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

Carbon dots-based dual-mode sensor for highly selective detection of nitrite in food substrates through diazo coupling reaction

As an antioxidant and preservative agent, nitrite (NO2-) plays an essential role in the food industry to maintain freshness or inhibit microbial growth. However, excessive addition of NO2- is detrimental to health, so accurate and portable detection of NO2- is critical for food quality control. Notably, the selectivity of most carbon dots (CDs)-based fluorescence sensors was not enough due to the nonspecific interaction mechanism of hydrogen bond, electrostatic interaction and inner filter effect etc. Herein, a novel fluorescence/UV-vis absorption (FL/UV-vis) dual-mode sensor was developed on basis of mC-CDs, which were prepared by simple solvothermal treatment of m-Phenylenediamine (m-PDA) and cyanidin cation (CC). The fluorescence of these mC-CDs could be selectively responded by NO2- through the specific diazo coupling reaction between NO2- and amino groups on the surface of mC-CDs, thus effectively improving the selectivity of NO2- detection. The CDs-based fluorescence sensor possessed a low detection limit of 0.091 μM and 0.143 μM for FL and UV-vis methods and the excellent linear range of 0.0-60.0 μM. Furthermore, the mC-CDs sensor was employed to detect NO2- in real samples with a recovery rate of 97.11 %-104.15 % for quantitative addition. Moreover, the smartphone-assisted fluorescence sensing platform developed could identify the subtle color changes that could not be distinguished by the naked eye, and had the advantages of fast detection speed and intelligence. More importantly, the portable solid phase sensor based on mC-CDs had been successfully applied to the specific fluorescence identification and concentration monitoring of NO2-. Accordingly, the designed sensor provided a new strategy for the highly selective and convenient sensing of NO2- in food substrates, and paved the way for the wide application of CDs-based nanomaterials in the detection of food safety.

Ultrabright silicon nanoparticles combined with o-phenylenediamine for ratiometric fluorescence and smartphone imaging dual-mode detection of nitrite

Nitrite (NO2-) is widely present in the natural environment and human daily life. Excessive NO2- can cause harm to the environment and human health. Herein, silicon nanoparticles (SiNPs) with a fluorescence quantum yield of up to 70 % were synthesised using a one-pot hydrothermal method and combined with the common and inexpensive o-phenylenediamine (OPD) to achieve the detection of NO2-. Upon the addition of NO2-, the blue fluorescence of the SiNPs was quenched due to static quenching and Förster resonance energy transfer (FRET), while the yellow fluorescence of benzotriazole, the reaction product of OPD and NO2-, was enhanced, resulting in the fluorescence color change from blue to yellow. Based on these phenomena, a ratiometric fluorescence sensor integrated with smartphone imaging technology was developed. This sensor is notable for its portability, cost-effectiveness, and satisfactory detection limits (0.016 μM for ratiometric fluorescence and 1.64 μM for smartphone imaging). Importantly, it demonstrates high reliability and practicability in detecting NO2- in real water and food samples. This broadens the application of SiNPs in the sensing field and introduces new possibilities for NO2- detection in complex sample matrices.

Rational design of carbon dot nanozymes for ratiometric dual-signal and smartphone-assisted visual detection of nitrite in food matrices

Developing reliable nitrite (NO2-) sensors is essential for food safety and reducing health risks from NO2- exposure. In this study, we strategically designed nitrogen-doped carbon dot (N-CD) nanozymes to establish an accessible dual-signal ratiometric sensing system for detecting NO2- in food matrices. This system utilizes the photoluminescence and enzyme-like properties of N-CD nanozymes combined with NO2--triggered diazotization reactions of substrates such as o-phenylenediamine (OPD) or 3,3',5,5'-tetramethylbenzidine (TMB). The resulting N-CD/OPD and N-CD/TMB composites provide dual-mode detection-fluorescence and colorimetric-with high selectivity for NO2- and excellent resistance to interference. These sensors exhibit clear color changes under both ultraviolet and visible light, and can be combined with smartphones for visual, on-site detection of NO2-. By incorporating a ratiometric strategy, dual-signal output, and smartphone compatibility, our system achieved a low detection limit (≤ 1.92 μM) and satisfactory recovery rates (85.6-115 %) in environmental water and food samples. This highlights the potential of smartphone-assisted sensors for environmental monitoring and food safety applications. Our carbon dot-based platform offers a practical and effective solution for on-site NO2- detection, contributing valuable insights to the field.

