Sulfur and nitrogen binary doped carbon dots derived from ammonium thiocyanate for selective probing doxycycline in living cells and multicolor cell imaging

Detection of Doxycycline Using Carbon Quantum dots as Probe Based on Internal Filtering Effect

The establishment of a convenient and effective detection method for doxycycline (DC) holds significant importance in drug monitoring and drug residue assessment. In this work, carbon quantum dots (CQDs) with excellent and stable luminescence performance (the quantum yield of CQDs was 21.8%) were synthesized by the melting method and employed as probes to monitor the fluorescence intensity variations before and after the introduction of DC. A fluorescence analytical method based on the internal filtration effect (IFE) was developed for DC determination. The mechanism of DC quenching CQDs was verified using fluorescence lifetime tests, absorption spectroscopy, and evaluation of internal filtration parameters. After optimizing experimental conditions, it was found that the DC concentration (CDC) exhibited a good linear relationship with the fluorescence quenching efficiency ((F0-F)/F0) of CQDs in the range of 5-30 µM. The fitted linear equation was Y = 0.01249*CDC+0.03625, R2 = 0.9987, and the detection limit was 2.343 µM (n = 8). This developed method has been successfully applied to accurately determine DC concentrations in both doxycycline hydrochloride tablets and human serum samples. It stands out for its simplicity, rapidity, and acceptable detection performance. Due to its advantages, this method holds great promise for application in the biomedical field for monitoring DC drug concentrations and ensuring quality control.

Synthesis of oil-soluble carbon dots via pyrolysis and their diverse applications in doxycycline detection, fluorescent ink and film

The overuse of doxycycline poses a risk for ecological environment. Advanced materials such as anti-counterfeiting and photovoltaic materials are urgently needed to develop innovative strategies for exploiting solar cells and protecting valuable products. Herein, oil-soluble CDs (o-CDs) were successfully fabricated from citric acid, tris-base and oleylamine as precursors via pyrolysis method. The o-CDs with uniform size distribution exhibited a high quantum yield of 0.48 and excellent photostability. The fluorescence of o-CDs was rapidly quenched by doxycycline at room temperature without further modification. Optimal conditions were selected to construct a fluorescence probe with high selectivity and good sensitivity to detect doxycycline. Interestingly, the probe achieved two linear ranges of 0.85--16.7 µM and 16.7--33.4 µM with a low detection limit of 0.26 µM. Furthermore, inner filter effect (IFE) was dominated in the process in which doxycycline interact with the oxygen-containing groups of o-CDs. This sensing platform has been further successfully applied to the detection of doxycycline in milk with recovery rates of 96.8%- 102.7% and relative standard deviations of 0.98%- 1.02%, suggesting that the novel probe has the potential to be applied in real samples. Moreover, o-CDs directly serve as fluorescence ink and work as fluorescence film using PVA as matrix because of strong fluorescence in the solid state, indicating that they have potential applications in anti-counterfeiting and photovoltaic materials. This is the first report that oil-soluble CDs via pyrolysis is applied in the detection of doxycycline in milk. Importantly, this work provides efficient strategies for the construction of anti-counterfeiting and photovoltaic materials.

Reactant conversion-intercalation strategy toward interlayer-expanded MoS2 microflowers with superior supercapacitor performance

In order to avoid the complicated control and fussy procedure associated with foreign species and templates in conventional synthesis strategies, a simple reactant conversion-intercalation strategy is developed to synthesize interlayer-expanded MoS₂ (E-MoS₂) by employing ammonium thiocyanate converted from a thiourea reactant as intercalator. In this strategy, the thiourea plays a bifunctionality role as reactant and intercalator precursor to ensure in situ embedding into the interlayers of MoS₂ to expand the interlayer spacing. The optimal E-MoS₂ obtained presents superior supercapacitor performance with a specific capacity of 246.8 F g^-1 at 0.5 A g^-1 in 1 M Na₂SO₄ electrolyte in a three-electrode system, outperforming pristine MoS₂ prepared by a conventional hydrothermal method (42.5 F g^-1 at 0.5 A g^-1). Furthermore, a symmetric supercapacitor based on an E-MoS₂ electrode delivers a high specific capacity of 261.3 F g^-1 and energy density of 13.3 W h kg^-1 at 0.5 A g^-1, and excellent cycling life with 81.7% capacity retention after 3000 cycles at 2 A g^-1. Density functional theory calculations reveal that the NH₄⁺ and SCN^- can be effectively adsorbed on the surface to be inserted into the interlayers during the growth of MoS₂, resulting in an expanded interlayer spacing of 9.4 Å, and the favorable electrochemical performance stems from the large Na⁺ adsorption capacitance and low diffusion barrier of the E-MoS₂. This work offers a novel intercalation strategy that may be generally applicable to other layer-structured materials, shedding some light on the development of high-performance electrode materials via interface engineering for energy applications.

