Concentration-dependent color tunability of nitrogen-doped carbon dots and their application for iron(III) detection and multicolor bioimaging

Carbon dots for staining bacterial dead cells and distinguishing dead/alive bacteria

The small molecular dyes such as propidium iodide (PI) always suffer from photo-bleaching and potential toxicity. To tackle the problems, a type of nontoxic carbon dots (CDs) was obtained for dead/alive bacterial distinguishing. This kind of carbon dots has an average size of 1.91 nm and owns carboxyl groups, emerging as excellent candidates for imaging bacterial cells. The negative charges of carboxyl groups lead their avoidance of alive cells while their small size facilitates penetration of dead cells. This kind of nontoxic CDs has effectively differentiated between and alive ones, presenting a highly promising green dye comparing with traditional small molecular dyes.

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

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

Facile synthesis of NS-doped carbon dots as sensitive "ON-OFF-ON" fluorescent sensor for Cu2+ and GSH detection

In this paper, a novel nitrogen and sulfur co-doped carbon quantum dots (NS-CQDs) were successfully prepared by a dehydration exothermic carbonization method. The NS-CQDs exhibited uniform size distribution, splendid photostability, and bright fluorescence emission with a fluorescence quantum yield of 24.1 %. It was found that Cu2+ could quench the fluorescence at 467 nm based on the static quenching effect when Cu2+ was added to the NS-CQDs. At this time, the fluorescence sensor changed from the "ON" state to the "OFF" state. When glutathione (GSH) was further introduced into the NS-CQDs/Cu2+ system, the fluorescence intensity of NS-CQDs was amazingly restored through the coordination reaction between GSH and Cu2+. The fluorescence sensor changed from the "OFF" state to the "ON" state. Therefore, NS-CQDs as an "ON-OFF-ON" fluorescence sensor was designed for sequential detection of Cu2+ and GSH. Furthermore, this study successfully demonstrated the sensor's ability to selectively detect Cu2+ and GSH within a wide concentration range. Specifically, the detection range for Cu2+ was 0.1 μM-200.0 μM with a detection limit of 0.07 μM, while the range for GSH was 0.6 μM-180.0 μM with a detection limit of 0.1 μM. Most importantly, the NS-CQDs nanosensor could reliably monitor Cu2+ and GSH levels in human serum samples, with significant potential for practical applications.

N, S/P co-doped hemicellulose-based carbon dots with tunable fluorescence for anti-counterfeiting

Based on the status quo of high energy consumption and low utilization of nonfibrous components in traditional pulp and paper industry, a sustainable and facile approach was proceeded to realize the high-value utilization of hemicelluloses from papermaking waste liquor. The hemicellulose waste produced by ethanol precipitation in pre-hydrolysis liquor (PHL), was directly used to fabricate carbon dots (CDs) via a hydrothermal method. The hydrothermal carbonization and heteroatoms doping contributed to the sp2 conjugated domains and surface defect states of CDs, thus creating the bright blue (N-CDs), deep cyan (N/S-CDs), and light cyan (N/P-CDs) fluorescence under UV radiation. The XPS analysis and density functional theory (DFT) calculations demonstrated that the large sp2 conjugated system and the synergistic effect of CO, N-(C)3, CS, and PO groups promoted the narrow of band gap and the red-shift of fluorescence emission. Importantly, the prepared CDs grew in situ on cotton fibers, showed excellent fluorescent performance. The obtained CDs could be also utilized to prepare anti-counterfeiting film and ink due to their excellent optical features, verifying the great potential application as security material. The feasible strategy of the high-value conversion of biomass waste opens a window of opportunity for the practical anti-counterfeiting utilizations.

Molecularly designed and synthesized of bright blue nitrogen-doped lignin-derived carbon dots applied in printable anti-counterfeiting

With the increased demand for green and sustainable development, the research of advanced biomass-based carbon dots (CDs) has drawn growing attention. Herein, a one-step green solvent integration strategy-assisted solvothermal method to preparing CDs from hydrolyzed lignin and ethylenediamine (EDA) in formamide (FA) was developed. The Schiff reaction between FA and EDA contributes to the formation of -C-N groups, further inducing the high photoluminescence quantum yield (up to 42.69 %)，obviously higher than NCDs prepared in H2O, EtOH and DMF systems (corresponding to H-NCDs, E-NCDs and D-NCDs, respectively). The analysis of structure, composition, photoluminescence (PL) behaviors and DFT calculations showed that F-NCDs have main blue fluorescent emission peak from 410 to 455 nm under 330-390 nm excitation due to the small sp2 structure in carbon core, and the large sp2 conjugated clusters and CO group related surface states leaded to the long wavelength emission. The F-NCDs with excellent optical properties was further used for preparing fluorescent film and invisible anti-counterfeiting ink, which exhibited outstanding fluorescence even at different temperatures and aging times. We provided a facile way for green facile preparation of lignin-based CDs and their sustainable anti-counterfeiting application.

