Preparation of highly crystalline nitrogen-doped carbon dots and their application in sequential fluorescent detection of Fe3+ and ascorbic acid

Ultrasensitive Fluorescence Detection of Ascorbic Acid Using Silver Ion-Modulated High-Quality CdSe/CdS/ZnS Quantum Dots

Improving the sensitivity of the fluorescence method for the detection of bioactive molecules is crucial in biochemical analysis. In this work, an ultrasensitive sensing strategy was constructed for the detection of ascorbic acid (AA) using high-quality 3-mercaptopropionic acid-capped CdSe/CdS/ZnS quantum dots (MPA-CdSe/CdS/ZnS QDs) as the fluorescent probe. The prepared water-soluble QDs exhibited a high photoluminescence quantum yield (PL QY) of up to 96%. Further, the fluorescence intensity of the QDs was intensively quenched through the dynamic quenching of Ag+ ions due to an efficient photoinduced electron transfer progress. While the existence of AA before adding Ag+ ions, Ag+ ions were reduced. Thus, the interaction of the QDs and Ag+ ions was destroyed, which led to the fluorescence distinct recovery. The detection limit of AA could be as low as 0.2 nM using this sensing system. Additionally, most relevant small molecules and physiological ions had no influence on the analysis of AA. Satisfactory results were obtained in orange beverages, showing its great potential as a meaningful platform for highly sensitive and selective AA sensing for clinical analysis.

Fabrication of chitosan-based fluorescent hydrogel membranes cross-linked with bisbenzaldehyde for efficient selective detection and adsorption of Fe2

Chitosan, as a natural biomass material, is green, recyclable, sustainable and well biocompatible. The molecular chain is rich in active groups such as amino and hydroxyl groups, and its preparation of fluorescent probes has the advantages of biocompatibility and efficient detection performance. In this study, a bis(benzaldehyde) (BHD) fluorescent functional molecule was designed. Then a series of fluorescent chitosan-based hydrogel films (CSBHD) were prepared using chitosan as raw material and BHD as cross-linking agent. As a fluorescent probe for metal ions, CSBHD was able to efficiently detect Fe2+ with a linear correlation of fluorescence intensity in the range of 0-160 μM, and the limit of detection (LOD) was 0.55 μM. Moreover, it has excellent adsorption performance for Fe2+ ions, with a maximum adsorption capacity of 223.5 g/mg at 500 mg/L Fe2+ concentration. Finally, we characterised the structure and microscopic morphology of CSBHD films and found that CSBHD as a hydrogel film has a high cross-linking density, good water resistance, excellent thermal stability, strong resistance to swelling, and excellent stability in cycling tests. Hence, it has great potential for application in adsorption and detection of Fe2+ ions. It also provides a good strategy for the application of chitosan based fluorescent probe materials in environmental monitoring and heavy metal ion adsorption.

Quantum dots as advanced nanomaterials for food quality and safety applications: A comprehensive review and future perspectives

The importance of food quality and safety lies in ensuring the best product quality to meet consumer demands and public health. Advanced technologies play a crucial role in minimizing the risk of foodborne illnesses, contamination, drug residue, and other potential hazards in food. Significant materials and technological advancements have been made throughout the food supply chain. Among them, quantum dots (QDs), as a class of advanced nanomaterials with unique physicochemical properties, are progressively demonstrating their value in the field of food quality and safety. This review aims to explore cutting-edge research on the different applications of QDs in food quality and safety, including encapsulation of bioactive compounds, detection of food analytes, food preservation and packaging, and intelligent food freshness indicators. Moreover, the modification strategies and potential toxicities of diverse QDs are outlined, which can affect performance and hinder applications in the food industry. The findings suggested that QDs are mainly used in analyte detection and active/intelligent food packaging. Various food analytes can be detected using QD-based sensors, including heavy metal ions, pesticides, antibiotics, microorganisms, additives, and functional components. Moreover, QD incorporation aided in improving the antibacterial and antioxidant activities of film/coatings, resulting in extended shelf life for packaged food. Finally, the perspectives and critical challenges for the productivity, toxicity, and practical application of QDs are also summarized. By consolidating these essential aspects into this review, the way for developing high-performance QD-based nanomaterials is presented for researchers and food technologists to better capitalize upon this technology in food applications.

A three-dimensional network structure of metal-based nanozymes for the construction of colorimetric sensors for the detection of antioxidants

Although many excellent nanozymes have been developed, designing and synthesizing highly active nanozymes is still challenging. Here, we developed a metal-based nanozyme (metal = Co, Fe, Cu, Zn) with a three-dimensional network structure. It possesses excellent peroxidase activity and catalyzes the reaction between H2O2 and TMB to produce blue oxTMB, while antioxidants have different reducing power on the oxidation product of TMB (oxTMB), which leads to different absorbance and color changes. Using these color reactions, different nanozymes were used to form a colorimetric sensor array with seven antioxidants, and seven antioxidants were sensitively identified. And the differences between the three nanozymes were compared by density functional theory calculations and enzyme kinetic curve results. In conclusion, the colorimetric sensor array based on metal-based nanozymes provides a good strategy for the identification and detection of antioxidants, which has a broad application prospect.

Schiff base fluorescent sensor with aggregation induced emission characteristics for the sensitive and specific Fe3+ detection

An aggregation induced emission based compound ((E)-4-((2-hydroxy-5-methoxybenzylidene)amino)benzoic acid) was synthesized through facile Schiff base condensation and characterized by various spectral techniques. The as-prepared compound represented a typical aggregation induced emission behavior in aqueous solution and exploited as a turn-off fluorescent sensor for Fe3+ detection in THF-H2O system (3:7, v/v) with high sensitivity and selectivity. The mechanism of the fluorescence quenching was intensively studied, which was attributed to both dynamic quenching and inner filter effect. The fluorescence probe displayed a highly broad dynamic response range (0.5-500 μM) for selective detection of Fe3+ with a limit of detection of 0.079 μM. The proposed method was successfully employed for detection and quantification of Fe3+ in human urine samples and proved to have potential for practical applications in biological field.

