Excitation-independent Emission Research Articles

Nitrogen-doped fluorescent carbon dots for highly sensitive and selective detection of tannic acid.

Herein, a facile approach for highly sensitive and selective detection of tannic acid (TA) was proposed with the nitrogen-doped fluorescent carbon dots (NCDs) as a novel fluorescent probe, using sodium citrate and aminopyrazine as precursors. The as-synthesized NCDs exhibited multiple advantages including high quantum yield (11.8%), good water solubility and satisfactory stability. In addition, the NCDs displayed excitation-independent emission behavior with fluorescence emission peak remaining at 389 nm under excitation of 270-350 nm. Significantly, the fluorescence quenching of as-fabricated NCDs was observed with the increasing TA concentration, and the calibration curve displayed a wide linear region ranging from 0.40 to 9.0 μmol L-1 with a detection limit of 0.12 μmol L-1. This fluorescent probe also performed well in determining TA in beer samples with average recoveries of TA ranging from 96.1% to 104.4% and the relative standard deviation less than 5%, which provided a reliable, rapid and simple method to determine TA in real samples. Thus, this method offered a valuable insight for its practical applications.

Structural and optical properties of upconversion CuInS/ZnS quantum dots

A facile one-pot method to synthesis CuInS/ZnS (CIS/ZnS) QDs was developed. The prepared CIS/ZnS QDs exhibited a high bright emission. Moreover, the upconversion photoluminescence (PL) of the CIS/ZnS QDs was explored. Interestingly, extraordinary excitation-independent emission for both up and down conversion fluorescence of the CIS/ZnS QDs were observed. Analysis of X-ray diffraction (XRD) of CIS/ZnS QDs showed chalcopyrite crystal structure. The high-resolution transmission electron Microscopy (HRTEM) images demonstrated crystalline CIS/ZnS QDs with spherical shape and average diameter sizes of 2.5 nm and 3.6 nm for CIS core and CIS/ZnS QDs core shell, respectively. The selected area electron diffraction (SAED) suggested that the prepared CIS/ZnS QDs are polycrystalline with about 0.32 nm lattice distance. The PL peaks position was almost fixed and exhibited a strong peak at about 640 nm for both up and down conversion emission with a linear relationship between the intensity of the PL emission peaks and excitation wavelengths.

Dual functionalized, stable and water dispersible CdTe quantum dots: Facile, one-pot aqueous synthesis, optical tuning and energy transfer applications

We report a facile one-pot synthesis of aqueously dispersible CdTe quantum dots (QDs) with tunable optical properties by using dual capping agents namely MPA and Citric acid. Optimum precursor ratio and pH > 8 at 80 °C facilitated particle size tunability and exceptional optical properties. QDs are ∼3.7 nm as seen from HR-TEM image; highly-luminescent showing absolute quantum-yield up to 54% and bear good photostability. CdTe QDs are highly stable in water with large negative Zeta potential. Optimum ratio of Cd2+:MPA:TeO32−:NaBH4:citric acid was 1:2:0.2:3.2:5.2, respectively. The pH dependent nucleation and growth of QDs subsequently lead to quasi CdTe/CdS structure as indicated by XRD, FTIR and particle size analysis. QDs showed excitation independent emission with tunable peak width. QDs can be stored over 6-months at 4 °C without causing agglomeration. Interaction of QDs with Rhodamine-6G and plasmonic silver nanoparticles via resonance energy transfer (RET) imply their suitability for sensors and biomedical applications.

One-step solvothermal synthesis of red emissive carbonized polymer dots for latent fingerprint imaging

Photoluminescent carbon-based nanomaterials (CNMs) have attracted much attention over the past decade due to their unique optical characteristics and negligible toxicity. However, it is very difficult to obtain red emission of CNMs. Bright red fluorescent carbonized polymer dots (CPDs) were prepared by facilely adjusting the reaction temperature through a one-step solvothermal route, using p-phenylenediamine (PPD) as precursor. The as-synthesized CPDs exhibit excitation-independent emission at about 608 nm under excitation from 365 to 550 nm. Loading CPDs in starch to form CPDs–starch powder, the resultant powder exhibits strong red emission and the typical aggregation-induced fluorescence quenching was effectively suppressed, thus allowing the CPDs to be used in solid state form. More importantly, latent fingerprints (LFPs) on surfaces of various substrates, including glass, stainless steel, marble, adhesive tape, aluminum foil, painted wood, compact disc (CD), and coin, are developed by CPDs–starch powder and exhibit clear patterns and satisfactory ridge details under a 365 nm UV lamp. Furthermore, fingerprints on surfaces with multicolored background and strong florescent properties could also be clearly observed. These results suggest that CPDs–starch powder can be used as a fluorescent labeling agent for the visualization of LFPs in forensic science.

