Carbon dots: surface engineering and applications

Electronic interaction-enhanced NO photorelease and photothermal conversion in N-doped carbon dot nanoconjugates

A nitric oxide (NO) photodonor (1) capable of releasing two NO molecules through a stepwise mechanism has been covalently grafted to blue-emitting N-doped carbon dots (NCDs). The resulting water-soluble nanoconjugate (NCDs-1), ca. 10 nm in diameter, exhibits a new absorption band not present in the simple physical mixture of the two components and is attributable to strong electronic interactions between them in the ground state. Blue light excitation of NCDs-1 leads to NO photogeneration with an efficiency almost one order of magnitude higher than that observed for 1 alone, probably due to a photoinduced electron transfer between the NCDs and the grafted 1. Photoexcitation of the nanoconjugate also results in effective photothermal conversion, which is negligible in the naked NCDs. Furthermore, in contrast to 1, the nanoconjugate liberates NO also under excitation with green light. Finally, the typical blue fluorescence of the NCDs is quenched in NCDs-1 but restored upon the photouncaging of the second NO molecule, providing readable and real-time information about the amount of NO photogenerated.

Carbon quantum dots from hemicucur[6]bit and the application for the detection of Pb2

A macrocyclic compound, hemicucurbit[6]uril (HemiQ[6]), is employed as the carbon source to produce a novel sort of carbon quantum dots (CQDs) with blue fluorescence in aqueous solution. The CQDs are fully identified by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Nuclear Magnetic Resonance (NMR), zeta potential, ultraviolet/visible (UV-vis) and photoluminescence spectroscopy (PL). The nanomaterial is developed for the analysis of Pb2+ in the light of the Resonance Rayleigh scattering (RRS) changes with the increasing Pb2+ concentration. The proposed probe emerges a high selectivity to Pb2+ and excellent sensitivity in the linear concentration range of 0-6 μM with a detection limit low to 0.42 μM, which is superior to the previous values of Pb2+ sensors, as well as the good anti-interference ability is confirmed by the specifical response to Pb2+ in the presence of other metal cations. Therefore, the proposed analysis of Pb2+ is explored for the application in real samples of tap water and lake water, in satisfied results of acceptable recoveries.

A review on green synthesis, biological applications of carbon dots in the field of drug delivery, biosensors, and bioimaging

Since the beginning of nanoscience and nanotechnology, carbon dots (CDs) have been the foundational idea and have dominated the growth of the nano-field. CDs are an intriguing platform for utilization in biology, technology, catalysis, and other fields thanks to their numerous distinctive structural, physicochemical, and photochemical characteristics. Since several carbon dots have already been created, they have been assessed based on their synthesis process, and luminescence characteristics. Due to their biocompatibility, less toxic effects, and most significantly their fluorescent features in contrast to other carbon nanostructures, CDs have several benefits. This review focuses on the most recent advancements in the characterization, applications, and synthesis techniques used for CDs made from natural sources. It will also direct scientists in the creation of a synthesis technique for adjustable carbon dots that is more practical, effective, and environmentally benign. With low toxicity and low cost, CDs are meeting the new era's requirements for more selectivity and sensitivity in the detection and sensing of various things, such as biomaterial sensing, enzymes, chemical contamination, and temperature sensing. Its variety of properties, such as optical properties, chemiluminescence, and morphological analysis, make it a good option to use in bioimaging, drug delivery, biosensors, and cancer diagnosis.

Metal-organic framework (MOF)/C-dots and covalent organic framework (COF)/C-dots hybrid nanocomposites: Fabrications and applications in sensing, medical, environmental, and energy sectors

Developing new hybrid materials is critical for addressing the current needs of the world in various fields, such as energy, sensing, health, hygiene, and others. C-dots are a member of the carbon nanomaterial family with numerous applications. Aggregation is one of the barriers to the performance of C-dots, which causes luminescence quenching, surface area decreases, etc. To improve the performance of C-dots, numerous matrices including metal-organic frameworks (MOFs), covalent-organic frameworks (COFs), and polymers have been composited with C-dots. The porous crystalline structures, which are constituents of metal nodes and organic linkers (MOFs) or covalently attached organic units (COFs) provide privileged features such as high specific surface area, tunable structures, and pore diameters, modifiable surface, high thermal, mechanical, and chemical stabilities. Also, the MOFs and COFs protect the C-dots from the environment. Therefore, MOF/C-dots and COF/C-dots composites combine their features while retaining topological properties and improving performances. In this review, we first compare MOFs with COFs as matrices for C-dots. Then, the recent progress in developing hybrid MOFs/C-dots and COFs/C-dots composites has been discussed and their applications in various fields have been explained briefly.

Colloidal Chiral Carbon Dots: An Emerging System for Chiroptical Applications

Chiral CDots (c-CDots) not only inherit those merits from CDots but also exhibit chiral effects in optical, electric, and bio-properties. Therefore, c-CDots have received significant interest from a wide range of research communities including chemistry, physics, biology, and device engineers. They have already made decent progress in terms of synthesis, together with the exploration of their optical properties and applications. In this review, the chiroptical properties and chirality origin in extinction circular dichroism (ECD) and circularly polarized luminescence (CPL) of c-CDots is briefly discussed. Then, the synthetic strategies of c-CDots is summarized, including one-pot synthesis, post-functionalization of CDots with chiral ligands, and assembly of CDots into chiral architectures with soft chiral templates. Afterward, the chiral effects on the applications of c-CDots are elaborated. Research domains such as drug delivery, bio- or chemical sensing, regulation of enzyme-like catalysis, and others are covered. Finally, the perspective on the challenges associated with the synthetic strategies, understanding the origin of chirality, and potential applications is provided. This review not only discusses the latest developments of c-CDots but also helps toward a better understanding of the structure-property relationship along with their respective applications.

Carboxylic Group Functionalized Carbon Quantum Dots inhibit Hen Egg White Lysozyme Amyloidogenesis, leading to the Formation of Spherical Aggregates with Reduced Toxicity and ROS Generation

INTRODUCTION

Proteinopathies are a group of diseases where the protein structure has been altered. These alterations are linked to the production of amyloids, which are persistent, organized clumps of protein molecules through inter-molecular interactions. Several disorders, including Alzheimer's and Parkinson's, have been related to the presence of amyloids. Highly ordered beta sheets or beta folds are characteristic of amyloids; these structures can further self- assemble into stable fibrils.

