Dual-Emission Fluorescent Probe for the Simultaneous Detection of Nitrite and Mercury(II) in Environmental Water Samples Based on the Tb3+-Modified Carbon Quantum Dot/3-Aminophenylboronic Acid Hybrid

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

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

pH-responsive dual-emission carbon dots for the ratiometric detection of organophosphorus pesticides in Brassica chinensis and Hg2+ in water

Accurate and rapid monitoring of organophosphorus pesticides (OPs) residues is crucial for regulating food safety. Herein, dual-emission carbon dots (de-CDs) were fabricated for the ratiometric detection of OPs and Hg2+. The de-CDs exhibited two emission peaks at 678 and 485 nm when excited with visible light. Interestingly, the fluorescence at 678 nm was significantly quenched by Hg2+ mainly because of the static quenching effect, whereas that at 485 nm exhibited a slight change. More significantly, the quenched fluorescence of the de-CDs recovered remarkably after introducing omethoate, diazinon and malathion. Accordingly, the ratiometric detection of the three OPs and Hg2+ was achieved with high selectivity and robust performance. In addition, the OPs residues assay in Brassica chinensis was successfully performed with satisfactory results. This study not only provides an attractive tool for the simple and rapid assay of OPs but also offers new insights into the fabrication of multi-functional carbon dots.

Real-time and specific monitoring of nitric oxide and evaluating of the oxidative stress in living cells and zebrafish under the pollutant exposure using a carbon dot-based composite fluorescent probe

Nitric oxide (NO) functions as an essential signalling molecule in various physiological and pathological pathways. In vitro and vivo redox processes mediated by reactive oxygen species (ROS) and nitric oxide (NO) directly influence the intracellular state. In this study, a red-emitting fluorescent nanoprobe, N,S-CDs@Zn-ICA, was synthesized to monitor NO fluctuations in living cells and zebrafish under the exposure to various pollutants. Red-emissive carbon dots (N,S-CDs) were synthesized by a hydrothermal method using o-phenylenediamine and urea as carbon / nitrogen sources, and H2SO4 as sulfur source. Glutathione (GSH) was introduced to link N,S-CDs with metal organic complexes (Zn-ICA) through an amidation reaction to fabricate a carbon dot-based composite fluorescent probe, which greatly improved the selectivity, stability, and response time of the N,S-CDs. The composite probe has high selectivity and sensitivity with limit of detection (LOD) of 96.0 nM. Furthermore, the proposed probe was successfully used to monitor the dynamic changes in NO levels and evaluate oxidative stress in MCF-7 cells and zebrafish under the exposure to various pollutants, including seven heavy metal ions (such as Pb2+, Cd2+, and Hg2+) and nine organic pollutants at different concentrations and exposure times. This work provides a novel strategy for constructing highly selective and red-emitting fluorescent probe for real-time and dynamic monitoring of NO and further evaluating oxidative stress induced by pollutants in vitro and in vivo via fluorescence imaging.

Multi-Emission Carbon Dots Combining Turn-On Sensing and Fluorescence Quenching Exhibit Ultrahigh Selectivity for Mercury in Real Water Samples

Mercury is a ubiquitous heavy-metal pollutant and poses serious ecological and human-health risks. There is an ever-growing demand for rapid, sensitive, and selective detection of mercury in natural waters, particularly for regions lacking infrastructure specialized for mercury analysis. Here, we show that a sensor based on multi-emission carbon dots (M-CDs) exhibits ultrahigh sensing selectivity toward Hg(II) in complex environmental matrices, tested in the presence of a range of environmentally relevant metal/metalloid ions as well as natural and artificial ligands, using various real water samples. By incorporating structural features of calcein and folic acid that enable tunable emissions, the M-CDs couple an emission enhancement at 432 nm and a simultaneous reduction at 521 nm, with the intensity ratio linearly related to the Hg(II) concentration up to 1200 μg/L, independent of matrix compositions. The M-CDs have a detection limit of 5.6 μg/L, a response time of 1 min, and a spike recovery of 94 ± 3.7%. The intensified emission is attributed to proton transfer and aggregation-induced emission enhancement, whereas the quenching is due to proton and electron transfer. These findings also have important implications for mercury identification in other complex matrices for routine, screening-level food safety and health management practices.

Zn-CD@Eu Ratiometric Fluorescent Probe for the Detection of Dipicolinic Acid, Uric Acid, and Ex Vivo Uric Acid Imaging

Development of reliable methods for the detection of potential biomarkers is of the utmost importance for an early diagnosis of critical diseases and disorders. In this study, a novel lanthanide-functionalized carbon dot-based fluorescent probe Zn-CD@Eu is reported for the ratiometric detection of dipicolinic acid (DPA) and uric acid (UA). The Zn-CD@Eu nanoprobe was obtained from a simple room-temperature reaction of zinc-doped carbon dots (Zn-CD) and the EDTA-Eu lanthanide complex. Under optimal conditions, a good linear response was obtained for DPA in two concentration ranges of 0-55 and 55-100 μM with a limit of detection of 0.53 and 2.2 μM respectively, which is significantly below the infectious dosage of anthrax (∼55 μM). Furthermore, the Zn-CD@Eu/DPA system was employed for the detection of UA with a detection limit of 0.36 μM in the linear range of 0-100 μM. The fluorescent probe was successfully implemented for determining DPA and UA in human blood serum, sweat, and natural water bodies with considerable recovery rates. In addition, the potential of the nanoprobe for ex vivo visualization of UA was demonstrated in fruit fly (Drosophila melanogaster) as a model organism.

Analyte-perturbed balance between reducibility and fluorescence of Ti3C2 MXene quantum dots for label-free, dual-mode detection of silver ions

BACKGROUND

As an emerging and attractive low-dimensional functional materials, Ti3C2 MXene quantum dots (QDs) enlarge the toolbox of fluorescence sensing. However, monochromatic fluorescence, which only provide one single signal, is often beset by challenges such as false-positive readouts and limitations in selectivity. Consequently, to improve the sensing accuracy by means of cross-verified dual-signal authentication, the endeavor to engineer dual-mode nanoprobes based on Ti3C2 QDs, incorporating both the capability of fluorescence and an alternative sensing mechanism, emerges as a compelling avenue.