Facile and Rapid Synthesis of Yellow-Emission Carbon Dots for White Light-Emitting Diodes

A one-pot microwave-assisted hydrothermal approach was developed to quickly synthesize yellow fluorescent carbon dots (CDs) using o-phenylenediamine and urea as raw materials. The optimized reaction temperature of 140°C and reaction time of 10 min were adopted to synthesize the CDs with photoluminescence quantum yield of 4.23%. The as-prepared CDs exhibit yellow fluorescence (λem ∼ 563 nm) and an excitation-independent PL emission in the region of 440–700 nm. The CDs were combined with a blue chip (λex = 420 nm) to fabricate white light-emitting diodes (WLEDs) and realize white light emission. The WLED has the color coordinates of (0.30, 0.30) and the correlated color temperature of 7915 K, which is suitable for office and outdoor illumination. The results demonstrate that as-obtained CDs can be a promising candidate as yellow-emitting phosphors for WLEDs.

Facile synthesis of orange fluorescence carbon dots with excitation independent emission for pH sensing and cellular imaging

High-efficient orange fluorescence nitrogen-doped carbon dots (N-CDs) were facilely prepared from P-phenylenediamine as a precursor via hydrothermal method. The as-prepared N-CDs with an average diameter of 3.65 nm displayed excitation-independent emission at 590 nm. The N-CDs demonstrated a remarkable fluorescence enhancement behavior with the increase of pH. A sigmoidal curve was well fitted using BiDoseResp equation with pKa1 3.57 and pKa2 6.01, which can be ascribed to the unique surface properties of N-CDs. Two-segment linear ranges of 2.6–4.6 and 5.0–6.8 broaden the response range to pH of the orange-emission N-CDs to some extent. The confocal fluorescent microscopic images of SMMC7721 cells were performed successfully, which demonstrating that N-CDs possess exceptional cell membrane permeability and can implement as biosensing platform to monitor pH fluctuations in living cells.

Metal ion sensing and light activated antimicrobial activity of aloe-vera derived carbon dots

Carbon dots (CDs) have emerged out as a potential material amongst the carbon family for a wide range of applications including chemical/biological sensing, photocatalysis, bioimaging, etc. The green synthesis of these CDs from natural sources is gaining the significant interest of peer community for their wide utility. Herein, we present a facile one-step pyrolysis method for CDs synthesis from Aloe-Vera extract, which show bright blue luminescence under UV light with a quantum yield of 12.3%. Further, ex-situ morphological, structural and optical characterizations reveal their high quality and excitation independent emission behavior with the presence of carboxyl, hydroxyl functional groups. Furthermore, these CDs were studied for Fe(III) sensing in water without any surface modifications and assessed for their light activated antibacterial activity against E.Coli and Staphylococcus aureus.

Facile one-pot synthesis of highly fluorescent nitrogen-doped carbon dots by mild hydrothermal method and their applications in detection of Cr(VI) ions

Nitrogen-doped carbon dots (N-CDs) with well-distribution size and strong blue emission were successfully synthesized via a simple mild hydrothermal strategy using citric acid and ethylenediamine as co-precursors. The highly fluorescent N-CDs exhibit high fluorescence quantum yield (QY, 58.6%), excitation-independent emission behavior, and good photostability. The experimental results showed that the N-CDs can be served as a fluorescent sensing platform for detection of Cr(VI) ions due to the effective fluorescence quenching effect of Cr(VI) ions. The quenching mechanism probably arises from the inner filter effect (IFE) and the electron transfer due to the strong interactions between functional groups (COOH, OH and NH2 groups) of the N-CDs and Cr(VI) ions. It is also found that the N-CDs showed high sensitivity toward Cr(VI) ions with a detection limit of 0.26 μM. Moreover, the obtained N-CD can be employed as chemsensor to detect Cr (VI) in real river water samples, which have potential applications in the environmental water.

