Reversible "Off-On" Fluorescence of Zn2+-Passivated Carbon Dots: Mechanism and Potential for the Detection of EDTA and Zn2

An artificial intelligence handheld sensor for direct reading of nickel ion and ethylenediaminetetraacetic acid in food samples using ratiometric fluorescence cellulose paper microfluidic chip

User-friendly in-field sensing protocol is crucial for the effective tracing of intended analytes under less-developed countries or resources-limited environments. Nevertheless, existing sensing strategies require professional technicians and expensive laboratory-based instrumentations, which are not capable for point-of-care on-site analyses. To address this issue, artificial intelligence handheld sensor has been designed for direct reading of Ni2+ and EDTA in food samples. The sensing platform incorporates smartphone with machine learning-driven application, 3D-printed handheld device, and cellulose paper microfluidic chip stained with ratiometric red-green-emission carbon dots (CDs). Intriguingly, Ni2+ introduction makes green fluorescent (FL) of CDs glow but red FL fade because of the coordination of Ni2+ with CDs verified by density functional theory (DFT), concurrently manifesting continuous FL colour transition from red to green. Subsequent addition of EDTA renders FL of CDs-Ni2+ recover owing to the capture of Ni2+ from CDs by EDTA based on strong chelation effect of EDTA on Ni2+ confirmed via DFT, accompanying with a noticeable colour returning from green to red. Inspired by above FL phenomena, CDs-based cellulose paper microfluidic chips are first fabricated to facilitate point-of-care testing of Ni2+ and EDTA. Designed fully-automatic handheld sensor is utilized to directly output Ni2+ and EDTA concentration in water, milk, spinach, bread, and shampoo based on wide linear ranges of 0-48 μM and 0-96 μM, and low limits of detection of 0.274 μM and 0.624 μM, respectively. The proposed protocol allows for speedy straightforward on-site determination of target analytes, which will trigger the development of automated and intelligent sensors in near future.

An Introductory Overview on Applications of Pyrazoles as Transition Metal Chemosensors

Utility of pyrazoles and their derivatives in constructing ordered porous materials with physicochemical characteristics such as chemosensors has undoubtedly created much interest in developing newer frameworks. A variety of pyrazole based chemosensors are known for their remarkable photophysical, pH sensitivity, solvatochromic, ion detection, high quantum yields and nonlinear optical behavior. Many of the transition metals have shown beneficial biological effects in biological systems. There is always a need of continuous monitoring to maintain an adequate range of all and specifically for the toxic ones like mercury. Pyrazoline nanoparticle probes have been reported for sensing/detection of Hg2+ions. Pyridinyl pyrazoline and benzimidazolyl pyrazole derived sensors are more selective and sensitive towards Zn2+and Fe3+ ions respectively. Pyrazole derived metal organic frameworks (MOF's) have been reported for environmental monitoring and biological imaging. Keeping in view of the enormous synthetic and biological importance of pyrazoles, herein, we are presenting an overview on applications of pyrazoles in transition metal chemosensors.

Carbon Quantum Dots: Properties, Preparation, and Applications

Carbon quantum dots are a novel form of carbon material. They offer numerous benefits including particle size adjustability, light resistance, ease of functionalization, low toxicity, excellent biocompatibility, and high-water solubility, as well as their easy accessibility of raw materials. Carbon quantum dots have been widely used in various fields. The preparation methods employed are predominantly top-down methods such as arc discharge, laser ablation, electrochemical and chemical oxidation, as well as bottom-up methods such as templates, microwave, and hydrothermal techniques. This article provides an overview of the properties, preparation methods, raw materials for preparation, and the heteroatom doping of carbon quantum dots, and it summarizes the applications in related fields, such as optoelectronics, bioimaging, drug delivery, cancer therapy, sensors, and environmental remediation. Finally, currently encountered issues of carbon quantum dots are presented. The latest research progress in synthesis and application, as well as the challenges outlined in this review, can help and encourage future research on carbon quantum dots.

