Fluorescence immunoassay based on the inner-filter effect of carbon dots for highly sensitive amantadine detection in foodstuffs

Sensitive colorimetric and fluorescence dual-mode detection of thiophanate-methyl based on spherical Fe3O4/GONRs composite nanozyme

Pesticide detection based on nanozyme is largely limited in terms of the variety of pesticides. Herein, a spherical and well-dispersed Fe3O4/graphene oxide nanoribbons (Fe3O4/GONRs) composite nanozyme was applied to firstly develop an enzyme-free and sensitive colorimetric and fluorescence dual-mode detection of thiophanate-methyl (TM). The synthesized Fe3O4/GONRs possess excellent dual enzyme-like activities (peroxidase and catalase) and can catalyze H2O2 to oxidize 3,3',5,5'-tetramethylbenzidine (TMB) into oxidized TMB (oxTMB). We found that Fe3O4/GONRs can adsorb TM through the synergistic effect of multiple forces, thereby inhibiting the catalytic activities of nanozyme. This inhibition can modulate the transformation of TMB to oxTMB, producing dual responses of absorbance decrease (oxTMB) and fluorescence enhancement (TMB). The limits of detection (LODs) of TM were 28.1 ng/mL (colorimetric) and 8.81 ng/mL (fluorescence), respectively. Moreover, the developed method with the recoveries of 94.8-100.8% also exhibited a good potential application in the detection of pesticides residues in water and food samples.

Highly efficient dual-mode detection of AFB1 based on the inner filter effect: Donor-acceptor selection and application

BACKGROUND

The utilization of inner filter effect (IFE) brings more opportunities for construction of fluorescence immunoassays but remains a great challenge, especially how to select best donor in the face of extensive fluorescent nanomaterials. Aflatoxin B1 possesses high toxicity among mycotoxins and is frequently found in agricultural products that may significantly threaten to human health. Therefore, with the help of signal transduction mechanism of IFE to develop a convenient and sensitive approach for AFB1 detection is of great significance in ensuring food safety.

RESULTS

Herein, the classical alkaline phosphatase (ALP) catalyzes hydrolysis of p-nitrophenylphosphate to produce p-nitrophenol (PNP) was employed as a model reaction, which intends to explore tunable multicolor fluorescence of gold nanoclusters (AuNCs) for matching PNP to maximize IFE efficiency. The luminescent green-emitting AuNCs were selected as an optimal donor in terms of excellent spectral overlap, high photoluminescence, and adequate system adaptability, thus achieving a 22-fold increase in sensitivity improvement compared to colorimetric method for ALP detection. The fluorescence quenching mechanism between PNP and AuNCs was validated as IFE by studying ultraviolet absorption, zeta potentials and fluorescence lifetime. In light of this, we integrated a highly specific antibody-antigen recognition system, efficient enzymatic reaction and excellent optical characteristics of AuNCs to develop dual-mode immunoassay for AFB1 monitoring. The sensitivity of fluorometric immunoassay was lower to 0.06 ng/mL, which obtained a 3.5-fold improvement compared to "gold standard" ELISA. Their practicability and applicability were confirmed in the tap water, corn, wheat and peanuts samples.

SIGNIFICANCE

This work provides an easy-to-understand screening procedure to select optimal donor-acceptor pairs in IFE analysis. Furthermore, we expect that integration of IFE-based signal conversion strategy into mature immunoassay not only extends the signal types, simplifies signal amplification steps, and reduces the false-positive/false-negative rates, but also provides a simple, convenient, and versatile strategy for monitoring of trace other contaminants.

Photostable yellow emissive carbon dots for iron-mediated reversible sensing of biothiols and cellular imaging

The different levels of biothiols in cells can not only screen cancer cells but also play a selective role in killing cancer cells. Therefore, accurate monitoring of biothiol in cancer cells is of great research significance. Herein, novel yellow emission CDs (Y-CDs) were prepared by a simple hydrothermal method using 2, 5-dihydroxyterephthalic acid (DHTA) as precursors. The Y-CDs as a highly efficient dual-mode sensor could detect Fe3+ and biothiols by colorimetric and fluorescence signals. Especially, with the addition of L-Cysteine, the quenched fluorescence could be quickly restored within 2 min and the detection limit was as low as 31.65 nM. Additionally, this sensor was utilized to sense biothiols in actual samples and living cells due to its eminent biocompatibility. Finally, the Y-CDs were successfully applied not only as fluorescent ink for message encryption but also as a portable solid hydrogels sensor for the detection of Fe3+ and biothiols. Therefore, these results suggested that Y-CDs could serve as a promising sensor for Fe3+ and biothiol detection in early cancer screening.

