The role of molecular fluorophores in the photoluminescence of carbon dots derived from citric acid: current state-of-the-art and future perspectives

The Emerging Star of Carbon Luminescent Materials: Exploring the Mysteries of the Nanolight of Carbon Dots for Optoelectronic Applications

Carbon dots (CDs), a class of carbon-based nanomaterials with dimensions less than 10 nm, have attracted significant interest since their discovery. They possess numerous excellent properties, such as tunability of photoluminescence, environmental friendliness, low cost, and multifunctional applications. Recently, a large number of reviews have emerged that provide overviews of their synthesis, properties, applications, and their composite functionalization. The application of CDs in the field of optoelectronics has also seen unprecedented development due to their excellent optical properties, but reviews of them in this field are relatively rare. With the idea of deepening and broadening the understanding of the applications of CDs in the field of optoelectronics, this review for the first time provides a detailed summary of their applications in the field of luminescent solar concentrators (LSCs), light-emitting diodes (LEDs), solar cells, and photodetectors. In addition, the definition, categories, and synthesis methods of CDs are briefly introduced. It is hoped that this review can bring scholars more and deeper understanding in the field of optoelectronic applications of CDs to further promote the practical applications of CDs.

Engineering Nitrogen-Doped Carbon Quantum Dots: Tailoring Optical and Chemical Properties through Selection of Nitrogen Precursors

The process of N-doping is frequently employed to enhance the properties of carbon quantum dots. However, the precise requirements for nitrogen precursors in producing high-quality N-doped carbon quantum dots (NCQDs) remain undefined. This research systematically examines the influence of various nitrogen dopants on the morphology, optical features, and band structure of NCQDs. The dots are synthesized using an efficient, eco- friendly, and rapid continuous hydrothermal flow technique. This method offers unparalleled control over synthesis and doping, while also eliminating convention-related issues. Citric acid is used as the carbon source, and urea, trizma base, beta-alanine, L-arginine, and EDTA are used as nitrogen sources. Notably, urea and trizma produced NCQDs with excitation-independent fluorescence, high quantum yields (up to 40%), and uniform dots with narrow particle size distributions. Density functional theory (DFT) and time-dependent DFT modelling established that defects and substituents within the graphitic structure have a more significant impact on the NCQDs' electronic structure than nitrogen-containing functional groups. Importantly, for the first time, this work demonstrates that the conventional approach of modelling single-layer structures is insufficient, but two layers suffice for replicating experimental data. This study, therefore, provides essential guidance on the selection of nitrogen precursors for NCQD customization for diverse applications.

Carbon dots on LAPONITE® hybrid nanocomposites: solid-state emission and inter-aggregate energy transfer

This study delves into the photoluminescent characteristics of solid-state hybrid carbon dots/LAPONITE® (CDLP). These hybrid materials were synthesized using the hydrothermal method with a precise pH control set at 8.5. The LAPONITE® structure remains intact without structural collapse, and we detected the possible deposition of carbon dots (CDs) aggregates on the clay mineral's edges. The use of different concentrations of citric acid (10-, 6-, 2- and 1-times weight/weight of LAPONITE® mass, maintaining the 1 : 1 molar ratio with ethylenediamine) during synthesis results in different CDs concentrations in CDLP-A (low precursors concentration) and CDLP-D (high concentration) with an amorphous structure and average size around 2.8-3.0 nm. The CDLP displayed visible photoluminescence emission in aqueous and powder, which the last underwent quenching according to lifetimes and quantum yield measurements. Low-temperature measurements revealed an enhancement of the non-radiative pathways induced by aggregation. Energy transfer modelling based on Förster-Dexter suggests an approximate mean distance of 9.5 nm between clusters of CDs.

Red-Emissive Center Formation within Carbon Dots Based on Citric Acid and Formamide

The formation of red-emissive optical centers in carbon dots based on citric acid and formamide was investigated by varying the synthesis parameters with focus on finding optimal─necessary and sufficient─amount of precursors to decrease byproduct amount and to increase the chemical yield of red-emissive carbon dots. The emission is observed at 640 nm excited at 590 nm and quantum yield reaches up 19%. A high chemical yield of carbon dots of 26% was achieved at an optimal molar ratio of citric acid to formamide of 1:4.

Biomass derived green carbon dots for sensing applications of effective detection of metallic contaminants in the environment

The rapid consumption of metals and unorganized disposal have led to unprecedented increases in heavy metal ion concentrations in the ecosystem, which disrupts environmental homeostasis and results in agricultural biodiversity loss. Mitigation and remediation plans for heavy metal pollution are largely dependent on the discovery of cost-effective, biocompatible, specific, and robust detectors because conventional methods involve sophisticated electronics and sample preparation procedures. Carbon dots (CDs) have gained significant importance in sensing applications related to environmental sustainability. Fluorescence sensor applications have been enhanced by their distinctive spectral properties and the potential for developing efficient photonic devices. With the recent development of biomass-functionalized carbon dots, a wide spectrum of multivalent and bivalent transition metal ions responsible for water quality degradation can be detected with high efficiency and minimal toxicity. This review explores the various methods of manufacturing carbon dots and the biochemical mechanisms involved in metal detection using green carbon dots for sensing applications involving Cu (II), Fe (III), Hg (II), and Cr (VI) ions in aqueous systems. A detailed discussion of practical challenges and future recommendations is presented to identify feasible design routes.

