Red emission nitrogen, boron, sulfur co-doped carbon dots for "on-off-on" fluorescent mode detection of Ag+ ions and l-cysteine in complex biological fluids and living cells

Dual-mode colorimetric and fluorescent detection of cobalt ions based on N, B co-doped carbon quantum dots and p-phenylenediamine derived nanoparticles

Nitrogen, boron co-doped carbon quantum dots (gCQDs), and a coloration probe (PPD-NPs) with response to cobalt ions (Co2+) were prepared by using 4-hydroxyphenylboric acid as the common precursor, with ethylenediamine and p-phenylenediamine (PPD) adopted as nitrogen-doped reagents, respectively. A noticeable brown-to-purple color change can be observed with the addition of Co2+, and a broad absorption band emerges at 535 nm. At the same time, gCQDs, which is introduced as the fluorescence signal source, will be significantly quenched due to the enhanced inner filtration effect, induced by the overlap between the emission spectrum of gCQDs and the emerging absorption band. Therefore, a colorimetric/fluorescent dual-mode sensing probe for Co2+ is constructed by combining the recognition unit PPD-NPs and the fluorescent gCQDs into PPD-NP/gCQD. Under the optimized experimental conditions, the calculated limits of detection are 1.51 × 10-7 M and 3.75 × 10-7 M for the colorimetric mode and the fluorescence mode, respectively, well qualified for the determination of Co2+ maximum permitted level in drinking water. The feasibility of the proposed method has been verified in tap water, lake water, and black tea samples.

A review on the chemical and biological sensing applications of silver/carbon dots nanocomposites with their interaction mechanisms

The development of new nanocomposites has a significant impact on modern instrumentation and analytical methods for chemical analysis. Due to their unique properties, carbon dots (CDs) and silver nanoparticles (AgNPs), distinguished by their unique physical, electrochemical, and optical properties, have captivated significant attention. Thus, combining AgNPs and CDs may produce Ag/CDs nanocomposites with improved performances than the individual material. This comprehensive review offers an in-depth exploration of the synthesis, formation mechanism, properties, and the recent surge in chemical and biological sensing applications of Ag/CDs with their sensing mechanisms. Detailed insights into synthesis methods to produce Ag/CDs are unveiled, followed by information on their physicochemical and optical properties. The crux of this review lies in its spotlight on the diverse landscape of chemical and biological sensing applications of Ag/CDs, with a particular focus on fluorescence, electrochemical, colorimetric, surface-enhanced Raman spectroscopy, and surface plasmon resonance sensing techniques. The elucidation of sensing mechanisms of the nanocomposites with various target analytes adds depth to the discussion. Finally, this review culminates with a concise summary and a glimpse into future perspectives of Ag/CDs aiming to achieve highly efficient and enduring Ag/CDs for various applications.

Nitrogen, sulfur-doped carbon quantum dots with large Stokes shift for real-time monitoring of pH in living cells

Construction of carbon quantum dots-based (CQDs) fluorescent probes for real-time monitoring pH in cells is still unsatisfied. Here, we propose the synthesis of nitrogen, sulfur-doped CQDs (N,S-CQDs) using one-pot hydrothermal treatment, and serve it as fluorescent probes to realize the real-time sensing of intracellular pH. These pH-responsive N,S-CQDs were proved exhibited a diversity of admirable properties, including great photostability, nontoxicity, favorable biocompatibility, and high selectivity. Particularly, due to the doping of nitrogen and sulfur, N,S-CQDs possessed long-wavelength emission and large Stokes Shift (190 nm), which could avoid self-absorption of tissue to realize high contrast and resolution bioimaging. The response of the probes to pH showed a good linear in range of 0.93-7.00 with coefficient of determination of 0.9956. Moreover, with advantages of high signal-to-noise ratio and stability against photobleaching, the as-prepared N,S-CQDs were successfully applied to monitor pH in living cells via bioimaging. All findings suggest that N,S-CQDs have significant potential for practical application for sensing and visualizing pH fluctuation in living systems.

o-phenylenediamine Derived Fluorescent Carbon Quantum dots for Detection of Hg(II) in Environmental Water

With the expansion of human activities, the consequent influx of mercury (Hg) into the food chain and the environment is seriously threatening human life. Herein, nitrogen and sulfur co-doped fluorescent carbon quantum dots (yCQDs) were prepared via a hydrothermal method using o-phenylenediamine (OPD) and taurine as precursors. The morphological characteristics as well as spectral features of yCQDs indicated that the photoluminescence mechanism should be the molecular state fluorophores of 2, 3-diaminophenothiazine (oxOPD), which is the oxide of OPD. The as-synthesized yCQDs exhibited sensitive recognition of Hg2+. According to the investigation in combination of UV-Vis absorption spectra, time-resolved fluorescence spectra and quantum chemical calculations, the abundant functional groups on the surface of yCQDs allowed Hg2+ to bind with yCQDs through various interactions, and the formed complexes significantly inhibited the absorption of excitation light, resulting in the static fluorescence quenching of yCQDs. The proposed yCQDs was utilized for Hg2+ sensing with the limit of detection calculated to be 4.50 × 10- 8 M. Furthermore, the recognition ability of yCQDs for Hg2+ was estimated in tap water, lake water and bottled water, and the results indicated that yCQDs have potential applications in monitoring Hg2+.

Carbon Dots based Tissue Equivalent Dosimeter as an Ionizing Radiation Sensor

This work explores the potential of carbon dots as a fluorescent probe in the determination of heavy ions and as an electrochemical biosensor. It also discusses how carbon dots can be introduced into the Fricke solution to potentially serve as an ionizing radiation sensor. The study presents a novel tissue equivalent dosimeter carbon dots-based as an ionizing radiation sensor. The methodology for the synthesis of Nitrogen-doped Carbon Dots N-CDs and the characterization of the material are described. The results show that the N-CDs have a high sensitivity to ionizing radiation and can be used as a dosimeter for radiation detection. The study also discusses the limitations and challenges of using carbon dots as a dosimeter for ionizing radiation. Overall, this study provides valuable insights into the potential applications of carbon dots in different fields and highlights the importance of further research in this area.

