Red-emitting Carbon Dots Research Articles

Facile synthesis of multifunctional red-emissive carbon dots for fluorescent quercetin and pH sensing

A convenient and cost-effective hydrothermal route was developed for the synthesis of red-emissive carbon dots (R-CDs) from neutral red and catechol. Superior properties of the R-CDs including high photostability, outstanding biocompatibility, and desirable biological penetration were found, which could realize the monitor of quercetin and pH. The quenching fluorescence of R-CDs was observed upon the addition of quercetin on account of a static quenching mechanism, achieving the concentration of quercetin in the scope of 0.66–111.33 μM with a detection limit of 0.19 μM. Furthermore, a smartphone based laboratory device and color processing software could be used for accurate testing of quercetin. Besides, the proposed method has been successfully quantified trace amounts of quercetin in red wine, lake water and human urine samples. Meanwhile, the R-CDs presented a pH response by both fluorescent and colorimetric mode. As the pH value increased, an excellent linear relationship was obtained against pH in the range of 6.20–7.60 accompanied with a significant color variation of the R-CDs solution from deep pink to yellow. Finally, the proposed R-CDs have also been applied to monitor the quercetin and pH in living cells due to its low cytotoxicity, providing a great capacity for various targets molecules detection.

Investigation on synthesis and luminescent properties of red-emitting carbon dots chemically functionalized by branched-polyethylenimine

Investigation on synthesis and luminescent properties of red-emitting carbon dots chemically functionalized by branched-polyethylenimine

Facile synthesis of red-emissive carbon dots with theoretical understanding for cellular imaging

Recently, red emissive carbon dots (R-CDs) have drawn widespread attention on account of their desirable fluorescence properties and good biocompatibility. Despite great efforts, facile synthesis of R-CDs for cellular imaging remains challenging and the fluorescence mechanism of R-CDs is still elusive. Herein, p-phenylenediamine-derived R-CDs with excitation-independency were successfully obtained through a facile solvothermal approach together with proportional precipitation. The fluorescent solvatochromism of R-CDs is realized, while high polarity leads to higher degree of dipole interaction between R-CDs and different solvents, favoring for emissive red-shift. Furthermore, density functional theory is adopted to explore the optical and electronic characteristics of some polycyclic aromatic molecules. Among different configurations, pyridine nitrogen and carbonyl bonds could relatively increase the charge density and significantly narrow the band gap, which can provide a crucial theoretical basis for the precise preparation of R-CDs. Moreover, R-CDs possess favorable cellular imaging ability, which indicates their potential for a promising candidate as fluorescence probes in bioimaging.

Red-Emissive Sulfur-Doped Carbon Dots for Selective and Sensitive Detection of Mercury (II) Ion and Glutathione.

Carbon dots (CDs) show great potential in bioimaging and biosensing because of their good biocompatibility and excellent optical properties. However, CDs with intense red emissions for sensitive and selective detection are rarely reported. Herein, we prepared the red-emissive carbon dots (RCDs) through a facile hydrothermal method using tetra (4-carboxyphenyl) porphyrin (TCPP) and thiourea as starting materials. The obtained RCDs were characterized by TEM, XRD, and XPS. RCDs exhibited high water solubility and strong red emission (λem = 650 nm), with the fluorescence quantum yield as high as 26.7%, which was greatly higher than that of TCPP. Moreover, the as-prepared RCDs could be acted as a highly selective and sensitive probe for the detection of Hg2+ and glutathione (GSH) through the fluorometric titration method. The detection limits of Hg2+ and GSH were calculated to be 1.73 and 1.6 nM, respectively. The cellular experiments demonstrated the good biocompatibility of RCDs and their feasibility in bioimaging. Thus, this work provided a simple strategy to design and synthesize the highly red-emissive carbon dots, which showed promising application in biological and environmental assays.

Synthesis of highly efficient green emissive carbon dots towards UV encryption fluorescent ink

Blue and red emitting carbon dots (CDs) have attracted extensive research interests, while the research on green emissive CDs (G-CDs) are often ignored. In this work, G-CDs with high photonluminescence quantum yield (PLQY) were synthesized through a microwave assisted solvothermal method, where 1-amino-2-naphthol-4-sulfonic acid (ANSA) and ethylenediamine were used as precursors, and ethanol as solvent. The chemical structure, surface composition and optical properties of G-CDs suggest that the G-CDs have a structure of carbon core coated by functional groups such as hydroxyl, sulfinyl, and amino groups. The mass fractions of C, N, O and S elements are 75.83%, 14.26%, 9.31% and 0.8%, respectively. The excitation and emission wavelengths are 439 nm and 528 nm, respectively. The G-CDs have considerable photonluminescence efficiency with QY up to 33.8%, which exceeds those of many other G-CDs. The G-CDs exhibit excitation wavelength-independent emission behavior, indicating that they possess a unique PL center. Furthermore, the possible synthesis mechanism and a simple application of the G-CDs are briefly introduced. This work is of great value for filling the gap of G-CDs with high PLQY and promoting the practical application of CDs.

