Aggregation-induced bimodal excitation of nitrogen-doped carbon dots for ratiometric sensing of new coccine and solid-state multicolor lighting

Ultra-thin Nb2O5 nanosheets construct 3D cross-linked architecture: Unraveling new coccine degradation pathways and toxicity changes

Photocatalytic technology offers a promising approach to address environmental and health challenges posed by the food colorant new coccine (NC). Nb2O5 is a notable candidate due to its stability and environmental compatibility, but faces limitations such as limited active sites and rapid charge carrier recombination. In the present study, we report a novel Nb2O5 catalyst featuring a three-dimensional (3D) cross-linked architecture constructed from ultra-thin nanosheets, with a catalyst thickness of less than 2 nm. This innovative structure offers an eminent superficial surface area combined with a substantial abundance of active sites, making it an efficient photocatalyst for the degradation of NC. The Nb2O5 3D catalyst demonstrated a remarkable degradation rate of 90.1 % for NC within just 30 min, accompanied by a rate constant of 73.5 × 10-3 min-1. This performance significantly surpasses that of three alternative Nb2O5 catalysts with varying morphologies (nanorods, nanoparticles, and nanospheres), which show rate constants more than seven times lower. Furthermore, we explore the degradation pathways associated with NC and provide a thorough examination of the toxicity changes occurring in its by-products. This work presents a promising framework for the development of advanced catalysts capable of effectively degrading NC, thereby contributing to the advancement of environmentally sustainable practices in the management of food colorants.

Concentration dependent carbon nanodots: Tunable luminescent color and fluorescence excitation-wavelength dependence

Carbon nanodots (CNDs) exhibiting concentration dependent properties have been synthesized through a one-pot hydrothermal reaction process utilizing diethylenetriamine and l-aspartic acid. At solid-state or high concentrations, the CNDs display excitation-wavelength independent fluorescence (FL) emissions, while at low concentrations, they exhibit excitation-wavelength dependent FL emissions. Detailed characterization of the structure and optical properties reveals that the concentration dependent FL properties can be ascribed to the intrinsic-state luminescence of the CNDs at low concentrations and the assembled-state luminescence at solid-state/high concentrations. With the increase of concentration, the self-assembly behavior of CNDs may cause the transition from intrinsic-state dominant luminescence to assembled-state dominant luminescence, and finally lead to the red-shift of FL color. Furthermore, the CNDs@Urea composites possess adjustable room-temperature phosphorescence (RTP) from turquoise to yellow by controlling the CNDs doping concentration. CNDs with concentration dependent optical properties have shown certain potentials in the fields of cell imaging, fingerprint recognition and anti-counterfeiting applications.

Precise Regulation Strategy for Fluorescence Wavelength of Aggregation-Induced Emission Carbon Dots

Aggregation-induced emission (AIE) carbon dot (CDs) in solid state with tunable multicolor emissions have sparked significant interest in multidimensional anti-counterfeiting. However, the realization of solid-state fluorescence (SSF) by AIE effect and the regulation of fluorescence wavelength in solid state is a great challenge. In order to solve this dilemma, the AIE method to prepare multi-color solid-state CDs with fluorescence wavelengths ranging from bright blue to red emission is employed. Specifically, by using thiosalicylic acid and carbonyl hydrazine as precursors, the fluorescence wavelength can be accurately adjusted by varying the reaction temperature from 150 to 230 °C or changing the molar ratio of the precursors from 1:1 to 1:2. Structural analysis and theoretical calculations consistently indicate that increasing the sp2 domains or doping with graphite nitrogen both cause a redshift in the fluorescence wavelength of CDs in the solid state. Moreover, with the multi-dimensional and adjustable fluorescence wavelength, the application of AIE CDs in the fields of multi-anti-counterfeiting encryption, ink printing, and screen printing is demonstrated. All in all, this work opens up a new way for preparing solid-state multi-color CDs using AIE effect, and further proposes an innovative strategy for controlling fluorescence wavelengths.

