Nitrogen and Sulfur Doped Carbon Dots from Amino Acids for Potential Biomedical Applications

Effect of temperature on the carbonization process of cationic carbon dots: a physicochemical and in vitro study

This work highlights the critical role of synthesis conditions in tuning the properties of carbon dots (CDs) for optimized performance in biomedical applications, offering valuable insights into the design of these carbon nanomaterials. Although various synthesis methods and carbon sources have been explored for CD production, few studies have investigated how synthesis temperature modulates and optimizes their physicochemical attributes. In this study, cationic CDs derived from poly(ethylene imine) (PEI) and chitosan (CS) were synthesized using a microwave-assisted hydrothermal method at different temperatures to explore this aspect. It was found that higher carbonization temperatures during the hydrothermal process resulted in smaller, more photoluminescent CDs. This increase in temperature significantly enhanced the biological interactions of the CDs, demonstrating notable biocompatibility. In contrast, the lowest hydrothermal temperature enhanced cytotoxic effects against the Gram-positive pathogen Staphylococcus aureus under light exposure. Furthermore, gastric cancer (AGS), colon cancer (HT-29), cervical cancer (HeLa), prostate cancer (PC-3), and breast epithelial (MCF-10) cell lines showed cytotoxicity that was dependent on the CDs synthesized at different temperatures.

Histidine-derived carbon dots as luminescent probes for detecting apoptosis

Histidine-derived carbon dots (His-CDs) were synthesized to detect staurosporine-induced apoptosis in T lymphoma (Jurkat) cells. The His-CDs were characterized for their physical and chemical properties including size, morphology, fluorescence, and surface functionality. Transmission electron microscopy (TEM) revealed a spherical morphology with an average size of 11.4 ± 3.4 nm. Fluorescence analysis showed maximum excitation at 338 nm and emission at 415 nm, attributed to the carbon dots' quantum confinement effect and surface defects. FTIR and SEM-EDS confirmed the presence of hydroxyl, amine, aromatic rings, and alkyl (C-H) functional groups and carbon, nitrogen, and oxygen elemental composition in ratios of 52%, 24.8%, and 23.3%, respectively. His-CDs were evaluated for cytotoxicity and apoptosis detection in Jurkat cells. Fluorescence microscopy and flow cytometry analysis demonstrated concentration-dependent fluorescence, suggesting effective cellular uptake of His-CDs. The apoptotic-sensing capability of His-CDs was tested using staurosporine, an apoptosis inducer. A concentration-dependent increase in fluorescence was observed with increasing staurosporine concentrations, indicating the His-CDs' sensitivity to apoptosis. The time-dependent fluorescence increases were noted with prolonged staurosporine exposure. Z-DEVD-FMK, a caspase-3 inhibitor, confirmed that the apoptosis detected by His-CDs was caspase-3 dependent, as co-treatment reduced His-CDs' fluorescence in the cell. In conclusion, these results demonstrate that His-CDs are biocompatible, sensitive apoptosis sensors and hold the potential for monitoring apoptotic pathways in cellular systems.

L-tryptophan carbon dots as a fluorescent probe for malachite green detection

Development of a rapid and sensitive detection method for hazardous dyes attracts considerable research interest. In this work, L-Tryptophan-based Carbon dots were developed as a fluorescence sensor for the detection of Malachite green (MG). Green fluorescent L-Trp-C-dots were synthesized by a simple pyrolysis technique using L-Trp as the starting precursor. L-Trp-C-dots exhibited different quenching responses to MG, and other interfering species, consequently offering a selective strategy to detect MG. The proposed sensor shows a limit of detection (LOD) of 0.06 μM and a limit of quantification (LOQ) of 0.22 μM with in the linearity range of 0 to 60 µM concentration. Additionally, the relative standard deviation (RSD) was found to be below 1.7 %. Furthermore, the recovery of MG from the real-time samples (green peas) was investigated.

A Simple Method for Synthesizing Nitrogen-Doped Carbon Quantum Dots for Fluorescent "Turn off" Mercury (II) Ion Sensing

Here, straightforward and environmentally friendly fluorescent nitrogen doped carbon quantum dots (N-CQDs) with a high blue fluorescence emission at 455 nm are used for ultrasensitive Hg2+ ion detection. Folic acid and urea are used as carbon sources in the carbonization process. Two broad absorption bands at around 280 and 370 nm from UV-Vis spectrum and characteristic absorption peaks from infrared spectrum confirms the successful synthesis of the N-CQDs. Energy dispersive X-Ray analysis confirmed the elemental composition of the N-CQDs. Transmission electron microscopy showed the homogeneous globular morphology of the N-CQDs with an average particle size of 65 nm. Zeta potential measurement established the stability and surface charge of N-CQDs. Dynamic light scattering measurement showed the average size of N-CQDs. With the addition of Hg2+ ion to N-CQDs, the blue fluorescence emission is quenched. Moreover, the N-CQDs can be applied to real water sample such as pond water, river water, and tap water. The detection limit is approximately calculated to be 12 nM and linear range is 0-30 parts per billion.