Applications of Functionalized Carbon-Based Quantum Dots in Fluorescence Sensing of Iron(III)

Iron, an essential trace element exhibits detrimental effects on human health when present at higher or lower concentration than the required. Therefore, there is a pressing demand for sensitive and selective detection of Fe3+ in water, food etc. Unfortunately, in several instances, the traditional approaches suffer from a number of shortcomings like complicated procedures, limited sensitivity, poor selectivity and more expensive and time consuming. The scope of optical tuning and excellent photophysical properties of carbon- based nanomaterials like carbon dots (C-dots) and graphene dots (g-dots) have made them promising optical sensors of metal ions. Moreover, high surface area, superior stability of such materials contributes towards the fruitful development of sensors. The present review offered critical information on the fabrication and fluorimetric applications of these functional nanomaterials for sensitive and selective detection of Fe3+. An in-depth discussion on fluorescent C-dots made from naturally occurring materials and chemical techniques were presented. Effect of doping in C-dots was also highlighted in terms of improved fluorescence response and selectivity. In a similar approach g-dots were also discussed. Many of these sensors exhibited great selectivity, superior sensitivity, high quantum yield, robust chemical and photochemical stability and real-time applicability. Further improvement in these factors can be targeted to develop new sensors.

A fluorescent dual-emitting platform for fluorescent "turn-on" ratiometric detection of ascorbic acid in beverages utilizing luminol-embedded iron-based metal-organic frameworks

A fluorescent dual-emitting platform for fluorescent "turn-on" ratiometric detection of ascorbic acid in beverages was developed utilizing luminol-embedded iron-based metal-organic frameworks (luminol@Fe-DOBDC MOFs). Luminol@Fe-DOBDC MOFs with fluorescent emissions at 430 nm and 540 nm under excitation wavelength of 365 nm were applied to detect ascorbic acid on the basis of ascorbic acid triggering the reduction of Fe3+ into Fe2+. In the presence of ascorbic acid, fluorescent intensity at 540 nm was increased significantly while fluorescent intensity at 430 nm was changed slightly. Two emission peaks separated by 110 nm can eliminate environmental interferences by built-in self-calibration of ratiometric signal, enhancing the sensitivity and accuracy. The increasing ratiometric fluorescent intensity (I540 nm/I430 nm) has linear relationship with the concentration of ascorbic acid from 0.2 to 30 μM with limit of detection of 70 nM. It is an efficient, sensitive and accurate platform to detect ascorbic acid in commercial beverages using transition-metal-based MOFs.

A sensitive sensor based on carbon dots for the determination of Fe3+ and ascorbic acid in foods

To develop a feasible, sensitive, and essential sensor is important for the identification of Fe3+ ions and ascorbic acid (AA). Herein, highly fluorescent heteroatom co-doped carbon dots (N,S-CDs) with a quantum yield (QY) of 24.6% were synthesized, using hydrothermal treatment of L-cysteine (Cys) and 1-amino-2-naphthol-4-sulfonic acid (ANSA). The fluorescence emission of the as-prepared N,S-CDs was quenched strongly by Fe3+ ions, and this was further recovered by the reduction effect of AA on Fe3+. Based on this, continuous fluorescence sensing of Fe3+ and AA with an "on-off-on" style was developed. The detection of Fe3+ and AA were in relatively wider linear ranges of 5.00-105 μmol L-1 and 4.97-54.8 μmol L-1, with a detection limit of 0.10 μmol L-1 and 2.4 nmol L-1 (S/N = 3), respectively. Then, the N,S-CDs were successfully used to measure Fe3+ ions and AA in some daily food samples, and this method exhibited some advantages over most other reported techniques in the term of response speed, quantum yield, and detection limit.

A Dual-Mode Detection Sensor Based on Nitrogen-Doped Carbon Dots for Visual Detection of Fe(III) and Ascorbic Acid via a Smartphone

Accurately and promptly detecting Fe3+ and ascorbic acid (AA) is a crucial objective. In this study, nitrogen-doped carbon dots (N-CDs) were synthesized using a one-step hydrothermal synthesis method with 6,9-diamino-2-ethoxyacridine lactate as the precursor. The introduction of Fe3+ and AA resulted in both fluorescence (FL) quenching and enhancement of the synthesized N-CDs. The fluorescent response of the N-CDs probe to Fe3+ was observed in the concentration range of 5-20 µM and 25-50 µM, with a limit of detection (LOD) of 290 nM. Remarkably, the fluorescence of the N-CDs was recovered upon the addition of AA to the N-CDs-Fe3+ system. Using the "off-on" fluorescent N-CDs probe, a linear range of 40-90 µM was achieved with an LOD of 0.69 µM. Additionally, the feasibility of employing a smartphone equipped with an RGB Color Picker was demonstrated for the analysis of Fe3+ and AA concentrations, providing a novel visual detection method. Furthermore, the application of N-CDs in solution demonstrated considerable potential for visually detecting Fe3+ and AA. The proposed dual-mode detection sensor was found to be simple, efficient, and stable, enabling the successful determination of Fe3+ and AA in practical samples with satisfactory results.

Fabrication of carbon dots-embedded luminescent transparent wood with ultraviolet blocking and thermal insulating capacities towards smart window application

To expand functions of transparent wood (TW) including fluorescence, ultraviolet blocking, heat preservation and insulation, we adopted carbon quantum dots (CQDs) to prepare luminescent transparent wood. CQDs with yellow/red fluorescence (YCD/RCD) were prepared by chitosan and o-phenylenediamine. Afterwards, Balsa woods were pretreated to obtain wood frameworks (DW/LW), which were further combined with epoxy resin for achieving transparent woods (DW-TW/LW-TW). Results showed LW retained more lignin, the LW-TW blocked more ultraviolet light, displaying the better visible transmission and mechanical strength than DW-TW. After adding YCD and RCD to LW-TW, the yellow and red fluorescence transparent woods with outstanding mechanical and ultraviolet blocking properties were prepared, especially the red fluorescence transparent wood (RTW). Specifically, the tensile strength and elongation at break of RTW reached up to 19.39 MPa and 5.35 %, respectively. Moreover, RTW could block 78.8 % of UV-B light and 78 % of UV-A light, respectively. Besides, RTW possessed excellent visible transmission (70.3 %) and UV blocking (88.87 %). Significantly, both RTW and YTW displayed outstanding water repellency, excellent durability, good thermal stability and insulation. Predictably, luminescent transparent woods certainly will enhance the adaptability of wood, and broaden its applications in green decoration, lighting setup, sensor and other fields.