Centrifugated lateral flow assay strips based on dual-emission carbon dots modified with europium ions for ratiometric determination and on-site discrimination of tetracyclines in environment

Due to the serious detrimental impact on human health, antibiotic pollution particularly tetracyclines residues has become a serious problem. Herein, a multiple response fluorescent probe consisted of dual-emission carbon dots and Eu3+ (D-CDs@Eu3+) is designed for the determination and discrimination of tetracyclines (TCs). Specifically, the carboxyl and amidogen group of dual-emission carbon dots (D-CDs) can coordinate with Eu3+ to form the D-CDs@Eu3+. Upon adding TCs, the fluorescence intensities of D-CDs at 405 nm and 495 nm are quenched due to inner filter effect (IFE) and the localization of fluorescence resonance energy transfer (L-FRET) between the D-CDs@Eu3+ and TC. Simultaneously, the D-CDs@Eu3+ may chelate with TCs to enhance the occurrence of antenna effect, while the characteristic peaks of Eu3+ at 590 nm and 615 nm are enhanced. On these bases, the TCs detection is achieved with low detection limits from 46.7 to 72.0 nM. Additionally, through the distinct efficiencies of L-FRET, the discrimination of TCs is achieved. Moreover, a novel centrifugated lateral flow assay strips (CLFASs) device is developed by integrating the D-CDs@Eu3+, lateral flow assay strips and smartphone using RGB variations for TCs detection, achieving remarkable recoveries (98.6-103.7 %) in real samples. Therefore, this CLFASs device provides a reliable approach for the TCs detection, demonstrating potential applications.

Concentration-Dependent Photoluminescence of Carbon Quantum Dots Useable in LED

We synthesized carbon quantum dots (CQDs) using a solvothermal method with o-phenylenediamine as the carbon and nitrogen source. The sample was characterized by transmission electron microscopy, X-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy. When we continued the optical characterization of the CQDs, we were surprised to discover that the colors of the synthesized CQDs changed with the dilution of the original solution. In addition, the photoluminescence (PL) of CQDs under 405 nm continuous wave laser excitation was also investigated. It was found that CQDs with different concentrations exhibited different PL spectra. In order to explain the mechanism of different PL spectra, chemical characterization of the CQDs at different concentrations was performed again, revealing that the color change is independent of particle size and surface functional groups. Systematic optical characterization and theoretical analysis indicate that this color change results from the interparticle distance. Furthermore, we investigated the PL lifetimes of CQDs using time-resolved PL measurements and found that the PL lifetime values change with the concentration of CQDs, which is attributed to nonradiative transitions. Finally, we fabricated warm white-light-emitting diodes with CQDs that are proportionally adjusted in concentrations. The investigation developed a simple and effective method to tune the color of CQDs by adjusting the concentration through dilution of the original solution, which provides a new approach for the preparation and regulation of multicolor CQDs.

Aggregation-induced-emission red carbon dots for ratiometric sensing of norfloxacin and anti-counterfeiting

The detection of trace antibiotic residues holds significant importance because it's related to food safety and human health. In this study, we developed a new high-yield red-emitting carbon dots (R-CDs) with aggregation-induced emission properties for ratiometric sensing of norfloxacin. R-CDs were prepared in 30 min using an economical and efficient microwave-assisted method with tartaric acid and o-phenylenediamine as precursors, achieving a high yield of 34.4 %. R-CDs showed concentration-dependent fluorescence and aggregation-induced-emission properties. A ratiometric fluorescent probe for detecting the norfloxacin was developed. In the range of 0-40 μM, the intensity ratio of two emission peaks (I445 nm/I395 nm) towards norfloxacin show good linear relationship with its concentrations and a low detection limit was obtained (36.78 nM). In addition, complex patterns were developed for anti-counterfeiting based on different emission phenomenon at different concentrations. In summary, this study designed a novel ratiometric fluorescent probe for detection of norfloxacin, which greatly shortens the detection time and improves efficiency compared with high-performance liquid chromatography and other methods. The study will promote the application of carbon dots in anti-counterfeiting and other related fields, laying the foundation for the preparation of low-cost photosensitive anti-counterfeiting materials.