Facile Synthesis of Carbon Dots from Amido Black 10b for Sensing in Real Samples

Herein, a one-step hydrothermal synthesis method was adopted to fabricate carbon dots (CDs) from amido black 10b in a sodium hydroxide solution. The morphology and composition of the CDs were investigated by XRD, FTIR TEM, XPS, UV-vis, and fluorescence spectroscopy. The obtained CDs (AB-CDs) with an average diameter of 19.4 nm displayed a well-dispersed characteristic in aqueous solutions. The as-prepared CDs showed bright blue fluorescence and good photostability, with a high quantum yield of 24.1%. AB-CDs displayed a selective and noticeable turn-off response to Fe³⁺. Accordingly, the quantitative detection of Fe³⁺ was achieved in the range of 5-200 μmol L^-1 with a detection limit of 1.84 μmol L^-1. The fluorescence response mechanism of Fe³⁺ to AB-CDs was ascribed to static quenching due to the emergence of the ground-state complex. Moreover, ascorbic acid could restore the fluorescence of AB-CDs quenched by Fe³⁺ by reducing Fe³⁺ to Fe²⁺. The developed nanoprobe was used to detect ascorbic acid with a limit of detection of 7.26 μmol L^-1 in the range of 20-300 μmol L^-1. Furthermore, the developed sensing system was successfully applied for an Fe³⁺ assay in a lake water sample and ascorbic acid detection in a human urine sample. The AB-CD-based analytical system showed its latent practical value in the chemical analysis and bioanalytical fields.

Application of sulfur and nitrogen doped carbon quantum dots as sensitive fluorescent nanosensors for the determination of saxagliptin and gliclazide

In this study, highly fluorescent sulfur and nitrogen doped carbon quantum dots (S,N-CQDs) were used as fluorescent nanosensors for direct spectrofluorimetric estimation of each of gliclazide (GLZ) and saxagliptin (SXG) without any pre-derivatization steps for the first time. S,N-CQDs were synthesized employing a simple hydrothermal technique using citric acid and thiosemicarbazide. The produced S,N-CQDs were characterized using different techniques including fluorescence emission spectroscopy, UV spectrophotometry, high-resolution transmission electron microscopy and FT-IR spectroscopy. Following excitation at 360 nm, S,N-CQDs exhibited a strong emission peak at 430 nm. The native fluorescence of S,N-CQDs was quantitatively enhanced by addition of increased concentrations of the studied drugs. The fluorescence enhancement of S,N-CQDs and the concentrations of the studied drugs revealed a wide linear relationship in the range of 30.0-500.0 µM and 75.0-600.0 µM with limits of detection of 5.0 and 10.15 µM for GLZ and SXG, respectively. The proposed method was efficiently used for determination of cited drugs in their commercial tablets with % recoveries ranging from 98.6% to 101.2% and low % relative standard deviation values (less than 2%). The mechanism of interaction between S,N-CQDs and the two drugs was studied. Validation of the proposed method was carried out in accordance with International Conference on Harmonization (ICH) guidelines.

Thiosemicarbazide functionalized carbon quantum dots as a fluorescent probe for the determination of some oxicams: application to dosage forms and biological fluids

In this study, highly fluorescent water-soluble nitrogen and sulfur doped carbon quantum dots (N, S-CQDs) were synthesized via a one-step hydrothermal process utilizing citric acid as a carbon source and thiosemicarbazide as a sulfur and nitrogen source. The obtained N, S-CQDs exhibited an intense emission band at 415 nm (λ ex = 345 nm). In the presence of either piroxicam, tenoxicam or lornoxicam, the emission band at 415 nm was significantly quenched which might be triggered due to destruction of the surface passivation layer of the N, S-CQDs. A linear correlation was found between the reduction in the fluorescence intensity of N, S-CQDs and the concentration of each drug in the ranges of 2.0-25.0 μM, 10.0-100.0 μM and 20.0-200.0 μM with correlation coefficients of more than 0.999 for all drugs. The detection limits were 0.49 μM, 1.58 μM and 4.63 μM for piroxicam, tenoxicam and lornoxicam, respectively. The effect of experimental parameters affecting the performance of the method was investigated and optimized. The developed sensor has the advantages of simplicity, time-saving, convenience and satisfactory selectivity for determination of the studied drugs in dosage forms with high % recoveries (98.86-101.69%). The method was extended for determination of piroxicam in spiked plasma with % recoveries ranging from 97.95-101.36%. The method was validated in accordance with International Council of Harmonization (ICH) standards, and the results obtained were compared statistically to those given by reported methods, indicating no significant differences in the level of accuracy and precision. The mechanism of the quenching process was studied and elucidated. The structure-activity relationship between the three drugs and the quenching efficiency was also studied and discussed.

