Recent Advances in Synthesis, Optical Properties, and Biomedical Applications of Carbon Dots

One-Step and One-Precursor Hydrothermal Synthesis of Carbon Dots with Superior Antibacterial Activity

Discovering efficient antibacterial materials is crucial in the area of increasing drug resistance. Herein, we synthesized carbon dots (C-dots) with superior antibacterial activity through a simple one-step hydrothermal method. In this method, p-phenylenediamine serves as not only the carbon source but also the origin for the functional group anchored on the obtained C-dots. The antibacterial activity of the obtained C-dots was tested against Staphylococcus aureus and Escherichia coli. The minimum bactericidal concentrations of the synthesized C-dots against S. aureus and E. coli were 2 and 30 μg/mL, respectively, which are lower than that of previously reported C-dots. The antibacterial mechanism was investigated, and the results indicated that a large number of -NH₃⁺ groups on the C-dots' surface enhanced their antibacterial activity. Besides, the C-dots exhibited negligible cytotoxicity.

Solvent Effect on Structural Elucidation of Photoluminescent Graphitic Carbon Nanodots

Photoluminescence (PL) of carbon nanodots (CNDs) is proposed to originate from the polycyclic aromatic carbon-core and in situ synthesized molecular fluorophores. This work reports the CNDs prepared by direct pyrolysis of citric acid only at a prolonged time, 40 h, and their fluorescence emission parameters in a variety of solvents by steady-state and time-resolved emission spectroscopies. The response of fluorescence emission lifetime and emission quenching rate constants to changes in solvent parameters such as polarity and tumbling lifetime were essentially independent, unlike molecular fluorophores that display solvent-dependent emission parameters. Fluorescence emission was quenched in nitromethane additionally indicating to the polycyclic aromatic carbon-core as a predominant structural feature of the CNDs. The quenching of CND emission in the presence of benzophenone that has a strong triplet component in the excited state was observed. Quenching demonstrates the Stern-Volmer behavior and reveals the additional nonradiative decay pathways of CNDs. The main photophysical features of CNDs are discussed in terms of fluorescence emission originating from the excited state of the polycyclic aromatic carbon-core where contribution from the potential molecular fluorophores is considered minimal.

The Puzzling Potential of Carbon Nanomaterials: General Properties, Application, and Toxicity

Being a member of the nanofamily, carbon nanomaterials exhibit specific properties that mostly arise from their small size. They have proved to be very promising for application in the technical and biomedical field. A wide spectrum of use implies the inevitable presence of carbon nanomaterials in the environment, thus potentially endangering their whole nature. Although scientists worldwide have conducted research investigating the impact of these materials, it is evident that there are still significant gaps concerning the knowledge of their mechanisms, as well as the prolonged and chronic exposure and effects. This manuscript summarizes the most prominent representatives of carbon nanomaterial groups, giving a brief review of their general physico-chemical properties, the most common use, and toxicity profiles. Toxicity was presented through genotoxicity and the activation of the cell signaling pathways, both including in vitro and in vivo models, mechanisms, and the consequential outcomes. Moreover, the acute toxicity of fullerenol, as one of the most commonly investigated members, was briefly presented in the final part of this review. Thinking small can greatly help us improve our lives, but also obliges us to deeply and comprehensively investigate all the possible consequences that could arise from our pure-hearted scientific ambitions and work.

Photonic Carbon Dots as an Emerging Nanoagent for Biomedical and Healthcare Applications

As a class of carbon-based nanomaterials, carbon dots (CDs) have attracted enormous attention because of their tunable optical and physicochemical properties, such as absorptivity and photoluminescence from ultraviolet to near-infrared, high photostability, biocompatibility, and aqueous dispersity. These characteristics make CDs a promising alternative photonic nanoagent to conventional fluorophores in disease diagnosis, treatment, and healthcare managements. This review describes the fundamental photophysical properties of CDs and highlights their recent applications to bioimaging, photomedicine (e.g., photodynamic/photothermal therapies), biosensors, and healthcare devices. We discuss current challenges and future prospects of photonic CDs to give an insight into developing vibrant fields of CD-based biomedicine and healthcare.