"Fluorescence-wavelength" label-free POCT tandem with "fluorescence-photothermal" nanobody-immunosensor for detecting BSA and β-lactoglobulin

Two carbon dots (CDs) (λEm = 525 nm, G-CDs and λEm = 640 nm, R-CDs) were synthesized from citric acid and urea. The bovine serum albumin (BSA) responsiveness of the R-CDs was used to develop a "fluorescence-wavelength" label-free point of care testing (POCT) for the detection of the milk quality marker BSA with the detection limit (LOD) of 4.89 μg/mL for fluorescence mode and 3.38 μg/mL for wavelength mode. In addition, R-CDs were found to have hydroxyl radical (·OH)-dependent fluorescence quenching properties, and a "fluorescence-photothermal" immunosensor based on nanobodies was constructed by introducing the fluorescence signal of R-CDs@BSA and the photothermal signal of oxTMB for the detection of β-lactoglobulin (β-LG) with the LOD of 0.034 ng/mL for fluorescence mode and 0.075 ng/mL for photothermal mode. The tandem detection of POCT and immunosensor enables the simultaneous and highly sensitive detection of BSA and β-LG after only simple dilution of less than 5 µL of sample.

Eu3+ Complex-Based Superhydrophobic Fluorescence Sensor for Cr(VI) Detection in Water

Cr(VI) compounds are bioaccumulative and highly toxic pollutants, and there is a need for simple and fast detection methods to monitor their trace levels. In this work, we developed a Eu3+ complex-based fluorescence sensor to easily detect Cr(VI) in water droplets. Our sensor consists of a nanofibrous membrane electrospun with a blend of polyvinylidene fluoride (PVDF), silica particles, and Eu3+ complex. Upon modifying the membrane surface with fluoroalkyl chemistry, the sensor displayed superhydrophobicity. When a water droplet with Cr(VI) was placed on such a superhydrophobic fluorescence sensor, the overlapping absorption of Cr(VI) and Eu3+ complex facilitated the inner filter effect, allowing the selective detection of Cr(VI) down to 0.44 µM (i.e., 45.76 µg L-1). We proposed and designed of new inexpensive and fast sensor for the detection of Cr(VI).

Biosensors Based on Inorganic Composite Fluorescent Hydrogels

Fluorescent hydrogels are promising candidate materials for portable biosensors to be used in point-of-care diagnosis because (1) they have a greater capacity for binding organic molecules than immunochromatographic test systems, determined by the immobilization of affinity labels within the three-dimensional hydrogel structure; (2) fluorescent detection is more sensitive than the colorimetric detection of gold nanoparticles or stained latex microparticles; (3) the properties of the gel matrix can be finely tuned for better compatibility and detection of different analytes; and (4) hydrogel biosensors can be made to be reusable and suitable for studying dynamic processes in real time. Water-soluble fluorescent nanocrystals are widely used for in vitro and in vivo biological imaging due to their unique optical properties, and hydrogels based on these allow the preservation of these properties in bulk composite macrostructures. Here we review the techniques for obtaining analyte-sensitive fluorescent hydrogels based on nanocrystals, the main methods used for detecting the fluorescent signal changes, and the approaches to the formation of inorganic fluorescent hydrogels via sol-gel phase transition using surface ligands of the nanocrystals.

Low temperature synthesis of carbon dots in microfluidic chip and their application for sensing cefquinome residues in milk

In this study, the N-doped carbon dots were continuously synthesized by a facile microfluidic strategy at 90 °C, and their quantum yields reached 19.2%. The characteristics of the obtained carbon dots could be real-time monitored in order to synthesize carbon dots with specific properties. By incorporating the carbon dots into a well-established enzymatic cascade amplification system, an inner filter effect-based fluorescence immunoassay was set up for ultrasensitive detection of cefquinome residues in milk samples. The developed fluorescence immunoassay provided a low detection limit of 0.78 ng/mL, which satisfied the maximum residue limit set by authorities. The fluorescence immunoassay had an 50% inhibition concentration of 0.19 ng/mL against cefquinome and showed a good linear relationship from 0.013 ng/mL to 1.52 ng/mL. While, the average recovery values ranged from 77.8% to 107.8% in spiked milk samples, with relative standard deviations ranging from 6.8% to 10.9%. Compared with conventional methods, the microfluidic chip was more flexible on carbon dots synthesis and the developed fluorescence immunoassay was more sensitive and eco-friendlier for ultra-trace cefquinome residue analysis.

