Near-infrared emissive carbon dots with 33.96% emission in aqueous solution for cellular sensing and light-emitting diodes

Carbonized polymer dots-based molecular imprinting: An adsorbent with enhanced selectivity for highly efficient recognition and removal of ceftiofur sodium from complex samples

Highly selective removal of residual cephalosporin antibiotics from complex systems is crucial for human health and ecological environment protection. Herein, a newly molecularly imprinted polymer adsorbent (CPDs-NH2@MIP) with enhanced selectivity for ceftiofur sodium (CTFS) was developed by using the special carbonized polymer dots (CPDs-NH2) as functional monomer. The CPDs-NH2 has a nano-spherical structure and functionalized groups (CC, -NH2) via the incomplete carbonization polymerization of citric acid, acrylamide and ethylenediamine, which can accurately interact with CTFS by overcoming steric hindrance, resulting in more precisely imprinted sites and reducing non-imprinted regions in MIP. The presented CPDs-NH2@MIP exhibited excellent adsorption capacity for CTFS (68.62 mg g-1), achieving equilibrium within 10 min, and highly selectivity in mixed solution containing five coexisting substances, with an imprinted factor (5.61). Compared with commercial adsorbents and MIPs prepared with traditional chain functional monomers, the CPDs-NH2@MIP showed significant advantage in selective recognition and separation of target. Analysis of microstructure and mechanism proved that usage of the spherical functional monomer generated precise imprinting sites and dense structure in CPDs-NH2@MIP, which effectively enhanced the selectivity in complex system combined with hydrogen bonding interaction. The idea of designing and using spherical functional monomer will promote the practicality of molecularly imprinted polymer adsorbents.

Near-Infrared Light Emitting Metal Halides: Materials, Mechanisms, and Applications

Near-Infrared (NIR) light emitting metal halides are emerging as a new generation of optical materials owing to their appealing features, which include low-cost synthesis, solution processability, and adjustable optical properties. NIR-emitting perovskite-based light-emitting diodes (LEDs) have reached an external quantum efficiency (EQE) of over 20% and a device stability of over 10,000 h. Such results have sparked an interest in exploring new NIR metal halide emitters. In this review, several different types of NIR-emitting metal halides, including lead/tin bromide/iodide perovskites, lanthanide ions doped/based metal halides, double perovskites, low dimensional hybrid and Bi3+/Sb3+/Cr3+ doped metal halides, are summarized, and their recent advancement is assessed. The characteristics and mechanisms of narrow-band or broadband NIR luminescence in all these materials are discussed in detail. Also, the various applications of NIR-emitting metal halides are highlighted and an outlook for the field is provided.

Effect of aggregation configuration of molecular fluorophore CZA on photoluminescence properties of carbon dots

The effect of aggregation configuration of molecular fluorophore citrazinic acid (CZA) on the photoluminescence (PL) properties of carbon dots (CDs) has been investigated using first-principles method. The structural stability of all aggregates has been analyzed, and the results show that the most stable structures are J-type CZA aggregates with head-to-tail configurations and the CZA/CD aggregates are bonded by replacing H atoms on the CD edges with de-OH from the pyridine ring of CZA. The luminescent properties of CZA/CD aggregates are mainly affected by the binding modes and binding sites. When the sites belong to electron-donating groups, electron-withdrawing groups or sp2 domain, the PL spectra of CDs are shifted and the luminescent intensities are significantly enhanced. The results suggest that covalently bonded CZA/CD aggregates are responsible for the high fluorescence quantum yield of CD. Moreover, the distance between the centers of the two pyridine rings in H-type CZA dimers less than 3.5 Å is prone to π-π stacking, leading to fluorescence quenching of aggregates. The present work is helpful in understanding the effect of molecular fluorophores on the PL properties of CDs and provides theoretical guidance for the controllable synthesis of CDs.

High SERS performance of functionalized carbon dots in the detection of dye contaminants

INTRODUCTION

The long-term overuse of malachite green (MG) has potential carcinogenic, teratogenic, and mutagenic effects. The functional nanocomposite is novel and challenging to construct and implement through surface enhanced Raman scattering (SERS) strategy to reveal the contributions in application.

OBJECTIVES

The novel Ag-CDs (carbon dots)-PBA (phenyl boric acid) nanocomposite was constructed by a facile route to detect toxic MG molecule with high SERS sensitivity and good uniformity.

METHODS

The enhanced substrate used for the detection of MG has been successfully constructed using PBA modulated Ag-CDs on a structured surface with rich binding sites.

RESULTS

The fabricated Ag-CDs-PBA substrate can be used to analyze various probe molecules exhibiting high sensitivity, good signal reproducibility, and excellent stability. The mechanism between components has been proved by calculations originating from the plasmonic Ag and active electronic transmission among the bridging CDs and PBA via the close spatial π-π effect. In addition, the accelerated separation of electron-hole pairs was triggered to further improve the SERS activity of the hybrid via a bidirectional charge transfer (CT) process. Significantly, the Ag-CDs-PBA system shows distinctive selectivity, in which PBA can hinder the interference of other species without specific hydroxyl groups.