Orange peel-derived carbon dots/Cu-MOF nanohybrid for fluorescence determination of l-ascorbic acid and Fe3. Anal Chim Acta 2024;1287:342066. [PMID: 38182373 DOI: 10.1016/j.aca.2023.342066]

Recycling and reuse of biomass waste in synthesis of nanomaterials have recently received much attention as an effective solution for environmental protection and sustainable development. Herein, nitrogen-doped carbon dots (N-CDs) with blue emission were synthesized from the orange peels as a precursor through a simple hydrothermal method and then, modified with ethylenediamine tetraacetic acid (N-CD@EDTA). The N-CD@EDTA was embedded as a fluorophore in Cu-based metal-organic framework (MOF-199) structure (N-CD@EDTA/MOF-199) to construct fluorescence sensor toward l-ascorbic acid (L-AA) determination. The N-CD@EDTA/MOF-199 nanohybrid significantly and selectively turned on toward L-AA determination during the fluorimetric experiments. Under optimal conditions, the probe showed a suitable linear response in the concentration range of 10 nM-100 μM with a low limit of detection (LOD) of 8.6 nM and high sensitivity of 0.201 μM-1. The possible mechanism of recognition and adsorption, including the reduction of Cu 2+ nodes in the MOF-199 structure in the presence of L-AA and the release of trapped N-CD@EDTA into the solution, was explored. Moreover, the N-CD@EDTA/MOF-199/L-AA (100 μM) system was further applied as a fluorescent "on-off" sensor for Fe3+ determination with a LOD of 1.15 μM. The proposed probe was successfully used in orange juice and water samples to determine L-AA and Fe3+ with satisfactory recovery, which displays the promising capability of sensor in real samples. The recoveries obtained by suggested method are consistent with that obtained from high performance liquid chromatography (HPLC) and atomic absorption spectroscopy which confirm the favorable characteristic of the sensor for accurate determination of L-AA and Fe3+ in practical applications.

Fluorescent carbon dots synthesized by waste wind turbine blade for photocatalytic degradation

Developing novel waste recycling strategies has become a feasible solution to overcome environmental pollution. In this work, a method of using waste wind turbine blade (WTB) as a carbon source to synthesize blue fluorescent carbon dots (B-CDs) by hydrothermal treatment is proposed. B-CDs are spherical and have an average particle size of 5.2 nm. The surface is rich in C-O, C=O, -CH3 , and N-H bond functional groups, containing five elements: C, O, N, Si, and Ca. The optimal emission wavelength of B-CDs is 463 nm, corresponding to an excitation wavelength of 380 nm. Notably, a relatively high quantum yield of 29.9% and a utilization rate of 40% were obtained. In addition, B-CDs can serve as a photocatalyst to degrade methylene blue dye, with a degradation efficiency of 64% under 40-min irradiation conditions. The presence of holes has a significant influence on the degradation process.

Ultrasound-Assisted Hydrothermal Synthesis of Highly Fluorescent Sulfur Quantum Dots for Fe3+ Ion and Ascorbic Acid Detection in Real Samples

In this work, the ultrasound-assisted hydrothermal synthesis method offers a facile method to synthesize highly efficient photoluminescence sulfur quantum dots (SQDs). Impressively, a switchable fluorescent "on-off-on" sensor was developed using the acquired SQDs, which are capable of sequentially detecting iron ions (Fe3+) and ascorbic acid (AA) with exceptional sensitivity and selectivity. Meanwhile, SQDs and Fe3+ formed complexes through coordination, causing the fluorescence quenching of SQDs because of the static quenching effect. Upon the addition of AA into the SQDs/Fe3+ system, a redox-reaction-mediated mechanism leads to the recovery of fluorescence. The fluorescence intensity of SQDs exhibits a linear relationship with the concentrations of Fe3+ and AA in the ranges 5-30 and 20-100 μM, respectively. Notably, the detection limits achieved are 14.31 nM for Fe3+ and 0.64 μM for AA. Moreover, the chemosensor was successfully employed for monitoring Fe3+ in real water samples and AA in fruits. These results demonstrate the excellent analysis and detection capabilities of SQDs in the complex environment.

Violuric acid carbon dots as a highly fluorescence probe for ultrasensitive determination of Zn (II) in tomato paste

Zinc is an essential metal since it plays an important role in biological systems, therefore, determination of zinc in food samples is important. Violuric acid was used to prepare highly fluorescent carbon dots (CDs), when it irradiated with ultraviolet radiation at 365 nm, a strong violet fluorescence was observed which caused by the increased amount of nitrogen in the CD structure, which were then successfully used for sensing zinc ion based on quenching of fluorescence. Violuric acid's hydrothermal carbonization reaction's temperature and time were simply optimized for better-quality performance of the CDs as-synthesized. The probe was characterized by HRTEM, SEM, XRD, EDX, fluorescence, UV-Visible absorption spectrophotometry, and FTIR. With a lower LOD 0.32 nM, the developed approach demonstrates an exceptional sensitivity and good selective response to the Zn²⁺ at 25℃. Compared to the results from ICP, the sensor was successfully used for determination of Zn²⁺ ions in tomato paste samples.

