Graphene oxide-assisted synthesis of N, S Co-doped carbon quantum dots for fluorescence detection of multiple heavy metal ions

Advancing analytical chemistry with carbon quantum dots: a comprehensive review

Carbon Quantum Dots (CQDs) have gained significant attention as versatile nanomaterials in analytical chemistry due to their strong fluorescence, high sensitivity, and biocompatibility features. This review explores the synthesis, functionalization, and broad applications of CQDs in various analytical domains, including bioimaging, diagnostics, and environmental monitoring. CQDs' unique properties, such as tunable emission and ease of surface modification, enhance their performance in fluorescence and electrochemical sensing. CQDs present emerging applications in single-cell analysis, point-of-care diagnostics, and food safety. Technological advancements in green synthesis and hybrid nanomaterial integration are paving the way for more sustainable, efficient, and scalable analytical tools. However, challenges related to reproducibility, stability, and large-scale production persist, highlighting the need for continued research. The present review provides a comprehensive overview of CQDs' impact, emphasizing their potential to transform analytical chemistry through innovative applications and future breakthroughs.

Application of carbon quantum dots as fluorescent probes in the detection of antibiotics and heavy metals

Carbon quantum dots (CQDs) are ideal fluorescent probes for rapid detection. This paper reviews the synthesis methods of CQDs, their application in the rapid detection of antibiotics and heavy metals in the environment and food, and the underlying detection mechanisms. The hydrothermal method is the most commonly used for synthesis, and CQDs doped with heteroatoms (such as N, P and S) exhibit superior fluorescence performance. In the presence of antibiotics and heavy metals, the fluorescence of CQDs can be quenched or enhanced. Single-signal and dual-signal probes can be developed using the fluorescence, phosphorescence and absorbance of CQDs, enabling rapid detection of various antibiotics (e.g., tetracycline, quinolone and beta-lactam antibiotics) and heavy metals (e.g., Cd2+, Cr6+, Fe3+, Hg2+, and Pb2+). With the combination of smartphones and fluorescent probe test strips developed based on CQDs, on-the-spot rapid detection can be realized. This review offers new insights into the rapid detection of CQDs.

Fluorescent carbon quantum dots for heavy metal sensing

Many heavy metals pose significant threats to the environment and human health. Traditional methods for detecting heavy metals are often limited by complex procedures, high costs, and challenges in field monitoring. Carbon quantum dots (CQDs), a novel class of fluorescent carbon nanomaterials, have garnered significant interest due to their excellent biocompatibility, low cost, and minimal toxicity. This paper reviews the primary synthesis methods, luminescence mechanisms, and fluorescence quenching mechanisms of CQDs, as well as their recent applications in detecting heavy metals. In heavy metal sensing applications, the simplest hydrothermal method is commonly employed for the one-step synthesis and surface modification of CQDs. Various green reagents and biomass materials, such as citric acid, glutathione, orange peel, and bagasse, can be used for CQDs' preparation. Quantum confinement effects and surface defects give CQDs their distinctive luminescence properties, enabling the detection of heavy metals through fluorescence quenching or enhancement. Additionally, CQDs can be applied in biological imaging and smart detection, and when combined with adsorption materials, they can offer multifunctional capabilities. This review also discusses the future development prospects of CQDs.

Engineering the future: Unveiling novel paths in heavy metal wastewater remediation with advanced carbon-based nanomaterials - Beyond performance comparison, tackling challenges, and exploring opportunities

This review addresses the pressing issue of heavy metal pollution in water, specifically focusing on the application of adsorption technology utilizing carbon materials such as biochar, carbon nanotubes, graphene, and carbon quantum dots. Utilizing bibliometric analysis with VOSviewer based on Web of Science core dataset, this study identifies research hotspots related to carbon-based materials in heavy metal applications over the past decade. However, existing literature still lacks sufficient comparative analysis of the potential of carbon-based materials' structural characteristics and inherent advantages in heavy metal applications. This review strategically addresses this gap, offering a comprehensive comparative analysis of these four materials from an engineering application perspective. It offers a thorough evaluation of their suitability for various water treatment applications, providing a detailed examination of their advantages and limitations in heavy metal application. Additionally, the review provides insights into performance comparisons, addresses challenges, and explores emerging opportunities in this field. Insights into potential application fields based on structural characteristics and inherent advantages are presented. This unique focus on a comprehensive comparative analysis distinguishes the article, offering a nuanced perspective on the strengths and future possibilities of carbon materials in tackling the global challenge of heavy metal pollution in water.