Nanobody-based fluorescent immunoassay using carbon dots anchored cobalt oxyhydroxide composite for the sensitive detection of fenitrothion

Fenitrothion (FN) residue in food is a serious threat to public health. Consequently, a sensitive, cost-effective, and convenient immunoassay for FN urgently needs to be fabricated to safeguard human health. Herein, a nanobody-alkaline phosphatase fusion protein (Nb-ALP)-based fluorescent ELISA using red emissive carbon dots (r-CDs) anchored cobalt oxyhydroxide nanosheet (CoOOH NS) composite was developed for detecting FN. Briefly, a Nb-ALP was obtained by autoinduction expression and employed as a recognition, signal transduction, and amplification element. As the fluorescence signal source, r-CDs were assembled with CoOOH NS to yield the r-CDs@CoOOH NS composite, leading to the fluorescence quenching of r-CDs via Förster resonance energy transfer (FRET). After competitive immunoreaction, the Nb-ALP bounded to the immobilized antigen can mediate the production of ascorbic acid, which can reduce the CoOOH NS to Co2+, breaking the FRET between r-CDs and CoOOH NS, accompanied by the fluorescence recovery of r-CDs. This fluorescent ELISA is highly sensitive to FN with a detection limit of 0.14 ng mL−1, which is 25-fold lower than that of conventional colorimetric ELISAs. The recovery test of food samples and the validation by GC−MS/MS further demonstrated the proposed assay was an ideal tool for detecting FN.

Surface state dominated and carbon core coordinated red-emitting carbon dots for the detection of Cr2O72- and cell imaging

Cr(VI) as a toxic heavy metal ion can easily enter into the body through drinking or eating and cause liver and kidney diseases as well as cancer. Considering its high biological toxicity and adverse effects on human body, it is desirable to develop a probe to monitor its level in the environment. Herein, a high-efficiency fluorescent nanoprobe based on red emissive carbon dots (R-CDs) was established through a convenient solvothermal strategy. The as-prepared CDs with excitation-independency had the fixed emission wavelength at 627 nm when the excitation wavelength was 560 nm. Further study manifested that the new surface state formed by nitrogen and sulfur doping and the increased conjugated system established through dehydration and carbonization were the main reasons for the fluorescence redshift. In this system, these R-CDs as a fluorescent probe exhibited high specificity and sensitivity to Cr2O72- with the linear range of 4–40 μΜ and the limit of detection could reach 80.00 nM. The quenching of these CDs by Cr2O72- was efficiently induced through a static quenching process. Meanwhile, the obtained CDs could enter into HeLa cells through endocytosis and exhibit bright red fluorescence in cells under a confocal laser scanning microscope. Thus, this work provided a promising probe not only for detecting Cr(VI) in natural environment but also for imaging in cells.

Polymer-Induced Emission-Active Fluorine-Embedded Carbon Dots for the Preparation of Warm WLEDs with a High Color Rendering Index.

Exploration of many strategies has continuously contributed to producing aggregation-induced red-emissive carbon dots (CDs). In this work, we designed fluorine-embedded (F-embedded) CDs from 1,2,4-triaminobenzene, thiourea, and ammonium fluoride (NH4F) exhibiting polymer-induced emission (PIE). The PIE phenomenon of fluorescent CDs is obtained in poly(vinyl alcohol) (PVA), showing emissions at 611 and 617 nm in the dispersed and solid states, respectively. The CDs exhibited a red shift of 28 nm in the PVA solution because PVA hydroxyl groups formed a robust bridge-like H-bonding network between CDs. The fluorine embedded in CDs enhanced the H-bond affinity toward PVA. It showed that this H-bond restricted the coupling of CDs' surface states and inhibited the nonirradiation transfer. For the solid state, surface PVA chains eliminated the π-π interaction of the conjugated core and constructed a self-quenching resistance polymeric system around CDs. As a result, CDs showed an unexpected red shift of fluorescence emission in PVA. Furthermore, white light-emitting diodes (WLEDs) have a correlated color temperature (CCT) of 5232 K, and a high color rendering index of 95 has been fabricated by integrating the red- and green-emissive films over the UV LEDs. Interestingly, the as-synthesized CDs showed room temperature phosphorescence (RTP), which enabled us to employ the CDs in double-security protection. Simultaneously, CDs have been used in fingerprint detection.