Carbon dots with enhanced red emission for ratiometric sensing and encryption applications

The excitation-dependent emission properties of carbon dots (Cdots) are extensively reported, but their red emission is often weak, limiting their wider application. Here we introduce ethidium bromide, as a functional precursor with red emission, to enhance the red emission for Cdots, with comparable intensity at a broad wavelength range to multi-emission Cdots (M-Cdots). We found that Cdots prepared with ethidium bromide/ethylenediamine exhibited strong blue and red emission at 440 and 615 nm, with optimal excitation at 360 and 470 nm as M-Cdots, respectively, but the Cdots from single ethidium bromide (EB-Cdots) possessed weak red emission. M-Cdots exhibited a broad absorption band at 478 nm, but a band blue-shifted to 425 nm was observed for EB-Cdots, while no absorption was observed at 478-425 nm for the Cdots prepared with citric acid and ethylenediamine. Thus, we proposed that C=O and C=N formed a π-conjugation structure as the absorption band at 478 nm for the red emission of M-Cdots, as also confirmed with the excitation at 470 nm. Moreover, the π-conjugation structure is fragile and sensitive to harsh conditions, so red emission was difficult to observe for the Cdots prepared with citric acid/ethylenediamine or single ethidium bromide. M-Cdots possess two centers for blue and red emission with different structures. The dual emission was therefore used for ratiometric sensing with dichromate (Cr2O72-) and formaldehyde (HCHO) as the targets using the intensity ratio of the emissions at 615 and 440 nm. Due to the comparable intensity at a broad wavelength range, we designed encryption codes with five excitations at 360, 400, 420, 450, and 470 nm as the inputs, and the emission colors were used for information decoding. Thus, we determined why red emission was difficult to realize for Cdots, and our results could motivate the design of red-emission Cdots for extensive applications.

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.

3D printing-based lateral flow visual assay utilizing the dual-excitation property of nitrogen-doped carbon dots for the ratiometric determination and chemometric discrimination of flavonoids

A ratiometric fluorescence sensor has been established based on dual-excitation carbon dots (D-CDs) for the detection of flavonoids (morin is chosen as the typical detecting model for flavonoids). D-CDs were prepared using microwave radiation with o-phenylenediamine and melamine and exhibit controllable dual-excitation behavior through the regulation of their concentration. Remarkably, the short-wavelength excitation of D-CDs can be quenched by morin owing to the inner filter effect, while the long-wavelength excitation remains insensitive, serving as the reference signal. This contributes to the successful design of an excitation-based ratiometric sensor. Based on the distinct and differentiated variation of excitation intensity, morin can be determined from 0.156 to 110 µM with a low detection limit of 0.156 µM. In addition, an intelligent and visually lateral flow sensing device is developed for the determination  of morin content in real samples with satisfying recoveries, which indicates the potential application for human health monitoring.

Facile Synthesis of B/P Co-Doping Multicolor Emissive Carbon Dots Derived from Phenylenediamine Isomers and Their Application in Anticounterfeiting

Carbon dots (CDs) possess a considerable number of beneficial features for latent applications in biotargeted drugs, electronic transistors, and encrypted information. The synthesis of fluorescent carbon dots has become a trend in contemporary research, especially in the field of controllable multicolor fluorescent carbon dots. In this study, an elementary one-step hydrothermal method was employed to synthesize the multicolor fluorescent carbon dots by co-doping unique phenylenediamine isomers (o-PD, m-PD, and p-PD) with B and P elements, which under 365 nm UV light exhibited signs of lavender-color, grass-color, and tangerine-color fluorescence, respectively. Further investigations reveal the distinctness in the polymerization, surface-specific functional groups, and graphite N content of the multicolor CDs, which may be the chief factor regarding the different optical behaviors of the multicolor CDs. This new work offers a route for the exploration of multicolor CDs using B/P co-doping and suggests great potential in the field of optical materials, important information encryption, and commercial anticounterfeiting labels.

Red Emitting Solid-State CDs/PVP with Hydrophobicity for Latent Fingerprint Detection

Fluorescent carbon dots (CDs) are a new type of photoluminescent nanomaterial. Solid-state CDs usually undergo fluorescence quenching due to direct π-π* interactions and superabundant energy resonance transfer. Therefore, the preparation of solid-state fluorescent CDs is a challenge, especially the preparation of long wavelength solid-state CDs. In this research, long wavelength emission CDs were successfully synthesized by solvothermal methods, and the prepared CDs showed good hydrophobicity. The composite solid-state CDs/PVP (Polyvinyl pyrrolidone) can emit strong red fluorescence, and the quantum yield (QY) of the CDs/PVP powder reaches 18.9%. The prepared CDs/PVP solid-state powder was successfully applied to latent fingerprint detection. The results indicate that the latent fingerprints developed by CDs/PVP powder have a fine definition and high contrast visualization effect, which proves that the prepared CDs/PVP has great application potential in latent fingerprint detection. This study may provide inspiration and ideas for the design of new hydrophobic CDs.