METHODS

Protein aggregation is caused by a wide variety of environmental and experimental factors, including mutations, high pH, high temperature, and chemical modification. Despite several efforts, a cure for amyloidosis has yet to be found. Due to its advantageous semi-conducting characteristics, unique optical features, high surface area-to-volume ratio, biocompatibility, etc., carbon quantum dots (CQDs) have lately emerged as key instruments for a wide range of biomedical applications. To this end, we have investigated the effect of CQDs with a carboxyl group on their surface (CQD-CA) on the in vitro amyloidogenesis of hen egg white lysozyme (HEWL).

RESULTS

By generating a stable compound that is resistant to fibrillation, our findings show that CQD-CA can suppress amyloid and disaggregate HEWL. In addition, CQD-CA caused the creation of non-toxic spherical aggregates, which generated much less reactive oxygen species (ROS).

CONCLUSION

Overall, our results show that more research into amyloidosis treatments, including surface functionalized CQDs, is warranted.

Amine Group Surface-Functionalized Carbon Quantum Dots Exhibit Anti-amyloidogenic Effects Towards Hen Egg White Lysozyme by Inducing Formation of Nontoxic Spherical Aggregates

The tendency of polypeptide chains to deviate from their conventional protein folding pathway and instead get trapped as off-pathway intermediates, has been a matter of great concern. These off-pathway intermediates eventually lead to the formation of insoluble, ordered fibrillar aggregates called amyloids, which are responsible for a host of neurodegenerative diseases like Alzheimer's disease, Parkinson's disease and Type II diabetes. In spite of extensive research, development of an effective therapeutic strategy against amyloidosis still remains elusive. In recent times, carbon quantum dots (CQD) have grabbed the attention of researchers against amyloidogenesis due to their ease of preparation, aqueous soluble nature, unique optical properties, high surface to volume ratio, physio-chemical properties, semi-conducting nature and mainly biocompatible. In the current study, we have reported an easy-to-prepare procedure for synthesis of amine group surface functionalized CQDs from commonly available kitchen spices with anti-oxidant properties. The as-synthesized CQDs were evaluated for their anti-amyloidogenic properties towards Hen Egg White Lysozyme (HEWL). Our results clearly show that the surfaced functionalized CQDs were able to interact with HEWL, thereby forming a stable complex, which was resistant towards amyloid formation and instead lead to the formation of non-toxic globular aggregates.

High-Performance Cannabinoid Sensor Empowered by Plant Hormone Receptors and Antifouling Magnetic Nanorods

The misuse of cannabinoids and their synthetic variants poses significant threats to public health, necessitating the development of advanced techniques for detection of these compounds in biological or environmental samples. Existing methods face challenges like lengthy sample pretreatment and laborious antifouling steps. Herein, we present a novel sensing platform using magnetic nanorods coated with zwitterionic polymers for the simple, rapid, and sensitive detection of cannabinoids in biofluids. Our technique utilizes the engineered derivatives of the plant hormone receptor Pyrabactin Resistance 1 (PYR1) as drug recognition elements and employs the chemical-induced dimerization (CID) mechanism for signal development. Additionally, the magnetic nanorods facilitate efficient target capture and reduce the assay duration. Moreover, the zwitterionic polymer coating exhibits excellent antifouling capability, preserving excellent sensor performance in complex biofluids. Our sensors detect cannabinoids in undiluted biofluids like serum, saliva, and urine with a low limit of detection (0.002 pM in saliva and few pM in urine and serum) and dynamic ranges spanning up to 9 orders of magnitude. Moreover, the PYR1 derivatives demonstrate high specificity even in the presence of multiple interfering compounds. This work opens new opportunities for sensor development, showcasing the excellent performance of antifouling magnetic nanorods that can be compatible with different recognition units, including receptors and antibodies, for detecting a variety of targets.

Comprehensive advances in the synthesis, fluorescence mechanism and multifunctional applications of red-emitting carbon nanomaterials

Red emitting fluorescent carbon nanomaterials have drawn significant scientific interest in recent years due to their high quantum yield, water-dispersibility, photostability, biocompatibility, ease of surface functionalization, low cost and eco-friendliness. The red emissive characteristics of fluorescent carbon nanomaterials generally depend on the carbon source, reaction time, synthetic approach/methodology, surface functional groups, average size, and other reaction environments, which directly or indirectly help to achieve red emission. The importance of several factors to achieve red fluorescent carbon nanomaterials is highlighted in this review. Numerous plausible theories have been explained in detail to understand the origin of red fluorescence and tunable emission in these carbon-based nanostructures. The above advantages and fluorescence in the red region make them a potential candidate for multifunctional applications in various current fields. Therefore, this review focused on the recent advances in the synthesis approach, mechanism of fluorescence, and electronic and optical properties of red-emitting fluorescent carbon nanomaterials. This review also explains the several innovative applications of red-emitting fluorescent carbon nanomaterials such as biomedicine, light-emitting devices, sensing, photocatalysis, energy, anticounterfeiting, fluorescent silk, artificial photosynthesis, etc. It is hoped that by choosing appropriate methods, the present review can inspire and guide future research on the design of red emissive fluorescent carbon nanomaterials for potential advancements in multifunctional applications.

Using 19F NMR to Investigate Cationic Carbon Dot Association with Per- and Polyfluoroalkyl Substances (PFAS)

There is much concern about per- and polyfluoroalkyl substances (PFAS) based on their environmental persistence and toxicity, resulting in an urgent need for remediation technologies. This study focused on determining if nanoscale polymeric carbon dots are a viable sorbent material for PFAS and developing fluorine nuclear magnetic resonance spectroscopy (19F NMR) methods to probe interactions between carbon dots and PFAS at the molecular scale. Positively charged carbon dots (PEI-CDs) were synthesized using branched polyethyleneimine to target anionic PFAS by promoting electrostatic interactions. PEI-CDs were exposed to perfluorooctanoic acid (PFOA) to assess their potential as a PFAS sorbent material. After exposure to PFOA, the average size of the PEI-CDs increased (1.6 ± 0.5 to 7.8 ± 1.8 nm) and the surface charge decreased (+38.6 ± 1.1 to +26.4 ± 0.8 mV), both of which are consistent with contaminant sorption. 19F NMR methods were developed to gain further insight into PEI-CD affinity toward PFAS without any complex sample preparation. Changes in PFOA peak intensity and chemical shift were monitored at various PEI-CD concentrations to establish binding curves and determine the chemical exchange regime. 19F NMR spectral analysis indicates slow-intermediate chemical exchange between PFOA and CDs, demonstrating a high-affinity interaction. The α-fluorine had the greatest change in chemical shift and highest affinity, suggesting electrostatic interactions are the dominant sorption mechanism. PEI-CDs demonstrated affinity for a wide range of analytes when exposed to a mixture of 24-PFAS, with a slight preference toward perfluoroalkyl sulfonates. Overall, this study shows that PEI-CDs are an effective PFAS sorbent material and establishes 19F NMR as a suitable method to screen for novel sorbent materials and elucidate interaction mechanisms.