Benefit of porous silica nanoreactor in preparation of fluorescence carbon dots from citric acid

A facile and robust synthesis of carbon dots (CDs) emitting blue-light in water without activation and stabilization has been developed by pyrolysis of citric acid (CA) adsorbed in silica gel (SiO2) pores. Effect of the host pore size on luminescent properties of SiO2@CDs nanocomposite as well as water suspension of CDs has been studied. The synthesis conditions such as concentration of the precursor, duration of synthesis also have been investigated. It has been demonstrated that upon the thermal treatment of silica gels saturated with CA (60% of maximum loading) at 170 °C for 5–600 min, luminescent CDs are shaped inside the nanoreactor pores. These SiO2@CDs emit photoluminescence centered at 450 nm. Silica-immobilized CDs can be separated from the source molecules and side-products by centrifugation, which allows avoiding the dialysis of the resulted mixture and so improve the scaling of the synthesis. The CDs can be easily released from SiO2@CDs by washing it with water. Water-eluted CDs demonstrate photoluminescence at 447 nm. The smaller pore size of the host and longer time of thermal treatment promote the formation of the CDs with better photoluminescent peak symmetry and higher quantum yield up to 10.1%.

A simple and ubiquitous device for picric acid detection in latent fingerprints using carbon dots

This work addresses the synthetic optimization of carbon dots (CDs) and their application in sensing picric acid from latent fingerprints by exploiting a smartphone-based RGB tool. The optimization of the synthesis of CDs is investigated towards achieving shorter reaction time, better product yield and fluorescence quantum efficiency. Precursors such as citric acid and thiourea were chosen for the synthesis of CDs. Among the various synthetic methodologies, it is found that the pyrolysis method offers ∼50% product yield within 15 min. The morphology and optical properties of the prepared CDs are characterized using the typical microscopic and spectroscopic techniques, respectively. The synthesized CDs exhibit quasi-spherical shape with an average particle size of 1.7 nm. The excitation dependent emissive properties of CDs are investigated by time resolved fluorescence spectroscopy. Furthermore, the excellent fluorescence properties (φ = 11%) of CDs are explored as a fluorescent fingerprint powder for the identification of latent fingerprints on various substrates. In addition, the presence of picric acid in latent fingerprints was detected. Furthermore, this study is extended to perform real time detection of fingerprints and harmful contaminants in fingerprints by utilizing a smartphone-based RGB color analysis tool. Based on these investigations, the prepared CDs could be a prospective fluorescent material in the field of forensics.

UV-Vis-NIR Full-Range Responsive Carbon Dots with Large Multiphoton Absorption Cross Sections and Deep-Red Fluorescence at Nucleoli and In Vivo

Carbon dots (CDs), with excellent optical property and cytocompatibility, are an ideal class of nanomaterials applied in the field of biomedicine. However, the weak response of CDs in the near-infrared (NIR) region impedes their practical applications. Here, UV-vis-NIR full-range responsive fluorine and nitrogen doped CDs (N-CDs-F) are designed and synthesized that own a favorable donor-π-acceptor (D-π-A) configuration and exhibit excellent two-photon (λ_ex = 1060 nm), three-photon (λ_ex = 1600 nm), and four-photon (λ_ex = 2000 nm) excitation upconversion fluorescence. D-π-A-conjugated CDs prepared by solvothermal synthesis under the assistance of ammonia fluoride are reported and are endowed with larger multiphoton absorption (MPA) cross sections (3PA: 9.55 × 10^-80 cm⁶ s² photon^-2 , 4PA: 6.32 × 10^-80 cm⁸ s³ photon^-3 ) than conventional organic compounds. Furthermore, the N-CDs-F show bright deep-red to NIR fluorescence both in vitro and in vivo, and can even stain the nucleoli of tumor cells. A plausible mechanism is proposed on the basis of the strong inter-dot and intra-dot hydrogen bonds through NH···F that can facilitate the expanding of conjugated sp² domains, and thus not only result in lower highest occupied molecular orbital-lowest unoccupied molecular orbital energy level but also larger MPA cross sections than those of undoped CDs.