Preparation, Physicochemical, and Cyto- and Genotoxic Characterisation of Polysaccharide Composites Containing Carbon Quantum Dots

Rapid industrial growth is associated with an increase in the production of environmentally harmful waste. A potential solution to significantly reduce pollution is to replace current synthetic materials with readily biodegradable plastics. Moreover, to meet the demands of technological advancements, it is essential to develop materials with unprecedented properties to enhance their functionality. Polysaccharide composites demonstrate significant potential in this regard. Polysaccharides possess exceptional film-forming abilities and are safe for human use, biodegradable, widely available, and easily modifiable. Unfortunately, polysaccharide-based films fall short of meeting all expectations. To address this issue, the current study focused on incorporating carbon quantum dots (CQDs), which are approximately 10 nm in size, into the structure of a starch/chitosan biocomposite at varying concentrations. This modification has improved the mechanical properties of the resulting nanocomposites. The inclusion of nanoparticles led to a slight reduction in solubility and an increase in the swelling degree. The optical characteristics of the obtained films were influenced by the presence of CQDs, and the fluorescence intensity of the nanocomposites changed due to the specific heavy metal ions and amino acids used. Consequently, these nanocomposites show great potential for detecting these compounds. Cellular viability assessments and comet assays confirm that the resulting nanocomposites do not exhibit any cytotoxic properties based on this specific analytic method. The tested nanocomposites with the addition of carbon quantum dots (NC/CD II and NC/CD III) were characterised by greater genotoxicity compared to the negative control. The positive control, the starch/chitosan composite alone, was also characterised by a greater induction of chromatin damage in mouse cells compared to a pure mouse blood sample.

Multi-functional ratiometric detection based on dual-emitting N-doped carbon dots

Ratiometric fluorescence probes based on multi-emission carbon dots improve accuracy and sensitivity on detecting various environment issues. Herein, a novel dual-emitting N-doped carbon dots (N-CDs) was synthesized from citric acid and urea via a solvothermal method in N,N-dimethylformamide (DMF). The blue and orange emissions of N-CDs in water were modulated, and pure white light-emitting with Commission Internationale de L'Eclairage (CIE) coordinates of (0.33, 0.33) was achieved. The two PL centers behaved differently for Fe3+, Cu2+ and Ag+ ions, with the limit of detection (LOD) of ppm as fluorescence probes. Additionally, N-CDs displayed unique solvatochromism phenomenon. A new green emission appeared in organic solvents and gradually quenched with the increase of solvent polarity. The ratiometric PL displayed an excellent linear response for detecting water, and the LOD was between 0.003 % and 0.3 % in DMF, ethanol, isopropanol and N-methylpyrrolidone. Furthermore, N-CDs exhibited pH-sensitive response in the range of 4.0-7.0 and temperature-dependent response during heating-cooling cycles between 15 and 70 °C. A simple, efficient and reliable multi-functional ratiometric probe for detecting metal ions, water content, pH and temperature simultaneously was realized. However, there is a need for future application research to overcome the limitation imposed by the excitation wavelength of 330 nm.