Synthesis and post-synthesis strategies for polychromatic carbon dots toward unique and tunable multicolor photoluminescence and associated emission mechanism

Carbon dots (CDs) with unique and tunable multicolor photoluminescence (PL) emission has attracted tremendous attention in the past few years due to their potential multifaceted application, specially in the biomedical and optoelectronic fields. There has been extensive deliberation and efforts to engineer the synthesis or post synthesis approach to obtain multicolor-emissive CDs and tune their optical properties toward longer wavelength. This review mainly focuses on the advancement of strategies for synthesis and post-synthesis techniques of CDs toward tunable multicolor emission. Based on the above discussion to achieve desired goals, several synthesis strategies (selection of proper benzenoid precursor, acid/base treatment of biomass, optimization of reaction conditions, optimization of the reagents, solvent engineering, acid strength regulation, reaction temperature regulation, chemical doping) and various post synthesis strategies (column chromatographic separation or purification, solvatochromism, pH variation, surface functionalization, concentration variation) have been reviewed. Although numerous research articles have been published on the synthesis of multicolor CDs for multifaceted application, there is still a lack of a concise review article focusing on systematic synthesis/post synthesis strategies with PL mechanism elucidation. Thus, we focused on providing a comprehensive overview of the state-of-the-art advances on the strategies for the preparation of polychromatic CDs with tunable emission and elucidating their emission mechanism.

Synthetic strategies, properties and sensing application of multicolor carbon dots: recent advances and future challenges

Recently, carbon dots (CDs) as newly developed carbon-based nanomaterials due to advantages such as excellent photostability and easy surface functionalization have generated wide application prospects in fields such as biological imaging and chemical sensing. The multicolor emission carbon dots (M-CDs) were acquired through the selection of different carbon source precursors, change of synthesis conditions and synthesis environment. Therefore, the aim of this review is to summarize the latest research progress in polychromatic CDs from the perspectives of synthesis strategies, luminescent mechanisms, luminescent properties and applications. This review focuses on how to prepare MCDs by changing raw materials and synthesis conditions such as reaction temperature, synthesis time, synthesis pH, and synthesis solvent. This review also presents the optical properties of MCDs, concentration effects, solvent effects, pH effects, elemental doping, and surface passivation on them, as well as their creative applications in the field of sensing applications. It is anticipated that this review will serve as a guide for the development of multifunctional M-CDs and inspire future research on controllable design and preparation of M-CDs.

Design of carbon dots for bioimaging and behavior regulation of stem cells

Carbon dots (CDs) have been widely used in bioimaging, biosensing and biotherapy because of their good biocompatibility, optical properties and stability. In this review, we comprehensively summarize the research on CDs in terms of synthesis methods, optical properties and biotoxicity. We describe and envisage the directions for CDs application in stem cell imaging and differentiation, with the aim of stimulating the design of future related CDs. We used 'carbon dots', 'stem cells', 'cell imaging', 'cell differentiation' and 'fate control' as keywords to search for important articles. The Web of Science database was used to extract vital information from a total of 357 papers, 126 review articles and 231 article proceedings within 12 years (2011-2022).

An Efficient Green Approach to Constructing Adenine Sulfate-Derived Multicolor Sulfur- and Nitrogen-Codoped Carbon Dots and Their Bioimaging Applications

A cost-effective and environmentally friendly approach is proposed for producing N- and S-codoped multicolor-emission carbon dots (N- and S-codoped MCDs) at a mild reaction temperature (150 °C) and relatively short time (3 h). In this process, adenine sulfate acts as a novel precursor and doping agent, effectively reacting with other reagents such as citric acid, para-aminosalicylic acid, and ortho-phenylenediamine, even during solvent-free pyrolysis. The distinctive structures of reagents lead to the increased amount of graphitic nitrogen and sulfur doping in the N- and S-codoped MCDs. Notably, the obtained N- and S-codoped MCDs exhibit considerable fluorescence intensities, and their emission color can be adjusted from blue to yellow. The observed tunable photoluminescence can be attributed to variations in the surface state and the amount of N and S contents. Furthermore, due to the favorable optical properties, good water solubility and biocompatibility, and low cytotoxicity, these N- and S-codoped MCDs, especially green carbon dots, are successfully applied as fluorescent probes for bioimaging. The affordable and environmentally friendly synthesis method employed to create N- and S-codoped MCDs, combined with their remarkable optical properties, offers a promising avenue for their use in various fields, particularly in biomedical applications.