CONCLUSION

Based on this deeper insight on plasmon-mediated mechanism, the SERS substrate was successfully practiced for quantitative determination in real water and fish samples. The strategy developed promises to be a new sensor technology and has great potential for environmental and food safety applications.

Novel solution and solid-state emissive long-wavelength carbon dots for water sensing and white LED applications

Carbon dots (CDs) are among the most popular nanomaterials due to their remarkable fluorescent and electronic properties, as well as their good biocompatibility and low cytotoxicity. Currently, CDs with aggregation-induced emission (AIE) are relatively rare and have become a significant research hotspot. This is because most conventional CDs suffer from severe quenching in a solid state. Herein, novel CDs with both solution and solid-state emissions were obtained using a facile one-step hydrothermal synthesis. Specifically, the CDs exhibit yellow solvent-dependent fluorescence in solution state (λem = 580 nm) and red AIE emission in solid state (λem = 640 nm). CDs powder is utilized as a red phosphor for light-emitting diode (LED). The resulting fabricated white LEDs (WLEDs) demonstrate good performance metrics, including a Color Rendering Index (CRI) of 89.5, Correlated Color Temperature (CCT) of 3876 K, and commission Internationale d'Éclairage (CIE) coordinates of (0.350, 0.331). Furthermore, the solvent-dependent phenomenon observed in AIE-CDs can be exploited to apply their solution as a highly sensitive fluorescence sensor for quantitatively detecting amounts of water in various organic solvents. The method offers both high accuracy and sensitivity, with R2 values varying from 0.853 to 0.994 and a low detection limit ranging from 0.296 to 3.23 % across a wide linear range. This inherent versatility makes the CDs suitable for a wide range of applications, including sensing and LED devices.

Synthesis Strategies, Optical Mechanisms, and Applications of Dual-Emissive Carbon Dots

Tuning the optical properties of carbon dots (CDs) and figuring out the mechanisms underneath the emissive phenomena have been one of the most cutting-edge topics in the development of carbon-based nanomaterials. Dual-emissive CDs possess the intrinsic dual-emission character upon single-wavelength excitation, which significantly benefits their multi-purpose applications. Explosive exploitations of dual-emissive CDs have been reported during the past five years. Nevertheless, there is a lack of a systematic summary of the rising star nanomaterial. In this review, we summarize the synthesis strategies and optical mechanisms of the dual-emissive CDs. The applications in the areas of biosensing, bioimaging, as well as photoelectronic devices are also outlined. The last section presents the main challenges and perspectives in further promoting the development of dual-emissive CDs. By covering the most vital publications, we anticipate that the review is of referential significance for researchers in the synthesis, characterization, and application of dual-emissive CDs.

Antibacterial Carbon Dots: Mechanisms, Design, and Applications

The increase in antibiotic resistance promotes the situation of developing new antibiotics at the forefront, while the development of non-antibiotic pharmaceuticals is equally significant. In the post-antibiotic era, nanomaterials with high antibacterial efficiency and no drug resistance make them attractive candidates for antibacterial materials. Carbon dots (CDs), as a kind of carbon-based zero-dimensional nanomaterial, are attracting much attention for their multifunctional properties. The abundant surface states, tunable photoexcited states, and excellent photo-electron transfer properties make sterilization of CDs feasible and are gradually emerging in the antibacterial field. This review provides comprehensive insights into the recent development of CDs in the antibacterial field. The topics include mechanisms, design, and optimization processes, and their potential practical applications are also highlighted, such as treatment of bacterial infections, against bacterial biofilms, antibacterial surfaces, food preservation, and bacteria imaging and detection. Meanwhile, the challenges and outlook of CDs in the antibacterial field are discussed and proposed.

RNA-Targeting Carbon Dots for Live-Cell Imaging of Granule Dynamics

It is significant to monitor the different RNA granules dynamics and phase separation process inside cells under various stresses, for example, oxidative stress. The current small-molecule RNA probes work well only in fixed cells and usually encounter problems such as insufficient stability and biocompatibility, and thus a specific RNA-targeting fluorescent nanoprobe that can be used in the living cells is urgently desired. Here, the de novo design and microwave-assisted synthesis of a novel RNA-targeting, red-emissive carbon dots (named as M-CDs) are reported by choosing neutral red and levofloxacin as precursors. The as-synthesized M-CDs is water-soluble with a high fluorescence quantum yield of 22.83% and can selectively bind to RNA resulting in an enhanced red fluorescence. More interestingly, such an RNA-targeting, red-emissive M-CDs can be fast internalized into cells within 5 s and thus used for real-time imaging the dynamic process of intracellular stress granules under oxidative stress, revealing some characteristics of granules that have not been identified by previously reported RNA and protein biomarkers. This research paves a new pathway for visualizing bulk RNA dynamics and studying phase-separation behaviors in living cells by rational design of the fluorescent carbon dots in terms of structure and functionality.

