In the field of food analysis, CQDs acts as a sensing probe to detect and analyze functional components of food such as protein, vitamins, phenolic compounds and so on, which not only has excellent optical properties but also has significant advantages of pro-environment, low price, convenience and speediness. Foods can provide enough energy and nutrients for human body, such as protein, fat, carbohydrates, vitamins and minerals, which are essential for normal life activities. As a result, the detection of nutrients in foods is very necessary.
The content of ovalbumin (OVA) is deemed as a reference for evaluating the quality of protein
[6]. Fu et al. synthesized a novel CQDs co-doped with N, O, P (NOP-CQDs) through one-step hydrothermal method and applied for quantitative detection of OVA in the egg products
[7] (). In the fluorescent resonance energy transfer (FRET) system composed of NOP-CQDs, graphene oxide and anti-OVA, the “on-off” sensing probe has achieved the selective recognition and capture of OVA based on the specific interaction of antigen-antibody, which achieved a limit of detection (LOD) of 153 µg L
−1. Purbia et al. developed a highly luminescent CQDs (Size: 1–6 nm) with green and blue fluorescent for detecting vitamin B
1 in commercial vitamin capsule (LOD: 280 nmol L
−1)
[8]. The detection principle is that the gradual addition of vitamin B
1 can restore the fluorescent of CQDs quenched by Cu
2+, thereby achieving the rapid detection of targeted component. Certain small molecular substances present in plant-derived foods have specific antibacterial, anti-inflammatory or anti-oxidant properties, which gives the foods special medicinal properties. These small molecules are defined as “functional components” and this kind of foods is called “medicine-homologous foods”
[9][10]. Qualitative or quantitative analysis of the functional components in medicinal-homologous foods is usually the main method to evaluate their quality. Fluorescent CQDs provides an effective, convenient and accurate strategy for the analysis and detection of such efficacy components. Chlorogenic acid has remarkable antibacterial and antiviral pharmacological effects, the amount of which is the main basis for evaluating the quality of honeysuckle (a medicinal-homologous food). Yang et al. synthesized the water-soluble CQDs with size of 2.1 nm and q
uantum yield (QY) of 16.5% via hydrothermal treatment of malic acid and urea and applied for the fluorescent sensing detection of chlorogenic acid in honeysuckle
[11] (). Due to the mechanism of i
nternal filter effect (IFE)
[12], the fluorescent of CQDs is effectively quenched as the concentration of chlorogenic acid increases in the linear range of 0.15–60 µmol L
−1, with a lower LOD of 45 nmol L
−1. By comparison with the h
igh performance liquid chromatography (HPLC) and HPLC-
mass spectrometry (HPLC-MS/MS) for chlorogenic acid detection
[13][14], the developed CQDs-based method not only has good improvement on the sensitivity but also significantly improves the detection speed, which is suitable for rapid screening of large quantities of honeysuckle samples. Curcumin (Cur) is an acidic polyphenolic substance extracted from the roots of ginger plants and its main chain is unsaturated aliphatic and aromatic groups. As a yellow pigment, it is often used as a meat coloring agent and acid-base indicator and has anti-inflammatory, antioxidant and other pharmacological effects
[15][16]. Liu et al. have rapidly synthesized one N and P dual-doped CQDs (NP-CQDs) using glucose as carbon source,
1,2-ethylenediamine as N-dopant and concentrated phosphoric acid as P-dopant, which was further utilized as a label-free sensor for Cur determination
[17] (), achieving a linear range of 0.5–20 µmol L
−1 and a LOD of 58 nmol L
−1. In practical samples (drinking water and foods), satisfactory relative standard deviations (RSD) and recoveries were 0.08–5.39% and 95.2–105.2%, respectively. Additionally, this NP-CQDs can be used as effective fluorescent agent for cellular imaging without noticeable cytotoxicity.
Figure 1. (
a) Schematic of the quantitative detection for ovalbumin (OVA) based on fluorescent resonance energy transfer (FRET) of N, O, P co-doped CQDs (NOP-CQDs). Reproduced with permission from
[7]. Copyright Sensors and Actuators B: Chemical, 2018; (
b) Schematic of the detection of chlorogenic acid in honeysuckle using CQDs. Reproduced with permission from
[11]. Copyright Spectrochimica Acta Part A-molecular and Biomolecular Spectroscopy, 2018; (
c) Schematic of the quenching mechanism of
Curcumin to NP-CQDs and the applications of cellular imaging. Reproduced with permission from
[17]. Copyright Talanta, 2018; (
d) Schematic of the synthesis pathway of the composite (CQDs@HP-Cu
2O/MWCNT) composed of CQDs, hexagonal porous Cu
2O (HP-Cu
2O) and multi-walled carbon nanotubes (MWCNT) and caffeic acid detection process. Reproduced with permission from
[18]. Copyright Composites Part B-Engineering, 2019.
Based on the luminescent characteristic of CQDs and the selective adsorption of m
olecularly imprinted polymers (MIPs), CQDs-embedded MIPs has provided new methods for the fluorescent analysis of trace substances in complex food matrices. Using the metal organic frameworks (MOFs) as the core of surface molecular imprinting, Xu et al. designed a novel nanocomposite of CQDs@MOF@MIP and further developed a fluorescent sensor for the detection of quercetin (QCT) in extract capsule of Ginkgo biloba. The fluorescent sensor showed remarkable sensitivity and selectivity to QCT in the wide concentration range of 0–50.0 µmol L
−1 with a LOD of 2.9 nmol L
−1 (S/N = 3)
[19]. This CQDs@MOF@MIP sensing model has high specific surface area and ample cavities and further possesses the ability of signal amplification and conversion, which can transform chemical signal into the detectable fluorescent signal by binding with target molecules, potentially becoming an innovative technology. has shown the synthesis process of the composite composed of the N-CQDs decorated hexagonal porous Cu
2O and
multi-walled carbon nanotubes (MWCNT) and the application of N-CQDs@HP-Cu
2O/MWCNT for the detection of caffeic acid (CA) in red wine
[18]. The fabricated fluorescent sensing device was demonstrated to possess high sensitivity, good repeatability and stability to CA. The doped CQDs and conductive MWCNT make the composite have higher specific surface area and porosity and further improve the electrocatalytic activity of Cu
2O-based materials
[20][21]. This study provides a strong guidance for the fabrication of various porous nanocomposites. Rutin is a flavonol glycoside widely present in plants and can be used as an edible antioxidant and nutrition enhancer. Sinduja et al. synthesized CQDs (Size: 7 nm) using the non-essential amino acid asparagine as a precursor and further exploited it for the determination of rutin by spectrofluorimetry based on the decrease in emission intensity at 441 nm
[22]. Good linear relationship (R
2 = 0.997) was obtained in the range of 0.5–15 µmol L
−1 with a LOD of 0.1 µmol L
−1. In this study, the intrinsic fluorescent characteristic of CQDs and the selective π-π interaction between rutin and the CQDs aromatic rings enhance the detection accuracy and reliability to the target.