Opposite regulation effects of Al3+ on different types of carbon quantum dots and potential applications in information encryption

Regulating the photoluminescence (PL) of carbon quantum dots (CQDs) through ion modification is a well-established and effective approach. Herein, we report the opposite regulation effects of Al3+ ions on the PL properties of two distinct types of CQDs (graphene quantum dots, GQDs, and nitrogen-doped carbon quantum dots of 2,3-diaminophenazine, DAP), and elucidate the underlying mechanism of the binding of Al3+ ions to different PL sites on CQDs by employing ultraviolet-visible spectroscopy, X-ray photoelectron spectroscopy, and density functional theory calculations. Specifically, Al3+ ions are primarily situated around the oxygen-containing groups, which do not impact the π-π regions of GQDs. However, Al3+ ions are preferentially adsorbed on the top of pyridine nitrogen in the phenazine rings of DAP, thus reducing the PL regions of DAP. Based on the opposite PL effects of Al3+ on GQDs and DAP, we explore potential applications of information encryption and successfully realize multi-level information encryption and decryption, which may provide new strategies for CQDs in information security.

Fe-doped carbon dots: a novel biocompatible nanoplatform for multi-level cancer therapy

BACKGROUND

Tumor treatment still remains a clinical challenge, requiring the development of biocompatible and efficient anti-tumor nanodrugs. Carbon dots (CDs) has become promising nanomedicines for cancer therapy due to its low cytotoxicity and easy customization.

RESULTS

Herein, we introduced a novel type of "green" nanodrug for multi-level cancer therapy utilizing Fe-doped carbon dots (Fe-CDs) derived from iron nutrient supplement. With no requirement for target moieties or external stimuli, the sole intravenous administration of Fe-CDs demonstrated unexpected anti-tumor activity, completely suppressing tumor growth in mice. Continuous administration of Fe-CDs for several weeks showed no toxic effects in vivo, highlighting its exceptional biocompatibility. The as-synthesized Fe-CDs could selectively induce tumor cells apoptosis by BAX/Caspase 9/Caspase 3/PARP signal pathways and activate antitumoral macrophages by inhibiting the IL-10/Arg-1 axis, contributing to its significant tumor immunotherapy effect. Additionally, the epithelial-mesenchymal transition (EMT) process was inhibited under the treatment of Fe-CDs by MAPK/Snail pathways, indicating the capacity of Fe-CDs to inhibit tumor recurrence and metastasis.

CONCLUSIONS

A three-level tumor treatment strategy from direct killing to activating immunity to inhibiting metastasis was achieved based on "green" Fe-CDs. Our findings reveal the broad clinical potential of Fe-CDs as a novel candidate for anti-tumor nanodrugs and nanoplatform.

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.

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.

Emerging and Versatile Platforms of Metal-Ion-Doped Carbon Dots for Biosensing, Bioimaging, and Disease Therapy

Metal ions possess abundant electrons and unoccupied orbitals, as well as large atomic radii, whose doping into carbon dots (CDs) is a facile strategy to endow CDs with additional physicochemical characteristics. After being doped with metal ions, CDs reveal obvious changes in their optical, electronic, and magnetic properties by adjustments to their electron density distribution and the energy gaps, leading them to be promising and competitive candidates as labeling probes, imaging agents, catalysts, nanodrugs, and so on. In this review, we summarize the fabrication methods of metal-ion-doped CDs (M-CDs), and highlight their biological applications including biosensing, bioimaging, tumor therapy, and anti-microbial treatment. Finally, the challenging future perspectives of M-CDs are analyzed. We hope this review will provide inspiration for further development of M-CDs in various biological aspects, and help readers who are interested in M-CDs and their biological applications.

Fluorescent Sensing of Ciprofloxacin and Chloramphenicol in Milk Samples via Inner Filter Effect and Photoinduced Electron Transfer Based on Nanosized Rod-Shaped Eu-MOF

Rapid, facile, and accurate detection of antibiotic residues is vital for practical applications. Herein, we designed a sensitive, visual, and rapid analytical method for sensitive detection of ciprofloxacin and chloramphenicol based on a nanosized rod-shaped Europium metal organic framework (Eu-MOF). The fluorescent Eu-MOF was firstly synthesized by a simple synthetic route at room temperature, which displays a red emission. The mechanisms of detecting ciprofloxacin and chloramphenicol were confirmed to be the inner filter effect (IFE) and photoinduced electron transfer (PET). Under the optimized experimental conditions, the detection limits of the developed method for ciprofloxacin and chloramphenicol detection were 0.0136 and 3.16 μM, respectively. Moreover, the sensor was effectively applied for quantitative determination of ciprofloxacin and chloramphenicol milk samples with satisfactory recoveries of 94.5-102% and 97-110%, respectively. This work developed a new method for rapid detection of ciprofloxacin and chloramphenicol residues. In addition, the established method has potential practical application value for on-site safety regulation on antibiotic residues in animal-derived food.

