Old silver mirror in qualitative analysis with new shoots in quantification: Nitrogen-doped carbon dots (N-CDs) as fluorescent probes for "off-on" sensing of formalin in food samples

A sensitive paper-based vapor-test kit for instant formalin detection in food products

A colorimetric sensing method based on a paper-based vapor-test kit was successfully developed for the selective and sensitive real-time monitoring of formalin in food samples. The device was specifically designed to efficiently extract and detect formalin simultaneously. A microcentrifuge tube was used as the sample solution container, with the inner cap serving as the reaction and detection zone. Formalin was converted into gaseous formaldehyde through controlled heating, which was then extracted and collected on a filter paper coated with Nash's reagent. The color change on paper was used for formalin quantification using a smartphone for detection and image analysis. Under optimal conditions, our method provided a linear range of 0.5-75 mg L-1 with a detection limit of 0.11 mg L-1. This method effectively determined formalin in fresh food and vegetable samples, with recoveries ranging from 92 to 111%, demonstrating comparable accuracy to the standard method for practical food quality control and safety.

Sustainable packaging film based on cellulose nanofibres/pullulan impregnated with zinc-doped carbon dots derived from avocado peel to extend the shelf life of chicken and tofu

A cellulose nanofiber (CNF)/pullulan (PUL) based multifunctional composite film was developed for active packaging applications by incorporating Zn-doped avocado-derived carbon dots (Zn-ACDs). The incorporation of Zn-ACDs improved the interfacial compatibility and produced a dense cross-sectional structure of the composite films. The Zn-ACDs added film showed no significant difference in water vapor permeability and surface hydrophilicity compared to the neat CNF/PUL film, but the tensile strength and elongation at break increased by ~45.4 % and ~64.1 %, respectively. The addition of 5 wt% Zn-ACDs to the CNF/PUL matrix resulted in 100.0 % UV blocking properties, excellent antioxidant activity (100.0 % for ABTS and 68.0 % for DPPH), and complete eradication of foodborne pathogens such as Listeria monocytogenes ATCC 15313 and Escherichia coli O157:H7 after 3 h of exposure. The CNF/PUL composite film with Zn-ACDs applied to the active packaging of chicken and tofu significantly reduced the total growth of aerobic microorganisms without significantly changing the actual color of the packaged chicken and tofu for 9 days at 10°C. This study demonstrates that CNF/PUL composite films with Zn-ACDs are a sustainable and environmentally friendly option for protecting food from microbial contamination.

One-step silver coating of polypropylene surgical mask with antibacterial and antiviral properties

Face masks can filter droplets containing viruses and bacteria minimizing the transmission and spread of respiratory pathogens but are also an indirect source of microbes transmission. A novel antibacterial and antiviral Ag-coated polypropylene surgical mask obtained through the in situ and one-step deposition of metallic silver nanoparticles, synthesized by silver mirror reaction combined with sonication or agitation methods, is proposed in this study. SEM analysis shows Ag nanoparticles fused together in a continuous and dense layer for the coating obtained by sonication, whereas individual Ag nanoparticles around 150 nm were obtained combining the silver mirror reaction with agitation. EDX, XRD and XPS confirm the presence of metallic Ag in both coatings and also oxidized Ag in samples by agitation. A higher amount of Ag nanoparticles is deposited on samples by sonication, as calculated by TGA. Further, both coatings are biocompatible and show antibacterial properties: coating by sonication caused 24 % and 40 % of bacterial reduction while coating by agitation 48 % and 96 % against S. aureus and E. coli, respectively. At 1 min of contact with SARS-CoV-2, the coating by agitation has an antiviral capacity of 75 % against 24 % of the one by sonication. At 1 h, both coatings achieve 100 % of viral inhibition. Nonetheless, larger samples could be produced only through the silver mirror reaction combined with agitation, preserving the integrity of the mask. In conclusion, the silver-coated mask produced by silver mirror reaction combined with agitation is scalable, has excellent physico-chemical characteristics as well as significant biological properties, with higher antimicrobial activities, providing additional protection and preventing the indirect transmission of pathogens.

