Photoluminescent Graphene Quantum Dots Research Articles

Bandgap engineering of coal-derived graphene quantum dots.

Bandgaps of photoluminescent graphene quantum dots (GQDs) synthesized from anthracite have been engineered by controlling the size of GQDs in two ways: either chemical oxidative treatment and separation by cross-flow ultrafiltration, or by a facile one-step chemical synthesis using successively higher temperatures to render smaller GQDs. Using these methods, GQDs were synthesized with tailored sizes and bandgaps. The GQDs emit light from blue-green (2.9 eV) to orange-red (2.05 eV), depending on size, functionalities and defects. These findings provide a deeper insight into the nature of coal-derived GQDs and demonstrate a scalable method for production of GQDs with the desired bandgaps.

An efficient edge-functionalization method to tune the photoluminescence of graphene quantum dots.

An efficient edge-functionalization strategy with high specificity was employed to study the effects of conjugated structures on photoluminescence (PL) properties of graphene quantum dots (GQDs). Both the experimental results and density functional theory (DFT)-based calculations suggested the mechanism for conjugated structures in GQDs to tune the band gap of GQDs.

Photoluminescent graphene quantum dots for in vivo imaging of apoptotic cells.

Apoptosis (programmed cell death) is linked to many incurable neurodegenerative, cardiovascular and cancer causing diseases. Numerous methods have been developed for imaging apoptotic cells in vitro; however, there are few methods available for imaging apoptotic cells in live animals (in vivo). Here we report a novel method utilizing the unique photoluminescence properties of plant leaf-derived graphene quantum dots (GQDs) modified with annexin V antibody (AbA5) to form (AbA5)-modified GQDs (AbA5-GQDs) enabling us to label apoptotic cells in live zebrafish (Danio rerio). The key is that zebrafish shows bright red photoluminescence in the presence of apoptotic cells. The toxicity of the GQDs has also been investigated with the GQDs exhibiting high biocompatibility as they were excreted from the zebrafish's body without affecting its growth significantly at a concentration lower than 2 mg mL(-1) over a period of 4 to 72 hour post fertilization. The GQDs have further been used to image human breast adenocarcinoma cell line (MCF-7 cells), human cervical cancer cell line (HeLa cells), and normal human mammary epithelial cell line (MCF-10A). These results are indispensable to further the advance of graphene-based nanomaterials for biomedical applications.

Photoluminescent graphene quantum dots for in vitro and in vivo bioimaging using long wavelength emission

The GQDs were exploited in bioimagingin vitroandin vivo: using the excitation-dependent PL, multi-color cell labeling was achieved and using long wavelength emission, thein vivoimaging was realized.

Charge and energy transfer interplay in hybrid sensitized solar cells mediated by graphene quantum dots

We explored the role of graphene quantum dots (GQDs) as co-sensitizers in hybrid dye sensitized solar cell (DSSC) architectures, focusing on various concurring mechanisms, such as: charge transfer, energy transfer and recombination rate, towards light harvesting improvement. GQDs were prepared by the hydrothermal method that allows the tuning of electronic levels and optical properties by employing appropriate precursors and synthesis conditions. The aim was to realize a type II alignment for TiO2/GQD/dye hybrid configuration, using standard N3 Ru-dye in order to improve charge transfer. When GQDs were used as co-sensitizers together with N3 Ru-dye, an improvement in power conversion efficiency was achieved, as shown by electrical measurements. The experimental analysis indicates that this improvement arises from the interplay of various mechanisms mediated by GQDs: (i) enhancement of charge separation and collection due to the cascaded alignment of the energy levels; (ii) energy transfer from GQDs to N3 Ru-dye due to the overlap between GQD photoluminescence and N3 Ru-dye absorption spectra; and (iii) reduction of the electron recombination to the redox couple due to the inhibition of the back electron transfer to the electrolyte by the GQDs.

