Single precursor-based luminescent nitrogen-doped carbon dots and their application for iron (III) sensing

Synthesis, properties and potential applications of photoluminescent carbon nanoparticles: A review

Photoluminescent-carbon nanoparticles (PL-CNPs) are a new class of materials that received immense interest among researchers due to their distinct characteristics, including photoluminescence, high surface-to-volume ratio, low cost, ease of synthesis, high quantum yield, and biocompatibility. By exploiting these outstanding properties, many studies have been reported on its utility as sensors, photocatalysts, probes for bio-imaging, and optoelectronics applications. From clinical applications to point-of-care test devices, drug loading to tracking of drug delivery, and other research innovations demonstrated PL-CNPs as an emerging material that could substitute conventional approaches. However, some of the PL-CNPs have poor PL properties and selectivity due to the presence of impurities (e.g., molecular fluorophores) and unfavourable surface charges by the passivation molecules, which impede their applications in many fields. To address these issues, many researchers have been paying great attention to developing new PL-CNPs with different composite combinations to achieve high PL properties and selectivity. Herein, we thoroughly discussed the recent development of various synthetic strategies employed to prepare PL-CNPs, doping effects, photostability, biocompatibility, and applications in sensing, bioimaging, and drug delivery fields. Moreover, the review discussed the limitations, future direction, and perspectives of PL-CNPs in possible potential applications.

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

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

Dual Fluorometric Detection of Fe3+ and Hg2+ Ions in an Aqueous Medium Using Carbon Quantum Dots as a "Turn-off" Fluorescence Sensor

The present study aimed to develop a carbon dots-based fluorescence (FL) sensor that can detect more than one pollutant simultaneously in the same aqueous solution. The carbon dots-based FL sensor has been prepared by employing a facile hydrothermal method using citric acid and ethylenediamine as precursors. The as-synthesized CDs displayed excellent hydrophilicity, good photostability and blue fluorescence under UV light. They have been used as an efficient "turn-off" FL sensor for dual sensing of Fe³⁺ and Hg²⁺ ions in an aqueous medium with high sensitivity and selectivity through a static quenching mechanism. The lowest limit of detection (LOD) for Fe³⁺ and Hg²⁺ ions was found to be 0.406 µM and 0.934 µM, respectively over the concentration range of 0-50 µM. Therefore, the present work provides an effective strategy to monitor the concentration of Fe³⁺ and Hg²⁺ ions simultaneously in an aqueous medium using environment-friendly CDs.

Effects of nitrogen/bismuth-doping on the photocatalyst composite of carbon dots/titanium dioxide nanoparticles (CDs/TNP) for enhanced visible light-driven removal of diclofenac

The present work demonstrates the coupling of titanium dioxide, TiO₂ nanoparticles (TNP) with N-doped, Bi-doped, and N-Bi co-doped rice husk-derived carbon dots (CDs) via a facile dispersion method, forming respective photocatalyst composites of CDs/TNP, N-CDs/TNP, Bi-CDs/TNP and N-Bi-CDs/TNP. Characterization analyzes verified the successful incorporation of respective CDs samples into TNP, forming photocatalyst composite with narrowed band gap and quenched photoluminescence intensity. Photocatalytic activity of TNP and the respective composites was investigated for photodegradation of diclofenac (DCF) under both simulated sunlight and natural sunlight irradiation. The as-prepared N-Bi-CDs/TNP composite showed the best photocatalytic performance among all composites, able to completely degrade 5 ppm of DCF within 60 min and 180 min under both types of visible light irradiation, respectively. The N-Bi-CDs/TNP composite also showed a TOC removal efficiency up to 87.63%. N-Bi-CDs, worked as photosensitizer and electron reservoir, contributed to the outstanding photocatalytic activity of N-Bi-CDs/TNP, whereby the recombination was prolonged and light absorption was shifted towards the visible light region. Furthermore, the composite of N-Bi-CDs/TNP also demonstrated good stability and reusability over repeated degradation cycles. The photodegradation of DCF resulted into several intermediates, which were identified from LC-MS analysis. The present work could provide an insight on the application of heteroatoms doped and co-doped carbon dots in semiconductor oxide as high performance photocatalysts.

