Formation and Band Gap Tuning Mechanism of Multicolor Emissive Carbon Dots from m-Hydroxybenzaldehyde

Recent advances in fluorescence and afterglow of CDs in matrices

Carbon dots (CDs) are novel nanomaterials with dimensions less than 10 nm that have attracted much attention due to their outstanding optical properties. However, the development of solid-state fluorescence and afterglow methods has been relatively slow, although the properties of these materials under liquid conditions have been extensively studied. In recent years, embedding CDs in a matrix has been shown to prevent aggregation quenching and inhibit nonradiative transitions, thus realizing solid-state fluorescence and afterglow, which has greatly broadened the research and application areas of CDs. In terms of hydrogen bonding, ionic bonding, covalent bonding and spatial confinement, the interactions between CDs and matrices can effectively realize and improve the solid-state fluorescence and afterglow effects of CDs. Recent applications of CDs in matrices in optoelectronics, information security, sensing, biotherapeutics and imaging are also summarized. Finally, we summarize the challenges and developments of CDs in matrices.

Hydrothermal synthesis of modified lignin-based carbon dots derived from biomass waste for fluorescence determination of valsartan

Recently, carbon dots (CDs) have been extensively investigated as potential tools for numerous applications. Modified lignin-based CDs have been synthesized and used in the field of drug detection. They were found to be highly selective and sensitive to valsartan (VAL). Using a simple hydrothermal method, phosphorus and chlorine co-doped CDs were synthesized using lignin extracted from date seeds. The fluorescence properties of the synthesized CDs are influenced by several factors, which were investigated in detail. The optimal synthesis conditions were 1.50 g of lignin, 18 mL of 2 M NaOH, 1 mM H3PO4, 3 mM HCl and the mixture was heated at 220 °C for 16 hours. The synthesized lignin-based CDs have excellent FL properties and are well soluble in water with reasonable stability. Characterization of the prepared CDs revealed that they have various functional groups with a graphene oxide-like structure. The developed CDs show a good quantum yield of 37.7%. The FL of the CDs is quenched by VAL at λ em 313 nm after λ ex at 275 nm by a combination of static and dynamic quenching mechanisms. The response of VAL was linear in the range of 4.0-100.0 μg mL-1. The detection and quantification limits of VAL were 1.23 and 3.71 μg mL-1, respectively. The nanoprobe was successfully used to analyze VAL in drug samples and provided satisfactory results.

Solid-State Red Carbon Dots Based on Biomass Furan Derivatives

In the preparation of carbon dots (CDs), precursors are crucial, and abundant precursors endow CDs with various structures and fluorescence characteristics. Furan (FU) and its derivatives are considered excellent carbonization materials due to their π conjugated structures and active functional groups, such as hydroxyl and aldehyde groups. Herein, we prepare FU-derivative-based CDs by a solvothermal method and investigate the influences of the precursor structure on the fluorescence characteristics. Surprisingly, CDs prepared from 5-hydroxymethylfurfural (HMF) with both aldehyde and hydroxyl groups exhibit red-shifted fluorescence characteristics in the solid state. We postulate that this solid-state fluorescence characteristic is due to the enhancement of supramolecular cross-linking fluorescence between CDs. The unique precursor structure leads to carboxyl groups on the surface of HMF-CDs that are conducive to the hydrogen bond formation. As the concentration of CDs increases, the hydrogen bonding effect increases, leading to a red-shift in the fluorescence wavelength. Therefore, basically full-color CDs/poly(vinyl alcohol) (PVA) phosphor-based light-emitting diodes can be achieved by controlling the degree of supramolecular cross-linking of CDs in PVA. This research provides a new approach for the preparation of solid-state luminescent CDs.

Machine learning-guided realization of full-color high-quantum-yield carbon quantum dots

Carbon quantum dots (CQDs) have versatile applications in luminescence, whereas identifying optimal synthesis conditions has been challenging due to numerous synthesis parameters and multiple desired outcomes, creating an enormous search space. In this study, we present a novel multi-objective optimization strategy utilizing a machine learning (ML) algorithm to intelligently guide the hydrothermal synthesis of CQDs. Our closed-loop approach learns from limited and sparse data, greatly reducing the research cycle and surpassing traditional trial-and-error methods. Moreover, it also reveals the intricate links between synthesis parameters and target properties and unifies the objective function to optimize multiple desired properties like full-color photoluminescence (PL) wavelength and high PL quantum yields (PLQY). With only 63 experiments, we achieve the synthesis of full-color fluorescent CQDs with high PLQY exceeding 60% across all colors. Our study represents a significant advancement in ML-guided CQDs synthesis, setting the stage for developing new materials with multiple desired properties.