Microwave bottom-up route for size-tunable and switchable photoluminescent graphene quantum dots using acetylacetone: New platform for enzyme-free detection of hydrogen peroxide

We report a microwave sequential bottom-up route to produce green and blue luminescent graphene quantum dots (g-GQDs and b-GQDs) with size-tunable and switchable functionalities by tailoring the diameter size and functional groups via microwave carbonization and aromatization processes from acetylacetone as a starting organic solvent. The b-GQDs as the final product show only one emission peak at 433nm and pH-independent blue luminescence, because two-step microwave irradiation could reduce the size and the oxygen-functional groups of the g-GQDs as an intermediate product. Also, the b-GQDs provide an exemplar enzyme-free platform for hydrogen peroxide detection through the electrochemical sensing due to much higher electron density and electron donating properties. In contrast, the g-GQDs show two different switchable photoluminescent emissions at ∼460nm (P1) and ∼500nm (P2): the P1 emission with sky-blue fluorescence originates from randomly conjugated oxygen-functional groups on the basal plane and/or edge of the g-GQDs and the P2 emission with green fluorescence results from quasi-molecular fluorophores formed by the electronic coupling of carboxylic acid groups.

Formation mechanism and optimization of highly luminescent N-doped graphene quantum dots

Photoluminescent graphene quantum dots (GQDs) have received enormous attention because of their unique chemical, electronic and optical properties. Here a series of GQDs were synthesized under hydrothermal processes in order to investigate the formation process and optical properties of N-doped GQDs. Citric acid (CA) was used as a carbon precursor and self-assembled into sheet structure in a basic condition and formed N-free GQD graphite framework through intermolecular dehydrolysis reaction. N-doped GQDs were prepared using a series of N-containing bases such as urea. Detailed structural and property studies demonstrated the formation mechanism of N-doped GQDs for tunable optical emissions. Hydrothermal conditions promote formation of amide between –NH2 and –COOH with the presence of amine in the reaction. The intramoleculur dehydrolysis between neighbour amide and COOH groups led to formation of pyrrolic N in the graphene framework. Further, the pyrrolic N transformed to graphite N under hydrothermal conditions. N-doping results in a great improvement of PL quantum yield (QY) of GQDs. By optimized reaction conditions, the highest PL QY (94%) of N-doped GQDs was obtained using CA as a carbon source and ethylene diamine as a N source. The obtained N-doped GQDs exhibit an excitation-independent blue emission with single exponential lifetime decay.

Hydrothermal Preparation of Photoluminescent Graphene Quantum Dots Characterized Excitation‐Independent Emission and its Application as a Bioimaging Reagent

Zero‐dimensional photoluminescent (PL) graphene quantum dots (GQDs) that can be used as the cell‐imaging reagent are prepared by a hydrothermal route using the graphene oxide (GO) as the carbon source. Under the optimized hydrothermal conditions, an initial hydrogen peroxide concentration of 0.5 mg mL−1 at 180 °C for 120 min, the GO sheets can be cut into nanocrystals with lateral dimensions in the range of 1.5–5.5 nm and an average thickness of around 1.1 nm. The as‐prepared GQDs exhibit an abundance of hydrophilic hydroxy and carboxyl groups and emit bright blue luminescence with up‐conversion properties in a water solution at neutral pH. Most interestingly, they indicate excitation‐independent emission characteristics, and the surface state is demonstrated to have a key role in the PL properties. The fluorescence quantum yield of the GQDs is tested to be around 6.99% using quinine sulfate as a standard. In addition, the as‐prepared GQDs can enter into HeLa cells easily as a fluorescent imaging reagent without any further functionalization, indicating they are aqueous stability, biocompatibility, and promising for potential applications in biolabeling and solution state optoelectronics.

Nanoporous graphene by quantum dots removal from graphene and its conversion to a potential oxygen reduction electrocatalyst via nitrogen doping