Multicolor emission-based nitrogen, sulfur and boron co-doped photoluminescent carbon dots for sequential sensing of Fe3+ and cysteine: RGB color sensor and live cell imaging

Herein, a simple hydrothermal synthesis is used to prepare multiple heteroatom-doped photoluminescent carbon dots (CDs) from thiourea (N and S source) and boric acid (B source) as precursors. The optical and physicochemical properties of the as-synthesized NSB-CDs were studied using UV-Vis, photoluminescence, TEM, FT-IR, XRD, Raman, and XPS analyses. The NSB-CDs exhibited excellent stability, high photostability, pH, and ionic strength tolerance; they retained their excellent stability independent of excitation. The NSB-CDs featured small sizes of approximately 3.2 ± 0.4 nm (range: 2.0-5.0 nm) as evidenced using TEM measurements. The NSB-CDs were used as a photoluminescent sensing platform to detect Fe3+ as well as cysteine (Cys) molecules. The competitive binding of Cys to Fe3+ resulted in NSB-CDs that retained their photoluminescence. For the rapid identification and quantification of Fe3+ and Cys, NSB-CDs were developed as a "switch-on" dual-function sensing platform. The linear detection range of Fe3+ was 0-20 μM (limit of detection [LOD]: 54.4 nM) and that of Cys was 0-50 μM (LOD: 4.9 nM). We also introduced a smartphone RGB analysis method for detecting low-concentration solutions based on digital images. The NSB-CDs showed no toxicity at 100 μg/mL. Photoluminescent probes for multicolor live-cell imaging can be used with NSB-CDs at this concentration, suggesting that NSB-CDs may be promising photoluminescent probes.

A ratiometric fluorescent probe for simultaneous detection of L-ascorbic acid and alkaline phosphatase activity based on red carbon dots/polydopamine nanocomposite

Herein, efficient red carbon dots (R-CDs) were synthesized by one-step hydrothermal treatment of N-(4-amino phenyl) acetamide and (2,3-difluoro phenyl) boronic acid. The optimal emission peak of R-CDs was at 602 nm (under 520 nm excitation) and the absolute fluorescence quantum yield of R-CDs was 12.9%. Polydopamine, which was formed by the self-polymerization and cyclization of dopamine in alkaline condition, emitted characteristic fluorescence with peak position of 517 nm (under 420 nm excitation) and affected the fluorescence intensity of R-CDs through inner filter effect. L-Ascorbic acid (AA), which was the hydrolysis product of L-ascorbic acid-2-phosphate trisodium salt under the catalytic reaction of alkaline phosphatase (ALP), effectively prevented the polymerization of dopamine. Combined with the ALP-mediated AA production and the AA-mediated polydopamine generation, the ratiometric fluorescence signal of polydopamine with R-CDs was correlated closely with the concentration of both AA and ALP. Under optimal conditions, the detection limits of AA and ALP were 0.28 μM during linear range of 0.5-30 μM and 0.044 U/L with linear range of 0.05-8 U/L, respectively. This ratiometric fluorescence detection platform can efficiently shield the background interference of sophisticated samples by introducing a self-calibration as reference signal in a multi-excitation mode, which can detect AA and ALP in human serum samples with satisfactory results. Such R-CDs/polydopamine nanocomposite provides a steadfast quantitative information and makes R-CDs be excellent candidate for biosensors via combining target recognition strategy.

The Green Synthesis of Carbon Quantum Dots through One-step Hydrothermal Approach by Orange Juice for Rapid, and Accurate Detection of Dopamine

In the current study, the fluorescent Carbon quantum dots (CDs) were synthesized through one-step hydrothermal approach by orange juice without any additional agents. The as-prepared green-CDs (GCDs) were quasi-spherical shape ranged from 2 to 8 nm with an average diameter of 5 nm, and emitted bright blue fluorescent (FL) under ultraviolet light irradiation (Uv). Different detailed analyses proved that the as-prepared GCDs had good morphologies, various functional groups, high water solubility, great optical features, and excellent stability towards diverse environmental conditions. The results indicated that the as-prepared GCDs can detect different concentrations of dopamine from 1 to 100 µM based on the quenching of their native fluorescent. Furthermore, the good linear relationship was obtained for dopamine in the broad range of concentrations from 1 to 100 µM with the limit of detection (LOD) of 0.81 µM. In addition, the as-prepared GCDs can be applied as a fluorescent probe for detection of dopamine in the different real samples.

Highly sensitive detection of microRNA-21 by nitrogen-doped carbon dots-based ratio fluorescent probe via nuclease-assisted rolling circle amplification strategy

Highly sensitive and selective detection of microRNA-21 (miRNA-21) in biological samples is critical for the disease diagnosis and cancer treatment. In this study, a nitrogen-doped carbon dots (N-CDs)-based ratio fluorescence sensing strategy was constructed for miRNA-21 detection with high sensitivity and excellent specificity. Bright-blue N-CDs (λ_ex/λ_em = 378 nm/460 nm) were synthesized by facile one-step microwave-assisted pyrolysis method by using uric acid as the single precursor, and the absolute fluorescence quantum yield and fluorescence lifetime of N-CDs were 35.8% and 5.54 ns separately. The padlock probe hybridized with miRNA-21 firstly and then was cyclized by T4 RNA ligase 2 to form a circular template. At the present of dNTPs and phi29 DNA polymerase, the oligonucleotide sequence in miRNA-21 was prolonged to hybridize with the surplus oligonucleotide sequences in circular template, generating long and reduplicated oligonucleotide sequences containing abundant guanine nucleotides. Separate G-quadruplex sequences were generated after the addition of Nt.BbvCI nicking endonuclease, and then hemin bound with G-quadruplex sequence to construct the G-quadruplex DNAzyme. Such G-quadruplex DNAzyme catalyzed the redox reaction of o-phenylenediamine (OPD) with H₂O₂, finally producing the yellowish-brown 2,3-diaminophenazine (DAP) (λ_em = 562 nm). Due to the inner filter effect between N-CDs and DAP, the ratio fluorescence signal of DAP with N-CDs was utilized for sensitive detection of miRNA-21 with detection limit of 0.87 pM. Such approach has practical feasibility and excellent specificity for miRNA-21 analysis during highly homological miRNA family in HeLa cell lysates and human serum samples.

