Nitrogen-doped carbon dots with excitation-independent long-wavelength emission produced by a room-temperature reaction

Full-color-emitting fluids from carbon dots stabilized in nonconventionally fluorescent micelles

Red-emitting carbon dots from pyrolysis of citric acid in formamide, which are intrinsically water-insoluble, were solubilized and stabilized by the fluorescent micelles formed by a nonionic silicone surfactant and an anionic surfactant, leading to the formation of full-color-emitting colloids with good biocompatibility and a variety of potential applications.

Red emitting carbon dots: surface modifications and bioapplications

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

Gram-Scale Room-Temperature Synthesis of Solid-State Fluorescent Carbon Nanodots for Bright Electroluminescent Light Emitting Diodes

The reaction conditions of high temperature and high pressure will introduce structural defects, high energy consumption, and security risks, severely hindering the industrial application of organic carbon nanodots (CDs). Moreover, the aggregation caused quenching effect also fundamentally limits the CDs based electroluminescent light emitting diodes (LEDs). Herein, for the first time, a rapid one-step room temperature synthetic strategy is introduced to prepare highly emissive solid-state-fluorescent CDs (RT-CDs). A strong oxidizing agent, potassium periodate (KIO₄ ), is adopted as a catalyst to facilitate the cyclization of o-phenylenediamine and 4-dimethylamino phenol in aqueous solution at room temperature for only 5 min. The resultant organic molecule, 2-(dimethylamino) phenazine, will self-assemble kinetically to generate supramolecular-structure CDs during crystallization. The elaborately arranged supramolecular structure (J aggregates) endows CDs with intense solid-state-fluorescence. Density functional theory (DFT) calculation shows that the excited state of RT-CDs exhibits charge transfer characteristic owing to the unique donor-Π-acceptor structure. A high-performance monochrome RT-CDs based electroluminescent LEDs (2967 cd m^-2 and 1.38 cd A^-1 ) were fabricated via systematic optimizations of device engineering. This work provides a concrete and feasible avenue for the rapid and massive preparation of CDs, advancing the commercialization of CDs based optoelectronic devices.

Epigallocatechin gallate-derived carbonized polymer dots: A multifunctional scavenger targeting Alzheimer's β-amyloid plaques

The design of high-efficiency scavengers targeting β-amyloid protein (Aβ) plaques in the progress of Alzheimer's disease (AD) has been recognized as an effective way to prevent and treat AD. Herein, epigallocatechin gallate (EGCG)-derived carbonized polymer dots (E-CPDs) were synthesized for the first time via a hydrothermal method using EGCG, an Aβ inhibitor, as one of the raw materials. The inhibitory efficiency and fluorescent property of E-CPDs were elegantly modulated by adjusting the molar ratio of EGCG to nitrogen-containing dopant, o-phenylenediamine (oPD), and 75E-CPDs fabricated with 75 mM EGCG and 50 mM oPD showed the highest inhibitory capability. The multifunctionality of 75E-CPDs on inhibition of Aβ fibrillization, Aβ fibrils disaggregation, amyloid fluorescent detection, and intracellular reactive oxygen species scavenging was demonstrated. 75E-CPDs inhibited the formation of β-sheet-rich Aβ aggregates, alleviated Aβ-induced cytotoxicity of cultured cells from 47% to 15%, and prolonged the lifespan of AD nematodes by scavenging in vivo amyloid plaques, demonstrating much higher performance than either EGCG or EGCG-free carbon dots. Notably, 75E-CPDs could rapidly disaggregate Aβ fibrils on "second" scale, faster than any other disaggregating agents. The aromatic structure as well as hydroxyl and carboxyl groups existing on 75E-CPDs surface, which would interact with Aβ species via hydrogen bonding, electrostatic interactions, and hydrophobic interactions, played critical roles in their inhibition and disaggregation capabilities. This work reveals that potent CDs can be fabricated by using an Aβ inhibitor as the precursor, providing a new perspective for the design of multifunctional scavengers targeting amyloid plaques. STATEMENT OF SIGNIFICANCE: Alzheimer's disease (AD) is one of the top ten causes of death worldwide and seriously threatens human health. Recently, carbon nanomaterials have attracted much attention because of their good biocompatibility and capability in modulating Aβ aggregation via multiple interactions. This work has for the first time fabricated epigallocatechin gallate-derived carbonized polymer dots (E-CPDs) and revealed the multifunctional potency of E-CPDs on alleviating the multifaced symptoms associated with β-amyloid protein (Aβ) fibrillization in the progression of AD. Notably, E-CPDs exhibited enhanced fluorescence emission upon binding to Aβ fibrils, possessing potential as Aβ fluorescent probes. It is believed that this work would open a new horizon in the design of multifunctional carbon nanomaterials as a potent amyloid scavenger for AD theranostics.

Fabrication of Carbon-Based Quantum Dots via a "Bottom-Up" Approach: Topology, Chirality, and Free Radical Processes in "Building Blocks"

The discovery of carbon-based quantum dots (CQDs) has allowed opportunities for fluorescence bioimaging, tumor diagnosis and treatment, and photo-/electro-catalysis. Nevertheless, in the existing reviews related to the "bottom-up" approaches, attention is mainly paid to the applications of CQDs but not the formation mechanism of CQDs, which mainly derived from the high complexities during the synthesis of CQDs. Among the various synthetic methods, using small molecules as "building blocks", the development of a "bottom-up" approach has promoted the structural design, modulation of the photoluminescence properties, and control of the interfacial properties of CQDs. On the other hand, many works have demonstrated the "building blocks"-dependent properties of CQDs. In this review, from one of the most important variables, the relationships among intrinsic properties of "building blocks" and photoluminescence properties of CQDs are summarized. The topology, chirality, and free radical process are selected as descriptors for the intrinsic properties of "building blocks". This review focuses on the induction and summary of recent research results from the "bottom-up" process. Moreover, several empirical rules pertaining thereto are also proposed.