Construction of a three-mode sensor based on gold nanoparticles and carbon quantum dots as probes for the detection of thiosemicarbazone

In this study, ginkgo leaves were used as a carbon source to synthesize carbon quantum dots (CQDs) with uniform particle size, high fluorescence (FL) intensity and strong stability, using a hydrothermal method. FL could be quenched by the FL resonance energy transfer effect between CQDs and gold nanoparticles (AuNPs), an important FL quenching agent. The electrostatic attraction between thiosemicarbazone (TSC) and citrate on the surface of AuNPs and the formation of a stable Au-S bond between TSC and AuNPs led to the aggregation of AuNPs and thus weakened the quenching effect on CQDs and partly recovered the FL. A sensor in FL mode for the detection of TSC was constructed based on the above-mentioned FL "off" and "on" phenomena. The results showed a good linear correlation in the concentration range 0-1.75 μM, and the limit of detection was as low as 0.05 μM. In addition, the aggregation of AuNPs caused by TSC also led to a change in the absorbance curve and color of the solution; colorimetric and chrominance detection modes were also constructed using these two types of changes, with sensitive responses ranging 0-2.25 μM and 0-1.60 μM and the limits of detection of 0.03 μM and 0.08 μM, respectively. More importantly, these three detection modes obtained satisfactory recovery rates in the detection of the TSC content in river water, liquor and wheat samples, and the detection results were mutually verified (95.18% to 104.96%).

A portable fluorescence detection device based on a smartphone employing carbon nanodots for Mn2+ sensing

The measurement of fluorescence emission for quantitative analysis is typically based on a traditional spectrofluorometer, which limits an onsite detection approach. Thus, an alternative device should be developed for fulfilling this analysis outside of the laboratory. Therefore, a low-cost, portable, and low-energy consumption fluorescence reader-based smartphone device was developed. An ultraviolet light-emitting diodes (UV-LED) was used to construct the fluorescence device-based smartphone as a low-power excitation light source. The smartphone camera was used as a detector for detecting photons from the fluorescence emission process of the fluorescence probe and was connected to a digital image platform. Transparent acrylic with orange and yellow colors was employed as a filter for reducing the interference from light source intensity. The obtained digital image was converted to red, green and blue (RGB) intensity using a custom-designed smartphone application. N,S-doped carbon nanodots (N,S-CDs) were demonstrated to be a good fluorescence indicator for determining trace quantities of Mn2+ in cosmetics. The approach exhibited high selectivity and sensitivity, detecting and quantifying analytes at 1-5 μM concentrations. Furthermore, the method's detection limit of 0.5 μM reflects its capacity to detect trace amounts of a target analyte. Mn2+ in cosmetic products was successfully analyzed using this device with high accuracy comparable with the results from inductively coupled plasma-optical emission spectroscopy (ICP-OES).

Boosting the Methanol Oxidation Reaction Activity of Pt-Ru Clusters via Resonance Energy Transfer

The sluggish kinetics of the methanol oxidation reaction (MOR) with PtRu electrocatalyst severely hinder the commercialization of direct methanol fuel cells (DMFCs). The electronic structure of Pt is of significant importance for its catalytic activity. Herein, it is reported that low-cost fluorescent carbon dots (CDs) can regulate the behavior of the D-band center of Pt in PtRu clusters through resonance energy transfer (RET), resulting in a significant increase in the catalytic activity of the catalyst participating in methanol electrooxidation. For the first time, the bifunction of RET is used to provide unique strategy for fabrication of PtRu electrocatalysts, not only tunes the electronic structure of metals, but also provides an important role in anchoring metal clusters. Density functional theory calculations further prove that charge transfer between CDs and Pt promotes the dehydrogenation of methanol on PtRu catalysts and reduces the free energy barrier of the reaction associated with the oxidation of CO* to CO2 . This helps to improve the catalytic activity of the systems participating in MOR. The performance of the best sample is 2.76 times higher than that of commercial PtRu/C (213.0 vs 76.99 mW cm - 2 mg Pt - 1 ${\rm{mW\ cm}}^{ - 2}{\rm{\ mg}}_{{\rm{Pt}}}^{ - 1}$ ). The fabricated system can be potentially used for the efficient fabrication of DMFCs.

Modification of Carbon Dots for Metal‐Ions Detection

It is very appealing to develop cheap, highly sensitive and efficient metal ion fluorescence sensors as traditional instrument methods are inherently costly and time‐consuming. Carbon dots (CDs) are widely used for sensing metal ions, attributing to their merits of good biocompatibility, low toxicity, easy surface modification and excellent photostability. This article reviewed the research progress of CDs as metal ion sensors in recent years, and studied their modification methods and detection performances. Finally, the challenges and opportunities of CDs as metal ion sensors were also analyzed. This article is expected to provide inspiration and help for researchers focusing on CDs as metal ion sensors.