Carbon Dots: An Excellent Fluorescent Probe for Contaminant Sensing and Remediation

Pollution-induced degradation of the environment is a serious problem for both developing and developed countries. Existing remediation methods are restricted, necessitating the development of novel remediation technologies. Nanomaterials with unique characteristics have recently been developed for remediation. Quantum dots (QDs) are semiconductor nanoparticles (1-10 nm) with optical and electrical characteristics that differ from bigger particles owing to quantum mechanics, making them intriguing for sensing and remediation applications. Carbon dots (CDs) offer better characteristics than typical QDs, such as, CdSe QDs in terms of contaminant sensing and remediation. Non-toxicity, chemical inertness, photo-induced electron transfer, good biocompatibility, and adjustable photoluminescence behavior are all characteristics of CDs. CDs are frequently made from sustainable raw materials as they are cost-effective, environmentally compactable, and excellent in reducing waste generation. The goal of this review article is to briefly describe CDs fabrication methods, to deeply investigate the criteria and properties of CDs that make them suitable for sensing and remediation of contaminants, and also to highlight recent advances in their use in sensing and remediation of contaminants.

The facile synthesis of nitrogen and sulfur co-doped carbon dots for developing a powerful "on-off-on" fluorescence probe to detect glutathione in vegetables

To determine the glutathione (GSH) content in vegetables, an "on-off-on" fluorescence probe was developed by a synthesis of nitrogen and sulfur co-doped carbon dots (N,S-CDs) using the microwave pyrolysis considering citric acid and L-cysteine as precursors. The fluorescence of N,S-CDs was quenched by adding Cu²⁺ at a concentration of 20-200 μmol/L due to the inner filter effect. The quenched fluorescence of N,S-CDs@Cu²⁺ system was recovered by adding the GSH at a concentration of 10-150 μmol/L due to the sulfhydryl-metal compound mechanism. By observing the GSH concentrations measured by our N,S-CDs@Cu²⁺ system vs. a traditional fluorescent chelating method, the two measurements provided the GSH data with a good consistence by showing the RSD range of 1.86%-2.27%. This indicates the validation and novelty of our N,S-CDs@Cu²⁺ system as being a powerful fluorescent probe for effectively and efficiently determining the GSH in vegetables.

Ratiometric detection of doxycycline in pharmaceutical based on dual ligands-enhanced copper nanoclusters

A water-soluble, stable, simple and dual ligands (bovine serum albumin and L-histidine)-enhanced copper nanoclusters (BSA-CuNCs@L-His) was synthesized by one-step wet chemical method. Interestingly, the introduction of L-His ligand could improve evidently the quantum yields (QYs, 3.47%) and stability of BSA-CuNCs due to forming the stronger interaction of L-His and Cu and producing bigger diameter CuNCs by coordination-induced aggregation. Thus, a new ratiometric fluorescent probe (RF-probe) was successfully exploited for sensitively and selectively mensurating doxycycline (DOX) because DOX could simultaneously regulate the fluorescence (FL) intensities of BSA-CuNCs@L-His at 410 and 520 nm. The FL quenching of BSA-CuNCs@L-His at 410 nm by DOX was mainly originated from the static quenching process, while DOX could bind to Trp-212 in BSA from the skeleton of BSA-CuNCs@L-His by electrostatic interaction causing the appearance of new emission peak at 520 nm. The content of DOX was monitored by the RF-probe with a linear range of 0.05-14.0 μM and a LOD (limit of detection) and LOQ (limit of quantification) of 6.4 and 21.3 nM (at 3σ/slope and 10σ/slope). Moreover, compared to the standard HPLC method, the proposed RF-probe was extended to the detection of DOX in doxycycline hydrochloride (DOXH) tablets, DOXH injections and DOXH capsules with satisfactory results.

Novel ratiometric probe based on the use of rare earth-carbon dots nanocomposite for the visual determination of doxycycline

Rare earth-carbon dots (RE-CDs) hybrid nanomaterials with the merits of both RE and CDs have rapidly emerging as highly promising functional materials in biochemical analysis. In this work, a new kind of water-soluble RE-CDs nanocomposite (CDs@CaF₂:Eu³⁺) was developed for the ratiometric determination of doxycycline (DOX). The CDs@CaF₂:Eu³⁺ under the excitation at 365 nm displayed blue emission of CDs at 440 nm and no obvious emission of Eu³⁺. With the addition of DOX, substantial fluorescence quenching of the CDs at 440 nm and enhancement of Eu³⁺ at 613 nm were observed, resulting in a ratiometric fluorescent response toward DOX. A wide linear range from 0.1 µM to 30 µM was achieved in the detection of DOX with a lowest detection limit of 43 nM. In particular, the probe could discriminate DOX from other tetracycline antibiotics through unique fluorescence response. Moreover, we have successfully applied the method for the determination of DOX in milk and honey samples.

Subcellular imaging and diagnosis of cancer using engineered nanoparticles

The advances in the synthesis of nanoparticles with engineered properties are reported to have profound applications in oncological disease detection via optical and multimodal imaging and therapy. Among various nanoparticle-assisted imaging techniques, engineered fluorescent nanoparticles show great promise from high contrast images and localized therapeutic applications. Of all the fluorescent nanoparticles available, the gold nanoparticles, carbon dots, and upconversion nanoparticles are emerging recently as the most promising candidates for diagnosis, treatment, and cancer monitoring. This review addresses the recent progress in engineering the properties of these emerging nanoparticles and their application for cancer diagnosis and therapy. In addition, the potential of these particles for subcellular imaging is also reviewed here.