Intramolecular hydrogen bond-tuned thermal-responsive carbon dots and their application to abnormal body temperature imaging

A steric hindrance strategy was used to prepare intramolecular hydrogen bond-controlled thermosensitive fluorescent carbon dots (CDs) via the solvothermal treatment of o-phenylenediamine respectively with three dihydroxybenzene isomers. The CDs obtained from different isomers have very similar morphology, surfaces, and photophysical properties but exhibited different thermal sensitivities. Meanwhile, the orange-emitting CDs (p-CDs) obtained from o-phenylenediamine and p-hydroquinone exhibited an optimal thermal sensitivity of 1.1%/°C. Comprehensive experimental characterizations and theoretical calculations revealed that even a small difference in substituent locations in the phenyl ring of the precursors can considerably affect the formation of intramolecular hydrogen bonds and that the CDs with strong intramolecular hydrogen bonds exhibited poor thermosensitivity. The p-CDs were incorporated with reference CDs (B-CDs) that exhibited heating-quenching blue emission through electrostatic self-assembly to construct a dual-emission probe (p-CDs/B-CDs), which exhibited a thermal sensitivity of 2.0%/°C. Test strips based on the p-CDs/B-CDs were prepared to measure temperature fluctuations based on sensitive and instant fluorescence color evolution. Further, this fluorescent colorimetry was successfully applied to a test strip-integrated wearable wristband to measure the body temperature. This study establishes an inherent relationship between precursors and the resulting intramolecular hydrogen bonds for precisely tuning the thermal sensitivity of CDs. It also offers a visual quantitative strategy for the early warning of abnormal body temperatures.

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

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

Sequential Injection Analysis for Rapid Determination of Mercury in Skincare Products Based on Fluorescence Quenching of Eco-Friendly Synthesized Carbon Dots

This work reports the analysis of mercury using a spectrofluorometric method combined with a sequential injection analysis (SIA) system. This method is based on the measurement of fluorescence intensity of carbon dots (CDs), which is quenched proportionally after adding mercury ions. Herein, the CDs underwent environmentally friendly synthesis using a microwave-assisted approach that provides intensive and efficient energy and shortens reaction time. After irradiation at 750 W for 5 min in a microwave oven, a dark brown CD solution with a concentration of 2.7 mg mL^-1 was obtained. The properties of the CDs were characterized by transmission electron microscopy, X-ray diffractometry, X-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy, and UV-vis spectrometry. We presented for the first time the use of CDs as a specific reagent for the determination of mercury in skincare products with the SIA system to achieve rapid analysis and full automatic control. The as-prepared CD stock solution was diluted 10 times and used as a reagent in the SIA system. Excitation and emission wavelengths at 360 and 452 nm, respectively, were used to construct a calibration curve. Physical parameters affecting the SIA performance were optimized. In addition, the effect of pH and other ions was investigated. Under the optimum conditions, our method showed a linear range from 0.3 to 600 mg L^-1 with an R ² of 0.99. The limit of detection was 0.1 mg L^-1. Relative standard deviation was 1.53% (n = 12) with a high sample throughput of 20 samples per hour. Finally, the accuracy of our method was validated by comparison using inductively coupled plasma mass spectrometry. Acceptable recoveries were also presented without a significant matrix effect. This method was also the first time that uses the untreated CDs for the determination of mercury(II) in skincare products. Therefore, this method could be an alternative for mercuric toxic control in other sample applications.

Biomimetic-compartmented nanoprobe for in-situ imaging of iron storage and release from ferritin in cells

Ferritin plays an important role in regulating the homeostasis of iron in cells by storing/releasing iron. Current methods usually explored the determination of iron content, but in-situ imaging of the iron storage/release from ferritin in cells cannot be achieved. Hence, an engineered self-assembled biomimetic-compartmented nanoprobe (APO@CDs) has been constructed. The protein shell of APO (apoferritin) acted as ion channel module to control iron ions entering/exiting ferritin cavity; the inner core of CDs (carbon dots) acted as signal module for iron ions response. Compared with CDs, the response sensitivity and specificity to iron ions (Fe3⁺) have been improved by using APO@CDs, and the cytotoxicity was significantly reduced. Additionally, compared with cells containing APO@CDs alone, the normalized fluorescence gray value of Fe³⁺-treated cells was significantly decreased (0.275), indicating that Fe³⁺ has effectively entered the ferritin. Furtherly, that of Fe³⁺-treated cells incubated with deferoxamine (DFO) was significantly enhanced (0.712), showing that Fe³⁺ was released from ferritin under the mediation of DFO. The results demonstrate that APO@CDs can be successfully applied to in-situ imaging of iron storage/release from ferritin in cells, providing a potential platform for the in-situ dynamic study of the iron storage/release in biomedical field.

