Carbon dots based dual-emission silica nanoparticles as ratiometric fluorescent probe for nitrite determination in food samples

Rapid, portable and visualizing nitrite detection enabled by a rationally designed meso-aminoindole substituted pyronine-based fluorescent probe

Nitrite (NO2-) widely exists in our daily diet, and its excessive consumption can lead to detrimental effects on the human central nervous system and an elevated risk of cancer. The fluorescence probe method for the determination of nitrite has developed rapidly due to its simplicity, rapidity and sensitivity. Despite establishing various nitrite sensing platforms to ensure the safety of foods and drinking water, the simultaneous achievement of rapid, specific, affordable, visualizing, and on-site nitrite detection remains challenging. Here, we designed a novel fluorescent probe by using Rhodamine 800 as the fluorescent skeleton and 5-aminoindole as the specific reaction group to solve this problem. The probe shows a maximal fluorescence emission at 602 nm, thereby avoiding background emission interference when applied to food samples. Moreover, this unique probe exhibited excellent sensing capabilities for detecting nitrite. These included: a rapid response time within 3 min, a noticeable color change that the naked eye can observe, a low detection limit of 13.8 nM, and a remarkable selectivity and specificity to nitrite. Besides that, the probe can detect nitrite quantitatively in barreled drinking water, ham sausage, and pickles samples, with good recoveries ranging from 89.0 % to 105.8 %. More importantly, based on the probe fixation and signal processing technology, a portable and smart sensing platform was fabricated and made convenient and rapid analysis the content of NO2- in real samples possible. The results obtained in this work provide a new strategy for the design of high-performance nitrite probes and feasible technology for portable, rapid and visual detection of nitrite, and this probe holds the potential as a practical tool for alleviating concern regarding nitrite levels.

Metal-organic framework-based ratiometric point-of-care testing for quantitative visual detection of nitrite

Nitrite (NO2-) is categorized as a carcinogenic substance and is subjected to severe limitations in water and food. To safeguard the public's health, developing fast and convenient methods for determination of NO2- is of significance. Point-of-care testing (POCT) affords demotic measurement of NO2- and shows huge potential in future technology beyond those possible with traditional methods. Here, a novel ratiometric fluorescent nanoprobe (Ru@MOF-NH2) is developed by integrating UiO-66-NH2 with tris(2,2'-bipyridyl)ruthenium(II) ([Ru(bpy)3]2+) through a one-pot approach. The special diazo-reaction between the amino group of UiO-66-NH2 and NO2- is responsible for the report signal (blue emission) with high selectivity and the red emission from [Ru(bpy)3]2+ offers the reference signal. The proposed probe shows obviously distinguishable color change from blue to red towards NO2- via naked-eye. Moreover, using a smartphone as the detection device to read color hue, ultra-sensitive quantitative detection of NO2- is achieved with a low limit of detection at 0.6 μΜ. The accuracy and repeatability determined in spiked samples through quantitative visualization is in the range of 105 to 117% with a coefficient of variation below 4.3%. This POCT sensing platform presents a promising strategy for detecting NO2- and expands the potential applications for on-site monitoring in food and environment safety assessment.

A simple and efficient alginate hydrogel combined with surface-enhanced Raman spectroscopy for quantitative analysis of sodium nitrite in meat products

Sodium nitrite is a commonly used preservative and color protectant in the food industry. Conventional analytical methods are highly susceptible to food matrix interference, time-consuming and costly. In this study, the ion cross-linking method was employed to prepare alginate hydrogel substrates, and phenosafranin was chosen as a single-molecule probe to analyze sodium nitrite. Our investigation centered on elucidating the effects of alginate and cross-linking ion concentrations on Raman signal characteristics. The optimal Raman response was observed in the precursor solution with 1% sodium alginate and 0.1 mol L-1 cross-linking ions. The relative standard deviations (RSDs) of the feature peaks from the three substrate batches ranged from 1.22% to 16.30%, attesting the robustness and consistency of the substrates. The signal reduction of the substrates after a four-week storage period remained below 10%, indicating that the substrates had good reproducibility and stability. The limits of detection (LODs) for sodium nitrite in extracts from cured meat, luncheon meat, and sliced ham were determined to range from 3.75 mg kg-1 to 8.11 mg kg-1, with low interference from the food matrix. The support vector machine algorithm was utilized to train and predict the data, which proved to be more accurate (98.6%-99.8% recovery) than the traditional linear regression model (81.9%-112.7% recovery) in predicting the spiked samples. The application of hydrogel-based surface-enhanced Raman spectroscopy (SERS) substrates for nitrite detection in food, combined with machine learning for regression prediction in data processing, collectively augmented the potential of SERS technology in the field of food analysis.

