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

Facile Green Gamma Irradiation of Water Hyacinth Derived-Fluorescent Carbon Dots Functionalized Thiol Moiety for Metal Ion Detection

Fluorescent sensor-based carbon dots (CDs) have significantly developed for sensing metal ions because of their great physical and optical properties, including tunable fluorescence emission, high fluorescence quantum yield, high sensitivity, non-toxicity, and biocompatibility. In this research, a green synthetic approach via simple gamma irradiation for the carbon dot synthesis from water hyacinth was developed since water hyacinth has been classified as an invasive aquatic plant containing cellulose, hemicellulose, and lignin. The thiol moiety (SH) was further functionalized on the surface functional groups of CDs as the "turn-off" fluorescent sensor for metal ion detection. Fluorescence emission displayed a red shift from 451 to 548 nm when excited between 240 and 500 nm. The quantum yield of CDs-SH was elucidated to be 13%, with strong blue fluorescence emission under ultraviolet irridiation (365 nm), high photostability and no photobleaching. The limit of detection was determined at micromolar levels for Hg2+, Cu2+, and Fe3+. CDs-SH could be a real-time monitoring sensor for Hg2+ and Cu2+ as fluorescence quenching was observed within 2 min. Furthermore, paper test-strip based CDs-SH could be applied to detect these metal ions.

Recent Progress of Fluorescent Carbon Dots and Graphene Quantum Dots for Biosensors: Synthesis of Solution Methods and their Medical Applications

In the fields of health and biology, fluorescent nanomaterials have emerged as highly potential and very useful candidates for use in biosensor applications. These typical highly powerful nanomaterials are carbon dots (CDs) and graphene quantum dots (GQDs) among many other metallic nanomaterials. In the context of medical biosensors, this review article investigates the techniques of synthesis, and many uses of these nanomaterials, the obstacles that they face, and the potential for their future. We cover the significance of fluorescent nanomaterials, their use in the medical field, as well as the several techniques of synthesis for CDs and GQDs, including ultrasonication, hydrothermal, electrochemical method, surface modification, and solvothermal. In addition, we also discuss their biomedical applications, which include biomolecule detection, disease diagnosis and examine the obstacles and prospective possibilities for development of ultra-bright, ultra-sensitive, and selective biosensors for use in in-vivo research.Fluorescent carbon dots and graphene quantum dots is synthesized by using several types of raw material and methods. These Carbon dots and graphene quantum dots are used in the medical field includes detection of biomaterials, detection of cancer, virus and mutation in DNA.

Recent advancements and prospects in carbon-based nanomaterials derived from biomass for environmental remediation applications

The conversion of waste biomass into a value-added carbonaceous nanomaterial highlights the appealing power of biomass valorization. The advantages of using sustainable and cheap biomass precursors exhibit the tremendous opportunity for boosting energy production and their application in environmental remediation processes. This review emphasis the development and production of carbon-based nanomaterials derived from biomass, which possess favourable characteristics such as biocompatibility and photoluminescence. The advantages and limitations of various nanomaterials synthesised from different precursors were also discussed with insights into their physicochemical properties. The surface morphology of the porous nanomaterials is also explored along with their characteristic properties like regenerative nature, non-toxicity, ecofriendly nature, unique surface area, etc. The incorporation of various functional groups confers superiority of these materials, resulting in unique and advanced functional properties. Further, the use of these biomass derived nanomaterials was also explored in different applications like adsorption, photocatalysis and sensing of hazardous pollutants, etc. The challenges and outcomes obtained from different carbon-based nanomaterials are briefly outlined and discussed in this review.

Multifunctional natural derived carbon quantum dots from Withania somnifera (L.) - Antiviral activities against SARS-CoV-2 pseudoviron

