentally friendly and required the use of nitric acid.To solve the above problem [72], Li et al [73] used the peptone hydrothermal method to prepare CQDs with a particle size of 1.43 nm and a large number of hydrophilic groups on the surface.This method is more environmentally friendly than the nitric acid oxidation method, and the obtained linear range was in the range 8∼65 μg ml −1 , and the detection limit was 1.8 μg ml −1 .
The fluorescent 'on-off' principle is also used in the detection of other substances.Hu et al [74] prepared N-CQDs based on coal tar pitch by chemical oxidation, which can be used for the detection of metal ions Fe 3+ and Cu 2+ in water samples or food.The fluorescence of N-CQDs was quenched when N-CQDs were mixed with Fe 3+ , and when L-ascorbic acid was present in the system, it acted as a reducing agent and interacted with Fe 3+ , which led to the detachment of Fe 3+ from N -CQDs to detach and fluorescence was restored.Ru Fan et al [75] also used the mechanism of redox reaction between ascorbic acid and Fe 3+ to prepare CQDs for the detection of ascorbic acid using waste cardamom.Zhang et al [76] prepared CQDs by hydrothermal method using black soybean as raw material, and the fluorescence of CQDs was quenched due to the electron transfer (ET) effect between CQDs and Ce 4+ .Cefixime can effectively block the ET effect between CQDs and Ce 4+ , resulting in the recovery of fluorescence and the detection of the antibiotic cefixime, and then realize the 'on-off-on' of fluorescence.
Through the investigation of the mechanism of oxalic acid 'on-off' detection method, it was found that the fluorescence of CQDs 'on-off' method is due to the fluorescence quenching when CQDs coexist with metal ions in the solution.This is due to the transfer of electrons from the excited state CQDs to the metal ions, resulting in the electrostatic effect of non-covalent force; and the formation of complexes between oxalic acid, ascorbic acid and metal ions through the action of ligand bonding, which weakens the degree of fluorescence quenching of the CQDs-metal ions system.The detection mechanism is shown in figure 6.

Antigen-antibody immune cross-linking
Morphine is an opioid receptor agonist used medically for anesthesia, analgesia and antidiarrhea.It is also a major component of opium (4∼21%).The illegal addition of small amounts of morphine to food can cause food dependence and addiction in those who consume it.Masteri-Farahani and Mosleh [77] developed a low-cost and selective rapid morphine assay by cross-linking CQDs with anti-morphine antibodies using NHS and EDC.The CQDs-morphine antibodies did not interact significantly with other psychotropic drugs such as codeine and methamphetamine, and the detection limit for morphine was 0.06 μM.Zhang et al [78] used glutaraldehyde, another bifunctional cross-linker isotype, to cross-link CQDs with morphine antibodies.The CQDs-morphine antibodies were captured by immobilizing the encapsulated antibodies on a 96-well plate to form a FLISA assay plate that could be used for the rapid and inexpensive detection of morphine as an illegal additive in food.The detection range achieved was 3.2 × 10 -4 to 10 mg L −1 (R 2 = 0.992) with a detection limit of 3 × 10 -4 mg l −1 .Harpreet Singh et al [79] similarly used NHS and EDC to link CQDs with anti-aflatoxin M1 (AFM1) antibodies for detection of AFM1 levels in dairy products.Dong et al [80] also used NHS, EDC and magnetic nanoparticles based on carbon dots as well as anti-chloramphenicol (CA) antibodies to prepare composites obtained for the detection of CA in food.
The mechanism of cross-linking CQDs and morphine antibodies by NHS and EDC is as follows.There is a large number of carboxyl groups on the surface of morphine antibodies, while there are abundant amino groups on the surface of the CQDs.The reaction conditions for the carboxyl group and amino group are inconducive, so it is difficult for there to be any chemical reaction in general.EDC can cause the carboxyl group of the antibody to form an unstable active ester, while NHS can bind to this ester through the hydroxyl group on the surface to form a more stable active ester, thus avoiding hydrolysis.The active ester on the surface of the antibody can then be easily amidated by the amino group on the surface of the CQDs to form an amide bond.Cross-linking of morphine antibodies to the CQD nanoparticles is achieved.The cross-linking mechanism of glutaraldehyde is very different.Research into the mechanism revealed that glutaraldehyde has a symmetrical structure and is able to form Schiff bases (-N = C-) with both antibodies and CQDs with amino groups on the surface, linking the antibodies to the CQDs in a five-carbon bridge, and NaBH 4 is added to make the whole cross-linking more stable.
By exploring the mechanism of the morphine immunoassay, it was found that the specific recognition ability of the antibody itself could enhance the selection and anti-interference ability of the CQDs-morphine antibody.The cross-linking of the CQDs and the morphine antibodies, which enabled Försterian fluorescence resonance energy transfer (FRET) of the CQDs fluorescence, resulted in a reduction in fluorescence intensity.The added morphine binds specifically to the morphine antibody, FRET disappears and fluorescence is restored.