Co, N co-doped porous carbon-based nanozyme as an oxidase mimic for fluorescence and colorimetric biosensing of butyrylcholinesterase activity

A Co, N co-doped porous carbon-based nanozyme (Co-N-C nanozyme) has been fabricated. Taking advantages of the excellent oxidase catalytic activity and significant stability of Co-N-C nanozyme, we propose a fluorescence and colorimetric system based on Co-N-C nanozyme and red-emitting carbon quantum dots (RCDs) for butyrylcholinesterase (BChE) sensing. As the chromogenic substrate 3,3',5,5'-tetramethylbenzidine (TMB) was catalyzed and oxidized by Co-N-C nanozyme, the generated oxTMB had a new absorption peak at 652 nm, which resulted in the significant quenching of the fluorescence of the carbon quantum dots at 610 nm. Under the catalysis of BChE, thiocholine was generated from the hydrolysis of S-butyrylthiocholine iodide (BTCh), and the as-generated thiocholine effectively inhibited the oxidation of TMB catalyzed by Co-N-C nanozyme, leading to a decrease of the absorption of oxTMB at 652 nm and effective fluorescence recovery of RCDs. By measuring the absorbance of produced oxTMB at 652 nm and the fluorescence of RCDs at 610 nm, the fluorescence and colorimetric system both exhibited an outstanding linear response to the activity of BChE in the range 0.5 to 40 U L^-1, with a detection limit of 0.16 U L^-1 and 0.21 U L^-1, respectively. Furthermore, this established dual-channel biosensing strategy has been successfully applied to the determination of BChE in human serum samples. The present work has effectively expanded the development and application of nanozyme in biosensing.

Carbon dots-based nanomaterials for fluorescent sensing of toxic elements in environmental samples: Strategies for enhanced performance

Rapid industrialization and manufacturing expansion have caused heavy metal pollution, which is a critical environmental issue faced by global population. In addition, the disadvantages presented by conventional detection methods such as the requirement of sophisticated instruments and qualified personnel have led to the development of novel nanosensors. Recently, carbon dots (CDs) have been presented as a multifunctional nanomaterial alternative for the accurate detection of heavy metal ions in water systems. The capacity of CDs to detect contaminants in wastewater -including heavy metals- can be found in the literature; however, to the best of our knowledge, none of them discusses the most recent strategies to enhance their performance. Therefore, in this review, beyond presenting successful examples of the use of CDs for the detection of metal ions, we further discuss the strategies to enhance their photoluminescence properties and their performance for environmental monitoring. In this manner, strategies such as heteroatom-doping and surface passivation are reviewed in detail, as well as describing the mechanisms and the effect of precursors and synthesis methods. Finally, the current challenges are described in detail to propose some recommendations for further research.

Analysis of Mn2+ and Zn2+ Ions in Macroalgae with Heteroelement-Doped Carbon-Based Fluorescent Probe

Kelp and laver are large economic macroalgae in China, which are rich in nutrients, especially Mn and Zn. Excessive intake of Mn and Zn can be harmful to the human body. Therefore, it is necessary to develop a convenient and efficient method to detect the contents of Mn and Zn in macroalgae. In this experiment, red carbon dots (R-CDs) doped with N and S elements were prepared by the thermal solvent method. The obtained R-CDs displayed excitation wavelength-independent fluorescent emission in the red spectral region. The R-CDs were used to construct a fluorescent probe for specific recognition of Mn²⁺ and Zn²⁺, achieving high-sensitivity detection of Mn²⁺ and Zn²⁺. The detection results showed a good linear relationship between fluorescence intensity and Mn²⁺ concentration, and the calculated detection limit was 0.23 nmol/L. For the detection of Zn²⁺, the detection limit was estimated as 19.1 nmol/L. At the same time, the content distribution of Mn and Zn elements in macroalgae produced in Fujian was investigated by the constructed fluorescence probe. It was found that kelp, laver, and their products are rich in Mn and Zn elements, and the content of Mn and Zn elements in laver is higher than that in kelp, which can be used as the optimal food supplement for Mn and Zn elements.