In-situ synthesis of ionic liquid-based-carbon quantum dots as fluorescence probe for hemoglobin detection

Carbon quantum dots (CQDs) have emerged as a potential fluorescent probe in bio/analytical chemistry in the present decade. The optical characteristics of CQDs may be tuned by their functional groups, which can also be used to selectively produce stable bonds with target molecules. Along with them, ionic liquids (ILs) are now demonstrating their important relevance in the field of pharmaceuticals for the creation of potent therapeutics. In the article, we have discussed the use of high fluorescent ILs-decorated-CQDs (CQDs-IM@OTf) as a straightforward and quick-acting fluorescence probe for sensitive and precise hemoglobin (Hb) determination with minimum detectability of 6.7 nM. The proposed mechanism behind this involves static mode of quenching which leads to the formation of a ground state complex [CQDs-IM@OTf-Hb complex] between the Hb protein and the drug. Despite the fact that Hb can quench the fluorescence of CQDs due to the inner filter effect (IFE) of the protein, which effects both the excitation and emission spectra of the CQDs, the addition of H₂O₂ improved the sensitivity of Hb detection. The present assay predicated on Hb interaction with H₂O₂, which produces reactive oxygen species such as hydroxyl (OH^.) and superoxide (O₂^.-) radicals under heme degradation and/or iron release from Hb. The subsequent reaction of hydroxyl radicals with CQDs, which acts as a strong oxidising agent, causes a high fluorescence quenching. The designed fluorescence probe was used to measure Hb in the concentration range of 3-90 nM with a precise detection limit of 0.33 nM. The quantification of hemoglobin (Hb) in diluted human blood samples is done using this observation.

A ratiometric fluorescent nanoprobe based on CdSe quantum dots for the detection of Ag+ in environmental samples and living cells

With the widespread application of Ag⁺ in modern life and industry, the potential hazardous effects of Ag⁺ to environment and humans have attracted great concerns. Thus, effective and rapid strategies for Ag⁺ detection are highly desirable. In this paper, a novel ratiometric fluorescence sensor using CdSe quantum dots (QDs) has been constructed for sensitive and selective detection of Ag⁺, which is based on the formation of Ag₂Se QDs. CdSe QDs were initially prepared and showed single wavelength emission at 510 nm. When Ag⁺ exists, a rising peak appeared at 650 nm and the emission at 510 nm declined, exhibiting distinct ratiometric fluorescence emission (I₆₅₀/I₅₁₀) characteristic with a linear response over the Ag⁺ concentration range of 0.01-4 μM. Significantly, the fluorescence changed from green to red. The detection limit of the constructed sensor is 1.4 nM. Furthermore, the sensing assay can be successfully applied to detect Ag⁺ in real water samples and living cells.

Carbon Dots-Based Fluorescence Assay for the Facile and Reliable Detection of Ag(+) in Natural Water and Serum Samples

In this report, red-emissive carbon dots (C-dots) were facilely prepared from o-phenylenediamine via microwave-assisted hydrothermal treatment. The C-dots demonstrated excitation wavelength-independent emission with maximums at 621 nm that could be effectively quenched by Ag⁺ via static quenching. This phenomenon was exploited to establish a sensitive fluorescence assay with a low detection limit (0.37 μM) and wide linear range (0-50 μM). In addition, this assay demonstrated excellent selectivity toward Ag⁺, free from the interference of 16 commonly seen metal ions. Most importantly, the assay demonstrated high reliability toward samples in deionized water, mineral water, lake water, and serum, which could indicate potential applications for Ag⁺ monitoring in complicated natural and biological environments.

Facile hydrothermal synthesis and purification of fluorescent carbon dots for food colorant tartrazine detection based on a dual-mode nanosensor

Colorant tartrazine is widely used in the food industry, but its long-term and excessive consumption is harmful to human health. Therefore, it is necessary to establish a sensitive detection method for tartrazine. Blue fluorescent carbon dots with L-arginine and o-phenylenediamine as precursors, namely L-Arg/oPD-CDs, were prepared via the hydrothermal method. Then, L-Arg/oPD-CDs were further purified by dialysis, thin layer chromatography and column chromatography. A dual-mode nanosensor based on fluorescent and UV absorption was successfully developed. Excellent linear ranges of 0-5 μM and 10-50 μM were obtained with a low detection limit of 42.3 nM based on fluorescence. A good linear range of 0-50 μM was obtained with a low detection limit of 130.15 nM based on UV absorption. The quenching mechanism of tartrazine towards L-Arg/oPD-CDs fluorescence was the inner filter effect. In addition, a dual-mode nanosensor was used for tartrazine determination in millet, maize flour, carbonated drink, and sugar samples. This study provides new insight into the detection of tartrazine by applying a dual-mode nanosensor.