Fabrication of a Heterojunction by Coupling a Metal-Organic Framework and N-Doped Carbon for the Photocatalytic Removal of Antibiotic Drugs with High Efficiency

Norfloxacin (NOR) and tetracycline (TC), two widely used antibiotic drugs released to the aquatic environment, induce harm to ecosystems. In this study, an effective method was developed successfully to remove NOR and TC by photocatalysis with a novel heterojunction NC/NH₂-MIL-53(Fe), which was fabricated by combining a metal-organic framework (MOF) material (NH₂-MIL-53(Fe)) and N-doped carbon (NC) nanoparticles via a facile solvent thermal method. The prepared product exhibits outstanding photocatalytic efficiencies toward degradation of NOR and TC that are 15 and 6 times higher than those of pure NH₂-MIL-53(Fe), respectively. Moreover, it is higher than those of the related materials reported previously. The greatly enhanced photocatalytic performance is assigned to the fabricated heterojunction with well-matched energy band gaps, where the NC acts as an efficient electron transfer/reservoir material to effectively promote the migration and transfer and restrain the recombination of charge carriers. In addition, the formed heterojunction increases specific surface area and light absorbance. The photocatalytic activity enhanced mechanism, degradation products, and pathway were investigated. The present study offers a novel strategy to significantly improve the photocatalytic performances of MOFs for highly efficient photocatalytic removal of antibiotic drugs in wastewater.

Synthesis and photoluminescence properties of orange-red carbon dots from the paper tissues as the precursor

In recent years, stable and photoluminescence (PL) tunable fluorescent materials known as carbon dots (CDs) have seen rapid development. Here, a simple, low-cost, and environmentally friendly approach has been developed to synthesize the orange-red CDs (OR-CDs) with paper tissues as the precursor in the presence of ethanol and concentrated sulfuric acid. The average diameter of the OR-CDs was measured at around 1.25 nm. The as-prepared OR-CDs are composed of carbon cores with the graphite structure distributed in the middle and wrapped or entangled by polymers for the outside. Moreover, the PL characteristics of the OR-CDs under the dilute solution state and the concentrated solution state, respectively, were explored. The former with the blue light showed prominent excitation-dependent characteristics in the range from 365 to 420 nm; the latter rendered the excitation-independent property with distinguished orange-red fluorescence emission. With an increase in the concentration, the emission fluorescence intensities gradually increased under the excitation wavelength of 540 nm, which can be attributed to the aggregation-induced emission property of the OR-CDs. By virtue of their excellent PL properties and low toxicity, we believe that the OR-CDs have potential applications in various optoelectronic areas, especially in white LED fields.

A multifunctional chemical toolbox to engineer carbon dots for biomedical and energy applications

Photoluminescent carbon nanoparticles, or carbon dots, are an emerging class of materials that has recently attracted considerable attention for biomedical and energy applications. They are defined by characteristic sizes of <10 nm, a carbon-based core and the possibility to add various functional groups at their surface for targeted applications. These nanomaterials possess many interesting physicochemical and optical properties, which include tunable light emission, dispersibility and low toxicity. In this Review, we categorize how chemical tools impact the properties of carbon dots. We look for pre- and postsynthetic approaches for the preparation of carbon dots and their derivatives or composites. We then showcase examples to correlate structure, composition and function and use them to discuss the future development of this class of nanomaterials.

Green Synthesis of Tunable Fluorescent Carbon Quantum Dots from Lignin and Their Application in Anti-Counterfeit Printing

Lignin converted to carbon quantum dots (CQDs) attracts tremendous attention for large-scale production of carbon nanomaterials and value-added disposal of biomass wastes (such as the black liquor from pulping industry and the residue from hydrolysis of biomass). The green synthesis of lignin-derived CQDs is reported via a facile two-step method with the adjustment of acid additives containing N or S. The resulting series of CQDs exhibit bright fluorescence in gradient colors from blue to yellowish green, among which the N, S co-doped CQDs with the addition of 2,4-diaminobenzene sulfonic acid show an optimal fluorescence quantum yield (QY) of 30.5%. The red-shift photoluminescence emission behaviors of these CQDs can be attributed to the increased graphitization degree and reduced optical energy band gaps (2.47 → 2.17 eV) with regard to the incorporation of various heteroatoms. The improved fluorescence QYs are consistent with the variation trend of the increased N/C content in the CQDs. The yellowish green-emissive CQDs with bright fluorescence, strong water solubility, and excellent chemical stability perform well in anti-counterfeiting printing. The promising and sustainable approach for the synthesis of tunable fluorescent CQDs exhibits the value-added utilization of lignin for the fluorescence ink production.

Fluorescent Egg White-Based Carbon Dots as a High-Sensitivity Iron Chelator for the Therapy of Nonalcoholic Fatty Liver Disease by Iron Overload in Zebrafish

Iron overload is the direct cause of many ferroptosis diseases, and it is essential to maintain iron homeostasis. In this paper, we report the Fe³⁺ chelation and therapy of the iron overload nonalcoholic fatty liver disease (NAFLD) by the fluorescent egg white-based carbon dots (EWCDs) obtained through the microwave-assisted pyrolysis method. As a high-sensitivity sensor, EWCDs show a high correlation between fluorescence emission and the concentration of Fe³⁺ (R² = 0.993) in low concentration ranges of 0-25 μM. In vivo and in vitro, the EWCDs show characteristics of high biocompatibility and specific binding of Fe³⁺. As a novel type of the nano-iron-chelator, EWCDs can successfully attenuate the production of lethal reactive oxygen species. EWCDs not only alleviate the endoplasmic reticulum stress response but also regulate the NF-κB signaling pathway downstream of the Nrf2 signaling pathway. EWCDs prevent hepatocyte apoptosis, regulate fatty acid metabolism, and alleviate inflammation. Ultimately, they alleviate NAFLD induced by iron overload in zebrafish. This work may provide a new idea and method for the application of carbon dots in the field of disease detection and treatment.