Organosilicon-Based Carbon Dots and Their Versatile Applications

Carbon dots (CDs) are a newly discovered type of fluorescent material that has gained significant attention due to their exceptional optical properties, biocompatibility, and other remarkable characteristics. However, single CDs have some drawbacks such as self-quenching, low quantum yield (QY), and poor stability. To address these issues, researchers have turned to organosilicon, which is known for its green, economical, and abundant properties. Organosilicon is widely used in various fields including optics, electronics, and biology. By utilizing organosilicon as a synthetic precursor, the biocompatibility, QY, and resistance to self-quenching of CDs can be improved. Meanwhile, the combination of organosilicon with CDs enables the functionalization of CDs, which significantly expands their original application scenarios. This paper comprehensively analyzes organosilicon in two main categories: precursors for CD synthesis and matrix materials for compounding with CDs. The role of organosilicon in these categories is thoroughly reviewed. In addition, the paper presents various applications of organosilicon compounded CDs, including detection and sensing, anti-counterfeiting, optoelectronic applications, and biological applications. Finally, the paper briefly discusses current development challenges and future directions in the field.

Prospects and hazards of silica nanoparticles: Biological impacts and implicated mechanisms

With the thrive of nanotechnology, silica nanoparticles (SiNPs) have been extensively adopted in the agriculture, food, cosmetic, and even biomedical industries. Due to the mass production and use, SiNPs inevitably entered the environment, resulting in ecological toxicity and even posing a threat to human health. Although considerable investigations have been conducted to assess the toxicity of SiNPs, the correlation between SiNPs exposure and consequent health risks remains ambiguous. Since the biological impacts of SiNPs can differ from their design and application, the toxicity assessment for SiNPs may be extremely difficult. This review discussed the application of SiNPs in different fields, especially their biomedical use, and documented their potential release pathways into the environment. Meanwhile, the current process of assessing SiNPs-related toxicity on various model organisms and cell lines was also detailed, thus estimating the health threats posed by SiNPs exposure. Finally, the potential toxic mechanisms of SiNPs were also elaborated based on results obtained from both in vivo and in vitro trials. This review generally summarizes the biological effects of SiNPs, which will build up a comprehensive perspective of the application and toxicity of SiNPs.

Carbon quantum dots for fluorescent detection of nitrite: A review

NO₂^- is commonly found in foods and the environment, and excessive intake of NO₂^- poses serious hazards to human health. Thus, rapid and accurate assay of NO₂^- is of considerable significance. Traditional instrumental approaches for detection of NO₂^- faced with limitations of expensive instruments and complicated operations. Current gold standards for sensing NO₂^- are Griess assay and 2,3-diaminonaphthalene assay, which suffer from slow detection kinetics and bad water solubility. The newly emerged carbon quantum dots (CQDs) exhibit integrated merits including easy fabrication, low-cost, high quantum yield, excellent photostability, tunable emission behavior, good water solubility and low toxicity, which make CQDs be widely applied to fluorescent assay of NO₂^-. In this review, synthetic strategies of CQDs are briefly presented. Advances of CQDs for fluorescent detection of NO₂^- are systematically highlighted. Lastly, the challenges and perspectives in the field are discussed.

Cobalt-Nitrogen Co-Doped Carbon as Highly Efficient Oxidase Mimics for Colorimetric Assay of Nitrite

Transition metal-N-doped carbon has been demonstrated to mimic natural enzyme activity; in this study, cobalt-nitrogen co-doped carbon (Co-N-C) nanomaterial was developed, and it could be an oxidase mimic. Firstly, Co-N-C with oxidase-like activity boosts the chromogenic reaction of 3,3',5,5'-tetramethylbenzidine (TMB) to produce the oxidized TMB (oxTMB). And the aromatic primary amino group of oxTMB reacts with nitrite (NO₂^-) to form diazo groups. Based on this background, we developed a cascade system of a Co-N-C-catalyzed oxidation reaction and a diazotization reaction for nitrite determination. The low detection limit (0.039 μM) indicates that Co-N-C is superior compared with the vast majority of previously reported nitrite assays. This study not only provides a novel nanozyme with sufficiently dispersed active sites, but it also further applies it to the determination of nitrite, which is expected to expand the application of nanozymes in colorimetric analysis.

A fluorescent carbon dots synthesized at room temperature for automatic determination of nitrite in Sichuan pickles

In this paper, highly fluorescent carbon dots were synthesized from sodium ascorbate and polyethyleneimine at room temperature (R-CDs). The proposed green synthesis method was energy-saving, environmentally friendly and easy online. R-CDs exhibit an optimal emission peak of 490 nm under excitation at 380 nm with a quantum yield of 32 %. R-CDs morphology, composition, and properties were characterized using TEM, FTIR, XPS, UV-vis and fluorescence spectroscopy. The study revealed that nitrite quenched the fluorescence of R-CDs under acidic conditions. Subsequently, this discovered reaction of R-CDs and nitrite was combined with flow-injection technology, and a simple, precise and automatic fluorescence strategy for nitrite determination was accomplished. The response to nitrite was linear in 5-300 μg·L^-1 concentration range and the limit of detection was 2.85 μg·L^-1 (3.3 S/k). This method was applied to nitrite determination in Sichuan pickles during the pickling process and results were consistent with the standard method, demonstrating its feasibility in practical applications.