Dual-mode immunochromatographic assay based on dendritic gold nanoparticles with superior fluorescence quenching for ultrasensitive detection of E. coli O157:H7

We presented a colorimetric/fluorescent dual-mode immunochromatographic assay (ICA) for the sensitive detection of Escherichia coli O157:H7. The use of polydopamine (PDA)-modified gold nanoparticles (AuNPs) with broadband absorption allowed for excellent colorimetry signals for the ICA detection. Moreover, the absorption spectrum of PDA-AuNPs significantly overlaps with the excitation and emission spectra of ZnCdSe/ZnS quantum dots (QDs), resulting in effective quenching of the QDs fluorescence due to the inner filter effect. The fluorescence intensity changes induced by PDA-AuNPs were utilized for the sensitive detection of E. coli O157:H7, achieving a detection limit of 9.06 × 10¹ CFU/mL, which was 46-fold lower than that of traditional AuNPs-based immunoassay. The proposed immunosensor exhibited the recovery rate between 80.12% and 114.69% in detecting actual samples, indicating its reliability and satisfactory accuracy. This study provides insights into dual-mode signal outputs and the ICA development for food safety applications.

Ionic liquid-based carbon dots as highly biocompatible and sensitive fluorescent probe for the determination of vitamin P in fruit samples

Vitamin P (VP) known as rutin is one of the common flavonoids, which widely exists in fruits and vegetables and often used as a dietary additive. The rapid and accurate detection of VP in food matrices is critical for evaluating food quality and guiding diet. Herein, a rapid, accurate, and selective detection scheme for VP in fruit samples was proposed for the first time using ionic liquid-based carbon dots (IL-CDs). The synthesized IL-CDs exhibited great biocompatibility and excellent optical properties including high fluorescence intensity, high quantum yield, and good fluorescence stability. Through an internal filtering effect (IFE), VP could greatly reduce the fluorescence of these CDs. In the present study, this probe demonstrated good sensitivity and excellent selectivity toward VP with a low detection limit of 60.0 nmol/L. Moreover, this approach was effectively applied to detect VP in food samples with a recovery range of 97 % to 119 %. More interestingly, the results of cell imaging suggested that IL-CDs were expected to be promising material for bioimaging.

Recent advances in quantum dot-based fluorescence-linked immunosorbent assays

Fluorescence immunoassays have been given considerable attention among the quantitative detection methods in the clinical medicine and food safety testing fields. In particular, semiconductor quantum dots (QDs) have become ideal fluorescent probes for highly sensitive and multiplexed detection due to their unique photophysical properties, and the QD fluorescence-linked immunosorbent assay (FLISA) with high sensitivity, high accuracy, and high throughput has been greatly developed recently. In this manuscript, the advantages of applying QDs to FLISA platforms and some strategies for their application to in vitro diagnostics and food safety are discussed. Given the rapid development of this field, we classify these strategies based on the combination of QD types and detection targets, including traditional QDs or QD micro/nano-spheres-FLISA, and multiple FLISA platforms. In addition, some new sensors based on the QD-FLISA are introduced; this is one of the hot spots in this field. The current focus and future direction of QD-FLISA are also discussed, which provides important guidance for the further development of FLISA.

A versatile platform for colorimetric, fluorescence and photothermal multi-mode glyphosate sensing by carbon dots anchoring ferrocene metal-organic framework nanosheet

Concerns regarding pesticide residues have driven attempts to exploit accurate, prompt and straightforward approaches for food safety pre-warning. Herein, a nanozyme-mediated versatile platform with multiplex signal response (colorimetric, fluorescence and temperature) was proposed for visual, sensitive and portable detection of glyphosate (GLP). The platform was constructed based on a N-CDs/FMOF-Zr nanosensor that prepared by in situ anchoring nitrogen-doped carbon dots onto zirconium-based ferrocene metal-organic framework nanosheets. The N-CDs/FMOF-Zr possessed excellent peroxidase (POD)-like activity and thus could oxide colorless 3, 3', 5, 5'-tetramethylbenzidine (TMB) into a blue oxidized TMB (oxTMB) in presence of H₂O₂. Intriguingly, owing to the blocking effect triggered by multiple interaction between GLP and N-CDs/FMOF-Zr, its POD-like activity of the latter was remarkably suppressed, which can modulate the transformation of TMB into oxTMB, generating tri-signal responses of fluorescence enhancement, absorbance and temperature decrease. More significantly, the temperature mode can be facilely realized by a portable home-made mini-photothermal device and handheld thermometers. The proposed multimodal sensing was capable of providing sensitive results by fluorescence mode and simultaneously realized visual/portable testing by colorimetric and photothermal channels. Consequently, it exhibited more adaptability for practical applications, which can satisfy different testing requirements according to sensitivity and available instruments/meters, presenting a new horizon for exploiting multifunctional sensors.