Multiple Stimuli-Response Polychromatic Carbon Dots for Advanced Information Encryption and Safety

Optical information encryption and safety have aroused great attention since they are closely correlated to data protection and information safety. The development of multiple stimuli-response optical materials for constructing large-capacity information encryption and safety is very important for practical applications. Carbon dots (CDs) have many gratifying merits, such as polychromatic emission, diverse luminous categories, and stable physicochemical properties, and are considered as one of the most ideal candidates for information protection. Herein, carbon core, functional groups, solvents, and other crucial factors are reviewed for outputting polychromatic emission of multiple luminous categories. In particular, substrate engineering strategies have been emphasized for their critical role in yielding excellent optical features of multiple luminous categories. High-capacity information encryption and safety strategies are reviewed by relying on the rich optical properties of CDs, such as polychromatic emission, multiple luminous categories of fluorescence, afterglow, and upconversion, as well as external-stimuli-assisted optical changes. Some perspectives for preparing excellent CDs and further developing information security strategies are proposed. This review provides a good reference for the manipulation of polychromatic CDs and the development of next-generation information encryption and safety.

Energy transfer mediated rapid and visual discrimination of tetracyclines and quercetin in food by using N, Cu Co-doped carbon dots

The appearance of multi-drug resistant Escherichia coli makes the combination of tetracyclines (TCs) and quercetin (QCT) more common to fight stubborn bacterial infections so that the effective detections of TCs and QCT are essential and necessary. Here, a novel fluorescence probe for differentiating TCs and QCT is developed based on the nitrogen and copper co-doped carbon dots (N, Cu-CDs). The N, Cu-CDs are prepared from ethylene diamine tetraacetic acid (EDTA) and anhydrous copper chloride as precursors through hydrothermal process and exhibit bright blue fluorescence with excellent optical stability. With the presence of four tetracyclines (DOX, TC, CTC and OTC), the fluorescence intensity of N, Cu-CDs is quenched directly due to the internal filtration effect (IFE), and the detection limit obtained through single-signal fluorescence sensing is as low as 23.8 nM for DOX, 37.2 nM for TC, 43.8 nM for OTC and 28.8 nM for CTC. More remarkably, three dimensional ratiometric fluorescence probe for detecting QCT is proposed based on the appearance of another emission at (410 nm, 490 nm) due to electron transform (ET) process. This new method shows a good linear relationship in the range of 10-100 μM with a low detection limit of 59.3 nM. Furthermore, a dual-channel fluorescence sensing platform based on microfluidics paper-based analytical devices (μPADs) is developed for simultaneously visual discrimination of TCs (DOX is chosen as the typical detecting model for TCs) and QCT. This investigation provides a new way for the development of CDs as multifunction fluorescence probes.

Biogenic Synthesis of Fluorescent Carbon Dots (CDs) and Their Application in Bioimaging of Agricultural Crops

Fluorescent nanoparticles have a transformative potential for advanced sensors and devices for point-of-need diagnostics and bioimaging, bypassing the technical burden of meeting the assay performance requirements. Carbon dots (CDs) are rapidly emerging carbon-based nanomaterials. Regardless of their fate, they will find increasing applications. In this study, a simple approach for synthesizing CDs from fruit peels was developed. The CDs were fabricated from Annona squamosa (L.) peels using a carbonization technique through microwave-assisted hydrothermal digestion at temperatures around 200 °C. Synthesized CDs were detected using a UV transilluminator for the preliminary confirmation of the presence of fluorescence. UV-Vis spectrophotometry (absorbance at 505 nm) analysis, zeta potential measurement (-20.8 mV), nanoparticles tracking analysis (NTA) (average size: 15.4 nm and mode size: 9.26 nm), photoluminescence, and Fourier transform infrared (FT-IR) analysis were used to identify the capping functional groups on the CDs. The total quantum yield exhibited was 8.93%, and the field emission scanning electron microscopy (FESEM) showed the size range up to 40 nm. The germinating mung bean (Vigna radiata (L.)) seeds were incubated with biogenically synthesized CDs to check the absorption of CDs by them. The fluorescence was observed under a UV-transilluminator in the growing parts of seeds, indicating the absorption of CDs during the germination, development, and growth. These fluorescent CDs could be used as a bioimaging agent. This novel method of synthesizing CDs was found to be eco-friendly, rapid, and cost-effective.

Milk-Derived Carbon Quantum Dots: Study of Biological and Chemical Properties Provides Evidence of Toxicity

Carbon dots (CDs) are carbon-based zero-dimensional nanomaterials that can be prepared from a number of organic precursors. In this research, they are prepared using fat-free UHT cow milk through the hydrothermal method. FTIR analysis shows C=O and C-H bond presence, as well as nitrogen-based bond like C-N, C=N and -NH₂ presence in CDs, while the absorption spectra show the absorption band at 280 ± 3 nm. Next, the Biuret test was performed, with the results showing no presence of unreacted proteins in CDs. It can be said that all proteins are converted in CDs. Photo luminance spectra shows the emission of CDs is 420 nm and a toxicity study of CDs was performed. The Presto Blue method was used to test the toxicity of CDs for murine hippocampal cells. CDs at a concentration of 4 mg/mL were hazardous independent of synthesis time, while the toxicity was higher for lower synthesis times of 1 and 2 h. When the concentration is reduced in 1 and 2 h synthesized CDs, the cytotoxic effect also decreases significantly, ensuring a survival rate of 60-80%. However, when the synthesis time of CDs is increased, the cytotoxic effect decreases to a lesser extent. The CDs with the highest synthesis time of 8 h do not show a cytotoxic effect above 60%. The cytotoxicity study shows that CDs may have a concentration and time-dependent cytotoxic effect, reducing the number of viable cells by 40%.