Preparation of sulphuric acid-mediated N,S-codoped red emissive carbon dots: Applications in food dyes detection, solid-state luminescence and cell imaging

As diseases such as cardiovascular disease and cancer caused by food problems are more and more frequent, food safety has received great attention. Among them, the safety problem caused by food dyes is more prominent. Thus, it is of great value to develop sensitive detection methods for food dyes. In present study, sulphuric acid-mediated N,S-codoped red emissive carbon dots (namely as R-CDs) had been manufactured by using N-phenyl-o-phenylenediamine as precursor, sulfuric acid as additive for the first time. The structural and fluorescence properties of R-CDs had been systematically studied. The results demonstrated that R-CDs showed uniform spherical morphology and had a graphite-like structure, for which the average diameters size was 5.05 nm. Due to the various functional groups such as hydroxyl, pyridinic N, pyrrolic N and -C-SO4, R-CDs emitted bright red fluorescence. Importantly, because of the interactions between the functional groups of R-CDs with the selected food dyes, three dyes including amaranth, brilliant blue FCF and methylene blue can sensitively quench the fluorescence of R-CDs through IFE and static quenching effects. The linearity ranges of them were separately detected as 0.20 μM −20 μM, 10 nM-1 μM and 60 nM-8 μM. The limits of detection (LODs) of them were 70 nM, 4 nM and 20 nM, respectively. Further, R-CDs was successfully applied to the sensitive detection of three dyes from various food samples. To maximize the fluorescence properties of R-CDs, a R-CDs/PVA composite gel was fabricated to make R-CDs fluoresce in solid state condition. The potential of R-CDs/PVA composite gel for preliminary visualization analysis of three dyes was studied. Finally, ascribed to the low toxicity and good biocompatibility, the potential of R-CDs as probe for cell imaging was explored preliminarily.

Design and Use of a Gold Nanoparticle-Carbon Dot Hybrid for a FLIM-Based IMPLICATION Nano Logic Gate.

The interest in nanomaterials resides in the fact that they can be used to create smaller, faster, and more portable systems. Nanotechnology is already transforming health care. Nanoparticles are being used by scientists to target malignancies, improve drug delivery systems, and improve medical imaging. Integration of biomolecular logic gates with nanostructures has opened new paths in illness detection and therapy that need precise control of complicated components. Most studies have used fluorescence intensity techniques to implement the logic function. Its drawbacks, mainly when working with nanoparticles in intracellular media, include fluctuations in excitation power, fluorophore concentration dependence, and interference from cell autofluorescence. We suggest using fluorescence lifetime imaging microscopy (FLIM) in order to circumvent these constraints. Designing a nanohybrid composed of gold nanoparticles (AuNPs) and red-emitting carbon dots (CDs) can be used to develop a FLIM-based logic gate that can respond to multiple input parameters. Our findings indicate a nanohybrid that can serve as a nano-computer to receive and integrate chemical and biochemical stimuli and produce a definitive output measured by FLIM. This can open a new research avenue for enhanced diagnostics and therapy that require complicated factor handling and precise control. The AuNPs are conjugated to CDs’ surfaces through a strong covalent linkage. The AuNP–CD nanohybrid shows fluorescence lifetime (FLT) quenching of pristine CDs after conjugation to AuNPs. The FLT was reduced from 3.61 ± 0.037 to 2.48 ± 0.040 ns. This quenched FLT can be recovered back by using trypsin as a recovering agent, giving us a reversible logic output. The FLT was recovered to 3.01 ± 0.01 ns after trypsin addition. This “on–off–on” response can be used to construct the IMPLICATION logic gate.