Influence of synthesis temperature on the active performance of doped‑carbon dots embedded in polyvinyl alcohol and their potential for active food packaging

This study explores how synthesis temperature influences the properties and functionalities of sulfur‑nitrogen doped carbon dots (S,N-doped CDs) derived from citric acid and cysteine. The focus lies on their potential in bionanocomposite films for active packaging. Bionanocomposite films were prepared by incorporating CDs synthesized at different temperatures (160, 180, and 200 °C) into a polyvinyl alcohol (PVOH) matrix. CD160 displayed a crystalline structure below 10 nm and demonstrated the strongest antifungal and antibacterial activity, as evidenced by the largest clear zones. This superior performance is presumably due to their high N and S content, as shown by the larger redshift in their photoluminescence spectra upon longer wavelength excitation and elemental analysis. CD180 lacked well-defined lattice fringes, indicating an amorphous structure with larger cluster sizes ranging from 10 to 100 nm. Interestingly, UV-blocking and antibacterial efficacy of both films remained high (over 95% UVA block and 5 log10 CFU/mL reduction in bacteria) regardless of CD crystallinity and size. However, the PVOH-CD180 film exhibited more photoluminescence, most likely due to larger defect-related emission centers. In contrast, CD200 had the weakest antifungal and antibacterial activity. While the PVOH-CD200 film achieved a 5 log10 CFU/mL reduction in Gram-positive bacteria, it only managed a 4 log10 CFU/mL reduction in Gram-negative bacteria under contact conditions at 37 °C for 24 h. In addition, the PVOH-CD200 film had the highest antioxidant activity, 88% DPPH radical scavenging, whereas the others had 80%. These findings underscore the critical role of synthesis temperature in tailoring S,N-doped CDs for specific applications within active packaging, where the desired properties dictate the optimal temperature. Preliminary studies suggest that PVOH-CD films have the potential to extend the shelf-life of sweet bread, demonstrating their promise as bionanocomposite active food packaging materials.

Design of Fluorescence Enhancing Sensor for Mercury Detection via Bamboo Cellulose-Derived Carbon Dots

Mercury contamination of the environment is extremely hazardous to human health because of its significant toxicity, especially in water. Biomass-derived fluorophores such as carbon dots (CDs) have emerged as eco-friendly and cost-effective alternative sensors that provide comparable efficacy while mitigating the environmental and economic drawbacks of conventional methods. In this work, we report the fabrication of a selective fluorescence-enhancing sensor based on sulfur-doped carbon dots (SCDs) using waste bamboo-derived cellulose and sodium thiosulfate as the soft base dopant, which actively complexes with mercury ions for detection. SCDs with an average size of 4 nm were synthesized hydrothermally, and the sulfur doping was confirmed quantitatively with an atomic percentage of 6.5%. Optical studies reveal an abnormal fluorescence enhancement of SCD in the presence of mercury due to the aggregation of carbon dots via sulfur-containing functional groups. The fabricated sensor exhibits a low detection limit of 5.16 nM, suggesting its application potential as a reliable mercury sensor. Real-time analyses carried out using tap water samples spiked with mercury and industry samples showed high efficiency for Hg(II) detection. The sensing performance was also demonstrated by using SCD-coated filter paper strips.

Blazing Carbon Dots: Unfolding its Luminescence Mechanism to Photoinduced Biomedical Applications

Carbon dots (CDs) are carbon-based nanomaterials that have garnered immense attention owing to their exceptional photophysical and optoelectronic properties. They have been employed extensively for biomedical imaging and phototherapy due to their superb water dispersibility, low toxicity, outstanding biocompatibility, and exceptional tissue permeability. This review summarizes the structural classification of CDs, the classification of CDs according to precursor sources, and the luminescence mechanism of CDs. The modification in CDs via various doping routes is comprehensively reviewed, and the effect of such alterations on their photophysical properties, such as absorbance, photoluminescence (PL), and reactive oxygen species generation ability, is also highlighted. This review strives to summarize the role of CDs in cellular imaging and fluorescence lifetime imaging for cellular metabolism. Subsequently, recent advancements and the future potential of CDs as nanotheranostic agents have been discussed. Herein, we have discussed the role of CDs in photothermal, photodynamic, and synergistic therapy of anticancer, antiviral, and antibacterial applications. The overall summary of the review highlights the prospects of CD-based research in bioimaging and biomedicine.

Carbon dot-based fluorescent sensors for pharmaceutical detection: Current innovations, challenges, and future prospects

Environmental contamination by pharmaceuticals has become a matter of concern as they are released in sewage systems at trace levels, thus impacting biological systems. Increasing concerns about the low-level occurrence of pharmaceuticals in the environment demands sensitive and selective monitoring. Owing to their high sensitivity and specificity carbon dots (CDs) have emerged as suitable fluorescent sensors. This review discusses the current scenario of the status of pharmaceuticals in the environment, limitations associated with traditional techniques employed for their detection, and benefits offered by CDs like easy surface modification and tunable optical properties for sensing applications. Several representative means by which CDs interact with other molecules such as inner filter effect (IFE), dynamic quenching (DQ), static quenching (SQ), Förster resonance energy transfer (FRET), among others, are also discussed along with co-referencing fluorophores to design sensors. Based on developments described herein, CDs-based sensors can be expected to sense pharmaceuticals ranging from nanogram to picogram, target real-time industrial and spiked sample analysis, etc., which provides direction for future research.

Synthesis of nanohybrid consisting of taurine derived carbon dots and nanoceria for anticancer applications

Here, we first synthesized carbon dots (CDs) from taurine and then a nanohybrid with ceria (CeO2) nanoparticles using thermal decomposition method for checking their antineoplastic efficacy against human triple negative mammary carcinoma cells, MDA-MB-231. The in vitro study demonstrated significant dose-dependent antineoplastic activity of the nanohybrids within a range of concentration from 10 to 50 μg/mL after 48 h of treatment. The cellular morphology analysis clearly depicted substantial amount of cell death which seems to stem from increased intracellular reactive oxygen species (ROS) production. However, the maximum anticancer activity of the nanohybrid as compared to bare CDs and CeO2 is supposed to be due to the combined anticancer effect of both CDs and CeO2 (a well-established antitumor agent). Further we have performed molecular docking study to reveal the anticancer mechanism of CDs which exhibited high binding capacity towards several proapoptotic and antiapoptotic protein molecules. The binding affinity values were found to be - 8.7 kcal/mol, - 7.9 kcal/mol, - 9.6 kcal/mol, - 9.5 kcal/mol, - 12 kcal/mol and - 11.1 kcal/mol for BAD, BCl-2, p53, Caspase-8, Caspase-9 and Caspase-3 respectively. Taken together, our synthesized CDs-CeO2 nanohybrid could be thought as a potential anticarcinogenic option in the field of breast cancer therapeutics.