Bifunctional Carbon Dots Derived From an Anaerobic Bacterium of Porphyromonas gingivalis for Selective Detection of Fe3+ and Bioimaging

In this study, for the first time, Porphyromonas gingivalis, an anaerobic bacterium, was selected to synthesize carbon dots. The achieved P. gingivalis-carbon dots (Pg-CDs) exhibited strong fluorescence and high stability with capability for dual function as Fe³⁺ sensor and intracellular imaging agent. The detection limit for Fe³⁺ was as low as 1.85 µm. On the other hand, the prepared Pg-CDs were an excellent candidate for biosensor with high biocompatibility.

Green one-pot synthesis of nitrogen and sulfur co-doped carbon quantum dots as new fluorescent nanosensors for determination of salinomycin and maduramicin in food samples

A simple green hydrothermal method was proposed for synthesis of highly fluorescent nitrogen and sulfur co-doped carbon quantum dots (N,S-CQDs) using citric acid and thiosemicarbazide. The produced N,S-CQDs were subjected to extensive spectroscopic characterization and applied as fluorescent nanosensors for the sensitive spectrofluorimetric determination of salinomycin and maduramicin directly without prior derivatization for the first time. The obtained N,S-CQDs showed strong emission band at 430 nm after excitation at 360 nm. The native fluorescence of N,S-CQDs was found to be quenched by the addition of increased concentrations of each drug. Method validation revealed a wide linear relationship between the fluorescence quenching of N,S-CQDs and the concentration of each drug in the range of 10.0-300.0 μM with detection limits of 2.07 μM and 1.34 μM for salinomycin and maduramicin, respectively. The developed method has been efficiently applied for estimation of analytes in six raw matrices with high recoveries.

A Novel Eplerenone Ecofriendly Fluorescent Nanosensor Based on Nitrogen and Sulfur-Carbon Quantum Dots

In this work, a novel ecofriendly optical nanosensor for detection of Eplerenone (EPL) in biological samples was reported. Highly luminescent water-soluble nitrogen and sulfur doped carbon quantum dots (N, S-CQDs) have been prepared successfully. The synthesis was based on the reaction of thiosemicarbazide (TS) as source of N and S and citric acid (CA) as source of carbon in one-step aqueous base reflux treatment. The produced N, S-CQDs have a small particle size in the range of 4.7 nm with a high quantum yield (58.5%) and high emission intensity at 446 nm under excitation wavelength of 370 nm. The unique properties of N, S-CQDs make them useful tool as a nano fluorescent probe for sensitive determination of EPL. EPL has been found to decrease the fluorescence of S, N-CDs significantly through static quenching according to the Stern - Volmer plot. The decreased intensity of S, N-CDs fluorescence was proportional to EPL in the 0.2-3.0 μM range. The limit of detection and quantitation were 0.05 and 0.15 μM, respectively. The assay of EPL by this approach was successfully done in drug formulations and in spiked human serum samples.

Selective detection for seven kinds of antibiotics with blue emitting carbon dots and Al3+ ions

In this work, we presented a facile microwave method to prepare blue emitting carbon dots (CDs) using lysine as carbon source and realized the specific detection of seven types of antibiotics by CDs and Al³⁺ ions via a two-step method. The CDs have good solubility in water and their excitation spectra are exactly coincided with the absorption of some typical antibiotics, which leads to the fluorescence quenching of CDs (OFF state). The inhibition mechanism of fluorescence is induced by the combination of inner filtering effect (IFE) and static quenching effect (SQE). In addition, the quenched fluorescence can be recovered by adding Al³⁺ ions (On state), and seven types of antibiotics can be distinguished exactly according to the emission peak position and intensity. It not only provides a new and convenient method for the detection of antibiotics, but also provides a new idea for the further application of CDs in optical sensing.

Dual-ligand functionalized carbon nanodots as green fluorescent nanosensors for cellular dual receptor-mediated targeted imaging

The conjugation of ligands to nanoparticles as drug delivery systems that target specific cells is a promising approach for the delivery of therapeutic agents to tumor cells. Herein, we prepared green-emission fluorescent carbon nanodots (CNDs) by a facile hydrothermal method with d-(+)-glucosamine hydrochloride and l-aspartic acid as the precursors, then covalently conjugated with folate (FA), polyethyleneimine (PEI) and hyaluronic acid (HA) to develop dual ligand-decorated nanocarriers (FA-PEI-HA-CNDs) for the targeted imaging of cancer cells. FA-PEI-HA-CNDs integrated the excellent fluorescence property of CNDs, and can be used for the real-time and noninvasive location tracking of cancer cells. The cellular uptake study demonstrated that FA-PEI-HA-CNDs markedly improved the internalization efficiency in A-549 cells via folate/CD44 receptor-mediated endocytosis in comparison with that of the A549 cells pretreated with excess FA, HA, and FA and HA. Therefore, these dual folate/CD44 receptor-targeted CNDs (FA-PEI-HA-CNDs) show promising potential for cancer detection, drug delivery, and individualized treatment as performance platforms.