M. SK, Baek KH. Assessing the Food Quality Using Carbon Nanomaterial Based Electrodes by Voltammetric Techniques

The world is facing a global financial loss and health effects due to food quality adulteration and contamination, which are seriously affecting human health. Synthetic colors, flavors, and preservatives are added to make food more attractive to consumers. Therefore, food safety has become one of the fundamental needs of mankind. Due to the importance of food safety, the world is in great need of developing desirable and accurate methods for determining the quality of food. In recent years, the electrochemical methods have become more popular, due to their simplicity, ease in handling, economics, and specificity in determining food safety. Common food contaminants, such as pesticides, additives, and animal drug residues, cause foods that are most vulnerable to contamination to undergo evaluation frequently. The present review article discusses the electrochemical detection of the above food contaminants using different carbon nanomaterials, such as carbon nanotubes (CNTs), graphene, ordered mesoporous carbon (OMC), carbon dots, boron doped diamond (BDD), and fullerenes. The voltammetric methods, such as cyclic voltammetry (CV) and differential pulse voltammetry (DPV), have been proven to be potential methods for determining food contaminants. The use of carbon-based electrodes has the added advantage of electrochemically sensing the food contaminants due to their excellent sensitivity, specificity, large surface area, high porosity, antifouling, and biocompatibility.

Production and Characterization of Biotinylated Anti-fenitrothion Nanobodies and Development of Sensitive Fluoroimmunoassay

A simple and sensitive fluoroimmunoassay (FIA) based on a heavy-chain antibody (VHH) for rapid detection of fenitrothion was developed. A VHH library was constructed from an immunized alpaca, and one clone recognizing fenitrothion (namely, VHHjd8) was achieved after careful biopanning. It was biotinylated by fusing with the Avi tag and biotin ligase to obtain a fusion protein (VHHjd8-BT), showing both binding capacity to fenitrothion and the streptavidin poly-horseradish peroxidase conjugate (SA-polyHRP). Based on a competitive assay format, the absorbance spectrum of oxidized 3,3',5,5'-tetramethylbenzidine generated by SA-polyHRP overlapped the emission spectrum of carbon dots, which resulted in quenching of signals due to the inner-filter effect. The developed FIA showed an IC₅₀ value of 1.4 ng/mL and a limit of detection of 0.03 ng/mL, which exhibited 15-fold improvement compared with conventional enzyme-linked immunosorbent assay. The recovery test of FIA was validated by standard GC-MS/MS, and the results showed good consistency, indicating that the assay is an ideal tool for rapid screening of fenitrothion in bulk food samples.

Synthesis and characterization of tracers and development of a fluorescence polarization immunoassay for amantadine with high sensitivity in chicken

Fluorescence polarization immunoassay (FPIA) is a homogeneous and rapid analytical method that is suitable for high-throughput screening of large numbers of samples. However, FPIA typically suffers from lower sensitivity than the well-established enzyme-linked immunosorbent assay (ELISA), limiting its wide application as an analytical tool that can be run with trace levels of an analyte. Herein, a highly sensitive FPIA for detecting amantadine (AMD) in chicken is described. To achieve high sensitivity, nine chemical tracers of AMD that employ different fluoresceins, fluorescein derivatives, and haptens were synthesized and paired with four previously produced monoclonal antibodies (mAbs). The effect of the tracer structure on the sensitivity of FPIA was investigated and discussed. We found that the tracers with a linear and shorter bridge between adamantane and fluorescein generally provided higher sensitivity. After optimization, N'-(1-adamantyl) ethylenediamine (AEDA), an AMD structural analogue labeled with fluorescein isothiocyanate (FITC), achieved the lowest IC₅₀ value (1.0 ng/ml) in the FPIA, which was comparable to that of the heterologous ELISA format that used the same mAb7G2. We also investigated the possible recognition mechanism of mAbs in terms of conformational and electronic aspects. The developed FPIA was applied to chicken to detect AMD residue, demonstrating a limit of detection (LOD) of 0.9 µg/kg with recoveries of 76.5-89.3% and coefficients of variation (CVs) below 14.5%. These results show that the proposed FPIA is an efficient, accurate, and convenient method for the rapid screening of AMD residues in chicken. PRACTICAL APPLICATION: The fluorescence polarization immunoassay (FPIA) was developed to determine and quantify amantadine (AMD) in chicken samples with high sensitivity. This homogeneous method avoids coating and washing steps and may provide high-throughput AMD screening in chicken in 10 min with high accuracy and precision. FPIA can be used as a monitoring tool and contribute significantly to the rapid detection of AMD in chicken.

