Red emissive boron and nitrogen co-doped “on-off-on” carbon dots for detecting and imaging of mercury(II) and biothiols

Microwave-assisted Facile Synthesized Carbon Dots as "on-off-on" Fluorescence Probes for Mercury and Iodine Ions in Bio-samples and Cell Imaging

A kind of nitrogen and sulfur co-doped CDs (N, S-CDs) was facilely synthesized using thiourea and citric acid as precursors, which established an "on-off-on" fluorescence probe to sequential detecting mercury and iodine ions inside water and biology samples. Under 360 nm excitation, CDs emit blue fluorescence with an optimal emission peak of 425 nm (on). The fluorescence of CDs experiences a significant quenching effect upon interaction with Hg2+ ions due to the electron transfer between CDs and Hg2+. This quenching effect is subsequently recovered upon the addition of I- owing to the formation of complexes between Hg2+ and I-. The probe exhibits high selectivity and sensitivity toward Hg2+ and I- with broad linearity in the range of 5-50 μM and 15-60 μM, respectively, and a low detection limit of 14.336 nM and 38.213 nM, respectively. The constructed fluorescence probe N, S-CDs has been successfully applied to the detection of Hg2+ and I- in water and biological samples with great recoveries. More importantly, the bioimaging study demonstrated that N, S-CDs are suitable for live monitoring in biological imaging scenarios of Hg2+ and I- in living cells.

Synthesis of N,Si co-doped carbon dots to establish a fluorescent sensor for Hg(II) detection with triple signal output

Mercury is a heavy metal with extreme toxicity. Thus, it is of significance to develop an effective method for mercury ion detection with high performance. In this study, carbon dots doped with nitrogen and silicon (N,Si/CQDs) were successfully prepared from folic acid and N-[3-(trimethoxysily)propyl]-ethylenediamine. The N,Si/CQDs show an obvious cyan fluorescence of 460 nm with the radiation of 350 nm. The existence of mercury ions induces the fluorescence quenching of N,Si/CQDs due to photoinduced electron transfer, which was applied for the sensitive sensing of Hg(II). More importantly, the practical application of the N,Si/CQD probe was confirmed by measurements of Hg(II) in real samples of lake water, sorghum and rice. In addition, the N,Si/CQD nanoprobe was integrated on a sensing strip for specific detection of Hg(II). Quantitative measurement of Hg(II) was realized by the outstanding linearity between the diameter (or fluorescence intensity) of the fluorescence quenching ring and the concentration of mercury ions. The sensor shows potential for rapid detection with a triple signal readout on-site and represents a larger step towards practical applications.

Rapid Preparation of Long-Wavelength Emissive Carbon Dots for Information Encryption Using the Microwave-Assisted Method

The synthesis of long-wavelength emission fluorescent carbon dots is not common, and it is even more difficult to quickly synthesize within 10 min. In this experiment, yellow, orange, and red B, N codoped fluorescent carbon dots were successfully synthesized using a microwave-assisted method with o-phenylenediamine as the carbon-nitrogen source, boric acid as the boron source, and potassium chloride as the catalyst in just 7 min. Based on the different contents of B, N element doping, there are differences in their surface structures, resulting in differences in the luminescence properties of the synthesized carbon dots. Long-wavelength carbon dots can avoid interference from the blue fluorescence of filter papers and have a clearer display in information encryption. Therefore, three types of carbon dots were mixed with PVP to produce fluorescent inks, and anticounterfeiting and encryption patterns were designed on the filter paper, displaying different fluorescence information under sunlight and UV light. In addition, the rich fluorescent colors were combined ingeniously to enable secondary encryption of information in the form of binary codes that increase the difficulty of decoding. These indicate that the three synthesized long-wavelength carbon dots have good application prospects in information encryption.

Carbon dots as specific fluorescent sensors for Hg2+ and glutathione imaging

Nitrogen-doped carbon dots (NCDs) have been constructed in which coal washing wastewater is used as carbon precursor, tryptophan is added for nitrogen doping and surface functional together with polyethylene glycol. The nitrogen doping and surface functional with electron rich groups resulted in excellent fluorescent properties regarding stability, reversibility, printability with high quantum yield which not only enable the NCDs as fluorescent ink for advanced message encryption, but also realize specific on-off-on fluorescent sensing of Hg2+ and GSH as solution, hydrogel and filter paper sensors. The NCDs had a linear range of 0.01-100 μM and a detection limit of 6.27 nM (RSD 0.33%) for Hg2+ and the NCDs@Hg2+ had a linear range of 0.01-60 μM and a detection limit of 3.53 nM (RSD 1.53%) for GSH in sensing studies with aqueous solutions. In addition, with the low cytotoxicity and good biocompatibility NCDs have been successfully used for imaging Hg2+ and GSH in living MG-63 cells. The presented NCDs recycle waste coal washing water into worthwhile material which can be implemented as promising anti-counterfeiting and message encryption candidates as well as effective Hg2+ and GSH sensing, tracking and removing tools in complicated environmental and biological systems.