One-Step Facile Synthesis of Nitrogen-Doped Carbon Dots: A Ratiometric Fluorescent Probe for Evaluation of Acetylcholinesterase Activity and Detection of Organophosphorus Pesticides in Tap Water and Food

Evaluation of acetylcholinesterase (AChE) activity and determination of organophosphorus pesticides (OPs) are of great importance for the clinical diagnosis of several serious diseases correlated with their variations in human blood serum. In this study, a highly selective and sensitive ratiometric fluorescent probe was innovatively fabricated for the evaluation of AChE activity and the determination of OPs in tap water and food on the basis of the inner filter effect (IFE) between nitrogen-doped carbon dots (N-CDs) and 2,3-diaminophenazine (DAP). N-CDs were synthesized via a one-pot hydrothermal method by using pancreatin and 1,2-ethanediamine as precursors. N-CDs showed excellent fluorescence properties and negligible cytotoxicity on human cervical carcinoma HeLa cells and human embryonic kidney 293T cells, suggesting their further biological applications. Upon the addition of AChE and choline oxidase, acetylcholine was catalyzed to produce choline that was further oxidized to produce H₂O₂. In the presence of horseradish peroxidase, o-phenylenediamine reacted with H₂O₂ to produce fluorescent DAP. Therefore, a ratiometric fluorescent probing platform existed via IFE between N-CDs with a fluorescence signal at 450 nm and DAP with a fluorescence signal at 574 nm. OPs irreversibly impeded the catalytic activity of AChE, finally leading to the decrease of DAP amount and the variation of ratiometric fluorescent signal. Under optimal conditions, such a fluorescent probe showed relatively low detection limits of 0.38 U/L for AChE, 3.2 ppb for dichlorvos, and 13 ppb for methyl-parathion. Practical application of this ratiometric fluorescent probe to detect OPs was further verified in tap water and food samples with satisfying results that were highly consisted with the results obtained by GC-MS.

Influence of nitrogen/phosphorus-doped carbon dots on polyamide thin film membranes for water vapor/N2 mixture gas separation

Nanoparticles have been attracting attention because they can significantly improve the performance of membranes when added in small amounts. In this study, the effect of polyamide membranes incorporating hydrophilic nitrogen/phosphorus-doped carbon dots (NP-CDs) to enhance water vapor/N₂ separation has been investigated. NP-CD nanoparticles with many hydrophilic functional groups are synthesized from chitosan by a one-pot green method and introduced to the surface of the polysulfone (PSf) substrates by interfacial polymerization reaction. The mean particle diameter of NP-CDs, estimated from transmission electron microscopy images, is 2.6 nm. By adding NP-CDs (0–1.5 wt%) to the polyamide layer, the contact angles of the membranes dramatically decreased from 65° (PSf) to <9° (thin film nanocomposite (TFN)), which means that the TFN membranes become significantly hydrophilic. From the water vapor separation results, the addition of NP-CDs in the polyamide layer improves the water vapor permeance from 1511 (thin film composite (TFC) without nanoparticles) to 2448 GPU (TFN with 1.0 wt% NP-CD loading, CD-TFN(1.0)) and the water vapor/N₂ selectivity from 73 (TFC) to 854 (CD-TFN(1.0)). To our knowledge, this is the first study of highly functionalized NP-CD-incorporated polyamide membranes to enhance water vapor separation.

Nanoparticles have been attracting attention because they can significantly improve the performance of membranes when added in small amounts.

N,S co-doped carbon dots as a dual-functional fluorescent sensor for sensitive detection of baicalein and temperature

In this work, nitrogen and sulfur dual-doped CDs (N,S-CDs) were prepared via a facile one-pot hydrothermal method from citric acid and N-acetyl-L-cysteine with a high quantum yield (QY) of 49%. As-fabricated N,S-CDs had a size around 2.5 nm and exhibited excitation-independent emission and excellent luminescent properties. The fluorescent sensor based on the N,S-CDs showed a highly sensitive detection of baicalein with a detection limit (LOD) of 0.21 μmol L^-1 in the linear range from 0.69 to 70.0 μmol L^-1. The fluorescence of the N,S-CDs could be effectively quenched by baicalein based on static quenching. In addition, the temperature sensor based on the synthesized N,S-CDs showed a good linear relationship between temperature and fluorescence (FL) intensity with a temperature range from 5 °C to 75 °C. Furthermore, the synthesized N,S-CDs were successfully applied to the measurement of baicalein in real samples. In a word, the N,S-CDs had great potential to be worked as fluorescence sensors to monitor the concentration of baicalein and temperature.

Fluorescent carbon dots functionalization

Carbon dots (CDs), as a new type of luminescent zero-dimensional carbon nanomaterial, have been applied in a variety of fields. Currently, functionalization of CDs is an extremely useful method for effectively tuning their intrinsic structure and surface state. Heteroatom doping and surface modification are two functionalization strategies for improving the photophysical performance and broadening the range of applications for fluorescent CDs. Heteroatom doping in CDs can be used to tune their intrinsic properties, which has received significant research interests because of its simplicity. Surface modification can be applied for varying active sites and the functional groups on the CDs surface, which can endow fluorescent CDs with the unique properties resulting from functional ligand. In this review, we summarize the structural and physicochemical properties of functional CDs. We focused our review on the latest developments in functionalization strategies for CDs and discuss the detailed characteristics of different functionalization methods. Ultimately, we hope to inform researchers on the latest progress in functionalization of CDs and provide perspectives on future developments for functionalization of CDs and their potential applications.

