Supercapacitors (SCs), including electric double-layer capacitors (EDLCs), pseudocapacitors, and hybrid capacitors, are esteemed for their high power density and attractive features such as robust safety, fast charging, low maintenance, and prolonged cycling lifespan, sparking significant interest. Carbon quantum dots (CQDs) are fluorescent nanomaterials with small size, broad excitation spectrum, stable fluorescence, and adjustable emission wavelengths. They are widely used in optoelectronics, medical diagnostics, and energy storage due to their biocompatibility, low toxicity, rich surface functional groups, abundant electron-hole pairs, large specific surface area, and tunable heteroatom doping. In this short review, we briefly discussed the advantages and disadvantages of bottom-up and top-down of CQD synthesis methods. The arc-discharge technique, laser ablation technique, plasma treatment, ultrasound synthesis technique, electrochemical technique, chemical exfoliation, and combustion are among the initial top-down approaches. The subsequent section delineates waste-derived and bottom-up methods, encompassing microwave synthesis, hydrothermal synthesis, thermal pyrolysis, and the metal-organic framework template-assisted technique. In addition, this short review focuses on the operational mechanism of supercapacitors, their properties, and the utilization of CQDs in supercapacitors.

This study employed a green microwave synthesis technique to produce carbon quantum dots (CQDs) from araucaria heterophylla gum extract. The produced CQDs emit a distinct blue fluorescent light, contributing a remarkable quantum yield of 14.69%. Their average particle size measures at 1.62 ± 0.39 nm. Furthermore, these CQDs demonstrate excellent water solubility and maintain high fluorescence stability despite ionic strength, pH and time variations. Moreover, we present here for the first time that the synthesized CQDs demonstrate a rapid, exceptionally sensitive, and discerning fluorescence quenching phenomenon (IFE) concerning Cefprozil (CPR). The fluorescent probe was sensitive and specific with good linear relationships for CPR in the 0-18 µM range. The limit of detection for relationships for CPR was 2.51 µM. This study provides novel opportunities for producing high-quality luminescent CQDs that meet the requirements for various biological and environmental applications.

Carbon quantum dots (CQDs) have attracted more attentions due to their multiple performances. However, the fabrication of long-wavelength emitting CQDs with aliphatic precursors still remains a challenge, mainly because it is difficult to generate large sp2 domains to reduce energy gap, which is not conducive to a redshift of the luminescence peak. Hereon, by regulating the pH of citric acid and thiourea mixture, a N, S co-doped CQD emitting bright red fluorescence at 635 nm is successfully fabricated through the solvothermal reaction under acidic condition, achieving a high quantum yield of 32.66%. Solvatochromic effects of the CQDs are discussed through theoretical equations and models, which confirm that the hydrogen-bonding interaction dominates the fluorescence emission behavior of CQDs in polar solvents. Besides, a feasible strategy is proposed to prepare an anti-counterfeiting textile via the deposition of red-emitting CQDs onto cotton fibers, through rapidly evaporating the preferred organic solvent. As expected, the CQD-decorated textiles exhibit encouraging anti-counterfeiting and security-warning functions, along with underwater and long-distance detectability, washability, and sun resistance. It is worth noting that the present work is innovative in realizing the application of red-light-emitting CQDs in the fields of security-warning textiles.

Carbon-based nanomaterials, such as carbon quantum dots (CQDs) and carbon 2D nanosheets (graphene, graphene oxide, and graphdiyne), have shown remarkable potential in various biological applications. CQDs offer tunable photoluminescence and excellent biocompatibility, making them suitable for bioimaging, drug delivery, biosensing, and photodynamic therapy. Additionally, CQDs\' unique properties enable bioimaging-guided therapy and targeted imaging of biomolecules. On the other hand, carbon 2D nanosheets exhibit exceptional physicochemical attributes, with graphene excelling in biosensing and bioimaging, also in drug delivery and antimicrobial applications, and graphdiyne in tissue engineering. Their properties, such as tunable porosity and high surface area, contribute to controlled drug release and enhanced tissue regeneration. However, challenges, including long-term biocompatibility and large-scale synthesis, necessitate further research. Potential future directions encompass theranostics, immunomodulation, neural interfaces, bioelectronic medicine, and expanding bioimaging capabilities. In summary, both CQDs and carbon 2D nanosheets hold promise to revolutionize biomedical sciences, offering innovative solutions and improved therapies in diverse biological contexts. Addressing current challenges will unlock their full potential and can shape the future of medicine and biotechnology.