A simple way to produce an efficient metal-free oxygen reduction electrocatalyst from graphene by generating nanopores in the matrix and subsequently establishing nitrogen-doped active sites along the pore openings is demonstrated. Well-structured nanoporous graphene (pGr) and photoluminescent graphene quantum dots (GQDs) could be simultaneously generated by a chemically assisted oxidative treatment of graphene. The process helped to knock out small pieces of Gr through epoxide formation, which subsequently resulted in the generation of GQD and pGr simultaneously. A longer oxidation time increased the quantity of GQDs and also resulted in a higher photoluminescent (PL) quantum yield. The PL quantum yield of GQD formed after 72 h of the oxidative treatment (GQD-72) was 15.8%, which is greater than the previous reported values. The TEM images showed matching sizes for GQDs and the pores present in pGr, implying that the pores are generated by the removal of GQDs from graphene during the oxidative treatment. Since pore openings are expected to give higher levels of unsaturation and defect sites in the system and are thus being treated as fertile regions for heteroatom doping, pGr-72 was further subjected to nitrogen (NpGr-72). NpGr-72 displayed excellent activity towards the electrochemical oxygen reduction reaction (ORR) compared to nitrogen-doped non-porous graphene (NGr) and many other reported nitrogen-doped carbon materials. A distinct 50 mV gain in the overpotential and 2.5 times increment in the kinetic current density (jk) have been achieved in the case of NpGr-72 compared to NGr. Interestingly, unlike NGr, NpGr-72 effectively reduced the oxygen molecule with a greater involvement of the preferred four-electron pathway. Additionally, the overpotential difference of NpGr-72 with respect to 20 wt% Pt/C is only 60 mV. Additionally, in a single cell evaluation under anion exchange membrane fuel cell (AEMFC) conditions, NpGr-72 exhibited a maximum power density of 27 mW cm−2, which is significantly higher than the corresponding value of 10 mW cm−2 obtained for NGr. Thus, the overall enhancement in the performance characteristics of NpGr-72 is attributed to the higher content of nitrogen (7.8 wt%) and its large proportion of desired chemical environment, which could be established by utilizing the high level of carbon unsaturation around the pore openings.

Preparation of Excitation‐Independent Photoluminescent Graphene Quantum Dots with Visible‐Light Excitation/Emission for Cell Imaging

We report the first pyrrole-ring surface-functionalized graphene quantum dots (p-GQDs) prepared by a two-step hydrothermal approach under microwave irradiation in an ammonia medium. The most distinct feature of the functionalized GQDs is that both the excitation and emission wavelengths fall into the visible-light region. The p-GQDs are excited by visible light at λ(ex) 490 nm (2.53 eV) to emit excitation-independent photoluminescence at a maximum wavelength of λ(em) 550 nm. This is thus far the longest emission wavelength reported for GQDs. Stable photoluminescence is achieved at pH 4-10 with an ionic strength of 1.2 mol L(-1) KCl. These features make the p-GQDs excellent probes for bio-imaging and bio-labeling, which is demonstrated by imaging live HeLa cells.

Modulated photoluminescence of graphene quantum dots in the vicinity of an individual silver nano-octahedron

We propose an effective strategy to enhance and modulate the photoluminescence (PL) of graphene quantum dots (GQDs) in the vicinity of a single silver nano-octahedron (SNO) utilizing three-dimensional finite-difference time-domain calculations. The SNO is designed to act as a multifrequency plasmonic antenna with multiple plasmon resonance modes covering multiple emission peaks of GQDs. The spectral modifications of spontaneous emission are investigated with the variations of the GQD's position and dipole moment orientation relative to the SNO. The PL colour of the GQD can be precisely adjusted between blue and green through the strong interaction with the designed antenna. The multicolour features of GQDs will also facilitate their potential applications as eco-friendly and multifunctional optical probes. The study contributes to a deeper understanding of the PL properties of GQDs near the metallic nanoparticles.

The electron-transfer based interaction between transition metal ions and photoluminescent graphene quantum dots (GQDs): A platform for metal ion sensing

The electron-transfer based quenching effect of commonly encountered transition metal ions on the photoluminescence of grapheme quantum dots (GQDs) was for the first time investigated, and was found to be associated with electron configuration of the individual metal ion. Ethylene diamine tetraacetic acid (EDTA), the metal ion chelator, can competitively interact with metal ions to recover the quenched photoluminescence of GQDs. Basically, metal ions with empty or completely filled d orbits could not quench the photoluminescence of GQDs, but this quenching effect was observed for the metal ions with partly filled d orbits. Based on the quenching-recovering strategy, a simple optical metal sensing platform was established by taking Ni(2+) as an example. Using the nickel ion-specific chelating reagent, dimethylglyoxime (DMG), to replace EDTA, a detection limit of 4.1 μM was obtained in standard solution. This proposed strategy does not need further functionalization of GQDs, facilitating the application for simple, fast and cost-effective screening of metal ions.