Molecular imprinting template
Sol-gel technology can be used to encapsulate CQDs in molecularly imprinted polymers (MIPs), and the resulting CQDs@MIPs composites can be used to detect specific substances, which is a further application after the functionalization of CQDs.
Sun et al [81] used mango peel as the carbon source and obtained CQDs by hydrothermal reaction.CQDs@MIPs for the specific detection of mesotrione were prepared using APTES with TEOS.The method showed good selectivity for mesotrione (imprinting factor 5.6) with a detection range of 15-3000 nmol l −1 and a detection limit of 4.7 nmol l −1 .Amjadi and Jalili [82] prepared a MIP@CDs/QDs composite material using CQDs and CdTe/CdS quantum dots (with APTES and TEOS as modifying agents) for the specific detection of the broad-spectrum fungicide diniconazole (DNZ).Hou et al [60] prepared CQDs by microwave, and used APTES and TEOS to prepare CDs@MIPs for the specific detection of tetracycline in milk.The detection range achieved was 20 nM to 14 μM, and the lowest detection limit was 5.48 nM.Zhang et al [83] formed composite materials by embedding CQDs in a covalent organic framework and then combining them with MIPs using tryptamine as a template.The composites were used to detect tryptamine in food with high selectivity and high sensitivity.Tryptamines are mostly found as biogenic amines in protein-rich meat or dairy products, and in food processing and production tryptamine is an important health regulatory indicator [84].These composites were shown to be an effective method for the convenient detection of tryptamine in food products.Excessive levels of domoic acid (DA) in edible shellfish and water bodies would be life-threatening.Wang et al [85] prepared CQDs@MIPs with APTES and TEOS for the specific detection of DA.
Exploration of the detection mechanism of the composite material CQDs@MIPs showed that the composite material quenches the fluorescence by binding the template molecules via the electrostatic force between specific recognition sites, and thereby realizes the specific detection of the template molecules.APTES, as a modifying agent, is able to undergo silylation with the hydroxyl group on the surface of the CQDs to form a siloxane bond, while the other end of the molecule is bonded to the template molecule.Then, the composite material CQDs@MIPs, which can specifically detect the template molecules, is synthesized with CQDs using TEOS as the precursor.The reaction mechanism is shown in figure 7.

Nucleic acid aptamer specific recognition
The application of CQDs in combination with aptamers for sensing is an important strategy of application that has been developed in the last few years [86].An aptamer (Apt) is a highly specific and selective oligonucleotide sequence with high affinity and specificity for specific screening targets [87].Du [88] prepared composite materials using CQDs and DNA tetrahedral signal amplifiers for the detection of Staphylococcus aureus and Salmonella typhimurium.Suitable sequences were screened for the synthesis of DNA tetrahedra as well as aptamers using the NUPACK nucleic acid analysis design software.The Apt and DNA tetrahedron (Td) were combined with CQDs and streptavidin magnetic beads (SA-MB), respectively, to selectively extract and identify pathogenic bacteria using a 'sandwich' method, and the fluorescent detection signal was amplified by the DNA tetrahedron.An aptamer sensor for sensitive detection of S. aureus was designed by Cui et al [89].The CQDs were modified on the aptamer as fluorescent donors, and were then bound to Fe 3 O 4 and the fluorescence quenched.The binding ability of S. aureus to aptamer was stronger than the binding ability of Fe 3 O 4 , resulting in the shedding of Fe 3 O 4 and the display of fluorescence.The linear range of the method was 50∼107 CFU ml −1 , and the detection limit was 8 CFU ml −1 .A schematic diagram of detection process is shown in figure 8.
Through exploring the biological detection mechanism of the combination of CQDs and the aptamer, it was found that the sandwich detection of CQDs and the aptamer results in higher detection specificity due to the different binding forces between different epitopes of the aptamer.At the same time, the simultaneous existence of aptamer functionalized CQDs and up-conversion nanoparticles provides a guarantee for parallel multichannel detection of multiple analytes [90].The use of aptamer sensors with CQD FRET and fluorescence 'onoff' methods allow for quantitative detection of bacteria with a certain degree of avoidance of background interference.