Near-Ultraviolet to Near-Infrared Fluorescent Nitrogen-Doped Carbon Dots with Two-Photon and Piezochromic Luminescence.

Carbon dots (CDs) have gained intensive interests owing to their unique structure and excellent optoelectronic performances. However, to acquire CDs with a broadband emission spectrum still remains an issue. In this work, nitrogen-doped CDs (N-CDs) with near-ultraviolet (NUV), visible, and near-infrared (NIR) emission were synthesized via one-pot solvothermal strategy, and the excitation-independent NUV and NIR emission and excitation-dependent visible emission were observed in the photoluminescence (PL) spectra of N-CDs. Moreover, the as-synthesized N-CDs displayed two-photon fluorescence emission. It is important to note that N-CDs also exhibited piezochromic luminescence with reversibility, in which the red- and blue-shifted PL with increasing applied pressure (0.07-5.18 GPa) and the red- and blue-shifted PL with releasing applied pressure (5.18 GPa to 1 atm) were developed for the first time. Combined with good hydrophilicity, high photobleaching resistance, and low toxicity, the piezochromic luminescence would greatly boost the valuable applications of N-CDs.

High fluorescent sulfur regulating graphene quantum dots with tunable photoluminescence properties

Sulfur-doped graphene quantum dots (S-GQDs) were synthesized by two facile hydrothermal technologies. The photoluminescence (PL) properties of the GQDs and S-GQDs samples were mainly investigated. Through regulating the content of S powders in S-GQDs synthesizing process, the optimal S-GQDs have a high S/C atomic ratio of 19.53%. The S doping introduce more functional groups on the C sp2 skeleton of S-3 sample and result in the appearance of the strong absorption band in the UV region. In comparison with other reported S-GQDs, the S-GQDs exhibit overwhelming high fluorescence quantum yield (57%) and excitation-independent emission, resulting from the outcome of the doped sulfur atoms. Moreover, the PL intensity of GQDs can be increased by doping it with S and the increasing efficiency depends on the thiophene sulfur content.

Capacitively Coupled Plasma Discharge of Ionic Liquid Solutions to Synthesize Carbon Dots as Fluorescent Sensors.

Oxygen and nitrogen capacitively coupled plasma (CCP) was used to irradiate mixtures of aliphatic acids in high boiling point solvents to synthesize fluorescent carbon dots (C-dots). With a high fluorescence intensity, the C-dots obtained from the O2/CCP radiation of a 1-ethyl-3-methylimidazolium dicyanamide ionic liquid solution of citric acid were characterized with an average diameter of 8.6 nm (σ = 1.1 nm), nitrogen and oxygen bonding functionalities, excitation-independent emissions, and upconversion fluorescence. Through dialysis of the CCP-treated C-dots, two emissive surface states corresponding to their respective functionalities and emissions were identified. The fluorescence spectrum of the CCP-treated C-dots was different from that of the microwave irradiation and possessed higher intensity than that of hydrothermal pyrolysis. By evaluation of the fluorescence quenching effect on flavonoids and metal ions, the CCP-treated C-dots showed a high selectivity for quercetin and sensitivity to Hg2+. Based on the Perrin model, a calibration curve (R2 = 0.9992) was established for quercetin ranging from 2.4 μM to 119 μM with an LOD (limit of detection) = 0.5 μM. The quercetin in the ethanol extract of the sun-dried peel of Citrus reticulata cv. Chachiensis was determined by a standard addition method to be 4.20 ± 0.15 mg/g with a matrix effect of 8.16%.

Sunlight-Induced Photocatalytic Degradation of Pollutant Dye by Highly Fluorescent Red-Emitting Mg-N-Embedded Carbon Dots

A straightforward and simpler use of an age-old technique was utilized for the fabrication of “red-emitting magnesium-nitrogen-embedded carbon dots” (r-Mg-N-CD) from the leaves extract of Bougainvillea plant as a natural source of carbon. This technique is similar to the solvent-based technique, which is used for the extraction of fragrances and essential oils from flowers and leaves. The as-derived leaves extract was further carbonized using a simple domestic microwave to obtain the small-sized red-emitting carbonaceous material as r-Mg-N-CD. The r-Mg-N-CD showed excitation-independent emissions at ∼678 nm with excellent photostability and a high quantum yield value (∼40%). Moreover, the important perspective of the present finding is to use this r-Mg-N-CD as a potential photocatalyst material for the degradation of pollutant dye (methylene blue) under the presence of sunlight. To infer the significant influence of using natural sunlight in the process of dye degradation, a comparative analysis was perfo...