B, N co-doped carbon dots as efficient nanozymes for colorimetric and fluorometric dual-mode detection of cholesterol

In this work, the B, N co-doped carbon dots (B, N-CDs) were synthesized via facile hydrothermal approach with 6-aminopyridine boronic acid as precursor. In addition to emitting intense blue luminescence when exposed to ultraviolet light, the prepared B, N-CDs displayed remarkable peroxidase-like activity, which could efficiently catalyze the oxidation of 3, 3', 5, 5' -tetramethylbenzidine (TMB) to blue ox-TMB in the presence of hydrogen peroxide (H2O2). Furthermore, the fluorescence intensity of B, N-CDs increased gradually upon the addition of H2O2. Since cholesterol oxidase (ChOx) can catalyze the oxidation of cholesterol to form H2O2, the as-prepared B, N-CDs was then used as both colorimetric and fluorometric sensors for the detection of cholesterol with detection limit of 0.87 and 2.31 μM, respectively. Finally, the dual-mode approach based on B, N-CDs was effectively utilized for detecting cholesterol levels in serum samples, proving the potential application of B, N-CDs in the field of biological assay.

Development of an accurate hand-held sensing platform for nitrite detection based on nitrogen-doped carbon dots

As is well known, excessive nitrite can seriously pollute the environment and can harm human health. Although existing methods can be used to determine nitrite content, they still have some drawbacks, such as relatively complicated operation and expensive equipment. Herein, a hand-held sensing platform (HSP) for NO2- determination was developed. First, ammonia-rich nitrogen-doped carbon dots with orange-yellow emission were designed and synthesised, which were suitable as fluorescent probes because of their good optical properties and stability. Then, the HSP based on fluorescence using photoelectric conversion technology was designed and manufactured using three-dimensional printing technology. Under optimum conditions, the voltage (V/V0) of the proposed HSP showed good linearity for NO2- detection in the range of 10-500 μM, with a detection limit of 1.95 μM. This portable sensor showed good stability, accuracy and reliability in detecting actual water and meat samples, which may ensure food safety in practical applications. Moreover, the HSP is compact, portable and easily assembled and is suitable for on-site real-time detection, which shows great application potential and prospects.

Hydrophobic carbon quantum dots with red fluorescence: An optical dual-mode and smartphone imaging sensor for identifying Chinese Baijiu quality

Chinese Baijiu (Liquor) is a popular alcoholic beverage, and the ethanol content in Baijiu is closely related to its quality; therefore, it is of great significance to explore a facile, sensitive, and rapid method to detect ethanol content in Baijiu. Hydrophobic carbon quantum dots (H-CQDs) with bright red fluorescence (24.14 %) were fabricated by hydrothermal method using o-phenylenediamine, p-aminobenzoic acid, manganese chloride, and hydrochloric acid as reaction precursors. After the introduction of ultrapure water into the ethanol solution dissolved with H-CQDs, the aggregated H-CQDs resulted in significant changes in fluorescence intensity and absorbance. On this basis, a sensor for detecting ethanol by optical dual-mode and smartphone imaging was constructed. More importantly, the sensor can be used for detecting ethanol content in Chinese Baijiu with satisfactory results. This sensing platform has great potential for quality identification in Chinese Baijiu, broadening the application scope of CQDs in food safety detection.