Primary Amine Functionalized Carbon Dots for Dead and Alive Bacterial Imaging

Small molecular dyes are commonly used for bacterial imaging, but they still meet a bottleneck of biological toxicity and fluorescence photobleaching. Carbon dots have shown high potential for bio-imaging due to their low cost and negligible toxicity and anti-photobleaching. However, there is still large space to enhance the quantum yield of the carbon quantum dots and to clarify their mechanisms of bacterial imaging. Using carbon dots for dyeing alive bacteria is difficult because of the thick density and complicated structure of bacterial cell walls. In this work, both dead or alive bacterial cell imaging can be achieved using the primary amine functionalized carbon dots based on their small size, excellent quantum yield and primary amine functional groups. Four types of carbon quantum dots were prepared and estimated for the bacterial imaging. It was found that the spermine as one of precursors can obviously enhance the quantum yield of carbon dots, which showed a high quantum yield of 66.46% and high fluorescence bleaching-resistance (70% can be maintained upon 3-h-irradiation). Furthermore, a mild modifying method was employed to bound ethylenediamine on the surface of the spermine-carbon dots, which is favorable for staining not only the dead bacterial cells but also the alive ones. Investigations of physical structure and chemical groups indicated the existence of primary amine groups on the surface of spermine-carbon quantum dots (which own a much higher quantum yield) which can stain alive bacterial cells visibly. The imaging mechanism was studied in detail, which provides a preliminary reference for exploring efficient and environment-friendly carbon dots for bacterial imaging.

The Emerging Development of Multicolor Carbon Dots

As a relatively new type of fluorescent carbon-based nanomaterials, multicolor carbon dots (MCDs) have attracted much attention because of their excellent biocompatibility, tunable photoluminescence (PL), high quantum yield, and unique electronic and physicochemical properties. The multicolor emission characteristics of carbon dots (CDs) obviously depend on the carbon source precursor, reaction conditions, and reaction environment, which directly or indirectly determines the multicolor emission characteristics of CDs. Therefore, this review is the first systematic classification and summary of multiple regulation methods of synthetic MCDs and reviews the recent research progress in the synthesis of MCDs from a variety of precursor materials such as aromatic molecules, small organic molecules, and natural biomass, focusing on how different regulation methods produce corresponding MCDs. This review also introduces the innovative applications of MCDs in the fields of biological imaging, light-emitting diodes (LEDs), sensing, and anti-counterfeiting due to their excellent PL properties. It is hoped that by selecting appropriate adjustment methods, this review can inspire and guide the future research on the design of tailored MCDs, and provide corresponding help for the development of multifunctional MCDs.

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.

One-pot room temperature synthesis of orange-emitting carbon dots for highly-sensitive vitamin B12 sensing

Fluorescent carbon dots (CDs) with the long-wavelength emission have received increasing attention due to their promising application prospects in the biological field. The synthesis of CDs with long wavelength emission mainly focuses on the high temperature method, but the room temperature synthesis is still rarely studied. Herein, a simple room temperature strategy is developed for the preparation of orange-emitting CDs (O-CDs) by Schiff base crosslinking reaction between methyl-p-benzoquinone and triethylenetetramine. The proposed O-CDs show a strong excitation-dependent emission with a relative quantum yield of about 6.56%. Because of the strong inner filter effect, O-CDs can be well applied for the highly-sensitive detection of vitamin B12 (VB12). The intensity ratio (F/F₀) of O-CDs is linear against the concentration of VB12 from 50 nM to 200 μM with a low detection limit of 10 nM. Therefore, the obtained O-CDs nanoprobes provide a promising platform for pharmaceutical analysis applications.

High-Performance Hydrogel Based on Modified Chitosan for Removal of Heavy Metal Ions in Borehole: A Case Study from the Bahariya Oasis, Egypt

Globally, there is a rising demand for water purification. This demand is driven by numerous factors, including economic growth, increasing population, water shortage, and deterioration of water quality. The current work highlights the manufacturing of environmentally friendly and highly efficient sorbent based on chitosan nanoparticles after successive crosslinking (using glutaraldehyde) and modification through grafting of 4-aminoazobenzene-3,4′-disulfonic acid (AZDS) as a source of sulfonic groups. First, the produced sorbent was thoroughly specified using FTIR, TGA, SEM, SEM-EDX, pHpzc, BET (nitrogen sorption desorption isotherms), and elemental analyses (EA). The sorbent was tested for the sorption of Fe(III) before application to highly contaminated iron water well samples. Next, the sorption was improved as the sulfonation process was conducted under the selected experimental conditions within 25 and 20 min with a maximum capacity of 2.7 and 3.0 mmol Fe g−1 in visible light and under UV, respectively. Then, the uptake kinetics for both techniques were fitted by the pseudo-first-order rate equation (PFORE), in which the effect of the resistance to intraparticle diffusion has remained an unneglected factor, while the Langmuir equation has fitted the sorption isotherms. After that, the efficient desorption was achieved by using 0.2 M hydrochloric acid solution, and the desorption process was as fast as the sorption process; 15 min was sufficient for complete desorption. The sorbent shows high selectivity for heavy metal ions compared to the representative elements. Finally, the sorbent was used for the removal of heavy metal ions from a highly contaminated water well in the Bahariya Oasis and appeared to be highly efficient for heavy metal removal even in a diluted solution. Accordingly, it can be implemented in the task of water treatment.