Fluorescence and Colorimetric Dual-Readout Detection of Tetracycline Using Leucine-Conjugated Iron Oxide Quantum Dots

This study developed a dual-readout system utilizing fluorescence and colorimetry based on iron oxide quantum dots (IO-QDs) for detecting tetracycline (TCy). IO-QDs were synthesized within 6 h using L-leucine as a surface modifier, achieving a more efficient route. Upon interaction with TCy, IO-QDs exhibited a significant decrease in fluorescence response and observable color changes, while fluorescence lifetime remained consistent regardless of TCy presence. Moreover, IO-QDs' fluorescence response remained stable across various temperatures. The Förster resonance energy transfer distance of less than 2 nm and a quenching constant of 2.90 × 1012 M-1s-1 indicated static quenching in the presence of TCy. Additionally, significant changes in observed and corrected fluorescence efficiency suggested the involvement of the inner filter effect in the fluorescence quenching of IO-QDs. The synthesized IO-QDs were then utilized for selective and rapid fluorescence-based TCy detection, showing a linear range of 0.5 to 80 μM. Simultaneously, a colorimetric method for TCy detection was established, demonstrating a good linear relationship within the range of 0.5 to 20 μM. The detection limits for TCy were determined as 0.539 and 0.329 μM using fluorescence and colorimetric approaches, respectively. Furthermore, IO-QDs were applied to detect real samples, and the dual-readout probe exhibited satisfactory recoveries, confirming the practical reliability of the developed method for analyzing milk and drinking water samples.

Rapid environmentally benign label free detection of heparin using highly fluorescent N,S-CDs sensing probe through a turn-on mechanism

Heparin (HEP) is one of the oldest anticoagulant drugs that currently still in widespread clinical use. It lacks chromophore and not easily derivatized due to its hydrophilic nature. In this work we developed a green, selective, and sensitive fluorescence sensor for detection of HEP in its injection dosage forms. The sensor is composed of nitrogen and sulfur co-doped carbon quantum dots (N,S-CDs) semi quenched by Fe3+. The N,S-CDs were prepared using microwave assisted pyrolysis in 3.5 min and exhibited high emission at 425 nm after excitation at 350 nm with high quantum yield of 96%. Owing to the anionic nature of HEP, it could compete with N,S-CDs for Fe3+ complexation resulting in turning-on the quenched fluorescence. This fluorescence enhancement was linear over a concentration range between 6 and 20 μg/mL (R2 = 0.99) with a limit of detection of 1.41 µg/ml. The accuracy and precision of the proposed sensor were indicated by percentage recovery values between 98% -102% and %RSD less than 2, respectively. Furthermore, the proposed sensor was successfully applied for determination of HEP in injection dosage form. The developed sensor showed excellent greenness on analytical eco-scale (score 93%) and GAPI scale.

Carbon quantum dots in bioimaging and biomedicines

Carbon quantum dots (CQDs) are gaining a lot more attention than traditional semiconductor quantum dots owing to their intrinsic fluorescence property, chemical inertness, biocompatibility, non-toxicity, and simple and inexpensive synthetic route of preparation. These properties allow CQDs to be utilized for a broad range of applications in various fields of scientific research including biomedical sciences, particularly in bioimaging and biomedicines. CQDs are a promising choice for advanced nanomaterials research for bioimaging and biomedicines owing to their unique chemical, physical, and optical properties. CQDs doped with hetero atom, or polymer composite materials are extremely advantageous for biochemical, biological, and biomedical applications since they are easy to prepare, biocompatible, and have beneficial properties. This type of CQD is highly useful in phototherapy, gene therapy, medication delivery, and bioimaging. This review explores the applications of CQDs in bioimaging and biomedicine, highlighting recent advancements and future possibilities to increase interest in their numerous advantages for therapeutic applications.

Ultrasensitive fluorescent carbon dot sensor for quantification of soluble and insoluble Cr(VI) in particulate matter

This study investigates advanced functional materials to address the need for practical and affordable analytical techniques for monitoring large amounts of insoluble Cr(VI). N,S-doped fluorescent carbon dots (f-CDs) were fabricated through microwave synthesis, with an average diameter of 10 nm. These f-CDs were explored as potential sensors for detecting Cr(VI) in ambient particulate matter (PM). Laboratory experiments yielded positive results, showing average recoveries of 106.0%, 102.3%, 96.4%, and 101.7% for PbCrO4, BaCrO4, CaCrO4, and (NH4)2CrO4, respectively. Applying the fluorescence method to field PM samples, a method detection limit (MDL) of 0.32 ng/m3 for total Cr(VI) quantification was achieved. The fluorescence decay of carbon dots remained stable over time, suggesting that Cr(VI) quenching primarily occurred due to the internal filter effect through a static quenching mechanism. These f-CDs exhibited advantageous properties, including affordability, solubility, luminescence, and sensitivity, positioning them as a promising alternative for Cr(VI) detection in ambient air particulates. This study contributes to further developing carbon-based functional materials for detecting metallic air pollutants.

Multifunctional Carbon Dots for Biomedical Applications: Diagnosis, Therapy, and Theranostic

Designing suitable nanomaterials is an ideal strategy to enable early diagnosis and effective treatment of diseases. Carbon dots (CDs) are luminescent carbonaceous nanoparticles that have attracted considerable attention. Through facile synthesis, they process properties including tunable light emission, low toxicity, and light energy transformation, leading to diverse applications as optically functional materials in biomedical fields. Recently, their potentials have been further explored, such as enzyme-like activity and ability to promote osteogenic differentiation. Through refined synthesizing strategies carbon dots, a rich treasure trove for new discoveries, stand a chance to guide significant development in biomedical applications. In this review, the authors start with a brief introduction to CDs. By presenting mechanisms and examples, the authors focus on how they can be used in diagnosing and treating diseases, including bioimaging failure of tissues and cells, biosensing various pathogenic factors and biomarkers, tissue defect repair, anti-inflammation, antibacterial and antiviral, and novel oncology treatment. The introduction of the application of integrated diagnosis and treatment follows closely behind. Furthermore, the challenges and future directions of CDs are discussed. The authors hope this review will provide critical perspectives to inspire new discoveries on CDs and prompt their advances in biomedical applications.