Applications of Carbon Dots for the Photocatalytic and Electrocatalytic Reduction of CO2

The photocatalytic and electrocatalytic conversion of CO2 has the potential to provide valuable products, such as chemicals or fuels of interest, at low cost while maintaining a circular carbon cycle. In this context, carbon dots possess optical and electrochemical properties that make them suitable candidates to participate in the reaction, either as a single component or forming part of more elaborate catalytic systems. In this review, we describe several strategies where the carbon dots participate, both with amorphous and graphitic structures, in the photocatalysis or electrochemical catalysis of CO2 to provide different carbon-containing products of interest. The role of the carbon dots is analyzed as a function of their redox and light absorption characteristics and their complementarity with other known catalytic systems. Moreover, detailed information about synthetic procedures is also reviewed.

Preparation and Characterization of a Novel Amidoxime-Modified Polyacrylonitrile/Fly Ash Composite Adsorbent and Its Application to Metal Wastewater Treatment

The polyacrylonitrile/fly ash composite was synthesized through solution polymerization and was modified with NH2OH·HCl. The amidoxime-modified polyacrylonitrile/fly ash composite demonstrated excellent adsorption capacity for Zn2+ in an aqueous medium. Fourier transform-Infrared spectroscopy, thermogravimetric analysis, nitrogen adsorption, X-ray diffraction, and scanning electron microscopy were used to characterize the prepared materials. The results showed that the resulting amidoxime-modified polyacrylonitrile/fly ash composite was able to effectively remove Zn2+ at pH 4-6. Adsorption of Zn2+ was hindered by the coexisting cations. The adsorption kinetics of Zn2+ by Zn2+ followed the pseudo-second order kinetic model. The adsorption process also satisfactorily fit the Langmuir model, and the adsorption process was mainly single layer. The Gibbs free energy ΔG0, ΔH0, and ΔS0 were negative, indicating the adsorption was a spontaneous, exothermic, and high degree of order in solution system.

A ratiometric fluorescence strategy based on polyethyleneimine surface-modified carbon dots and Eosin Y for the ultrasensitive determination of protamine and trypsin

A ratiometric fluorescent nanoprobe for protamine and trypsin detection with excellent biocompatibility and high sensitivity was successfully constructed based on CDs-PEI and Eosin Y.

In Situ Femtosecond-Laser-Induced Fluorophores on Surface of Polyvinyl Alcohol for H2O/Co2+ Sensing and Data Security

In situ fluorophores were induced on polyvinyl alcohol (PVA) bulk materials by direct femtosecond laser writing. The generation of fluorophores was ascribed to localized laser-assisted carbonization. The carbonization of PVA polymers was confirmed through X-ray photoelectron spectroscopy analysis. The distinct fluorescence responses of fluorophores in various solutions were harnessed for implementing in situ reagent sensors, metal ion sensors, data encryption, and data security applications. The demonstrated water detection sensor in acetone exhibited a sensitivity of 3%. Meanwhile, a data encryption scheme and a "burn after reading" technique were demonstrated by taking advantage of the respective reversible and irreversible switching properties of the in situ laser-induced fluorophores. Taking a step further, a quantitative cobalt ion measurement was demonstrated based on the concentration-dependent fluorescence recovery. Combined with a laser-induced hydrophilic modification, our scheme could enable "lab-on-a-chip" microfluidics sensors with arbitrary shape, varied flow delay, designed reaction zones, and targeted functionalities in the future.

An Overview of the Recent Developments in Carbon Quantum Dots—Promising Nanomaterials for Metal Ion Detection and (Bio)Molecule Sensing

The fluorescent carbon quantum dots (CQDs) represent an emerging subset of carbonaceous nanomaterials, recently becoming a powerful tool for biosensing, bioimaging, and drug and gene delivery. In general, carbon dots are defined as zero-dimensional (0D), spherical-like nanoparticles with <10 nm in size. Their unique chemical, optical, and electronic properties make CQDs versatile materials for a wide spectrum of applications, mainly for the sensing and biomedical purposes. Due to their good biocompatibility, water solubility, and relatively facile modification, these novel materials have attracted tremendous interest in recent years, which is especially important for nanotechnology and nanoscience expertise. The preparation of the biomass-derived CQDs has attracted growing interest recently due to their low-cost, renewable, and green biomass resources, presenting also the variability of possible modification for the enhancement of CQDs’ properties. This review is primarily focused on the recent developments in carbon dots and their application in the sensing of different chemical species within the last five years. Furthermore, special emphasis has been made regarding the green approaches for obtaining CQDs and nanomaterial characterization toward better understanding the mechanisms of photoluminescent behavior and sensing performance. In addition, some of the challenges and future outlooks in CQDs research have been briefly outlined.