Cadmium induced aggregation of orange-red emissive carbon dots with enhanced fluorescence for intracellular imaging

Cadmium is a notorious toxic heavy metal, that poses serious threat to human health. Sensitive and selective detection of cadmium in cells is of great significance in poison screening and disease diagnosis. Orange-red emissive carbon dots (OR-CDs), prepared from the calcination of selected carbon sources 5-amino-1, 10-phenanthroline (Aphen) and salicylic acid (SA), were found to act as a "turn on" type fluorescence probe for Cd²⁺ detection. The structure and optical properties of OR-CDs were comprehensively investigated by both experimental characterizations and density functional theory (DFT) calculations. The OR-CDs consist of a basic unit of nine aromatic rings, and the N/O binding sites on the OR-CDs can specifically bind with Cd²⁺, leading to aggregation induced enhanced emission (AIEE). A detection limit of 0.30 μM was achieved for Cd²⁺ with a linear range of 0.80-100 μM. OR-CDs can not only be used for intracellular Cd²⁺ imaging but also have the potential to alleviate cadmium poison in living organisms.

Dispersion-assisted tunable fluorescence from carbon dots

In this study, carbon dots (CDs) synthesized by hydrothermal method with amino-rich surface exhibit tunable fluorescence across entire visible range by simply controlling the concentration. A comprehensive comparison has been performed for the first time between concentration-induced aggregation of the single-type CDs and electrostatic-induced agglomeration of opposite-charged CDs in terms of their fluorescence properties. Experimental results show that both the aggregation of CDs and internal absorption filtration are possible causes of the concentration-dependent fluorescence emission. Subsequently, the inter distance of adjacent CDs in their aggregates was enlarged by forming rigid double-stranded DNA (dsDNA) between adjacent CDs through base pairing. It is clear that the contact of CDs induces the changes of fluorescence emission and light absorption. Through a better understanding of the mechanisms behind concentration-induced multicolor emission, this work can provide a novel strategy to develop the advanced applications of CDs.

Ultra-bright carbon quantum dots for rapid cell staining

A schematic illustration of the synthesis of G-CDs and cell imaging under one-photon and two-photon conditions.

Surface state modulation of blue-emitting carbon dots with high quantum yield and high product yield

Carbon dots (CDs) with high quantum yield (QY) and product yield (PY) were achieved benefited from higher content of CO bond, pyrrolic N, and pyridinic N, as well as lower content of amino N and crosslinking structure of CDs.

Fluorescence Enhancement of Lignin-Based Carbon Quantum Dots by Concentration-Dependent and Electron-Donating Substituent Synergy and Their Cell Imaging Applications

Black liquor is an important pollutant in the pulp industry, but it also has the potential for high-value utilization. In this study, lignin extracted from black liquor was hydrothermally prepared into lignin-based carbon quantum dots (L-CQDs) using a one-pot method. Physicochemical characterization suggested that the L-CQDs exhibited a lamellar core-shell multilayered graphene structure surrounded by oxygen-containing functional groups. The fluorescence intensity of the L-CQDs was strengthened depending on their own concentration dependence and the doping of external groups. The fluorescence intensity of L-CQDs varied between 89.09 and 183.66 under different concentrations, and the most intense fluorescence (183.66) was obtained at 0.1 mg mL^-1. At hydroxyl and amino adsorption capacities of 11.08 and 0.98 mmol g^-1, the hydroxylated RL-CQDs-5 and aminated NL-CQDs-3 exhibited the highest fluorescence intensities at 689.22 and 605.39, respectively. Moreover, when pristine L-CQDs were sequentially aminated and hydroxylated, the NRL-CQDs' fluorescence intensity reached 1224.92. Cell imaging experiments proved that cells cultivated with NRL-CQDs have brighter fluorescence compared with L-CQDs. The results will render L-CQDs more suitable for practical applications.

Aptamer-superparamagnetic nanoparticles capture coupling siderophore-Fe3+ scavenging actuated with carbon dots to confer an "off-on" mechanism for the ultrasensitive detection of Helicobacter pylori

The detection of Helicobacter pylori infection in human feces is an appropriate non-invasive diagnostic method. However, the antibody-dependent stool antigen immunoassay bears many challenges. Therefore, we developed an antibody-independent biosensing platform. The core of this platform was a triple-module biosensor. The first module was Ca²⁺-doped superparamagnetic nanoparticles modified with an H. pylori-specific aptamer, functioning to selectively capture H. pylori cells from samples. The second module was a bifunctional co-polymer of chloroprotoporphyrin IX iron (III)-polyethylene glycol-desferrioxamine, which could bind to H. pylori with high affinity and chelate Fe³⁺ from the third module of Fe³⁺-quenched carbon dots (CDs) solution. When the formed module 1-H. pylori-module 2 complexes reacted with module 3, a subsequent magnetic separation could scavenge Fe³⁺, causing fluorescence recovery from quenched CDs as the transducing mechanism. This transducer could respond to tiny changes in Fe³⁺ concentration with distinguishable fluorescence differences, thus conferring the biosensor with high sensitivity, a wide detection range of 10-10⁷ CFU/mL and a limit of detection (LOD) as low as 1 CFU/mL. From simulated human stool samples, H. pylori was enriched with a centrifugal microfluidic plate to eliminate any interference from matrices, and the bacteria were subjected to detection using the biosensor. The actual LOD for the biosensing platform coupling microfluidics and the biosensor was 10¹, and the total time taken was 65 min. This work showcases an instant, accurate, and ultra-sensitive diagnosis of H. pylori in feces.