Deciphering the Relaxation Mechanism of Red-Emitting Carbon Dots Using Ultrafast Spectroscopy and Global Target Analysis

Red-emitting carbon dots (C-dots) have tremendous potential for bioimaging and optoelectronic applications. Here, we investigated the structural modification of red-emitting C-dots due to boron doping and their ultrafast relaxation dynamics. It is evident from the X-ray photoelectron spectroscopy study that the relative percentage of pyrridinic nitrogen is increased at the expense of amino nitrogen and graphitic nitrogen in B-doped C-dots. Transient absorption spectroscopy and global target analysis reveal the formation of an additional excited-state level that takes away a significant amount of the excited-state population after boron doping. This new excited state slows the initial relaxation process toward the emissive state from 317 to 750 fs and increases the overall lifetime from 1.03 to 1.45 ns in B-doped C-dots.

Facile synthesis of orange fluorescence multifunctional carbon dots for label-free detection of vitamin B12 and endogenous/exogenous peroxynitrite

In this work, orange emission fluorescent multifunctional carbon dots (O-CDs) were designed for the label-free detection of vitamin B₁₂ (VB₁₂)，endogenous/exogenous peroxynitrite (ONOO^-) sensing, cell imaging, and fluorescent flexible film preparation. The O-CDs with excitation-independent were prepared using safranine T and ethanol as precursors via one-step hydrothermal process. VB₁₂ was utilized as a quencher to quench the fluorescence of O-CDs due to the internal filtration effect (IFE). Two-segment linear ranges are 1-65 μM and 70-140 μM, and the detection limit was calculated as 0.62 μM. Besides, ONOO^- can reduce the fluorescence intensity of O-CDs based on static quenching (SQ). The linear ranges are 0.3-9 μM and 9-48 μM, and the detection limit was 0.06 μM. Moreover, the O-CDs were exploited as a cellular imaging reagent for intracellular VB₁₂ and endogenous/exogenous ONOO^- imaging owing to its great biocompatibility, low toxicity and strong photostability. These results indicate that O-CDs have the potential to be used as a sensitive fluorescence probe to rapidly monitor VB₁₂ and endogenous/exogenous ONOO^- with high selectivity in living cells. Also, the as-proposed O-CDs can be employed to fabricate O-CDs/PVA composites as fluorescent flexible films. All of the above prove that the O-CDs present great prospect in multiple applications such as biosensing, cellular labeling, biomedical optical imaging, and fluorescent films.

One-step synthesis of red emission multifunctional carbon dots for label-free detection of berberine and curcumin and cell imaging

In this work, the red emission multifunctional carbon dots (R-CDs) were prepared via one-pot hydrothermal strategy of neutral red (NR) and ethylenediamine (EDA) for the label-free detection of berberine and curcumin, cell imaging, and fluorescent flexible film. The as-fabricated R-CDs not only possess good water dispersibility and excellent fluorescence stability, but also were successfully employed as a photoluminescent nanoprobe for label-free monitoring of berberine (BRH) and curcumin (Cur) based on dynamic quenching and internal filter effect (IFE), respectively. More importantly, as-proposed R-CDs displayed outstanding cellular permeability and lower cytotoxicity for cellular applications, which was consistent with the results of confocal fluorescence imaging and cell viability measurement of SMMC7721 cells. Thus, the multifunctional R-CDs may provide a rich tool library for biosensing and cellular imaging reagent applications. Interestingly, R-CDs were also used to manufacture R-CDs/PVA composites as fluorescent flexible films. To the best of our knowledge, this is the first demonstration of a label-free multifunctional fluorescent nanoprobe for berberine and curcumin based on red emission CDs.

A facile synthesis of long-wavelength emission nitrogen-doped carbon dots for intracellular pH variation and hypochlorite sensing

Intracellular pH and hypochlorite (ClO^-) concentration play an important role in life activities, so there is an urgent need to develop a valid strategy to monitor pH and ClO^- in biological systems with high sensitivity and specificity. In this study, we report long-wavelength emission nitrogen-doped carbon dots (N-CDs) and their potential applications in intracellular pH variation, ClO^- sensing and cell imaging. The N-CDs were prepared via a facile one-pot hydrothermal method of neutral red (NR) and glutamine (Gln). N-CDs exhibited a pH-sensitive response in the range of 4.0-9.0 and a good linear relationship in the range of 5.6-7.4, which indicated that N-CDs are an ideal agent for monitoring pH fluctuations in living cells. In addition, ClO^- was capable of reducing the photoluminescence of N-CDs based on static quenching. The linear range is 1.5-112.5 μM and 112.5-187.5 μM, and the LOD is 0.27 μM. Besides, the as-fabricated N-CDs have been smoothly achieved to monitor pH and ClO^- in PC-12 living cells due to their great biocompatibility and lower cytotoxicity, demonstrating their promising applications in the biomedical field. Compared with other CD-based methods, the as-proposed N-CDs have a longer fluorescence emission, which makes them potentially valuable in biological systems. The results pave a way towards the construction of long-wavelength carbon-based nanomaterials for fluorescence sensing and cell imaging.