Eutectic Media Open a Synthetic Route to Oligocitrazinic Acid Fluorophores of Purple Hue

Under isochoric and solvent-free conditions, the reaction between ammonium formate and citric acid results in a deeply purple reaction product with fluorescent properties. This brings this reaction in the realm of bio-based fluorophores and bottom-up carbon nanodots from citric acid. The reaction conditions are optimized in terms of UV-vis spectroscopic properties and, subsequently, the main reaction product is separated. While the structural analysis does not give any indication for carbon nanodots in a general sense, it points towards the formation of molecular fluorophores that consist of oligomerized citrazinic acid derivatives. Furthermore, EPR spectroscopy reveals the presence of stable free radicals in the product. We hypothesize that such open-shell structures may play a general role in molecular fluorophores from citric acid and are not yet sufficiently explored. Therefore, we believe that analysis of these newly discovered fluorophores may contribute to a better understanding of the properties of fluorophores and CND from citric acid in general.

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.

Carbon Dots from Lycium barbarum Attenuate Radiation-Induced Bone Injury by Inhibiting Senescence via METTL3/Clip3 in an m6A-Dependent Manner

Radiation-induced bone injury management remains a challenge in clinical practice, and there is no effective medicine. Recently, biomass-derived carbon dots (CDs) have attracted attention in biomedical engineering due to the advantages of abundant heteroatoms, low toxicity, and no need to drug loading. Here, we report that CDs, synthesized from Lycium barbarum via hydrothermal strategy, can effectively alleviate radiation-induced bone injury. CCK-8, apoptosis analysis, β-galactosidase staining, quantitative polymerase chain reaction, and western blots demonstrate that CDs can mediate radiation-induced damage and senescence of bone marrow mesenchymal stem cells (BMSCs). CDs regulate osteogenic- and adipogenic-balance after irradiation, shown by alizarin red and oil red O staining. In vivo experiments reveal that CDs prevent the occurrence of osteoradionecrosis in rats, demonstrated by micro-CT and histology examination. The osseointegration of titanium implants installed in irradiated bone is promoted by CDs. Mechanistically, CDs increase the N6-methyladenosine (m6A) level of irradiated BMSCs via the increased methyltransferase-like 3 (METTL3). High-throughput sequencing facilitates detection of increased m6A levels located in the 3'-untranslated regions (UTR) of the CAP-Gly domain containing linker protein 3 (Clip3) mRNA. The dual-luciferase reporter assay shows that 3'UTR is the direct target of METTL3. Subsequently, the increased m6A modification led to enhanced degradation of mRNA and downregulated CLIP3 expression, eventually resulting in the alleviation of radiation-induced bone injury. Interfering with the METTL3/Clip3 axis can antagonize the effect of CDs, indicating that CDs mediate radiation-induced bone injury via the METTL3/Clip3 axis. Taken together, CDs from L. barbarum alleviate radiation-induced bone injury by inhibiting senescence via regulation of m6A modification of Clip3. The present study paves a new pathway for the management of radiation-induced bone injury.

Poly(octamethylene citrate) Modified with Glutathione as a Promising Material for Vascular Tissue Engineering

One of the major goals of vascular tissue engineering is to develop much-needed materials that are suitable for use in small-diameter vascular grafts. Poly(1,8-octamethylene citrate) can be considered for manufacturing small blood vessel substitutes, as recent studies have demonstrated that this material is cytocompatible with adipose tissue-derived stem cells (ASCs) and favors their adhesion and viability. The work presented here is focused on modifying this polymer with glutathione (GSH) in order to provide it with antioxidant properties, which are believed to reduce oxidative stress in blood vessels. Cross-linked poly(1,8-octamethylene citrate) (cPOC) was therefore prepared by polycondensation of citric acid and 1,8-octanediol at a 2:3 molar ratio of the reagents, followed by in-bulk modification with 0.4, 0.8, 4 or 8 wt.% of GSH and curing at 80 °C for 10 days. The chemical structure of the obtained samples was examined by FTIR-ATR spectroscopy, which confirmed the presence of GSH in the modified cPOC. The addition of GSH increased the water drop contact angle of the material surface and lowered the surface free energy values. The cytocompatibility of the modified cPOC was evaluated in direct contact with vascular smooth-muscle cells (VSMCs) and ASCs. The cell number, the cell spreading area and the cell aspect ratio were measured. The antioxidant potential of GSH-modified cPOC was measured by a free radical scavenging assay. The results of our investigation indicate the potential of cPOC modified with 0.4 and 0.8 wt.% of GSH to produce small-diameter blood vessels, as the material was found to: (i) have antioxidant properties, (ii) support VSMC and ASC viability and growth and (iii) provide an environment suitable for the initiation of cell differentiation.

Recent progress of second near-infrared (NIR-II) fluorescence microscopy in bioimaging

Visualizing biological tissues in vivo at a cellular or subcellular resolution to explore molecular signaling and cell behaviors is a crucial direction for research into biological processes. In vivo imaging can provide quantitative and dynamic visualization/mapping in biology and immunology. New microscopy techniques combined with near-infrared region fluorophores provide additional avenues for further progress in vivo bioimaging. Based on the development of chemical materials and physical optoelectronics, new NIR-II microscopy techniques are emerging, such as confocal and multiphoton microscopy, light-sheet fluorescence microscopy (LSFM), and wide-field microscopy. In this review, we introduce the characteristics of in vivo imaging using NIR-II fluorescence microscopy. We also cover the recent advances in NIR-II fluorescence microscopy techniques in bioimaging and the potential for overcoming current challenges.