Carbon Dots-Types, Obtaining and Application in Biotechnology and Food Technology

Materials with a "nano" structure are increasingly used in medicine and biotechnology as drug delivery systems, bioimaging agents or biosensors in the monitoring of toxic substances, heavy metals and environmental variations. Furthermore, in the food industry, they have found applications as detectors of food adulteration, microbial contamination and even in packaging for monitoring product freshness. Carbon dots (CDs) as materials with broad as well as unprecedented possibilities could revolutionize the economy, if only their synthesis was based on low-cost natural sources. So far, a number of studies point to the positive possibilities of obtaining CDs from natural sources. This review describes the types of carbon dots and the most important methods of obtaining them. It also focuses on presenting the potential application of carbon dots in biotechnology and food technology.

Metal-organic frameworks (MOFs) based luminescent and electrochemical sensors for food contaminant detection

With the increasing population, food toxicity has become a prevalent concern due to the growing contaminants of food products. Therefore, the need for new materials for toxicant detection and food quality monitoring will always be in demand. Metal-organic frameworks (MOFs) based on luminescence and electrochemical sensors with tunable porosity and active surface area are promising materials for food contaminants monitoring. This review summarizes and studies the most recent progress on MOF sensors for detecting food contaminants such as pesticides, antibiotics, toxins, biomolecules, and ionic species. First, with the introduction of MOFs, food contaminants and materials for toxicants detection are discussed. Then the insights into the MOFs as emerging materials for sensing applications with luminescent and electrochemical properties, signal changes, and sensing mechanisms are discussed. Next, recent advances in luminescent and electrochemical MOFs food sensors and their sensitivity, selectivity, and capacities for common food toxicants are summarized. Further, the challenges and outlooks are discussed for providing a new pathway for MOF food contaminant detection tools. Overall, a timely source of information on advanced MOF materials provides materials for next-generation food sensors.

Chitosan/Starch-Based Active Packaging Film with N, P-Doped Carbon Dots for Meat Packaging

Nitrogen, phosphorus-doped green-tea-derived carbon dots (NP-CDs) incorporated chitosan/starch (Chi/St) based multifunctional nanocomposite films were prepared. FE-SEM images verified a homogeneous distribution of CDs with minimum aggregation in the fabricated films. Incorporating NP-CDs led to enhanced UV-light blocking (93.1% of UV-A and ∼99.7% of UV-B) without significantly affecting the films' water transparency and water vapor permeability. Besides, incorporating NP-CDs into the Chi/St films enhanced antioxidant activity (98.0% for ABTS and 71.4% for DPPH) and displayed strong antibacterial activity against L. monocytogenes, E. coli, and S. aureus. Wrapping the meat in the prepared film and storing it at 20 °C has been shown to reduce bacterial growth (less than 2.5 Log CFU/g after 48 h) without significantly altering the actual color of the wrapped meat. The Chi/St film loaded with NP-CD has high potential as an active packaging material to ensure safety and extend the shelf life of meat products.

The Development of the Fluorescence-based Portable Device for Lead (II) and Formalin Determination in Food Samples by Using Nitrogen-Doped Carbon Dots (N-CDs)

The fluorescence-based portable device for the determination of lead (Pb²⁺) and formalin (FA) in food samples by using Nitrogen-doped carbon dots (N-CDs) as a fluorescence probe was developed. The proposed approach, Pb^2+, and FA were determined based on the photo-induced electron transfer (PET) mechanism and the silver mirror reaction. The fluorescence intensity of the N-CDs decreased with the increase of Pb²⁺ concentration and increased with the increasing FA concentration. The fluorescence intensity of N-CDs after the reactions were measured by a filter-free fluorometer platform using a commercial camera module and a Raspberry Pi, a compact computer, as a detector and processor. The experimental results were obtained using control samples with known Pb²⁺ and FA concentrations in the 0.01-10 mg L^- 1 and 25-150 mg L^- 1, respectively. The proposed approach is simple, low-cost, and accurate for the on-site monitoring of Pb²⁺ and FA in various food samples. Of utmost importance, the proposed approach is expected to be a pioneering model for the future development of other analytes with a broad range of practical applications.