Bioinhibition
Qureshi et al [91] used waste pomegranate peel as the carbon source (pomegranate peel contains flavonoids, alkaloids, organic acids and other substances, which have insect repellent and antibacterial effects) to prepare CQDs with sizes ranging from 5 to 9 nm, and studied the antibacterial effect of the CQDs.The minimum inhibitory concentration (MIC) test for S. aureus, Klebsiella pneumoniae, Pseudomonas aeruginosa and Escherichia coli found that the MICs of the CQDs against S. aureus, K. pneumoniae and P. aeruginosa were 67, 72 and 84 μg ml −1 , respectively, but there was no effect on E. coli.Zhang et al [92] prepared La-CQDs using ATP and LaCl 3 •2H 2 O and the hydrothermal method, and N and P-CQDs using ATP and the hydrothermal method, and tested their antibacterial effect on S. aureus and E. coli.They found that the La-CQDs had a significant inhibitory effect on both E. coli and S. aureus.Liu et al [93] used the hydrothermal method to prepare N,Cl-CQDs from the stems of Impatiens balsamina, and studied their antibacterial activity against bacteria.It was found that the N,Cl-CQDs had an obvious inhibitory effect on Gram-positive bacteria but no obvious effect on Gram-negative bacteria.Lai et al [94] prepared blue fluorescent CQDs by hydrothermal method, and the CQDs showed strong antimicrobial effect against E. coli and S. aureus with MIC of 1.14 mg ml −1 .Gao et al [95] prepared CQDs from acetone and composite nanomaterials made with Ag 2 S were used as inhibitors of Staphylococcus aureus, Escherichia coli and methicillin-resistant Staphylococcus aureus.There are two reasons for the antibacterial effect of CQDs.One is that CQDs can induce reactive oxygen species through the effect of light [96], which can break the cell wall/membrane of the attached bacteria, causing damage to and inhibition of the bacterial DNA/RNA or protein, and thus achieving an antibacterial effect.The other reason is that the surface of CQDs is positively charged and has abundant amino groups, which makes the dots more liable to absorption by Gram-positive bacteria with a lot of carboxyl groups in their cell walls.The CQDs enter the cell mechanically, causing the cell wall to break down and the bacteria to die.
The fluorescent properties of carbon quantum dots can be used in the field of food safety detection for objects such as heavy metal ions, illegal additives, antibiotics, bacteria, etc, as well as in the fields of food biocontrol and photocatalytic degradation of organics.Carbon quantum dots are expected to provide new ideas in food safety detection, assurance and realization.

Conclusion and future perspectives
As a new type of fluorescent nanomaterial, CQDs have been widely used in the fields of detection sensing, bioinhibition and photocatalytic degradation.
HTC, pyrolysis and microwaves are three common methods for preparing CQDs.HTC and pyrolysis are used in industrial production because they do not require the use of complex or harmful solvents (or even no solvents) in the preparation process.HTC is the more advantageous method when preparing CQDs with abundant functional groups on the surface.Microwave preparation of CQDs is a simple process to carry out, but it is not suitable for the preparation of CQDs with metal doping, and has the shortcomings of low quantum yield and inhomogeneous particles.Metal/non-metallic doping and cross-linking agents such as NHS, APTES, EDC, TEOS and glutaraldehyde are important ways to realize functionalization of CQDs.modification of CQDs particles by surface or internal doping is still the main method used for functionalization.The most important factors in the preparation of CQDs are optimization of the quantum yield, improvement of the uniformity of dot size and the development of efficient and large-scale industrial production methods.
Research and development of the application of CQDs in the food industry is mainly focused on detection sensing technology.CQDs can be modified using the fluorescence 'on-off' method, antibody cross-linking method, molecular imprinting template method, nucleic acid aptamer modification method etc to achieve rapid and specific detection of target objects.At the same time, CQDs are also widely used in biological bacteriostasis and photocatalytic degradation of pollutants.Further exploration of their mechanism of action will create more applications of CQDs in detection sensing, photocatalysis and bacteriostasis.The exploration of functional materials or methods that can improve the optical properties or enhance the specific recognition ability of CQDs is an important research topic for the wider application of CQD technology in the field of food safety.

Figure 3 .
Figure 3.The equipment used for (a) HTC and (b) pyrolysis.

Figure 7 .
Figure 7. Schematic diagram of the reaction mechanism of CQDs@MIPs.

Figure 8 .
Figure 8. Schematic diagram of a CQD DNA tetrahedral composite assay.