Evaluation of N and S Codoped Carbon Dots as an Environment Friendly and High-Efficiency Inhibitor for X65 Steel in an Acidic Medium

The high content of nitrogen and sulfur-doped carbon dots (N, S-CDs) was designed to prevent the corrosion of X65 steel in an acidic medium. The corrosion-inhibiting abilities of related nanomaterials for X65 steel were acquired by electrochemical experiments, and the corroded products were investigated by FT-IR, XPS, and Raman analysis. The conclusions confirm that the N, S-CDs are a high-efficiency inhibitor. When the concentration is 200 mg/L, the inhibitive efficiency of X65 steel can reach up to 99.1% and it interacts with X65 steel through chemical and physical adsorption. Additionally, results from the spectroscopic studies show that the S-group is the main contributor to the chemical adsorption process.

Enhanced ferrate(VI) oxidation of organic pollutants through direct electron transfer

Fe(VI) is a versatile agent for water purification, and various strategies have been developed to improve its pollutant removal efficiency. Herein, it was found that in addition to intermediate iron species [Fe(IV)/Fe(V)], direct electron transfer (DET) played a significant role in the abatement of organic pollutants in Fe(VI)/carbon quantum dots (CQDs) system. Around 86, 83, 73, 64, 52, 45 and 17% of BPA, DCF, SMX, 4-CP, phenol, p-HBA, and IBP (6 μM) could be oxidized by 30 μM of Fe(VI), whereas with the addition of CQDs (4 mg/L), the oxidation ratio of these pollutants increased to 98, 99, 80, 88, 87, 66 and 57%, respectively. The negative impact induced by solution pH and background constituents on Fe(VI) abatement of pollutants could be alleviated by CQDs, and CQDs acted as catalysts for mediating DET from organic pollutants to Fe(VI). Theoretical calculation revealed that iron species [Fe(VI)/Fe(V)/Fe(IV)] was responsible for the oxidation of 36% of phenol, while DET contributed to the oxidation of 64% of phenol in the Fe(VI)/CQDs system. Compared with iron species oxidation, the CQDs mediated DET from pollutants to Fe(VI) was more efficient for utilizing the oxidation capacity of Fe(VI). The DET mechanism presented in the study provides a prospective strategy for improving the pollution control potential of Fe(VI).

Red emissive carbon dots: a promising next-generation material with intracellular applicability

The accidental discovery of carbon dots (CDs) back in 2004 has led to their widespread use in the biomedical field. CDs have demonstrated their effectiveness in reporting 3D structures of biological specimens, identifying normal and cancer cells, and even detecting analytes within cells. However, the limitations of blue-green emitting CDs, such as their shallow penetration, photodamage, and auto-fluorescence, have hindered their practical applications. To overcome these limitations, red emissive CDs (RCDs) have been developed, which have deep tissue penetration, minimal photo-damage, low auto-fluorescence, and high imaging contrast. In this article, we present a thorough review on the use of RCDs in biomedical applications, including in vivo and in vitro bioimaging, photoacoustic imaging, monitoring temperature and polarity changes in living cells, tumour therapy, and drug delivery. With the rapid progress being made in the development of RCDs for intracellular applications, their clinical application is expected to become a reality in the near future.

Gadolinium-Doped Carbon Nanodots as Potential Anticancer Tools for Multimodal Image-Guided Photothermal Therapy and Tumor Monitoring

This study focuses on the synthesis and characterization of gadolinium-doped carbon nanodots (CDs-Gd) and their potential applications in multimodal imaging and precision cancer therapy. CDs-Gd were synthesized through a solvothermal decomposition method combining citric acid, GdCl3, and urea. The incorporation of Gd3+ ions within the carbonaceous structure resulted in stable CDs-Gd with a peculiar architecture that retained optical and paramagnetic properties. Combined characterization techniques confirmed the presence of pH-sensitive COOH functions on the CDs-Gd surface along with the unique lattice structure induced by Gd3+ doping. The optical properties of CDs-Gd exhibited a tunable emission spectrum displaying blue-green emission with pH-dependent behavior. Additionally, CDs-Gd exhibited contrast-enhancing properties in T1-weighted magnetic resonance imaging (MRI) experiments. MRI acquisitions at different Gd3+ concentrations and pH values demonstrated the potential of CDs-Gd as contrast agents for monitoring pH changes in an aqueous environment. We found that the relaxivity of CDs-Gd at pH 5.5 (tumor, 11.3 mM-1 s-1) is roughly 3-fold higher than that observed at pH 7.4 (physiological, 5.0 mM-1 s-1) and outperformed clinical standards such as γ-butyrol (3.3 mM-1 s-1). Monitoring pH changes in tumor microenvironment (TME) is crucial for evaluating the effectiveness of anticancer treatments and understanding tumor progression. Furthermore, CDs-Gd demonstrated concentration-dependent photothermal conversion ability in the near-infrared (NIR) region, allowing for efficient heat generation under laser irradiation. This indicates the potential application of CDs-Gd in image-guided photothermal therapy (IG-PTT) for cancer treatment. The in vitro studies on MCF-7 (breast cancer) and 16-HBE (healthy bronchial epithelium) cell lines demonstrated that CDs-Gd exhibited high biocompatibility (cell viability >80%). However, upon NIR activation, they showed potent anticancer effects by inhibiting tumor cell proliferation and inducing apoptosis selectively in cancer cells. In conclusion, the synthesized CDs-Gd nanoparticles possess unique optical, photothermal, and MRI contrast properties, making them promising candidates for multimodal imaging-guided precision cancer therapy applications.