Natural carbon dots (NCQDs) are expediently significant in the photo-, nano- and biomedical spheres owing to their facile synthesis, optical and physicochemical attributes. In the present study, three NCQDs are prepared and optimized from Withania somnifera (ASH) by one-step hydrothermal (bottom-up) method: HASHP (without dopant), nitrogen doped HASHNH3 (surface passivation using ammonia) and HASHEDA (surface passivation with ethylenediamine). The HR-TEM images reveal that HASHP, HASNH3, HASHEDA are spherically shaped with 2.5 ± 0.5 nm, 4 ± 1 nm and 5 ± 2 nm particle size, respectively, whereas FTIR confirms the aqueous solubility and nitrogen doping. The XRD patterns ensure that the NCQDs are amorphous and graphitic in nature. Comparatively, HASHNH3 (32.5%) and HASHEDA (27.6%) portray better fluorescence quantum yield than HASHP (5.6%). The increase in quantum yield for the doped NCQDs can be attributed to the surface passivation using ammonia and ethylenediamine. Surface passivation plays a crucial role in enhancing the fluorescence properties of quantum dots. The introduction of nitrogen through ammonia and ethylenediamine provides additional electronic states, possibly reducing non-radiative recombination sites and hence boosting the QY. In addition, an antiviral study unveils the striking potential of surface passivated NCQDs to curb Covid-19 crises with around 85% inhibition of SARS-CoV pseudoviron cells, which is better in comparison to the non-doped NCQDs. Hence, to understand the paramount efficacy of these NCQDs, a hypothesis on their possible mechanism of action against Covid-19 is discussed.

Biomass derived green carbon dots for sensing applications of effective detection of metallic contaminants in the environment

The rapid consumption of metals and unorganized disposal have led to unprecedented increases in heavy metal ion concentrations in the ecosystem, which disrupts environmental homeostasis and results in agricultural biodiversity loss. Mitigation and remediation plans for heavy metal pollution are largely dependent on the discovery of cost-effective, biocompatible, specific, and robust detectors because conventional methods involve sophisticated electronics and sample preparation procedures. Carbon dots (CDs) have gained significant importance in sensing applications related to environmental sustainability. Fluorescence sensor applications have been enhanced by their distinctive spectral properties and the potential for developing efficient photonic devices. With the recent development of biomass-functionalized carbon dots, a wide spectrum of multivalent and bivalent transition metal ions responsible for water quality degradation can be detected with high efficiency and minimal toxicity. This review explores the various methods of manufacturing carbon dots and the biochemical mechanisms involved in metal detection using green carbon dots for sensing applications involving Cu (II), Fe (III), Hg (II), and Cr (VI) ions in aqueous systems. A detailed discussion of practical challenges and future recommendations is presented to identify feasible design routes.

Simple and green approach for photoluminescent carbon dots prepared from faba bean seeds as a luminescent probe for determination of Hg+ ions and cell imaging

In this research, for the first time, a dedicated sensor was designed to detect Hg+ ions using photoluminescent carbon dots (CDs). Due to the preferred green synthesis of CDs from bio-resources, carbohydrate-rich faba bean seeds as a potential carbon precursor were applied to the synthesis of CDs. The CDs were prepared from the faba bean seeds using the hydrothermal method in an aqueous solution in the absence of substances such as an acid or base and any other additives. The synthesized CDs exhibited maximum emission intensity at 387 nm when excited at 310 nm and their luminescence quantum yield was calculated to be ~5.94%. Then, the fluorescence emission of CDs was examined in the presence of different metal ions. Results revealed that the CDs had good selectivity towards the Hg+ ions, so the fluorescence emission was significantly changed in the presence of these ions with a limit of detection (LOD) as low as 0.35 μM. Furthermore, because of their very low cytotoxicity, these CDs can be applied for cell imaging.

Green synthesis of fluorescent carbon dots from Robinia hispida L. leaves for selective detection of Hg (II)

In this study, Robinia hispida L leaves (RH) was used as a precursor for the first time to synthesize fluorescent carbon dots (CDs) with stable blue fluorescence by a single-step hydrothermal synthesis method. Notably, the innovative approach eliminates the necessity for toxic chemicals or hazardous substances, marking a significant advancement in the field. The synthesized CDs demonstrate CDs demonstrates the predominance of spherical shapes with an average size of 11.63 ± 1.92 nm. The CDs not only exhibit an enhanced fluorescent efficiency with a relatively high quantum yield of up to 6.8%, but they also possess the potential for direct utilization in the selective determination of Hg(II) through fluorescence quenching, even without any functionalization. Under the optimized conditions at a pH of 7.0, a robust linear correlation was found to exist between the fluorescence intensity and the concentration of Hg (II) within the range of 5-17.5μM, exhibiting a detection limit (3σ) of 1.5μM. Additionally, this methodology was effectively employed to successfully detect Hg (II) ions in various aqueous samples, including tap water, spring water, drinking water, and a certified reference material (CRM-SA-C Sandy Soil C). The spike recoveries of 97.6%-101.6% with less than 2.7% variability were performed on all samples.