Preparation of highly crystalline nitrogen-doped carbon dots and their application in sequential fluorescent detection of Fe3+ and ascorbic acid

Carbon dots (CDs) have been a new class of fascinating carbon-based fluorescent nanomaterials. In the present work, new N-doped CDs with highly crystalline graphite structures are prepared from renewable precursors, chitosan and tartaric acid, and are well characterized. The prepared CDs are applied as a biocompatible fluorescent sensor for the sequential detection of Fe³⁺ and AA. Among various transition metal ions, Fe³⁺ can selectively quench the fluorescence of CDs. Upon the further addition of AA, the quenched fluorescence of CDs is then restored as Fe³⁺ is reduced to Fe²⁺ by AA, which can be utilized for the fluorescent determination of AA. A good linear relationship in the range of 0-150 μM of AA concentration is established with a low detection limit of 0.02 µM. Moreover, the practical applications of this fluorescent sensing method in measurement of AA in food samples are successfully realized with satisfactory results.

Purple-emissive carbon dots enhance sensitivity of Si photodetectors to ultraviolet range

In this work, we achieved the synthesis of purple-emissive carbon dots (p-CDs) by a simple and highly reproducible route using folic acid as the sole precursor. The emission of the p-CDs is located around 390 nm, and is independent of the excitation wavelength with a high photoluminescence quantum yield of 54.6%, thus complementing an emission color palette of brightly emitting carbon dots with purple. The purple-emissive CDs are highly stable in both the colloidal state and in polymer films. A carbon dot luminescence down-shifting layer is used to sensitize a Si photodetector to the UV range. As an example, p-CDs with an excitation maximum at 330 nm were integrated into a Si photodetector, resulting in an improvement in the photoresponsivity in a UV range from 0.8 to 2.5 mA W^-1, with a relative enhancement of 203.8%. This work is a cheap, scalable, and environmentally friendly way to create purple-emissive carbon dots, which enhance the photoresponsivity of commercial photodetectors in the UV range, thus being suitable for optical power meters, optical wireless communication systems, sunlight sensors, spectrophotometers, or radiation detectors.

Polyethylene Glycol6000/carbon Nanodots as Fluorescent Bioimaging Agents

Photoluminescent nanomaterials have immense potential for use in biological systems due to their excellent fluorescent properties and small size. Traditional semiconductor quantum dots are heavy-metal-based and can be highly toxic to living organisms, besides their poor photostability and low biocompatibility. Nano-sized carbon quantum dots and their surface-modified counterparts have shown improved characteristics for imaging purposes. We used 1,3, 6-trinitropyrene (TNP) and polyethylene glycol6000 (PEG6000) in a hydrothermal method to prepare functional polyethylene glycol6000/carbon nanodots (PEG6000/CDs) and analyzed their potential in fluorescent staining of different types of bacteria. Our results demonstrated that PEG6000/CDs stained the cell pole and septa of gram-positive bacteria B. Subtilis and B. thuringiensis but not those of gram-negative bacteria. The optimal concentration of these composite nanodots was approximately 100 ppm and exposure times varied across different bacteria. The PEG6000/CD composite had better photostability and higher resistance to photobleaching than the commercially available FM4-64. They could emit two wavelengths (red and green) when exposed to two different wavelengths. Therefore, they may be applicable as bioimaging molecules. They can also be used for differentiating different types of bacteria owing to their ability to differentially stain gram-positive and gram-negative bacteria.