Nitrogen-doped carbon dots: Recent developments in its fluorescent sensor applications

Doped carbon dots have attracted great attention from researchers across disciplines because of their unique characteristics, such as their low toxicity, physiochemical stability, photostability, and outstanding biocompatibility. Nitrogen is one of the most commonly used elements for doping because of its sizeable atomic radius, strong electronegativity, abundance, and availability of electrons. This distinguishes them from other atoms and allows them to perform distinctive roles in various applications. Here, we have reviewed the most current breakthroughs in nitrogen-doped CDs (N-CDs) for fluorescent sensor applications in the last five years. The first section of the article addresses several synthetic and sustainable ways of making N-CDs. Next, we briefly reviewed the fluorescent features of N-CDs and their sensing mechanism. Furthermore, we have thoroughly reviewed their fluorescent sensor applications as sensors for cations, anions, small molecules, enzymes, antibiotics, pathogens, explosives, and pesticides. Finally, we have discussed the N-CDs' potential future as primary research and how that may be used. We hope that this study will contribute to a better understanding of the principles of N-CDs and the sensory applications that they can serve.

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

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

A colorimetric and fluorometric dual-mode carbon dots probe derived from phenanthroline precursor for the selective detection of Fe2+ and Fe3

Iron's metabolism is heavily involved in the regulation of redox balance for cell functions, however, the simultaneous monitoring of Fe^2+/3+ concentration is still a great challenge due to their transitional nature in biological systems. A novel type of carbon dots (CDs) was synthesized by solvothermal treatment with 5-amino-1,10-phenanthroline (Aphen) and salicylic acid as precursors, and the resulting targeted CDs (T-CDs) were used to simultaneously detect Fe²⁺ and Fe³⁺. Comprehensive experimental characterizations revealed that the strong binding affinity of Aphen moiety to Fe²⁺ leads to the formation of rigid T-CDs aggregates, which causes a substantial enhancement of fluorescence intensity, whereas Fe³⁺ could cause the fluorescence quenching of T-CDs due to the oxidation-reduction induced electron transfer. These different fluorescence responses allow T-CDs to sensitively differentiate Fe²⁺ from Fe³⁺, and give the limit of detection (LOD) of 1.78 and 2.78 μM for Fe²⁺ and Fe³⁺, respectively. Furthermore, the Aphen dominated structure endows the T-CDs with a colorimetric response to Fe²⁺ with a LOD of 0.13 μM, which is very different from Fe³⁺. Thus, the dynamic changes of Fe²⁺ and Fe³⁺ in solution can be accurately monitored by T-CDs within the total iron concentration of 50 μM, which is probably the most sensitive dual-mode probe reported so far. In addition, this probe is successfully applied to detect the Fe^2+/3+ concentration in cells, demonstrating a huge application potential in the sensing of the dynamic equilibrium of these important transition metals during the cell metabolism or stimulated process. The dynamic changes of Fe²⁺ and Fe³⁺ in solution can be accurately monitored by carbon dots based on the colorimetric and fluorometric dual-mode.

Green synthesis of CQDs for determination of iron and isoniazid in pharmaceutical formulations

Camphor leaves were used as the precursor for the hydrothermal synthesis of carbon quantum dots. The preparation method is simple and rapid, and the raw material is environmentally friendly and easy to obtain. Without additional modification, the carbon quantum dots were used as fluorescent probes for the sensitive and selective detection of Fe³⁺ and isoniazid at different excitation wavelengths. For Fe³⁺, at the excitation wavelength of 320 nm, the ratio of fluorescence intensity of CQD solution after adding Fe³⁺ to CQD solution without Fe³⁺ addition, F/F₀, and Fe³⁺ concentration showed a good linear relationship in the range of 2.72 × 10^-5 to 1.00 × 10^-4 mol L^-1 (R² = 0.9912), and the limit of detection was 8.16 μmol L^-1. For isoniazid, at the excitation wavelength of 270 nm, the ratio of fluorescence intensity of CQDs solution with isoniazid to CQDs solution without isoniazid, F/F₀, and isoniazid concentration showed good linear relationships in the range of 3.81 × 10^-6 to 1.00 × 10^-5 mol L^-1 (R² = 0.9941) and 1.00 × 10^-5 to 2.10 × 10^-4 mol L^-1 (R² = 0.9910) respectively, and the limit of detection was 1.14 μmol L^-1. A fluorescence method for the determination of Fe and isoniazid content was proposed. The method has been used to detect iron in iron supplement tablets and isoniazid in isoniazid tablets with satisfactory results.

Chitosan-Based Carbon Dots with Applied Aspects: New Frontiers of International Interest in a Material of Marine Origin

Carbon dots (CDs) have attracted significant research attention worldwide due to their unique properties and advantageous attributes, such as superior optical properties, biocompatibility, easy surface functionalization, and more. Moreover, biomass-derived CDs have attracted much attention because of their additional advantages related to more environmentally friendly and lower-cost synthesis. In this respect, chitosan has been recently explored for the preparation of CDs, which in comparison to other natural precursors exhibited additional advantages. Beyond the benefits related to the eco-friendly and abundant nature of chitosan, using it as a nanomaterial precursor offers additional benefits in terms of structure, morphology, and dopant elements. Furthermore, the high content of nitrogen in chitosan allows it to be used as a single carbon and nitrogen precursor for the preparation of N-doped CDs, significantly improving their fluorescent properties and, therefore, their performances. This review addresses the most recent advances in chitosan-based CDs with a special focus on synthesis methods, enhanced properties, and their applications in different fields, including biomedicine, the environment, and food packaging. Finally, this work also addresses the key challenges to be overcome to propose future perspectives and research to unlock their great potential for practical applications.

Multifunctional N,S-doped and methionine functionalized carbon dots for on-off-on Fe3+ and ascorbic acid sensing, cell imaging, and fluorescent ink applying

Fluorescent carbon dots (CDs) have been identified as potential nanosensors and attracted tremendous research interests in wide areas including anti-counterfeiting, environmental and biological sensing and imaging in considering of the attractive optical properties. In this work, we present a CDs based fluorescent sensor from polyvinylpyrrolidone, citric acid, and methionine as precursors by hydrothermal approach. The selective quantifying of Fe3+ and ascorbic acid (AA) are based on the fluorescent on-off-on process, in which the fluorescent quenching is induced by the coordination of the Fe3+ on the surface of the CDs, while the fluorescence recovery is mainly attributed to redox reaction between Fe3+ and AA, breaking the coordination and bringing the fluorescence back. Inspired by the good water solubility and biocompatibility, significant photostability, superior photobleaching resistance as well as high selectivity, sensitivity, and interference immunity, which are constructed mainly from the N,S-doping and methionine surface functionalization, the CDs have not only been employed as fluorescence ink in multiple anti-counterfeiting printing and confidential document writing or transmitting, but also been developed as promising fluorescence sensors in solution and solid by CDs doped test strips and hydrogels for effectively monitoring and removing of Fe3+ and AA in environmental aqueous solution. The CDs have been also implemented as effective diagnostic candidates for imaging and tracking of Fe3+ and AA in living cells, accelerating the understanding of their function and importance in related biological processes for the prevention and treatment specific diseases.