Smartphone-Based Techniques Using Carbon Dot Nanomaterials for Food Safety Analysis

The development of portable and efficient nanoprobes to realize the quantitative/qualitative onsite determination of food pollutants is of immense importance for safeguarding human health and food safety. With the advent of the smartphone, the digital imaging property causes it to be an ideal diagnostic substrate to point-of-care analysis probes. Besides, merging the versatility of carbon dots nanostructures and bioreceptor abilities has opened an innovative assortment of construction blocks to design advanced nanoprobes or improving those existing ones. On this ground, massive endeavors have been made to combine mobile phones with smart nanomaterials to produce portable (bio)sensors in a reliable, low cost, rapid, and even facile-to-implement area with inadequate resources. Herein, this work outlines the latest advancement of carbon dots nanostructures on smartphone for onsite detecting of agri-food pollutants. Particularly, we afford a summary of numerous approaches applied for target molecule diagnosis (pesticides, mycotoxins, pathogens, antibiotics, and metal ions), for instance microscopic imaging, fluorescence, colorimetric, and electrochemical techniques. Authors tried to list those scaffolds that are well-recognized in complex media or those using novel constructions/techniques. Lastly, we also point out some challenges and appealing prospects related to the enhancement of high-efficiency smartphone based carbon dots systems.

Bridging biological and food monitoring: A colorimetric and fluorescent dual-mode sensor based on N-doped carbon dots for detection of pH and histamine

Rapid and sensitive monitoring of pH and histamine is crucial for bridging biological and food systems and identifying corresponding abnormal situations. Herein, N-doped carbon dots (CDs) are fabricated by a hydrothermal method employing dipicolinic acid and o-phenylenediamine as precursors. The CDs exhibit colorimetric and fluorescent dual-mode responses to track pH and histamine variations in living cells and food freshness, respectively. The aggregation-induced emission enhancement and intramolecular charge transfer result in a decrease in absorbance and an increase in fluorescence, which become readily apparent as the pH changes from acidic to neutral. This property enables precise differentiation between normal and cancerous cells. Furthermore, given the intrinsic basicity of histamine, pH-responsive CDs are advantageous for additional colorimetric and fluorescent monitoring of histamine in food freshness, achieving linearities of 25-1000 µM and 30-1000 µM, respectively, which are broader than those of alternative nanoprobes. Interestingly, the smartphone-integrated sensing platform can portably and visually evaluate pH and histamine changes due to sensitive color changes. Therefore, the sensor not only establishes a dynamic connection between pH and histamine for the purposes of biological and food monitoring, but also presents a novel approach for developing a multifunctional biosensor that can accomplish environmental monitoring and biosensing simultaneously.

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.

FeMo2Ox(OH)y-based mineral hydrogels as a novel POD nanozyme for sensitive and selective detection of aromatic amines contaminants via a colorimetric sensor array

Developing convenient pathways to discriminate and identify multiple aromatic amines (AAs) remains fascinating and critical. Here, a novel three-channel colorimetric sensor array based on FeMo2Ox(OH)y-based mineral (FM) hydrogels is successfully constructed to monitor AAs in tap water. Benefiting from the substantial oxygen vacancies (VO), FM nanozymes exhibit extraordinary peroxidase (POD)-like activities with Km of 0.133 mM and Vmax of 2.518 × 10-2 mM·s-1 toward 3,3',5,5'-tetramethylbenzidine (TMB), which are much better than horseradish peroxidase and most of POD mimics. This reveals that doping Cu and Co into FM (FM-Cu and FM-Co) can change POD activity. Based on various POD activities, TMB and H2O2 are used to generate fingerprint colorimetry signals from the colorimetry sensor array. The analytes can accurately discriminate through linear discriminant analysis, with a detection limit as low as 2.12 × 10-2-0.14 μM. The sensor array can effectively identify and discriminate AA contaminants and their mixtures and has performed well in real sample tests.

A Portable Integrated Electrochemical Sensing System for On-Site Nitrite Detection in Food

Ensuring an appropriate nitrite level in food is essential to keep the body healthy. However, it still remains a huge challenge to offer a portable and low-cost on-site food nitrite analysis without any expensive equipment. Herein, a portable integrated electrochemical sensing system (IESS) is developed to achieve rapid on-site nitrite detection in food, which is composed of a low-cost disposable microfluidic electrochemical patch for few-shot nitrite detection, and a reusable smartphone-assisted electronic device based on self-designed circuit board for signal processing and wireless transmission. The electrochemical patch based on MXene-Ti3C2Tx/multiwalled carbon nanotubes-cyanocobalamin (MXene/MWCNTs-VB12)-modified working electrode achieves high sensitivity of 10.533 µA mm-1 and low nitrite detection limit of 4.22 µm owing to strong electron transfer ability of hybrid MXene/MWCNTs conductive matrix and high nitrite selectivity of VB12 bionic enzyme-based ion-selective layer. Moreover, the portable IESS can rapidly collect pending testing samples through a microfluidic electrochemical patch within 1.0 s to conduct immediate nitrite analysis, and then wirelessly transmit data from a signal-processing electronic device to a smartphone via Bluetooth module. Consequently, this proposed portable IESS demonstrates rapid on-site nitrite analysis and wireless data transmission within one palm-sized electronic device, which would pave a new avenue in food safety and personal bespoke therapy.