Ion-selective electrodes based on laser-induced graphene as an alternative method for nitrite monitoring

Nitrite is an important food additive for cured meats; however, high nitrite levels pose adverse health effects to humans. Hence, monitoring nitrite concentration is critical to comply with limits imposed by regulatory agencies. Laser-induced graphene (LIG) has proven to be a scalable manufacturing alternative to produce high-performance electrochemical transducers for sensors. Herein, we expand upon initial LIG studies by fabricating hydrophilic and hydrophobic LIG that are subsequently converted into ion-selective sensors to monitor nitrite in food samples with comparable performance to the standard photometric method (Griess method). The hydrophobic LIG resulted in an ion-selective electrode with improved potential stability due partly to a decrease in the water layer between the electrode and the nitrite poly(vinyl) chloride-based ion-selective membrane. These resultant nitrite ion-selective sensors displayed Nernstian response behavior with a sensitivity of 59.5 mV dec-1, a detection limit of 0.3 ± 0.1 mg L-1 (mean ± standard deviation), and a broad linear sensing range from 10-5 to 10-1 M, which was significantly larger than currently published nitrite methods. Nitrite levels were determined directly in food extract samples of sausage, ham, and bacon for 5 min. These sensor metrics are significant as regulatory agencies limit nitrite levels up to 200 mg L-1 in finished products to reduce the potential formation of nitrosamine (carcinogenic compound). These results demonstrate the versatility of LIG as a platform for ion-selective-LIG sensors and simple, efficient, and scalable electrochemical sensing in general while demonstrating a promising alternative to monitor nitrite levels in food products ensuring regulatory compliance.

Accurate quantification, naked eyes detection and bioimaging of nitrite using a colorimetric and near-infrared fluorescent probe in food samples and Escherichia coli

Nitrite (NO₂^-) is an inorganic contaminant that exists widely in the environment including water and food products, excessive amounts of NO₂^- would threaten humans and aquatic life. Developing a rapid and convenient sensing method for NO₂^- remains a great challenge. Herein, a colorimetric and near-infrared fluorescent probe (TBM) was synthesized and applied for sensitively and selectively detecting NO₂^- in water, food samples and Escherichia coli (E. coli). With the addition of NO₂^-, the probe TBM solution has a distinct visual color changed from red to colorless and fluorescence intensity at 620 nm quickly decreased. The probe TBM could detect NO₂^- quantitatively with a detection limit of 85 nM based on a 3σ/slope. Under optimum conditions, TBM has been successfully used to detect NO₂^- in real-world environmental and dietary samples, with positive results. Besides, paper strips loaded with TBM have been used to visually determine NO₂^- levels. Most importantly, TBM has also been proven to be able to discriminate from different concentrations of NO₂^- in E. coli by fluorescence imaging. In summary, the probe TBM was successfully developed for the accurate quantification, naked eyes detection and bioimaging of NO₂^- in water, food samples and E. coli, which provides a useful tool to better guarantee the quality and safety of daily life and food industry.

Detection of Heavy Metal Ions by Ratiometric Photoelectric Sensor

In recent years, heavy metal pollution has become increasingly serious. Heavy metals exist in an environment mainly in the form of ions (heavy metal ions, HMs). They can contaminate food, water, soil, and the atmosphere, leading to serious harm to plants and animals. With high bioavailability and nonbiodegradability, HMs can accumulate through biomagnification. Consequently, heavy metal pollution has become the cause of many fatal diseases threatening human health and ecological environment. Therefore, the accurate detection of HMs is vital and necessary. In this paper, the harm and limit standards of heavy metals were systematically summarized and the common analysis methods were overviewed and compared. Specifically, the latest research progress of ratiometric photoelectric sensor, including optical and electrical sensor, were mainly described. The research status and advantages and disadvantages of a photoelectric sensor were summarized. Furthermore, the future directions were proposed, which provided the reference for the further research and application of the ratiometric photoelectric sensor.