RESULTS

Here, based on the alterations in colorimetric and fluorescent signals of Ti3C2 QDs with the addition of Ag+, we propose a dual-mode sensor obviating the necessity for nanoprobe labeling. Owing to the decent reducibility of Ti3C2 QDs, Ag+ is adsorbed and reduced, resulting in the generation of plasmonic Ag nanoparticles (NPs), which simultaneously trigger colorimetric responses of the solution and enhance the fluorescent emission of Ti3C2 QDs. The confluence of colorimetry and fluorometry within this strategy optimally harnesses the modulating role of the acquired Ag NPs on the reducing capability and fluorescence characteristics of Ti3C2 QDs. The equilibrium imparts versatility and promising prospects to this analyte-triggered label-free method, which enables a remarkable specificity and an excellent detecting limit (0.45 μM) for Ag+.

SIGNIFICANCE

The balance between reducibility and fluorescence of Ti3C2 QDs for dual-mode detection is inventively demonstrated. With the exemplification of a direct influence of both features of the nanoprobe via the introduction of analytes, this study opens the feasibility of the analyte-perturbed felicitous equilibrium, which endows label-free methods with versatility and promising prospects. This design may evoke more biosensing strategies with the function of double-signal mutual verification.

Fluorescent ovalbumin-functionalized gold nanocluster as a highly sensitive and selective sensor for relay detection of salicylaldehyde, Hg(II) and folic acid

A sensitive and selective relay-based scheme for the detection of salicylaldehyde, Hg2+, and folic acid (FA) has been demonstrated using fluorescent ovalbumin functionalized gold nanoclusters (OVA-AuNCs, λem = 655 nm) in this article. The OVA-AuNCs were conjugated to salicylaldehyde via an imine linkage to form Salic_OVA-AuNCs conjugate. The molecular docking study reveals that multiple functional groups and amino acid residues are involved in the interaction between salicylaldehyde and the OVA-AuNCs. The coupling of salicylaldehyde with OVA-AuNCs results in fluorescence quenching at 655 nm and concomitant formation of an emission band at 500 nm, which have leveraged to detect salicylaldehyde down to 2.02 µM. Following that, the Salic_OVA-AuNCs has been used for the detection of Hg2+ and FA. Several processes, such as internal charge transfer (ICT), photoinduced electron transfer (PET) and metallophilic interactions, are involved between the Salic_OVA-AuNCs nanoprobe and the analytes, which allowed to detect Hg2+ and FA down to 0.13 nM and 0.11 nM, respectively. The Salic_OVA-AuNCs nanoprobe has an additional naked-eye utility when applied to paper-strip sensing strategy for Hg2+ and FA detection.

Preparation and application of carbon quantum dot fluorescent probes combined with rare earth ions

Globally, antibiotic abuse, organic contamination, and excessive heavy metal ion pollution pose serious threats to human health. In this case, ratiometric fluorescent probes can eliminate the errors caused by environmental factors and provide more accurate detection results than single-emission intensity nanoprobes. Accordingly, based on the excellent biocompatibility and abundant surface functional groups of carbon quantum dots (CQDs) and the properties of large Stokes shifts and narrow emission bands of rare earth ions (RE3+), RE-CQD fluorescent probes have attracted widespread attention. Herein, firstly we review the combination of carbon quantum dots with rare earth ions (rare earth complexes) using various functionalization approaches to improve the defects of rare earth complexes and realize the functionalization of carbon quantum dots and their applications in many fields, such as biology and environmental science. In addition, we classify the methods for the synthesis of RE-CQD hybrids into three groups according to the different binding modes of the RE and CQDs, including doping, covalent grafting, and direct coordination. The excellent properties of these fluorescent probes are also briefly described. Finally, a comprehensive overview of the important applications of RE-CQD fluorescent probes in the fields of public safety sensing, chemical sensing, biomedical sensing, temperature sensing, and pH sensing is presented. In this review, the recent research progress in the field of ratiometric fluorescence sensing based on carbon quantum dots and rare earth ions is summarized and an outlook on the future development of RE-CQD fluorescent probes regarding their construction and potential applications is provided.

Zero-Dimensional Carbon Nanomaterials for Fluorescent Sensing and Imaging

Advances in nanotechnology and nanomaterials have attracted considerable interest and play key roles in scientific innovations in diverse fields. In particular, increased attention has been focused on carbon-based nanomaterials exhibiting diverse extended structures and unique properties. Among these materials, zero-dimensional structures, including fullerenes, carbon nano-onions, carbon nanodiamonds, and carbon dots, possess excellent bioaffinities and superior fluorescence properties that make these structures suitable for application to environmental and biological sensing, imaging, and therapeutics. This review provides a systematic overview of the classification and structural properties, design principles and preparation methods, and optical properties and sensing applications of zero-dimensional carbon nanomaterials. Recent interesting breakthroughs in the sensitive and selective sensing and imaging of heavy metal pollutants, hazardous substances, and bioactive molecules as well as applications in information encryption, super-resolution and photoacoustic imaging, and phototherapy and nanomedicine delivery are the main focus of this review. Finally, future challenges and prospects of these materials are highlighted and envisaged. This review presents a comprehensive basis and directions for designing, developing, and applying fascinating fluorescent sensors fabricated based on zero-dimensional carbon nanomaterials for specific requirements in numerous research fields.