A sensitive lateral flow test strip sensor for visual detection of acid red 18 in food using bicentric-emission carbon dots

Hazardous synthetic colorants have found widespread use in food production, and excessive consumption of these pigments can pose potential risks to human health. In this study, we propose an ultrasensitive fluorescence method for the analysis of Acid Red 18 (AR18) in food products. The method is based on the nitrogen-doped carbon dots (N-CDs) derived from tris and resorcinol through a hydrothermal way. The as-synthesized N-CDs exhibit two emission centers at 425 nm and 541 nm, corresponding to the excitation wavelengths of 377 nm and 465 nm, respectively. Upon the addition of AR18, the fluorescence intensity at 541 nm significantly decreases with a simultaneous, though less pronounced, reduction in the intensity at 425 nm, which is attributed to the localization of fluorescence resonance energy transfer (L-FRET). Specifically, a novel ratiometric fluorescent probe was constructed based on the extracted data from the 3D fluorescence excitation-emission matrix. This probe demonstrates a wide linear range from 0.0539 to 30 μM and a low limit of detection (LOD) of 53.9 nM. For practical applications, a portable fluorescent sensor based on a lateral flow test strip (LFTS) was designed for real-time monitoring of AR18. Color channel values were determined using a smartphone application, resulting in a satisfactory LOD of 75.3 nM. Furthermore, the suitability of the proposed ratiometric fluorescent probe was validated through the detection of AR18 in real food samples, consistently achieving recovery rates in the range of 99.7-101.4%. This research not only expands the scope of CDs in sensing fields, but also provides an effective strategy for the development of an excellent platform for real-time AR18 detection, contributing to public food safety.

Bell pepper derived nitrogen-doped carbon dots as a pH-modulated fluorescence switching sensor with high sensitivity for visual sensing of 4-nitrophenol

Recently, converting bio-waste into bio-asset and implementing a portable sensing instrument for pollutant monitoring has been highly desirable and challenging. Herein, biomass-derived nitrogen-doped carbon dots (CDs) are prepared hydrothermally and emit blue fluorescence (470 nm) with a high quantum yield of 23.2%. Significantly, CDs can serve as a pH-modulated fluorescence switching nano-sensor to detect 4-NP from 0.054 to 68 μM with low detection limit (LOD, 54 nM) and limit of quantification (LOQ, 181 nM) based on inner filter effect. Moreover, the satisfactory recovery of 101.8-107.5% is gained in practical sample monitoring. Furthermore, a smartphone-integrated optosensing device with CDs-based film is developed for detecting 4-NP with LOD and LOQ of 0.110 μM and 0.350 μM. Concomitantly, the practicability of this device is further validated in several crop samples with satisfactory recovery rates of 101.6-108.6%. Therefore, this work provides a reliable way and a prospective application for on-site 4-NP monitoring in food.

An optical and visual multi-mode sensing platform base on nitrogen, sulfur, boron co-doped carbon dots for rapid and simple determination of ferric ions in water