Green preparation of carbon dots from Momordica charantia L. for rapid and effective sensing of p-aminoazobenzene in environmental samples

p-Aminoazobenzene (pAAB) is a hazardous azo dye that causes considerable harm to human health and the environment. The development of novel and sensitive sensors for the rapid detection of pAAB is in high demand. In this study, a simple fluorescent sensor for pAAB detection is designed based on carbon dots (CDs) which are prepared using green carbon source of Momordica charantia L. via a facile hydrothermal approach. The fluorescence spectra of CDs exhibit considerable overlap with the absorption band of pAAB, and the fluorescence is specifically suppressed in the presence of pAAB ascribed to the inner filter effect. Good and wide linearity is observed in the pAAB concentration range of 0.01-12.5 μg mL^-1 with a lower detection limit of 3.9 ng mL^-1. The established method achieves good results with a rapid analysis of pAAB in different practical water and soil samples. The as-constructed fluorescent sensor provides a simple, rapid, economical and eco-friendly platform and possesses prospective applications for the effective, selective and sensitive detection of pAAB in the environmental field.

CDs-MnO2-TPPS Ternary System for Ratiometric Fluorescence Detection of Ascorbic Acid and Alkaline Phosphatase

Manganese dioxide (MnO₂) nanosheet-based fluorescence sensors often use oxidase-like activity or wide absorption spectrum for detection of antioxidants. In those strategies, MnO₂ nanosheets were reduced to Mn²⁺ by antioxidants. However, few strategies emphasize the role of Mn²⁺ obtained from MnO₂ reduction in the design of the fluorescence sensor. Herein, we expanded the application of a MnO₂ nanosheet-based fluorescence sensor by involving Mn²⁺ in the detection process using ascorbic acid (AA) as a model target. In this strategy, carbon dots (CDs), MnO₂ nanosheets, and tetraphenylporphyrin tetrasulfonic acid (TPPS) comprise a ternary system for ratiometric fluorescence detection of AA. Initially, CDs were quenched by MnO₂ nanosheets based on the inner filter effect, while TPPS maintained its fluorescence intensity. After the addition of AA, MnO₂ nanosheets were reduced to Mn²⁺ so that the fluorescence intensity of CDs was recovered and TTPS was quenched by coordination with Mn²⁺. Overall, AA triggered an emission intensity increase at 440 nm for CDs and a decrease at 640 nm for TPPS. The ratio intensity of CDs to TPPS (F ₄₄₀/F ₆₄₀) showed a good linear relationship from 0.5 to 40 μM, with a low detection limit of 0.13 μM for AA detection. By means of the alkaline phosphatase (ALP)-triggered generation of AA, this strategy can be applied for the detection of ALP in the range of 0.1-100 mU/mL, with a detection limit of 0.04 mU/mL. Furthermore, this sensor was applied to detect AA and ALP in real, complex samples with ideal recovery. This novel platform extended the application of MnO₂ nanosheet-based fluorescence sensors.

Development of a molecularly imprinted microspheres-based microplate fluorescence method for detection of amantadine and rimantadine in chicken

In this study, molecularly imprinted microspheres of a type capable of recognising amantadine and rimantadine were first synthesised, and three fluorescent tracers based on dansyl chloride, fluorescein isothiocyanate and 5-carboxytetramethylrhodamine were also synthesised. These reagents were used to develop and optimise a direct competitive fluorescence method on conventional 96-well microplate for detection of the two analytes. Results showed that this method achieved simple operation procedure, rapid assay process (30 min), high sensitivity (limits of detection 0.04-0.05 ng mL^-1) and acceptable recycle performance (five times). After optimisation of several parameters, this method was used to detect amantadine and rimantadine in chicken muscle samples. Their recoveries from standards fortified blank samples were in the range of 62.3-93.7%. The analysis results for some real chicken samples were consistent with a confirmatory LC-MS/MS method. Therefore, this method could be used as a rapid, simple and accurate tool for routine screening the residues of amantadine and rimantadine in a large number of chicken muscle samples.