Dual-ligand lanthanide metal-organic framework probe for ratiometric fluorescence detection of mercury ions in wastewater

By serving dipyridylic acid (DPA) and 2,5-dihydroxyterephthalic acid (DHTA) as the biligands, a novel lanthanide (Eu3+) metal-organic framework (MOF) namely Eu-DHTA/DPA was prepared for specific Hg2+ fluorescence determination. The dual-ligand approach can endows the resulting luminescent MOF with dual emission of ratiometric fluorescence and uniform size. Eu3+ produces intense red fluorescence when activated by the ligand DPA, while the other ligand DHTA produces yellow fluorescence. Under 273 nm excitation, the presence of Hg2+ in the monitoring environment causes an increase in the intensity of the DHTA fluorescence peak at 559 nm and a decrease in the intensity of the Eu3+ fluorescence peak at 616 nm. Hg2+ effectively quenches the fluorescence emission of the central metal Eu3+ in Eu-DHTA/DPA at 616 nm through a dynamic quenching effect. This recognition process occurs due to the coordination of Hg2+ with ligands such as benzene rings, carboxyl groups, and pyridine N in three-dimensional space. Hg2+ was detected by measuring the ratio between two fluorescence peaks (I559 nm/I616 nm) within the range 2-20 μM, achieving a remarkably low detection limit of 40 nM. The established ratiometric fluorescence method has been successfully applied to the determination of Hg2+ in industrial wastewater of complex composition. The method plays a crucial role in the rapid and sensitive monitoring of Hg2+ in real environmental samples. The recoveries ranged from 92.82% to 112.67% (n = 3) with relative standard deviations (RSD) below 4.8%. This study offers a convenient and effective method for constructing probes for Hg2+ monitoring, with practical applications in environmental monitoring.

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.

Nitrogen and sulfur co-doped carbon quantum dots fluorescence quenching assay for detection of mercury (II)

Mercury is a highly toxic and potentially bioaccumulative heavy metal ion that can cause severe health problems in humans even at very low concentrations. Thus, the development of a simple, rapid, and sensitive assay for the effective detection of mercury ions at trace levels is of great importance. Here, nitrogen and sulfur co-doped carbon quantum dots (N,S-CQD) were synthesized by a simple hydrothermal treatment of chitosan in the presence of thiourea and citric acid with a quantum yield (QY) up to 33.0 % and used as a selective fluorescent probe to detect mercury ions (Hg²⁺). The effect of pH, ionic strength, and time on the fluorescence intensity of N,S-CQD were investigated and optimized. The synthesized N,S-CQD showed ultrasensitive ability to detect Hg²⁺ ions in the water samples, also in the presence of 11 interfering metal ions, with a low detection limit (∼4 nM) over a wide linear range from ∼5-160 nM. The sensing performance of N,S-CQD probe in real sample applications was evaluated by the detection of Hg²⁺ in lake water samples, which confirmed its potential application in environmental analysis.

One-step hydrothermal synthesis of near-infrared emission carbon quantum dots as fluorescence aptamer sensor for cortisol sensing and imaging

Fluorescence carbon quantum dots (CQDs) have been widely applied to sensing and bioimaging. In this paper, near-infrared carbon quantum dots (NIR-CQDs) were prepared through a simple one-step hydrothermal approach using reduced glutathione and formamide as raw materials. Based on NIR-CQDs, aptamer (Apt) and graphene oxide (GO) has been applied to fluorescence sensing cortisol. NIR-CQDs-Apt adsorbed to the surface of GO through π-π stacking and an inner filter effect (IFE) occurred between NIR-CQDs-Apt and GO leading to NIR-CQDs-Apt fluorescence "off". The IFE process is disrupted in the presence of cortisol, allowing NIR-CQDs-Apt fluorescence "on". This led us to construct a detection method with excellent selectivity over other cortisol sensors. The sensor can detect cortisol from 0.4 to 500 nM and has a detection limit as low as 0.13 nM. Importantly, this sensor can be used to detect intracellular cortisol with excellent biocompatibility and cellular imaging capabilities, which is promising for biosensing.

Real-time monitoring of mercury(II) in water and food samples using a quinoline-based ionic probe

Herein, a novel ionic fluorescent probe for mercury(II) detection is presented consisting of a functional quinoline-based IL. Interestingly, the probe displayed high sensitivity (0.8 nM) and selectivity through the regulation function of electrostatic attraction, where its performance was significantly superior to that of quinoline probes without negative charge. Furthermore, the probe was found to exhibit two different fluorescent signals and colorimetric signals in the presence of different concentrations of mercury(II), which was consistent with the reaction mechanisms of the generation of large conjugated systems and the formation of anion-mercury(II) complexes. Moreover, this probe could be further loaded on a simple filter paper to serve as a visual paper sensor due to its adequate response time of less than 5 s. This regulation function strategy of electrostatic attraction has excellent potential for use in the precise detection of targeted analytes in real complex samples with improved accuracy and selectivity.

Lanthanide ternary complex as a fluorescent probe for highly sensitive and selective detection of copper ions based on selective recognition and photoinduced electron transfer

Copper ions have a very important role in human health, industrial and agricultural production. Herein, lanthanide ternary complex of 2,6-pyridinedicarboxylic acid (DPA)-Eu³⁺-polyethyleneimine (PEI) as a fluorescent probe was thus fabricated for highly sensitive and selective detection of copper ions. PEI itself is non-fluorescent, the PEI-Eu³⁺complex is also non-fluorescent, and PEI has specific recognition to copper ions due to its higher affinity ability to copper ion than other metal ions. It was found that Cu²⁺ ions cannot quench the characteristic fluorescence of Eu³⁺ in the DPA-Eu³⁺ system, while in the DPA-Eu³⁺-PEI system, Cu²⁺ ions can greatly quench the characteristic fluorescence of Eu³⁺ due to photoinduced electron transfer (PET). The luminescent and quenching mechanism was also discussed in detail. The DPA-Eu³⁺-PEI probe not only has high sensitivity and selectivity, but also has very rapid fluorescence response and the response time is only 1 min. A good linear relationship between the fluorescence ratios of F₀/F and the concentrations of Cu²⁺ was obtained in the range of 0.02 ∼ 10.0 μM (R² = 0.998), and the limit of detection (LOD) is 8.0 nM. The probe was successfully applied for the detection of Cu²⁺ ions in the lake and river water samples, wastewater and urine samples. This work may provide a new strategy for fabricating simple and effective fluorescence probe and a promising application for the rapid and on-site detection in environmental monitoring and biological fluids.