Baicalin functionalized PEI-heparin carbon dots as cancer theranostic agent

The worldwide prevalence of cancer and its significantly rising risks with age have garnered the attention of nanotechnology for prompt detection and effective therapy with minimal or no adverse effects. In the current study, heparin (HP) polymer derived heteroatom (N, S-) co-doped CDs were synthesized using hydrothermal synthesis method to efficiently deliver natural anticancer compound baicalin (BA). Heparin carbon dots (HCDs) were passivated with polyethylenimine (PEI) to improve its fluorescence quantum yield. The surface passivation of CDs by polycationic PEI polymer not only facilitated loading of BA, but also played a crucial role in the pH-responsive drug delivery. The sustained release of BA (up to 80 %) in mildly acidic pH (5.5 and 6.5) conditions endorsed its drug delivery potential for cancer-specific microenvironments. BA-loaded PHCDs exhibited enhanced anticancer activity as compared to BA/PHCDs indicating the effectiveness of the nanoformulation, Furthermore, the flow cytometry analysis confirmed that BA-PHCDs treated cells were arrested in the G2/M phase of cell cycle and had a higher potential for apoptosis. Bioimaging study demonstrated the excellent cell penetration efficiency of PHCDs with complete cytoplasmic localization. All this evidence comprehensively demonstrates the potency of BA-loaded PHCDs as a nanotheranostic agent for cancer.

Nanostructured N/S doped carbon dots/mesoporous silica nanoparticles and PVA composite hydrogel fabrication for anti-microbial and anti-biofilm application

Regarding the convergence of the worldwide epidemic, the appearance of bacterial infection has occasioned in a melodramatic upsurge in bacterial pathogens with confrontation against one or numerous antibiotics. The implementation of engineered nanostructured particles as a delivery vehicle for antimicrobial agent is one promising approach that could theoretically battle the setbacks mentioned. Among all nanoparticles, silica nanoparticles have been found to provide functional features that are advantageous for combatting bacterial contagion. Apart from that, carbon dots, a zero-dimension nanomaterial, have recently exhibited their photo-responsive property to generate reactive oxygen species facilitating to enhance microorganism suppression and inactivation ability. In this study, potentials of core/shell mesoporous silica nanostructures (MSN) in conjugation with carbon dots (CDs) toward antimicrobial activity against Staphylococcus aureus, Pseudomonas aeruginosa and Escherichia coli have been investigated. Nitrogen and sulfur doped CDs (NS/CDs) conjugated with MSN which were cost effective nanoparticles exhibited much superior antimicrobial activity for 4 times as much as silver nanoparticles against all bacteria tested. Among all nanoparticles tested, 0.40 M NS/CDs@MSN showed the greatest minimal biofilm inhibitory at very low concentration (< 0.125 mg mL-1), followed by 0.20 M NS/CDs@MSN (0.5 mg mL-1), CD@MSN (25 mg mL-1), and MSN (50 mg mL-1), respectively. Immobilization of NS/CDs@MSN in polyvinyl alcohol (PVA) hydrogel was performed and its effect on antimicrobial activity, biofilm controlling efficiency, and cytotoxicity toward fibroblast (NIH/3 T3 and L-929) cells was additionally studied for further biomedical applications. The results demonstrated that 0.40 M NS/CDs-MSN@PVA hydrogel exhibited the highest inhibitory effect on S. aureus > P. aeruginosa > E. coli. In addition, MTT assay revealed some degree of toxicity of 0.40 M NS/CDs-MSN@PVA hydrogel against L-929 cells by a slight reduction of cell viability from 100% to 81.6% when incubated in the extract from 0.40 M NS/CDs-MSN@PVA hydrogel, while no toxicity of the same hydrogel extract was detected toward NIH/3 T3 cells.

Rapid Pathogen Purge by Photosensitive Arginine-Riboflavin Carbon Dots without Toxicity

Photo-activatable antipathogenic carbon dots (CDs) were prepared by carbonization of citric acid and arginine (Arg) via 3 min microwave treatment for use in the eradication of common microorganisms. Nitrogen-doped Arg CDs were spherical in shape with a size range of 0.5 to 5 nm. The Arg CDs were modified with fluorescent dyes, such as fluorescein sodium salt (FSS, as Arg-FSS) and riboflavin (RBF, as Arg-RBF), to improve antimicrobial potency by enhancing their application in photodynamic therapy. The modified Arg CDs afforded fluorescence emission properties at 520 nm in the green region in addition to excellent blue fluorescence intensity at 420 nm under 345 nm excitation upon their FSS and RBF conjugation, respectively. Although the cytotoxicity of Arg CDs was decreased for Arg-RBF CDs to 91.2 ± 0.7% cell viability for fibroblasts, the Arg-based CDs could be safely used for intravenous applications at 1000 μg/mL concentration. The Arg CDs showed broad-spectrum antimicrobial activity against common pathogens and the minimum inhibitory concentration of Arg CDs was almost two-fold decreased for the modified forms without UV light. However, faster and more effective antibacterial activity was determined for photosensitive Arg-RBF CDs, with total bacterial eradication upon UV-A light exposure for 30 min.

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.