Novel β-cyclodextrin doped carbon dots for host-guest recognition-assisted sensing of isoniazid and cell imaging

In the present study, novel β-cyclodextrin doped carbon dots (CCDs) were prepared via a simple one-pot hydrothermal method at a mild temperature (140 °C), using mixtures of β-cyclodextrin and citric acid as precursors. By characterizing the chemical properties of CCDs prepared at 140 °C and 180 °C, the importance of low-temperature reaction for preservation of the specific structure of β-CD was elucidated. The CCDs showed excellent optical properties and were stable to changes in pH, ionic strength and light irradiation. Since the fluorescence of the CCDs could be selectively quenched by isoniazid (INZ) through specific host-guest recognition effects, a convenient isoniazid fluorescence sensor was developed. Under the optimal conditions, the sensor exhibited a relatively low detection limit of 0.140 μg mL^-1 and a wide detection range from 0.2 μg mL^-1 to 50 μg mL^-1 for INZ detection. Furthermore, the sensor was employed successfully for the determination of INZ in urine samples with satisfactory recovery (91.1-109.5%), displaying potential in clinical applications. Finally, low cytotoxicity of the prepared CCDs was confirmed using the CCK-8 method, followed by application in HepG2 cell imaging.

Carbon Quantum Dots from Pomelo Peel as Fluorescence Probes for “Turn-Off–On” High-Sensitivity Detection of Fe3+ and L-Cysteine

This study designed a “turn-off–on” fluorescence analysis method based on carbon quantum dots (CQDs) to detect metal ions and amino acids in real sample systems. CQDs were derived from green pomelo peel via a one-step hydrothermal process. The co-doped CQDs with N and S atoms imparted excellent optical properties (quantum yield = 17.31%). The prepared CQDs could be used as fluorescent “turn-off” probes to detect Fe³⁺ with a limit of detection of 0.086 µM, a linear detection range of 0.1–160 µM, and recovery of 83.47–106.53% in water samples. The quenched CQD fluorescence could be turned on after adding L-cysteine (L-Cys), which allowed detection of L-Cys with a detection limit of 0.34 µM and linear range of 0.4–85 µM. Recovery of L-Cys in amino acid beverage was 87.08–122.74%. Visual paper-based testing strips and cellulose/CQDs composite hydrogels could be also used to detect Fe³⁺ and L-Cys.

Fluorine and nitrogen co-doped near-infrared carbon dots for fluorescence "on-off-on" determination of nitrite

A fluorescence "on-off-on" strategy was established for the determination of nitrite in aqueous solution based on fluorine and nitrogen co-doped near-infrared carbon dots (NIR-CDs). NIR-CDs were prepared via one-step hydrothermal method by using N-(4-aminophenyl)-acetamide and 4,5-difluorobenzene-1,2-diamine as precursors. The photoluminescence quantum yield of NIR-CDs reaches to 17.4%, and the optimal emission peak of NIR-CDs is 675 nm under excitation of 530 nm. The Stokes shift of NIR-CDs (145 nm) is higher than that of some CDs with longer emission wavelengths. The red bathophenanthroline disulfonic acid (BPS)-Fe²⁺ complex can quench the fluorescence of NIR-CDs via inner filter effect and static quench modes. Nitrite can oxidize Fe²⁺ to produce Fe³⁺ in acidic environment, resulting in not only the formation of colorless and unstable BPS-Fe³⁺ complex but also the fluorescence recovery of NIR-CDs. This fluorescence "on-off-on" phenomenon also comes with the color variation of the mixture, resulting in both the fluorescence and the visual determination of nitrite. Under optimal conditions, this assay exhibits a good linear range from 1 to 50 μM and a low detection limit of 0.056 μM for nitrite determination. The method showed good applicability for nitrite determination in soil extract, human urine, and water samples with acceptable results. A convenient fluorescence "on-off-on" strategy for nitrite detection based on fluorine and nitrogen co-doped near-infrared carbon dots (NIR-CDs) and bathophenanthroline disulfonic acid (BPS)-Fe²⁺ complex was innovatively established. This probe showed a low detection limit of 0.056 μM for nitrite in authentic samples, which offered a new sight for fluorescent and visual detection of nitrite in environmental protection and human health areas.

Smartphone-based fluorescence detection of bilirubin using yellow emissive carbon dots

Development of highly sensitive and selective fluorescent probes for biomolecule detection has significant implications in clinical diagnosis and bioanalysis. In this study, yellow emissive carbon dots (Y-CDs, λ_ex 430 nm, λ_em 550 nm) are synthesized utilizing a one-pot solvothermal approach with o-phenylenediamine (oPDA) as a precursor. The fluorescence of Y-CDs was quenched with the addition of bilirubin due to the inner filter effect mechanism. The fluorescence intensity of Y-CDs decreases as bilirubin concentration increases and can be completely quenched with approximately 90 μM bilirubin. Over other coexisting interferents (26 interferents), the Y-CD probe exhibited great selectivity for bilirubin. More crucially, a smartphone can capture the visible color intensity change of the Y-CD probe under a 365 nm UV lamp and later with the aid of computer software, RGB (red/green/blue) analysis was performed for the quantification of colors. This provides computer vision-based detection and sensitive bilirubin assay with a linear range of 4.0-225 μM and a limit of detection of 1.37 μM. Furthermore, the proposed fluorescent probe was applied in real samples (newborn serum, serum and urine of adults with hyperbilirubinemia) with satisfactory recoveries (96-102%). Based on the validation findings, solution and computer vision-based methods have the potential to be used as fast detection methods for bilirubin in biological samples at the bedside. For the first time, a fluorescent probe based on yellow emissive CDs and RGB analysis for bilirubin recognition has been reported.