Multicolor emissive sulfur, nitrogen co-doped carbon dots and their application in ion detection and solid lighting

Multicolor emissive carbon dots (CDs) have potential applications in many fields such as photoelectric display, light-emitting devices, and bioimaging. Such CDs by heteroatom doping of nitrogen elements assisted with various solvents has achieved recently, yet generally low efficiency in red-light region. Herein, we developed a facile route to synthesize multicolor-emissive CDs governed by a solvothermal method of tetrahydrofuran solvent. The maximum emission locates at 432, 510, and 584 nm with the absolute FL QYs up to 21.1%, 11.0%, and 55.4% for selected B-CDs, G-CDs, and YG-CDs, respectively. By means of the analysis on their PL spectra, fluorescence lifetimes, and microstructures, the different graphitic degrees and surface states formed under the participation of S and N elements in as-prepared CDs determine the fluorescent color, and large sp²-conjugated domains within the YR-CDs is closely related to high QY. They, likewise, endow the YR-CDs with high sensitivity for ion detections of Ag⁺ and Fe³⁺, which was further illustrated to have different quenching mechanisms each other due to the affinity interaction with different surface groups of the CDs. High PL QYs of these CDs emission are beneficial to application in solid lighting, phosphors dried by these CDs solutions were mixed with PVP water solution to fabricate CDs/PVP films. The films exhibited stable fluorescence, and three phosphors were also mixed together in proportion to obtain white-light film with a CIE coordinates of (0.34, 0.34), which demonstrates that these CDs are potentially fluorescent nanomaterial in the solid-state lighting field.

Porphin-Based Carbon Dots for "Turn Off-On" Phosphate Sensing and Cell Imaging

Porphin-based carbon dots (denoted as PCDs) are prepared through a one-step hydrothermal method by using meso-tetra (4-carboxyphenyl) porphin (TCPP), citric acid, and ethanediamine as precursor. PCDs give rise to the optimal photoluminescence at λex/λem = 375/645 nm, exhibit an excitation-independent property, excellent water solubility, and good biocompatibility, which provide red emission and avoid the autofluorescence as an efficient fluorescent imaging probe. On the other hand, when Eu3+ is added into PCDs, the carboxylate groups located on the surface of PCDs exhibit high affinity to Eu3+, resulting in the fluorescence of PCDs turning off via static quenching. In the presence of phosphate, owing to the strong coordination with Eu3+, the fluorescence of PCDs turns on. Based on this performance, a novel "turn off-on" phosphate sensing system is developed. The detection limit of this sensing system can attain 3.59 × 10-3 μmol L-1. This system has been utilized for the detection of phosphate in real samples successfully, which further demonstrates potential applications in biological diagnostic and environmental analysis.

Carbon and graphene quantum dots: a review on syntheses, characterization, biological and sensing applications for neurotransmitter determination

Neuro-transmitters have been considered to be essential biochemical molecules, which monitor physiological and behavioral function in the peripheral and central nervous systems. Thus, it is of high pharmaceutical and biological significance to analyze neuro-transmitters in the biological samples. So far, researchers have devised a lot of techniques for assaying these samples. It has been found that electro-chemical sensors possess features of robustness, selectivity, and sensitivity as well as real-time measurement. Graphene quantum dots (GQDs) and carbon QDs (CQDs) are considered some of the most promising carbon-based nanomaterials at the forefront of this research area. This is due to their characteristics including lower toxicity, higher solubility in various solvents, great electronic features, strong chemical inertness, high specific surface areas, plenty of edge sites for functionalization, and versatility, in addition to their ability to be modified via absorbent surface chemicals and the addition of modifiers or nano-materials. Hence in the present review, the synthesis methods of GQDs and CQDs has been summarized and their characterization methods also been analyzed. The applications of carbon-based QDs (GQDs and CQDs) in biological and sensing areas, such as biological imaging, drug/gene delivery, antibacterial and antioxidant activity, photoluminescence sensors, electrochemiluminescence sensors and electrochemical sensors, have also been discussed. This study then covers sensing features of key neurotransmitters, including dopamine, tyrosine, epinephrine, norepinephrine, serotonin and acetylcholine. Hence, issues and challenges of the GQDs and CQDs were analyzed for their further development.

Carbon and graphene quantum dots for biological and sensing applications of neurotransmitters.

Disulfide bond-based self-crosslinked carbon-dots for turn-on fluorescence imaging of GSH in living cells

A self-quenched nanoprobe built on a disulfide bond-based crosslink of carbon-dots has been constructed for intracellular GSH sensing.