A susceptible coordination hybrid based terbium sensibilization coupled ESIPT effects for pattern discrimination of analogues

Multianalyte detection and analogue discrimination are extremely valuable frontier areas for their wide applications in environmental, medical, clinical and industrial analyses. Nowadays, researchers rack their brains on how to develop excellent multianalyte chemosensors that have presented huge challenges in designing high-efficient fluorescent sensing materials and constructing high-throughput detection methods. In this paper, we propose a novel strategy to utilize the dual-emission fluorescent detection platform as a lab-on-a-molecule, arising from the disalicylaldehyde-coordinated hybrid H₂Q_j3/Tb based terbium sensibilization coupled excited-state intramolecular proton transfer effects. Using the statistical analysis (PCA and HCA) for sensing signals of three fluorescence channels (431, 543 and 583 nm), we demonstrate this elaborate chemosensor with multianalyte detection of three species (solvents, anions and cations) and pattern discrimination of analogues. As a result, the H₂Q_j3/Tb shows great lab-on-a-molecule characters for each set of species, resulting in the easier identification of many critical analytes (e.g., H₂O, NO₂^- and Fe³⁺) and discrimination of analogues. In addition, it is also proven to be able to provide reliable content determination for an analyte, especially the NO₂^- (LOD = 0.37 μM), and discrimination for mixed analogues. A combination of easy-to-implement preparation procedure and data analysis technique makes this work promising for not only designing similar lanthanide-based materials but also realizing more high-efficient multianalyte sensing systems towards various potential applications.

Dual-response fluorescent probe based on nitrogen-doped carbon dots and europium ions hybrid for ratiometric and on-site visual determination of oxytetracycline and tetracycline

Tetracyclines residues, particularly oxytetracycline (OTC) and tetracycline (TC), have raised extensive concern because of their serious adverse effects on human health. Herein, a dual-response fluorescent probe based on nitrogen-doped carbon dots (N-CDs) and Eu³⁺ hybrid (N-CDs-Eu³⁺) was developed to selectively determine OTC and TC. The N-CDs act as ancillary ligands of Eu³⁺ and recognition units of OTC/TC, while the Eu³⁺ ions chelated with N-CDs can also specifically recognize OTC/TC. Upon inclusion of OTC/TC, an enhancement in Eu³⁺ emission occurs due to the energy transfer from OTC/TC to Eu³⁺ and the efficient elimination of quenching effect caused by H₂O molecule, which is attributed to the incorporation of N-CDs; while the blue fluorescence emitted by the N-CDs decreases under the inner filter effect and static quenching effect caused by OTC/TC. Based on the double and reverse response signals, the ratiometric detection of OTC and TC in the range of 0.1-45 μΜ and 0.1-30 μΜ is achieved with a detection limit of 0.017 and 0.041 μM, respectively. In addition, the noticeable variation in fluorescence color of the probe is integrated with a smartphone-assisted analysis device for the rapid on-site quantitative assay of OTC, where the detection limit is 0.15 μΜ. The results show that this probe performs with excellent specificity and anti-interference for both OTC and TC, and satisfactory detection results are obtained in lake water, milk, and honey samples, thereby confirming that the probe exhibits promising application in food safety and environmental monitoring.

Facile synthesis of nitrogen-doped carbon dots as sensitive fluorescence probes for selective recognition of cinnamaldehyde and l-Arginine/l-Lysine in living cells

The disorder of amino acid metabolism and the abuse of small molecule drugs pose serious threats to public health. However, due to the limitations of existing detection technologies in sensing cinnamaldehyde (CAL) and l-Arginine/l-Lysine (l-Arg/l-Lys), there is an urgent need to develop new sensing strategies to meet the severe challenges currently facing. Herein, nitrogen-doped carbon dots (N-CDs) were developed using a simple one-pot hydrothermal carbonization method. These N-CDs exhibited numerous distinctive characteristics such as excellent photoluminescence, high water dispersibility, favorable biocompatibility, and superior chemical inertness. Strikingly, the as-prepared CDs as a highly efficient fluorescent probe possessed significant sensitivity and selectivity toward CAL and l-Arg/l-Lys over other analytes with a low detection limit of 58 nM and 16 nM/18 nM, respectively. The fluorescence of N-CDs could be quenched by CAL through an electron transfer process. Then, the strong electrostatic interaction between l-Arg/l-Lys and N-CDs induced the efficient fluorescence recovery. More importantly, the outstanding biosafety and excellent analyte-responsive fluorescence characteristics of N-CDs have also been verified in living cells as well as in serum and urine. Overall, the N-CDs had a wide application prospect in the diagnosis of amino acid metabolic diseases and small molecule drug sensing.

The synthesis of carbon dots by folic acid and utilized as sustainable probe and paper sensor for Hg2+ sensing and cellular imaging

In this work, the blue emission carbon dots (FA-CDs) are synthesized by one-pot solvothermal method by using folic acid as precursor. The FA-CDs emitted bright emission at 445 nm when excited at 360 nm with the QY of 31.2 %. The FA-CDs exhibit sensitive quenching response to Hg²⁺ with variable concentrations systematically, which determined FA-CDs can be employed as fluorescent probe, with a reliable linear relationship between fluorescence intensity and Hg²⁺ concentration, and a limit of detection (LOD) of 1.29 nM. Notably, the quenched FA-CDs can be recovered by using EDTA saturated solution with the emission comparable to initial in succession. The FA-CDs based paper sensor can be explored with similar detection performance, and it can also be restored by EDTA saturated solution. Both the restored CDs and paper sensor can be reused in the next turn for detecting Hg²⁺, which allowed the FA-CDs and their paper sensor can be serviced as sustainable probe for Hg²⁺ detection. The visual LOD of paper sensor can be determined at 0.1 μM, notably, the paper sensor can be reused at least 3 times with good performance, which is beneficial to environmental protection and saving resources. Possess excellent water solubility and non-toxic properties, the cellular imaging of FA-CDs was evaluated with excellent quality fluorescent image results. The FA-CDs provide a promising convenient fluorescent probe for multi-application in detection and imaging.