Nitrogen and sulfur co-doped carbon quantum dots fluorescence quenching assay for detection of mercury (II)

Mercury is a highly toxic and potentially bioaccumulative heavy metal ion that can cause severe health problems in humans even at very low concentrations. Thus, the development of a simple, rapid, and sensitive assay for the effective detection of mercury ions at trace levels is of great importance. Here, nitrogen and sulfur co-doped carbon quantum dots (N,S-CQD) were synthesized by a simple hydrothermal treatment of chitosan in the presence of thiourea and citric acid with a quantum yield (QY) up to 33.0 % and used as a selective fluorescent probe to detect mercury ions (Hg²⁺). The effect of pH, ionic strength, and time on the fluorescence intensity of N,S-CQD were investigated and optimized. The synthesized N,S-CQD showed ultrasensitive ability to detect Hg²⁺ ions in the water samples, also in the presence of 11 interfering metal ions, with a low detection limit (∼4 nM) over a wide linear range from ∼5-160 nM. The sensing performance of N,S-CQD probe in real sample applications was evaluated by the detection of Hg²⁺ in lake water samples, which confirmed its potential application in environmental analysis.

Specific discrimination of zinc and manganese ions by label free dual emissive carbon dots by ratio-metric mode

Due to the worldwide ecological and environmental issues induced by heavy metal pollution, including zinc and manganese, the ratio-metric discrimination of Zn²⁺ and Mn²⁺ based on CDs is urgently required. In this work, reduced CDs (re-CDs) with the intrinsic dual emissive peaks are obtained, and specific discrimination of Zn²⁺ and Mn²⁺ is realized by re-CDs with ratio-metric mode. With the addition of Zn²⁺, the fluorescent (FL) intensity at 650 nm increases obviously, while that at 680 nm progressively decreases. However, the presence of Mn²⁺ would induce the quenching of FL intensity at 680 nm while that at 650 nm remains constant. Then the Zn²⁺ and Mn²⁺ can be separately determined with the ratio of FL intensity at 650 nm to that at 680 mm (F₆₅₀/F₆₈₀). Under optimal conditions, the limit of detection (LOD) of Zn²⁺ is determined to be 9.09 nmol/L, and that for Mn²⁺ is estimated to be 0.93 nmol/L, which is much lower than previously reported work and standard level of Zn²⁺ and Mn²⁺ permitted in drinking water by WHO. Moreover, the specific recognition of Mn²⁺ and Zn²⁺ can be realized via the addition of different masking agents (ethylenediamine for Zn²⁺ and triethanolamine for Mn²⁺). Furthermore, our results reveal that the structural changes from -NH-CO to -NC-OH induced by Zn²⁺ contribute to the shift of FL peak from 680 to 650 nm while both static and dynamic quenching processes are involved in the detection of Mn²⁺. The ratio-metric probe was successfully applied to Zn²⁺ and Mn²⁺ determination in human serum samples and Sandy Lake water.

Reflective epoxy resin/chitosan/PAA composite-functionalized fiber-optic interferometric probe sensor for sensitive heavy metal ion detection

A highly sensitive label-free chemical sensing platform for the detection of various metal ions is demonstrated. The chemical sensor was derived from a single-mode fiber that is inserted into the ceramic tube with epoxy resin (ER) on the end face for reflecting light and forms the Fabry-Perot (F-P) interferometric cavity. Multilayer chitosan (CS)/polyacrylic acid (PAA) were coated on the surface of the epoxy resin and act as the sensitive film. Based on the analysis of the sensing principle and the F-P cavity structure, the parameters were numerically simulated and experimentally evaluated, which enables ease of fabrication and real-time modulation of the cavity length. The sensitivity of sensing Ni²⁺, Zn²⁺, and Na⁺ reached 9.95 × 10^-4 nm ppb^-1, 2.31 × 10^-4 nm ppb^-1, and 4 × 10^-4 nm ppb^-1, respectively, and the sensing results were theoretically analyzed by the Langmuir adsorption model, which corresponds to the surface atom percentage results obtained by SEM and EDS measurements for sensing three types of metal ions. The proposed ER/CS/PAA multilayer film-coated F-P sensor can be employed as a probe, which features label-free, highly sensitivity, real-time monitoring, ease of measurement, stability, and therefore provides a remarkable analytical platform for chemical applications.