Functionalized Carbon Dots on Graphene as Outstanding Non-Metal Bifunctional Oxygen Electrocatalyst

Carbon-based bifunctional electrocatalysts for both oxygen reduction and evolution reactions are potentially cost-effective to replace noble metals in energy devices such as fuel cells, metal-air batteries, and photoelectrochemical converters, but enrichment of active sites holds the key to efficiency. Here, graphene frameworks with heteroatom-doped carbon dots (CDs) are developed via a hydrothermal route followed by pyrolysis. The CDs are rationally prepared with careful selection of heteroatoms, embedded on the substrate to provide enriched active sites. Structural characterizations (e.g., transmission electron microscopy and X-ray photoelectron spectroscopy) reveal the successful addition of CDs with nitrogen and sulfur species. Especially, a heat-treated N,S codoped sample, NS-CD@gf_a900, exhibits the optimum oxygen electrocatalysis, even closer to noble-metal counterparts, as a result of the effect of active sites of the CDs and the synergistic behavior of N and S. Considering the importance of size and dopants of the material, this approach not only suggests a straightforward preparation route of nanocarbons, but also appoints the utilization of a new class of non-metal species as efficient oxygen electrocatalysts.

One-pot synthesis of dual carbon dots using only an N and S co-existed dopant for fluorescence detection of Ag. SPECTROCHIMICA ACTA. PART A, MOLECULAR AND BIOMOLECULAR SPECTROSCOPY 2019;208:162-171. [PMID: 30312843 DOI: 10.1016/j.saa.2018.10.004]

Luminescent carbon-based nanoparticles, named often as carbon dots (CDs), were synthesized from citric acid (CA) and guanidine thiocyanate (GITC) via an N and S co-doped hydrothermal procedure. In the present structure characterization, N and S elements could be sufficiently doped by means of the heteroatom or the functional groups bonded on the surface of CDs. The as-prepared CDs solution showed blue color fluorescence under ultraviolet excitation, yet the PL spectra exhibited a repetitive emission process from excitation-independent to excitation-dependent. In view of the triexponential feature of fluorescence lifetimes of CDs, one possibility was proposed to be co-existence of two types of CDs with different surface states. Additionally, the as-prepared CDs were used as a sensing probe for the detection of Ag⁺ taking into consideration of the possible interactions between Ag⁺ and various fluorophores attached to the CD surface. As expected, the changes of fluorescence intensities were linearly proportional to the different concentration ranges of Ag⁺, which suggests the complex nature of the quenching mechanism. And for the first time, the SCN group was found to accelerate the quenching of CDs towards Ag⁺, promising a new approach for efficient detection of Ag⁺ for the application in industrial pollutants.

Amino-Functionalized Al-MOF for Fluorescent Detection of Tetracyclines in Milk

A fluorescent method for detection of tetracyclines (TCs) in milk was developed by using the NH₂-MIL-53(Al) nanosensor synthesized via a one-pot hydrothermal method. The nanosensor had a crystalline nanoplates structure with rich groups of -NH₂ and -COOH. The -NH₂/-COOH of NH₂-MIL-53(Al) reacted with the -CO-/-OH of TCs to form a complex. The electron of -NH₂/-COOH from the NH₂-BDC ligand transferred to the -CO-/-OH of TCs. -NH₂ of the NH₂-MIL-53(Al) interacted with the -CO-/-OH of TCs by hydrogen bonding. The quenching efficiency of the inner filter effect (IFE) was calculated to contribute 57-89%. The synergistic effect of photoinduced electron transfer (PET) and IFE account for fluorescence quenching. TCs were quantitatively detected in milk samples with recoveries of 85.15-112.13%; the results were in great accordance with high-performance liquid chromatography (HPLC) ( P > 0.05), confirming the NH₂-MIL-53(Al) nanosensor has potential applicability for the detection of TCs in food matrix.

Highly fluorescent carbon dots as an efficient nanoprobe for detection of clomifene citrate

Highly fluorescent carbon dots (CDs) were synthesized through facile hydrothermal carbonization and ethylenediamine passivation of an easily available prawn shell precursor. The as-prepared CDs exhibit high water solubility, wavelength-tunable fluorescence with quantum yield up to 68.9%, high photostability and resistance against biomolecules, thus enabling the application as viable fluorescent nanoprobes for detection of guest quenchers. The fluorescence of the CDs can be effectively quenched by clomifene citrate (CC, a common drug for infertility) through static quenching, and therefore can serve as a simple and efficient fluorescent nanoprobe for determination of CC with wide linear range (0.25–10 μg mL⁻¹) and low detection limit (0.2 μg mL⁻¹). The CDs also showed low cytotoxicity, which enables the safe and accurate fluorescent detection of spiked CC in human serum, demonstrating their potential as a credible fluorescent CC nanoprobe in clinical examination.