Electronic Supplementary Material

Supplementary material (fluorescence spectra: UV and Xe irradiation, TG, thermo stability, ionic strength, relationship between fluorescence responses at different concentrations of Fe3+ and AA, reaction time-dependent fluorescent responses; XPS spectra of CDs + Fe3+ and Fe3+@CDs + AA; structural characterization; equations about fluorescence lifetime, quantum yield and LOD; comparison of the CDs for the detection of Fe3+ and AA with reported methods; detection of Fe3+ and AA in real samples; absorption of Fe3+ in environmental samples and MTT assay results) is available in the online version of this article at 10.1007/s12274-022-5107-7.

An inner filter effect-based nitrogen-doped carbon dots-CoOOH nanoflakes fluorescence probe for detection of ascorbic acid by chemical redox modulation

BACKGROUND

Ascorbic acid (AA) is an essential nutrient for humans, which must be obtained from vegetables, fruits, and other foods. The content of AA has become an important standard to evaluate the quality and nutritional value of food. The fluorescence sensing method based on nanomaterials is a good alternative for the rapid detection of AA. In this study, we developed an inner filter effect-based fluorescent probe that hybridized nitrogen-doped carbon dots (NCDs) with cobalt oxyhydroxide nanoflakes (CoOOH NFs).

RESULTS

An optimal NCDs was successfully selected because it has a strong fluorescence at 430 nm and the most significant quenching phenomenon with CoOOH NFs due to the inner filter effect. When adding AA into the NCDs-CoOOH NFs probe solution, a specific redox reaction will occur between the enediol group of AA and the CoOOH NFs to interfere with the quenching ability of CoOOH NFs and recover the fluorescence of NCDs. The recovered fluorescence intensities demonstrated a linear relationship with the concentrations of AA. The assay based on the NCDs-CoOOH NFs probe allows AA to be tested in a wide range of 5-200 μmol L^-1 with a detection limit of 2.31 nmol L^-1 . Furthermore, to evaluate its practical application, the NCDs-CoOOH NFs fluorescence probe was utilized to analyze AA in vegetable, fruit, and serum matrixes with satisfactory results.

CONCLUSION

An inner filter effect-based fluorescence probe for the rapid detection of AA was developed, and it has a good potential to be applied in both food and clinical testing. © 2022 Society of Chemical Industry.

Cerium-based nanoparticles triggered catalytic reaction for the colorimetric and ratiometric fluorimetric dual-signal sensing of vitamin C

The construction of multi-modal detection methods has attracted widespread attention in the field of biosensing due to their high sensitivity and strong anti-interference ability. In this manuscript, we developed colorimetric and ratiometric fluorescence dual-signal optical methods based on cerium-based nanoparticles (Ce NPs) for the sensitive detection of vitamin C (VC). The catalysis of Ce NPs with excellent peroxidase-like activity upon the reaction of H₂O₂ with OPD was occurred, promoting the oxidation of o-phenylenediamine (OPD) to generate 2,3-diaminophennazine (OPDox) with an obvious absorption peak at 420 nm and an emission peak at 565 nm. In the presence of VC, VC not only inhibited the generation of OPDox, but also induced the formation of quinoxaline with an obvious absorption peak at 336 nm and an emission peak at 430 nm. This can be visually observed and monitored by measuring the absorbance of peak at 336 nm (A₃₃₆) and the ratiometric fluorescence intensity (F₄₃₀/F₅₆₅). Therefore, the dual-signal methods are constructed for the detection of VC. The detection lower detection limits are 8.0 μM and 8.4 μM when using the fluorescence and colorimetric signals, respectively. Furthermore, the proposed methods are successfully applied to the detection of VC in practical samples with satisfactory results.

Enhanced detection of ascorbic acid with cascaded fluorescence recovery of a dual-nanoquencher system

An innovative strategy with target-triggered cascade fluorescence recovery of a dual-nanoquencher system was developed to detect ascorbic acid (AA). Herein, manganese dioxide (MnO₂) nanosheets and gold nanoparticles (AuNPs) were used as nanoquenchers simultaneously. Owing to their synergistic effects, the fluorescence of 2,3-diaminophenazine (DAP) was decreased efficiently, thus minimizing the background fluorescence. The introduction of AA triggered the decomposition of MnO₂ into Mn²⁺, which induced the aggregation of AuNPs. Both the decomposed MnO₂ and aggregated AuNPs possess weak quenching abilities towards DAP. Such a cascade amplification strategy enhanced the detection sensitivity for AA with a LOD as low as 6.7 nM, which was two orders of magnitude lower than that of MnO₂-based fluorescence assay. Furthermore, this amplification strategy was successfully applied to detect AA in food samples.

One-pot synthesis of robust dendritic sulfur quantum dots for two-photon fluorescence imaging and "off-on" detection of hydroxyl radicals and ascorbic acid

The straightforward preparation of fluorescent sulfur quantum dots (SQDs) with good photostability and biocompatibility and multifunction remains a challenge. Herein, a simple method to improve the performance of SQDs is reported, that is, using hyperbranched polyglycerol (HPG) as a ligand to direct the synthesis of dendritic HPG-SQD nanocomposites from cheap elemental sulfur. Thanks to the protection of HPG, the HPG-SQDs show much better biocompatibility and photostability as compared with the widely reported polyethylene glycol (PEG) ligand-capped SQDs (PEG-SQDs). In addition, the HPG-SQDs also present excellent aqueous solubility, stable fluorescence against environmental variation, good cell uptake capability, and strong single- and two-photon fluorescence. Moreover, the HPG-SQDs display sensitive and selective fluorescence "off-on" behavior to hydroxyl radicals (˙OH) and ascorbic acid (AA), respectively, and thereby hold potential as a fluorescent switch to detect ˙OH and AA. For the first time, the utilization of two-photon fluorescence of HPG-SQDs to monitor ˙OH and AA in cells is demonstrated in this study.