Fluorescent Carbon Dots for Sensitive and Rapid Monitoring of Intracellular Ferrous Ion

Although iron is an essential constituent for almost all living organisms, iron dyshomeostasis at a cellular level may trigger oxidative stress and neuronal damage. Hence, there are numerous reported carbon dots (CDs) that have been synthesized and applied to determine intracellular iron ions. However, among reported CDs focused to detect Fe3+ ions, only a few CDs have been designed to specifically determine Fe2+ ions over Fe3+ ions for monitoring of intracellular Fe2+ ions. We have developed the nitrogen-doped CDs (NCDs) for fluorescence turn-off detection of Fe2+ at cellular level. The as-synthesized NCDs exhibit a strong blue fluorescence and low cytotoxicity, acting as fluorescence probes to detect Fe2+ as low as 0.702 µM in aqueous solution within 2 min and visualize intracellular Fe2+ in the concentration range from 0 to 500 µM within 20 min. The as-prepared NCDs possess some advantages such as high biocompatibility, strong fluorescence properties, selectivity, and rapidity for intracellular Fe2+ monitoring, making NCDs an excellent nanoprobe for biosensing of intracellular ferrous ions.

A facile green synthesis of functionalized carbon quantum dots as fluorescent probes for a highly selective and sensitive detection of Fe3+ ions

In this study, we have reported an economical, easy, greener and non-toxic synthesis route of water soluble carbon quantum dots (CQDs) through hydrothermal treatment using gelatin as precursor. Under the UV lamp of wavelength 365 nm, the as-prepared CQDs exhibit strong blue fluorescence along with CIE coordinate index of (0.17, 0.14) and possess a quantum yield of 22.7% with rhodamine B as standard. The morphology of as-synthesized CQDs as investigated by TEM measurement confirmed their spherical shape and also revealed that their sizes varied in the scale of 0.5-5 nm. Furthermore, the CQDs showed excitation dependent fluorescence emission behaviour in range of 280 nm to 420 nm as a result of quantum confinement effect. Apart from this, in CQDs solution, the addition of Fe³⁺ ion lead to fluorescence quenching effect. These results revealed that the as-synthesized CQDs have a sensitive response towards the Fe³⁺ ion. The calculated limit of detection (LOD) is 0.2 μM with correlation coefficient R² = 0.996 in the concentration range 0 to 50 μM. More remarkably, the application of CQDs for monitoring the trace level of Fe³⁺ ion in tap water yielded acceptable recoveries (103.33%-105%). Therefore, this work provides a novel additional fluorescent probe for the detection of Fe³⁺ ion in real world.

Biomass-derived carbon quantum dots: a novel and sustainable fluorescent "ON-OFF-ON" sensor for ferric ions

Fluorescent carbon dot sensing probes have attracted much attention in recent times due to their amazing properties regarding chemical inertness, solubility, non-toxicity, optoelectronic behavior, and charge transport functionality. Herein, we report the green synthesis of lotus stem-derived carbon dots (LS-CQDs) from the naturally available lotus stem by a simple and economical hydrothermal method without the use of an oxidizing agent. HR-TEM and DLS measurements confirm the quasi-spherical shaped LS-CQDs, with a 2.5 nm average diameter. The LS-CQDs possess better aqueous dispersibility and stability due to the presence of hydrophilic hydroxyl, carboxyl, and amine surface functional groups, as manifested by FT-IR analysis. The LS-CQDs demonstrate excellent fluorescence properties that are sensitive to conditions of pH, time, and temperature. Furthermore, the prepared LS-CQDs display an interesting fluorescence "ON-OFF-ON" property. The LS-CQDs depict a selective and sensitive fluorescence quenching response in the presence of ferric ions. Moreover, the prepared LS-CQDs exhibit a quantum yield of about 0.44%. The LS-CQDs show an excellent sensing response with the limit of detection (LOD) equal to 0.212 ppm. The promising sensitivity and selectivity of LS-CQDs were utilized for the detection of ferric ions in the water samples collected from three polluted sources viz. lake water (Dal lake), underground water (tube well), and stream water. For all the collected water samples the results were reasonably good with the achievement of recovery factor above 1. Therefore, we strongly believe that the present study will serve as a good guiding star for the selective and sensitive detection of ferric ions from various polluted water bodies.

Production of nitrogen-doped carbon quantum dots with controllable emission wavelength, excellent sensing of Fe3+ in aqueous solution, and potential application for stealth quick response coding in the visible regime

Nitrogen-doped carbon quantum dots (N-CQDs) exhibit a high quantum yield with controllable emission wavelength and intensity in the blue-green regime. N-CQDs were tested and determined to be thermally and optically stable during 150 °C heat treatment and prolonged UV irradiation. Potential applications of N-CQDs were demonstrated, including excellent Fe³⁺ sensing in aqueous solution, fluorescent polymer fibres, and stealth quick response coding at visible wavelengths.