Carbon quantum dots (CQDs) have been investigated for biomedical applications in medical imaging due to their fluorescent properties, overall long-term stability, and excellent cytocompatibility and biocompatibility. Lignin is an organic polymer in the tissues of woody plants. It is also considered a byproduct of the wood and pulp industries. Hence, it presents as a renewable source of carbon nanoparticles. In this study, we report the synthesis and material and biological characterization of two colloidal suspensions of CQDs in water derived from lignin-based carbon. One was the native form of CQDs derived from lignin carbon, and the second was doped with nitrogen to evaluate material differences. Material characterization was carried out using various commonly used techniques, including Fourier transform infrared spectroscopy (FTIR), emission and absorbance spectra, zeta potential, transmission electron microscopy (TEM), and scanning electron microscopy (SEM). Thin films of CQDs were formed on glass and silicon substrates to assess the in vitro cytocompatibility with human mesenchymal stem cells (hMSCs). Observations suggest that the two forms of CQDs promote cell attachment within 24 h and sustain it for at least 7 days. The overall structure and shape of cells suggest a lack of any adverse or toxic effects of CQDs. The data lay down the novel foundation to support the use of lignin-derived CQDs in tissue engineering applications.

Cancer is the second most fatal disease among women. In recent years, utilizing strategies based on carbon quantum dots (CQDs) as targeted drug delivery systems has had a significant impact on advancing and improving cancer treatment. This study is focused on the development of a nanocarrier, based on CQDs, for improving the therapeutic efficiency of mitoxantrone (MTX). Hence, the N-doped CQDs were synthesized by a hydrothermal method. Following its purification, MTX was loaded to the CQD, resulting in an increase in the size from 36.78 ± 0.9 nm to 157.8 ± 12.18 nm, with an ideal drug entrapment efficiency of 97 %. Drug release investigation showed a pH-dependent improvement, from 8 % at pH 7.4 to 11 % at pH 5.2 after 48 h. Based on the Methylthiazolyldiphenyl-tetrazolium bromide (MTT) results after 5 h of treatment on MCF-7 breast cancer cells, the N-doped CQD showed no significant effect on the cancer cells, whereas a half maximal Inhibitory Concentration (IC50) was achieved with the N-doped CQD-MTX complex at a concentration between 0.5 to 0.8 μM. Therefore, the newly developed drug delivery complex was capable of providing a rather identical influence on MCF-7 cells, as the free MTX, however, improving the pharmacokinetic of the drug by its controlled and on-target drug release, due to an alteration in distribution and absorption parameters.

In recent years, carbon quantum dots (CQDs) have garnered considerable attention as a promising material for biomedical applications because of their unique optical and biological properties. In this study, CQDs were derived from the leaves of Hibiscus rosa-sinensis Linn. via microwave-assisted technique and characterized using different techniques such as ultraviolet-visible, Fourier transform infrared, fluorescence spectrometry, X-ray diffraction, dynamic light scattering, transmission electron microscopy and energy-dispersive X-ray spectroscopy. Subsequently, their potential for biomedical applications was investigated through in vitro assays assessing scratch healing, anti-inflammatory, antibacterial, and cytotoxicity properties. It was found that the CQDs were fluorescent, polycrystalline, quasi-spherical, ~ 12 nm in size with presence of -OH and -COOH groups on their negatively charged surfaces, and demonstrated good anti-inflammatory by inhibiting protein denaturation, cyclooxygenase-2 and regulating inflammatory cytokines. The CQDs also exhibited antimicrobial activity against Klebsiella pneumoniae and Bacillus cereus, good biocompatibility, along with excellent promotion of cell proliferation in vitro, indicating their potential as a anti-inflammatory and wound healing material. The properties were more enhanced than their precursor, H. rosa-sinensis leaf extract. Hence, the CQDs synthesized from the leaves of H. rosa-sinensis can serve as a potential biomedical agent.