Tuning the Luminescence of Microwave-Assisted N-Doped Fluorescent Carbon Dots: Bioimaging Applications and Label-Free Anti-Cancer Drug Delivery

Nanosized fluorescent carbon dots (Cdots) have gained a lot of attention in the recent years because of their superior properties, such as good biocompatibility, low toxicity, excellent chemical stability, resistance to photobleaching, and ease of chemical modification. Cdots are promising candidates for considerable applications in various fields: sensors, bioimaging, and drug delivery. Specifically, nitrogen-doped Cdots have attracted a huge interest because of their applicability in bioimaging and drug delivery. Conventional methods for the synthesis of Cdots have drawbacks, such as the use of organic solvents, the presence of side products, and the time required for synthesis. Keeping all these points in mind, herein, we report green methodology for the synthesis of water-soluble, blue-emitting, nitrogen-doped multifunctional Cdots under microwave irradiation within 3 min. The Cdots were prepared using citric acid and arginine as source materials and were characterized using various physicochemical techniques. A pH-responsive drug delivery system was then designed using anticancer drug doxorubicin and the synthesized Cdots. The biocompatibility of synthesized Cdots was analyzed against L929 normal cell line. The Cdots-DOX conjugates exhibited efficient anticancer activity against HeLa cells and also acted as excellent bioimaging agents.

Abnormal neural differentiation in response to graphene quantum dots through histone modification interference

Graphene quantum dots (GQDs) have been broadly applied in biomedicine in recent years, and their environmental exposure and toxicological impacts have raised increasing concerns. The nanosafety assessment on the nervous system is one of the most important aspects, and potential effects of GQDs on neurodevelopment and the underlying mechanism are still elusive. In this study, the neural developmental toxicities of OH-GQDs and NH₂-GQDs were investigated using the mouse embryonic stem cells (mESCs). The results revealed that OH-GQDs significantly inhibited the ectoderm development, and reduced the neural precursor formation and neurogenesis during the neural differentiation of the mESCs. The exploration on the mechanism uncovered that the increased enrichment of H3K27me3 at the promoter region of the Smad6 gene was involved in histone modification-activated BMP signal pathway, which consequently influenced its regulatory effects on neural differentiation. Additionally, OH-GQDs elicited a stronger effect on inducing the imbalance of histone modification, and resulted in higher latency of neural differentiation disturbance than did NH₂-GQDs, suggesting surface functionalization-specific effects of GQDs on neurodevelopmental toxicity. This study would provide new insights in not only the adverse effects of GQDs on neurodevelopment, but also the influence from the chemical modification of GQDs on their bioactivities.

Carbon Quantum Dots from Amino Acids Revisited: Survey of Renewable Precursors toward High Quantum-Yield Blue and Green Fluorescence

Carbon quantum dots (CQDs) were synthesized via a green, one-step hydrothermal method. As CQD precursors, nine amino acids of different structural descriptors (negatively/positively charged in water, polar, hydrophobic, sulfur-containing, and other/complex ones) were surveyed: Asp, Cys, Gly, His, Leu, Lys, Phe, Pro, and Ser. The reactions were performed in an autoclave in the presence of citric acid at 180 °C for 24 h and yielded core-shell CQDs. CQDs were comprehensively characterized by transmission electron microscopy, dynamic light scattering, Raman, UV/Vis, infrared, X-ray photoelectron spectroscopy, and fluorescence spectroscopy. At the excitation wavelength of λ_ex = 350 nm, Cys-, Phe-, Leu-, and Lys-based CQDs displayed the highest quantum yield blue fluorescence-90 ± 5, 90 ± 4, 87 ± 5, and 67 ± 3%, respectively-superior to the conventional fluorescent dyes. Strikingly, for Lys- and Phe-CQDs, dissimilar trends in the excitation-emission wavelength relationships were identified, that is, constantly strong red shifts versus excitation wavelength-independent emission. Cys- and Lys-CQDs were water-dispersible toward the narrow unimodal distribution of hydrodynamic diameters-0.6 and 2.5 nm, respectively. Additionally, Lys- and Cys-CQDs, with high absolute zeta potential values, formed stable aqueous colloids in a broad range of pH (2, 7, and 12). The results constitute important premises for water-based applications of CQDs, such as bioimaging or photocatalysis.

Light-Activated Modified Arginine Carbon Dots as Antibacterial Particles

Nitrogen-doped arginine carbon dots (Arg CDs) as light-sensitive antibacterial agents were prepared by using citric acid as the carbon source and arginine amino acid as the nitrogen source via a microwave-assisted synthesis method. Dynamic light scattering (DLS) measurements and TEM images revealed that the Arg CDs were in the 1–10 nm size range with a graphitic structure. To improve their antibacterial capability, the Arg CDs were modified with ethyleneimine (EDA), pentaethylenehexamine (PEHA), and polyethyleneimine (PEI) as different amine sources, and the zeta potential value of +2.8 ± 0.6 mV for Arg CDs was increased to +34.4 ± 4.1 mV for PEI-modified Arg CDs. The fluorescence intensity of the Arg CDs was significantly enhanced after the modification with EDA, and the highest antibacterial effect was observed for the PEI-modified Arg CDs. Furthermore, the photodynamic antibacterial capacity of bare and EDA-modified Arg CDs was determined upon light exposure to show their light-induced antibacterial effects. Photoexcited (315–400 nm, UVA, 300 W), EDA-modified Arg CDs at 5 mg/mL concentration were found to inhibit about 49 ± 7% of pathogenic bacteria, e.g., Escherichia coli, with 5 min of light exposure. Furthermore, the biocompatibilities of the bare and modified Arg CDs were also investigated with blood compatibility tests via hemolysis and blood clotting assays and cytotoxicity analysis on L929 fibroblast cells.