Carbon Quantum Dots with High Photothermal Conversion Efficiency and Their Application in Photothermal Modulated Reversible Deformation of Poly(N-isopropylacrylamide) Hydrogel

The fluorescence of carbon quantum dots (CQDs) has been paid a lot of attention, but its photothermal performance attracts less attention since preparing CQDs with high photothermal conversion efficiency (PCE) is a big challenge. In this work, CQDs with an average size of 2.3 nm and a PCE of up to 59.4% under 650 nm laser irradiation were synthesized by a simple one-pot microwave-assisted solvothermal method using citric acid (CA) and urea (UR) as the precursors and N,N-dimethylformamide as the solvent under an optimized condition (CA/UR = 1/7, 150 °C, and 1 h). The as-prepared CQDs were demonstrated to have unique surface chemical states; i.e., abundant pyrrole, amide, carboxyl, and hydroxyl groups were found on the surfaces of CQDs, which ensure a high PCE. These CQDs were introduced into a thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) to form a CQDs@PNIPAM nanocomposite, and then, a bilayer hydrogel composed of CQDs@PNIPAM and polyacrylamide (PAM) was fabricated. The bilayer hydrogel can be reversibly deformed just by a light switching on/off operation. Based on the excellent photothermal performance, the developed CQDs are expected to be used in photothermal therapy, photoacoustic imaging, and other biomedical fields, and the CQDs@PNIPAM hydrogel nanocomposite is promising to be applied in intelligent device systems as a light-driven smart flexible material.

Red emitting carbon dots: surface modifications and bioapplications

Quantum dots (QDs), and carbon quantum dots (CDs) in particular, have received significant attention for their special characteristics. These particles, on the scale of several nanometers, are often produced using simple and green methods, with naturally occurring organic precursors. In addition to facile production methods, CDs present advantageous applications in the field of medicine, primarily for bioimaging, antibacterial and therapeutics. Also, CDs present great potential for surface modification through methods like doping or material mixing during synthesis. However, the bulk of current literature focuses on CDs emitting in the blue wavelengths which are not very suitable for biological applications. Red emitting CDs are therefore of additional interest due to their brightness, photostability, novelty and deeper tissue penetration. In this review article, red CDs, their methods of production, and their biological applications for translational research are explored in depth, with emphasis on the effects of surface modifications and doping.

Microwave-assisted one-step synthesis of water-soluble manganese-carbon nanodot clusters

Using metal coordination to assemble carbon nanodots (CND) into clusters can enhance their photophysical properties for applications in sensing and biomedicine. Water-soluble clusters of CNDs are prepared by one-step microwave synthesis starting from ethylenediaminetetraacetic acid, ethylenediamine and MnCl2·4H2O as precursors. Transmission electron microscopy and powder X-Ray diffraction techniques indicate that the resulting clusters form spherical particles of 150 nm constituted by amorphous CNDs joined together with Mn ions in a laminar crystalline structure. The nanomaterial assemblies show remarkable fluorescence quantum yields (0.17-0.20) and magnetic resonance imaging capability (r1 = 2.3-3.8 mM-1.s-1). In addition, they can be stabilized in aqueous solutions by phosphate ligands, providing a promising dual imaging platform for use in biological systems.

A Green Synthesis Route to Derive Carbon Quantum Dots for Bioimaging Cancer Cells

Carbon quantum dots (CQDs) are known for their biocompatibility and versatile applications in the biomedical sector. These CQDs retain high solubility, robust chemical inertness, facile modification, and good resistance to photobleaching, which makes them ideal for cell bioimaging. Many fabrication processes produce CQDs, but most require expensive equipment, toxic chemicals, and a long processing time. This study developed a facile and rapid toasting method to prepare CQDs using various slices of bread as precursors without any additional chemicals. This fast and cost-effective toasting method could produce CQDs within 2 h, compared with the 10 h process in the commonly used hydrothermal method. The CQDs derived from the toasting method could be used to bioimage two types of colon cancer cells, namely, CT-26 and HT-29, derived from mice and humans, respectively. Significantly, these CQDs from the rapid toasting method produced equally bright images as CQDs derived from the hydrothermal method.

Progress on the luminescence mechanism and application of carbon quantum dots based on biomass synthesis

With the continuous development of carbon-based materials, a variety of new materials have emerged one after another. Carbon Quantum Dots (CQDs) have succeeded in standing out from the crowd of new materials due to their better optical properties in biomedicine, ion detection, anti-counterfeiting materials and photocatalysis. In recent years, through the continuous exploration of CQDs, research scholars have found that the organic substances or heavy metals contained in traditional ones can cause irreversible harm to people and the environment. Therefore, the application of traditional CQDs in future studies will be gradually limited. Among various new materials, biomass raw materials have the merits of good biocompatibility, lower toxicity and green and environmental protection, which largely overcome the defects of traditional materials and have attracted many scholars to focus on the research and development of various biomass CQDs. This paper summarises the optical properties, fluorescence mechanisms, synthetic methods, functionalisation modulation of biomass CQDs and their relevant research progress in the fields of ion detection, bioimaging, biomedicine, biosensing, solar cells, anti-counterfeit materials, photocatalysis and capacitors. Finally, the paper concludes with some discussion of the challenges and prospects of this exciting and promising field of application.

Two-Birds-with-One-Stone Synthesis of Hydrophilic and Hydrophobic Fluorescent Carbon Nanodots from Dunaliella salina Biomass as 4-Nitrophenol Nanoprobes Based on Inner Filter Effect and First Derivative Redshift of Emission Band

4-Nitrophenol (4-NP) has been listed as a priority pollutant and has also been reported as a human urinary metabolite used as a marker to evaluate exposure to certain pesticides. In the work herein, a solvothermal approach is applied to the one-pot synthesis of both hydrophilic and hydrophobic fluorescent carbon nanodots (CNDs), utilizing the halophilic microalgae Dunaliella salina as a biomass precursor. Both kinds of the produced CNDs showed appreciable optical properties and quantum yields, good photostability and they were capable of probing 4-NP by quenching their fluorescence through the inner filter effect. Interestingly, a prominent 4-NP concentration-dependent redshift of the corresponding emission band of the hydrophilic CNDs was noticed, which was further exploited, for the first time, as an analytical platform. Capitalizing on these properties, analytical methods were developed and applied to a variety of matrixes, such as tap water, treated municipal wastewater and human urine. The method based on the hydrophilic CNDs (λ_ex/λ_em: 330/420 nm) was linear in the range of 0.80-45.0 μM and showed acceptable recoveries (from 102.2 to 113.7%) with relative standard deviations of 2.1% (intra-day) and 2.8% (inter-day) for the quenching-based detection mode and 2.9% (intra-day) and 3.5% (inter-day) for the redshift one. The method based on the hydrophobic CNDs (λ_ex/λ_em: 380/465 nm) was linear in the range of 1.4-23.0 μM, with recoveries laying within the range of 98.2-104.5% and relative standard deviations of 3.3% and 4.0% for intra-day and inter-day assays, respectively.