Green Synthesis of Nitrogen–Doped Carbon Dots from Fresh Tea Leaves for Selective Fe3+ Ions Detection and Cellular Imaging

In this research, we successfully developed a green, economical and effective one–step hydrothermal method for the synthesis of fluorescent nitrogen–doped carbon dots (N–CDs) by utilizing fresh tea leaves and urea as the carbon and nitrogen sources, respectively. The obtained N–CDs were characterized by TEM, XPS and FT–IR. We found that the N–CDs were near–spherical with an average size of about 2.32 nm, and contained abundant oxygen and nitrogen functional groups. The N–CDs exhibited bright blue fluorescence under ultraviolet illumination, with the maximum emission at 455 nm. Meanwhile, the as–prepared N–CDs could be selectively quenched by Fe3+ ions. The quenching of N–CDs is linearly correlated with the concentration of Fe3+ in the range of 0.1–400 μM with a low detection limit of 0.079 μM. Significantly, the N–CDs present excellent biocompatibility and high photostability. The results also depict that multicolor fluorescence is displayed under a fluorescence microscope and successfully applied for the detection of intracellular Fe3+. To sum up, the fluorescent N–CDs are expected to be a sensitive detection probe for Fe3+ in biological systems.

Engineering the Local Atomic Environments of Indium Single-Atom Catalysts for Efficient Electrochemical Production of Hydrogen Peroxide

The in-depth understanding of local atomic environment-property relationships of p-block metal single-atom catalysts toward the 2 e^- oxygen reduction reaction (ORR) has rarely been reported. Here, guided by first-principles calculations, we develop a heteroatom-modified In-based metal-organic framework-assisted approach to accurately synthesize an optimal catalyst, in which single In atoms are anchored by combined N,S-dual first coordination and B second coordination supported by the hollow carbon rods (In SAs/NSBC). The In SAs/NSBC catalyst exhibits a high H₂ O₂ selectivity of above 95 % in a wide range of pH. Furthermore, the In SAs/NSBC-modified natural air diffusion electrode exhibits an unprecedented production rate of 6.49 mol peroxide g_catalyst ^-1  h^-1 in 0.1 M KOH electrolyte and 6.71 mol peroxide g_catalyst ^-1  h^-1 in 0.1 M PBS electrolyte. This strategy enables the design of next-generation high-performance single-atom materials, and provides practical guidance for H₂ O₂ electrosynthesis.

Facile one-step fabrication of Cu-doped carbon dots as a dual-selective biosensor for detection of pyrophosphate ions and measurement of pH

High-performance determination of pyrophosphate ions (PPi) and pH is an important goal in biological systems. In this work, Cu-doped carbon dots (Cu-CDs) were synthesized rapidly and simply via a one-pot hydrothermal method. The as-obtained Cu-CDs, with an average size of 2.55 nm, exhibit an excitation-independent fluorescence emission and possess desirable functional groups of carboxyl and amine, which can be served as fluorescence nanoprobes for detection of PPi based on surface passivation. Under the optimal condition, the linear range for detection of PPi was 0.05-20 µM, and the corresponding limit of detection (LOD) was 0.013 µM, indicative of a promising assay for the PPi. Moreover, the fluorescent intensity of the Cu-CDs is linear against pH value from 6 to 8.7 in buffer solution, suggesting the feasibility as a pH sensor. The synthesized Cu-CDs coated fluorescent paper indeed can monitor pH in urine with satisfaction by naked eyes through ultraviolet irradiation. The successful detection of PPi and the visual detection of pH value suggest a highly promising application of Cu-CDs in the field of biosensing.

A label-free fluorescence nanosensor based on nitrogen and phosphorus co-doped carbon quantum dots for ultra-sensitive detection of new coccine in food samples

In this paper, an innovative method for the sensitive detection of new coccine using N, P-doped carbon quantum dots (N,P-CQDs) as fluorescent nanosensor is reported for the first time. The sensing mechanism is based on the fluorescence quenching of N,P-CQDs by new coccine through inner filter effect (IFE). N,P-CQDs were prepared by simple hydrothermal treatment of citric acid, phosphoric acid and ethylenediamine. Under the optimal conditions, the new coccine has two good linear responses in the concentration range of 0.2-100 and 100-200 μM, and the detection limits are as low as 24.8 and 9.4 nM, respectively. Our developed nanosensor has been successfully used for the determination of new coccine in food samples with good precision and high accuracy. This work highlights the economic, rapid, simple, selective and ultra-sensitive for new coccine detection, and opens up a new way for the monitoring of new coccine in actual food samples.

Integration detection of mercury(ii) and GSH with a fluorescent "on-off-on" switch sensor based on nitrogen, sulfur co-doped carbon dots

Using aurine and citric acid as precursors, we have synthesized stable blue-fluorescent nitrogen and sulfur co-doped carbon dots (NS-CDs), with a high quantum yield of up to 68.94% via a thermal lysis method. The fluorescent NS-CDs were employed as a sensitive sensor for the integration detection of Hg2+ and glutathione (GSH). This was attributed to Hg2+ effectively quenching the fluorescence of the NS-CDs by static quenching, and then GSH was able to recover the fluorescence owing to the stronger binding between Hg2+ and the sulfhydryl of GSH. Based on the "on-off-on" tactic, the detection limits of Hg2+ ions and GSH were 50 nM and 67 nM respectively. The fluorescence sensor was successfully applied to detect Hg2+ ions and GSH in actual samples (tap water and fetal bovine serum). Furthermore, we have proved that the sensor had good reversibility. Overall, our NS-CDs can serve as effective sensors for environmental and biological analysis in the future.

Carbon dot composites for bioapplications: a review

Recent advancements in the synthesis of carbon dot composites and their applications in biomedical fields (bioimaging, drug delivery and biosensing) have been carefully summarized. The current challenges and future trends of CD composites in this field have also been discussed.