Highly efficient red-emitting carbon dots as a “turn-on” temperature probe in living cells

Nanothermometers, which can precisely detect the intracellular temperature changes, have great potential to solve questions concerning the cellular processes. Thus, the temperature sensors that provide fluorescent “turn-on” signals in the biological transparency window are of highly desirable. To meet these criteria, this work reported a new “turn-on” carbon dot (CD)-based fluorescent nanothermometry device for sensing temperature in living cells. The CDs that emit bright red fluorescence (R-CDs; λmax = 610 nm in water) were synthesized with o-phenylenediamine as carbon precursor via a facile solvothermal method. The R-CDs in water were almost nonfluorescent at 15 °C. As the temperature increased, the fluorescence intensity of R-CDs exhibited a gradual increase and the final enhancement factor was greater than 21-fold. The fluorescence intensity exhibited a linear response to temperature and a high-sensitive variation of ≈13.3 % °C−1 was detected within a broad temperature range of 28–60 °C. Moreover, the R-CD thermal sensors also exhibited high storage stability, excellent response reversibility and superior photo- and thermo-stability. Due to its good biocompatibility and “intelligent” response to external temperature, the nanothermometer could be applied for sensing temperature changes in biological media.

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.

A design of red emission CDs-based aptasensor for sensitive detection of insulin via fluorescence resonance energy transfer

We successfully designed an aptasensor based on the red emission carbon dots (R-CDs) and effectively detected insulin (INS) via fluorescence resonance energy transfer (FRET). In the process, the aptamer (apt) labeled with R-CDs (R-CDs@apt) was used as fluorescence donor and graphene oxide (GO) was used as fluorescence receptor. The successful detection due to the aptamer sequence has a certain affinity for Go and INS, while the affinity for INS is stronger than that of GO. When INS is not added to the detection system, the aptamer is adsorbed onto the surface of GO, shortening the distance between R-CDs@apt and GO, resulting in FRET and the quenching of fluorescence of R-CDs@apt. When INS was added to the detection system, the aptamer selectively bound INS and separated from the adsorption of GO, FRET gradually disappeared and the fluorescence of R-CDs@apt/GO/INS system was restored. By comparing the changes of fluorescence intensity before and after adding INS, the detection of INS was implemented. The aptasensor has a good linear curve with the detection limit of as low as 1.1 nM when the concentration of INS reached 1.3–150 nM. This method has excellent selectivity and anti-interference. Therefore, it is a potential method for detecting substances in biological fluids.

Red emissive N–S co-doped carbon dots for live imaging of tumor spheroid in the microfluidic device

Here, we present a one-pot, water-based synthesis of red emissive biocompatible carbon dots (R-CDs) for live bioimaging of U87 microglia tumor spheroids produced in a microwell-based microfluidic device. R-CDs were synthesized by the hydrothermal method, using thiourea and para-phenylenediamine as the starting precursors. The mold of the microfluidic platform was produced by a lab-made high-resolution optical 3D printer and CNC micro-milling. The optical and physical properties of synthesized R-CDs were validated by different analytical methods such as fluorescent and absorption spectroscopy, FT-IR, DLS, and TEM. The prepared R-CDs emit red light with a maximum fluorescent wavelength of 584 nm. The prepared R-CD has low cytotoxicity and has been successfully applied for staining the glioblastoma spheroid without any fixation step. Therefore, it is a promising fluorescent probe in live 3D cell imaging.

One step synthesis of efficient red emissive carbon dots and their bovine serum albumin composites with enhanced multi-photon fluorescence for in vivo bioimaging

Efficient red emissive carbon dots (CDs) in aqueous solutions are very scarce for high performance bioimaging applications. In this work, we report a one-step solvothermal treatment to synthesize pure red emissive CDs (FA-CDs) from citric acid and urea in formic acid without complicated purification procedures. Photoluminescence quantum yield (PLQY) of 43.4% was observed in their dimethyl sulfoxide solutions. High PLQY up to 21.9% in aqueous solutions was achieved in their bovine serum albumin (BSA) composites (FA-CDs@BSA) with significantly enhanced multi-photon fluorescence. The strong surface electron-withdrawing structure of FA-CDs caused by the high content of C = O groups contributes for their pure red emission. Owing to the significantly enhanced single and multi-photon red fluorescence and enlarged particle sizes after composing with BSA, in vivo tumor imaging and two-photon fluorescence imaging of blood vessels in mouse ear have been realized via intravenous injection of FA-CDs@BSA aqueous solutions.