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

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

A Highly Sensitive and Selective Nano-Fluorescent Probe for Ratiometric and Visual Detection of Oxytetracycline Benefiting from Dual Roles of Nitrogen-Doped Carbon Dots

The specific detection of oxytetracycline (OTC) residues is significant for food safety and environmental monitoring. However, rapid specific determination of OTC from various tetracyclines is still challenging due to their similar chemical structures. Here, nitrogen-doped carbon dots (NCDs) with excitation and pH-dependent optical properties and a high-fluorescence quantum yield were successfully synthesized, which were directly employed to fabricate a dual-response fluorescence probe by self-assembly with Eu³⁺ (NCDs/Eu³⁺) for the ratiometric determination of OTC. The addition of OTC into the probe greatly enhances the characteristic emission of Eu³⁺ due to the "antenna effect", and the incorporation of NCDs into the probe further improves the Eu³⁺ fluorescence by remarkably weakening the quenching effect caused by H₂O molecules and efficiently shortening the distance of energy transfer from OTC to Eu³⁺. Meanwhile, the fluorescence of NCDs apparently decreases due to aggregation-caused quenching. The results demonstrate that a ratiometric detection of OTC (0.1-25 µM) with a detection limit of 29 nM based on the double response signals is achieved. Additionally, visual semi-quantitative assay of OTC can be realized with the naked eye under a 365 nm UV lamp according to the fluorescence color change of the as-fabricated probe. This probe exhibits acceptable specificity and anti-interference for OTC assay, holding promise for the fast detection of OTC in real water and milk samples.

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.

Carbon dots combined with masking agent for high selectivity detection of Cr(VI) to overcome interference associated challenges

Hexavalent chromium (Cr(VI)) determination is of great importance to the public health because of its extensive sources and high toxicity. However, interference from non-target ions and complex matrix remains challenges for Cr(VI) detection. In this work, we constructed a novel sensing system for high selectivity detection of Cr(VI), which is composed of strong emitting carbon dots (CE-CDs) and a specific masking agent. The detection conditions, anti-interference capability and the sensing and masking mechanisms of CE-CDs-based sensing method were systematically investigated. The results revealed that the optimal detection conditions included pH 4-10, reaction time 180 s and CE-CDs concentration 18 mg/L. Under optimal conditions, the linear range of the method was up to 500 µm, and the detection limit was as low as 23 nM. In addition, the interference of Hg(II) can be accurately eliminated by using DMPS, an effective masking agent. During the sensing process, inner filter effect and ion-molecular interaction between Cr(VI) and CE-CDs accounted for the fluorescence quenching mechanism, while the efficient masking was attributed to the strong coordination interaction between Hg(II) and DMPS. Most notably, this method had broad applicability, even for the trace detection of Cr(VI) in colored leather with complex matrix. These findings indicate that this approach is expected to open up new avenues for Cr(VI) detection.

Lanthanide Molecular Species Generated Fe3O4@SiO2-TbDPA Nanosphere for the Efficient Determination of Nitrite

The presence of nitrite (NO₂⁻) in water and food leads to serious problems in public health and the environment. Therefore, it is important to develop a rapid and efficient method for the selective detection of NO₂⁻. In this work, the synthesis and characterization of magnetic Fe₃O₄@SiO₂-TbDPA nanoprobe have been carried out. The Fe₃O₄@SiO₂-TbDPA aqueous solution exhibits a strong green emission. Due to the addition of various concentrations of NO₂⁻ (0–100 μM), the fluorescence intensity has been suppressed. The nanoprobe Fe₃O₄@SiO₂-TbDPA exhibits excellent selectivity and sensitivity toward NO₂⁻ ions. Excellent linearity is obtained in the range of 5–80 μM with a detection limit of 1.03 μM. Furthermore, the presence of magnetic Fe₃O₄ nanoparticles in Fe₃O₄@SiO₂-TbDPA nanospheres will also facilitate the effective separation of Fe₃O₄@SiO₂-TbDPA from the aqueous solution. Our proposed strategy is expected to fabricate an organic-inorganic hybrid magnetic nanomaterial and can be used as an efficient sensor. It has been shown that this new strategy has numerous advantages, such as high stability, selectivity, and simplicity of operation. It demonstrates great potential for simple and convenient NO₂⁻ detection. It may expand to a variety of ranges in environmental monitoring and biomedical fields.

Intelligent multicolor nano-sensor based on nontoxic dual fluoroprobe and MOFs for colorful consecutive detection of Hg2+ and cysteine

Ultrasensitive detection of Hg²⁺ in aquatic ecosystems is of great significance due to its high toxicity and ubiquity in water. Herein, using a one-step in-situ synthesis method, blue fluorescent carbon dots (CDs), red fluorescent InP/ZnS quantum dots (InPQDs), and MOFs (ZIF-8) integrated multicolor nano-sensor CDs/InPQDs@ZIF-8 was constructed for consecutive visual detection of Hg²⁺ and Cys. The InPQDs can act as the response unit for Hg²⁺ and Cys, with the limit of detection (LOD) of 8.68 and 37.96 nM, respectively. Significantly, the low detection limit combines with good specificity and accuracy of the nano-sensor meet the requirement for the safety monitoring and control of Hg²⁺ in drinking and environmental water. Moreover, the color recognition and processing software installed on smart phone can realize the real-time and rapid sensing of Hg²⁺ and Cys. A logic gate circuit was also devised, providing the possibilities for the application of the nano-sensor in the field of intelligent devices. As far as we know, this was the first example to apply InPQDs to the continuous multicolor visual detection of Hg²⁺ and Cys, which provided reference for the construction of environmentally-friendly dual emission fluorescent sensors for hazardous substance monitoring.

Colloidal synthesis of perovskite-type lanthanum aluminate incorporated graphene oxide composites: Electrochemical detection of nitrite in meat extract and drinking water

A novel electrochemical method has been developed for determination of nitrite using La-based perovskite-type lanthanum aluminate nanorod-incorporated graphene oxide nanosheets (LaAlO₃@GO). Morphological and structural analyses of the prepared perovskite-type electrocatalyst, with and without a glassy carbon electrode (GCE) surface, were performed using various techniques, including transmission electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffractometry, Raman spectroscopy, and electrochemical impedance spectroscopy. Under optimal conditions, the LaAlO₃@GO composite-modified GCE (LaAlO₃@GO/GCE) exhibited excellent electrocatalytic performance toward the electrooxidation of nitrite (pH = 7.0), with a significant increase in anodic peak currents compared with the bare GCE. Using amperometry, the fabricated sensor exhibited a wide nitrite determination range from 0.01 to 1540.5 µM, with a detection limit of 0.0041 µM. Notably, the proposed LaAlO₃@GO/GCE electrode demonstrated a good nitrite detection performance in different meat and water samples. In addition, the LaAlO₃@GO/GCE electrode displayed excellent selectivity, repeatability, reproducibility, storage, and operational stability toward nitrite detection.