Energy-level engineering of carbon dots through a post-synthetic treatment with acids and amines

Chemically synthesized carbon dots (CDs) have attracted a lot of attention as an eco-friendly and cost-efficient light-emitting material, and functionalization of CD surfaces with additives of different natures is a useful way to control their properties. In this study, we show how a post-synthetic treatment of CDs with citric acid, benzoic acid, urea and o-phenylenediamine changes their chemical composition and optical properties. In particular, it results in the formation of carboxyl/imide/carbonyl groups at the CD surface, leading to the appearance of additional blue (or for CDs treated with phenylenediamine, blue and green) emissive optical centers on top of the remaining emission from the original CDs. Most importantly, the increased oxidation degree alongside a decreased relative amount of carbon and nitrogen in such treated CDs decreases their highest occupied molecular orbital (HOMO) energy level by up to 0.9 eV (the maximal value was observed for CDs treated with o-phenylenediamine). Moreover, the Fermi energy level shifted above the lowest unoccupied molecular orbital (LUMO) energy level for some of the treated CD samples. Thus, the energy structure of CDs can be tuned and optimized for further applications through the functionalization of their surface with organic additives.

Synthesis and application of quantum dots in detection of environmental contaminants in food: A comprehensive review

Environmental pollutants can accumulate in the human body through the food chain, which may seriously impact human health. Therefore, it is of vital importance to develop quick, simple, accurate and sensitive (respond quickly) technologies to evaluate the concentration of environmental pollutants in food. Quantum dots (QDs)-based fluorescence detection methods have great potential to overcome the shortcomings of traditional detection methods, such as long detection time, cumbersome detection procedures, and low sensitivity. This paper reviews the types and synthesis methods of QDs with a focus on green synthesis and the research progress on rapid detection of environmental pollutants (e.g., heavy metals, pesticides, and antibiotics) in food. Metal-based QDs, carbon-based QDs, and "top-down" and "bottom-up" synthesis methods are discussed in detail. In addition, research progress of QDs in detecting different environmental pollutants in food is discussed, especially, the practical application of these methods is analyzed. Finally, current challenges and future research directions of QDs-based detection technologies are critically discussed. Hydrothermal synthesis of carbon-based QDs with low toxicity from natural materials has a promising future. Research is needed on green synthesis of QDs, direct detection without pre-processing, and simultaneous detection of multiple contaminants. Finally, how to keep the mobile sensor stable, sensitive and easy to store is a hot topic in the future.

Carbon-Based Fluorescent Nano-Biosensors for the Detection of Cell-Free Circulating MicroRNAs

Currently, non-communicable diseases (NCDs) have emerged as potential risks for humans due to adopting a sedentary lifestyle and inaccurate diagnoses. The early detection of NCDs using point-of-care technologies significantly decreases the burden and will be poised to transform clinical intervention and healthcare provision. An imbalance in the levels of circulating cell-free microRNAs (ccf-miRNA) has manifested in NCDs, which are passively released into the bloodstream or actively produced from cells, improving the efficacy of disease screening and providing enormous sensing potential. The effective sensing of ccf-miRNA continues to be a significant technical challenge, even though sophisticated equipment is needed to analyze readouts and expression patterns. Nanomaterials have come to light as a potential solution as they provide significant advantages over other widely used diagnostic techniques to measure miRNAs. Particularly, CNDs-based fluorescence nano-biosensors are of great interest. Owing to the excellent fluorescence characteristics of CNDs, developing such sensors for ccf-microRNAs has been much more accessible. Here, we have critically examined recent advancements in fluorescence-based CNDs biosensors, including tools and techniques used for manufacturing these biosensors. Green synthesis methods for scaling up high-quality, fluorescent CNDs from a natural source are discussed. The various surface modifications that help attach biomolecules to CNDs utilizing covalent conjugation techniques for multiple applications, including self-assembly, sensing, and imaging, are analyzed. The current review will be of particular interest to researchers interested in fluorescence-based biosensors, materials chemistry, nanomedicine, and related fields, as we focus on CNDs-based nano-biosensors for ccf-miRNAs detection applications in the medical field.

Research Progress in the Synthesis of Carbon Dots and Their Application in Food Analysis

Food safety is connected to public health, making it crucial to protecting people's health. Food analysis and detection can assure food quality and effectively reduce the entry of harmful foods into the market. Carbon dots (CDs) are an excellent choice for food analysis and detection attributable to their advantages of good optical properties, water solubility, high chemical stability, easy functionalization, excellent bleaching resistance, low toxicity, and good biocompatibility. This paper focuses on the optical properties, synthesis methods, and applications of CDs in food analysis and detection, including the recent advances in food nutritional composition analysis and food quality detection, such as food additives, heavy metal ions, foodborne pathogens, harmful organic pollutants, and pH value. Moreover, this review also discusses the potentially toxic effects, current challenges, and prospects of CDs in basic research and applications. We hope that this review can provide valuable information to lay a foundation for subsequent research on CDs and promote the exploration of CDs-based sensing for future food detection.