Water-soluble green-emitting carbon nanodots with enhanced thermal stability for biological applications

High stability and water solubility of fluorescent nanomaterials are considered key factors to evaluate their feasibility for fundamental applications. Herein, water-soluble and thermally stable, green-emitting carbon nanodots (CNDs) have been synthesized via a facile hydrothermal method with an average size of 1.9 nm. CNDs showed green emission centered at 544 nm with the photo-luminescence quantum yield (PLQY) of up to 10.1% under the excitation of 400 nm. The obtained CNDs demonstrated high resistance towards photo-bleaching and an ionic (KCl) environment. Moreover, the aqueous solution of CNDs exhibited excellent stability under harsh thermal conditions from 10 °C to 80 °C. The as-prepared CNDs showed stable performance at high temperatures, even after keeping them at 80 °C for 30 min. Furthermore, the green emissive CNDs were incubated in T-ca cancer cells for bio-imaging applications. The results indicated that CNDs can served as an effective thermally-stable bio-imaging agent in T-ca cells at the physiological temperature range of 25 °C-45 °C. Green emission and excellent thermal stability make these CNDs promising fluorescent materials for potential applications in the medical field, which requires long-wavelength fluorescence and high-temperature imaging.

ESIPT fluorophores derived from 2,3-dichloro-5,6-dicyano-p-benzoquinone based carbon dots for dual emission and multiple anti-counterfeiting

Fluorophores and hydrogen bonding interactions play key roles in the fluorescence properties of bottom-up carbon dots. In this work, an excited-state intramolecular proton-transfer (ESIPT) active fluorophore, 5-chloro-6-ethoxy-4,7-dihydroxyisoindoline-1,3-dione (CEDD) and a non-ESIPT 7-cyano-5,8-dihydroxyquinoxaline-6-carboxamide (CDQC) are extracted from 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) based carbon dots. The enol form of CEDD shows a weak blue, small Stokes shift and short lifetime emission under the aprotic or alkali conditions, but the keto form exhibits a strong green, large Stokes shift and long lifetime emission in a protic or an acidic environment. Due to the lack of the ESIPT process, CDQC has no dual emission characteristics, but shows efficient solid-state emission. By virtue of the ESIPT ability of CEDD, multiple anti-counterfeiting methods are achieved by using hydrogen chloride, ammonia, and fluorescence lifetime imaging, as well as dimethyl sulfoxide as the encryption/decryption tools.

Yellow emission carbon dots for highly selective and sensitive OFF-ON sensing of ferric and pyrophosphate ions in living cells

A simple "OFF-ON" fluorescent system was proposed for selective and sensitive detection of ferric ion (Fe³⁺) and pyrophosphate (PPi) in living cells. The method was constructed based on the bright yellow emission of carbon dots (y-CDs), which were prepared using o-phenylenediamine (OPD) as the precursor via a facile hydrothermal treatment. The as-obtained y-CDs, with an average size of 2.6 nm, exhibited an excitation-independent emission peak at 574 nm. The fluorescence of y-CDs can be remarkably quenched by Fe³⁺ with high selectivity and sensitivity. Interestingly, the quenched fluorescence can be recovered regularly upon addition of PPi, showing a promising detection for PPi. The linear ranges for Fe³⁺ and PPi detections were 0.05-80 and 0.5-120 μM, respectively, and the corresponding limit of detections (LODs) were 22.1 and 73.9 nM. As we proved the y-CDs have negligible cytotoxicity and excellent biocompatibility, further application to the fluorescence imaging of intracellular Fe³⁺ and PPi were conducted, suggesting the prepared y-CDs can be used to monitor Fe³⁺ and PPi variation in living cells. Overall, our developed y-CDs-based OFF-ON switch fluorescent probe has the advantages of simplicity, agility, high sensitivity and selectivity, which provides a promising platform for environmental and biology applications, and paves a new avenue for monitoring the hydrolysis process of adenosine triphosphate disodium salt (ATP) by detection of PPi in organisms.