Abnormal iron ions levels may lead to some diseases and serious environmental pollution. Herein, optical and visual detection strategies of Fe3+ in water based on co-doped carbon dots (CDs) were established in the present study. Firstly, a one-pot synthetic strategy for the preparation of the N, S, B co-doped CDs with a home microwave oven was developed. Secondly, the optical properties, chemical structures, and morphology of CDs were further characterized by fluorescence spectroscopy, Uv-vis absorption spectroscopy, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and transmission electron microscope. Finally, the results indicated that the fluorescence of the co-doped CDs was quenched by ferric ions via the static mechanism and the aggregation of CDs, accompanied by the increased red color. The multi-mode sensing strategies of Fe3+ with fluorescence photometer, UV-visible spectrophotometer, portable colorimeter and smartphone had the advantages of good selectivity, excellent stability and high sensitivity. Fluorophotometry based on co-doped CDs was a powerful probe platform for measuring lower concentrations of Fe3+ due to its higher sensitivity, better linear relationship, lower limit of detection (0.27 μM) and limit of quantitation (0.91 μM). In addition, the visual detection methods with a portable colorimeter and smartphone had been proven to be very suitable for rapid and simple sensing of higher concentrations of Fe3+. Moreover, the co-doped CDs utilized for Fe3+ probes in tap water and boiler water obtained satisfactory results. Consequently, the efficient, versatile optical and visual multi-mode sensing platform could be extended to apply such a visual analysis of ferric ions in the biological, chemical and other fields.

Ultra-fast solvent-free protocol remodels the large-scale synthesis of carbon dots for nucleolus-targeting and white light-emitting diodes

Carbon dots (CDs) provides unprecedented opportunities for optical applications due its unique properties, but the energy-extensive consumption, high-risk factor and time-consuming synthesis procedure greatly hinders its industrialization process. Herein, we proposed an ultra-low energy consumption solvent-free synthetic strategy for fast preparing green/red fluorescence carbon dots (G-/R-CDs) using m-/o-phenylenediamine and primary amine hydrochloride. The involvement of primary amine hydrochloride can improve the formation rate of G-CDs/R-CDs through effectively absorbing microwave energy and providing acid react environment. The developed CDs exhibit good fluorescence efficiency, optical stability and membrane permeability for dexterous bioimaging in vivo. Based on inherently high nitrogen content, the G-CDs/R-CDs possess excellent nuclear/nucleolus targeting ability, and were successfully applied for screening cancer and normal cells. Furthermore, the G-CDs/R-CDs were further applied for fabricating high-safety and high-color rendering index white light-emitting diodes, providing a perfect candidate for indoor lighting. This study opens up new horizons for advancing practical applications of CDs in related fields of biology and optics.

Machine Learning-Assisted High-Throughput Strategy for Real-Time Detection of Spermine Using a Triple-Emission Ratiometric Probe

In this study, we designed and fabricated a spermine-responsive triple-emission ratiometric fluorescent probe using dual-emissive carbon nanoparticles and quantum dots, which improve the sensor's accuracy and reduce interfering environmental effects. The probe is advantageous for the proportionate detection of spermine because it has good emission resolution, and the maximum points of the two emission peaks differ by 95 nm. As a proof of concept, cuvettes and a 96-well plate were combined with a smartphone and YOLO series algorithms to accomplish real-time, visual, and high-throughput detection of seafood and meat freshness. In addition, the reaction mechanism was verified by density functional theory and fundamental characterizations. Upon exposure to different amounts of spermine, the intensity of the fluorescent probe changed linearly, and the fluorescent color shifted from yellow-green to red, with a limit of detection of 0.33 μM. To enable visual identification of food-originated spermine, a hydrogel-based visual sensing platform was successfully developed utilizing the triple-emission fluorescent probe. Consequently, spermine could be identified and quantified without complicated equipment.

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.

Highly sensitive and ratiometric detection of nitrite in food based on upconversion-carbon dots nanosensor

Nitrite (NO₂^-) is extensively found in the daily dietary environment. However, consuming too much NO₂^- can pose serious health risks. Thus, we designed a NO₂^--activated ratiometric upconversion luminescence (UCL) nanosensor which could realize NO₂^- detection via the inner filter effect (IFE) between NO₂^--sensitive carbon dots (CDs) and upconversion nanoparticles (UCNPs). Due to the exceptional optical properties of UCNPs and the remarkable selectivity of CDs, the UCL nanosensor exhibited a good response to NO₂^-. By taking advantage of NIR excitation and ratiometric detection signal, the UCL nanosensor could eliminate the autofluorescence thereby increasing the detection accuracy effectively. Additionally, the UCL nanosensor proved successful in detecting NO₂^- quantitatively in actual samples. The UCL nanosensor provides a simple as well as sensitive sensing strategy for NO₂^- detection and analysis, which is anticipated to extend the utilization of upconversion detection in food safety.