Fighting Non-Small Lung Cancer Cells Using Optimal Functionalization of Targeted Carbon Quantum Dots Derived from Natural Sources Might Provide Potential Therapeutic and Cancer Bio Image Strategies

Non-small cell lung cancer (NSCLC) is an important sub-type of lung cancer associated with poor diagnosis and therapy. Innovative multi-functional systems are urgently needed to overcome the invasiveness of NSCLC. Carbon quantum dots (CQDs) derived from natural sources have received interest for their potential in medical bio-imaging due to their unique properties, which are characterized by their water solubility, biocompatibility, simple synthesis, and low cytotoxicity. In the current study, ethylene-diamine doped CQDs enhanced their cytotoxicity (98 ± 0.4%, 97 ± 0.38%, 95.8 ± 0.15%, 86 ± 0.15%, 12.5 ± 0.14%) compared to CQDs alone (99 ± 0.2%, 98 ± 1.7%, 96 ± 0.8%, 93 ± 0.38%, 91 ± 1.3%) at serial concentrations (0.1, 1, 10, 100, 1000 μg/mL). In order to increase their location in a specific tumor site, folic acid was used to raise their functional folate recognition. The apoptotic feature of A549 lung cells exposed to N-CQDs and FA-NCQDs was characterized by a light orange-red color under fluorescence microscopy. Additionally, much nuclear fragmentation and condensation were seen. Flow cytometry results showed that the percentage of cells in late apoptosis and necrosis increased significantly in treated cells to (19.7 ± 0.03%), (27.6 ± 0.06%) compared to untreated cells (4.6 ± 0.02%), (3.5 ± 0.02%), respectively. Additionally, cell cycle arrest showed a strong reduction in cell numbers in the S phase (14 ± 0.9%) compared to untreated cells (29 ± 0.5%). Caspase-3 levels were increased significantly in A549 exposed to N-CQDs (2.67 ± 0.2 ng/mL) and FA-NCQDs (3.43 ± 0.05 ng/mL) compared to untreated cells (0.34 ± 0.04 ng/mL). The functionalization of CQDs derived from natural sources has proven their potential application to fight off non-small lung cancer.

Nitrogen doped carbon quantum dots (N-CQDs) with high luminescence for sensitive and selective detection of hypochlorite ions by fluorescence quenching

A highly luminescent nitrogen-doped carbon quantum dots (N-CQDs) with a quantum yield of 44% was prepared by a facile hydrothermal synthesis method using citric acid (CA) and ethylenediamine (EDA) with a molar ratio of 1:1 at 200 °C for 5 h. The hypochlorite (ClO^-) ions significantly quench the fluorescence of the N-CQDs according to a pseudo-second-order kinetic model. A sensitive and selective quantification method with an excellent linearity in the range of 1.0-10.0 μM was developed to detect ClO^- ions based on the fluorescence quenching. The limit of detection (LOD) of 0.43 μM and the limit of quantification (LOQ) of 1.04 μM were achieved, respectively. This approach was successfully applied to detect the residual ClO^- ions in local tap water and in swimming pool water. In addition, the developed fluorescence quenching method was also successfully applied in anti-counterfeiting and paper encryption. Both of the applications in real world suggest that the as-prepared N-CQDs is a kind of promising fluorescence probe for rapid detecting ClO^- ions in environment fields, and has potential applications in text secrecy fields.

State-of-the-art developments in carbon quantum dots (CQDs): Photo-catalysis, bio-imaging, and bio-sensing applications

Carbon quantum dots (CQDs), the intensifying nanostructured form of carbon material, have exhibited incredible impetus in several research fields such as bio-imaging, bio-sensing, drug delivery systems, optoelectronics, photovoltaics, and photocatalysis, thanks to their exceptional properties. The CQDs show extensive photonic and electronic properties, as well as their light-collecting, tunable photoluminescence, remarkable up-converted photoluminescence, and photo-induced transfer of electrons were widely studied. These properties have great advantages in a variety of visible-light-induced catalytic applications for the purpose of fully utilizing the energy from the solar spectrum. The major purpose of this review is to validate current improvements in the fabrication of CQDs, characteristics, and visible-light-induced catalytic applications, with a focus on CQDs multiple functions in photo-redox processes. We also examine the problems and future directions of CQD-based nanostructured materials in this growing research field, with an eye toward establishing a decisive role for CQDs in photocatalysis, bio-imaging, and bio-sensing applications that are enormously effective and stable over time. In the end, a look forward to future developments is presented, with a view to overcoming challenges and encouraging further research into this promising field.

Green synthesis of carbon quantum dots and their environmental applications

Green synthesis of scalable, high-quality, fluorescent carbon quantum dots (CQDs) from natural biomass remains attractive due to their outstanding environmental application. CQDs are an emerging class of zero-dimensional carbon nanomaterials (<10 nm) that have recently attracted much attention due to their strong optical properties, biocompatibility, nontoxicity, uniform particle size, high photostability, low-cost synthesis, and highly tunable photoluminescence. The unique properties of CQDs possess a broad range of prospective applications in a number of fields such as metal ions detection, photocatalysis, sensing, medical diagnosis, bioimaging, and drug delivery. CQD nanostructures are synthesized using various techniques such as hydrothermal method, laser ablation, microwave irradiation, electrochemical oxidation, reflux method, and ultrasonication. However, this type of fabrication approach requires several chemical reactions including oxidation, carbonization, and pyrolysis. Green synthesis of CQDs has several advantages such as the use of low-cost and non-toxic raw materials, renewable resources, simple operations, and being environment-friendly. This review article will discuss the physicochemical properties of CQDs techniques used in the production of CQDs, and the stability of CQDs along with their applications in wastewater treatment and biomedical fields.

The Transformation of 0-D Carbon Dots into 1-, 2- and 3-D Carbon Allotropes: A Minireview

Carbon dots (CDs) represent a relatively new type of carbon allotrope with a 0-D structure and with nanoparticle sizes < 10 nm. A large number of research articles have been published on the synthesis, characteristics, mechanisms and applications of this carbon allotrope. Many of these articles have also shown that CDs can be synthesized from “bottom-up” and “top-down” methods. The “top-down” methods are dominated by the breaking down of large carbon structures such as fullerene, graphene, carbon black and carbon nanotubes into the CDs. What is less known is that CDs also have the potential to be used as carbon substrates for the synthesis of larger carbon structures such as 1-D carbon nanotubes, 2-D or 3-D graphene-based nanosheets and 3-D porous carbon frameworks. Herein, we present a review of the synthesis strategies used to convert the 0-D carbons into these higher-dimensional carbons. The methods involve the use of catalysts or thermal procedures to generate the larger structures. The surface functional groups on the CDs, typically containing nitrogen and oxygen, appear to be important in the process of creating the larger carbon structures that typically are formed via the generation of covalent bonds. The CD building blocks can also ‘aggregate’ to form so called supra-CDs. The mechanism for the formation of the structures made from CDs, the physical properties of the CDs and their applications (for example in energy devices and as reagents for use in medicinal fields) will also be discussed. We hope that this review will serve to provide valuable insights into this area of CD research and a novel viewpoint on the exploration of CDs.