Highly fluorescent carbon dots (CDs) were synthesized through facile hydrothermal carbonization and ethylenediamine passivation of an easily available prawn shell precursor.

Biomass-based quantum dots co-doped with sulfur and nitrogen for highly sensitive detection of thrombin and its inhibitor

A biomass-based quantum dots co-doped with sulfur and nitrogen was prepared by using garlic and ginger as precursors, and used for the detection of thrombin and its inhibitor based on the inner filter effect (IFE).

Fluorescent carbon dots synthesized by microwave-assisted pyrolysis for chromium(VI) and ascorbic acid sensing and logic gate operation

Herein, nitrogen, sulfur co-doped fluorescent carbon dots (N,SCDs) were synthesized by simple and facile microwave-assisted pyrolysis using ammonium citrate and cysteamine hydrochloride as precursors. The obtained N,SCDs exhibited outstanding photostability, excitation-wavelength independence, excellent fluorescence properties with fluorescence quantum yield (FLQY) up to 54.8%. Significantly, the fluorescence of N,SCDs was effectively quenched by Cr(VI) based on inner filter effect (IFE). Subsequently, the fluorescence of the N,SCDs-Cr(VI) system successfully recovered with addition of ascorbic acid (AA) owing to redox reaction between Cr(VI) and AA. Therefore, N,SCDs could be employed as an efficient fluorescent "turn-off" probe for highly sensitive detection of Cr(VI), with a liner detection range from 0.35 to 126.0 μmol L^-1 and a detection limit of 0.11 μmol L^-1. Meanwhile, the N,SCDs-Cr(VI) system could be used as an fluorescent "turn-on" sensor for detection of AA with a detection limit of 0.17 μmol L^-1 and a liner detection range of 0.57-264.0 μmol L^-1. Interestingly, a molecular logic gate was constructed based on the fluorescence recover of the N,SCDs-Cr(VI) system in the presence of AA. Most importantly, this fluorescent probe has been applied for determination of Cr(VI) and AA in environmental and biological fields with satisfactory results.

Nitrogen and sulfur co-doped carbon nanodots toward bovine hemoglobin: A fluorescence quenching mechanism investigation

A deep understanding of the molecular interactions of carbon nanodots with biomacromolecules is essential for wider applications of carbon nanodots both in vitro and in vivo. Herein, nitrogen and sulfur co-doped carbon dots (N,S-CDs) with a quantum yield of 16% were synthesized by a 1-step hydrothermal method. The N,S-CDs exhibited a good dispersion, with a graphite-like structure, along with the fluorescence lifetime of approximately 7.50 ns. Findings showed that the fluorescence of the N,S-CDs was effectively quenched by bovine hemoglobin as a result of the static fluorescence quenching. The mentioned quenching mechanism was investigated by the Stern-Volmer equation, temperature-dependent quenching, and fluorescence lifetime measurements. The binding constants, number of binding sites, and the binding average distance between the energy donor N,S-CDs and acceptor bovine hemoglobin were calculated as well. These findings will provide for valuable insights on the future bioapplications of N,S-CDs.

Carbon Dots from Sugars and Ascorbic Acid: Role of the Precursors on Morphology, Properties, Toxicity, and Drug Uptake

There is the need for reproducible, simple, high-yielding synthetic protocols aimed at obtaining carbon dots (CDs) with controlled fluorescence, photothermal and photochemical behavior, surface properties, biocompatibility, tumor targeting ability, drug absorption biodistribution, and tumor uptake. This Letter describes a systematic study on the effect of glucose, fructose, and ascorbic acid as starting materials for the preparation of highly luminescent CDs, characterized by a blue emission. Their composition and morphology are investigated by titration of OH surface groups, spectroscopic techniques, and high-resolution transmission electron microscopy (HR-TEM), and their toxicity was tested toward HeLa cells. CDs made using fructose were toxic, while those made from glucose and ascorbic acid showed good biocompatibility. The reproducible and simple synthetic procedure yields luminescent biomass-derived CDs for combined cancer therapy and diagnostics. Their doxorubicin (DOX) drug uptake was measured by spectrofluorimetry, indicating a crucial role of the morphologies of the CDs in controlling DOX loading. The glucose derived CDs showed up to 28% w/w of DOX loading.

Carbon dots doped with nitrogen and boron as ultrasensitive fluorescent probes for determination of α-glucosidase activity and its inhibitors in water samples and living cells

An ultrasensitive fluorometric assay is described for the determination of the activity of the enzyme α-glucosidase in waters and living cells. Carbon dots doped with nitrogen and boron (N,B-CDs) were prepared that have excitation/emission peaks at 400/510 nm and a fluorescence quantum yield of 47%. 4-Nitrophenylglucoside is added and then hydrolyzed by α-glucosidase to form yellow 4-nitrophenol which screens off fluorescence due to an inner filter effect. The method was applied to the determination of α-glucosidase activity and has a 3 mU mL^-1 detection limit. It was subsequently applied to the determination of the α-glucosidase inhibitor acarbose which can be determined in a concentration as low as 10 nM (at three times the standard deviation versus slope). The method was also applied to the determination of α-glucosidase activity and acarbose in living HeLa cells and MCF-7 cells. The method is simple, sensitive, and excellently selective. Graphical abstract N,B-CDs as ultrasensitive fluorescence probe for α-glucosidase activity and its inhibitor in waters and living cells based on IFE.