Crystallization-induced emission from F-doped carbon dots

Herein, F-doped CDs with bright red SSF were synthesized by a solvothermal method using trifluoroethanol as the solvent and m-hydroxybenzaldehyde as the carbon source. Strong F-F interactions are vital for inducing crystallization, and solid luminescence is achieved by blocking the nonradiative energy dissipation pathways of crystalline organizations.

Application of Multicolor Fluorescent Carbon Dots Based on Tea Polyphenols in a White Light-Emitting Diode and Room-Temperature Phosphorescence

Carbon dots (CDs) are new carbon nanomaterials, among which those prepared from biomass are popular due to their excellent optical properties and environmental friendliness. As representative natural phenolic compounds, tea polyphenols are ideal precursors with fluorescent aromatic rings and phenolic hydroxyl structures. Usually, polyphenolic precursors can only be used to produce blue or green fluorescent CDs, and fluorescence in long wavelength domains, such as orange or red, cannot be achieved. Herein, the high reactivity of the phenolic hydroxyl groups in tea polyphenols with o-phthalaldehyde was exploited to modulate the pH during the carbonation process, which led to redshifts of the fluorescence wavelengths. Different pH values during the reaction caused the precursors to take different reaction paths and form fluorescent groups exhibiting different conjugated structures, resulting in carbon dots providing different fluorescent colors. Finally, by utilizing the in situ hydrolysis of ethyl orthosilicate, the tea polyphenol-based carbon dots were embedded into a silica matrix, inducing phosphorescence of the carbon dots. This study provides a new approach for green preparation and application of natural polyphenolic CDs.

Room temperature cost-effective synthesis of carbon quantum dots for fluorescence pattern recognition of metal ions

Herein, a type of low-consuming carbon quantum dot (CD) has been synthesized at room temperature in just 45 minutes via Schiff base reaction between o-phthalaldehyde (OPA) and polyethyleneimine (PEI). These CDs are pH-dependent, so a novel label-free florescent sensor array can be constructed by utilizing buffers with various pH levels, which leads to distinctive fluorescence response patterns upon being challenged with metal ions for their pattern recognition. The results demonstrate that large-scale detection of metal ions can be achieved with as little as 3 types of sensors. Additionally, the sensors are able to discriminate between various metal ion concentrations and mixtures of different metal ions. The technique demonstrates potential uses in water quality monitoring by promising straightforward, quick, sensitive, and potent discrimination of metal ions.

Photothermal Carbon Dots Chelated Hydroxyapatite Filler: High Photothermal Conversion Efficiency and Enhancing Adhesion of Hydrogel

The synthesis of photothermal carbon/hydroxyapatite composites poses challenges due to the binding modes and relatively low photothermal conversion efficiency. To address these challenges, the calcium ions chelated by photothermal carbon dots (PTC-CDs) served as the calcium source for the synthesis of photothermal carbon dots chelated hydroxyapatite (PTC-HA) filler via the coprecipitation method. The coordination constant K and chelation sites of PTC-HA were 7.20 × 102 and 1.61, respectively. Compared to PTC-CDs, the coordination constant K and chelation sites of PTC-HA decreased by 88 and 35% due to chelating to hydroxyapatite, respectively. PTC-HA possesses fluorescence and photothermal performance with a 62.4% photothermal conversion efficiency. The incorporation of PTC-HA filler significantly enhances as high as 76% the adhesion performance of the adhesive hydrogel. PTC-HA with high photothermal conversion efficiency and enhancing adhesion performance holds promise for applications in high photothermal conversion efficiency, offering tissue adhesive properties and fluorescence capabilities to the hydrogel.