A Brief Review of Carbon Dots-Silica Nanoparticles Synthesis and their Potential Use as Biosensing and Theragnostic Applications

Carbon dots (CDs) are carbon nanoparticles with sizes below 10 nm and have attracted attention due to their relatively low toxicity, great biocompatibility, water solubility, facile synthesis, and exceptional photoluminescence properties. Accordingly, CDs have been widely exploited in different sensing and biomedical applications, for example, metal sensing, catalysis, biosensing, bioimaging, drug and gene delivery, and theragnostic applications. Similarly, the well-known properties of silica, such as facile surface functionalization, good biocompatibility, high surface area, and tunable pore volume, have allowed the loading of diverse inorganic and organic moieties and nanoparticles, creating complex hybrid nanostructures that exploit distinct properties (optical, magnetic, metallic, mesoporous, etc.) for sensing, biosensing, bioimaging, diagnosis, and gene and drug delivery. In this context, CDs have been successfully grafted into diverse silica nanostructures through various synthesis methods (e.g., solgel chemistry, inverse microemulsion, surfactant templating, and molecular imprinting technology (MIT)), imparting hybrid nanostructures with multimodal properties for distinct objectives. This review discusses the recently employed synthesis methods for CDs and silica nanoparticles and their typical applications. Then, we focus on combined synthesis techniques of CD-silica nanostructures and their promising biosensing operations. Finally, we overview the most recent potential applications of these materials as innovative smart hybrid nanocarriers and theragnostic agents for the nanomedical field.

Intelligently design primary aromatic amines derived carbon dots for optical dual-mode and smartphone imaging detection of nitrite based on specific diazo coupling

Primary aromatic amines derived carbon dots (PAA-CDs) with the protonated amino groups and high quantum yield of 46% were favorably obtained by one-step solvothermal treatment of m-phenylenediamine (m-PDA) in acidic environment. The interaction between the PAA-CDs and nitrite (NO₂^-) was inherited the characteristic reaction of m-PDA (a primary aromatic amine) and NO₂^-, resulting in strong fluorescence quenching and obvious absorption variation of the PAA-CDs. Meanwhile, a chromogenic reaction of diazo coupling can cause significant color changes. Hence, the PAA-CDs were developed for an optical dual-mode and smartphone imaging sensor for NO₂^- detection in the range of 3.0 ~ 40.0 μM with high selectivity, good sensitivity, and excellent anti-interference capability. A limit of detection (LOD) of 0.024 μM and 0.16 μM was implemented by fluorometry and colorimetry, respectively. For smartphone imaging colorimetry, the LODs of 0.46 μM (visible color) and 0.99 μM (fluorescence color) were acquired. More importantly, the established sensor has been successfully applied for the dynamic detection of NO₂^- in various food samples with the satisfying results. A smartphone imaging colorimetry method based on the CDs was firstly proposed to visually and quantitatively detect NO₂^-, which will broaden the application range of the CDs in food safety inspection.

New analytical strategies amplified with carbon-based nanomaterial for sensing food pollutants

The most significant topic currently under the moonlight is Nanobiotechnology and engineered nanomaterials. The novel characteristics displayed by engineered Nanomaterials, especially carbon-based nanomaterials, have spurred interest in its potential application in the food industry. It has provided opportunities for finding solutions to the long-standing challenges in the food industry to assess food safety, maintain food quality, extend the shelf life of produce, and efficiently deliver nutrients. Nanomaterials can be incorporated in food sensors facilitating efficient monitoring of crop maturity and detecting biological and chemical contaminants. When integrated into food packages, nanomaterials could aid in assessing the freshness and improving the quality of packaged foods. In addition, more efficient delivery of nutrients could be possible in foods fortified using nano compounds. The initial section of this review gives an overview of the broad application of nanotechnology in the food industry and carbon-based nanomaterials. The latter part focuses on nanotechnology in biosensors for food safety and quality monitoring.

Ratiometric fluorescent sensors for nitrite detection in the environment based on carbon dot/Rhodamine B systems

A novel carbon dot/Rhodamine B-based ratiometric fluorescent probe was developed for a highly sensitivity and selective detection of nitrite (NO₂⁻). The probe showed colour changes from blue to orange under ultraviolet light in response to NO₂⁻ with a detection limit as low as 67 nM in the range of 0 to 40 μM. A ratiometric fluorescent test paper was successfully prepared using the probe solution, which demonstrated its feasibility towards a rapid and semi-quantitative detection of NO₂⁻ in real samples.

A visual ratiometric fluorescent sensor based on blue carbon dot/Rhodamine B is used to selectively detect NO₂⁻ in the environment.

A novel highly specific colorimetric fluorescent probe for the detection of nitrite in aqueous solution

Developing an effective method for the detection of nitrite (NO₂ ^- ions) in natural environment especially environmental waters and soils is very necessary, because it will cause serious damage to human health once excess NO₂ ^- ions enters the human body. Herein, a new colorimetric fluorescent probe NB-NO₂ ^- for determining NO₂ ^- ions was designed, and it possesses good water-solubility and pleasurable selectivity over others common ions for NO₂ ^- ions. The addition of NO₂ ^- ions changed the color of solution from blue to colorless by naked-eye. And through the test and calculation, the detection limit of the probe NB-NO₂ ^- is 129 nM. Based on the above excellent characteristics, the probe NB-NO₂ ^- was successfully used for monitoring NO₂ ^- ions in environmental waters and soils.