On-off-on nanosensors of carbon quantum dots derived from coal tar pitch for the detection of Cu2+, Fe3+, and L-ascorbic acid

Novel nitrogen-doped carbon quantum dots (N-CQDs) were synthesized by a chemical oxidation method using medium-low temperature coal tar pitch as the raw material. Such quantum dots were developed as a highly sensitive fluorescent "on-off-on" switch sensor for the selective and simultaneous sensing of Cu²⁺ and Fe³⁺. The as-prepared N-CQDs, which emit blue light, were characterized by TEM images, FT-IR spectra, Raman spectroscopy, XPS analysis, fluorescence spectra, and UV-vis absorption spectra. The results showed that the N-CQDs exhibit outstanding optical properties and high optical stability within the pH range of 4-10, with a quantum yield of approximately 7%. Additionally, the material performed as an "on-off" sensor which can be dramatically extinguished by Cu²⁺ and Fe³⁺. A linear relationship between Cu²⁺ and Fe³⁺ ion concentration and fluorescence intensity was observed in the range from 0 to 50 μM. The limits of detection of the fluorescent sensor toward Cu²⁺ and Fe³⁺ were 0.16 μM and 0.173 μM, respectively. The Fe³⁺-quenched N-CQDs were restored after adding L-ascorbic acid, due to the redox reaction between Fe³⁺ and L-ascorbic acid, which resulted in the detachment of Fe³⁺ from the surface of the N-CQDs. The linear range for the detection of L-ascorbic acid was 0-28 μM. Therefore, the amount of L-ascorbic acid can be measured by using the sensing system consisting of Fe³⁺ and N-CQDs. In consequence, N-CQDs are considered an important material for the detection of Cu²⁺ and Fe³⁺ in water samples or L-ascorbic acid in drugs.

Carbon dots for specific "off-on" sensing of Co2+ and EDTA for in vivo bioimaging

Fluorescent carbon dots (CDs) were hydrothermally synthesized from a mixture of frozen tofu, ethylenediamine and phosphoric acid in an efficient 64% yield. The resulting CDs exhibit good water solubility, low cytotoxicity, high stability, and excellent biocompatibility. The CDs selectively and sensitively detect Co²⁺ through fluorescent quenching with a detection limit of 58 nM. Fluorescence can be restored through the introduction of EDTA, and this phenomenon can be used to quantify EDTA in solution with a detection limit of 98 nM. As both analytes are detected by the same CD platform, this is an "off-on" fluorescence sensor for Co²⁺ and EDTA. The technique's robustness for real-world samples was illustrated by quantifying cobalt in tap water and EDTA in contact lens solution. The CDs were also evaluated for in vivo imaging as they show low cytotoxicity and excellent cellular uptake. In a zebrafish model, the CDs are rapidly adsorbed from the intestine into the liver, and are essentially cleared from the body in 24 h with no appreciable bioaccumulation. Their simple and efficient synthesis, combined with excellent physical and chemical performance, renders these CDs attractive candidates for theranostic applications in targeted "smart" drug delivery and bioimaging.

Hydrothermal synthesis of N-doped carbon quantum dots and their application in ion-detection and cell-imaging

Carbon quantum dots (CQDs), owing to their characteristic luminescent properties, have become a new favorite in the field of luminescence. They have been widely used in light emitting diode, ion detection, cell-imaging, ect. Herein a facile synthesis method of nitrogen-doped carbon quantum dots (N-CQDs) has been developedviaa one-step hydrothermal of glucose and m-phenylenediamine. The chemical composition, surface functional groups, and crystal structure of so prepared N-CQDs were systematically characterized. The characterizations indicate that nitrogen has been chemically doped in the CQDs and the N-CQDs crystallize in a graphene structure. Photoluminescence (PL) measurements show that the N-CQDs emit strong blue emission under the irradiation of ultraviolet. The emission is excitation-dependent, is resistant to photo bleaching and high ionic strength, and slightly decreases with the increase of temperature. The quantum yield of them is about 17.5%. The PL intensity of N-CQDs quenches linearly with the increase of the concentrations of Fe³⁺(0.5-1.0 mM) and CrO₄^2-(0.3-0.6 mM), which are a kind of excellent fluorescent probe for the detection of Fe³⁺ and CrO₄^2-. The quenching mechanism of Fe³⁺ and CrO₄^2-is verified to be a static quenching mechanism based on inner filter effect. The N-CQDs are also found to be a good cell-imaging reagent of Hela cells.