Fluorimetric and colorimetric analysis of total iron ions in blood or tap water using nitrogen-doped carbon dots with tunable fluorescence

Nitrogen-doped Cdots were fabricated with tunable blue-green fluorescence and changing of color for fluorimetric and colorimetric assays for total iron.

A dual-emission nano-rod MOF equipped with carbon dots for visual detection of doxycycline and sensitive sensing of MnO4−

Herein, ethanediamine-modified carbon dots (CDs) were encapsulated into luminescent MOF(Eu), which was designed for a dual-emission hybrid material (CDs@MOF(Eu)) with diverse fluorescence applications. This material exhibited high selectivity and sensitivity towards doxycycline. With an increasing concentration of doxycycline, the blue light emission of CDs could be quenched, whereas the red light emission of MOF(Eu) was enhanced. In view of this result, more convenient “test paper” was used first as a new tool for doxycycline detection, the colour of which turned from blue-purple to red as observed by the naked eyes under 365 nm UV-irradiation. This hybrid material also was a probe for sensing MnO₄⁻ with a low limit of detection and good anti-interference performance. We propose that CDs can improve detection sensitivity compared with the original MOF(Eu). The possible sensing mechanism was discussed in detail. Importantly, the feasibility of this composite for sensing doxycycline in a simulated biological system and sensing MnO₄⁻ in tap water was investigated.

A dual-emission hybrid material could detect doxycycline and MnO₄⁻ sensitively. Test paper was regarded initially as a tool for doxycycline visual detection. A lower LOD of MnO₄⁻ showed that carbon dots can accelerate quenching speed of MOF(Eu).

N,S co-doped carbon dots as a stable bio-imaging probe for detection of intracellular temperature and tetracycline

N,S-CDs display an unambiguous bioimaging ability in the detection of intracellular temperature and tetracycline with satisfactory results.

Heteroatom-doped carbon dots: synthesis, characterization, properties, photoluminescence mechanism and biological applications

Heteroatom-doped carbon dots (CDs), due to their excellent photoluminescence (PL) properties, attracted widespread attention recently and demonstrated immense promise for diverse applications, particularly for biological applications. The objective of this feature article is to provide a comprehensive overview of the recent progress in the research and development of heteroatom-doped CDs and a detailed description of the influence of single or co-doping heteroatoms on their PL behavior. The most recent understanding and critical insights into the PL mechanism of heteroatom-doped CDs are also highlighted. Moreover, potential bio-related applications of heteroatom-doped CDs in biosensing, bioimaging, and theranostics are also reviewed. This state-of-the-art review will provide a platform for understanding the intricate details of heteroatom-doped CDs, a summary of the latest progress in the field, and related applications in biology and is expected to inspire further developments in this exciting class of materials.

Nitrogen-doped carbon dots originating from unripe peach for fluorescent bioimaging and electrocatalytic oxygen reduction reaction

This paper reports the robust hydrothermal synthesis of nitrogen doped carbon dots (N-CDs) using the unripe fruit of Prunus persica (peach) as the carbon precursor and aqueous ammonia as the nitrogen source. The optical properties of synthesized N-CDs were characterized by ultraviolet visible (UV-Vis) and fluorescence spectroscopy techniques. The synthesized N-CDs were emitted blue light when excitated with a portable UV lamp. The materials with the optical properties were characterized further by high resolution transmission electron microscope (HRTEM), X-ray diffraction (XRD), Raman, Fourier transform infrared (FT-IR) and X-ray photoelectron spectroscopy (XPS). The mean size of the N-CDs was approximately 8nm, as calculated from the HRTEM image. The d-spacing of N-CDs, calculated using Bragg law, was approximately 0.21nm, which was consistent with the interlayer distance calculated from the HRTEM image. FT-IR spectroscopy and XPS revealed the presence of the phytoconstituents functionalities of peach fruit over the N-CDs surface and a high level of nitrogen doping on carbon dots (CDs) was confirmed by XPS studies. These results suggest that the unripe fruit extract of peach is an ideal candidate for the preparation of N-CDs. The resulting N-CDs showed excellent optical properties in water. The synthesized N-CDs exhibited a high fluorescence quantum yield and low cytotoxicity, and can be used as fluorescence imaging probes. In addition, the N-CDs were catalytically activite towards the oxygen reduction reaction (ORR). The N-CDs exhibited good catalytic activity in an alkaline medium (0.1M KOH) with a remarkable ORR of approximately 0.72V vs reversible hydrogen electrode (RHE), and O2 reduction follows mainly a 2 electron pathway by being reduced to hydrogen peroxide. The 2-electron reduction pathway is used in industry for H2O2 production.