Cyanine dye-assembled composite upconversion nanoparticles for the sensing and cell imaging of nitrite based on a single particle imaging method

An upconversion luminescence total internal reflection single particle imaging method was developed for the sensing and cell imaging of nitrite.

Ratiometric Colorimetric Detection of Nitrite Realized by Stringing Nanozyme Catalysis and Diazotization Together

Due to the great threat posed by excessive nitrite in food and drinking water to human health, it calls for developing reliable, convenient, and low-cost methods for nitrite detection. Herein, we string nanozyme catalysis and diazotization together and develop a ratiometric colorimetric approach for sensing nitrite in food. First, hollow MnFeO (a mixture of Mn and Fe oxides with different oxidation states) derived from a Mn-Fe Prussian blue analogue is explored as an oxidase mimic with high efficiency in catalyzing the colorless 3,3′,5,5′-tetramethylbenzidine (TMB) oxidation to blue TMBox, presenting a notable signal at 652 nm. Then, nitrite is able to trigger the diazotization of the product TMBox, not only decreasing the signal at 652 nm but also producing a new signal at 445 nm. Thus, the analyte-induced reverse changes of the two signals enable us to establish a ratiometric colorimetric assay for nitrite analysis. According to the above strategy, facile determination of nitrite in the range of 3.3–133.3 μM with good specificity was realized, providing a detection limit down to 0.2 μM. Compared with conventional single-signal analysis, our dual-signal ratiometric colorimetric mode was demonstrated to offer higher sensitivity, a lower detection limit, and better anti-interference ability against external detection environments. Practical applications of the approach in examining nitrite in food matrices were also verified.

Reusable 3D silver superposed silica SERS substrate based on the Griess reaction for the ratiometric detection of nitrite

When nitrite is ingested and absorbed by the body, it can be converted into highly toxic nitrosamines (carcinogens, teratogens, and mutagens), posing health risks to the general population. Therefore, it calls for establishing a method for determination of nitrite. In this paper, the glass-SiO₂-Ag surface-enhanced Raman scattering (SERS) substrate with a large number of "hot spots" were prepared by two kinds of silane coupling agents. The SERS substrate had high sensitivity and repeatability. Silicon dioxide supported the silver nanoparticles (Ag NPs), which increased surface roughness of the substrate, generated a great quantity of hot spots and enhanced the SERS signal. In the SERS spectrum, the intensity ratio of the two characteristic peaks (1287 cm^-1 and 1076 cm^-1) had a good linear correlation with the logarithm of the concentration of nitrite, R² = 0.9652. The recoveries of 50 μM and 100 μM nitrite in three kinds of foods, three kinds of cosmetics and tap water were 90.9-105.3%.

Headspace Solid-Phase Microextraction Following Chemical Vapor Generation for Ultrasensitive, Matrix Effect-Free Detection of Nitrite by Microplasma Optical Emission Spectrometry

A new chemical vapor generation method coupled with headspace solid-phase microextraction miniaturized point discharge optical emission spectrometry (HS-SPME-μPD-OES) for the sensitive and matrix effect-free detection of nitrite in complex samples is described. In an acidic medium, the volatile cyclohexene was generated from cyclamate in the presence of nitrite, which was volatilized to the headspace of the container, efficiently separated, and preconcentrated by HS-SPME. Consequently, the SPME fiber was transferred to a laboratory-constructed thermal desorption chamber wherein the cyclohexene was thermally desorbed and swept into μPD-OES for its sensitive quantification via monitoring the carbon atomic emission line at 193.0 nm. As a result, the quantification of nitrite was accomplished through the determination of cyclohexene. The application of HS-SPME as a sampling technique not only simplifies the experimental setup of μPD-OES but it also preconcentrates and separates cyclohexene from N₂ and sample matrices, thus eliminating the interference from water vapor and N₂ and significantly improving the analytical performance on the determination of nitrite. Under the optimum experimental conditions, a limit of detection of 0.1 μg L^-1 was obtained, which is much better than that obtained by conventional methods. The precision, expressed as relative standard deviation, was better than 3.0% at a concentration of 10 μg L^-1. The proposed method provides several advantages of portability, simplicity, high sensitivity, and low energy consumption and eliminates expensive instruments and matrix interference, thus retaining a promising potential for the rapid, sensitive, and field analysis of nitrite in various samples.