Graphene Quantum Dots from Polycyclic Aromatic Hydrocarbon for Bioimaging and Sensing of Fe3+ and Hydrogen Peroxide

An easy approach for large‐scale and low‐cost synthesis of photoluminescent (PL) graphene quantum dots (GQDs) based on the carbonization of commercially available polycyclic aromatic hydrocarbon (PAH) precursors with strong acid and followed by hydrothermal reduction with hydrazine hydrate is reported. Transmission electron microscopy (TEM) and atomic force microscopy (AFM) characterizations indicate that the size and height of GQDs are in the range of 5–10 nm and 0.5–2.5 nm, respectively. PAH, which has more benzene rings, generally forms GQDs with relatively larger size. The GQDs show high water solubility, tunable photoluminescence, low cytotoxicity, and good optical stability, which makes them promising fluorescent probes for cellular imaging. In addition, the fluorescence of GQDs shows a sensitive and selective quenching effect to Fe3+ with a detection limit of 5 × 10−9m. By combination with the Fe2+/Fe3+ redox couple, the PL GQDs are able to detect oxidant, using H2O2 as an example. This study opens up new opportunities to make full use of GQDs because of their facile availability, cost‐effective productivity, and robust functionality.

Surface Coating of Graphene Quantum Dots Using Mussel-Inspired Polydopamine for Biomedical Optical Imaging

Because of the superiority of GQDs (graphene quantum dots) in biomedical imaging, in terms of biocompatibility and toxicity of semiconductor quantum dots, GQDs bring new opportunities for the diagnosis and detection of diseases. In this study, we synthesized photoluminescent (PL) graphene quantum dots (GQDs) through a simple exfoliation and oxidation process, and then coated them with polydopamine (pDA) for enhanced stability in water and low toxicity in vivo. From the results, the GQDs coated with pDA showed an excellent stability of PL intensity. It showed that the PL intensity of noncoated GQDs in PBS solution rapidly decreased with time, resulting in a 45% reduction of the PL intensity for 14 days of incubation in PBS solution. After coating with polydopamine, PL intensities of polydopamine-coated GQDs was maintained more stably for 14 days compared with uncoated GQDs. We have observed the in vitro and in vivo biocompatibility of pDA-coated GQDs in nude mice. The overall observation revealed that pDA-coated GQDs could be used as a long-term optical imaging agent as well as a biocompatible drug carrier.

Blue and green luminescence of reduced graphene oxide quantum dots

We report on a new method for synthesising strongly blue and green photoluminescent graphene quantum dots (GQDs). Graphene was prepared by a new feasible method using an intensive cavitation field in a pressurised ultrasonic batch reactor. The prepared graphene was quantitatively converted to graphene oxide using our modified, safer Hummer’s method. Graphene oxide was characterised by microscopic (AFM and TEM) and spectral (infrared and Raman) methods, and the thermal stability of graphene oxide was determined using thermal analysis (DTA-TG). GQDs were prepared by a one-pot reaction, refluxing graphene oxide in different solvents (ethylene glycol, polyethylene glycol, dimethylformamide, dimethyl sulfoxide and N-methyl-2-pyrrolidone) at atmospheric pressure. The synthesised GQDs were characterised by infrared, UV–Vis absorption and photoluminescence spectroscopy, X-ray photoelectron spectroscopy (XPS) and AFM microscopy.

In Vivo Biodistribution and Toxicology of Carboxylated Graphene Quantum Dots

Photoluminescent graphene quantum dots (GQDs) have fascinating optical and electronic properties with numerous promising applications in biomedical engineering. In this work, we first studied the in vivo biodistribution and the potential toxicity of carboxylated photoluminescent GQDs. KB, MDA-MB231, A549 cancer cells, and MDCK normal cell line were chosen as in vitro cell culture models to examine the possible adverse effects of the carboxylated photoluminescent GQDs. The carboxylated GQDs are desirable for increased aqueous solubility. All cancer cells efficiently took up the carboxylated GQDs. No acute toxicity or morphological changes were noted in either system at the tested exposure levels. A long-term in vivo study revealed that the GQDs mainly accumulated in liver, spleen, lung, kidney, and tumor sites after intravenous injection. To reveal any potential toxic effect of the GQDs on treated mice, serum biochemical analysis and histological evaluation were performed. The toxicity results from serum biochemistry and complete blood count study revealed that the GQDs do not cause appreciable toxicity to the treated animals. Finally, we observed no obvious organ damage or lesions for the GQDs treated mice after 21 days of administration at 5 mg/kg or 10 mg/kg dosages. With adequate studies of toxicity, both in vitro and in vivo, photoluminescent GQDs may be considered for biological application.