Influence of Nitrogen-Doped Carbon Quantum Dots on the Electrocatalytic Performance of the CoP Nanoflower Catalyst for OER

Cobalt phosphides modified by nitrogen-doped carbon quantum dots (CoP-NCQDs) were successfully constructed by a facile and low-cost hydrothermal treatment, which is expected to replace traditional noble-metal oxygen evolution reaction electrode materials. Detailed experiments and findings show that nitrogen-doped carbon quantum dots (NCQDs) have a significant impact on the morphology of the CoP catalyst, and nitrogen doping can regulate the surface-active sites to obtain the catalyst with abundant structural defects. Simultaneously, nitrogen doping can regulate the content of pyridinic N and pyrrolic N, which exerts positive effects on the formation of the bond structure and electron conduction between NCQDs and CoP.

Color Conversion Light-Emitting Diodes Based on Carbon Dots: A Review

This paper reviews the state-of-the-art technologies, characterizations, materials (precursors and encapsulants), and challenges concerning multicolor and white light-emitting diodes (LEDs) based on carbon dots (CDs) as color converters. Herein, CDs are exploited to achieve emission in LEDs at wavelengths longer than the pump wavelength. White LEDs are typically obtained by pumping broad band visible-emitting CDs by an UV LED, or yellow-green-emitting CDs by a blue LED. The most important methods used to produce CDs, top-down and bottom-up, are described in detail, together with the process that allows one to embed the synthetized CDs on the surface of the pumping LEDs. Experimental results show that CDs are very promising ecofriendly candidates with the potential to replace phosphors in traditional color conversion LEDs. The future for these devices is bright, but several goals must still be achieved to reach full maturity.

Photoluminescence mechanisms of red-emissive carbon dots derived from non-conjugated molecules

Red-emissive carbon dots (R-CDs) have been widely studied because of their potential application in tissue imaging and optoelectronic devices. At present, most R-CDs are synthesized by using aromatic precursors, but the synthesis of R-CDs from non-aromatic precursors is challenging, and the emission mechanism remains unclear. Herein, different R-CDs were rationally synthesized using citric acid (CA), a prototype non-aromatic precursor, with the assistance of ammonia. Their structural evolution and optical mechanism were investigated. The addition of NH₃·H₂O played a key role in the synthesis of CA-based R-CDs, which shifted the emission wavelength of CA-based CDs from 423 to 667 nm. Mass spectrometry (MS) analysis indicated that the amino groups served as N dopants and promoted the formation of localized conjugated domains through an intermolecular amide ring, thereby inducing a significant emission redshift. The red-emissive mechanism of CDs was further confirmed by control experiments using other CA-like molecules (e.g., aconitic acid, tartaric acid, aspartic acid, malic acid, and maleic acid) as precursors. MS, nuclear magnetic resonance characterization, and computational modeling revealed that the main carbon chain length of CA-like precursors tailored the cyclization mode, leading to hexatomic, pentatomic, unstable three/four-membered ring systems or cyclization failure. Among these systems, the hexatomic ring led to the largest emission redshift (244 nm, known for CA-based CDs). This work determined the origin of red emission in CA-based CDs, which would guide research on the controlled synthesis of R-CDs from other non-aromatic precursors.

Self-Supporting Carbon Nanofibers with Ni-Single-Atoms and Uniformly Dispersed Ni-Nanoparticles as Scalable Multifunctional Hosts for High Energy Density Lithium-Sulfur Batteries

The energy density of lithium-sulfur batteries (LSBs) is currently hampered by modest sulfur loadings and high electrolyte/sulfur ratios (E/S). These limitations can potentially be overcome using easy-to-infiltrate sulfur hosts with high catalytic materials. However, catalytic materials in such hosts are very susceptible to agglomeration due to the lack of efficient confinement in easy-to-infiltrate structures. Herein, using carbon dots as an aggregation limiting agent, the successful fabrication of self-supporting carbon nanofibers (CNF) containing Ni-single-atoms (Ni_SA ) and uniformly dispersed Ni-nanoparticles (Ni_NP ) of small sizes as multifunctional sulfur hosts is reported. The Ni_SA sites coordinated by such Ni_NP offer outstanding catalytic activity for sulfur reactions and CNF is an easy-to-infiltrate sulfur host with a large-scale preparation method. Accordingly, such hosts that can be prepared on a large scale enable sulfur cathodes to exhibit high sulfur utilization (66.5 mAh cm^-2 at ≈0.02 C) and cyclic stability (≈86.1% capacity retention after 100 cycles at ≈0.12 C) whilst operating at a high sulfur loading (50 mg cm^-2 ) and low E/S (5 µL mg^-1 ). This work provides a blueprint toward practical LSBs with high energy densities.