A facile dual-function fluorescent probe for detection of phosgene and nitrite and its applications in portable chemosensor analysis and food analysis

Due to the potential threats of phosgene and nitrite to public health and safety, in this work, we first proposed the application of a facile dual-function fluorescent probe 2-(1H-Benzimidazol-2-yl)Aniline (BMA) for the detection of phosgene and nitrite in different solvent environments. BMA had fast response (1 min), high selectivity and sensitivity (the limit of detection was 1.27 nM) to phosgene in CH₃CN solution (containing 10% DMSO), which manifested as a ratiometric fluorescent mode from 416 nm to 480 nm. The response of BMA to nitrite in HCl solution (pH = 1, containing 10% CH₃CN) was also highly selective and sensitive (the limit of detection was 60.63 nM), which shown as a turn-off fluorescent mode at 485 nm. In addition, two portable chemosensors (BMA-loaded TLC plates and test strips) had also been successfully manufactured for the detection of phosgene in the gas phase and nitrite in solution, which displayed good responses. Most importantly, BMA had also been successfully used for detection of nitrite in food samples, and a good recovery (88.5%-107.2%) was obtained by adding standard sodium nitrite.

The synthesis of a chemically reactive and polymeric luminescent gel

In the past, chemically reactive polymeric interfaces have been considered to be of potential interest for developing functional materials for a wide range of practical applications. Furthermore, the rational incorporation of luminescence properties into such chemically reactive interfaces could provide a basis for extending the horizon of their prospective utility. In this report, a simple catalyst-free chemical approach is introduced to develop a chemically reactive and optically active polymeric gel. Branched-polyethyleneimine (BPEI)-derived, inherently luminescent carbon dots (BPEI-CDs) were covalently crosslinked with pentaacrylate (5Acl) through a 1,4-conjugate addition reaction under ambient conditions. The synthesized polymeric gel was milky white under visible light; however, it displayed fluorescence under UV light. Additionally, the residual acrylate groups in the synthesized fluorescent gel allowed its chemical functionality to be tailored through facile, robust 1,4-conjugate addition reactions with primary-amine-containing small molecules under ambient conditions. The chemical reactivity of the luminescent gel was further employed for a proof-of-concept demonstration of portable and parallel 'ON'/'OFF' toxic chemical sensing (namely, the sensing of nitrite ions as a model analyte). First, the chemically reactive luminescent gel derived from BPEI-CDs was covalently post-modified with aniline for the selective synthesis of a diazo compound in the presence of nitrite ions. During this process, the color of the gel under visible light changed from white to yellow and, thus, the colorimetric mode of the sensor was turned 'ON'. In parallel, the luminescence of the gel under UV light was quenched, which was denoted as the 'OFF' mode of the sensor. This parallel and unambiguous 'ON'/'OFF' sensing of a toxic chemical (nitrite ions, with a detection limit of 3 μM) was also achieved even in presence of other relevant interfering ions and at concentrations well below the permissible limit (65 μM) set by the World Health Organization (WHO). Furthermore, this chemically reactive luminescent gel could be of potential interest in a wide range of basic and applied contexts.

Effect of fermentation by various bacterial strains on quality of dried duck meat slice

The effects of fermentation on the sensory qualities, lipid oxidation, harmful substances, and microbial growth of dried duck meat slice (DDMS) were investigated. The results showed that the optimal fermentation was controlled at 22.18 °C for 49.15 h with the mixed inoculation (7.09 log CFU/g) of Lactobacillus acidophilus and Pediococcus pentosaceus (2:1). Under the optimal fermentation conditions, the fermented DDMS presented higher scores of color (9.0 ± 0.16), aroma (8.8 ± 0.35), and total (8.9 ± 0.24) with lower hardness (5316 ± 98.80 g), compared to control (8.6 ± 0.21, 8.3 ± 0.26, 8.4 ± 0.08, and 7016 ± 114.17 g, respectively). Meanwhile, the histamine content decreased, and the nitrite content was reduced by nearly 60% in fermented DDMS. The lipid oxidation and microbial growth (Escherichia coli, mold, and yeast) in DDMS were also inhibited by fermentation. It provides useful data for improving the quality and safety of meat products.

Engineering of a Dual-Recognition Ratiometric Fluorescent Nanosensor with a Remarkably Large Stokes Shift for Accurate Tracking of Pathogenic Bacteria at the Single-Cell Level