A Review on Carbon Dots: Synthesis, Characterization and Its Application in Optical Sensor for Environmental Monitoring

The development of carbon dots (CDs), either using green or chemical precursors, has inevitably led to their wide range application, from bioimaging to optoelectronic devices. The reported precursors and properties of these CDs have opened new opportunities for the future development of high-quality CDs and applications. Green precursors were classified into fruits, vegetables, flowers, leaves, seeds, stem, crop residues, fungi/bacteria species, and waste products, while the chemical precursors were classified into acid reagents and non-acid reagents. This paper quickly reviews ten years of the synthesis of CDs using green and chemical precursors. The application of CDs as sensing materials in optical sensor techniques for environmental monitoring, including the detection of heavy metal ions, phenol, pesticides, and nitroaromatic explosives, was also discussed in this review. This profound review will offer knowledge for the upcoming community of researchers interested in synthesizing high-quality CDs for various applications.

New bidental sulfur-doped graphene quantum dots modified with gold as a catalyst for hydrogen generation

Disadvantages of fossil fuels encourage researchers to develop clean combustion sources, in particular, H₂ due to the high potential energy and safe by-products. Herein, Au was deposited on S-doped graphene quantum dots to obtain a heterogeneous photocatalyst for the degradation of formic acid toward H₂ generation. Insertion of thiol groups on graphene quantum dots was carried out by self-condensation reaction of citric acid in the presence of Dimaval, as the thiol groups carrier. After the complexation of Au with the prepared S-doped graphene quantum dots, the catalytic activity of composite was evaluated in formic acid degradation to generate H₂ under visible light. Au@S-doped graphene quantum dots demonstrated superior catalytic activity with the turnover frequency up to 112 h^-1. The reaction enjoys significant benefits such as stability and recyclability of the catalyst, excellent reaction rate, and mild reaction conditions.

Highly Photostable Carbon Dots from Citric Acid for Bioimaging

Bioimaging supported by nanoparticles requires low cost, highly emissive and photostable systems with low cytotoxicity. Carbon dots (C-dots) offer a possible solution, even if controlling their properties is not always straightforward, not to mention their potentially simple synthesis and the fact that they do not exhibit long-term photostability in general. In the present work, we synthesized two C-dots starting from citric acid and tris (hydroxymethyl)-aminomethane (tris) or arginine methyl ester dihydrochloride. Cellular uptake and bioimaging were tested in vitro using murine neuroblastoma and ovine fibroblast cells. The C-dots are highly biocompatible, and after 24 h of incubation with the cells, 100% viability was still observed. Furthermore, the C-dots synthesized using tris have an average dimension of 2 nm, a quantum yield of 37%, high photostability and a zeta potential (ζ) around −12 mV. These properties favor cellular uptake without damaging cells and allow for very effective bioimaging.

Bismuth- and gadolinium-codoped carbon quantum dots with red/green dual emission for fluorescence/CT/T1-MRI mode imaging

The synthesized novel Bi,Gd-CQDs exhibit red and green fluorescence, enabling CT and MR imaging, and providing an approach for multifunctional biological imaging.

Versatile Fluorescent Carbon Dots from Citric Acid and Cysteine with Antimicrobial, Anti-biofilm, Antioxidant, and AChE Enzyme Inhibition Capabilities

Nanostructured fluorescent particles derived from natural molecules were prepared by a green synthesis technique employing a microwave method. The precursors citric acid (CA) and cysteine (Cys) were used in the preparation of S- and N-doped Cys carbon dots (Cys CDs). Synthesis was completed in 3 min. The graphitic structure revealed by XRD analysis of Cys CDs dots had good water dispersity, with diameters in the range of 2-20 nm determined by TEM analysis. The isoelectric point of the S, N-doped CDs was pH value for 5.2. The prepared Cys CDs displayed excellent fluorescence intensity with a high quantum yield of 75.6 ± 2.1%. Strong antimicrobial capability of Cys CDs was observed with 12.5 mg/mL minimum bactericidal concentration (MBC) against gram-positive and gram-negative bacteria with the highest antimicrobial activity obtained against Staphylococcus aureus. Furthermore, Cys CDs provided total biofilm eradication and inhibition abilities against Pseudomonas aeruginosa at 25 mg/mL concentration. Cys CDs are promising antioxidant materials with 1.3 ± 0.1 μmol Trolox equivalent/g antioxidant capacity. Finally, Cys CDs were also shown to inhibit the acetylcholinesterase (AChE) enzyme, which is used in the treatment of Alzheimer's disease, even at the low concentration of 100 μg/mL.

Multifunctional Biomedical Applications of Nitrogen and Sulfur Co-Doped Carbon Dots

Multifunctional carbon dots have drawn considerable attention due to their potential biomedical application value. We report the preparation of blue-green fluorescence-emitting, multifunctional, nitrogen-and-sulfur co-doped carbon dots (N, S-CDs) synthesized via a one-step process using 1-thioglycerol as a sulfur source, glucose and citric acid as carbon sources, and polyethyleneimine as a nitrogen source. Because of abundant amino and sulfur content, the CDs exhibited high sensibility and selectivity for detecting Cu₂₊ (detection limit: 0.01 μM, linear range: 0.025 to 50 μM). Fast and sensitive detection of tiopronin was also achieved on the basis of the fluorescence "off-on" mode considering the strong affinity between tiopronin and Cu₂₊. The N, S-CDs exhibited good biocompatibility as determined by fluorescence imaging using onion epidermal cells and gram-positive bacteria. The CDs also exhibited excellent antimicrobial ability against the gram-positive bacteria. Our results indicate that these novel N, S-CDs could be ideal candidates for several biochemical applications such as antibacterial treatment and detection of small biomolecules.