Carbon Dots: An Emerging Smart Material for Analytical Applications

Carbon dots (CDs) are optically active carbon-based nanomaterials. These nanomaterials can change their light emission properties in response to various external stimuli such as pH, temperature, pressure, and light. The CD's remarkable stimuli-responsive smart material properties have recently stimulated massive research interest for their exploitation to develop various sensor platforms. Herein, an effort has been made to review the major advances made on CDs, focusing mainly on its smart material attributes and linked applications. Since the CD's material properties are largely linked to their synthesis approaches, various synthesis methods, including surface passivation and functionalization of CDs and the mechanisms reported so far in their photophysical properties, are also delineated in this review. Finally, the challenges of using CDs and the scope for their further improvement as an optical signal transducer to expand their application horizon for developing analytical platforms have been discussed.

Carbon Dots: A New Type of Carbon-Based Nanomaterial with Wide Applications

Carbon dots (CDs), as a new type of carbon-based nanomaterial, have attracted broad research interest for years, because of their diverse physicochemical properties and favorable attributes like good biocompatibility, unique optical properties, low cost, ecofriendliness, abundant functional groups (e.g., amino, hydroxyl, carboxyl), high stability, and electron mobility. In this Outlook, we comprehensively summarize the classification of CDs based on the analysis of their formation mechanism, micro-/nanostructure and property features, and describe their synthetic methods and optical properties including strong absorption, photoluminescence, and phosphorescence. Furthermore, the recent significant advances in diverse applications, including optical (sensor, anticounterfeiting), energy (light-emitting diodes, catalysis, photovoltaics, supercapacitors), and promising biomedicine, are systematically highlighted. Finally, we envisage the key issues to be challenged, future research directions, and perspectives to show a full picture of CDs-based materials.

An ultrasensitive chemiluminescence assay for 4-nitrophenol by using luminol-NaIO4 reaction catalyzed by copper, nitrogen co-doped carbon dots

A chemiluminescence (CL) assay on the basis of the tremendous enhancing effect of copper and nitrogen co-doped carbon dots (Cu,N-CDs) on the luminol-NaIO₄ reaction was introduced for the determination of nanomolar levels of 4-nitrophenol (4-NP). Cu,N-CDs were synthesized by a hydrothermal approach and characterized by TEM, XRD, EDX, and FTIR analysis. The potential CL mechanism was elucidated by recording the CL spectrum and by evaluating the influence of reactive oxygen species. It was found that 4-NP remarkably inhibited the luminol-NaIO₄-Cu,N-CDs reaction and reduced the CL signal of the reaction. This fact was applied for developing a CL assay for 4-NP. Under the optimized conditions, 4-NP could be determined in the concentration range of 0.25 to 150 nM, with a detection limit as low as 0.06 nM. This assay was successfully exploited for the analysis of 4-NP in real environmental samples.

Red-emission carbon dots-quercetin systems as ratiometric fluorescent nanoprobes towards Zn2+ and adenosine triphosphate

Carbon dots (CDs) emitting red fluorescence (610 nm) were synthesized by solvent thermal treatment of p-phenylenediamine in toluene. Upon 440 nm excitation, quercetin (QCT) alone endowed slight effects on the red fluorescence of CDs. Once Zn²⁺ was further introduced, the QCT-Zn²⁺ complex was quickly formed. This complex absorbs excitation light and emits bright green fluorescence at 480 nm. The red fluorescence of CDs was greatly quenched owing to the inner-filter effect. The ratio of fluorescence intensity at 480 nm and 610 nm (I₄₈₀/I₆₁₀) gradually increases with increasing concentration (c) of Zn²⁺. Al³⁺ exhibits the same phenomen like Zn²⁺. Fluoride ions form a more stable complex with Al³⁺ than QCT-Al³⁺ complex but have a negligible effect on the QCT-Zn²⁺ complex. The possible interference of Al³⁺ on Zn²⁺ can thus be avoided by adding certain amount of F^-. The CD-QCT-F^- system was constructed as a ratio-metric fluorescent nanoprobe toward Zn²⁺ with determination range of 0.14-30 μM and limit of detection (LOD) of 0.14 μM. Due to the stronger affinity of adenosine triphosphate (ATP) to Zn²⁺ than QCT, the I₄₈₀/I₆₁₀ value of CD-QCT-F^--Zn²⁺ system gradually decreases with increasing c_ATP. The ratiometric fluorescent nanoprobe toward ATP was established with detection ranges of 0.55-10 and 10-35 μM and a LOD of 0.55 μM. The above two probes enable the quantitative determination of Zn²⁺ and ATP in tap and lake water samples with satisfactory recoveries. Graphical abstract Schematic representation of the ratiometric fluorescent nanoprobes based on the carbon dots (CDs)-quercetin (QCT) system towards Zn²⁺ and adenosine triphosphate (ATP) with high selectivity and sensitivity.