A rotary multi-positioned cloth/paper hybrid microfluidic device for simultaneous fluorescence sensing of mercury and lead ions by using ion imprinted technologies

A novel rotary cloth/paper hybrid microfluidic analytical device (μCPAD) was proposed via the synergy of the fluorescence sensing cloth-based component and rotary paper-based microfluidic analytical device (μPAD) for simultaneous detection of mercury (Hg²⁺) and lead (Pb²⁺) ions. Fluorescence sensing cloth-based component was prepared by grafting quantum dots onto cotton cloth and then modifying with ion imprinted polymers (IIP). Because the cloth has good ductility and durability, it can bear strong oscillation during the fabrication of grafting quantum dots and IIP, and brings a lot of convenience to the production process. At the same time, because rotary μCPAD was stacked by three-layer papers with designed hydrophilic channels and hydrophobic barriers, it could realize simultaneous detection of Hg²⁺ and Pb²⁺ ions by rotating top layer counterclockwise or clockwise. The fluorescence signals were obtained through quantum dots' electron transfer fluorescence quenching effect with the limits of detection were 0.18 and 0.07 μg/L, respectively. This method successfully realized the transference of specific and sensitive fluorescence sensing materials (quantum dots) onto the microfluidic device to improve the portability and expanded applications. Moreover, the novel microfluidic device may have great potential in point-of-care testing of heavy metal ions in environmental monitoring fields.

Quantification of 2-chlorohydroquinone based on interaction between N-doped carbon quantum dots probe and photolysis products in fluorescence system

As a member of chlorophenolic compounds, 2-chlorohydroquinone (H₂QCl) has been widely used as intermediates in various chemical industries and leaded to serious threat on the environment. It is urgent to develop simple and robust analytical method for sensitive and selective determination of H₂QCl. Carbon quantum dots (CQDs), a promising photoluminescence nanomaterial, have gained sufficient concern as optical sensors owing to their outstanding photochemical properties. In this work, nitrogen doped carbon quantum dots (N-CQDs) were successfully synthesized by a simple secondary hydrothermal method and applied as a fluorescent probe for the quantitation of H₂QCl. A new fluorescence region centered at excitation wavelength of 310 nm and emission wavelength of 390 nm appeared after nitrogen doping. It was found that the N-CQDs exhibited a high selectivity towards H₂QCl with sensitive fluorescence response and the fluorescence quenching of N-CQDs was linear with the concentration of H₂QCl in the range of 30-90 μM (Y = 0.0049X + 0.1255, R² = 0.996). This is the first time that the dual role of excitation light was observed in the fluorescence detection system. The ultraviolet light acted as not only the excitation energy source for N-CQDs photoluminescence, but also the light source for photolysis of H₂QCl. In the detection process, H₂QCl was degraded to p-benzoquinone by light, and then the CQDs combined with p-benzoquinone through Michael addition reaction under the action of doped nitrogen. The electron transfer from N-CQDs to the linked p-benzoquinone caused the quenching of fluorescence originated from the edge state of N-CQDs. Furthermore, this established method can be applied for the quantitative determination of H₂QCl in environmental water samples with satisfactory recoveries between 94.31 and 105.51%.

Silicon nanoparticles / gold nanoparticles composite as a fluorescence probe for sensitive and selective detection of Co2+ and vitamin B12 based on the selective aggregation and inner filter effect

Cobalt as a transition metal ion is a biologically essential trace element, and plays an important role in various biological systems. The silicon nanoparticles (SiNPs) / gold nanoparticles (AuNPs) composite as a simple and efficient fluorescent probe was developed to detect Co²⁺ and vitamin B₁₂ (VB₁₂) based on the selective aggregation and inner filter effect (IFE). The green-emitting SiNPs were synthesized by one-pot hydrothermal method, and the AuNPs were synthesized and modified with thioglycolic acid and cetyltrimethylammonium bromide. The fluorescent probe was fabricated by simple mixing the SiNPs and AuNPs together. In the presence of Co²⁺/VB₁₂, AuNPs are selectively aggregated, which results in the enhancement of the local surface plasmon resonance absorption centered at 520 nm and the green fluorescence of SiNPs is significantly quenched via IFE. The fluorescence quenching efficiency of the probe is linearly proportional to the concentration of Co²⁺ in the range of 0.1-80 µM with a low detection limit of 60 nM, which is far lower than the guideline value of Co²⁺ in drinking water (1.7 µM). For vitamin B₁₂ (VB₁₂), its linear relationship is in the range of 0.1-100 µM, and the limit of detection is 69 nM. Furthermore, the proposed method shows good selectivity for the detection of Co²⁺ and VB₁₂, and does not need sophisticated pretreatment, only through simple filter. It has been applied in actual environmental water samples and drug tablets with satisfactory results.

A Fluorescent Nanosensor for Silver (Ag+) and Mercury (Hg2+) Ions Using Eu (III)-Doped Carbon Dots

Carbon dots doped with Eu³⁺ ions (Eu-Cdots) were prepared by a hydrothermal treatment, using citric acid and urea as precursors and Eu (NO₃)₃ as a europium source. The Eu³⁺ ions are strongly coordinated with the carboxylate groups at the surface of the Cdots and incorporated within the nanographene network in the carbon core. Vibrational spectroscopy provides evidence of such interaction with identification of bands assigned to the stretching of the Eu-O bond. Eu³⁺ doped Cdots have larger diameters then undoped Cdots, but they are divided into smaller domains of sp² carbon. The UV-vis excitation spectrum provides evidence of energy transfer from the Cdots to the Eu³⁺. The luminescence spectrum shows the characteristic sharp peaks of Eu³⁺ in the red part of the visible spectrum and a broad emission of Cdots centered at 450 nm. The luminescence of the Cdots is strongly quenched by Hg²⁺ and Ag⁺, but not by other cations. The quenching mechanism differs significantly depending on the nature of the ion. Both the blue emission of Cdots and the red emission of Eu³⁺ are quenched in the presence of Hg²⁺ while only the emission of the Cdots is affected by the presence of Ag⁺. A ratiometric sensor can be built using the ratio of luminescence intensities of the Cdots to the Eu³⁺ peaks.