Synthesis and characterization of high efficient photoluminescent sunlight driven photocatalyst of N-Carbon Quantum Dots

A one-step simple, novel and efficient route for the preparation of N-doped carbon quantum dots (N-CQDs) has been established through the hydrothermal method, using diethylenetriamine (DETA) and citric acid (CA) as the surface passivation agent and carbon source, respectively. The as-prepared N-CQDs exhibit excellent photostability emission property and excitation-independent emission behavior with a high quantum yield up to 95%, which is the maximum value attained until now. As well as the Photoluminescence (PL) intensity of the obtained N-CQDs is pH independent over a range of 4–12 and their PL intensity retains unchanged even after several hours of UV excitation and over six months of storage, presenting an outstanding stability. Most importantly, the experimental results showed that N-CQDs have exceptional photocatalytic performance in the photodegradation of Methyl Blue (MB) under visible and natural sunlight by in situ photogenerated of H2O2 in reaction media. The improved photocatalytic performance of N-CQDs was attributed to the important role of N dopant into CQDs.

Photostable and Low-Toxic Yellow-Green Carbon Dots for Highly Selective Detection of Explosive 2,4,6-Trinitrophenol Based on the Dual Electron Transfer Mechanism.

Advances in the development of fluorescent carbon dots (CDs) for detecting nitro-explosives have attracted great interest. However, developing long-wavelength luminescence CDs for highly selective determination of 2,4,6-trinitrophenol (TNP) and getting insight into the detection mechanism remain further to be investigated. Here, excitation-independent yellow-green emission CDs with good photostability and low biotoxicity were introduced for detecting TNP selectively. Then, two types of electron transfer (ET) processes including hydrogen-bond interaction-assisted ET and proton transfer-assisted ET are suggested to be responsible for their photophysical behavior. Finally, the visual detection of TNP has been successfully developed by a CD-based indicator paper. The facile, highly sensitive, and selective detection for TNP in both of a solution and a solid phase makes CDs potentially useful in environmental sensor applications.

Zinc and nitrogen ornamented bluish white luminescent carbon dots for engrossing bacteriostatic activity and Fenton based bio-sensor

Carbon dots with heteroatom co-doping associated with consummate luminescence features are of acute interest in diverse applications such as biomolecule markers, chemical sensing, photovoltaic, and trace element detection. Herein, we demonstrate a straightforward, highly efficient hydrothermal dehydration technique to synthesize zinc and nitrogen co-doped multifunctional carbon dots (N, Zn-CDs) with superior quantum yield (50.8%). The luminescence property of the carbon dots can be tuned by regulating precursor ratio and surface oxidation states in the carbon dots. A unique attribution of the as-prepared carbon dots is the high monodispersity and robust excitation-independent emission behavior that is stable in enormously reactive environment and over a wide range of pH. These N, Zn-CDs unveils captivating bacteriostatic activity against gram-negative bacteria Escherichia coli. Furthermore, the excellent luminescence properties of these carbon dots were applied as a platform of sensitive biosensor for the detection of hydrogen peroxide. Under optimized conditions, these N, Zn-CDs reveals high sensitivity over a broad range of concentrations with an ultra-low limit of detection (LOD) indicating their pronounced prospective as a fluorescent probe for chemical sensing. Overall, the experimental outcomes propose that these zero-dimensional nano-dots could be developed as bacteriostatic agents to control and prevent the persistence and spreading of bacterial infections and as a fluorescent probe for hydrogen peroxide detection.

An insight into electronic and optical properties of multilayer graphene quantum dots synthesized by hydrothermal approach

The carbonaceous small sized graphene quantum dots (GQDs) synthesized by hydrothermal route manifest the existence of band gap by cutting the graphene sheets into quantum dots (QDs). Consequently, the tuning of band gap from 0–3.2 eV occurs by decreasing the size of graphene owe to the quantum confinement effect. The small sized uniform particles corroborated from TEM analysis exhibits the excitation independent emission spectra in the UV excitation range. In addition, GQDs unveil the excitation dependent behavior by red shift in emission wavelength alongwith the decrease in intensity of photoluminescence upon excitation in the visible region. It happens due to the presence of varying size particles. The work function of GQDs was analysed by UPS spectroscopy and was found to be 4.48 eV. These electronic energy levels relies on the size and band gap of GQDs. The distinct optical and electronic properties leads to GQDs as an absorber, emissive and transport material in optoelectronic devices.