Osteogenic potential of Zn2+-passivated carbon dots for bone regeneration in vivo

Carbon dots are a new kind of nanomaterial which has great potential in biomedical applications. Previously, we have synthesized novel Zn2+-passivated carbon dots (Zn-CDs) which showed good osteogenic activity in vitro. In this study, we will further investigate the osteogenic effects of Zn-CDs in vivo which is essential before their clinical use. Herein, Zn2+-passivated carbon dots (Zn-CDs) are prepared and characterized as previously reported. Then, the optimum dose for inducing osteoblasts was evaluated by MTS assay, intracellular reactive oxygen species (ROS) detection, alkaline phosphatase (ALP) activity test and alizarin red staining in vitro. Finally, a 5 mm diameter calvarial bone defect model was created in rats and Zn-CDs were applied for repairing the critical bone defect. It was shown that zinc gluconate (Zn-G) and Zn-CDs promoted the survival of bone marrow stromal cells (BMSCs) when the zinc ion concentration was 10-4 mol L-1 (Zn-G: 45.6 μg mL-1) and 10-5 mol L-1 (Zn-CDs: 300 μg mL-1) or below respectively. With regard to the osteogenic capability, the ALP activity induced by Zn-CDs was significantly higher than that by Zn-G. Besides, the results of alizarin red staining showed that the area of calcified nodules was increased in a dose-dependent manner in the Zn-CD group. Moreover, there were more calcium nodules in the Zn-CD group than in the Zn-G group at the same concentration of Zn2+ (10-5 mol L-1). Taken together, Zn-CDs achieved the highest osteogenic effect at the concentration of 10-5 mol L-1 without affecting cell proliferation in long-term stimulation. Importantly, the volume of new bone formation in the Zn-CD group (6.66 ± 1.25 mm3) was twice higher than that in the control group (3.33 ± 0.94 mm3) in vivo. Further histological evaluation confirmed the markedly new bone formation at 8 weeks in the Zn-CD group. The in vitro and in vivo experiments revealed that Zn-CDs could be a new predictable nanomaterial with good biocompatibility and fluorescence properties for guiding bone regeneration.

Deep Red Emissive Carbonized Polymer Dots with Unprecedented Narrow Full Width at Half Maximum

Development of high-performance carbon dots (CDs) with emission wavelength longer than 660 nm (deep red emission) is critical in deep-tissue bioimaging, yet it is still a major challenge to obtain CDs with both narrow full width at half maximum (FWHM) and high deep red/near-infrared emission yield. Here, deep red emissive carbonized polymer dots (CPDs) with unprecedented FWHM of 20 nm are synthesized. The purified CPDs in dimethyl sulfoxide (DMSO) solution possess quantum yield (QY) as high as 59% under 413 nm excitation, as well as recorded QY of 31% under 660 nm excitation in the deep red fluorescent window. Detailed characterizations identify that CPDs have unique polymer characteristics, consisting of carbon cores and the shells of polymer chains, and π conjugated system formed with N heterocycles and aromatic rings governs the single photoluminescence (PL) center, which is responsible for high QY in deep red emissive CPDs with narrow FWHM. The CPDs exhibit strong absorption and emission in the deep red light region, low toxicity, and good biocompatibility, making them an efficient probe for both one-photon and two-photon bioimaging. CPDs are rapidly excreted via the kidney system and hepatobiliary system.