Covalent and Noncovalent Loading of Doxorubicin by Folic Acid-Carbon Dot Nanoparticles for Cancer Theranostics

With special properties such as excellent fluoresce features, low toxicity, good biocompatibility, permeability, and easy clearance from the body, carbon dot (CD)-based nanoparticles (NPs) have the potential to deliver drugs and use in vivo diagnostics through molecular imaging. In this work, folic acid-CD (FA-CD) NPs were prepared to deliver doxorubicin (Dox) covalently and noncovalently as cancer theranostics. FA was conjugated to the surface of CDs for targeting cancer cells with overexpressing folate receptors. CDs prepared with various amounts of precursors lead to their associated NPs with different photoluminescence properties and drug release profiles. The loading of Dox and its releasing data depends on the linkage of drug Dox to FA-CD and CD composition. All NPs were characterized by UV-vis, Fourier transform infrared spectroscopy, and dynamic light scattering. The noncovalent FA-CD-Dox NPs were preferred with a simple preparation process, excellent photoluminescence, and in vitro drug release properties. The noncovalent FA-CD-Dox showed the best efficacy against MDA-MB-231 compared to the CD-Dox and covalent FA-CD-Dox.

Measuring the refractive index and sub-nanometre surface functionalisation of nanoparticles in suspension

Direct measurements to determine the degree of surface coverage of nanoparticles by functional moieties are rare, with current strategies requiring a high level of expertise and expensive equipment. Here, a practical method to determine the ratio of the volume of the functionalisation layer to the particle volume based on measuring the refractive index of nanoparticles in suspension is proposed. As a proof of concept, this technique is applied to poly(methyl methacrylate) (PMMA) nanoparticles and semicrystalline carbon dots functionalised with different surface moieties, yielding refractive indices that are commensurate to those from previous literature and Mie theory. In doing so, it is demonstrated that this technique is able to optically detect differences in surface functionalisation or composition of nanometre-sized particles. This non-destructive and rapid method is well-suited for in situ industrial particle characterisation and biological applications.

Small variations in reaction conditions tune carbon dot fluorescence

The development of robust and reproducible synthetic strategies for the production of carbon dots (CDs) with improved fluorescence quantum yields and distinct emission profiles is of great relevance given the vast range of applications of CDs. The fundamental understanding at a molecular level of their formation mechanism, chemical structure and how these parameters are correlated to their photoluminescence (PL) properties is thus essential. In this study, we describe the synthesis and structural characterization of a range of CDs with distinct physico-chemical properties. The materials were prepared under three minutes of microwave irradiation using the same common starting materials (D-glucosamine hydrochloride 1 and ethylenediamine 2) but modifying the stoichiometry of the reagents. We show that small variation in reaction conditions leads to changes in the fluorescent behaviour of the CDs, especially in the selective enhancement of overlapped fluorescence bands. Structural analysis of the different CD samples suggested different reaction pathways during the CD formation and surface passivation, with the latter step being key to the observed differences. Moreover, we demonstrate that these materials have distinct reversible response to pH changes, which we can be attribute to different behaviour towards protonation/deprotonation events of distinct emission domains present within each nanomaterial. Our results highlight the importance of understanding the reaction pathways that lead to the formation of this carbon-based nanomaterials and how this can be exploited to develop tailored materials towards specific applications.

DNA Nanotechnology-Based Supramolecular Assemblies for Targeted Biomedical Applications

DNA is a polyanionic, hydrophilic, and natural biopolymer that offers properties such as biodegradability, biocompatibility, non-toxicity, and non-immunogenicity. These properties of DNA as an ideal biopolymer offer modern-day researchers' reasons to exploit these to form high-order supramolecular assemblies. These structures could range from simple to complex and provide various applications. Among them, supramolecular assemblies like DNA hydrogels (DNA-HG) and DNA dendrimers (DNA-DS) show massive growth potential in the areas of biomedical applications such as cell biology, medical stream, molecular biology, pharmacology, and healthcare product manufacturing. The application of both of these assemblies has seen enormous growth in recent years. In this focused review on DNA-based supramolecular assemblies like hydrogels and dendrimers, we present the principles of synthesis and characterization, key developments with examples and applications, and conclude with a brief perspective on challenges and future outlook for such devices and their subsequent applications.

Recent Progress on Carbon Quantum Dots Based Photocatalysis

As a novel carbon allotrope, carbon quantum dots (CQDs) have been investigated in various fields, including photocatalysis, bioimaging, optoelectronics, energy and photovoltaic devices, biosensing, and drug delivery owing to their unique optical and electronic properties. In particular, CQDs' excellent sunlight harvesting ability, tunable photoluminescence (PL), up-conversion photoluminescence (UCPL), and efficient photo-excited electron transfer have enabled their applications in photocatalysis. This work focuses on the recent progress on CQDs-related materials' synthesis, properties, and applications in photocatalysis.

Photodynamic Anti-Bacteria by Carbon Dots and Their Nano-Composites

The misuse of many types of broad-spectrum antibiotics leads to increased antimicrobial resistance. As a result, the development of a novel antibacterial agent is essential. Photodynamic antimicrobial chemotherapy (PACT) is becoming more popular due to its advantages in eliminating drug-resistant strains and providing broad-spectrum antibacterial resistance. Carbon dots (CDs), zero-dimensional nanomaterials with diameters smaller than 10 nm, offer a green and cost-effective alternative to PACT photosensitizers. This article reviewed the synthesis methods of antibacterial CDs as well as the recent progress of CDs and their nanocomposites in photodynamic sterilization, focusing on maximizing the bactericidal impact of CDs photosensitizers. This review establishes the base for future CDs development in the PACT field.