Electron-phonon coupling-assisted universal red luminescence of o-phenylenediamine-based carbon dots

Due to the complex core-shell structure and variety of surface functional groups, the photoluminescence (PL) mechanism of carbon dots (CDs) remain unclear. o-Phenylenediamine (oPD), as one of the most common precursors for preparing red emissive CDs, has been extensively studied. Interestingly, most of the red emission CDs based on oPD have similar PL emission characteristics. Herein, we prepared six different oPD-based CDs and found that they had almost the same PL emission and absorption spectra after purification. Structural and spectral characterization indicated that they had similar carbon core structures but different surface polymer shells. Furthermore, single-molecule PL spectroscopy confirmed that the multi-modal emission of those CDs originated from the transitions of different vibrational energy levels of the same PL center in the carbon core. In addition, the phenomenon of "spectral splitting" of single-particle CDs was observed at low temperature, which confirmed these oPD-based CDs were unique materials with properties of both organic molecules and quantum dots. Finally, theoretical calculations revealed their potential polymerization mode and carbon core structure. Moreover, we proposed the PL mechanism of red-emitting CDs based on oPD precursors; that is, the carbon core regulates the PL emission, and the polymer shell regulates the PL intensity. Our work resolves the controversy on the PL mechanism of oPD-based red CDs. These findings provide a general guide for the mechanism exploration and structural analysis of other types of CDs.

Reversible Zn2+-induced 3D self-assembled aerogel of carboxyl modified copper indium diselenide quantum dots: mechanism and application for inkjet printing anti-counterfeiting

Three-dimensional (3D) self-assembled quantum dot (QD) aerogels have attracted attention due to the combined properties of both QDs and porous materials. However, the difficulty and complexity of structural composition control limit the practical application of 3D self-assembled QDs. Hence, convenient, available and multifunction QD aerogels need to be explored to promote broader practical applications. Herein, we propose a universal and facile self-assembly method of copper indium selenium (CISe) QD aerogels based on coordination interaction between Zn²⁺ and carboxyl. Both experiments and Monte Carlo simulations indicate that QDs are aggregated into oligomers by Zn²⁺, and then the oligomers are gradually interconnected to each other to form a 3D network as the concentration of Zn²⁺ increases. Moreover, Zn²⁺-induced 3D self-assembled aerogel could be depolymerized by EDTA reversibly. In combination with CISe QDs, Zn-CISe aerogel has been successfully applied in green pollution-free environment-friendly anti-counterfeiting and encryption systems.

Modulating the Carbonization Degree of Carbon Dots for Multicolor Afterglow Emission

Organic afterglow materials based on carbon dots (CDs) have aroused extensive attention for their potential applications in sensing, photoelectric devices, and anticounterfeiting. Effective methods to control the CD structure and modulate the energy levels are critical but still challenging. Here, we demonstrate a method to modulate the afterglow emission of CDs@SiO₂ composites by controlling the carbonization degree of CDs with variable calcining temperatures. The CDs@SiO₂-Raw prepared with a hydrothermal bottom-up synthesis method shows a more polymerized structure of CDs with low carbonization degree, which emits long-lived thermally activated delayed fluorescence (TADF) with the lifetime of 252 ms. After calcination at 550 °C, CDs@SiO₂-550 exhibits a larger conjugated π-domain structure with higher carbonization degree, thus inducing room-temperature phosphorescence (RTP) emission with a lifetime of 451 ms. The transformation of the carbonization degree of CD structures leads to changes in energy levels and ΔE_ST, which affect their afterglow luminescence behaviors. This work proposes a new concept to modulate the afterglow emission of CDs@SiO₂ composites and forecasts potential applications of CD-based afterglow materials.

Comprehensive exploration of long-wave emission carbon dots for brain tumor visualization

Carbon dot (CD)-based tumor imaging has been proven to be a reliable nanodiagnostic technique. Although abundant types of CDs have been developed, it is still a major challenge to synthesize long-wavelength CDs with high quality and superior repetition due to the complicated synthetic process. Here, stable long-wavelength red-light emission carbon dots (R-CDs) have been synthesized using appropriate carbon sources via a solvothermal method, which enables effective visualization of deep brain glioblastoma (GBM) by a liposome-formulated delivery system. The luminescence phenomenon and structural growth characteristics of R-CDs have been fully investigated and it has been found that R-CDs exhibit different optical behaviors in different pH and solvent environments. In vitro and in vivo models have proved their excellent cell targeting capacity, bioluminescence imaging potential, and biosafety for GBM visualization. Considering their stability and biocompatibility, the in-depth tissue imagining and other extensive applications of R-CDs are strongly recommended.