Nitrogen-doped carbon dots: Recent developments in its fluorescent sensor applications

Doped carbon dots have attracted great attention from researchers across disciplines because of their unique characteristics, such as their low toxicity, physiochemical stability, photostability, and outstanding biocompatibility. Nitrogen is one of the most commonly used elements for doping because of its sizeable atomic radius, strong electronegativity, abundance, and availability of electrons. This distinguishes them from other atoms and allows them to perform distinctive roles in various applications. Here, we have reviewed the most current breakthroughs in nitrogen-doped CDs (N-CDs) for fluorescent sensor applications in the last five years. The first section of the article addresses several synthetic and sustainable ways of making N-CDs. Next, we briefly reviewed the fluorescent features of N-CDs and their sensing mechanism. Furthermore, we have thoroughly reviewed their fluorescent sensor applications as sensors for cations, anions, small molecules, enzymes, antibiotics, pathogens, explosives, and pesticides. Finally, we have discussed the N-CDs' potential future as primary research and how that may be used. We hope that this study will contribute to a better understanding of the principles of N-CDs and the sensory applications that they can serve.

Recent advances in green carbon dots (2015-2022): synthesis, metal ion sensing, and biological applications

Carbon dots (CDs) show extensive potential in various fields such as sensing, bioimaging, catalysis, medicine, optoelectronics, and drug delivery due to their unique properties, that is, low cytotoxicity, cytocompatibility, water-solubility, multicolor wavelength tuned emission, photo-stability, easy modification, strong chemical inertness, etc. This review article especially focuses on the recent advancement (2015-2022) in the green synthesis of CDs, their application in metal ions sensing and microbial bioimaging, detection, and viability studies as well as their applications in pathogenic control and plant growth promotion.

One step synthesis of ultra-high quantum yield fluorescent carbon dots for "on-off-on" detection of Hg2+ and biothiols

In this paper, the carbon dots (CDs) with strong blue fluorescence were synthesized through hydrothermal method, which using folic acid, ammonium citrate and ethylenediamine as precursors. The prepared CDs with a high absolute quantum yield of 81.94% and showed excellent stability in high concentration salt solution and different pH conditions. With the addition of Hg²⁺, the signal of CDs was selectively quenched. At the same time, the CDs-Hg²⁺ system could be recovered after the introduction of biothiols. Moreover, the fluorescence of CDs showed a good linear relationship with Hg²⁺ (1-15 µM), and the detection limit as low as 0.08 µM. In addition, the prepared CDs with low toxicity could be used to detect Hg²⁺ in living cells and actual water samples.

Natural and Engineered Nanomaterials for the Identification of Heavy Metal Ions—A Review

In recent years, there has been much interest in developing advanced and innovative approaches for sensing applications in various fields, including agriculture and environmental remediation. The development of novel sensors for detecting heavy metals using nanomaterials has emerged as a rapidly developing research area due to its high availability and sustainability. This review emphasized the naturally derived and engineered nanomaterials that have the potential to be applied as sensing reagents to interact with metal ions or as reducing and stabilizing agents to synthesize metallic nanoparticles for the detection of heavy metal ions. This review also focused on the recent advancement of nanotechnology-based detection methods using naturally derived and engineered materials, with a summary of their sensitivity and selectivity towards heavy metals. This review paper covers the pros and cons of sensing applications with recent research published from 2015 to 2022.