Nitrogen and sulfur co-doped carbon dots with bright fluorescence for intracellular detection of iron ion and thiol

Carbon dots (CDs) have been widely used in recent years because of their excellent water solubility and abundant surface functional groups. However, compared with quantum dots or biological probes, the quantum yield of CDs is lower, and the fluorescence mainly concentrated in the blue-green range, which significantly limits the biological applications of CDs. Heteroatoms doping is the most common method to improve the luminescence of CDs. In this work, nitrogen and sulfur co-doped luminescent CDs were successfully synthesized by microwave assisted method using glutathione (GSH) and p-phenylenediamine (PPD) as raw materials. It can emit bright green fluorescence in ethanol solution, and the maximum emission wavelength is 535 nm when excited at 374 nm, and the absolute quantum yield is as high as 63%. Iron ion (Fe³⁺) can interact with the functional groups on the surface of the CDs to form CDs/Fe³⁺, which is a non-fluorescence complex, and Fe³⁺ can be reduced to ferrous ion (Fe²⁺). In other words, the reaction mechanism of CDs and Fe³⁺ is a combination of dynamic quenching and static quenching. The fluorescence of CDs quenched by Fe³⁺ can be restored by thiol, because there is a stronger binding force between sulfhydryl (-SH) on the surface of thiol and Fe³⁺, which enables CDs to be released. In addition, the CDs has good biocompatibility and stability, indicating that it has excellent potential in bioimaging. This discovery will expand the application of CDs in the fields of biosensing and imaging.

Nanosensors Based on Structural Memory Carbon Nanodots for Ag+ Fluorescence Determination

Ag⁺ pollution is of great harm to the human body and environmental biology. Therefore, there is an urgent need to develop inexpensive and accurate detection methods. Herein, lignin-derived structural memory carbon nanodots (C_SM-dots) with outstanding fluorescence properties were fabricated via a green method. The mild preparation process allowed the C_SM-dots to remain plentiful phenol, hydroxyl, and methoxy groups, which have a specific interaction with Ag⁺ through the reduction of silver ions. Further, the sulfur atoms doped on C_SM-dots provided more active sites on their surface and the strong interaction with Ag nanoparticles. The C_SM-dots can specifically bind Ag⁺, accompanied by a remarkable fluorescence quenching response. This “turn-off” fluorescence behavior was used for Ag⁺ determination in a linear range of 5–290 μM with the detection limit as low as 500 nM. Furthermore, findings showed that this sensing nano-platform was successfully used for Ag⁺ determination in real samples and intracellular imaging, showing great potential in biological and environmental monitoring applications.

Simple and Sensitive Multi-components Detection Using Synthetic Nitrogen-doped Carbon Dots Based on Soluble Starch

Although carbon dots (CDs) as fluorescent sensors have been widely exploited, multi-component detection using CDs without tedious surface modification is always a challenging task. Here, two kinds of nitrogen-doped CDs (NCD-m and NCD-o) based on soluble starch (SS) as carbon source were prepared through one-pot hydrothermal process using m-phenylenediamine and o-phenylenediamine as nitrogenous dopant respectively. Through fluorescence "on-off" mechanism of CDs, NCD-m and NCD-o could be used as a fluorescence sensor for detection of Fe ³⁺ and Ag ⁺ with LOD of 0.25 and 0.51 μM, respectively. Additionally, NCD-m could be used for indirect detection of ascorbic acid (AA) with LOD of 5.02 μM. Moreover, fluorescence intensity of NCD-m also exhibited the sensitivity to pH change from 2 to 13. More importantly, Both NCD-m and NCD-o had potential application for analysis of complicated real samples such as tap water, Vitamin C tablets and orange juice. Ultimately, the small size of NCD-m could contribute to reinforcing intracellular endocytosis, which allowed them to be used for bacteria imaging. Obviously, these easily obtainable nitrogen-doped CDs were able to be used for multi-components detection. Strategy for synthesis of nitrogen-doped carbon dots (NCDs) and a schematic for fabrication of as-prepared NCDs for detection of Fe ³⁺, Ag ⁺ and ascorbic acid (AA).

Towards Red Emissive Systems Based on Carbon Dots

Carbon dots (C-dots) represent an emerging class of nontoxic nanoemitters that show excitation wavelength-dependent photoluminescence (PL) with high quantum yield (QY) and minimal photobleaching. The vast majority of studies focus on C-dots that exhibit the strongest PL emissions in the blue/green region of the spectrum, while longer wavelength emissions are ideal for applications such as bioimaging, photothermal and photodynamic therapy and light-emitting diodes. Effective strategies to modulate the PL emission of C-dot-based systems towards the red end of the spectrum rely on extensive conjugation of sp2 domains, heteroatom doping, solvatochromism, surface functionalization and passivation. Those approaches are systematically presented in this review, while emphasis is given on important applications of red-emissive suspensions, nanopowders and polymer nanocomposites.

Phenanthroline Derivative Fluorescent Probe for Rapid and Sensitive Detection of Silver(I)

In the present work, a phenanthroline derivative (2-(2-methoxyphenyl)-4-phenyl-1,10-phenanthroline, MPP), as a fluorescent probe, was synthesized to realize a rapid, simple and sensitive detection of silver(I). The detection conditions of Ag⁺ were optimized. This fluorescent probe has the advantages of a fast reaction time, a wide pH applicable range, and a low detection limit, exhibiting a good linear response between the fluorescence intensity and the concentration in the range of 0.05 - 1.5 μmol/L for Ag⁺. The detection limit is as low as 3.38 × 10^-8 mol/L (S/N = 3). This probe had been used to detect Ag⁺ in real samples, and the recovery efficiency of spiked Ag⁺ had been also tested. The recovery efficiency is satisfactory, ranging from 92.0 to 105.4%. Therefore, this fluorescent probe should provide a new choice for the quantitative detection of silver ions in environmental water samples.