Porphyrin structure carbon dots under red light irradiation for bacterial inactivation

Porphyrin structure carbon dots were synthesized and applied for bacterial inactivation under red light irradiation.

Fluorine-containing graphene quantum dots with a high singlet oxygen generation applied for photodynamic therapy

Compared with graphene quantum dots (GQDs), fluorine-containing GQDs (F-GQDs) present higher ¹O₂ generation under light irradiation and thus cause obvious toxicity to HepG2 cells. F-GQDs can be used as a photosensitizer for photodynamic therapy.

Biomass-Derived Sulfur, Nitrogen Co-Doped Carbon Dots for Colorimetric and Fluorescent Dual Mode Detection of Silver (I) and Cell Imaging

A method for green synthesis of sulfur, nitrogen co-doped photoluminescence carbon dots (S,N/CDs) originating from two natural biomass was proposed. By simple hydrothermal heating of bean pod and onion, blue emission CDs were prepared. Ag+ can effectively quench the as-prepared S,N/CDs. Under optimized conditions, the linear range of the established method for Ag+ detection was 0.1-25 μM, and the detection of limit based on 3S/N was 37 nM. More interestingly, the addition of Ag+ can induce an evident color change of S,N/CDs from yellow to brown under sunlight. The developed method was applied for detection of Ag+ in river water and tap water samples. Satisfied recoveries ranging from 96.0 to 102.0% with precision below 4.1% were obtained. S,N/CDs showed low toxicity toward 4T1 cells, which also can be extended to cellular imaging and intracellular Ag+ detection. The simple and green approach proposed here could meet the requirements for bioimaging and environmental monitoring.

Nano-Bioconjugate Film from Aloe vera To Detect Hazardous Chemicals Used in Cosmetics

It is of utmost importance to detect hazardous chemicals that affect human health. In this work, a simple method has been developed using a traditional medicinal herb Aloe vera as a carbon source to fabricate a nano-bioconjugate film. The nano-bioconjugate system comprises of A. vera gel itself and sodium alginate to form a fluorescent nano-bioconjugate film. The film was successfully used as an optical "turn-off" sensor in detecting analytes viz. para-Aminobenzoic acid (PABA), benzophenone, hydroquinone, and propylparaben, which are used in cosmetics and are listed as "red-listed" chemicals. The applicability of the fluorescent film in detecting these hazardous chemicals was even assessed with some locally purchased cosmetic samples. Mechanistic insight into the fluorescent quenching shown by nano-bioconjugate film is also discussed. Developments of such a detection system from sustainable sources make it an interesting option for fabricating sensors for hazardous chemicals.

Plasmonic Silver Nanoprism-Induced Emissive Mode Control between Fluorescence and Phosphorescence of a Phosphorescent Palladium Porphyrin Derivative

We have succeeded in significantly enhancing fluorescence from intrinsically phosphorescent palladium octaethylporphyrin (Pd-porphyrin) that has an intersystem crossing efficiency of ∼1 by using silver nanoprisms (AgPRs). This was achieved by controlling the wavelength of the localized surface plasmon (LSP) resonance of AgPRs and the distance between the Pd-porphyrin molecules and the AgPR surfaces. In addition to enhancing phosphorescence by spectrally overlapping the phosphorescence band with the LSP resonance band, tuning the LSP wavelength to approximately 520 nm led to the appearance of a new emission band around the wavelength corresponding to the fluorescent radiation. The appearance of fluorescence suggests that the nonradiative energy transfer from the singlet excited state of Pd-porphyrin to the LSP of AgPRs overcame the ultrafast intramolecular intersystem crossing to the triplet excited state, manifesting the spectral properties of the singlet excited state of Pd-porphyrin. The fluorescence nature of this radiation was strongly supported by lifetime measurements of the hybrids of Pd-porphyrin and AgPRs. Furthermore, the dependence of the emissive intensities on the distance between the Pd-porphyrin molecules and the AgPR surfaces showed interesting opposite trends. The fluorescence intensity was increased as the distance between the molecules and the AgPRs was decreased from 10.5 to 1 nm, while the phosphorescence intensity was decreased, which indicates that the LSP-induced fluorescence radiation process from Pd-porphyrin near the AgPRs outweighed the quenching by the AgPRs, even though the phosphorescence significantly suffered quenching.