Efficient Reduction of Cr(VI) with Carbon Quantum Dots

Hexavalent chromium (Cr(VI)) pollution is a global problem, and the reduction of highly toxic Cr(VI) to less toxic Cr(III) is considered to be an effective method to address Cr(VI) pollution. In this study, low-toxicity carbon quantum dots (CQDs) were used to reduce Cr(VI) in wastewater. The results show that CQDs can directly reduce Cr(VI) at pH 2 and can achieve a reduction efficiency of 94% within 120 min. It is observed that under pH higher than 2, CQDs can activate peroxymonosulfate (PMS) to produce reactive oxygen species (ROS) for the reduction of Cr(VI) and the reduction efficiency can reach 99% within 120 min even under neutral conditions. The investigation of the mechanism shows that the hydroxyl groups on the surface of CQDs can be directly oxidized by Cr(VI) because of the higher redox potential of Cr(VI) at pH 2. As the pH increases, the carbonyl groups on the surface of CQDs can activate PMS to generate ROS, O₂ ^•-, and ¹O₂, which result in Cr(VI) being reduced. To facilitate the practical application of CQDs, the treatment of Cr(VI) in real water samples by CQDs was simulated and the method reduced Cr(VI) from an initial concentration of 5 mg/L to only 8 μg/L in 150 min, which is below the California water quality standard of 10 μg/L. The study provides a new method for the removal of Cr(VI) from wastewater and a theoretical basis for practical application.

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

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

Carbon quantum dots with blue/near infrared emissions for ratiometric fluorescent lornoxicam sensing and bio-imaging

An economical and eco-friendly hydrothermal method for the preparation of nitrogen-doped carbon quantum dots (N-CQDs) was studied with rambutan peel and lysine. The morphology, structure, and optical properties of N-CQDs were characterized by transmission electron microscopy, Fourier transform infrared spectrometry, X-ray powder diffractometer, X-ray photoelectron spectrometry, and UV spectrophotometry. The synthesized N-CQDs have excellent characteristics such as strong fluorescence, good dispersion, high stability, and excellent water solubility. The absolute fluorescence quantum yield is 1.02%, the average particle size is 1.63 nm, and the maximum excitation wavelength is 340 nm. The maximum emission wavelengths are 430 nm and 800 nm. As a quencher, lornoxicam (LNX) was used to quench the fluorescence of N-CQDs with the mechanism of inner filter effect. The fluorescence ratio of N-CQDs (F₄₃₀/F₈₀₀) shows a good linear relationship to the concentration of LNX. The linear range and the detection limit of LNX are 0.01‒100 and 0.003 μmol/L, respectively. An effective ratiometric fluorescence probe for the detection of LNX was constructed. The method has the advantages of low detection limit, high sensitivity, wide linear range, and can be applied to the determination of LNX in real samples. Moreover, according to the excitation-dependent fluorescence behavior, dual-wavelength emission, and biocompatibility of N-CQDs, it has been applied to cell imaging.

Multiple heteroatom-doped photoluminescent carbon dots for ratiometric detection of Hg2+ ions in cell imaging and environmental applications

Photoluminescence detection and imaging of Hg²⁺ ions in the biochemical living system are of great importance. In this study, a new photoluminescent probe based on nitrogen (N), sulfur (S), and boron (B) multiple heteroatom co-doped carbon dots (NSB-CDs) is synthesized for the ratiometric detection of Hg²⁺ ions. The prepared NSB-CDs possess good aqueous solubility, excellent pH and ionic stability, excitation dependency, and high quantum yield (QY = 17.6%). The ratiometric photoluminescent sensor NSB-CDs exhibit high selectivity, sensitivity, and interference towards Hg²⁺ ions over other metal ions. After adding Hg²⁺ ions, the emission intensity of the NSB-CDs exhibits a large redshift from 452 to 496 nm (up to 44 nm), corresponding to a notable change from blue to green emission in aqueous solutions. The association constant (K_a), the limit of detection (LOD), and the limit of quantification (LOQ) for NSB-CDs/Hg²⁺ complex are calculated to be 3.6 × 10⁴ M^-1, 3.1 × 10^-9 M, and 10.4 × 10^-9 M, respectively, in the range of 0-30 × 10^-6 M. The live cell bioimaging of HCT-116 cells with NSB-CDs validates the application of multicolor imaging for the detection of Hg²⁺ ions in aqueous media and biological systems. Moreover, the potential use of the NSB-CDs/Hg²⁺ complex for real sample analysis is demonstrated.

Fluorescent Carbon Dots and their Applications in Sensing of Small Organic Molecules

Background:

Fluorescence-based sensing is considered highly sensitive and fluorescent probes with improved properties are always desired. Fluorescent carbon dots (CDs) are newly emerging quasi-spherical nanoparticles of less than 10 nm in size and belong to the carbon nano-material’s family. CDs have great potential as fluorescent probes and currently are under open deliberation by the researchers due to their striking properties such as low environmental hazard, high selectivity, greater sensitivity, good biocompatibility, tunable fluorescent properties and excitation dependent multicolor emission behavior.