Biocompatible sulfur nitrogen co-doped carbon quantum dots for highly sensitive and selective detection of dopamine

In this work, sulfur and nitrogen co-doped carbon quantum dots (S,N-CQDs) were prepared via one-pot hydrothermal treatment of EDTA disodium and sodium sulfide. The prepared S,N-CQDs were characterized by TEM, XRD, FT-IR, XPS, UV-vis absorption and fluorescence spectra to characterize their morphology, crystal structure, functional groups, elemental composition, and optical properties. It was found that S and N elements were successfully doped into the CQDs and the morphology was approximately spherical with an average particle size of 2.16 nm, in which the excitation/emission wavelengths were 350 and 420 nm, respectively. Compared with single element doped CQDs, double element doped CQDs have a higher quantum yield and excellent optical stability. Cell experiments showed that S,N-CQDs had good biocompatibility because they had no obvious toxicity on both normal cell lines and cancer cell lines. More importantly, based on the synergy of static quenching and dynamic quenching, the S,N-CQDs were used as effective fluorescent probes for sensitive detection of DA, with high anti-interference and low limit of detection. Based on the good biocompatibility of S,N-CQDs, the detection of dopamine in actual serum samples were carried out and the results showed an excellent recovery rate. Therefore, this work provides a dopamine sensor with a practical application prospect.

Doped-carbon dots: Recent advances in their biosensing, bioimaging and therapy applications

As a fascinating class of fluorescent carbon dots (CDs), doped-CDs are now sparked intense research interest, particularly in the diverse fields of biomedical applications due to their unique advantages, including low toxicity, physicochemical, photostability, excellent biocompatibility, and so on. In this review, we have summarized the most recent developments in the literature regarding the employment of doped-CDs for pharmaceutical and medical applications, which are published over approximately the past five years. Accordingly, we discuss the toxicity and optical properties of these nanomaterials. Beyond the presentation of successful examples of the application of these multifunctional nanoparticles in photothermal therapy, photodynamic therapy, and antibacterial activity, we further highlight their application in the cellular labeling, dual imaging, and in vitro and in vivo bioimaging by use of fluorescent-, photoacoustic-, magnetic-, and computed tomography (CT)-imaging. The potency of doped-CDs was also described in the biosensing of ions, small molecules, and drugs in biological samples or inside the cells. Finally, the advantages, disadvantages, and common limitations of doped-CD technologies are reviewed, along with the future prospects in biomedical research. Therefore, this review provides a concise insight into the current developments and challenges in the field of doped-CDs, especially for biological and biomedical researchers.

A sensitive turn-off-on fluorometric sensor based on S,N co-doped carbon dots for environmental analysis of Hg(II) ion

A simple and sensitive fluorescence turn-off-on sensor was established by means of S,N co-doped carbon dots (S,N-CDs) and Ag nanoparticles (AgNPs) for the determination of Hg²⁺ . For this purpose, blue emissive S,N-CDs were hydrothermally synthesized and characterized using transmission electron microscopy, Fourier transform infrared spectroscopy, and energy dispersive X-ray spectroscopy. We observed that the fluorescence intensity of the as-prepared S,N-CDs was impressively quenched by AgNPs. The quenching mechanism was studied and attributed to nanosurface energy transfer and the inner filter effect between S,N-CDs and AgNPs. Furthermore, by adding Hg²⁺ , the fluorescence intensity of S,N-CDs/AgNPs was restored as a result of aggregation of AgNPs in the presence of Hg²⁺ . Based on these facts, S,N-CDs and AgNPs were exploited to design a sensitive turn-off-on sensor for analysis of Hg²⁺ . The recovered fluorescence signal was proportional to the concentration of Hg²⁺ in the range 1.5-2000 nM with a detection limit of 0.51 nM. The established sensor was used with satisfactory results for measurement of Hg²⁺ in environmental water samples.

Rapid microwave-assisted green synthesis of guanine-derived carbon dots for highly selective detection of Ag+ in aqueous solution

In this work, a simple and green synthetic approach of novel guanine decorated carbon dots (G-CDs) using guanosine 5'-monophosphate and ethylenediamine through a domestic microwave oven was established for the first time. The as-prepared fluorescent G-CDs were characterized by transmission electron microscopy, X-ray photoelectron spectroscopy, UV-vis spectroscopy, and fluorescence spectroscopy. The obtained fluorescent G-CDs with a uniform morphology had desirable functional groups and excellent optical performances. Furthermore, the fluorescence intensity of G-CDs was remarkably quenched by Ag⁺ than that of other nucleotides-derived CDs. The density functional theory calculations were performed to confirm that the strong interaction of guanine-Ag⁺ was responsible for the remarkable fluorescence response of G-CDs towards Ag⁺. In addition, as a label-free fluorescence probe, the G-CDs displayed a good linear detection for highly selective Ag⁺ sensing over the range of 0-80 μM with the low detection limit of 90 nM. Therefore, the proposed G-CDs had the capacity for Ag⁺ detection in the real samples.

You Don't Learn That in School: An Updated Practical Guide to Carbon Quantum Dots

Carbon quantum dots (CQDs) have started to emerge as candidates for application in cell imaging, biosensing, and targeted drug delivery, amongst other research fields, due to their unique properties. Those applications are possible as the CQDs exhibit tunable fluorescence, biocompatibility, and a versatile surface. This review aims to summarize the recent development in the field of CQDs research, namely the latest synthesis progress concerning materials/methods, surface modifications, characterization methods, and purification techniques. Furthermore, this work will systematically explore the several applications CQDs have been subjected to, such as bioimaging, fluorescence sensing, and cancer/gene therapy. Finally, we will briefly discuss in the concluding section the present and future challenges, as well as future perspectives and views regarding the emerging paradigm that is the CQDs research field.