Fluorescence-Scattering Dual-Signal Response of Carbon Dots@ZIF-90 for Phosphate Ratiometric Detection

Ratiometric fluorescence has drawn extensive attention owing to its self-calibration property. However, it is difficult to obtain appropriate fluorescent materials that can be excited under one excitation and possess well-resolved signals simultaneously. In this work, with the optical properties of the fluorescence of carbon dots (CDs) and the second-order scattering (SOS) of ZIF-90 (zeolitic imidazole frameworks-90) nanoparticles, the synthesized CDs@ZIF-90 can be applied to phosphate (PO₄^3-) ratiometric detection. The fluorescence of CDs is greatly suppressed through encapsulating CDs into ZIF-90. Nevertheless, the SOS is quite obvious due to the high scattering intensity of large size ZIF-90. The competitive coordination between PO₄^3- and the metal node of ZIF-90 decomposes CDs@ZIF-90, leading to the restoration of fluorescence and the diminution of SOS. On the basis of the PO₄^3--induced ZIF-90 decomposition and CD release, a novel method for PO₄^3- ratiometric detection is developed through the dual-signal response of the fluorescence scattering. Under the optimal condition, the method shows a linear range from 1.0 to 50.0 μmol L^-1 with a detection limit of 0.23 μmol L^-1. Furthermore, the probes are employed to assess PO₄^3- in practical aqueous samples successfully. Compared with the traditional approach, which only records fluorescence signals, the method reported here provides a new strategy to design ratiometric sensors by fluorescence and scattering.

Synthesis of Carbon Dots and their Application as Turn Off-On Fluorescent Sensor for Mercury (II) and Glutathione

In this paper, we present a new method for the detection of mercury (II) and glutathione using carbon dots as fluorescent sensor. The synthesized carbon dots have the advantages of simple manipulation, low cost and the high fluorescence quantum yield of them which was22.79%. The combination of mercury (II) and carbon dots caused the turn off of carbon dots fluorescence. With the reaction between mercury (II) and glutathione, the carbon dots were released and the fluorescence was turned on when the glutathione added. According to this, the carbon dots could be developed to detect mercury (II) and glutathione specifically, and the detection limit of mercury (II) is as low as 0.41 μM.

Manganese oxide doped carbon dots for temperature-responsive biosensing and target bioimaging

We report on the synthesis of manganese oxide doped CDs (MnO_x-CDs) by a hydrothermal strategy using manganese (III) acetylacetonate (Mn(III) (C₅H₇O₂)₃) as the only raw materials. The MnO_x-CDs exhibit water solubility, favorable biocompatibility, low cytotoxicity, and show blue fluorescence with excitation/emission maxima at 326/442 nm with a quantum yield of 11.3%, allowing efficient cellular imaging. The MnO_x-CDs have a reversible temperature-sensitive fluorescent property in vitro within 10-60 °C, which can also be used as a sensitive thermometer in living cells. By a scratch assay, the MnO_x-CDs can restrain the migration of HepG2 cancer cells, which make the MnO_x-CDs be attractive candidates for liver cancer adjuvant treatment. Besides, the fluorescence of the MnO_x-CDs is quenched in the presence of Fe³⁺ due to the formation of a nonfluorescent MnO_x-CDs-Fe³⁺ complex between oxygen-containing groups on the surface of MnO_x-CDs and Fe³⁺, and the quenched fluorescence of MnO_x-CDs can be turn-on by dissociation of MnO_x-CDs-Fe³⁺ complexes by biothiols including L-cysteine, homocysteine and glutathione. Therefore, the Fe³⁺ and biothiols can be sequentially detected with high reliability and accuracy via exploiting the on-off-on nanosensor at room temperature, respectively. Further application to detection biothiols in human serum indicated that the probe was practicality and feasibility in medical field.

Identification of eight pathogenic microorganisms by single concentration-dependent multicolor carbon dots

Single concentration-dependent carbon dots were synthesized and applied to the rapid identification of eight kinds of pathogenic microorganisms.

High Stability and Strong Fluorescence of Carbon Nanodots as Nanosensor for Hg2+ in Environmental Waters

Pollution of toxic heavy-metal ions such as mercury ions (Hg²⁺) is well known to severely threaten ecological environment and human health. Correspondingly, development of a fast and sensitive method for detecting heavy-metal ions is urgently needed and has been received widespread attention in recent years. In this study, carbon nanodots (CDs) with strong blue fluorescence were synthesized by a microwave-assisted hydrothermal method. The as-prepared blue fluorescent CDs not only have excellent stability (e.g. photostability, salt stability and pH stability), but also have extremely high selectivity and sensitivity for probing Hg²⁺ via fluorescence quenching. Specifically, fluorescence of CDs is gradually quenched along with the increase in Hg²⁺ concentration, and a low concentration of Hg²⁺ can be identified (with low detection limit, 15 nM). Therefore, the novel fluorescent CDs could be developed for detecting Hg²⁺ in aqueous conditions, and have great potential for fast probing Hg²⁺ in environmental samples.