Review on Carbon Dot-Based Fluorescent Detection of Biothiols

Biothiols, such as cysteine (Cys), homocysteine (Hcy), and glutathione (GSH), play a vital role in gene expression, maintaining redox homeostasis, reducing damages caused by free radicals/toxins, etc. Likewise, abnormal levels of biothiols can lead to severe diseases, such as Alzheimer's disease (AD), neurotoxicity, hair depigmentation, liver/skin damage, etc. To quantify the biothiols in a biological system, numerous low-toxic probes, such as fluorescent quantum dots, emissive organic probes, composited nanomaterials, etc., have been reported with real-time applications. Among these fluorescent probes, carbon-dots (CDs) have become attractive for biothiols quantification because of advantages of easy synthesis, nano-size, crystalline properties, low-toxicity, and real-time applicability. A CDs-based biothiols assay can be achieved by fluorescent "Turn-On" and "Turn-Off" responses via direct binding, metal complex-mediated detection, composite enhanced interaction, reaction-based reports, and so forth. To date, the availability of a review focused on fluorescent CDs-based biothiols detection with information on recent trends, mechanistic aspects, linear ranges, LODs, and real applications is lacking, which allows us to deliver this comprehensive review. This review delivers valuable information on reported carbon-dots-based biothiols assays, the underlying mechanism, their applications, probe/CDs selection, sensory requirement, merits, limitations, and future scopes.

Red-Emissive Sulfur-Doped Carbon Dots for Selective and Sensitive Detection of Mercury (II) Ion and Glutathione

Carbon dots (CDs) show great potential in bioimaging and biosensing because of their good biocompatibility and excellent optical properties. However, CDs with intense red emissions for sensitive and selective detection are rarely reported. Herein, we prepared the red-emissive carbon dots (RCDs) through a facile hydrothermal method using tetra (4-carboxyphenyl) porphyrin (TCPP) and thiourea as starting materials. The obtained RCDs were characterized by TEM, XRD, and XPS. RCDs exhibited high water solubility and strong red emission (λem = 650 nm), with the fluorescence quantum yield as high as 26.7%, which was greatly higher than that of TCPP. Moreover, the as-prepared RCDs could be acted as a highly selective and sensitive probe for the detection of Hg2+ and glutathione (GSH) through the fluorometric titration method. The detection limits of Hg2+ and GSH were calculated to be 1.73 and 1.6 nM, respectively. The cellular experiments demonstrated the good biocompatibility of RCDs and their feasibility in bioimaging. Thus, this work provided a simple strategy to design and synthesize the highly red-emissive carbon dots, which showed promising application in biological and environmental assays.

A highly selective and sensitive "on-off" fluorescent probe for detecting cadmium ions and l-cysteine based on nitrogen and boron co-doped carbon quantum dots

Cadmium ions (Cd2+) have caused relatively serious pollution, threatening human health and ecosystems. l-Cysteine (l-Cys) is an essential amino acid in living organisms and concentration of l-Cys is closely related to some human diseases. In this work, we first introduced 2-amino-3-hydroxypyridine and sodium borohydride as the nitrogen source and boron source to fabricate boron and nitrogen co-doped carbon quantum dots (N,B-CQDs) with high fluorescence quantum yield (21.2%), which were synthesized through a simple, low-consumption and pollution-free one-pot hydrothermal method. The obtained N,B-CQDs are able to detect Cd2+ rapidly and sensitively through fluorescence enhancement, which may be ascribed to chelation enhanced fluorescence that is induced by the formation of the N,B-CQDs/Cd2+ complex. Simultaneously, N,B-CQDs can be used to detect l-cysteine because significant fluorescence quenching was observed when l-Cys was added into the N,B-CQDs/Cd2+ system. In the two fluorescence "turn-on" and "turn-off" processes, this fluorescent probe obtained a good linear relationship over Cd2+ concentration ranging from 2.5 µM to 22.5 µM with a detection limit of 0.45 µM, while the concentration of l-cysteine showed a linear relationship in the range of 2.5-17.5 µM with a detection limit of 0.28 µM. The sensor has been successfully used to detect Cd2+ and l-cysteine in real samples with satisfying results.