Carbon Dots as an Emergent Class of Antimicrobial Agents

Antimicrobial resistance is a recognized global challenge. Tools for bacterial detection can combat antimicrobial resistance by facilitating evidence-based antibiotic prescribing, thus avoiding their overprescription, which contributes to the spread of resistance. Unfortunately, traditional culture-based identification methods take at least a day, while emerging alternatives are limited by high cost and a requirement for skilled operators. Moreover, photodynamic inactivation of bacteria promoted by photosensitisers could be considered as one of the most promising strategies in the fight against multidrug resistance pathogens. In this context, carbon dots (CDs) have been identified as a promising class of photosensitiser nanomaterials for the specific detection and inactivation of different bacterial species. CDs possess exceptional and tuneable chemical and photoelectric properties that make them excellent candidates for antibacterial theranostic applications, such as great chemical stability, high water solubility, low toxicity and excellent biocompatibility. In this review, we will summarize the most recent advances on the use of CDs as antimicrobial agents, including the most commonly used methodologies for CD and CD/composites syntheses and their antibacterial properties in both in vitro and in vivo models developed in the last 3 years.

A Multi-Catalytic Sensing for Hydrogen Peroxide, Glucose, and Organophosphorus Pesticides Based on Carbon Dots

In this work, a facile one-pot hydrothermal route was employed to synthesize a series of fluorescent carbon dots (CDs) by using 20 natural amino acids, respectively, as the starting materials. It was found that the CDs synthesized using phenylalanine could possess the intrinsic peroxidase-like activity that could effectively catalyze a traditional peroxidase substrate like 3, 3', 5, 5'- tetramethylbenzidine (TMB) in the presence of H2O2 to produce a blue solution; thereby, a catalytic sensing system for H2O2 has been developed. On the basis of this catalytic reaction, together with the fact that glucose oxidase (GOx) can catalyze the hydrolysis of glucose to generate H2O2, a sensitive catalytic sensing system for glucose could be further established. Furthermore, based on this catalytic reaction, taken together with the two enzymatic catalytic systems of acetylcholinesterase (AChE) and choline oxidase (CHO), a highly sensitive multi-catalytic sensing system could be successfully developed for organophosphorus (OPs) pesticides such as dimethoate, DDVP, and parathion-methyl. Limit of detections (LODs) of H2O2 and glucose were estimated to be 6.5 and 0.84 μM, respectively. The limit of detection of the sub-nM level could be obtained for tested dimethoate, DDVP, and parathion-methyl OPs pesticides. The established sensing systems can exhibit good practical application performance in serum and several fruit samples.

Carbon Dot Nanoparticles: Exploring the Potential Use for Gene Delivery in Ophthalmic Diseases

Ocular gene therapy offers significant potential for preventing retinal dystrophy in patients with inherited retinal dystrophies (IRD). Adeno-associated virus (AAV) based gene transfer is the most common and successful gene delivery approach to the eye. These days, many studies are using non-viral nanoparticles (NPs) as an alternative therapeutic option because of their unique properties and biocompatibility. Here, we discuss the potential of carbon dots (CDs), a new type of nanocarrier for gene delivery to the retinal cells. The unique physicochemical properties of CDs (such as optical, electronic, and catalytic) make them suitable for biosensing, imaging, drug, and gene delivery applications. Efficient gene delivery to the retinal cells using CDs depends on various factors, such as photoluminescence, quantum yield, biocompatibility, size, and shape. In this review, we focused on different approaches used to synthesize CDs, classify CDs, various pathways for the intake of gene-loaded carbon nanoparticles inside the cell, and multiple studies that worked on transferring nucleic acid in the eye using CDs.

pH-responsive zwitterionic carbon dots for detection of rituximab antibody

Zwitterionic carbon dots (CDs) have received much attention as a result of good photostability, high biocompatibility, and high quantum yield. In this study, novel zwitterionic CDs were synthesized using a simple hydrothermal method of citric acid (CA) and l-histidine as carbon and nitrogen precursors, respectively. Prepared zwitterionic CDs have an average particle size of 4 nm diameter and showed green fluorescence with a peak at 530 nm when excited at 470 nm; quantum efficiency was 39.34% using rhodamine 6G as a baseline. The fluorescence intensity of zwitterionic CDs was quenched by rituximab in the range 0-400 μmol L^-1 , with a limit of detection of 27 μmol L^-1 . In addition, the synthesized zwitterionic CDs had low toxicity, good stability, and high selectivity and sensitivity sensing for rituximab, therefore zwitterionic CDs are a promising candidate for practical applications.

N, S doped carbon quantum dots inside mesoporous silica for effective adsorption of methylene blue dye

This study aimed to develop non-metal elements for doping carbon quantum dots (CQDs) with nitrogen and sulfur (N, S-CQDs), which loaded inside hexagonal mesoporous silica (HMS) in order to effectively remove methylene blue dye (MB) from an aqueous solution. The histidine and cysteine amino acids were used as the source for synthesis N, S-CQDs through the hydrothermal method. Morphology and structure of the N, S-CQDs, and adsorbent (N, S-CQDs/HMS) were characterized by using different microscopic and spectroscopic techniques. The adsorption parameters such as adsorbent dosage (0.25–1 g/L), pH (2–10), contact time (15–75 min), and initial MB dye concentration (20–300 mg/L) were investigated. The maximum adsorption capacity and removal efficiency of MB were determined at 370.4 mg/g and 97%, respectively, under optimum conditions at 303 K. The adsorption isotherm studies were fitted with the Freundlich isotherm equation, and the dye removal kinetics of the adsorbent followed the pseudo-second-order model. Thermodynamic studies showed that the adsorption process had exothermic and spontaneous behavior. The removal of MB next to the Rhodamine B and Reactive Black 5 dyes indicated that the N, S-CQDs/HMS had excellent selective behavior for MB absorption. This prepared adsorbent could be well recycled with suitable activity after four repeated adsorption–desorption cycles. Results revealed that the porous characters, surface area, charge properties, reduction in the bandgap, and quantum yield of the N, S-CQDs/HMS were essential factors that affected dye adsorption.