Highly photoluminescent carbon dots-based immunosensors for ultrasensitive detection of aflatoxin M1 residues in milk

Here, a facile hydrothermal method was used to synthesize highly photoluminescent N-doped carbon dots, and the quantum yields reached 97.1%. Then, a label-free immunosensor based on the inner filter effect of carbon dots was developed for ultrasensitive detection of aflatoxin M₁ residues in milk. The detection limit was 0.0186 ng/mL (equivalents to 18.10 ng/kg), which satisfied the most stringent maximum tolerable limit value of 25 ng/kg. Besides, the immunosensor showed a good linear relationship from 0.003 ng/mL to 0.81 ng/mL, and the average recoveries ranged from 79.6% to 112.5% for spiked milk samples, with relative standard deviations ranging from 6.7% to 13.3%. Compared with other immunoassays, the inner filter effect-based immunosensor incorporating fluorescent detection into conventional enzymatic cascade amplification systems and could be a reliable on-site screening method for aflatoxin M₁ residue analysis.

Hedgehog-like Bi2S3 nanostructures: a novel composite soft template route to the synthesis and sensitive electrochemical immunoassay of the liver cancer biomarker

A new route was proposed to synthesize novel hedgehog-like Bi₂S₃nanostructure using CTAB-trimellitic acid as a composite soft template and thiourea as the sulfur source, which was used  to construct an effective electrochemical AFP immunosensor.

A Perspective on Application of Carbon Quantum Dots in Luminescence Immunoassays

Quantum dots (QDs) have been established in our daily life, for instance, in display screens and light-harvesting technologies, mainly owing to their peculiar opto-electronic features. However, toxicity of inorganic QDs, such as CdSe, CdTe, and perovskites, limits their applications in biological environments for medical diagnosis and bio-imaging purposes. A new generation of QDs called carbon quantum dots (CQDs) have been progressing rapidly over the past few years. CQDs have become as popular as other carbon-based nanomaterials such as carbon nanotubes (CNTs), due to their ease of preparation, ultra-small size, biocompatibility, and bright luminescence.

Visual determination of azodicarbonamide in flour by label-free silver nanoparticle colorimetry

A very practical and competitive sensing strategy for the detection of azodicarbonamide in flour samples was developed by using label-free Ag NPs as a colorimetric probe. Well-dispersed Ag NPs in suspension can form aggregates upon reacting with glutathione (GSH) via Ag-SH covalent bonds and electrostatic attraction, with the color changing from bright yellow to red. However, azodicarbonamide can oxidize the -SH of GSH, preventing the aggregation of Ag NPs. Under the optimum conditions, the A₅₅₀/A₃₉₈ of Ag NPs is linearly related to the concentration of azodicarbonamide in the range of 0.33 μM to 1.7 μM. The proposed method can be used for the detection of azodicarbonamide in flour, with a detection limit of 0.09 μM and recovery between 95% and 97.4% (RSD < 6%). When the azodicarbonamide concentration reaches 0.33 μM, the color change can be detected by the naked eye.

A Review on the Use of Impedimetric Sensors for the Inspection of Food Quality

This paper exhibits a thorough review of the use of impedimetric sensors for the analysis of food quality. It helps to understand the contribution of some of the major types of impedimetric sensors that are used for this application. The deployment of impedimetric sensing prototypes has been advantageous due to their wide linear range of responses, detection of the target analyte at low concentrations, good stability, high accuracy and high reproducibility in the results. The choice of these sensors was classified on the basis of structure and the conductive material used to develop them. The first category included the use of nanomaterials such as graphene and metallic nanowires used to form the sensing devices. Different forms of graphene nanoparticles, such as nano-hybrids, nanosheets, and nano-powders, have been largely used to sense biomolecules in the micro-molar range. The use of conductive materials such as gold, copper, tungsten and tin to develop nanowire-based prototypes for the inspection of food quality has also been shown. The second category was based on conventional electromechanical circuits such as electronic noses and other smart systems. Within this sector, the standardized systems, such as electronic noses, and LC circuit -based systems have been explained. Finally, some of the challenges posed by the existing sensors have been listed out, along with an estimate of the increase in the number of sensors employed to assess food quality.