Fluorine-Nitrogen-Codoped Carbon Dots as Fluorescent Switch Probes for Selective Fe(III) and Ascorbic Acid Sensing in Living Cells

High-quality fluorescent probes based on carbon dots (CDs) have promising applications in many fields owing to their good stability, low toxicity, high quantum yield, and low raw material price. The fluorine- and nitrogen-doped fluorescent CDs (NFCDs) with blue fluorescence was successfully synthesized using 3-aminophenol and 2,4-difluorobenzoic acid as the raw material by the hydrothermal method. The NFCDs as probe can be used to directly and indirectly detect Fe3+ (detection range: 0.1–150 μM and detection limit: 0.14 μM) and ascorbic acid (AA) (detection range: 10–80 μM and detection limit: 0.11 μM). The NFCDs-based probe shows exceptional selectivity and strong anti-interference for Fe3+ and ascorbic acid (AA). In addition, we examined the response of NFCDs to Fe3+ and AA in living cells, which showed that the timely use of AA can reduce the effects of iron poisoning. This has important biological significance. This means that using NFCDs as fluorescent probes is beneficial for Fe3+ and AA detection and observing their dynamic changes in living cells. Thus, this work may contribute to the study of Fe3+- and AA-related diseases.

Direct preparation of solid carbon dots by pyrolysis of collagen waste and their applications in fluorescent sensing and imaging

The fluorescent carbon dots (CDs) have found their extensive applications in sensing, bioimaging, and photoelectronic devices. In general terms, the synthesis of CDs is straight-forward, though their subsequent purification can be laborious. Therefore, there is a need for easier ways to generate solid CDs with a high conversion yield. Herein, we used collagen waste as a carbon source in producing solid CDs through a calcination procedure without additional chemical decomposition treatment of the raw material. Considering a mass of acid has destroyed the original protein macromolecules into the assembled structure with amino acids and peptide chains in the commercial extraction procedure of collagen product. The residual tissues were assembled with weak intermolecular interactions, which would easily undergo dehydration, polymerization, and carbonization during the heat treatment to produce solid CDs directly. The calcination parameters were surveyed to give the highest conversion yield at 78%, which occurred at 300°C for 2 h. N and S atomic doping CDs (N-CDs and S-CDs) were synthesized at a similar process except for immersion of the collagen waste in sulfuric acid or nitric acid in advance. Further experiments suggested the prepared CDs can serve as an excellent sensor platform for Fe³⁺ in an acid medium with high anti-interference. The cytotoxicity assays confirmed the biosafety and biocompatibility of the CDs, suggesting potential applications in bioimaging. This work provides a new avenue for preparing solid CDs with high conversion yield.

Detection of Fe3+ ions in aqueous environment using fluorescent carbon quantum dots synthesized from endosperm of Borassus flabellifer

Natural products derived carbon quantum dots (CQDs) catch huge attention owing to their distinctive properties of smaller size, water dispersibility, high photostability, lower cost, tunable emission, biocompatibility, least toxicity, electrical conductivity, optical and catalytic properties, and easy modification. Herein high fluorescent CQDs were prepared using Borassus flabellifer (ice apple) as a carbon source utilizing the simplistic one-step hydrothermal method. The prepared CQDs possessed excellent photoluminescence, high photostability, and stability in an aqueous solution and harbored large of quantum yield and strong stability in high pH conditions with the characteristic strong blue fluorescence emission. With these superior properties, the CQDs have been used as sensing probes for the detection of Fe³⁺ ions having excellent selectivity and sensitivity with a 2.01 μM limit of detection. The CQDs decorated probe was found effective in detecting Fe³⁺ ions in the tap and drinking mineral water, suggesting the applicability of the prepared sensor. The developed sensor exhibited advantages, including simple, low-cost, label-free, rapid, and good sensitivity and selectivity towards Fe³⁺ ions, with a great application for detection of such ions in real water.

A novel biosensor for detecting vitamin C in milk powder based on Hg2+-mediated DNA structural changes

Background:

Detection of Vitamin C (Vc) is very important to protect human health. A lot of methods have been developed for the detection of Vc. However, many methods require complex material preparation and skilled operators. Thus, a simple, label-free biosensor is still urgently needed.

Methods:

In this work, N-methylmesoporphyrin IX (NMM)/G-quadruplex pair was used as a label-free signal reporter. Without Vc, the G-quadruplex DNA and its incomplete complementary chain could form duplex structure by T-Hg(II)-T mismatch. In this case, the G-quadruplex structure could not be formed. When Vc was added, the Hg2+ was reduced to Hg(0). Then, the G-quadruplex DNA became free and formed G-quadruplex structure to emit fluorescence signal.

Results:

Under optimal conditions, this biosensor showed a good linear response in the range of 0.2 - 4.0 μM and a low limit of detection (19.9 nM). This biosensor also had good selectivity towards Vc. Meanwhile, the satisfactory recovery rates (93.2%-102.8%) suggested that this biosensor had attractive potential for measuring Vc in real samples.

Conclusion:

In this work, a simple label-free fluorescent biosensor for the detection of Vc based on Hg2+-mediated DNA structural changes had been developed. The whole experiment was simple and all reagents were commercialized. The label-free detection was realized by NMM/G-quadruplex as a signal reporter. This biosensor was very sensitive with a low limit of detection. And it had a potential practical application for the Vc detection in milk powder.