Simple method for visual detection of nitrite using fluorescence and colorimetry by poly (tannic acid) nanoparticles

The nitration reaction of nitrite and phenolic substances was first used to identify and detect NO₂^- by taking fluorescent poly (tannic acid) nanoparticles (FPTA NPs) as sensing platform. With the low cost, good biodegradable and convenient water-soluble FPTA NPs, a fluorescent and colorimetric dual modes detecting assay was realized. In fluorescent mode, the linear detection range of NO₂^- was 0-36 μM, the LOD was as low as 3.03 nM, and the response time was 90 s. In colorimetric mode, the linear detection range of NO₂^- was 0-46 μM, and the LOD was as low as 27 nM. Besides, a smartphone with FPTA NPs@ agarose hydrogel formed a portable detection platform to test the fluorescent and visible color changes of FPTA NPs for NO₂^- sensing as well as for accurate visualization and quantitative detection of NO₂^- in actual water and food samples.

A smartphone-assisted ratiometric colorimetric and fluorescent probe for triple-mode determination of nitrite based on MnO2 nanoparticles and carbon quantum dots

A triple-mode colorimetric and fluorescent sensing scheme based on manganese dioxide nanoparticles (MnO₂NPs) and carbon quantum dots (CQDs) were developed to determine nitrite. MnO₂NPs can oxidize 3,3',5,5'-tetramethylbenzidine (TMB) into a blue oxidation product (TMBox), which is further oxidized into a yellow diimine derivative by nitrite. The ratio of absorbance at 652 nm to 452 nm was monitored as signal response for UV-vis detection mode. A "turn-off" CQDs fluorescence probe was also constructed for fluorescent detection mode. Smartphone tool kit was used to capture the color of sample for smartphone-based measurement. Various analytical performance under different detection modes were obtained and compared. The proposed methods were applied to food samples with satisfactory recoveries (83.3-106 %). The results were validated with AOAC standard spectrophotometric method. The current triple-mode detection were accurate, convenient, low-cost and fast for analyzing nitrite in foods and water samples on-site.

Controllable synthesis of bifunctional magnetic carbon dots for rapid fluorescent detection and reversible removal of Hg2

Mercury is one of the most toxic heavy metals, whose identification and separation are crucial for environmental remediation. Till now, it remains a significant challenge upon simultaneous detection and removal of Hg²⁺. Herein, bifunctional probe magnetic carbon dots were synthesized and optimized via systematic structure manipulation of the carbon and iron precursors towards fluorescence, Hg²⁺ adsorption and magnetic separation. The probe exhibited blue emission at 440 nm with high quantum yield of 55 % and a high paramagnetism with the saturation magnetization value of 22.70 emu/g. Furthermore, the fluorescent detection of Hg²⁺ with limit of 5.40 nM and high selectivity were achieved through surface structure manipulation with moderate -NH₂, -SH and Fe contents. As a result, the magnetic removal of Hg²⁺ was consecutively effectuated with high removal efficiency of 98.30 %. The detection and recovery of Hg²⁺ in real samples were further verified and demonstrated the excellent environmental tolerance of probe. The reusability was viable with recycling at least three turns by external magnet. This work not only provides a promising approach for simultaneous detection and removal of heavy metal pollution, but also provides an excellent example as a versatile platform for multifunction integration via the structure manipulation for other applications.

Nafee S, Alamro FS, Pashameah RA, Ahmed HA, Medany SS. Zinc Nanocomposite Supported Chitosan for Nitrite Sensing and Hydrogen Evolution Applications