Synthesis of metal ion-tolerant Mn-doped fluorescence silicon quantum dots with green emission and its application for selective imaging of ·OH in living cells

Monitoring hydroxyl radical (·OH) in living cells remains a big challenge on account of its high reactivity and short half-life. In this work, we designed a fluorescent probe based on manganese-doped silicon quantum dots (Mn-SiQDs) for detecting and imaging of ·OH with good water solubility. The manganese was doped in its ethylene diamine tetra-acetic acid (EDTA) complex form and effectively improved the metal ion tolerance of fluorescence of SiQDs. And m-dihydroxybenzene was used as the reductant to extend the emission of SiQDs to the green region at 515 nm when the excitation wavelength was 424 nm. Basing on the fluorescence quenching of Mn-SiQDs, a linear response of ·OH was observed in the range 0.8-50 μM with a limit of detection (LOD) of 88.4 nM, which is lower than those reported with SiQDs. The interference from other ROS or RNS has been assessed and no impact was found. In fully aqueous systems, the Mn-SiQDs have been applied to monitor and image the endogenous ·OH in HeLa cells. Our work provided a new strategy for designing SiQDs with good biocompatibility, high selectivity and long monitoring wavelength. Synthesis of green-emitting silicon quantum dots with N-[3 -(trimethoxysilyl) propyl] ethylenediamine (DAMO), Ethylenediamine tetraacetic acid disodium salt dehydrate (EDTA-2Na·2H₂O), manganese acetate tetrahydrate (Mn(CH₃COO)₂·4H₂0) and m-dihydroxybenzene. The green fluorescence of the silicon quantum dots can be selectively quenched by hydroxyl radicals.

High quantum yield nitrogen and boron co-doped carbon dots for sensing Ag+, biological imaging and fluorescent inks

Herein, bright blue-green fluorescent nitrogen and boron co-doped carbon dots (N, B-CDs) with a quantum yield (QY) up to 33.04% were synthesized viahydrothermal treatment from ammonium citrate tribasic and 3-aminophenylboronic acid. The synthesized N, B-CDs showed outstanding water solubility. According to the principle of the static quenching effect (SQE), the synthesized N, B-CDs were utilized as an efficient sensor for sensing Ag⁺. The linear range and limit of detection (LOD) of the sensor for Ag⁺ are 0.99-26.04 μM and 9.03 nM (3σ/m). The proposed method was successfully adopted to detect Ag⁺ in environmental water, which is of great significance to environmental detection. Furthermore, due to the excellent fluorescence performance, the N, B-CDs were found to be an effective tool for biological imaging and as a fluorescent ink, which widens the horizons for the multifunctional applications of N, B-CDs.

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.

Nitrogen and fluorine co-doped green fluorescence carbon dots as a label-free probe for determination of cytochrome c in serum and temperature sensing

As an important biomarker, the analysis of cytochrome c (Cyt c) plays a crucial role in cell-apoptosis or even cancer diagnosis. This work develops a label-free probe for Cyt c using the nitrogen and fluorine co-doped carbon dots (N, F-CDs) which were facile prepared through solvothermal method with 3, 4-difluorophenylhydrazine as precursor. The N, F-CDs have an average diameter of 3.4 nm, and can form a quite stable colloidal solution. The N, F-CDs show bright yellow-green fluorescence, excitation/emission wavelengths 475/530 nm, and a relatively high fluorescence quantum yield of 16.9%. Interestingly, the N, F-CDs indicate a linear and reversible variation of emission intensity with a sensitivity of -1.11% per ℃ in the temperature range from 25 to 60 ℃. Inner filter effect (IFE) between N, F-CDs and Cyt c turns the fluorescence of N, F-CDs from "on" to "off". The sensor possesses the excellent anti-interference ability towards the main components of plasma. Under optimum conditions, there is a linear relationship between fluorescence intensity function (F₀-F) and the concentration of Cyt c in the range of 0.5-25 μΜ with a limit of detection (LOD) (S/N = 3) of 0.25 μM. Finally, the developed method has been successfully used to detect Cyt c in human serum sample with satisfactory recoveries in a range of 93.14-110.40%.