Carbon quantum dots have received attention due to their environmental friendliness, low biological toxicity and production cost. Polymer–CQD composite fibers, detection of Fe³⁺, and stealthy fluorescent labels made by thermal transfer printing are potential applications.

Fluorescent films based on PVDF doped with carbon dots for evaluation of UVA protection of sunscreens and fabrication of cool white LEDs

The ultraviolet-A (UVA) radiation from sunlight that reaches the earth's surface can induce premature aging, immunosuppression, and skin cancer. Commercial sunscreen products offer limited information regarding protection against UVA light. Therefore, proposing new and practical alternatives to evaluate the UVA protection capacity of commercial sunscreens is highly imperative. This work presents a novel methodology for evaluating the quality of sunscreens using polyvinylidene fluoride (PVDF) based films doped with plant derived photoluminescent carbon dots (CDs). The bluish white light emitting (under UVA exposure) PVDF/CD films were used to evaluate the UVA protection capacity of 8 different commercial sunscreens. The evaluation of UVA protection is based on the fluorescence attenuation observed with the films coated with sunscreens. In addition, visual evaluation of the UVA protection capacity of the sunscreens and commercial glasses, using the same films and a commercial UV lamp at 365 nm, has also been demonstrated. Two sunscreens with limited UVA protection were identified using the proposed simple evaluation mechanisms without conventionally used expensive instruments and complex methodologies. Additionally, the capacity of PVDF/CD material has been explored for the possible fabrication of WLEDs with cool light emission.

Metformin derived carbon dots: Highly biocompatible fluorescent nanomaterials as mitochondrial targeting and blood-brain barrier penetrating biomarkers

Carbon dots (CDs) have been intensively studied since their discovery in 2004 because of their unique properties such as low toxicity, excellent biocompatibility, high photoluminescence (PL) and good water dispersibility. In this study metformin derived carbon dots (Met-CDs) were synthesized using a microwave assisted method. Met-CDs were meticulously characterized using ultra-violet spectroscopy (UV-vis), photoluminescence (PL), Fourier Transform Infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), atomic force (AFM) and transmission electron (TEM) microscopies. According to results of cytotoxicity studies, Met-CDs possess low-toxicity and excellent biocompatibility towards both non-tumor and tumor cell lines indicating that Met-CDs are outstanding candidates for living cell bioimaging studies. Furthermore, bioimaging studies have displayed that Met-CDs can penetrate the cell membrane and disperse throughout the cell structure including the nucleus and mitochondria. More specifically, Met-CDs tend to start localizing selectively inside the mitochondria of cancer cells, but not of non-tumor cells after 1 h of incubation. Finally, a zebrafish study confirmed that Met-CDs cross the blood-brain barrier (BBB) without the need of any other ligands. In summary, this study presents synthesis of Met-CDs which feature abilities such as mitochondrial and nucleus localizations along with BBB penetration.

Fluorescent recognition of Fe3+ in acidic environment by enhanced-quantum yield N-doped carbon dots: optimization of variables using central composite design

A versatile synthetic approach for development of highly fluorescent nitrogen-doped carbon dots (N-CDs) from carboxymethylcellulose in the presence of linear polyethyleneimine (LPEI) has been developed. According to single factor method, central composite design incorporated with response surface methodology matrix was applied to find and model optimal conditions for the temperature (220-260 °C), duration (1-3 h) and LPEI weight (0.5-1.5%). The statistical results show that duration was the most significant parameter for efficient carbonization conversion rate in comparison with temperature and LPEI weight. The reduced cubic model (R2 = 0.9993) shows a good correlation between the experimental data and predicted values. The optimal variables were temperature of 260 °C, duration of 2 h and LPEI weight of 1%. Under these conditions, quantum yield of up to 44% was obtained. The numerically optimized N-CDs have an average size of 3.4 nm with graphitic nature owing to the abundant amino species incorporated into the carbon core framework. The blue-green N-CDs possess emission dependent upon the solvent polarity, wide pH stability with enhanced emission in an acidic environment. Impressively, the N-CDs show long-shelf-life for up to 1 year with no noticeable precipitation. The N-CDs were able to recognize a high concentration of Fe3+ ions with a detection limit of 0.14 μM in acidic solution owing to the special coordination for Fe3+ to be captured by electron-donating oxygen/ amino groups around N-CDs. Moreover, the N-CDs can also be used as a new kind of fluorescent ink for imaging applications.

Efficient removal of Cu(ii) from aqueous systems using enhanced quantum yield nitrogen-doped carbon nanodots

Low value waste resources have been converted into value-added luminescence carbon dots for copper adsorption from contaminated water.