Nanoparticles of ZnO-Chitosan (Zn-Chit) composite were prepared using precipitation methods. Several analytical techniques, such as scanning electron microscope (SEM), transmitted electron microscope (TEM), powder X-ray diffraction (XRD), infrared spectroscopy (IR), and thermal analysis, were used to characterize the prepared composite. The activity of the modified composite was investigated for nitrite sensing and hydrogen production applications using various electrochemical techniques. A comparative study was performed for pristine ZnO and ZnO loaded on chitosan. The modified Zn-Chit has a linear range of detection 1–150 µM and a limit of detection (LOD) = 0.402 µM (response time ~3 s). The activity of the modified electrode was investigated in a real sample (milk). Furthermore, the anti-interference capability of the surface was utilized in the presence of several inorganic salts and organic additives. Additionally, Zn-Chit composite was employed as an efficient catalyst for hydrogen production in an acidic medium. Thus, the electrode showed long-term stability toward fuel production and enhanced energy security. The electrode reached a current density of 50 mA cm−2 at an overpotential equal to −0.31 and −0.2 V (vs. RHE) for GC/ZnO and GC/Zn-Chit, respectively. Electrode durability was studied for long-time constant potential chronoamperometry for 5 h. The electrodes lost 8% and 9% of the initial current for GC/ZnO and GC/Zn-Chit, respectively.

Nafee S, Alamro FS, Pashameah RA, Ahmed HA, Medany SS. Zinc Nanocomposite Supported Chitosan for Nitrite Sensing and Hydrogen Evolution Applications

Nanoparticles of ZnO-Chitosan (Zn-Chit) composite were prepared using precipitation methods. Several analytical techniques, such as scanning electron microscope (SEM), transmitted electron microscope (TEM), powder X-ray diffraction (XRD), infrared spectroscopy (IR), and thermal analysis, were used to characterize the prepared composite. The activity of the modified composite was investigated for nitrite sensing and hydrogen production applications using various electrochemical techniques. A comparative study was performed for pristine ZnO and ZnO loaded on chitosan. The modified Zn-Chit has a linear range of detection 1-150 µM and a limit of detection (LOD) = 0.402 µM (response time ~3 s). The activity of the modified electrode was investigated in a real sample (milk). Furthermore, the anti-interference capability of the surface was utilized in the presence of several inorganic salts and organic additives. Additionally, Zn-Chit composite was employed as an efficient catalyst for hydrogen production in an acidic medium. Thus, the electrode showed long-term stability toward fuel production and enhanced energy security. The electrode reached a current density of 50 mA cm-2 at an overpotential equal to -0.31 and -0.2 V (vs. RHE) for GC/ZnO and GC/Zn-Chit, respectively. Electrode durability was studied for long-time constant potential chronoamperometry for 5 h. The electrodes lost 8% and 9% of the initial current for GC/ZnO and GC/Zn-Chit, respectively.

A smartphone-based fluorospectrophotometer and ratiometric fluorescence nanoprobe for on-site quantitation of pesticide residue

Cost-effective and user-friendly quantitation at points-of-need plays an important role in food safety inspection, environmental monitoring, and biomedical analysis. This study reports a stand-alone smartphone-based fluorospectrophotometer (the SBS) installed with a custom-designed application (the SBS-App) for on-site quantitation of pesticide using a ratiometric sensing scheme. The SBS can collect fluorescence emission spectra in the wavelength range of 380-760 nm within 5 s. A ratiometric fluorescence probe is facilely prepared by directly mixing the blue-emissive carbon nanodots (the Fe3+-specific fluorometric indicator) and red-emissive quantum dots (the internal standard) at a ratio of 11.6 (w/w). Based on the acetylcholinesterase/choline oxidase dual enzyme-mediated cascade catalytic reactions of Fe2+/Fe3+ transformation, a ratiometric fluorescence sensing scheme is developed. The practicability of the SBS is validated by on-site quantitation of chlorpyrifos in apple and cabbage with a comparable accuracy to the GC-MS method, offering a scalable solution to establish a cost-effective surveillance system for pesticide pollution.

The Formation Process and Mechanism of Carbon Dots Prepared from Aromatic Compounds as Precursors: A Review

Fluorescent carbon dots are a novel type of nanomaterial. Due to their excellent optical properties, they have extensive application prospects in many fields. Studying the formation process and fluorescence mechanism of CDs will assist scientists in understanding the synthesis of CDs and guide more profound applications. Due to their conjugated structures, aromatic compounds have been continuously used to synthesize CDs, with emissions ranging from blue to NIR. There is a lack of a systematic summary of the formation process and fluorescence mechanism of aromatic precursors to form CDs. In this review, the formation process of CDs is first categorized into three main classes according to the precursor types of aromatic compounds: amines, phenols, and polycyclics. And then, the fluorescence mechanism of CDs synthesized from aromatic compounds is summarized. The challenges and prospects are proposed in the last section.