One-Step Hydrothermal Synthesis of Nitrogen-Doped Conjugated Carbonized Polymer Dots with 31% Efficient Red Emission for In Vivo Imaging.

Carbon dots with long-wavelength emissions, high quantum yield (QY) and good biocompatibility are highly desirable for biomedical applications. Herein, a green, facile hydrothermal synthesis of highly efficient red emissive nitrogen-doped carbonized polymer dots (CPDs) with optimal emission at around 630 nm are reported. The red emissive CPDs possess a variety of superior properties including excellent water dispersibility, good biocompatibility, narrow bandwidth emission, an excitation-independent emission, and high QY (10.83% (in water) and 31.54% (in ethanol)). Further studies prove that such strong red fluorescence is ascribed to the efficient conjugated aromatic π systems and hydrogen bonds of CPDs. And the fluorescence properties of CPDs can be regulated by adjusting the dosage of HNO3 before the reaction. Additionally, the as-prepared CPDs are successfully used as a fluorescent probe for bioimaging, both in vitro and in vivo. More importantly, biodistribution results demonstrate that most CPDs and their metabolites are not only excreted in urine but also excreted by hepatobiliary system in a rapid manner. Besides, the CPDs could easily cross the blood brain barrier, which may provide a valuable strategy for the theranostics of some brain diseases through real-time tracking.

Supramolecular nanodots derived from citric acid and beta-amines with high quantum yield and sensitive photoluminescence

Herein, we reported the characterization of a highly fluorescent and sensitive amorphous nanodots synthesized by hydrothermal treatment of citric acid in the presence of various β-amines below 200 °C. Five-membered ring fused 2-pyridone compounds are effectively produced in the condensation reaction, and serve as the main building blocks to self-assemble into supramolecular nanodots under hydrogen bonding. The 2-pyridone compounds are not the sole blue species, but effectively enhance the photoluminescence quantum yield of citric acid derived nanodots. Compared with the excitation-independent emission in dilute solution, the dense nanodots exhibit red-shifted, suppressed and unexpected excitation-dependent emission. Moreover, the supramolecular nanodots have high sensitivity in various ambient conditions, such as pH, solvent polarity and metal ions.

Exploring the Emissive States of Heteroatom-Doped Graphene Quantum Dots

The photoluminescence (PL) emission states of heteroatom-doped graphene quantum dots (GQDs) remain unknown, particularly the assignment of the low-energy excitation band (more than 330 nm). To address these issues, this work synthesized three different types of GQDs: undoped GQDs (UGQDs), nitrogen-doped GQDs (NGQDs), and boron-doped GQDs (BGQDs), with similar sizes, chemical compositions (types and compositions of surface functional groups), and defects using a constant potential electrolysis method. The PL emissive states in these GQDs and the effects of the dopant heteroatom on the PL were revealed based on the combination of spectroscopic methods and theoretical calculations. The results indicated that the GQDs exhibit multiemissive centers for the PL emission mechanism. An excitation-independent PL emission band (band I) results from a high-energy transition originating from the quantum confinement of the carbon core (carbon π–π* transitions in sp2 domain), and an excitation-dependent PL emission band...

Interaction of proteins with lemon-juice/glutathione-derived carbon nanodot: Interplay of induced-aggregation and co-solubilization

The accumulation of protein aggregates (tau) causes Alzheimer's disease (AD), Parkinson's disease (PD), and a range of neurodegenerative diseases. To develop a less toxic and bio-derived nanomaterials for inhibition of protein-aggregation, carbon nanodot has been used for this study. Nanodot have generated huge interest in biomedical applications owing to unique emission property and good biocompatibility. A carbon nanodot is synthesized from a natural resource-lemon juice and glutathione. The synthesized nanodot possesses excitation-independent emission and nano-sheet like with high graphitic content. Interaction of protein with CND is monitored by intrinsic fluorescence (trp residues), FT-IR and circular dichroism spectroscopy. Whereas it solubilizes the protein aggregates at its higher concentration. Both induced-aggregation and co-solubilization are sequence-independent and dictated by nanodot. The study may shed light on the role of glutathione in glutathione-dependent glyoxalase system toward defence against glycation product.