Green synthesis of multifunctional carbon quantum dots: An approach in cancer theranostics

Carbon quantum dots (CQDs) have gained significant growing attention in the recent past due to their peculiar characteristics including smaller size, high surface area, photoluminescence, chemical stability, facile synthesis and functionalization possibilities. They are carbon nanostructures having less than 10 nm size with fluorescent properties. In recent years, the scientific community is curiously adopting biomass precursors for the preparation of CQDs over the chemical compounds. These biomass sources are sustainable, eco-friendly, inexpensive, widely available and convert waste into valuable materials. Hence in our work the fundamental understating of diverse fabrication methodologies of CQDs, and the types of raw materials employed in recent times, are all examined and correlated comprehensively. Their unique combination of remarkable properties, together with the ease with which they can be fabricated, makes CQDs as promising materials for applications in diverse biomedical fields, in particular for bio-imaging, targeted drug delivery and phototherapy for cancer treatment. The mechanism for luminescence is of considerable significance for leading the synthesis of CQDs with tunable fluorescence emission. Therefore, it is aimed to explore and provide an updated review on (i) the recent progress on the different synthesis methods of biomass-derived CQDs, (ii) the contribution of surface states or functional groups on the luminescence origin and (iii) its potential application for cancer theranostics, concentrating on their fluorescence properties. Finally, we explored the challenges in modification for the synthesis of CQDs from biomass derivatives and the future scope of CQDs in phototherapy for cancer theranostics.

Carbon Dots in Biomedicine: A Review

Despite the rapid development of science and technology, the effective treatment of cancer still threatens human life and health. However, the success of cancer treatment is closely related to early diagnosis, identification, and effective treatment. In recent years, with the strengthening of the development and research of nanomaterials for cancer diagnosis and treatment, researchers have found that carbon dots (CDs) have the advantages of wide absorption, excellent biocompatibility, diverse imaging characteristics, and photostability and are widely used in various fields, such as sensing, imaging, and drug/gene transportation. Recently, researchers also discovered that CDs could be used as an effective photosensitizer to generate active oxygen or convert light energy into heat under the stimulation of the external lasers, making them have the effects of photothermal and photodynamic therapy for cancer. In this review, we first outline the single-modal and multimodal imaging analysis of CDs in cancer cells. After introducing diversified imaging functions, we focused on the design and the latest research progress of CDs in phototherapy and introduced in detail the strategies of CDs in phototherapy treatment and the challenges faced by clinical applications. We hope that this overview can provide important insights for researchers and accelerate the pace of research on CDs in imaging-guided phototherapy treatment.

The fluorescence mechanism of carbon dots based on the separation and identification of small molecular fluorophores

Carbon dots (CDs) have attracted much attention in theoretical researches and their practical applications due to their excellent optical properties, and many researchers discovered that flurophores play a very important role in synthesis process of CDs and the luminescence of prepared CDs. In this study, two CDs were pyrolysis with citric acid, N-acetyl-l-cysteine and glutathione derivatives as carbon sources. Four intermediate small molecules were separated from the prepared CDs through ultrafiltration and chromatography, and their chemical structures were determined. The formation process of CDs was monitored through identified small molecule intermediates and HPLC. It is speculated that the two CDs have the same formation pathway, including TPA (5-oxo-2,3-dihydro-5H-[1,3]thiazolo[3,2-a]pyridine-3,7-dicarboxylic acid) synthesis, fluorophore polymerization, carbon chain extension, and carbonization. It was also discovered that these two CDs have the same fluorescence properties, thiazolopyridone structure, and nitrogen-sulfur co-doped functional groups are important reasons for the mixed excitation dependence of CDs. This study would provide valuable theoretical basis for the studies on preparation of excellent CDs, raw material selection, and CDs formation mechanism.

Revealing the nature of optical activity in carbon dots produced from different chiral precursor molecules

Carbon dots (CDs) are light-emitting nanoparticles that show great promise for applications in biology and medicine due to the ease of fabrication, biocompatibility, and attractive optical properties. Optical chirality, on the other hand, is an intrinsic feature inherent in many objects in nature, and it can play an important role in the formation of artificial complexes based on CDs that are implemented for enantiomer recognition, site-specific bonding, etc. We employed a one-step hydrothermal synthesis to produce chiral CDs from the commonly used precursors citric acid and ethylenediamine together with a set of different chiral precursors, namely, L-isomers of cysteine, glutathione, phenylglycine, and tryptophan. The resulting CDs consisted of O,N-doped (and also S-doped, in some cases) carbonized cores with surfaces rich in amide and hydroxyl groups; they exhibited high photoluminescence quantum yields reaching 57%, chiral optical signals in the UV and visible spectral regions, and two-photon absorption. Chiral signals of CDs were rather complex and originated from a combination of the chiral precursors attached to the CD surface, hybridization of lower-energy levels of chiral chromophores formed within CDs, and intrinsic chirality of the CD cores. Using DFT analysis, we showed how incorporation of the chiral precursors at the optical centers induced a strong response in their circular dichroism spectra. The optical characteristics of these CDs, which can easily be dispersed in solvents of different polarities, remained stable during pH changes in the environment and after UV exposure for more than 400 min, which opens a wide range of bio-applications.