Introduction:

This review demonstrates various available methods for fabrication of fluorescent CDs, capping of CDs and characterization with various techniques including UV-visible, FT-IR, and TEM. Analytical applications using CDs for the sensing of small organic molecules, specifically nitroaromatic compounds in the environmental samples are complied.

Methods:

The review covers literature related to synthesis and characterization of carbon dots. It includes around 171 research articles in this field.

Results:

Carbon dots can be synthesized using numerous routes. In all cases CDs possess spectral properties with little variation in wavelength maxima. Optical properties of CDs can be tuned by compositing these with metallic quantum dots or by modifying their surface with desired functionalities. HR-TEM is needed to see the morphology and size of particles whereas UV-Visible and FTIR are indispensable tools for this kind of research. These particles are successfully applied to sense small molecules in some matrices.

Conclusion:

Carbon dots are bright stars in fluorescent sensing of small molecules. However, more research is needed to determine small organic molecules in diversified areas of analysis.

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

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

A fluorescence turn-on CDs-AgNPs composites for highly sensitive and selective detection of Hg2

In this paper, a simple and effective fluorescence turn-on approach for highly sensitive and selective monitoring Hg²⁺ ions was designed by using carbon dots (CDs) and silver nanoparticles (AgNPs). It reveals that the fluorescence of CDs solution can be quenched in the presence of AgNPs through inner filter effect (IFE) and the quenched CDs-AgNPs system is turned on after addition of Hg²⁺ ions, which is due to higher affinity of Hg²⁺ and AgNPs than that of CDs and AgNPs, thus resulting the disappearance of AgNPs from the CDs-AgNPs composites and leading to the fluorescence turn-on of CDs. The developed fluorescence turn-on approach exhibited high selectivity and sensitivity for detection of Hg²⁺. Under the optimum experimental conditions, good linearity was achieved over the range of 100-160 μM and the limit of detection (LOD) was estimated to be 2.22×10^-8 M for Hg²⁺. The recoveries of Hg²⁺ spiked in real samples ranged from 98.4% to 101.6%. Results of this study suggest that the fluorescence turn-on approach can be used to the detection of Hg²⁺ in real water samples.

A ratiometric fluorescence sensor based on carbon quantum dots realized the quantitative and visual detection of Hg2

In this paper, based on the fluorescence of carbon quantum dots (CQDs) quenched by mercury ions (Hg²⁺ ) and the nonresponse of Hg²⁺ to rhodamine B fluorescence, a dual emission ratio fluorescence sensor was constructed to realize the quantitative detection of Hg²⁺ . Under excitation at 365 nm, the fluorescence spectrum showed double emission peaks at 437 nm and 590 nm, corresponding to the fluorescence emissions of CQDs and rhodamine B, respectively. This method quantitatively detected Hg²⁺ based on the linear relationship between the ratio of the intensities of the two emission peaks F₄₃₇ /F₅₉₀ and the concentration of Hg²⁺ . The detection range was 10-70 nM, and the limit of detection (S/N = 3) was 3.3 nM. In addition, this method could also realize the qualitative and semiquantitative detection of Hg²⁺ according to the fluorescence colour change of the probe under ultraviolet light. After various evaluations, the method could be successfully applied to the quantitative and visual detection of Hg²⁺ in tap water, and demonstrated excellent selectivity, anti-interference performance, and repeatability of the method.

Highly fluorescent carbon dots from coix seed for the determination of furazolidone and temperature

In this work, blue emission fluorescent carbon dots (CDs) were fabricated by using the hydrothermal strategy from coix seed for the first time. We found that the prepared CDs possessed many excellent characteristics including excitation-dependent properties, good solubility and strong photostability. The optimal excitation and emission wavelength of CDs were 363 and 435 nm, respectively. Unbelievably, the fluorescence of CDs was selectively and effectively quenched with the addition of furazolidone (Fu). The quenching mechanisms might be assumed to the static quenching and inner filter effect (IFE). Based on this principle, a novel fluorescence probe was developed for the determination of Fu. At the same time, the proposed probe showed excellent sensitivity and selectivity towards Fu with a wide linear range from 0.5 to 100 μM, and the corresponding detection limit was 0.096 μM. Moreover, the CDs also could be applied for the sensing of temperature. The practical application of the CDs for Fu detection in real samples was also confirmed with the satisfactory recoveries changing from 96.6% to 108.5%, which provided huge possibility in the field of medical analysis.