Interparticle distance as a key factor for controlling the dual-emission properties of carbon dots

Due to their distinct and useful features, multi-color-emissive carbon dots (CDs) have gained much attention, however, their lack of red emission has so far limited their practical use for various applications, thus understanding of their emission origins is required. Concentration-dependent photoluminescence (PL) is a unique feature of CDs and is regarded as one way to realize tunable PL, providing information on their PL mechanism. In this work, we synthesized concentration-dependent blue/red dual-color-emissive CDs under single excitation at 365 nm. With the increase in concentration, the blue emission band showed a large red-shift from 431 to 500 nm, while the red emission band at around 650 nm showed only a relative intensity change, without a significant spectral shift. We performed spectroscopic analysis by changing the solution environment including the solvent type and pH to investigate the origins of the two emission bands. These results, combined with the zeta potential and fluorescence microscopy results, confirmed that the interparticle distance of CDs is a key factor for their dual-emissive PL behavior. With the advantage of unique PL features, we showed the potential of the CDs for white light-emitting diode (WLED) and ratiometric pH sensing applications.

High Quantum Yield Fluorescent Carbon Nanodots for detection of Fe (III) Ions and Electrochemical Study of Quenching Mechanism

Carbon nanodots (CNDs) offer potential applications in photocatalysis, optoelectronics, bio-imaging, and sensing due to their excellent photoluminescence (PL) properties, biocompatibility, aqueous solubility, and easy functionalization. Recent emphasis on CNDs in the selective detection of metal ions is due to the growing concern for human and environmental safety. In this work, two types of fluorescent carbon nanodots (CNDs) are synthesized economically from ethylene diamine (E-CNDs) or urea (U-CNDs) in a single step microwave process. The as-prepared CNDs exhibit excellent PL at an excitation wavelength of 350 nm with a quantum yield of 64% for E-CNDs and 8.4% for U-CNDs with reference to quinine sulfate. Both E-CNDs and U-CNDs demonstrate high selectivity towards Fe (III) ions among different metal ions, by fluorescence quenching in a dose dependent manner. The limit of detection of E-CNDs and U-CNDs is observed to be 18 nM and 30 nM, respectively, in the linear response range of 0-2000 μM with a short response time (seconds). The CNDs detect Fe (III) ions in tap water and serum sample with no spiking and the recovery was ~100% with the Fe (III) samples. Cellular internalization studies confirm the localization of the CNDs and the optical imaging sensing of Fe (III) ions inside living cells. A charge transfer fluorescence quenching mechanism, specifically between the CNDs and Fe (III), is proposed and examined using cyclic voltammetry. The overall characteristics of the E-CNDs provides a potential sensing platform in highly sensitive and selective detection of Fe (III) ions.

Tunable photoluminescence studies based on blue-emissive carbon dots and sequential determination of Fe(III) and pyrophosphate ions

Fluorescence has been well documented and the optical feature of carbon dots generates considerable interests. Here the nitrogen-doped carbon dots with a relative quantum yield of 25% have been prepared. It displays stable blue emission based on the excitation at 355 nm. The carbon nanomaterial is highly dispersible in aqueous solution and can be employed as an effective optical probe for label-free detection of Fe³⁺ (0.87 μM) via a switched off change. Additionally, such sensing nanoplatform can be recovered in the presence of pyrophosphate (PPi) and an "off-on" process has been identified. It is expected that this on-off-on strategy will allow new possibilities for developing efficient sensors in industrial fields.

A facile hydrothermal method for preparation of fluorescent carbon dots on application of Fe3+ and fingerprint detection

The carbon dots (CDs) was prepared by a facile hydrothermal treatment of citric acid and glycine at 180 °C. The CDs at around 3.2 nm was collected after filtration and dialysis. The sample displayed green fluorescence (G-CDs) with a quantum yield of 3.7% in high concentration and the strongest emission peak was at 545 nm under the excitation wavelength of 480 nm; the blue fluorescence CDs (B-CDs) with a quantum yield of 29.8% was obtained after diluted either in solution or in powder, the strongest emission peak was located at 475 nm under the excitation wavelength of 380 nm. The G-CDs possessed a high selectivity to Fe³⁺, which was in a linear range of 0-3.5 μM with the detection limit of 0.21 μM. The CDs powder with blue fluorescence at a relative low content was obtained and adaptable for the fingerprint detection on substrates of litmus paper, resin tabletop, glass, and orange plastic ruler.

Benzoxazine monomer derived carbon dots as a broad-spectrum agent to block viral infectivity

Multiple viruses can cause infection and death of millions annually. Of these, flaviviruses are found to be highly prevalent in recent years with no distinctive antiviral therapies. Therefore, there is a desperate need for broad-spectrum antiviral drugs that can be active against a large number of existing and emerging viruses. Herein, we prepared a kind of benzoxazine monomer derived carbon dots (BZM-CDs) and demonstrated their infection-blocking ability against life-threatening flaviviruses (Japanese encephalitis, Zika, and dengue viruses) and non-enveloped viruses (porcine parvovirus and adenovirus-associated virus). It was found that BZM-CDs could directly bind to the surface of the virion, and eventually the first step of virus-cell interaction was impeded. The developed nanoparticles are active against both flaviviruses and non-enveloped viruses in vitro. Thus, the application of BZM-CDs may constitute an intriguing broad-spectrum approach to rein in viral infections.