Mulberry-Leaves-Derived Red-Emissive Carbon Dots for Feeding Silkworms to Produce Brightly Fluorescent Silk

Fluorescent silk has promising applications in dazzling textiles, biological engineering, and medical products, but the natural Bombyx mori silk has almost no fluorescence. Here carbon dots (CDs) made from mulberry leaves are reported, which have a strong near-infrared fluorescence with absolute quantum yield of 73% and a full width at half maximum of 20 nm. After feeding with such CDs, silkworms exhibit bright red fluorescence, grow healthily, cocoon normally, and turn to moths finally. The cocoons are pink in daylight and show bright red fluorescence under ultraviolet light. After breaking out of such cocoons, the red-emissive moths can mate and lay fluorescent eggs which would hatch normally. The growth cycle of the second generation of the test silkworm is the same as that of the control group, which means such CDs have excellent biocompatiblility. Dissection and analyses on both the test silkworms and cocoons disclose the metabolic route of the CDs, that is, the fluorescent CDs are absorbed by silkworms from alimentary canals, then transferred to silk glands, and finally to cocoons, while those unabsorbed CDs are excreted with the feces. All experimental results confirm the excellent biocompatibility and fluorescence stability of such CDs.

Chiral carbon dots: synthesis, optical properties, and emerging applications

Carbon dots are luminescent carbonaceous nanoparticles that can be endowed with chiral properties, making them particularly interesting for biomedical applications due to their low cytotoxicity and facile synthesis. In recent years, synthetic efforts leading to chiral carbon dots with other attractive optical properties such as two-photon absorption and circularly polarized light emission have flourished. We start this review by introducing examples of molecular chirality and its origins and providing a summary of chiroptical spectroscopy used for its characterization. Then approaches used to induce chirality in nanomaterials are reviewed. In the main part of this review we focus on chiral carbon dots, introducing their fabrication techniques such as bottom-up and top-down chemical syntheses, their morphology, and optical/chiroptical properties. We then consider emerging applications of chiral carbon dots in sensing, bioimaging, and catalysis, and conclude this review with a summary and future challenges.

An Eco-Friendly Synthetic Approach for Copper Nanoclusters and Their Potential in Lead Ions Sensing and Biological Applications

A new preparation route for high-luminescent blue-emission pepsin copper nanoclusters (Pep-CuNCs) is introduced in this work. The synthesized nanoclusters are based on a pepsin molecule, which is a stomach enzyme that works to digest proteins that exist in undigested food. Here, we have developed an eco-friendly technique through microwave-assisted fast synthesis. The resulting copper nanoclusters (CuNCs) exhibit significant selectivity towards Pb(II) ions. The pepsin molecule was utilized as a stabilizer and reducing agent in the production procedure of Pep-CuNCs. The characteristics of the resulting Pep-CuNCs were studied in terms of size, surface modification, and composition using various sophisticated techniques. The CuNCs responded to Pb(II) ions through the fluorescence quenching mechanism of the CuNCs' fluorescence. Thus, great selectivity of Pep-CuNCs towards Pb(II) ions was observed, allowing sensitive determination of this metal ion at lab-scale and in the environment. The CuNCs have detection limits for Pb(II) in very tenuous concentration at a nanomalar scale (11.54 nM). The resulting Pep-CuNCs were utilized significantly to detect Pb(II) ions in environmental samples. Additionally, the activity of Pep-CuNCs on different human tumor cell lines was investigated. The data for the observed behavior indicate that the Pep-CuNCs displayed their activity against cancer cells in a dose dependent manner against most utilized cancer cell lines.

Giant two-photon absorption in MXene quantum dots

Looking for materials with compelling nonlinear optical (NLO) response is of great importance for next-generation nonlinear nanophotonics. We demonstrate an escalated two-photon absorption (TPA) in ultrasmall niobium carbide quantum dots (Nb₂C QDs) that is induced by a two-even-parity states transition. The TPA response of Nb₂C QDs was observed in the near-infrared band of 1064-1550 nm. Surprisingly, at 1064 nm, Nb₂C QDs shows an enhanced TPA response than other wavelengths with a nonlinear absorption coefficient up to a value of 0.52 ± 0.05 cm/GW. Additionally, the nonlinear optical response of Nb₂C changes to saturable absorption when the incident wavelength is between 400-800 nm wavelength. Density functional theory (DFT) validates that TPA, induced by two even-parity states transition, breaks the forbidden single-photon transition, enabling a tremendous TPA response in Nb₂C QDs at 1064 nm. It offers the possibility of manipulating the NLO response of Nb₂C via morphology or surface termination.

Carbon-Based Nanomaterials for Sustainable Agriculture: Their Application as Light Converters, Nanosensors, and Delivery Tools

Nano-enabled agriculture is now receiving increasing attentions. Among the used nanomaterials, carbon-based nanomaterials are good candidates for sustainable agriculture. Previous review papers about the role of carbon-based nanomaterials in agriculture are either focused on one type of carbon-based nanomaterial or lack systematic discussion of the potential wide applications in agriculture. In this review, different types of carbon-based nanomaterials and their applications in light converters, nanosensors, and delivery tools in agriculture are summarized. Possible knowledge gaps are discussed. Overall, this review helps to better understand the role and the potential of carbon-based nanomaterials for nano-enabled agriculture.