Ratiometric fluorescent nanoprobes based on Resonance Rayleigh Scattering and inner filter effect for detecting alizarin red and Pb2

A new ratiometric fluorescent strategy for detection of alizarin red (ARS) was designed based on the fluorescence of CDs and scattered light of scatterer. The CDs-ARS system can be used to detect Pb²⁺ based on that the complexation between ARS and Pb²⁺. With the addition of ARS, the fluorescence of CDs was apparently quenched via inner filter effect (IFE). Resonance Rayleigh Scattering (RRS) at 350 nm was enhanced by an increase in the number of scatterer. The value of ln(I₃₅₀/I₄₂₅) was linearly correlated with ARS concentration in the range of 0-80 μM, and the detection limit for ARS was calculated to be 68.1 nM. When Pb²⁺ was added to the CDs-ARS system, the complexation of ARS with Pb²⁺ increased the size of the scatterer, resulting in the increase of the RRS intensity at 350 nm. Due to the affinity between ARS and Pb²⁺, the overlap of the emission spectra of CDs and the absorption spectra of ARS was reduced, resulting in the IFE effect was inhibited and the recovery of the fluorescence of CDs. The value of I₃₅₀/I₄₂₅ linearly increased with the addition of Pb²⁺ within the range of 10-50 μM, the limit of detection was 36.8 nM. As for practical application, CDs and CDs-ARS were applied to detect ARS and Pb²⁺ in tap water and poor water, respectively. The recovery values were obtained to be 95.4-98.8% and 93.4-101.7%. Furthermore, the system of CDs-ARS has been successfully applied to H1299 cell imaging.

Synthesis of dual functional procaine-derived carbon dots for bioimaging and anticancer therapy

Aim: Procaine-derived carbon dots, termed P-dots, expectedly offer both fluorescent biomarker function and anticancer activity. Materials & methods: P-dots were synthesized by condensing procaine, citric acid and ethylenediamine via hydrothermal synthesis and characterized by transmission electron microscopy, Fourier transform infrared spectroscopy and x-ray photoelectron spectroscopy. The cellular uptake behavior and the bioimaging performance of P-dots were assessed by confocal laser scanning microscopy. Their antitumor activity was evaluated using the CCK-8 assays and in vivo antitumor experiments, and the underlying mechanism was evaluated by flow cytometry and western blotting. Results: P-dots exhibited excellent luminescence properties suitable for bioimaging and considerable anticancer activity associated with caspase-3-related cell apoptosis. Conclusion: The synthesized procaine-derived carbon dots presented a dual function consisting of bioimaging and anticancer activity, which may enable their implementation as safe and effective clinical nanotherapeutics.

A green carbon dots-based fluorescent sensor for selective and visual detection of nitrite triggered by the nitrite–thiol reaction

Selective and visual detection of nitrite is realized through the inner-filter effect triggered by the nitrite–thiol reaction based on green carbon dots.

Thieno[3,2-c]pyran: an ESIPT based fluorescence “turn-on” molecular chemosensor with AIE properties for the selective recognition of Zn2+ ion

Thieno[3,2-c]pyran was synthesized as a fluorescent turn-on chemosensor for the selective recognition of Zn²⁺ ions with a low detection limit (0.67 μM), and it also exhibited AIE properties.

Evolution and Synthesis of Carbon Dots: From Carbon Dots to Carbonized Polymer Dots

Despite the various synthesis methods to obtain carbon dots (CDs), the bottom-up methods are still the most widely administrated route to afford large-scale and low-cost synthesis. However, as CDs are developed with increasing reports involved in producing many CDs, the structure and property features have changed enormously compared with the first generation of CDs, raising classification concerns. To this end, a new classification of CDs, named carbonized polymer dots (CPDs), is summarized according to the analysis of structure and property features. Here, CPDs are revealed as an emerging class of CDs with distinctive polymer/carbon hybrid structures and properties. Furthermore, deep insights into the effects of synthesis on the structure/property features of CDs are provided. Herein, the synthesis methods of CDs are also summarized in detail, and the effects of synthesis conditions of the bottom-up methods in terms of the structures and properties of CPDs are discussed and analyzed comprehensively. Insights into formation process and nucleation mechanism of CPDs are also offered. Finally, a perspective of the future development of CDs is proposed with critical insights into facilitating their potential in various application fields.

A brand-new generation of fluorescent nano-neural tracers: biotinylated dextran amine conjugated carbonized polymer dots

We develop a novel fluorescent nano-neural tracer: BDA–CPDs, which can be anterogradely transported within the peripheral nervous system of rats.

Carbon-based quantum particles: an electroanalytical and biomedical perspective

Carbon-based quantum particles, especially spherical carbon quantum dots (CQDs) and nanosheets like graphene quantum dots (GQDs), are an emerging class of quantum dots with unique properties owing to their quantum confinement effect.

Adverse effects of fluorescent carbon dots from canned yellow croaker on cellular respiration and glycolysis

Adverse effects on cellular respiration and glycolysis were investigated for the fluorescent carbon dots from canned yellow croaker.