Facile Hydrothermal and Solvothermal Synthesis and Characterization of Nitrogen-Doped Carbon Dots from Palm Kernel Shell Precursor

Carbon dots (CDs), a nanomaterial synthesized from organic precursors rich in carbon content with excellent fluorescent property, are in high demand for many purposes, including sensing and biosensing applications. This research focused on preparing CDs from natural and abundant waste, palm kernel shells (PKS) obtained from palm oil biomass, aiming for sensing and biosensing applications. Ethylenediamine and L-phenylalanine doped CDs were produced via the hydrothermal and solvothermal methods using one-pot synthesis techniques in an autoclave batch reactor. The as-prepared N-CDs shows excellent photoluminescence (PL) property and a quantum yield (QY) of 13.7% for ethylenediamine (EDA) doped N-CDs (CDs-EDA) and 8.6% for L-phenylalanine (L-Ph) doped N-CDs (CDs-LPh) with an excitation/emission wavelength of 360 nm/450 nm. The transmission electron microscopy (TEM) images show the N-CDs have an average particle size of 2 nm for both CDs. UV-Visible spectrophotometric results showed C=C and C=O transition. FTIR results show and confirm the presence of functional groups, such as -OH, -C=O, -NH2 on the N-CDs, and the X-ray diffraction pattern showed that the N-CDs were crystalline, depicted with sharp peaks. This research work demonstrated that palm kernel shell biomass often thrown away as waste can produce CDs with excellent physicochemical properties.

Fluorescent and Colorimetric Dual-signal Enantiomers Recognition via Enzyme Catalysis: The Case of Glucose Enantiomers Using Nitrogen-doped Silicon Quantum Dots/Silver Probe Coupled with β-D-Glucose Oxidase

Chiral enantiomer recognition is important but facing tough challenges in the direct quantitative determination for complex samples. In this work, via chosing nitrogen-doped silicon quantum dots (N-SiQD) as optical nanoprobe and constructing N-SiQD/silver (N-SiQD/Ag NPs) complex, β-D-GOx as model enzyme and glucose enantiomers as analytes, a fluorescent and colorimetric dual-signal chiral sensing strategy was proposed herein for chiral recognition based on specific enzyme-catalyzed reaction. N-SiQD can exhibit intense fluorescence, while it can be quenched by Ag NPs owing to the formation of N-SiQD/Ag NPs. In the presence of glucose isomer, D-glucose is catalytically hydrolyzed by β-D-GOx to form H₂O₂ owing to the specific enzyme catalyzed reaction between D-glucose and β-D-GOx, and H₂O₂ can etch Ag NPs from the N-SiQD/Ag NPs probe to change the solution color from brown to colorless and restore the N-SiQD fluorescence; while these phenomena cannot be caused by L-glucose, a dual-signal sensing method was thus constructed for recognizing glucose enantiomers. It is believed that the chiral enantiomers recognition strategy via enzyme catalysis has great application for selective and quantificational detection of enantiomers in the complex sample system.

Indole Carbonized Polymer Dots Boost Full-Color Emission by Regulating Surface State

Carbonized polymer dots (CPDs) are impressive imaging probes with great potential for enriching the library of metal-free fluorescent materials, yet current strategies have struggled to achieve products that emit full-color light in a single reaction system. Establishing an efficient and robust synthesis approach that unlocks the color barrier to the luminescence centers of specific CPDs remains a challenge. Herein, the surface-state engineering of pyridine and amide in the indole system to create a palette of resolvable full-color light-emissive CPDs is reported. Detailed structural analysis revealed that cationic polymerization and oxidation reactions potentially contribute to the formation of the main frameworks and emission centers of the final CPDs, with emissive oxygen- and nitrogen-based centers fixed by cross-linked polymer structures. This study provides valuable insight into the energy absorbance and photoluminescence mechanism of CPDs and introduces additional reactants (benzo heterocycle) into CPD research.

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Achieving a palette of full-color light-emissive CPDs in a single reaction system

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Providing the surface-state engineering rules for CPDs' emission centers

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Enriching the guidance library for studying the fluorescence mechanism of CPDs

Synthesis of Self-Targeted Carbon Dot with Ultrahigh Quantum Yield for Detection and Therapy of Cancer

This study aims to engineer a new type of ultrahigh quantum yield carbon dots (CDs) from methotrexate (MTX-CDs) with self-targeting, imaging, and therapeutic effects on MDA-MB 231 breast cancer cells. CDs were synthesized via a straightforward thermal method using a methotrexate (MTX) drug source. The physicochemical characteristics of the prepared MTX-CDs were studied using Fourier transform infrared (FT-IR) spectroscopy, transmission electron microscopy (TEM), dynamic light scattering (DLS), X-ray powder diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). TEM and DLS revealed which MTX-CDs have homogeneous spherical morphology with a smaller average size of 5.4 ± 2.2 nm, polydispersity index (PDI) of 0.533, and positive surface charge of around +3.93 mV. Results of FT-IR spectroscopy and high-resolution XPS indicated the presence of residues of MTX on CDs. Therefore, the synthesized MTX-CDs could be targeted and be taken up by FR-positive cell lines without the aid of additional targeting molecules. In vitro epifluorescence images demonstrated high-contrast cytoplasm biodistribution of MTX-CDs after 2 h of treatment. A much stronger fluorescent signal was detected in MDA-MB 231 compared to MCF 7, indicating their ability to precisely target FR. The highest cytotoxic and apoptotic effects were observed in MTX-CDs compared to free MTX obtained by the MTT assay, cell cycle arrest, and annexin V-FITC apoptosis techniques. Results revealed that the novel engineered MTX-CDs were capable of inducing apoptosis (70.2% apoptosis) at a lower concentration (3.2 μM) compared to free MTX, which was proved by annexin V and cell cycle. This work highlights the potential application of CDs for constructing an intelligent nanomedicine with integration of diagnostic, targeting, and therapeutic functions.