Red emissive carbon dots: a promising next-generation material with intracellular applicability

The accidental discovery of carbon dots (CDs) back in 2004 has led to their widespread use in the biomedical field. CDs have demonstrated their effectiveness in reporting 3D structures of biological specimens, identifying normal and cancer cells, and even detecting analytes within cells. However, the limitations of blue-green emitting CDs, such as their shallow penetration, photodamage, and auto-fluorescence, have hindered their practical applications. To overcome these limitations, red emissive CDs (RCDs) have been developed, which have deep tissue penetration, minimal photo-damage, low auto-fluorescence, and high imaging contrast. In this article, we present a thorough review on the use of RCDs in biomedical applications, including in vivo and in vitro bioimaging, photoacoustic imaging, monitoring temperature and polarity changes in living cells, tumour therapy, and drug delivery. With the rapid progress being made in the development of RCDs for intracellular applications, their clinical application is expected to become a reality in the near future.

Ultra-Narrow-Bandwidth Deep-Red Electroluminescence Based on Green Plant-Derived Carbon Dots

Deep-red light-emitting diodes (DR-LEDs, >660 nm) with high color-purity and narrow-bandwidth emission are promising for full-color displays and solid-state lighting applications. Currently, the DR-LEDs are mainly based on conventional emitters such as organic materials and heavy-metal based quantum dots (QDs) and perovskites. However, the organic materials always suffer from the complicated synthesis, inferior color purity with full-width at half-maximum (FWHM) more than 40 nm, and the QDs and perovskites still suffer from serious problems related to toxicity. Herein, this work reports the synthesis of efficient and high color-purity deep-red carbon dots (CDs) with a record narrow FWHM of 21 nm and a high quantum yield of more than 50% from readily available green plants. Moreover, an exciplex host is further established using a polymer and small molecular blend, which has been shown to be an efficient strategy for producing high color-purity monochrome emission from deep-red CDs via Förster energy transfer (FET). The deep-red CD-LEDs display high color-purity with Commission Internationale de l'Eclairage (CIE) coordinates of (0.692, 0.307). To the best of the knowledge, this is the first report of high color-purity CD-LEDs in the deep-red region, opening the door for the application of CDs in the development of high-resolution light-emitting display technologies.

Synthetic strategies, properties and sensing application of multicolor carbon dots: recent advances and future challenges

Recently, carbon dots (CDs) as newly developed carbon-based nanomaterials due to advantages such as excellent photostability and easy surface functionalization have generated wide application prospects in fields such as biological imaging and chemical sensing. The multicolor emission carbon dots (M-CDs) were acquired through the selection of different carbon source precursors, change of synthesis conditions and synthesis environment. Therefore, the aim of this review is to summarize the latest research progress in polychromatic CDs from the perspectives of synthesis strategies, luminescent mechanisms, luminescent properties and applications. This review focuses on how to prepare MCDs by changing raw materials and synthesis conditions such as reaction temperature, synthesis time, synthesis pH, and synthesis solvent. This review also presents the optical properties of MCDs, concentration effects, solvent effects, pH effects, elemental doping, and surface passivation on them, as well as their creative applications in the field of sensing applications. It is anticipated that this review will serve as a guide for the development of multifunctional M-CDs and inspire future research on controllable design and preparation of M-CDs.

Preparation of Multicolor Fluorescent Carbon Dots Based on Catechol and o-Phthalaldehyde

As the foremost category of carbon materials, carbon dots (CDs) have been extensively applied in many domains because of their special fluorescence features and outstanding biocompatibility. However, in early studies of fluorescent CDs, as the fluorescence wavelength of most CDs was restricted to the blue or green region and was excitation dependent, the application of CDs was limited. In this study, three representative CDs, fluorescing yellow, green, and blue, were synthesized under alkaline, neutral, and acidic circumstances, respectively, while using a hydrothermal method in which catechol and phthalaldehyde acted as carbon sources and methanol functioned as the reaction solvent. The carbon nuclei of the three fluorescent CDs all had comparable graphite structures. The diversity of photoluminescence (PL) emission from these three CDs was attributed mainly to the different sizes of the sp² conjugated structures among them. Mixing synthesized CDs with epoxy resin, three colors (yellow, green, and blue) of LED using CIE coordinates (0.40, 0.44), (0.33, 0.46), and (0.21, 0.22), respectively, were successfully prepared.