Ratiometric fluorescent sensors for sequential on-off-on determination of riboflavin, Ag+ and l-cysteine based on NPCl-doped carbon quantum dots

The fluorescent sensor, especially ratiometric fluorescent sensor, is one of the most important applications for CQDs, which is becoming a research hotspot. Herein, carbon quantum dots co-doped with nitrogen, phosphorus and chlorine (NPCl-CQDs) were synthesized by acid-base neutralization reaction exothermic carbonization method. The as-fabricated NPCl-CQDs could emit blue fluorescence and possess excellent fluorescence properties. Based on the FRET, multifunctional and ratiometric fluorescent sensors for "on-off-on" sequential determination of riboflavin, Ag⁺, and Cys with good selectivity and high sensitivity were established. The linear range of riboflavin, Ag⁺, and Cys are 0.50-10.18 μM and 15.89-27.76 μM, 0.66-1.46 mM and 1.50-4.20 mM, and 0.01-0.15 μM and 0.15-0.36 μM with the limit of detection of 3.50 nM, 26.38 μM, and 0.96 nM, respectively. Furthermore, the sensors were successfully used to determine riboflavin, Ag⁺, and Cys in tablets, river water, and human urine with the recoveries of 95.2-104.0%, 95.6-102.0%, and 94.8-106.4%, respectively. More importantly, the as-constructed "on-off-on" NPCl-CQDs-based ratiometric fluorescent sensors were applied for detecting riboflavin, Ag⁺, and Cys in HeLa cells with satisfying results. The finding of this study shows the feasibility and effectiveness of the NPCl-CQDs as the available ratiometric fluorescent sensors for the determination of riboflavin, Ag⁺, and Cys in real samples and living cells.

Hydrothermal synthesis of fluorescent carbon dots from gardenia fruit for sensitive on-off-on detection of Hg2+ and cysteine

Nitrogen and sulfur co-doped carbon dots (N/S-CDs) were prepared by a simple hydrothermal method using gardenia fruit as precursor. The N/S-CDs are nearly spherical particles with a size of 2.1 nm and possess excellent fluorescence stability in a wide pH range and high NaCl concentrations, as well as under UV light irradiation. The absolute quantum yield (QY) without any surface modification reaches up to 10.7%. Meanwhile, the N/S-CDs can be quenched by Hg²⁺ (turn-off), while the quenched fluorescence can be recovered (turn-on) by introducing cysteine (Cys), with linear ranges of 2-20 μM for Hg²⁺and 0.1-2.0 μM for Cys. On the basis of the on-off-on sensing mode, the N/S-CDs can be used to detect Hg²⁺ and Cys. Hence, the N/S-CDs would be a promising sensor in environmental and biological analysis.

A mitochondrion-targeting fluorescent probe for hypochlorite anion in living cells

As momentous reactive oxygen species (ROS), it is necessary to develop high-sensitivity and high-specificity fluorescent probes for tracking hypochlorite anion (ClO^-) in environmental and biological systems. Herein, a kind of red luminescent carbon dots (NS-dots) was synthesized by one-step solvothermal method to detect ClO^- in PBS buffer solution (V_PBS:V_EtOH = 100:1, pH = 7.4). The NS-dots has high sensitivity and low detection limit (13.3 μmol/L) for detecting ClO^- with linear range from 6.7 × 10^-5 mol/L to 26.7 × 10^-5 mol/L. Using Rhodamine B (31% at 520 nm in water) as a reference, the NS-dots have a fluorescence quantum yield of 7.2%. Intracellular photostability, mitochondrial targeting properties and the fluorescence imaging towards intracellular ClO^- were demonstrated.

Fe3+ and intracellular pH determination based on orange fluorescence carbon dots co-doped with boron, nitrogen and sulfur

The fluorescent boron, nitrogen and sulfur co-doped carbon dots (BNSCDs) were prepared by simple hydrothermal reaction of 4-carboxyphenylboronic acid and 2,5-diaminobenzenesulfonic acid at 200 °C for 8 h. The fluorescence of the BNSCDs could be quenched by Fe³⁺ based on the electron transfer between Fe³⁺ and BNSCDs, so a label-free, good selectivity and high sensitivity method for Fe³⁺determination was established with linear range and LOD of 1.5-692 μmol/L and 87 nmol/L, respectively. And then the fluorescent probe was employed for detection of Fe³⁺ in tap water, coal gangue, fly ash and food samples successfully. Moreover, the as-prepared BNSCDs could serve as a novel pH fluorescent probe in the range of pH 1.60-7.00, which could be attributed to the proton transfer of carboxyl groups on the surface of BNSCDs. More importantly, the pH fluorescent probe possesses fast, real-time and low toxicity, applying for intracellular pH fluorescence imaging in HIC, HIEC, LO2 and SMMC7721 cells. In view of its simplicity, timely response and outstanding compatibility, the as-fabricated BNSCDs show the potential applications in water quality and solid waste monitoring, food detection, real-time measuring of intracellular pH change in vitro.

Carbon quantum dots derived from the extracellular polymeric substance of anaerobic ammonium oxidation granular sludge for detection of trace Mn(vii) and Cr(vi)

Carbon quantum dots (CQDs) were synthesized via a hydrothermal method, in which extracellular polymeric substance (EPS) from anaerobic ammonium oxidation (anammox) granular sludge was used as a carbon precursor, while citric acid and ethylenediamine were applied as auxiliary carbon source and passivation agent, respectively. The synthesized CQDs, with orderly spherical shape and mean size of 7.15 nm, emitted blue fluorescent light under UV radiation of 365 nm. The CQDs had a high fluorescence yield (40.84%), with good water solubility and excellent spectroscopic properties. In addition, the CQDs exhibited selective, sensitive and distinctive fluorescence quenching behaviors for Cr(vi) and Mn(vii) in a PBS buffer solution (NaH₂PO₄–Na₂HPO₄) of pH 7, with a detection limit of 5.8 nM for Cr(vi) and 2.3 nM for Mn(vii). Owing to the nitrogen components from the EPS of anammox granules, the CQDs were well nitrogen-doped, promoting electron-transfer and leading to reduction between the CQDs and Mn(vii)/Cr(vi). These results indicate that CQD-based chemical sensing is a simple and efficient means for the fluorescence detection of Mn(vii) and Cr(vi).

Fluorescence detection of trace Mn(vii) or Cr(vi) by the CQDs prepared from anaerobic ammonium oxidation granular sludge.