Effects of polydopamine-passivation on the optical properties of carbon dots and its potential use in vivo

Passivation of carbon dots via heteroatom doping has been shown to enhance their optical properties and tune their fluorescence signature. Additionally, the incorporation of polymeric precursors in carbon dot synthesis has gained considerable interest with benefits to biological applications namely bioimaging, drug delivery and sensing, among others. In order to combine the desirable attributes of both, fluorescence enhancement and increased biocompatibility, polymers composed of high aromaticity and nitrogen content can be used as efficient carbon dot passivating agents. Here, the synthesis of fluorescent polymer-passivated carbon dots was developed through a microwave-assisted pyrolysis reaction of galactose, citric acid and polydopamine. Passivation of the dots with polydopamine induces a 90 nm red-shift in the fluorescence maxima from 420 to 510 nm. Moreover, passivation results in excitation-independent fluorescence and a 3.5-fold increase in fluorescence quantum yield, which increases from 1.3 to 4.6%. The application of the carbon dots as imaging probes was investigated in in vitro and in vivo model systems. Cytotoxicity studies in J774 and CHO-K1 cell lines revealed reduced cell toxicity for the polydopamine-passivated carbon dots in comparison to their unpassivated counterpart. In BALB/c mice, biodistribution studies demonstrated that regardless of surface passivation, the dots predominantly remained in the circulatory system 90 minutes post inoculation suggesting their potential use for cardiovascular therapies.

Endogenous Fluorescence Carbon Dots Derived from Food Items

Background

Fluorescent carbon dots (CDs) are a novel class of carbon-based nanomaterials that were discovered in 2004. However, nobody knew that CDs existed in food items naturally until 2012. Properties of nanosize materials are distinct from those of their bulk materials due to the particle size and accordingly alter their bioavailability and/or biocompatibility. Therefore, the potential health risk of nanoparticles in food has drawn massive attention. Currently, almost all studies regarding the biosafety of nanoparticles in food have mainly focused on engineered nanoparticles used as food additives and have excluded the endogenous nanoparticles in food. Therefore, investigation of the properties of food-borne fluorescent CDs and their potential health risk to humans is of great significance.

Scope and approach

This review summarizes the existing literature on fluorescent carbon dots (CDs) in food, with particular attention to their properties, formation process, and the potential health risks posed to consumers. The knowledge gap between food-borne nanoparticles and their potential risks is identified, and future research is proposed.

Key findings and conclusions

The presence of fluorescent CDs in food produced during food processing has been summarized. Fluorescent CDs less than 10 nm in size mainly contain carbon, oxygen, hydrogen, and/or nitrogen. The presence of CDs in food items was first demonstrated in 2012, and their formation was attributed to heating of the starting material. The properties of CDs in food are different from the engineered nanoparticles used as food as additives and represent a novel kind of nanostructure in food. Further studies should focus on the chronic effects of CDs, although their toxicity is low, because investigations both in vivo and in vitro are limited.

Simple hydrothermal approach for synthesis of fluorescent molybdenum disulfide quantum dots: Sensing of Cr3+ ion and cellular imaging

Nowadays, fluorescent molybdenum disulfide quantum dots (MoS₂ QDs) have proven to be potential candidates in the sensing and bioimaging areas owing to their exceptional intrinsic characteristics. Here, a simple hydrothermal strategy was explored for the preparation of MoS₂ QDs using ammonium heptamolybdate and 6-mercaptopurine (6-MP) as precursors. The emission peak of MoS₂ QDs was significantly quenched in the presence Cr³⁺ ion due to the selective surface chemistry on the surfaces of MoS₂ QDs. The designed fluorescent MoS₂ QDs showed a linear fluorescence quenching response with increasing concentration of Cr³⁺ ion (0.1-10 μM), allowing to detect Cr³⁺ ion even at 0.08 μM. This fluorescent MoS₂ QDs were utilized for the quantification of Cr³⁺ ion in real samples (water and biological samples). Interestingly, the synthesized MoS₂ QDs exhibited negligible cytotoxicity on NRK cells and acted as good candidates for imaging of Trichoderma viride fungal cells.

Carbon dots-fed Shewanella oneidensis MR-1 for bioelectricity enhancement

Bioelectricity generation, by Shewanella oneidensis (S. oneidensis) MR-1, has become particularly alluring, thanks to its extraordinary prospects for energy production, pollution treatment, and biosynthesis. Attempts to improve its technological output by modification of S. oneidensis MR-1 remains complicated, expensive and inefficient. Herein, we report on the augmentation of S. oneidensis MR-1 with carbon dots (CDs). The CDs-fed cells show accelerated extracellular electron transfer and metabolic rate, with increased intracellular charge, higher adenosine triphosphate level, quicker substrate consumption and more abundant extracellular secretion. Meanwhile, the CDs promote cellular adhesion, electronegativity, and biofilm formation. In bioelectrical systems the CDs-fed cells increase the maximum current value, 7.34 fold, and power output, 6.46 fold. The enhancement efficacy is found to be strongly dependent on the surface charge of the CDs. This work demonstrates a simple, cost-effective and efficient route to improve bioelectricity generation of S. oneidensis MR-1, holding promise in all relevant technologies.

Bacterial fuel cells have generated attention with the prospect of green energy production; current research is focused on optimising the system to improve efficiency. Here, the authors report on the feeding of carbon dots to S. oneidensis MR-1 to enhance metabolic activity and bioelectric generation.

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.

Rational synthesis of highly efficient ultra-narrow red-emitting carbon quantum dots for NIR-II two-photon bioimaging

Despite the growing research interest in highly bio-compatible carbon quantum dots (CQDs) for bioimaging, the synthesis of red-emitting CQDs with high photoluminescence efficiency and a sharp emission spectrum remains a formidable challenge in this field. Herein, we established a rational strategy for the synthesis of highly efficient ultra-narrow red-emitting CQDs by adopting a conjugated aromatic amine precursor (tris(4-aminophenyl)amine, TAPA) and introducing oxidative radical reagents. The resultant CQDs, T-CQDs featured red PL (615 ± 2 nm) with a high photoluminescence quantum yield (84 ± 5%) and a narrow emission linewidth (FWHM = 27 ± 1 nm), which together represented one of the highest levels in the field of CQDs so far. The T-CQDs were then further analyzed from the spectral and structural aspects, and the repeatability and universality of this strategy have also been discussed. Finally, the T-CQDs were successfully applied for both one-photon imaging and two-photon imaging with various bio-samples, both in vitro and in vivo.