Promising instrument-free detections of various analytes using smartphones with Spotxel® Reader

In consideration of the problems related to food safety, environmental pollution, and the spread of infected diseases nowadays, we urgently need testing methods that can be easily performed by common people. Smartphone-based detections are promising for general applications. However, some of these analytical strategies require a combination of accessories and instruments, such as portable electrochemical workstations, mini multi-mode microplate readers, and complex temperature control devices, etc., which are small but still expensive. Herein, we comprehensively introduce a free app (Spotxel^® Reader) that can provide accurate data analysis for microplate or parallel-format test sensors without an instrument. By simulating the optical signal of the test samples through a smartphone, the sensing results can be obtained for free. We discuss the detection strategies involved in the reported smartphone-based analyses using Spotxel^® Reader. Prospects for the development of this free app for future detection applications are presented. This review aims to popularize free analysis software, so that ordinary people may realize convenient tests.

Recent Advances in Synthesis, Modification, Characterization, and Applications of Carbon Dots

Although there is significant progress in the research of carbon dots (CDs), some challenges such as difficulty in large-scale synthesis, complicated purification, low quantum yield, ambiguity in structure-property correlation, electronic structures, and photophysics are still major obstacles that hinder the commercial use of CDs. Recent advances in synthesis, modification, characterization, and applications of CDs are summarized in this review. We illustrate some examples to correlate process parameters, structures, compositions, properties, and performances of CDs-based materials. The advances in the synthesis approach, purification methods, and modification/doping methods for the synthesis of CDs are also presented. Moreover, some examples of the kilogram-scale fabrication of CDs are given. The properties and performance of CDs can be tuned by some synthesis parameters, such as the incubation time and precursor ratio, the laser pulse width, and the average molar mass of the polymeric precursor. Surface passivation also has a significant influence on the particle sizes of CDs. Moreover, some factors affect the properties and performance of CDs, such as the polarity-sensitive fluorescence effect and concentration-dependent multicolor luminescence, together with the size and surface states of CDs. The synchrotron near-edge X-ray absorption fine structure (NEXAFS) test has been proved to be a useful tool to explore the correlation among structural features, photophysics, and emission performance of CDs. Recent advances of CDs in bioimaging, sensing, therapy, energy, fertilizer, separation, security authentication, food packing, flame retardant, and co-catalyst for environmental remediation applications were reviewed in this article. Furthermore, the roles of CDs, doped CDs, and their composites in these applications were also demonstrated.

Temperature-Responsive Covalent Organic Framework-Encapsulated Carbon Dot-Based Sensing Platform for Pyrethroid Detection via Fluorescence Response and Smartphone Readout

In this paper, carbon dot (CD)-encapsulated 1,3,5-tris(4-formylphenyl)benzene (TFB)/2,5-dihydroxyterephthalohydrazide (DHTH) covalent organic frameworks (TDCOFs) grafted with thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) (CDs@TDCOFs@PNIPAM) were fabricated for the detection of pyrethroids. CDs@TDCOFs@PNIPAM achieved a temperature-responsive "on/off" detection of pyrethroids based on the target-triggered electron-transfer mechanism. The detection limit of pyrethroids was as low as 0.69 μg/L in the wide linear range of 5-400 μg/L (R² > 0.9523). Simultaneously, CDs@TDCOFs@PNIPAM with red, green, and blue (RGB) fluorescence emissions were integrated with a smartphone-assisted device, enabling the visual smart quantitative detection of pyrethroids with a detection limit of 4.875 μg/L. Ultimately, agricultural products were chosen as actual samples to verify the applicability of both recognition modes, and the calculated recovery rate was 105.48-113.40%. Accordingly, CDs@TDCOFs@PNIPAM featuring temperature-responsive switching behavior and RGB fluorescence emission provided a promising analytical strategy for ensuring agricultural and food safety.

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.