A Mini Review on Carbon Quantum Dots: Preparation, Properties, and Electrocatalytic Application

Luminescent carbon quantum dots (CQDs) represent a new form of nanocarbon materials which have gained widespread attention in recent years, especially in chemical sensor, bioimaging, nanomedicine, solar cells, light-emitting diode (LED), and electrocatalysis. CQDs can be prepared simply and inexpensively by multiple techniques, such as the arc-discharge method, microwave pyrolysis, hydrothermal method, and electrochemical synthesis. CQDs show excellent physical and chemical properties like high crystallization, good dispersibility, photoluminescence properties. In particular, the small size, superconductivity, and rapid electron transfer of CQDs endow the CQDs-based composite with improved electric conductivity and catalytic activity. Besides, CQDs have abundant functional groups on the surface which could facilitate the preparation of multi-component electrical active catalysts. The interactions inside these multi-component catalysts may further enhance the catalytic performance by promoting charge transfer which plays an important role in electrochemistry. Most recent researches on CQDs have focused on their fluorescence characteristics and photocatalytic properties. This review will summarize the primary advances of CQDs in the synthetic methods, excellent physical and electronic properties, and application in electrocatalysis, including oxygen reduction reaction (ORR), oxygen evolution reaction (OER), hydrogen evolution reduction (HER), and CO2 reduction reaction (CO2RR).

Copper-Doped Carbon Dots for Optical Bioimaging and Photodynamic Therapy

Carbon dots (CDs), as an effective bioimaging agent, have aroused widespread interest. With the increasing number of CDs used in photodynamic therapy (PDT), developing efficient CDs with multiple functions such as imaging and phototherapy has become a new challenge. Herein, a new type of copper-doped CDs (Cu-CDs) with a high fluorescence quantum yield of 24.4% was synthesized from a copper complex of poly(acrylic acid) through coordination between the carboxyl group and copper ions. Owing to their good solubility, bright fluorescence, and low cytotoxicity, the Cu-CDs can be used for fluorescence imaging in both the HeLa (human cervical cancer) cell line and SH-SY5Y (human neuroblastoma cells) multicellular spheroids (3D MCs). More importantly, the Cu-CDs show a high quantum yield of singlet oxygen (¹O₂; 36%), good photoinduced cytotoxicity, and effective inhibition of 3D MC growth. Therefore, the Cu-CDs can be used as a promising imaging-guided PDT agent. This study provides a new carbon-based nanomaterial for multifunctional photodiagnostic and therapeutic agents for biological applications.

Ultrabright Full Color Carbon Dots by Fine-Tuning Crystal Morphology Controllable Synthesis for Multicolor Bioimaging and Sensing

In this paper, two kinds of novel carbon nanocrystals (CNCs) with different crystal morphologies (the branch-chain young sprout form (CM1) and conifer-pine form (CM2)) were obtained in a controllable way. The mechanism of crystal morphological development was explored well. When the two kinds of the CNCs were dissolved in different polar solvents, they voluntarily become "ultrafine crystals" at the moment. After that, the ultrabright full color carbon dots (UBFCCDs) have been preliminarily prepared by fine-controlling. With the evaporation of the solvents, the CNCs crystallized again, which could repeat back and forth many times. After the conditions of preparing for CDs were optimized carefully, the as-prepared CDs exhibit ultrabright effects of multiexcitation and multiemission (from blue to red) and can show unique up-conversion luminescence characteristics under a lower excitation wavelength of 660 nm instead of a near-infrared wavelength of 980 or 808 nm. Significantly, the QY% of the UBFCCDs can reach 78.0%, which is higher than that of the traditional hydrothermal methods of discarding precipitation and carrying out dialysis (QY% = 69.0%). The as-prepared CDs can be used for multicolor biomedical imaging in vivo and in vitro and metal ion sensing and also show their potential value for industrial applications.