Rapid, accurate, reliable, and risk-free tracking of pathogenic microorganisms at the single-cell level is critical to achieve efficient source control and prevent outbreaks of microbial infectious diseases. For the first time, we report a promising approach for integrating the concepts of a remarkably large Stokes shift and dual-recognition into a single matrix to develop a pathogenic microorganism stimuli-responsive ratiometric fluorescent nanoprobe with speed, cost efficiency, stability, ultrahigh specificity, and sensitivity. As a proof-of-concept, we selected the Gram-positive bacterium Staphylococcus aureus (S. aureus) as the target analyte model, which easily bound to its recognition aptamer and the broad-spectrum glycopeptide antibiotic vancomycin (Van). To improve the specificity and short sample-to-answer time, we employed classic noncovalent π-π stacking interactions as a driving force to trigger the binding of Van and aptamer dual-functionalized near-infrared (NIR) fluorescent Apt-Van-QDs to the surface of an unreported blue fluorescent π-rich electronic carbon nanoparticles (CNPs), achieving S. aureus stimuli-responsive ratiometric nanoprobe Apt-Van-QDs@CNPs. In the assembly of Apt-Van-QDs@CNPs, the blue CNPs (energy donor) and NIR Apt-Van-QDs (energy acceptor) became close to allow the fluorescence resonance energy transfer (FRET) process, leading to a remarkable blue fluorescence quenching for the CNPs at ∼465 nm and a clear NIR fluorescence enhancement for Apt-Van-QDs at ∼725 nm. In the presence of S. aureus, the FRET process from CNPs to Apt-Van-QDs was disrupted, causing the nanoprobe Apt-Van-QDs@CNPs to display a ratiometric fluorescent response to S. aureus, which exhibited a large Stokes shift of ∼260 nm and rapid sample-to-answer detection time (∼30.0 min). As expected, the nanoprobe Apt-Van-QDs@CNPs showed an ultrahigh specificity for ratiometric fluorescence detection of S. aureus with a good detection limit of 1.0 CFU/mL, allowing the assay at single-cell level. Moreover, we also carried out the precise analysis of S. aureus in actual samples with acceptable results. We believe that this work offers new insight into the rational design of efficient ratiometric nanoprobes for rapid on-site accurate screening of pathogenic microorganisms at the single-cell level in the early stages, especially during the worldwide spread of COVID-19 today.

Application of a smartphone based spectrophotometer for rapid in-field determination of nitrite and chlorine in environmental water samples

In this paper, a low cost and portable smartphone-based spectrophotometer with the purpose of measuring trace levels of two important anions, chlorine and nitrite ions in water samples, is introduced. This home-made spectrophotometer is made of Plexiglas, equipped with two LEDs as a light source, and a piece of DVD is acted as light dispersing element. Battery of smartphone was used as its power supply and spectral analysis was performed by a free software downloadable from Google Playstore. By using this lightweight spectrophotometer, various environmental samples were analyzed for their NO₂^- and Cl₂ content in field. Good detection limits of 5.00 × 10^-2 mg L^-1 and 8.60 × 10^-3 mg L^-1 were obtained for chlorine and nitrite, respectively. The linear range for chlorine was 1.00-4.00 mg L^-1 and this range for nitrite was 0.05-1.20 mg L^-1. Reproducibility as relative standard deviation for both chlorine and nitrite was better than 8.75%. In order to investigate validity of data, results were compared to standard methods of measuring chlorine and nitrite, using both spectrophotometry and commercial kits which showed no difference between results obtained. This very simple to use and inexpensive device can be used many times, so can be considered as a low-cost alternative to the detection device of commercial kits.

A highly selective chromogenic probe for the detection of nitrite in food samples

A rapid, sensitive, and highly selective method for determining nitrite in food has been developed. This method is based on the reaction of nitrite with the amino group of 3,3,5,5-tetramethylbenzidine (TMB) to form a diazonium salt, and then the diazonium salt and glucosamine hydrochloride are coupled to each other to form an orange compound. The optimal conditions for maximum color and other analytical parameters were studied. A colorimetric method for nitrite detection has been developed with an outstanding correlation coefficient (R² = 0.9944), a wide linear range (1-75 μM) and 0.73 μM limit of detection (at S/N = 3) for nitrite ions. This method was successfully applied to the determination of nitrite in a variety of foods and gave recoveries in the range between 100.16% and 103.07%, demonstrating that the accuracy, reliability and potential application of this assay for monitoring nitrite in foods.

White pepper-derived ratiometric carbon dots for highly selective detection and imaging of coenzyme A

A new-style white pepper derived dual-emission carbon dots (CDs) with a quantum yield of 10.4% was designed and facile constructed with one-pot solvothermal method. The green emission (520 nm) had an efficient and special "turn-on" fluorescence sensing of coenzyme A (CoA) with the aid of Cu²⁺, while red emission (668 nm) barely changed and worked as reference. In the concentration range (0-150 µM), relative fluorescence intensity ratios (F₅₂₀/F₆₆₈) showed excellent linear correlation with concentrations of CoA, and detection limit was as low as 8.75 nm. Moreover, the strategy has been successfully applied for label-free detection of CoA in real pig liver samples with good recoveries (93.3-108.0%). Notably, the synthesized CDs had durable fluorescence, low cytotoxicity, and good biocompatibility for cellular imaging, which demonstrated wide and promising applicability for biosensing and bioimaging in the future.