Dual-mode detection sensor based on nitrogen-doped carbon dots from pine needles for the determination of Fe3+ and folic acid

In this study, nitrogen-doped carbon dots (N-CDs) from pine needles were obtained by one-step hydrothermal synthesis without any chemical reagents. The fluorescence quenching and absorbance enhancement of N-CDs occurred when Fe³⁺ and folic acid (FA) were added. Based on this, the dual-mode detection sensor by fluorescence and ultraviolet-visible (UV-Vis) spectrophotometry for the determination of Fe³⁺ and FA was established. Detected by the dual-mode detection sensor under the optimized condition, the linear range of Fe³⁺ was 0.1-540 μM and FA was 0.1-165 μM. At the same time, the two inputs "NOR" and "OR" logic gates are constructed successfully according to the dual-mode sensor signals. The proposed dual-mode detection sensor is simple, efficient and stable; it can be applied to determinate Fe³⁺ and FA in practical samples successfully and the results are satisfactory.

Nitrogen and Sulfur Co-doped Carbon Dots as a Turn-Off Fluorescence Probe for the Detection of Cerium and Iron

Carbon dots has becoming one of the most promising fluorescence sensors to determine the trace level of heavy metals in environments because of their advantages in optical properties, response time, and convenient operation procedures. Herein, a novel nitrogen and sulfur co-doped carbon dots (NS-CDs) were prepared though microwave assisted approach using DL-malic acid and allyl thiourea for the first time. Due to the existence of nitrogen and sulfur, the as-prepared NS-CDs exhibited bright blue fluorescence at 430 nm upon 330 nm excitation, with a fluorescence quantum yield of 19.8%. The sensitivity study of NS-CDs against metal ions and organic molecules has approved that the fluorescence could be further quenched by Ce⁴⁺ and Fe³⁺ ions, with the same linear detection ranges varying from 10 to 90 µM. The limits of detection (LOD) were determined as low as 0.75 µM and 0.67 µM for Ce⁴⁺ and Fe³⁺ ions, respectively. The possible quenching mechanism is explained by inner filter effect and static quenching mechanism for Ce⁴⁺ ions, while the quenching effect caused by Fe³⁺ ions is attributed to the inner filter effect, static and dynamic quenching mechanisms. Additionally, the developed sensor was used for the detection of Ce⁴⁺ and Fe³⁺ ions in tap water with satisfactory recoveries. Finally, the designed NS-CDs sensor possesses good biocompatibility against MA104 cells, suggesting the sensor can be potentially applied to detect Ce⁴⁺ and Fe³⁺ ions in environment and biological systems.

N, S-Doped Carbon Dots Prepared by Peanut Protein Isolates and Cysteamine as Highly Sensitive Fluorescent Sensors for Fe2+, Fe3+ and Lactoferrin

Lactoferrin (LF) is an iron-binding glycoprotein with various biological activities that has been extensively used in food and medical applications. Several methods for detecting LF have been reported, but they still face challenges in terms of sensitivity and simplicity of detection. To achieve an accurate and efficient detection of LF, we developed a method for the determination of LF in lactoferrin supplements using carbon dots (CDs) fluorescent probes. The N, S-doped PPI carbon dots (N, S-PPI-CDs) were prepared using a protein (peanut protein isolate) and cysteamine as precursors. The prepared N, S-PPI-CDs exhibited intense blue fluorescence and good biocompatibility, while the fluorescence intensity of the N, S-PPI-CDs showed a good linear relationship with Fe²⁺/Fe³⁺ concentration (0-2 μM). The N, S-PPI-CDs exhibited a high potential ability to rapidly detect Fe²⁺/Fe³⁺ within 30 s, with a limit of detection (LoD) of 0.21 μM/0.17 μM. Due to the reversible binding of LF to Fe, the N, S-PPI-CDs showed a high sensitivity and selectivity for LF, with a limit of detection (LoD) of 1.92 μg/mL. In addition, LF was quantified in real sample LF supplements and showed a fluctuation in recovery of less than 2.48%, further demonstrating the effectiveness of the fluorescent N, S-PPI-CDs sensor.