Wavelength-Tuneable Fluorescent Carbon Dots for Nucleic Acid Imaging

Nucleic acid is one of the most important substances in organisms, and its dynamic changes are closely related to physiological processes. Nucleic acid labeling is conducive to providing important information for the early diagnosis and treatment of pathophysiological processes. Here, we utilized the transfer mechanism between carbon sources and CDs to synthesize wavelength-adjustable N-CDs for the nucleic acid image. Along with the increased graphite nitrogen (from 10.6 to 30.1%) gradually by the precise design of the nitrogen structure in carbon sources (e.g., primary amines, secondary amines, tertiary amines, and liking graphite-nitrogen), the energy gap of CDs reduced, resulting in adjustable wavelength from visible to near-infrared range (from 461 nm/527 nm to 650 nm/676 nm). Furthermore, N-CDs exhibited a selective affinity for nucleic acids, especially RNA. Therefore, N-CDs support an efficient platform for real-time tracking of RNA dynamic changes in cells.

Bismuth, Nitrogen-Codoped Carbon Dots as a Dual-Read Optical Sensing Platform for Highly Sensitive, Ultrarapid, Ratiometric Detection of Doxorubicin

Doxorubicin (DOX) is a potent anticancer drug, but it has side effects on normal tissues, particularly myocardial cells. Therefore, it is crucial to detect the DOX concentration in body fluids for effective clinical treatment. In this work, N,Bi-codoped CDs (Bi,N-CDs) were synthesized through a one-step hydrothermal method to carbonize the raw materials of 2,4-dinitroaniline and bismuth nitrate. The resulting Bi,N-CDs showed a reduced emission at 490 nm and an enhanced emission at 590 nm in the presence of DOX. The ratio of fluorescence (FL) intensity (F590/F490) was found to be a reliable indicator of DOX concentration, ranging from 0.05 to 30 μM and 40-200 μM, with detection limits (LOD) of 34 and 24 nM, respectively. A ratiometric fluorescence nanoprobe was established for highly selective and sensitive detection of DOX using a specific electrostatic interaction and inner filter effect between Bi,N-CDs and DOX. Meanwhile, Bi,N-CDs exhibited a distinct color change ranging from yellow to orange-red when exposed to DOX, allowing for a colorimetric method to measure DOX levels in the range of 0.05-30 μM, with a detection limit of 169 nM. The probe was triumphantly used to monitor DOX in actual samples via a dual-mode optical sensing strategy. This study contributes to the development of heteroatom-doped CDs and expands their potential applications for detecting biological samples.

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.

Large-Scale Synthesis of Tunable Fluorescent Carbon Dots Powder for Light-Emitting Diodes and Fingerprint Identification

The emergence and fast development of carbon dots (CDs) provide an unprecedented opportunity for applications in the field of photoelectricity, but their practicability still suffers from complicated synthesis procedures and the substrate dependence of solid-state fluorescence. In this study, we design a unique microwave-assisted solid-phase synthesis route for preparing tunable fluorescent CD powders with yellow, orange, and red fluorescence (Y-CDs, O-CDs, R-CDs) by simply adjusting the mass ratio of reactants, a method which is suitable for the large-scale synthesis of CDs. The Y-/O-/R-CDs were systematically characterized using physics and spectroscopy techniques. Based on the perfect solid-state fluorescence performance of the proposed fluorescent CD powders, the Y-/O-/R-CDs were successfully applied for the construction of multi-color and white light-emitting diode devices at low cost. Furthermore, the Y-CDs displayed much higher yield and luminous efficiency than the O-CDs and R-CDs and were further used for fingerprint identification on the surfaces of glass sheets and tinfoil. In addition, the R-CD aqueous solution fluorescence is sensitive to pH, suggesting its use as a pH indicator for monitoring intracellular pH fluctuations. The proposed series of fluorescent powders composed of CDs may herald a new era in the application of optical components and criminal investigation fields.