Fluorescent carbon dots from Indian Bael patra as effective sensing tool to detect perilous food colorant

Herein a simple strategy has been demonstrated for the synthesis of environmentally amiable and highly fluorescent carbon dots from the most useful plant of Indian classical Ayurveda i.e. Bael patra fruit. The morphological features and chemical composition of the prepared carbon dots were characterized through High resolution transmission electron microscopy, Field emission scanning electron microscopy and X-ray photoelectron spectroscopy. Owing to their highly emission nature, the applicability of carbon dots was tested against various food colorant i.e. Allura red. Under the optimized conditions, the decreased fluorescence intensity exhibited a good linear relationship with increasing concentration of Allura red. Additionally, an extensive research was carried out to determine the adsorption efficiency of carbon dots for Allura red and heavy metals. Based on the context, here we report the novelty of this work, demonstrating the decontamination of various samples from Allura red and heavy metals with the application of carbon dots.

"ON-OFF-ON" fluorescence switches based on N,S-doped carbon dots: Facile hydrothermal growth, selective detection of Hg2+, and as a reversive probe for guanine

Water contamination due to heavy metal ions has become a major environmental problem worldwide. In this work, "on-off-on" fluorescence switches comprising N,S-doped carbon dots (N,S-CDs) have been developed for selective recognition of Hg²⁺ and as reversive probes for guanine. N,S-CDs were synthesized in a facile one-step hydrothermal approach using citric acid and methionine as precursors. The synthesized N,S-CDs display fluorescence with excitation/emission maxima of 370/440 nm and a quantum yield of 32.5%. Under the variable pH (2-12), the fluorescent N,S-CDs with a linear range from 0.05 to 100 μM displayed selective discrimination for Hg²⁺ with the limit of 6.24 nM over several other cations, anions, and neutral analytes resulting in the quenching of fluorescence response. Furthermore, the addition of guanine at the LOD of 6.4 nM can restore N,S-CDs' fluorescence in a reversible manner. For this kind of fluorescence switch, its purposed applications on environmental samples are employed successfully to detect Hg²⁺ in tap water and river water.

Herbal medicine derived carbon dots: synthesis and applications in therapeutics, bioimaging and sensing

Since the number of raw material selections for the synthesis of carbon dots (CDs) has grown extensively, herbal medicine as a precursor receives an increasing amount of attention. Compared with other biomass precursors, CDs derived from herbal medicine (HM-CDs) have become the most recent incomer in the family of CDs. In recent ten years, a great many studies have revealed that HM-CDs tend to be good at theranostics without drug loading. However, the relevant development and research results are not systematically reviewed. Herein, the origin and history of HM-CDs are outlined, especially their functional performances in medical diagnosis and treatment. Besides, we sort out the herbal medicine precursors, and analyze the primary synthetic methods and the key characteristics. In terms of the applications of HM-CDs, medical therapeutics, ion and molecular detection, bioimaging, as well as pH sensing are summarized. Finally, we discuss the crucial challenges and future prospects.

Carbon Dots: Classification, Properties, Synthesis, Characterization, and Applications in Health Care-An Updated Review (2018-2021)

Carbon dots (CDs) are usually smaller than 10 nm in size, and are meticulously formulated and recently introduced nanomaterials, among the other types of carbon-based nanomaterials. They have gained significant attention and an incredible interest in the field of nanotechnology and biomedical science, which is merely due to their considerable and exclusive attributes; including their enhanced electron transferability, photobleaching and photo-blinking effects, high photoluminescent quantum yield, fluorescence property, resistance to photo-decomposition, increased electrocatalytic activity, good aqueous solubility, excellent biocompatibility, long-term chemical stability, cost-effectiveness, negligible toxicity, and acquaintance of large effective surface area-to-volume ratio. CDs can be readily functionalized owing to the abundant functional groups on their surfaces, and they also exhibit remarkable sensing features such as specific, selective, and multiplex detectability. In addition, the physico-chemical characteristics of CDs can be easily tunable based on their intended usage or application. In this comprehensive review article, we mainly discuss the classification of CDs, their ideal properties, their general synthesis approaches, and primary characterization techniques. More importantly, we update the readers about the recent trends of CDs in health care applications (viz., their substantial and prominent role in the area of electrochemical and optical biosensing, bioimaging, drug/gene delivery, as well as in photodynamic/photothermal therapy).