Sustainable Preparation of Graphene Quantum Dots for Metal Ion Sensing Application

Over the past several years, graphene quantum dots (GQDs) have been extensively studied in water treatment and sensing applications because of their exceptional structure-related properties, intrinsic inert carbon property, eco-friendly nature, etc. This work reported on the preparation of GQDs from the ethanolic extracts of eucalyptus tree leaves by a hydrothermal treatment technique. Different heat treatment times and temperatures were used during the hydrothermal treatment technique. The optical, morphological, and compositional analyses of the green-synthesized GQDs were carried out. It can be noted that the product yield of GQDs showed the maximum yield at a reaction temperature of 300 °C. Further, it was noted that at a treatment period of 480 min, the greatest product yield of about 44.34% was attained. The quantum yields of prepared GQDs obtained after 480 min of treatment at 300 °C (named as GQD/300) were noted to be 0.069. Moreover, the D/G ratio of GQD/300 was noted to be 0.532 and this suggested that the GQD/300 developed has a nano-crystalline graphite structure. The TEM images demonstrated the development of GQD/300 with sizes between 2.0 to 5.0 nm. Furthermore, it was noted that the GQD/300 can detect Fe³⁺ in a very selective manner, and hence the developed GQD/300 was successfully used for the metal ion sensing application.

Facile and scalable synthesis of un-doped, doped and co-doped graphene quantum dots: a comparative study on their impact for environmental applications

In recent years, graphene quantum dots (GQDs) received huge attention due to their unique properties and potential applicability in different area. Here, we report simple and facile method for the synthesis of GQDs and their functionalization by doping and co-doping using different heteroatom under the optimized conditions. The doping and co-doping of GQDs using boron and nitrogen have been confirmed by FTIR and TEM. The UV-visible and fluorescence techniques have been used to study the optical properties and stability of functionalized GQDs. Further, the screening for enhancement of quantum yields of all GQDs were performed with fluorescence and UV-visible spectra under the optimized conditions. The average QY was obtained as 16.0%, 83.6%, 18.2% and 29.6% for GQDs, B-GQDs, N-GQDs and B,N-GQDs, respectively. The sensor was used to determine paraoxon in water samples. The LOD was observed to be 1.0 × 10-4 M with linearity range of 0.001 to 0.1 M. The RSD was calculated for the developed B,N-GQDs based sensor and observed to be 2.99% with the regression coefficient as 0.997. All the doped, co-doped and un-doped GQDs possess remarkable properties as a fluorescent probe.

A novel technique to overcome fluid flow influence in carbon quantum dots/paper-based analytical devices

Paper-based analytical devices are promising choices for rapid tests and lab-on-chip detection techniques. Carbon quantum dots (CQDs), on the other hand, are biocompatible nanomaterials, which are industrially promising, due to their fast and cost-effective gram-scale synthesis techniques, as well as their significantly high and stable photoluminescence (PL) properties, which are durable and reliable over a year. However, there have been limitations in the entrapment of CQDs on cellulose papers in a way that their PL is not influenced by the flowing of the CQDs with the stream of analyte fluid, making the sensors less accurate at very low concentrations of liquid analytes. Therefore, in this investigation, a polyvinyl alcohol/alkaline-based method was systematically generated and developed to entrap CQDs inside a 3D crystalline matrix on paper, in a way that they can be used directly as probes for a simple drop-and-detect method. As a proof of concept, N/P-doped CQD on cellulose paper was used to make fluorescent paper-based analytical devices for identifying traces of Hg²⁺ of around 100 ppb. The designed sensor was tested over several months, to study its durability and functionality over long periods, for potential industrial applications.