Far-Red to Near-Infrared Carbon Dots: Preparation and Applications in Biotechnology

As novel fluorescent nanomaterials, carbon dots (CDs) exhibit excellent photostability, good biocompatibility, and high quantum yield (QY). Their superior properties make them promising candidates for biomedical assays and therapy. Among them, the red-emission (>600 nm) CDs have attracted increasing attention in the past years due to their little damage to the biological matrix, deep tissue penetration, and minimum autofluorescence background of biosamples. This Review, summarizes the recent progress of far-red to near-infrared (NIR) CDs from the preparation and their biological applications. The challenges in designing far-red and NIR CDs and their further applications in biomedical fields are also discussed.

Preparation of Multicolor Photoluminescent Carbon Dots by Tuning Surface States

The achievements of multicolor photoluminescent (PL)-emissive carbon dots (CDs), particularly red to near infrared (NIR), are critical for their applications in optoelectronic devices and bioimaging, but it still faces great challenges to date. In this study, PL emission red-shifts were observed when tartaric acid (TA) was added into m-phenylenediamine (mPD) or o-phenylenediamine (oPD) solutions as carbon sources to prepare CDs, i.e., from blue to green for mPD and from yellow-green to red for oPD. Morphology and structure analyses revealed that the increased surface oxidation and carboxylation were responsible for the red-shifts of emission, indicating that TA played a key role in tuning the surface state of CDs. These factors could be employed as effective strategies to adjust PL emissions of CDs. Consequently, multicolor PL CDs (i.e., blue-, green-, yellow-green- and red-emissive CDs) can be facilely prepared using mPD and oPD in the absence and presence of TA. Particularly, the obtained red-emissive CDs showed a high PL quantum yield up to 22.0% and an emission covering red to NIR regions, demonstrating great potentials in optoelectronic devices and bioimaging. Moreover, multicolor phosphors were further prepared by mixing corresponding CDs with polyvinylpyrrolidone (PVP), among which the blue, green, and red ones could serve as three primary color phosphors for fabricating multicolor and white light-emitting diodes (LEDs). The white LED was measured to show a Commission Internationale de L'Eclairage (CIE) 1931 chromaticity coordinate of (0.34, 0.32), a high color rendering index (CRI) of 89, and a correlated color temperature (CCT) of 5850 K, representing one of the best performances of white LEDs based on CDs.

Intracellular ratiometric temperature sensing using fluorescent carbon dots

Highly sensitive non-invasive temperature sensing is critical for studying fundamental biological processes and applications in medical diagnostics. Nanoscale-based thermometers are promising non-invasive probes for precise temperature sensing with subcellular resolution. However, many of these systems have limitations as they rely on fluorescence intensity changes, deconvolution of peaks, or the use of hybrid systems to measure thermal events. To address this, we developed a fluorescence-based ratiometric temperature sensing approach using carbon dots prepared via microwave synthesis. These dots possess dual fluorescence signatures in the blue and red regions of the spectrum. We observed a linear response as a function of temperature in the range of 5-60 °C with a thermal resolution of 0.048 K-1 and thermal sensitivity of 1.97% C-1. Temperature-dependent fluorescence was also observed in HeLa cancer cells over a range of 32-42 °C by monitoring changes in the red-to-blue fluorescence signatures. We demonstrate that the ratiometric approach is superior to intensity-based thermal sensing because it is independent of the intracellular concentration of the optical probe. These findings suggest that dual-emitting carbon dots can be an effective tool for in vitro and possibly in vivo fluorescence nanothermometry.

Fe3+-Catalyzed low-temperature preparation of multicolor carbon polymer dots with the capability of distinguishing D2O from H2O

Carbon polymer dots (CPDs) exhibit differential optical responses to H₂O and D₂O due to the different surface states of CPDs.

A facile synthesis of hybrid silicon quantum dots and fluorescent detection of bovine hemoglobin

A new type of hybrid SiQDs was synthesized with a higher nitrogen content, fluorescence intensity and longer fluorescence lifetime.

Tuning Carbon Dots' Optoelectronic Properties with Polymers

Due to their unique properties of photoluminescence, biocompatibility, photostability, ease of preparing, and low cost, carbon dots have been studied extensively over the last decade. Soon after their discovery, it was realized that their main optical attributes may be protected, enhanced, and tuned upon proper surface passivation or functionalization. Therefore, up to date, numerous polymers have been used for these purposes, resulting to higher-quality carbon dots regarding their quantum yield or further emission-related aspects and compared to the primitive, bare ones. Hence, this review aims to clarify the polymers' role and effect on carbon dots and their features focusing on the quality characteristics of their photoluminescence upon passivation or functionalization. Given in fact the numbers of relevant publications, emphasis is given on recent articles capturing the latest advances for polymers in carbon dots for expanding emission lifetimes, advancing quantum yields, tuning emission wavelengths, enhancing specific spectral range absorption, and tailoring optoelectronic properties in general.