A Tb3+ functionalized triazine-porous organic framework as a ratiometric fluorescent sensor for determination of ciprofloxacin in aquatic products

A Tb3+ functionalized triazine-porous organic framework (Tb3+/TAPOF) was prepared by introducing Tb3+ into a triazine-porous organic framework through a coordination bond.

Innovative ratiometric optical strategy: Nonconjugated polymer dots based fluorescence-scattering dual signal output for sensing mercury ions

A new ratiometric platform was developed for sensing Hg²⁺, which combined fluorescence and scattering simultaneously. This ratiometric strategy reflected superiorities over conventional methods, since the two independent signals at irrelevant categories meet the requirements of sufficient wavelength separation, stimulation under one excitation, and collection on single instrument. Herein, nonconjugated polymer dots (N-PDs) were served as the recognition unit for Hg²⁺ with turn-off fluorescence and turn-on scattering. Additionally, two signal collection tactics were proposed to achieve fluorescence and scattering in a window: one was to record down-conversion fluorescence and second-order scattering spectra (FL@SOS), and the other was to gather the fluorescence excited by second-order diffraction light and first-order scattering (SODL-FL@FOS). This ratiometric sensor exhibited outstanding performance toward Hg²⁺ in the range of 0.1-50 μM with the detection limit of 27 nM. By contrast, the present proposal provided a more ingenious and scalable way to construct ratiometric sensor than traditional approach.

AIEgen-Based Lifetime-Probes for Precise Furin Quantification and Identification of Cell Subtypes

Biochemical sensing probes based on aggregation-induced-emission luminogens (AIEgens) are widely used in biological imaging and therapy, chemical sensing, and material sciences. However, it is still a great challenge to quantify the targets through fluorescence intensity of AIEgen probes due to their undesirable aggregations. Here, a PyTPA-ZGO probe with three lifetime signals for precise quantification of furin is constructed: the lifetime signal 1 and signal 2 comes from AIEgen PyTPA-P (τ_Pn ) and inorganic nanoparticles Zn₂ GeO₄ :Mn²⁺ -NH₂ (τ_Zn ), respectively, while the lifetime signal 3 is marked as the composite dual-lifetime signal (CDLS_n , C D L S n = τ Z n τ P n ). In contrast, the fluorescence intensity signal of PyTPA-P shows defectively quantitative performance. Furthermore, it is found that the CDLS_n exhibits higher significant differences than the two other lifetime signals (τ_Pn and τ_Zn ) thanks to its wide range between the maximum and minimum signal values and small standard deviation. Therefore, CDLS_n is further used to accurately identify cell subtypes based on the specific concentration of furin in each subtype. The lifetime criterion can realize precise quantification, and it should be a promising direction of AIEgen-based quantitative analysis in the future.

Fluorescent nitrogen-doped carbon dots for high selective detecting p-nitrophenol through FRET mechanism

A facile, friendly and one-step hydrothermal protocol was used to synthesize nitrogen-doped carbon dots (N-CDs) by utilizing hexamethylenetetramine and ethanediamine as the carbon and nitrogen sources. It demonstrated good water solubility and fluorescence properties were stable, whether in acidic or alkaline. Quantum yield (QY) of N-CDs was 8.3% at an excitation wavelength of 325 nm with maximum emission at 425 nm. The fluorescence of N-CDs achieved very high fluorescence quenching of 60% in the detection of p-nitrophenol (p-NP) in aqueous medium via fluorescence resonance energy transfer (FRET) mechanisms. Under optimum conditions, fluorescence probs of N-CDs had strong selectivity to p-NP, and the fluorescence intensity was linearly proportional to p-NP concentration from 0.5 to 70.0 μM with a detection limit of 0.201 μM. The corresponding cell experiments were also performed, indicating that the prepared N-CDs possessed low cytotoxicity and good biocompatibility. Meanwhile, the N-CDs can be used for the determination of p-NP in river water and industrial wastewater.

Ultrasensitive colorimetric strategy for Hg2+ detection based on T-Hg2+-T configuration and target recycling amplification

A novelty aptasensor for ultrasensitive detection of Hg²⁺ is developed, exploiting the combination of plasmonic properties of gold nanoparticles (AuNPs) and exonuclease III (Exo III)-assisted target recycling for signal amplification. In the presence of Hg²⁺, a DNA duplex can be formed due to the strong coordination of Hg²⁺ and T bases of single-stranded DNA (ssDNA) probe. Exo III digests the DNA duplex from the 3' to 5' direction, resulting in the releasing of Hg²⁺. Then, the released Hg²⁺ binds with another ssDNA probe through T-Hg²⁺-T coordination. After Exo III-assisted Hg²⁺ cycles, numerous ssDNA probes are exhausted, which promotes poly(diallyldimethylammonium chloride) (PDDA)-induced AuNP aggregation, leading to an obvious color change and aggregation-induced plasmon red shift of AuNPs (from 520 to 610 nm). Therefore, this biosensor is ultrasensitive, which is applicable to the detection of trace level of Hg²⁺ with a linear range from 5 pM to 0.6 nM and an ultralow detection limit of 0.2 pM. Furthermore, it enables visual detection of Hg²⁺ as low as 50 pM by the naked eye. More importantly, the assay can be applied to the reliable determination of spiked Hg²⁺ in sea water samples with good recovery.

A mitochondria-targeted fluorescent probe for monitoring endogenous cysteine in living cells and zebrafish

At the organelle level, pathogenesis due to abnormal concentrations of cysteine (Cys) is of great significance for the early diagnosis and treatment of related diseases. Generally speaking, organelle localization requires the participation of specific target groups, which increases the difficulty of synthesis. Herein, through simple synthesis, a novel biflavone derivative (BFD) that exhibits excited-state intramolecular proton transfer (ESIPT) was obtained and successfully located in mitochondria without target groups. The probe BFD can distinguish Cys from Hcy and GSH with a rapid response (< 5 s) and showed visual detection for Cys with a large Stokes shift (about 260 nm). Because of its nanomorphology in solution and surface functional groups, the probe BFD can enter the cell smoothly and achieve mitochondrial localization. Owing to its excellent optical performance, the probe BFD was successfully applied to the imaging of endogenous Cys in HeLa cells and zebrafish.