This study aims to develop a microelectrode array-based neural probe that can record dopamine activity with high stability and sensitivity. To mimic the high stability of the gold standard method (carbon fiber electrodes), the microfabricated platinum microelectrode is coated with carbon-based nanomaterials. Carboxyl-functionalized multi-walled carbon nanotubes (COOH-MWCNTs) and carbon quantum dots (CQDs) were selected for this purpose, while a conductive polymer like poly (3-4-ethylene dioxythiophene) (PEDOT) or polypyrrole (PPy) serves as a stable interface between the platinum of the electrode and the carbon-based nanomaterials through a co-electrodeposition process. Based on our comparison between different conducting polymers and the addition of CQD, the CNT-CQD-PPy modified microelectrode outperforms its counterparts: CNT-CQD-PEDOT, CNT-PPy, CNT-PEDOT, and bare Pt microelectrode. The CNT-CQD-PPy modified microelectrode has a higher conductivity, stability, and sensitivity while achieving a remarkable limit of detection (LOD) of 35.20 ± 0.77 nM. Using fast-scan cyclic voltammetry (FSCV), these modified electrodes successfully measured dopamine\'s redox peaks while exhibiting consistent and reliable responses over extensive use. This electrode modification not only paves the way for real-time, precise dopamine sensing using microfabricated electrodes but also offers a novel electrochemical sensor for in vivo studies of neural network dynamics and neurological disorders.

Modification and functionalization of porous aromatic framework (PAF) materials have emerged as crucial research directions in various fields. In this study, we employed a hydrothermal method to synthesize a carbon quantum dots (CQDs) solution. By loading different amounts of CQDs onto the surface of PAF-45 material through ultrasonic and hydrothermal treatments, we successfully formed CQDs/PAF-45 composite materials. The introduction of CQDs effectively transformed the hydrophobic nature of PAF-45 into a hydrophilic material, thereby overcoming the challenge of achieving efficient contact between PAF catalysts and reactants in aqueous solutions. In the photocatalytic degradation experiments of Rhodamine B (RhB), tetracycline, CQDs/PAF-45 composite materials surpassed that of the pristine PAF-45 material. Notably, the 1 wt% CQDs/PAF-45 composite material exhibited the highest photocatalytic activity, with degradation efficiencies for Rhodamine B, tetracycline, and phenol approximately 1.4 times, 1.5 times and 1.5 times higher than those of the PAF-45 material, respectively.

The modification of titanium dioxide (TiO2) is a strategy to maximize the utilization of sunlight. Carbon quantum dots (CQDs) are carbon nanomaterials with outstanding optical and electronic properties that are suitable for that purpose. In this work, three types of hybrid TiO2/CQD photoelectrodes were synthesized following different methods: 1) deposition of a CQD layer on top of TiO2 (labelled as TiO2-CQD); 2) deposition of a TiO2 layer on top of CQDs (labelled as CQD-TiO2) and; 3) deposition of a mixed CQD + TiO2 layer (labelled as CQD + TiO2). The photoelectrodes were investigated for the photoelectrocatalytic degradation of phenol as model pollutant under simulated solar light and TiO2-CQD showed the highest apparent reaction rate constant of kapp = 0.0117 min-1 with 40% of TOC removal in 6 h of treatment. CQDs were found to enhance photon absorption in the visible region of the electromagnetic spectrum and in turn phenol degradation by promoting the separation of photogenerated charge carriers through electron transfer via the Ti-O-C bonds formed at the TiO2-CQD interface. Finally, the performance of the TiO2-CQD photoanode was evaluated for the treatment of real wastewater from the membrane fabrication sector, confirming its photoelectrocatalytic efficiency under solar radiation with 93% of TOC removal in 8 h of treatment and kapp = 0.0058 min-1.