Double-signal quantification of amoxicillin based on interaction with 4-aminoantipyrine at copper and nitrogen co-doped carbon quantum dots as an artificial nanozyme

An one-pot hydrothermal method was developed for synthesis of carbon quantum dots co-doped with copper and nitrogen (Cu, N@CQDs). The synthesized Cu, N@CQDs has unique advantages such as high fluorescence quantum yield (39.1%) and high catalytic activity. Oxidative coupling of amoxicillin (AMX) with 4-aminoantipyrine (4-NH₂-APE) in the presence of H₂O₂ as an oxidant to produce pink quinoneimine chromogen was carried out with the aid of Cu, N@CQDs as a peroxidase-like catalyst. This system was used for the colorimetric and fluorometric assays of AMX with reliable results. Colorimetric method is based on the measurement of a pink-colored product at λ_max = 505 nm while the fluorometric assay is based on the quenching of the fluorescence emission of Cu, N@CQDs at 440 nm after excitation at 370 nm. For the colorimetric method, the absorption intensity linearly increased over the concentration range 4.3-110.0 µM with LOD (S/N = 3) of 1.3 µM. For the fluorometric method, the emission intensity of Cu, N@CQDs linearly decreased upon addition of AMX in the concentration range 0.2-120.0 µM with a limit of detection (LOD, S/N = 3) of 0.06 µM. The proposed system was applied to the determination of AMX in different real samples such as pharmaceutical capsules, human serum, milk, and conduit water samples with recoveries in the range 95.8-104.1% and relative standard deviation (RSD %) less than 4.1%.

Yeast powder derived carbon quantum dots for dopamine detection and living cell imaging

Dopamine (DA) is an important neurotransmitter in the brain of mammals. There is a critical need for fast and sensitive determination approaches to monitor these potential diseases due to various weaknesses in clinical trials of the existing methods for DA detection. DA can effectively quench the fluorescence of carbon quantum dots (CDs) through the inner filter effect and static quenching. In this work, fluorescent yeast CDs (Y-CDs) were prepared via a simple hydrothermal approach of using yeast powder and regarded as the fluorescent nanoprobe to directly monitor the DA concentration. The as-prepared detection platform exhibited excellent sensitivity and selectivity toward DA with a low detection limit of 30 nM and a wide linear range of 0.05-150 μM. Benefiting from these outstanding features, the developed label-free method has been successfully applied for fast DA detection in human serum samples with satisfactory recoveries. Furthermore, it demonstrated that the Y-CDs were well suitable for live cell imaging and showed low toxicity toward MCF-7 cells. Consequently, this work will facilitate the great potential of the versatile Y-CDs in developing biosensors for clinical diagnosis and other biological applications.

Water stable, red emitting, carbon nanoparticles stimulate 3D cell invasion via clathrin-mediated endocytic uptake

Bright fluorescent nanoparticles with excitation and emission towards the red end of the spectrum are highly desirable in the field of bioimaging. We present here a new class of organic carbon-based nanoparticles (CNPs) with a robust quantum yield and fluorescence towards the red region of the spectrum. Using organic substrates such as para-phenylenediamine (PPDA) dispersed in diphenyl ether under reflux conditions, we achieved scalable amounts of CNPs with an average size of 27 nm. These CNPs were readily taken up by different mammalian cells, and we show that they prefer clathrin-mediated endocytosis for their cellular entry route. Not only can these CNPs be specifically taken up by cells, but they also stimulate cellular processes such as cell invasion from 3D spheroid models. This new class of CNPs, which have sizes similar to those of proteinaceous ligands, hold immense potential for their surface functionalization. These could be explored as promising bioimaging agents for biomedical imaging and intracellular drug delivery.

Superior resistance-thermal stability of carbon dots@NaBiF4 nanocomposite: facile synthesis and surface configurations

Carbon dots (CDs), emerging as promising materials for optoelectronic and biomedicine applications, are widely investigated due to their distinct merits of facile preparation, biocompatibility, and environment-friendliness. Here, a unique strategy...

Carbon dot composites for bioapplications: a review

Recent advancements in the synthesis of carbon dot composites and their applications in biomedical fields (bioimaging, drug delivery and biosensing) have been carefully summarized. The current challenges and future trends of CD composites in this field have also been discussed.

Carbon dot/inorganic nanomaterial composites

The preparation methods, formation mechanism, properties and applications of carbon dot/inorganic nanohybrid materials are reported.

Photothermal-enhanced peroxidase-like activity of CDs/PBNPs for the detection of Fe3+ and cholesterol in serum samples

Carbon dots/Prussian blue nanoparticles (CDs/PBNPs) with fluorescence (FL) performance and peroxidase-like activity are synthesized by a simple two-step method. The FL of CDs/PBNPs can be effectively quenched by Fe³⁺. Fe³⁺ can accelerate the peroxidase-like activity of CDs/PBNPs. More excitingly, the peroxidase-like activity of CDs/PBNPs could be further enhanced due to the influence of the photothermal effect. Based on the FL property and enhanced peroxidase-like activity, a cascade strategy is proposed for detection of Fe³⁺ and free cholesterol. CD/PBNPs act as FL probe for detection of Fe³⁺. The enhanced peroxidase-like activity of CDs/PBNPs can also be used as colorimetric probe for the detection of free cholesterol. The detection ranges of Fe³⁺ and free cholesterol are 4-128 μM and 2-39 μM, and the corresponding limit of detections are 2.0 μM and 1.63 μM, respectively. The proposed strategy has been verified by the feasibility determination of Fe³⁺ and free cholesterol, suggesting its potential in the prediction of disease.

Fabrication of fluorescence sensor based on molecularly imprinted polymer on amine-modified carbon quantum dots for fast and highly sensitive and selective detection of tannic acid in food samples

Developing a rapid and accurate method for tannic acid (TA) detection and measurement is necessary due to its extensive applications in the food industry. In this work, a fluorescence sensor with a low limit of detection was synthesized for TA for the first time. First, amine-modified carbon quantum dots (a-CQDs) with high-quantum yield were synthesized by the hydrothermal method. A layer of molecularly imprinted polymer (MIP) was then placed on a-CQDs by the surface printing method to increase the sensor selectivity. The mechanism of TA detection by the prepared a-CQDs/MIPs was quenching the fluorescence intensity of a-CQDs in the presence of TA due to the transfer of electrons from the TA to the a-CQDs. The linear range of the sensor response was at the TA concentration of 1-200 nmol L^-1 and its detection limit was 0.6 nmol L^-1 under optimal conditions. Finally, the sensor was used to measure TA in grape juice, green tea, and black tea samples using the recovery method. Recovery values between 97.4 and 103.6% and RSDs less than 3.8% indicated the high potential of the prepared sensor for TA analysis in complex food samples.