Hydrothermal synthesis of green fluorescent nitrogen doped carbon dots for the detection of nitrite and multicolor cellular imaging

A fluorescent probe for the determination of nitrite (NO₂^-) was fabricated by using green fluorescent nitrogen doped carbon dots (NCDs). The NCDs were synthesized via a one-pot hydrothermal carbonization of citric acid in the presence of p-phenylenediamine as the nitrogen source. The N content of the NCDs was high to 17.09% and consisted of a variety of functional groups on the NCDs surface, including sp²-hybridized CN, porphyrin C-N-C and amino N in N-(C) ₃ or H-N-(C) ₂ et al. N atoms were also doped within the framework of the NCDs. The almost monodisperse NCDs (average particle diameter = 3.67 nm) exhibited green photoluminescence (PL) with excitation/emission maxima of 360/505 nm. The PL of the NCDs was dependent on both excitation wavelength and solution pH. The NCDs showed a strong PL quenching response to NO₂^- under acidic conditions (pH = 2.5). The PL intensity of the NCDs was inversely proportional to the concentration of NO₂^- between 0.02 and 40 μM (R² = 0.992), with a detection limit of 21.2 nM. The practical use of the nanoprobe for NO₂^- determination in food samples was also demonstrated, successfully. NCD-nitroso compounds formed because of reaction between the abundant amide groups on the surface of NCDs with the NO₂^-, which caused an inner filter effect and static PL quenching. Importantly, the NCDs had low cellular toxicity and were successfully used as a multicolor cellular imaging agent for Hepg2 cells.

Fluorometric determination of nitrite through its catalytic effect on the oxidation of iodide and subsequent etching of gold nanoclusters by free iodine

A method for sensitive detection of nitrite is presented. It is found that the red fluorescence of gold nanoclusters (with excitation/emission maxima at 365/635 nm) is quenched by traces of iodine via etching. Free iodide is formed by oxidation of iodide by bromate anion under the catalytic effect of nitrite. This catalytic process provides a sensitive means for nitrite detection. Under the optimal conditions, fluorescence linearly dropos in the 10 nM to 0.8 μM nitrite concentration range. The limit of detection is 1.1 nM. This is a few orders of magnitude lower than the maximum concentration allowed by authorities. Graphical abstract Schematic representation of a method for detection of nitrite via a redox reaction. Iodine was produced in the reaction and subsequently quenched the fluorescence from gold nanoclusters by etching their metallic cores, and a sensitive assay for nitrite down to 1.1 nM was developed.

Dual emission carbon dots from carotenoids: Converting a single emission to dual emission

Dual emission carbon dots have a high potential for use as fluorescence-based sensors with higher selectivity and sensitivity. This study demonstrated the possibility of conversion of a biological molecular system with a single emission peak to a double emission carbon dots system. This report is the first to describe the synthesis of dual emission carbon dots by tuning the electronic environment of a conjugated system. Here we prepared carbon dots from a natural extract, from which carotenoids were used as a new source for carbon dots. Formation of the carbon dots was confirmed by images obtained under a transmission electron microscope as well as from a dynamic light scattering study. The prepared carbon dots system was characterized and its optical property was monitored. The study showed that, after irradiation with microwaves, the fluorescence intensity of the whole system changed, without any change in the original peak position of the carotenoid but with the appearance of an additional peak. A Fourier transform infrared study confirmed breaking of the conjugated system. When using ethylene glycol as a surface passivating agent added to these carotenoid carbon dots, the dual emission spectra became more distinct.

Modification-Free Fabricating Ratiometric Nanoprobe Based on Dual-Emissive Carbon Dots for Nitrite Determination in Food Samples

Great challenges still exist for facilely fabricating ratiometric fluorescent nanoprobes. Fortunately, the appearance of dual-emissive carbon dots (CDs) offers a glimmer of hope for the fabrication of modification-free ratiometric nanoprobe. The chemical and electronic structure characteristics of the dual-emissive CDs might be modulated by conjugated structures of carbon sources and the doped nitrogen and sulfur atoms, and the surface state also contributed to the fluorescence properties via surface functional groups. Herein, we report a one-pot strategy for simultaneous preparation of two kinds of CDs named RYDE CDs and RODE CDs, showing dual-emissive fluorescent peaks with long wavelength by 2,3-diaminobenzoic acid hydrochloride for the first time. Notably, trace nitrite determination with high sensitivity and selectivity was realized for the first time based on the modification-free ratiometric fluorescent nanoprobe fabricated rapidly and directly by the as-prepared RYDE CDs at constant room temperature (20 °C). Under the optimal conditions, the limit of detection for nitrite was 31.61 nM, with a wide concentration linear range of 0.1-100 μM. Furthermore, this ratiometric nanoprobe was successfully applied for nitrite analysis in bacon, sausage, pickle, and milk samples. Additionally, the nanoprobe was also capable of visually monitoring temperature fluctuations and cell imaging.

A colorimetric and fluorescent lighting-up sensor based on ICT coupled with PET for rapid, specific and sensitive detection of nitrite in food

A colorimetric and fluorogenic sensor exhibiting rapid, specific and sensitive detection of potentially toxic nitrite in food is described.