Simple One-step Solid-state Synthesis of Highly Crystalline N Doped Carbon Dots As Selective Turn Off-sensor for Picric Acid and Metanil Yellow

A simple one-step solid-state pyrolysis method has been employed to synthesize highly crystalline nitrogen-doped carbon dots using adipic acid and urea as carbon and nitrogen sources. The prepared carbon dots displayed UV emission ( λ_ex = 290 nm and λ_em = 370 nm) and blue fluorescence emission ( λ_ex = 360 nm and λ_em = 420 nm). These crystalline nitrogen-doped carbon dots exhibited a quantum yield of 6% with tryptophan as standard at 370 nm emission and 14% with quine sulfate as standard at 420 nm emission. The synthesized carbon dots were spherical, having a mean particle diameter of 2.56 ± 0.57 nm. The prepared carbon dots have large functional groups on their surface, which renders excellent water solubility to them. Carbon dots was used as selective and sensitive turn off sensor for detection of picric acid Metanil yellow with the linear response for picric acid ranging from 2 μM to 22 μM and 2-45 μM with a detection limit of 0.06 μM and 0.45 μM and for Metanil yellow ranging from 1 μM to 30 μM with a detection limit of 0.32 μM. The mechanism for detecting metanil yellow is proposed to be the inner filter effect. At the same time, it is both the inner filter effect and FRET for picric acid. The actual sample application of carbon dots as a nanosensor was tested to detect metanil yellow as an adulterant in turmeric powder.

Reuse of waste Myrica rubra for green synthesis of nitrogen-doped carbon dots as an "on-off-on" fluorescent probe for Fe3+ and ascorbic acid detection

As one kind of high nutrition fruits, abandoned Myrica rubra causes great waste due to short storage period. For resource utilization, we herein fabricated the Myrica rubra-based N-doped carbon dots (MN-CDs) by a facile/green hydrothermal method. MN-CDs, fabricated from four regions of China, displayed significant differences in their corresponding fluorescence intensities (FIs). Interestingly, different batches of waxberry samples from the same region (Wenzhou, China) exhibited slight differences in their FIs, and also an excellent anti-photobleaching and anti-salt capacity. Based on Fe³⁺-triggered quenching effect and fluorescent recovery by redox reaction of AA and Fe³⁺, MN-CDs were employed to construct an "on-off-on" switch probe for sequential detection of Fe³⁺ and ascorbic acid (AA). Through Zeta potential, UV spectrum, Stern-Volmer equation, and valence-conduction band theory, the Fe³⁺-triggered quenching belonged to a static quenching process, which resulted from the synergistic contribution of inner filtering effect and photo-induced electron transfer mechanisms. The linear ranges for Fe³⁺ and AA detections were 1-1000 and 0.1-1000 mM. The limits of detection were 0.3 μM for Fe³⁺ in environmental waters, and 0.03 μM for AA in pharmaceutical tablets and fruit juice samples. Under 365-nm UV lamp, the color changes of test papers were easily observed from dark blue and bright blue in the presence of Fe³⁺ and AA, and thus the MN-CDs-based switch probe could be satisfactorily used for visually qualitative detection of Fe³⁺ and AA outdoor with our naked eyes. To sum up, MN-CDs not only realize resource reutilization of abandoned Myrica rubra, but also offer an convenient outdoor approach for qualitative detection of Fe³⁺ and AA in complex matrices.

Sustainable and Green Synthesis of Waste-Biomass-Derived Carbon Dots for Parallel and Semi-Quantitative Visual Detection of Cr(VI) and Fe3

Carbon dot (CD)-based multi-mode sensing has drawn much attention owing to its wider application range and higher availability compared with single-mode sensing. Herein, a simple and green methodology to construct a CD-based dual-mode fluorescent sensor from the waste biomass of flowers of wintersweet (FW-CDs) for parallel and semi-quantitative visual detection of Cr(VI) and Fe3+ was firstly reported. The FW-CD fluorescent probe had a high sensitivity to Cr(VI) and Fe3+ with wide ranges of linearity from 0.1 to 60 µM and 0.05 to 100 µM along with low detection limits (LOD) of 0.07 µM and 0.15 µM, respectively. Accordingly, the FW-CD-based dual-mode sensor had an excellent parallel sensing capacity toward Cr(VI) and Fe3+ with high selectivity and strong anti-interference capability by co-using dual-functional integration and dual-masking strategies. The developed parallel sensing platform was successfully applied to Cr(VI) and Fe3+ quantitative detection in real samples with high precision and good recovery. More importantly, a novel FW-CD-based fluorescent hydrogel sensor was fabricated and first applied in the parallel and semi-quantitative visual detection of Cr(VI) and ferrous ions in industrial effluent and iron supplements, further demonstrating the significant advantage of parallel and visual sensing strategies.

Iron ion sensing and in vitro and in vivo imaging based on bright blue-fluorescent carbon dots

Herein, we propose an eco-friendly synthesis of carbon dots (CDs) and ingeniously design a rapid and label-free "turn-off" sensing platform for ultrasensitive recognition of Fe³⁺ in vitro and in vivo. CDs with extraordinary advantages involving exceptional stability, ultra-low toxicity as well as admirable biocompatibility were simply prepared via one-step hydrothermal strategy of Caulis polygoni multiflora. Result indicated that as-acquired CDs not only exhibit excitation dependency, but also have a high quantum yield of (QY) up to 42%. Miraculously, the fluorescence of CDs can be extinguished sharply by Fe³⁺ because of static quenching effect with linear range of 0-400 µM, yielding a detection limit of 0.025 μM. Benefiting from these characteristics, CDs have been extended for multicolourful imaging and tracking Fe³⁺ fluctuations in living cells. Bioimaging of zebrafish larvae exposed to CDs confirmed that it is smoothly circulated to other tissues and organs owing to their small size. Eventually, as-prepared CDs have been implemented for the real-time detection of Fe³⁺ in nude mice.