Carbon dots/ZnO quantum dots composite-based white phosphors for white light-emitting diodes

A facile synthetic approach to prepare environmentally friendly white fluorescent carbon dots (CDs)/ZnO quantum dots (QDs) composites through electrostatic force is described. This method can be used for fabrication of high-performance white light-emitting diodes (WLEDs). The fluorescence emission spectra of WLED devices covered a range from 425 nm to 750 nm. WLEDs had a CIE chromaticity coordinate of (0.30, 0.34) and a color temperature of 7093 K.

Carbon Dots with an Emission in the Near Infrared Produced from Organic Dyes in Porous Silica Microsphere Templates

Carbon dots (CDs) with an emission in the near infrared spectral region are attractive due to their promising applications in bio-related areas, while their fabrication still remains a challenging task. Herein, we developed a template-assisted method using porous silica microspheres for the formation of CDs with optical transitions in the near infrared. Two organic dyes, Rhodamine 6G and IR1061 with emission in the yellow and near infrared spectral regions, respectively, were used as precursors for CDs. Correlation of morphology and chemical composition with optical properties of obtained CDs revealed the origin of their emission, which is related to the CDs’ core optical transitions and dye-derivatives within CDs. By varying annealing temperature, different kinds of optical centers as derivatives of organic dyes are formed in the microsphere’s pores. The template-assisted method allows us to synthesize CDs with an emission peaked at 1085 nm and photoluminescence quantum yield of 0.2%, which is the highest value reported so far for CDs emitting at wavelengths longer than 1050 nm.

Anhydride-Terminated Solid-State Carbon Dots with Bright Orange Emission Induced by Weak Excitonic Electronic Coupling

In this work, fluorescent solid carbon dots (CDs) welcome a new member, namely anhydride-terminated CDs, which have a photoluminescence quantum yield (PLQY) of 28% for orange-emitted CDs at 580 nm in powder form. For the first time, we revealed that the electronic coupling of the functional groups should be a crucial factor affecting the optical properties of solid CDs. Due to the negligible hydrogen bonding interaction between the anhydride groups, the electronic coupling of excitons between neighboring anhydride groups is weak, leading to a high PLQY of 28% and an immobile emission peak at 580 nm in solid state. Anhydride-terminated CDs can be partly converted into carboxyl-terminated CDs after dispersion in ethanol. However, the strong electronic coupling of carboxyl groups at high concentration generates the stacking mode of J-aggregates, giving rise to a red-shifted emission from 450 to 515 nm as well as quenched fluorescence in solid state. In comparison, a useful blue emission for solid-state CDs occurs from low sp² hybridized carbon atoms, which possess weak electronic coupling and a stationary emission band at 450 nm in both solution and solid state. By adjusting the feed ratio of the reactants, the relevant intensities between the emission from low sp² hybridized carbon atoms at 450 nm and the emission from anhydride groups at 580 nm can be controlled. As a result, single-component anhydride-terminated CD powder with tunable emission color from orange to white light can be achieved. As-prepared anhydride-terminated CDs can be used for fabricating light-emitting diodes (LEDs), white LEDs, and luminescent solar concentrators (LSCs).

Solution-processable carbon dots with efficient solid-state red/near-infrared emission

Carbon dots (CDs) emerge as promising luminescent materials for potential applications in optoelectronics on basis of their merits including low cost, eco-friendliness and strong, color-tunable photoluminescence (PL). However, the research on solid-state emissive CDs is still at the primary stage because of the aggregation-caused quenching (ACQ) of PL and their poor film-formation ability. In this work, we produce CDs with branched-polyethylenimine (b-PEI) chemically functionalized on the surfaces. The thus newly synthesized P-CDs successfully overcome the bottleneck of ACQ effect and display efficient red and NIR emission in aggregate state. Under the excitation of 520 nm, a strong red emission (maxima of 640 nm) with a high photoluminescence quantum yield (PLQY) of 21% was observed for the P-CDs in neat film. Moreover, this design strategy endows the P-CDs with good film-formation ability via solution spin-coating, which significantly increases its value for the film-based optoelectronic devices.

Preparation, characterization and cell labelling of strong pH-controlled bicolor fluorescence carbonized polymer dots

As a class of important carbon nanomaterial, carbonized polymer dots (CPDs), also called carbon dots (CDs), have aroused wide interest owing to their unique water solubility, fluorescence properties, and rich surface functional groups. However, the directional tuning of the fluorescence properties of CPDs remains incomplete because of the influence of many factors like diameter, solvent and surface groups. Particularly, most carbonized polymer dots are synthesized in a neutral pH environment. Herein, by modulating the pH (strongly acidic or alkaline) of dextrin water solution, bicolor fluorescence emission (blue and yellow) CPDs were prepared by a hydrothermal reaction. Through systematic characterization, it was found that the different fluorescence properties are regulated by the diameters and surface groups of the carbon cores. Simultaneously, the pH value affected the nucleation process. Based on the excellent fluorescence properties, cell fluorescence imaging and cytotoxicity were tested. The bicolor fluorescence CPDs obtained by tuning the pH provide an important theoretical basis for the design of broadband CPDs.

Multicolor fluorescence N-doped CPDs from dextrin water solution in strong acidic and alkaline environments were synthesized and characterized, which revealed that pH value plays a vital role in the process of CPD growth.

Carbon dot composites for bioapplications: a review

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