Microarray analysis of gene expression differences in microglia after exposure to graphene quantum dots

Graphene quantum dots (GQDs) have been broadly applied in biomedicine in recent years. So far, researches have reported that GQDs might contribute to the injury of the central nervous system (CNS), yet the latent toxicological mechanism is not clear. This study aims to investigate the underlying biological mechanism responsible for the neurotoxicity of nitrogen-doped GQDs (N-GQDs) and amino-functionalized GQDs (A-GQDs) by use of genome-wide transcription microarray. The findings showed that 174 and 1341 genes were altered significantly in the BV2 cells treated by 25 μg/mL N-GQDs and 100 μg/mL N-GQDs compared with the control, respectively. As for the BV2 cells exposed to 100 μg/mL A-GQDs, 1396 diversely expressed genes were detected. By comparing the 100 μg/mL N-GQDs exposed group with 100 μg/mL A-GQDs exposed group, the expression of 256 genes was extensively altered, including 58 upregulated genes and 198 downregulated genes. From Gene Ontology (GO) analysis, the altered genes were mainly enriched in functions of ion channel activation and cellular processes. Based on the KEGG pathway and signal-net analysis, the toxicity of GQDs in BV2 cells was closely related to calcium signaling pathway, cell cycle and endocytosis. And the pathways that the shared mRNAs involved all served as the crucial roles in the neurotoxicity of GQDs despite the chemical functionalization (N-GQDs or A-GQDs). In addition, the consequences from qRT-PCR, Western blot, intracellular calcium level measurements and comet assay further confirmed that calcium dyshomeostasis, DNA damage and cell cycle arrest were the key factors responsible for the GQDs-induced neurotoxicity through affecting several classical signaling pathways. In conclusion, our research will supply essential data for further studies on mechanisms of GQDs-induced neurotoxicity by use of genome-wide screening.

Carbon Dots Fabrication: Ocular Imaging and Therapeutic Potential

Vision loss is a major complication in common ocular infections and diseases such as bacterial keratitis, age-related macular degeneration (AMD) and diabetic retinopathy (DR). The prevalence of such ophthalmic diseases represents an urgent need to develop safe, effective, and long-term treatments. Current therapies are riddled with drawbacks and limitations which calls for the exploration of alternative drug delivery mechanisms. Toxicity of the inorganic metals and metal oxides used for drug delivery raise safety concerns that are alleviated with the alternate use of, a natural and organic polymer which is both biocompatible and environmentally friendly. Carbon dots (CDs) represent a great potential in novel biomedical applications due to their tunable fluorescence, biocompatibility, and ability to be conjugated with diverse therapeutic materials. There is a growing interest on the exploitation of these properties for drug delivery with enhanced bio-imaging. However, there are limited reports of CD applications for ophthalmic indications. In this review, we focus on the CD potential and the development of translational therapies for ophthalmic diseases. The current review presents better understanding of fabrication of CDs and how it may be useful in delivering anti-bacterial agents, anti-VEGF molecules as well as imaging for ophthalmic applications.

Recent Developments of Carbon Dots in Biosensing: A Review

Early diagnosis of diseases is of great importance because it increases the chance of a cure and significantly reduces treatment costs. Thus, development of rapid, sensitive, and reliable biosensing techniques is essential for the benefits of human life and health. As such, various nanomaterials have been explored to improve performance of biosensors, among which, carbon dots (CDs) have received enormous attention due to their excellent performance. In this Review, the recent advancements of CD-based biosensors have been carefully summarized. First, biosensors are classified according to their sensing strategies, and the role of CDs in these sensors is elaborated in detail. Next, several typical CD-based biosensors (including CD-only, enzymatic, antigen-antibody, and nucleic acid biosensors) and their applications are fully discussed. Last, advantages, challenges, and perspectives on the future trends of CD-based biosensors are highlighted.

Induction of ferroptosis in response to graphene quantum dots through mitochondrial oxidative stress in microglia

BACKGROUND

Graphene quantum dots (GQDs) provide a bright prospect in the biomedical application because they contain low-toxic compounds and promise imaging of deep tissues and tiny vascular structures. However, the biosafety of this novel QDs has not been thoroughly evaluated, especially in the central nervous system (CNS). The microarray analysis provides a hint that nitrogen-doped GQDs (N-GQDs) exposure could cause ferroptosis in microglia, which is a novel form of cell death dependent on iron overload and lipid peroxidation.

RESULTS

The cytosolic iron overload, glutathione (GSH) depletion, excessive reactive oxygen species (ROS) production and lipid peroxidation (LPO) were observed in microglial BV2 cells treated with N-GQDs, which indicated that N-GQDs could damage the iron metabolism and redox balance in microglia. The pre-treatments of a specific ferroptosis inhibitor Ferrostatin-1 (Fer-1) and an iron chelater Deferoxamine mesylate (DFO) not only inhibited cell death, but also alleviated iron overload, LPO and alternations in ferroptosis biomarkers in microglia, which were caused by N-GQDs. When assessing the potential mechanisms of N-GQDs causing ferroptosis in microglia, we found that the iron content, ROS generation and LPO level in mitochondria of BV2 cells all enhanced after N-GQDs exposure. When the antioxidant ability of mitochondria was increased by the pre-treatment of a mitochondria targeted ROS scavenger MitoTEMPO, the ferroptotic biological changes were effectively reversed in BV2 cells treated with N-GQDs, which indicated that the N-GQDs-induced ferroptosis in microglia could be attributed to the mitochondrial oxidative stress. Additionally, amino functionalized GQDs (A-GQDs) elicited milder redox imbalance in mitochondria and resulted in less ferroptotic effects than N-GQDs in microglia, which suggested a slight protection of amino group functionalization in GQDs causing ferroptosis.

CONCLUSION

N-GQDs exposure caused ferroptosis in microglia via inducing mitochondrial oxidative stress, and the ferroptotic effects induced by A-GQDs were milder than N-GQDs when the exposure method is same. This study will not only provide new insights in the GQDs-induced cell damage performed in multiple types of cell death, but also in the influence of chemical modification on the toxicity of GQDs.