N,Cl-Codoped Carbon Dots from Impatiens balsamina L. Stems and a Deep Eutectic Solvent and Their Applications for Gram-Positive Bacteria Identification, Antibacterial Activity, Cell Imaging, and ClO- Sensing

In this study, we first synthesized metal-free N,Cl-doped carbon dots (N,Cl-CDs) using Impatiens balsamina L. stems as green precursors in a deep eutectic solvent (DES). The obtained N,Cl-CDs were characterized through transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Fourier-transform infrared (FT-IR) spectroscopy, fluorescence (FL) spectroscopy, and ultraviolet (UV) spectroscopy. In addition to the common features of carbon dots (CDs), such as high light stability, small size, low toxicity, good aqueous solubility, and favorable biocompatibility, these N,Cl-CDs exhibited excellent recognition and selectivity for Gram-positive bacteria by doping with N and Cl elements using DES. The N,Cl-CDs with positive charge cannot only differentiate Gram-positive bacteria by selective fluorescence imaging but also have antibacterial effects on Gram-positive bacteria. Through potential, ROS, and morphological analyses of bacteria before and after treatment with N,Cl-CDs, the antibacterial mechanisms of bacteriostasis, Enterococcus faecium, Staphylococcus aureus, Bacillus subtilis, Escherichia coli, Pseudomonas aeruginosa, and Salmonella were explored. In addition, N,Cl-CDs demonstrated low cytotoxicity and good cell imaging ability in cancer and normal cells. Moreover, they can be used as a fluorescence sensor for the detection of ClO- with a detection range from 100 nM to 40 μM and a limit of detection (LOD) of 30 nM. In summary, the prepared N,Cl-CDs could be applied as environmentally friendly Gram-positive bacterial identification and antibacterial agents. Additionally, their cell imaging and ClO- detection abilities were outstanding.

Gram-scale synthesis of nitrogen-doped carbon dots from locusts for selective determination of sunset yellow in food samples

Locust powder was converted into water-soluble fluorescent nitrogen-doped carbon dots (N-CDs) with gram-scale yield through a self-exothermic reaction between nitric acid and diethylenetriamine (DETA) within 10 min. The morphology, elemental information, and optical properties of the N-CDs were characterized using high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, and Fourier-transform infrared, ultraviolet-visible and fluorescence spectroscopy. Spectroscopic investigation indicated that the fluorescence emission behaviour of N-CDs is excitation wavelength dependent, with the strongest emission peak at 470 nm using a 390 nm excitation wavelength. The strong absorption peak of sunset yellow (SY) at 482 nm overlaps substantially with the blue emission peak (470 nm) of N-CDs. This enables the fluorescence emission of N-CDs to be obviously quenched by SY through the inner filter effect. There was a good linear relationship between the fluorescence quenching degree and the concentrations of SY within the range 0.5-40 μM. The detection limit of developed fluorescence assay for SY is 28 nM, and the relative standard deviation is 2.3% (c = 10 μM). The N-CDs derived from locusts by the self-exothermic reaction are highly selective and sensitive fluorescent probes for SY, which were applied to the fluorescence sensing of SY in different food samples with satisfactory results.

Dual-excitation and dual-emission carbon dots for Fe3+ detection, temperature sensing, and lysosome targeting

Dual-excitation and dual-emission carbon dots (CDs) have been prepared by simple one-step hydrothermal treatment of p-phenylenediamine and 5-aminosalicylic acid. The as-prepared CDs emit bright green fluorescence under excitation at 320-400 nm and bright orange fluorescence under excitation at 490-560 nm. Interestingly, the CDs can be employed as a dual-excitation and dual-emission fluorescent probe for Fe³⁺ detection in aqueous solution and living cells. Furthermore, the obtained CDs can function as a promising dual-excitation and dual-emission temperature sensor. Additionally, the CDs can be utilized for lysosomal targeting.

Multi-functionalized carbon aerogels derived from chitosan

Carbon aerogels are prepared by a thermal treating-freeze drying approach from chitosan, with glycine hydrochloride ionic liquid (IL) acting as solvent and nitrogen source. Different post-treatments such as ball milling and high temperature carbonization are employed to functionalize the obtained carbon aerogels with tuned properties, making it promising candidates as fluorescence material (NACs-Q), electrode material (FDC-800) and catalyst support (NAC_Pd-C). NACs-Q is water-soluble quantum dot with average particle sizes of 3.8 nm, presenting excitation-/emission-independent and pH-sensitive properties, which could be used as sensor for testing acetone vapor or an "on-off-on" sensor for detections of Fe³⁺ and vitamin C in fruits. FDC-800 exhibits fluffy lamellar structure with developed micro-mesopores and nitrogen-containing groups on their surfaces, which is beneficial for building flexible solid-state supercapacitor with excellent performance, delivering a capacitance of 208F/g at 0.5 A/g, and achieving an energy density of 7.2 W h/kg at a power density of 50 W/kg. Moreover, NAC_Pd-C can be used as catalyst for phenol hydrogenation, and phenol conversion of 100% with cyclohexanone selectivity of 98.3% is achieved, due to the synergetic effects of the Pd active-site, the N-containing groups, and the Lewis acid sites on the support.

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

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

Ultra-high quantum yield nitrogen-doped carbon quantum dots and their versatile application in fluorescence sensing, bioimaging and anti-counterfeiting

The exploration of carbon quantum dots (CQDs) with ultra-high quantum yield, simple synthesis path, and satisfying output to facilitate their wide applications in numerous fields are always the research focus. In this work, nitrogen-doped carbon quantum dots (N-CQDs) with strong blue fluorescence were synthesized with a simple one-step hydrothermal method using citric acid and o-phenylenediamine as raw materials, and the absolute quantum yield was as high as 92.1%. The detailed research results demonstrate that the N-CQDs have outstanding fluorescence stability, high selectivity, and anti-interference in Hg²⁺ detection. The obtained N-CQDs also possess excellent biocompatibility, which can also be successfully applied in cell imaging and intracellular Hg²⁺ detection. Most importantly, due to their high quantum yield and excellent dispersibility, the N-CQDs solution can be used as a quick-drying fluorescent ink for ink-jet printing. Therefore, the as-prepared N-CQDs have great potential in fluorescence sensing, biomedical diagnosis, data encryption, and anti-counterfeiting.