One-Pot Green Synthesis of Ultrabright N-Doped Fluorescent Silicon Nanoparticles for Cellular Imaging by Using Ethylenediaminetetraacetic Acid Disodium Salt as an Effective Reductant

Because of excellent photoluminescence properties, robust chemical inertness, and low cytotoxicity of silicon nanoparticles (Si NPs), exploration of their applications in bioimaging is of great interest. Up to date, a method to synthesis Si NPs with high fluorescence quantum yield (QY) is still challenging. This situation limits the further applications of Si NPs. In this work, we report a mild, simple, and green one-pot method to synthesis N-doped fluorescent Si NPs with an ultrahigh QY up to 62%, using ethylenediaminetetraacetic acid disodium salt as an effective reductant. The obtained ultrabright Si NPs have properties such as relative small size (about 2 nm), water dispersibility, robust stability, and biocompatibility. The as-prepared Si NPs were further applied for cellular imaging with satisfactory results, indicating their great potential in bioimaging applications.

Facile preparation of bright orange fluorescent carbon dots and the constructed biosensing platform for the detection of pH in living cells

Long wavelength (i.e., orange- to red-light) fluorescence emission and functionality are critically longing for the development and applications of carbon dots (CDs) toward biosensor and bioimaging analysis. Herein, the N-doped carbon dots (N-CDs) with bright orange fluorescence emission at 592 nm were facilely synthesized via p-phenylenediamine as a carbon precursor by microwave method. The as-prepared N-CDs exhibit favorable biocompatibility, excellent water solubility, highly optical stabilities and display sensitive pH response behavior. When the pH is decreased from 9.45 to 2.45, its emission fluorescence intensity will be significantly enhanced. The pKa value of N-CDs is 5.80 and it shows linear response to the physiological pH range of 4.45-7.00. Moreover, the N-CDs also possess high selectivity of hydrogen ions over common metal ions and some bioactive molecules, excellent photostability and reversibility. Based on this, a fluorescence sensing platform is established for the detection of pH in the environment. The N-CDs were successfully used as the fluorescent probe for cellular imaging and applied to monitor pH fluctuations in live cells based on its biocompatibility and cell membrane permeability. It is also anticipated that the N-CDs are potential bio-nano-materials for real-time tracking of the intracellular pH especially under physiological conditions in the disease diagnosis, biosensing and biomedical fields.

Brightly Fluorescent Zinc-Doped Red-Emitting Carbon Dots for the Sunlight-Induced Photoreduction of Cr(VI) to Cr(III)

The present finding deals with a simple and low-cost fabrication of surface-passivated, brightly fluorescent zinc-oxide-decorated, red-emitting excitation-independent ultrafluorescent CDs, denoted as "CZnO-Dots". Surface doping of zinc oxide significantly improved the quantum yield by up to ∼72%, and these brightly fluorescent red-emitting CZnO-Dots have been employed for the aqueous-phase photoreduction of 100 ppm hexavalent chromium(VI) to trivalent chromium(III) under the influence of sunlight irradiation. The overall utility of the prepared CZnO-Dots can be ascertained by their recyclability over seven cycles.

Concentration-induced multi-colored emissions in carbon dots: origination from triple fluorescent centers

Color-tunable carbon dots have been synthesized via a one pot hydrothermal synthesis and re-dispersed in dimethylformamide solution after purification. Structural and optical property characterizations indicate that the concentration-dependent photoluminescent properties can be ascribed to the existence of multi-emissive centers in the carbon dots from core states, edge states and surface states. Therefore, a multi-center fluorescent mechanism for the carbon dots has been proposed according to the preservation effect and inductive effect of the solvent. The emission wavelength of the carbon dots with different concentrations can be tuned from 585 to 514 nm under the fixed excitation of 420 nm blue light, and a warm white light-emitting diode with a CIE color coordinate at (0.42, 0.35) is fabricated on a InGaN blue chip emitting 420 nm blue light.

Room temperature synthesis of pH-switchable polyaniline quantum dots as a turn-on fluorescent probe for acidic biotarget labeling

The synthesis of well-defined light-element-derived quantum dots (LEQDs) with advanced optical properties under mild conditions is highly desirable yet challenging. Here, a polyaniline (PANI) structure is introduced into carbon-rich LEQDs to yield well-defined, fluorescent polyaniline quantum dots (PAQDs), PAQD24, through a one-pot room temperature reaction. The mild synthetic conditions effectively minimize the defects introduced during the conventional synthesis and endow PAQD24 with desirable optical properties, including a narrow emission band (full width at half maximum = 55 nm), an optimal quantum yield of 32.5% and two-photon fluorescence. Furthermore, the bandgap of PAQD24 is highly sensitive toward pH variations in the near-neutral region, due to the proton doping and dedoping of the PANI structure. Such unique properties together with its fine bio-compatibility enable the application of this material as a turn-on fluorescent probe for the labeling of acidic biotargets from sub-cellular to organ levels, providing potential applications in diagnosis and surgery guidance for certain diseases.

Synthesis of Luminescent Carbon Dots with Ultrahigh Quantum Yield and Inherent Folate Receptor-Positive Cancer Cell Targetability

Carbon dots (CDs) have a wide range of applications in chemical, physical and biomedical research fields. We are particularly interested in the use of CDs as fluorescence nanomaterials for targeted tumor cell imaging. One of the important aspects of success is to enhance the fluorescence quantum yields (QY) of CDs as well as increase their targetability to tumor cells. However, most of the reported CDs are limited by relative low QY. In the current study, for the first time, one-step synthesis of highly luminescent CDs by using folic acid (FA) as single precursor was obtained in natural water through hydrothermal method. The as-prepared CDs exhibited QY as high as 94.5% in water, which is even higher than most of organic fluorescent dyes. The obtained CDs showed excellent photoluminescent activity, high photostability and favorable biocompatibility. The FA residuals in CDs led to extraordinary targetability to cancer cells and promoted folate receptor-mediated cellular uptake successfully, which holds a great potential in biological and bioimaging studies.