Improvement of Photoluminescence of Graphene Quantum Dots with a Biocompatible Photochemical Reduction Pathway and Its Bioimaging Application

As a rising star in the family of fluorescent material, graphene quantum dots (GQDs) have attracted great attention because of their excellent properties such as high photostability against photobleaching and blinking, biocompatibility, and low toxicity. Herein, blue luminescent GQDs were prepared by photo-reducing GQDs with isopropanol. After photochemical reduction, the increasing of sp(2) domains and the formed hydroxyl in pGQDs can enhance the photoluminescence of GQDs. The quantum yield of the photo-reduced GQDs (pGQDs) was increased 3.7 fold. Because of its less negative surface charges and lower cytotoxicity than chemical reduced GQDs (cGQDs), the pGQDs were more easily uptaken by cells. This work may provide a simple and green pathway to enhance the QY of GQDs with satisfactory biocompatibility as fluorescent nanoprobes.

Tuning the Photoluminescence of Graphene Quantum Dots through the Charge Transfer Effect of Functional Groups

The band gap properties of graphene quantum dots (GQDs) arise from quantum confinement effects and differ from those in semimetallic graphene sheets. Tailoring the size of the band gap and understanding the band gap tuning mechanism are essential for the applications of GQDs in opto-electronics. In this study, we observe that the photoluminescence (PL) of the GQDs shifts due to charge transfers between functional groups and GQDs. GQDs that are functionalized with amine groups and are 1-3 layers thick and less than 5 nm in diameter were successfully fabricated using a two-step cutting process from graphene oxides (GOs). The functionalized GQDs exhibit a redshift of PL emission (ca. 30 nm) compared to the unfunctionalized GQDs. Furthermore, the PL emissions of the GQDs and the amine-functionalized GQDs were also shifted by changes in the pH due to the protonation or deprotonation of the functional groups. The PL shifts resulted from charge transfers between the functional groups and GQDs, which can tune the band gap of the GQDs. Calculations from density functional theory (DFT) are in good agreement with our proposed mechanism for band gap tuning in the GQDs through the use of functionalization.

Surface Chemistry Routes to Modulate the Photoluminescence of Graphene Quantum Dots: From Fluorescence Mechanism to Up‐Conversion Bioimaging Applications

AbstractThe bandgap in graphene‐based materials can be tuned from 0 eV to that of benzene by changing size and/or surface chemistry, making it a rising carbon‐based fluorescent material. Here, the surface chemistry of small size graphene (graphene quantum dots, GQDs) is tuned programmably through modification or reduction and green luminescent GQDs are changed to blue luminescent GQDs. Several tools are employed to characterize the composition and morphology of resultants. More importantly, using this system, the luminescence mechanism (the competition between both the defect state emission and intrinsic state emission) is explored in detail. Experiments demonstrate that the chemical structure changes during modification or reduction suppresses non‐radiative recombination of localized electron‐hole pairs and/or enhances the integrity of surface π electron network. Therefore the intrinsic state emission plays a leading role, as opposed to defect state emission in GQDs. The results of time‐resolved measurements are consistent with the suggested PL mechanism. Up‐conversion PL of GQDs is successfully applied in near‐IR excitation for bioimaging.

Blue luminescent graphene quantum dots and graphene oxide prepared by tuning the carbonization degree of citric acid

An easy bottom–up method for the preparation of photoluminescent (PL) graphene quantum dots (GQDs) and graphene oxide (GO) has been developed by tuning the carbonization degree of citric acid and dispersing the carbonized products into alkaline solutions. The GQDs are nanosheets ∼15nm in width, and 0.5–2.0nm in thickness. They show a relatively strong (9.0%) PL quantum yield and an excitation-independent PL emission activity. In contrast, the GO nanostructures consist of sheets that are hundreds of nanometers in width and ∼1nm in height. They exhibit a relatively weak (2.2%) PL quantum yield and an excitation-dependent PL emission activity.