A novel carbon dot/polyacrylamide composite hydrogel film for reversible detection of the antibacterial drug ornidazole

A carbon dot/polyacrylamide (CDs/PAM) composite hydrogel film with stable fluorescence performance was fabricated by merging a hydrogel film and carbon dots (CDs) with blue fluorescence, which were prepared by hydrothermal synthesis using anhydrous citric acid and acrylamide as carbon sources. The obtained CDs/PAM composite hydrogel film exhibited a good fluorescence quenching effect on ornidazole (ONZ), and can be used for the quantitative detection of ONZ. In the ONZ concentration range of 5–60 μM, a good linear relationship between the fluorescence quenching efficiency of the CDs/PAM composite hydrogel film and the concentration of ONZ solution was obtained with a low detection limit of 2.35 μM. In addition, the detection system has good selectivity and strong anti-interference capacity, and can be used in repeated cycles for detection.

A novel carbon dot/polyacrylamide composite hydrogel film with stable blue fluorescence performance was fabricated by merging a hydrogel film and carbon dots, which was used for highly selective and quantitative detection of ONZ in real samples.

Spectroscopic detection of Hg2+ in water samples using fluorescent carbon quantum dots as sensing probe

Mercury (Hg2+) is remarked as toxic and hazardous element to global environment. Here, carbon quantum dots (CQDs) were synthesized by simple microwave assisted technique for Hg2+ detection in water samples via. fluorescence quenching and FT-IR spectroscopic approach. The morphology and chemical structure of synthesized CQDs was investigated by TEM, FT-IR, 13C-NMR, fluorescence and UV-vis spectroscopic technique. The resultant CQDs bears spherical morphology with an average size of 2–4 nm. The binding parameters, as Stern-Volmer quenching constant (Ksv) and binding constant for CQDs-Hg system was investigated by fluorescence method, whereas UV-vis techniques was employed for determination of thermodynamic parameters, as Gibb’s free energy (ΔG), enthalpy (ΔH) and entropy (ΔS) at three different temperature (295, 298 and 305 K). Moreover, selectivity assay for Hg2+ detection has been studied in presence of other metal ions by FT-IR as well as fluorescence spectroscopy. Analytical assay was also successfully applied for Hg2+ in spiked water samples collected from different areas of Chhattisgarh, with 98–99 recovery %. The detection of Hg2+ has been demonstrated in the range of 0 to 5.0μM with 3.25 nM detection limit. The present method is found to be simple, highly sensitive and selective for sensing of Hg2+ in aquatic environmental samples using CQDs as sensing probe.

Chitosan-derived N-doped carbon dots for fluorescent determination of nitrite and bacteria imaging

N-doped carbon dots (N-CDs) were successfully synthesized via simple one-step hydrothermal carbonization using chitosan as carbon and nitrogen sources. The obtained N-CDs contained a variety of functional groups on the NCDs surface, and exhibited excitation-independent behavior and strong blue fluorescence with a relatively higher fluorescence quantum yield (QY = 35%). It also presented excellent water solubility, resistance to pH change, high ion strength and UV irradiation. Since the fluorescence of the N-CDs could be selectively quenched by NO₂^-, they could act as a fluorescent sensor for the determination of NO₂^- in real tap water and lake water samples with a wide linear range (1-500 μM) and low detection limit (0.1 μM). They could also be used for bacterial imaging as multicolor fluorescent probes. The results indicated that N-CDs could be a promising candidate material for biomedical applications.

Copper nanoclusters with/without salicylaldehyde-modulation for multifunctional detection of mercury, cobalt, nitrite and cyanide ions in aqueous solution and bioimaging

The sensitive determination of multiple heavy metal ions and toxic anions is important in biological and environmental fields. Here we report a facile strategy to construct a multifunctional chemosensor for the detection of Hg²⁺, [Formula: see text]Co²⁺, and CN^- in aqueous solution based on the fluorescent copper nanoclusters (Cu NCs). It was interesting to find that salicylaldehyde (SA) could effectively modulate the fluorescence property and sensing behavior of Cu NCs. In the absence of SA, Cu NCs showed 'on-off' fluorescence responses at the addition of Hg²⁺ and [Formula: see text] under different quenching mechanisms. Upon the presence of SA, Cu NCs exhibited a strong intramolecular charge transfer emission at 500 nm, accompanied by the decrease of the initial fluorescence of Cu NCs at 430 nm. This fluorescence on-state of Cu NC-SA at 500 nm was found to be exclusively turned off by Co²⁺ and enhanced by CN^-. Spectroscopy results combined with thermodynamic analysis provided sufficient information to deduce the sensing mechanisms. Finally, the Cu NCs showed high biocompatibility and were able to be used for fluorescence bioimaging in living cells. This study provided a novel and simple strategy to construct the multifunctional chemosensors for bioanalytical applications.

When rare earth meets carbon nanodots: mechanisms, applications and outlook

Rare earth (RE) elements are widely used in the luminescence and magnetic fields by virtue of their abundant 4f electron configurations. However, the overall performance and aqueous stability of single-component RE materials need to be urgently improved to satisfy the requirements for multifunctional applications. Carbon nanodots (CNDs) are excellent nanocarriers with abundant functional surface groups, excellent hydrophilicity, unique photoluminescence (PL) and tunable features. Accordingly, RE-CND hybrids combine the merits of both RE and CNDs, which dramatically enhance their overall properties such as luminescent and magnetic-optical imaging performances, leading to highly promising practical applications in the future. Nevertheless, a comprehensive review focusing on the introduction and in-depth understanding of RE-CND hybrid materials has not been reported to date. This review endeavors to summarize the recent advances of RE-CNDs, including their interaction mechanisms, general synthetic strategies and applications in fluorescence, biosensing and multi-modal biomedical imaging. Finally, we present the current challenges and the possible application perspectives of newly developed RE-CND materials. We hope this review will inspire new design ideas and valuable references in this promising field in the future.