Leftover Kiwi Fruit Peel-Derived Carbon Dots as a Highly Selective Fluorescent Sensor for Detection of Ferric Ion

Recently, the use of natural products for the synthesis of carbon dots (CDs) has received much attention. Herein, leftover kiwi (Actinidia Deliciosa) fruit peels were successfully turned into beneficial fluorescent carbon dots (KN-CDs) via the hydrothermal-carbonization route. KN-CDs 1 and KN-CDs 2 were prepared without and with ammonium hydroxide, respectively. KN-CDs 1 and KN-CDs 2 were systematically characterized by various analytical techniques. Synthesized KN-CDs showed spherical-shaped morphology with narrow size distribution and excellent optical properties with excitation-independent behaviors. The quantum yields of KN-CDs 1 and KN-CDs 2 were calculated as 14 and 19%, respectively. Additionally, the KN-CDs possess excellent prolonging and photostability. Because of the excellent optical properties of KN-CDs, they were utilized as fluorescent sensors. The strong fluorescence of the KN-CDs was selectively quenched by Fe3+ ion, and quenching behavior showed a linear correlation with the concentrations of Fe3+ ion. KN-CDs 1 and KN-CDs 2 showed the detection of Fe3+ ions within the concentration range of 5–25 µM with the detection limit of 0.95 and 0.85 µM, respectively. Based on the turn-off sensing by the detection of Fe3+ ions, KN-CDs would be a promising candidate as a selective and sensitive fluorescent sensor.

Natural source of carbon dots from part of a plant and its applications: a review

Carbon dots (CDs) are carbon nanoparticles with a size of less than 10 nm, and are synthesized from various sources; they have been of great interest to scientists worldwide due to their unique optical, electrical, and chemical properties. Sources of carbon are inexpensive and can be classified as a renewable natural resources. Many researchers use CDs because of their low toxicity, better water solubility, high biocompatibility, and stable photoluminescence. The simple methods for producing CDs are hydrothermal and use inexpensive equipment, have low energy consumption, simple manipulation, and one-step preparation. Since the discovery of CDs, researchers have used them in various applications such as sensing, bioimaging, drug delivery, and catalysis. In this review, CDs synthesized from natural resources such as samples from herbs, roots, leaves, flowers, and fruit and some applications are described. This review provides a summary of carbon dots that is expected to provide further information for development of new CDs.

Green Sources Derived Carbon Dots for Multifaceted Applications

For the past decade, the Carbon dots (CDs) a tiny sized carbon nanomaterial are typically much attentive due to their outstanding properties. Nature is a fortune of exciting starting materials that provides many inexpensive and renewable resources which have received the topmost attention of researchers because of non-hazardous and eco-friendly nature that can be used to prepare green CDs by top-down and bottom-up synthesis including hydrothermal carbonization, microwave synthesis, and pyrolysis due to its simple synthetic process, speedy reactions and clear-cut end steps. Compared to chemically derived CDs, green CDs are varied by their properties such as less toxicity, high water dispersibility, superior biocompatibility, good photostability, bright fluorescence, and ease of modification. These nanomaterials are a promising material for sensor and biological fields, especially in electrochemical sensing of toxic and trace elements in ecosystems, metal sensing, diagnosis of diseases through bio-sensing, and detection of cancerous cells by in-vitro and in-vivo bio-imaging applications. In this review, the various synthetic routes, fluorescent mechanisms, and applications of CDs from discovery to the present are briefly discussed. Herein, the latest developments on the synthesis of CDs derived from green carbon materials and their promising applications in sensing, catalysis and bio-imaging were summarized. Moreover, some challenging problems, as well as upcoming perspectives of this powerful and tremendous material, are also discussed.

Ultrasensitive Fe3+ ion detection based on pH-insensitive fluorescent graphene nanosensors in strong acid and neutral media

Schematic illustration of the preparation of FRGO and the detection of Fe³⁺ ions in strong acid and neutral media.

Fluorescence quenching mechanism and the application of green carbon nanodots in the detection of heavy metal ions: a review

This review article highlights the quenching mechanism and applications of green CNDs for the detection of metal ions.