Microwave-assisted synthesis of highly luminescent N- and S-co-doped carbon dots as a ratiometric fluorescent probe for levofloxacin

Uniform N- and S-co-doped carbon dots (NSCDs) with fluorescence quantum yields of up to 64% were synthesized via a one-step microwave-assisted method. Ammonium citrate and L-cysteine act as precursors, and synthesis is completed in 2.5 min using a 750 W microwave oven to give a 62% yield. The NSCDs show bright blue fluorescence (with excitation/emission peaks at 353/426 nm) and have narrow size distribution. On exposure to levofloxacin (LEV), the emission maximum shifts to 499 nm. This effect was used to design ratiometric (2-wavelength) assays for LEV. The fluorometric method (based on measurement of the fluorescence intensity ratio at 499 and 426 nm) has a detection limit of 5.1 μg·L^-1 (3σ/k) and a linear range that extends from 0.01 to 70 mg·L^-1. The method was applied to the determination of LEV in three kinds of spiked water samples and has recoveries in the range from 98.6 to 106.8%. The fluorescent probe described here is highly selective and sensitive. Graphical Abstract Highly luminescent N- and S-co-doped carbon dots were synthesized using AC (ammonium citrate) and Cys (L-cysteine) by microwave-assisted method, and were applied to the visual and ratiometric fluorescence determination of LEV (levofloxacin).

Facile one-step synthesis of highly luminescent N-doped carbon dots as an efficient fluorescent probe for chromium(vi) detection based on the inner filter effect

Label-free fluorescence assay system is designed for Cr(iv) detection.

Graphitic Nitrogen Triggers Red Fluorescence in Carbon Dots

Carbon dots (CDs) are a stable and highly biocompatible fluorescent material offering great application potential in cell labeling, optical imaging, LED diodes, and optoelectronic technologies. Because their emission wavelengths provide the best tissue penetration, red-emitting CDs are of particular interest for applications in biomedical technologies. Current synthetic strategies enabling red-shifted emission include increasing the CD particle size (sp² domain) by a proper synthetic strategy and tuning the surface chemistry of CDs with suitable functional groups (e.g., carboxyl). Here we present an elegant route for preparing full-color CDs with well-controllable fluorescence at blue, green, yellow, or red wavelengths. The two-step procedure involves the synthesis of a full-color-emitting mixture of CDs from citric acid and urea in formamide followed by separation of the individual fluorescent fractions by column chromatography based on differences in CD charge. Red-emitting CDs, which had the most negative charge, were separated as the last fraction. The trend in the separation, surface charge, and red-shift of photoluminescence was caused by increasing amount of graphitic nitrogen in the CD structure, as was clearly proved by XPS, FT-IR, Raman spectroscopy, and DFT calculations. Importantly, graphitic nitrogen generates midgap states within the HOMO-LUMO gap of the undoped systems, resulting in significantly red-shifted light absorption that in turn gives rise to fluorescence at the low-energy end of the visible spectrum. The presented findings identify graphitic nitrogen as another crucial factor that can red-shift the CD photoluminescence.

Scalable synthesis of organic-soluble carbon quantum dots: superior optical properties in solvents, solids, and LEDs

Carbon quantum dots (CQDs) have attracted much attention owing to their unique optical properties and a wide range of applications. The fabrication and control of CQDs with organic solubility and long-wavelength emission are still urgent issues to be addressed for their practical use in LEDs. Here, organic-soluble CQDs were produced at a high yield of ∼90% by a facile solvent engineering treatment of 1,3,6-trinitropyrene, which were simultaneously used as the nitrogen and carbon sources. The optical properties of the organic-soluble CQDs (o-CQDs) were investigated in nonpolar and polar solvents, films, and LED devices. The CQDs have a narrow size distribution around 2.66 nm, and can be dispersed in different organic solvents. Significantly, the as-prepared CQDs present an excitation-independent emission at 607 nm with fluorescence quantum yields (QYs) up to 65.93% in toluene solution. A pronounced solvent effect was observed and their strong absorption bands can be tuned in the whole visible region (400-750 nm) by changing the solvent. The CQDs in various solvents can emit bright, excitation-independent, long-wavelength fluorescence (orange to red). Furthermore, benefiting from the unique oil-solution properties, the as-prepared CQDs can be processed in thin film and device forms to meet the requirements of various applications, such as phosphor-based white-light LEDs. The color coordinate for these CQD modified LEDs is realized at (0.32, 0.31), which is close to pure white light (0.33, 0.33).

Carbon dots: materials, synthesis, properties and approaches to long-wavelength and multicolor emission

Herein, we summarize recent research developments and progress of carbon dots (CDs), which have been attracting considerable attention as a new type of photoluminescent material. Raw materials, from single carbonaceous compounds to colorful natural substances, for the synthesis of CDs are discussed. A range of diverse synthetic methodologies to achieve better photoluminescence performance and more advanced functions are summarized, and these are basically divided into two classes: top-down and bottom-up. The inspiring properties, mainly including composites, optical properties and cytotoxicity, are listed. In particular, the luminescence mechanism and surface functionalization of the CDs are briefly discussed. Moreover, on the basis of the above, the long-wavelength and multicolor emission properties of CDs and ways to achieve these goals including surface state and size controlled by synthesis strategies, proper precursors, chemical doping and modification, solvatochromic effects and energy transfer are reviewed in detail.

N-Doped carbon dots: a metal-free co-catalyst on hematite nanorod arrays toward efficient photoelectrochemical water oxidation

N-Doped carbon dots (N-CDs) have been demonstrated as an effective co-catalyst for α-Fe₂O₃ nanorod arrays on a Ti sheet for photoelectrochemical water oxidation.

A novel carbon dots derived from reduced l-glutathione as fluorescent probe for the detection of the l-/d-arginine

Novel carbon dots (CDs) were fabricated by a hydro-thermal method, in which reduced l-glutathione was considered as the precursor and ethylenediamine as the passivating agent. Subsequently, the chemical structure and fluorescence stability of the CDs were thoroughly investigated.