Highly specific esterase activated AIE plus ESIPT probe for sensitive ratiometric detection of carbaryl

Fabrication of facile, sensitive, and accurate pesticide detection strategies plays crucial roles in food safety, environmental protection, and human health. Here, a novel esterase activatable aggregation-induced emission (AIE) plus excited-state intramolecular proton transfer (ESIPT) probe, kaempferol tetraacetate, was designed and synthesized from purified natural kaempferol for ratiometric sensing of carbaryl. Acetate groups are introduced as the esterase reactive sites and AIE plus ESIPT initiator. Kaempferol tetraacetate is an aggregation-caused quenching compound that shows fluorescent (FL) emission at 415 nm. Esterase specifically hydrolyzes kaempferol tetraacetate to kaempferol with AIE plus ESIPT characteristics (distinct FL emission, 530 nm; a large Stokes shift, 165 nm within a short time (8 min). Molecular docking and kinetics performance indicate the high affinity and specific hydrolysis of esterase and kaempferol tetraacetate. Carbaryl inhibits the activity of esterase to efficiently suppress the production of kaempferol. Thus, a facile ratiometric assay strategy is constructed for carbaryl detection. By measuring the FL intensity ratio, the proposed strategy presents high selectivity and reliability with a wide linear range from 0.02 to 2.00 μg L^-1 and a very low limit of detection at 0.007 μg L^-1. Furthermore, appropriate recovery from 93.75% to 108.67% with a relative standard deviation less than 5.66% for real sample analysis indicates good accuracy and precision. All results indicate that the fabricated strategy offers a new way for facile, sensitive, and accurate detection of carbaryl in real complex samples.

Carbon quantum dots with blue/near infrared emissions for ratiometric fluorescent lornoxicam sensing and bio-imaging

An economical and eco-friendly hydrothermal method for the preparation of nitrogen-doped carbon quantum dots (N-CQDs) was studied with rambutan peel and lysine. The morphology, structure, and optical properties of N-CQDs were characterized by transmission electron microscopy, Fourier transform infrared spectrometry, X-ray powder diffractometer, X-ray photoelectron spectrometry, and UV spectrophotometry. The synthesized N-CQDs have excellent characteristics such as strong fluorescence, good dispersion, high stability, and excellent water solubility. The absolute fluorescence quantum yield is 1.02%, the average particle size is 1.63 nm, and the maximum excitation wavelength is 340 nm. The maximum emission wavelengths are 430 nm and 800 nm. As a quencher, lornoxicam (LNX) was used to quench the fluorescence of N-CQDs with the mechanism of inner filter effect. The fluorescence ratio of N-CQDs (F₄₃₀/F₈₀₀) shows a good linear relationship to the concentration of LNX. The linear range and the detection limit of LNX are 0.01‒100 and 0.003 μmol/L, respectively. An effective ratiometric fluorescence probe for the detection of LNX was constructed. The method has the advantages of low detection limit, high sensitivity, wide linear range, and can be applied to the determination of LNX in real samples. Moreover, according to the excitation-dependent fluorescence behavior, dual-wavelength emission, and biocompatibility of N-CQDs, it has been applied to cell imaging.

Nitrogen-doped carbon dots/Ni-MnFe-layered double hydroxides (N-CDs/Ni-MnFe-LDHs) hybrid nanomaterials as immunoassay label for low-density lipoprotein detection

Nitrogen-doped carbon dots/Ni-MnFe-layered double hydroxides (N-CDs/Ni-MnFe-LDHs) are demonstrated as superior peroxidase mimic antibody labels alternative to horseradish peroxidase (HRP) in an immunoassay, potentially overcoming some of the inherent disadvantages of HRP and other enzyme mimicking nanomaterials. They revealed efficient peroxidase-like activity and catalyzed the oxidation of colorless 3,3',5,5'-tetramethylbenzidine (TMB) to form the intense blue product (at 620 nm) in the presence of hydrogen peroxide (H₂O₂). Using low-density lipoprotein (LDL) as a model target, an ultra-low limit of detection (0.0051 mg/dL) and a linear range of 0.0625-0.750 mg/dL were achieved, exhibiting higher sensitivity than the HRP-based immunoassay. Thus, the proposed N-CDs/Ni-MnFe-LDHs can be used as HRP mimicking analogs for developing highly sensitive colorimetric immunosensors for detection of biomarkers, as well as trace chemical analysis.