Colloidal gold-based immunochromatographic strip assay for the rapid detection of bacitracin zinc in milk

In this study, a murine monoclonal antibody (mAb) of 6D2-G10 against bacitracin zinc (BAC) was produced and applied to an immunochromatographic strip (ICS) for the initial detection of BAC in milk. The ICS with a cut-off value of 25 ng/mL could be perceived by the naked eye within 10 min. With the assist of the strip reader, the limit of detection (LOD) was measured as 0.82 ng/mL, the half-maximal inhibitory concentration (IC₅₀) was recorded as 3.16 ng/mL, and the linear detection range was from 0.97 to 10.30 ng/mL. The recoveries ranged from 87.7% to 96.0% with the highest coefficient of variation (CV) of 9.1% in the intra-assay and from 84.3% to 90.2% with the highest CV of 10.7% in the inter-assay. In short, the established ICS provided a serviceable analytical tool for qualitatively and quantitatively monitoring BAC in milk.

Vinyl Phosphate-Functionalized, Magnetic, Molecularly-Imprinted Polymeric Microspheres' Enrichment and Carbon Dots' Fluorescence-Detection of Organophosphorus Pesticide Residues

The rapid detection of organophosphorus pesticide residues in food is crucial to food safety. One type of novel, magnetic, molecularly-imprinted polymeric microsphere (MMIP) was prepared with vinyl phosphate and 1-octadecene as a collection of dual functional monomers, which were screened by Gaussian09W molecular simulation. MMIPs were used to enrich organic phosphorus, which then detected by fluorescence quenching in vinyl phosphate-modified carbon dots (CDs@VPA) originated from anhydrous citric acid. MMIPs and CDs@VPA were characterized by TEM, particle size analysis, FT-IR, VSM, XPS, adsorption experiments, and fluorescence spectrophotometry in turn. Through the fitting data from experiment and Gaussian quantum chemical calculations, the molecular recognition properties and the mechanism of fluorescence detection between organophosphorus pesticides and CDs@VPA were also investigated. The results indicated that the MMIPs could specifically recognize and enrich triazophos with the saturated adsorption capacity 0.226 mmol g^-1, the imprinting factor 4.59, and the limit of recognition as low as 0.0006 mmol L^-1. Under optimal conditions, the CDs@VPA sensor has shown an extensive fluorescence property with a LOD of 0.0015 mmol L^-1 and the linear range from 0.0035 mmol L^-1 to 0.20 mmol L^-1 (R² = 0.9988) at 390 nm. The mechanism of fluorescence detection of organic phosphorus with CDs@VPA sensor might be attributable to hydrogen bonds formed between heteroatom O, N, S, or P, and the O-H group, which led to fluorescent quenching. Meanwhile, HN-C=O and Si-O groups in CDs@VPA system might contribute to cause excellent blue photoluminescence. The fluorescence sensor was thorough successfully employed to the detection of triazophos in cucumber samples, illustrating its tremendous value towards food sample analysis in complex matrix.

Carbon-Based Nanomaterials in Sensors for Food Safety

Food safety is one of the most important and widespread research topics worldwide. The development of relevant analytical methods or devices for detection of unsafe factors in foods is necessary to ensure food safety and an important aspect of the studies of food safety. In recent years, developing high-performance sensors used for food safety analysis has made remarkable progress. The combination of carbon-based nanomaterials with excellent properties is a specific type of sensor for enhancing the signal conversion and thus improving detection accuracy and sensitivity, thus reaching unprecedented levels and having good application potential. This review describes the roles and contributions of typical carbon-based nanomaterials, such as mesoporous carbon, single- or multi-walled carbon nanotubes, graphene and carbon quantum dots, in the construction and performance improvement of various chemo- and biosensors for various signals. Additionally, this review focuses on the progress of applications of this type of sensor in food safety inspection, especially for the analysis and detection of all types of toxic and harmful substances in foods.

Co2+ detection, cell imaging, and temperature sensing based on excitation-independent green-fluorescent N-doped carbon dots

Green-fluorescent N-doped carbon dots (N-CDs) have been successfully fabricated using hydrothermal treatment of tyrosine and urea.