Dual-reverse-signal ratiometric fluorescence method for malachite green detection based on multi-mechanism synergistic effect

The proposition of ratiometric detection mode has demonstrated great superiority in improving analysis accuracy by forming self-calibration. Herein, the novel dual-reverse-signal ratiometric fluorescence detection for malachite green (MG) was first achieved based on synergistic effect of fluorescence resonance energy transfer (FRET) and inner filter effect (IFE). The ratiometric fluorescence probe (B-RCDs) was self-assembled via electrostatic attraction between blue-emission carbon dots (BCDs) and red-emission carbon dots (RCDs), followed with FRET effect from BCDs to RCDs and exhibited dual-emission at 450 nm and 627 nm. In the presence of MG, the IFE effect between MG and RCDs quenched the fluorescence at 627 nm and restored the fluorescence at 450 nm, sending out two reverse signals along with an obvious color change from pink to purple (302 nm UV lamp). This ratiometric method not only simplified the preparation process, but also improved the detection sensitivity, showing a low limit of detection (LOD) of 41.8 nM, which exhibited superiority than that of single-signal RCDs (157.3 nM). This method held a rapid response of 10 min and represented satisfactory recoveries (99.14%−109.08%) in real water samples, revealing it was a promising candidate in the fast, sensitive and practical detection of MG. Moreover, the design of synergistic effect supplied a new perspective for the development of ratiometric sensing in the future.

Reversible fluorescent test strip with red fluorescent carbon dots for monitoring water in organic solvents: Visual detection via a smartphone

Herein, a novel type of red-emitting carbon dots called TN-CDs was created via a one-step hydrothermal approach using neutral red and tartaric acid as raw materials. The fluorescence of TN-CDs was gradually quenched as the amount of water increased, and the color of the solution changed from yellow to pink mauve (or purple to pink). The reaction could be completed within only 5 s in various organic solvents such as N,N-Dimethylformamide (DMF), methanol (MeOH), acetonitrile (ACN), and ethanol (EtOH) with linear detection ranges of 1.2%–35.0%, 0.5%–20.0%, 0.25%–5.0% and 0%–16.0%, respectively. In addition, we prepared a reusable test strip and then combined it with TN-CDs to detect water content in DMF, as well as integrated it with smartphone software, a UV lamp, and a dark chamber for real-time, on-site, visual quantitative detection of the water content.

Gadolinium doped red-emissive carbon dots as targeted theranostic agents for fluorescence and MR imaging guided cancer phototherapy

As a new type of fluorescent agents, carbon dots (CDs) have revealed great potential in biomedical imaging applications attributed to their unique optical properties and good biocompatibility. However, engineering a versatile CDs nanoplatform integrating multifunctional imaging and therapeutic functionalities has always been an intractable challenge in targeted cancer treatment based on carbon nanomaterials. Herein, a gadolinium-doped CD conjugated with AS1411 aptamers (AS1411-Gd-CD) was prepared via facile solvothermal approach for targeted fluorescence (FL)/MR imaging guided photothermal therapy (PTT). The synthesized AS1411-Gd-CDs exhibited uniform nanostructure, good biocompatibility, strong red-fluorescence emission, significant MR contrast and efficient photothermal conversion. Benefiting from these outstanding properties, the AS1411-Gd-CDs were successfully employed in tumor-specific FL/MR dual-modal imaging in vitro and in vivo. Furthermore, under the guidance of FL/MR imaging, the AS1411-Gd-CDs demonstrated significant tumor inhibition effect owing to the outstanding photothermal effect. Thus, this study provided a versatile CDs nanoplatform as a multifunctional theranostic agent for synergistic FL/MR imaging and PTT in cancer treatment.

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 20nm. 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.

Solvent-dependent red emissive carbon dots and their applications in sensing and solid-state luminescence

Herein, a kind of red emissive carbon dots (BNCDs) based on 1,1′-binaphthyl-2,2′-diamine (BNDM) and citric acid as carbon source were prepared through one-pot solvothermal method. The synthesized BNCDs have uniform size distribution, photobleaching resistance, and excitation wavelength-independent emission performance with the optimal fluorescence at λex/λem = 540/603 nm. The composite films of BNCDs in polyvinyl alcohol (PVA) matrix exhibited bright red emission under ultraviolet irradiation, indicating their potential in solid-state luminescence. Also, BNCDs shows unique solvent-dependent phenomenon, the optical behavior can be tuned by the change of solvent polarity. Another striking observation was that BNCDs can be applied as a fluorescence sensor for highly sensitive quantitative detection of trace water in organic solvents because it’s polarity-sensitive property. Through fluorescence “on–off–on” mechanism, BNCDs could be used as a fluorescence sensor for detection of Fe3+ and F- with LOD of 0.219 and 0.0104 μmol L−1, respectively. This strategy offers an efficient approach for preparing red emissive carbon dots and lays a foundation for its multi-functional application.