Colorimetric determination of cysteine based on inhibition of GSH-Au/Pt NCs as peroxidase mimic

In this work, we reported a facile and highly sensitive strategy for colorimetric detection of cysteine (Cys) based on the inhibition of catalytic activity of bimetallic nanoclusters induced by Cys. Glutathione-modified gold-platinum nanoclusters (GSH-Au/Pt NCs) with different Au/Pt molar ratios were prepared via one-pot approach and utilized as peroxidase mimics to catalyze the oxidation of 3,3',5,5'-tetramethylbenzidine (TMB) by H₂O₂. It has been found that Cys could inhibit the peroxidase-like activity of GSH-Au/Pt NCs efficiently, which leads to a decrease of the absorption intensity of the system at 652 nm with a fading of the blue color. These findings provide a worthy method for visualization and quantitative detection of Cys with different concentrations in the range from 0.5 to 30 μM, and the detection limit is 0.154 μM. Moreover, this method displays a promising application in colorimetric analysis of Cys in urine samples.

Synthesis of blue fluorescent carbon dots and their application in detecting mercury and iodine based on "off-on" mode

In this study, a type of blue-emission fluorescent carbon dot was synthesized using malic acid, urea, and water. This material demonstrates strong stability to heat, ionic strength, and time. It was found that mercury ions can quench the blue fluorescence of the material, and using iodine ions, the fluorescence can be recovered. Hence, we designed an "off-on" mode to detect mercury and iodine ions using the carbon dots. The results showed that this material exhibits good selectivity and recovery rate. Concurrently, imaging experiments showed that this material demonstrates low cytotoxicity and can be used in cell fluorescence imaging. The study concludes that this material has wider application prospects in the future.

Cu(II)-assisted orange/green dual-emissive carbon dots for the detection and imaging of anthrax biomarker

The spores of Bacillus anthracis are highly deadly to human beings and animals, and are concurrently potential biological warfare agents. Hence, the rapid and sensitive monitoring Bacillus anthracis biomarker, dipicolinic acid (DPA), is very desirable. Herein, orange/green dual-emissive carbon dots (OG-CDs) were synthesized via the hydrothermal approach. The OG-CDs not only emitted dual fluorescence at 527 and 590 nm under the single 503 nm excitation, but also exhibited excellent water solubility, good photostability and great salt tolerance. The fluorescence of the OG-CDs at 527 nm can be completely quenched when chelated with Cu(II). However, because of the stronger chelation between DPA and Cu(II), the fluorescence restored rapidly on subsequent addition of DPA. As such, the CD-Cu(II) system can be used for determination of DPA based on the fluorescence "off-on" response. Under optimum conditions, the detection limit for DPA was 56 nM, with a linear range of 0.5-12.5 μM. The established CD-Cu(II) based spectrofluorometric method has been applied to the analysis of DPA in real water samples with recoveries of 93.6%-104.3%. More remarkably, the CD-Cu(II) probe also has been successfully applied for the imaging of DPA in Escherichia coli with excellent bio-compatibility.

A one-pot synthesis of fluorescent N,P-codoped carbon dots for vitamin B12 determination and bioimaging application

N,P-codoped carbon dots were synthesised using l-arginine and phosphoric acid and explored for the detection of vitamin B₁₂ (VB₁₂) and bioimaging.

Dual Photoluminescence Emission Carbon Dots for Ratiometric Fluorescent GSH Sensing and Cancer Cell Recognition

We developed a facile strategy for the fabrication of dual-emission carbon nanodots (CDs) and demonstrated their applications for ratiometric glutathione (GSH) sensing and for differentiating cancer cells from normal cells. Dual-emission CDs were synthesized using a simple hydrothermal treatment of alizarin carmine as the carbon source, manifesting intriguing dual-emission behavior at 430 and 642 nm. With increasing GSH concentration, the fluorescence band at 430 nm increased gradually, whereas that at 642 nm decreased slightly. With monitoring of the intrinsic ratiometric fluorescence variation (I_430nm/I_642nm), as-prepared CDs were developed as an effective platform for ratiometric fluorescent GSH sensing, with a linear range of 1-10 to 25-150 μM and a detection limit of 0.26 μM. More importantly, confocal fluorescent imaging of cancer cells and normal cells indicated that obtained CDs could be implemented as an effective tool to visualize cancer cells with overexpressing GSH.