Photoluminescent Graphene Quantum Dots Research Articles

Ab-initio Calculations of Vibrational Fingerprints in the Photoluminescence of Graphene Quantum Dots

Ab-initio Calculations of Vibrational Fingerprints in the Photoluminescence of Graphene Quantum Dots

Opening band gap of multi-color graphene quantum dots using D-fructose as a green precursor

Graphene was successfully isolated by Professors Andre Geim and Kostya Novoselov in 2004. Graphene has attracted considerable research importance for its distinctive properties and potential applications. In this study, low-cost and high blue, yellow, and green photoluminescent (PL) graphene quantum dots (GQDs) were synthesized with deionized water (DI) and D-fructose as precursors by the dehydration method in the presence of hydrochloric acid. Transmission electron microscopy imaging proved the changing size of the GQDs and, by nuclear magnetic resonance spectroscopy, we observed the hydrogen-functionalized GQDs. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of the synthesized GQDs were determined, and an energy bandgap was obtained from optical investigations.

Planted Graphene Quantum Dots for Targeted, Enhanced Tumor Imaging and Long-Term Visualization of Local Pharmacokinetics.

While photoluminescent graphene quantum dots (GQDs) have long been considered very suitable for bioimaging owing to their protein-like size, superhigh photostability and in vivo long-term biosafety, their unique and crucial bioimaging applications in vivo remain unreachable. Herein, planted GQDs are presented as an excellent tool for in vivo fluorescent, sustainable and multimodality tumor bioimaging in various scenarios. The GQDs are in situ planted in the poly(ethylene glycol) (PEG) layer of PEGylated nanoparticles via a bottom-up molecular approach to obtain the NPs-GQDs-PEG nanocomposite. The planted GQDs show more than four times prolonged blood circulation and 7-8 times increased tumor accumulation than typical GQDs in vivo. After accessible specificity modification, the multifunctional NPs-GQDs-PEG provides targeted, multimodal molecular imaging for various tumor models in vitro or in vivo. Moreover, the highly photostable GQDs enable long-term, real-time visualization of the local pharmacokinetics of NPs in vivo. Planting GQDs in PEGylated nanomedicine offers a new strategy for broad in vivo biomedical applications of GQDs.

Folic Acid Modified Graphene Quantum Dots from Konjac Glucomannan for Cell Imaging and Targeted Drug Delivery

AbstractGreen and low‐cost photoluminescent graphene quantum dots (GQDs) were prepared by a one‐step hydrothermal method with konjac glucomannan (KGM), a natural polysaccharide, as the precursors. Then, the folic acid (FA)‐functionalized GQDs (FA‐GQDs) was formed by the amidation of GQDs with FA, and the drug carrier named as FA‐GQDs‐DOX was obtained by loading doxorubicin (DOX) on FA‐GQDs. Due to the inherent fluorescence properties of GQDs and DOX in FA‐GQDs‐DOX, the resultant samples could be used to selectively label cancer cells without external dyes, and the cellular uptake and the drug‐releasing behavior could be visualized. The coupling of FA with GQDs and the loading of DOX on FA‐GQDs were confirmed by various means. In addition, the cell imaging showed that the FA‐GQDs could be used as a fluorescent probe to recognize folate receptor (FR)‐positive cancer cells and that the FA‐GQDs‐DOX could selectively deliver DOX to FR‐positive cancer cells. As a result, the FA‐GQDs‐DOX could be applied in early diagnosis and targeted therapy of cancer.

Gamma-Ray-Induced Structural Transformation of GQDs towards the Improvement of Their Optical Properties, Monitoring of Selected Toxic Compounds, and Photo-Induced Effects on Bacterial Strains.

Structural modification of different carbon-based nanomaterials is often necessary to improve their morphology and optical properties, particularly the incorporation of N-atoms in graphene quantum dots (GQDs). Here, a clean, simple, one-step, and eco-friendly method for N-doping of GQDs using gamma irradiation is reported. GQDs were irradiated in the presence of the different ethylenediamine (EDA) amounts (1 g, 5 g, and 10 g) and the highest % of N was detected in the presence of 10 g. N-doped GQDs emitted strong, blue photoluminescence (PL). Photoluminescence quantum yield was increased from 1.45, as obtained for non-irradiated dots, to 7.24% for those irradiated in the presence of 1 g of EDA. Modified GQDs were investigated as a PL probe for the detection of insecticide Carbofuran (2,2-Dimethyl-2,3-dihydro-1-benzofuran-7-yl methylcarbamate) and herbicide Amitrole (3-amino-1,2,4-triazole). The limit of detection was 5.4 μmol L−1 for Carbofuran. For the first time, Amitrole was detected by GQDs in a turn-off/turn-on mechanism using Pd(II) ions as a quenching agent. First, Pd(II) ions were quenched (turn-off) PL of GQDs, while after Amitrole addition, PL was recovered linearly with Amitrole concentration (turn-on). LOD was 2.03 μmol L−1. These results suggest that modified GQDs can be used as an efficient new material for Carbofuran and Amitrole detection. Furthermore, the phototoxicity of dots was investigated on both Gram-positive and Gram-negative bacterial strains. When bacterial cells were exposed to different GQD concentrations and illuminated with light of 470 nm wavelength, the toxic effects were not observed.

Preparation and Characterization of Photoluminescent Graphene Quantum Dots from Watermelon Rind Waste for the Detection of Ferric Ions and Cellular Bio-Imaging Applications.

Graphene quantum dots (GQDs) were synthesized using watermelon rind waste as a photoluminescent (PL) agent for ferric ion (Fe) detection and in vitro cellular bio-imaging. A green and simple one-pot hydrothermal technique was employed to prepare the GQDs. Their crystalline structures corresponded to the lattice fringe of graphene, possessing amide, hydroxyl, and carboxyl functional groups. The GQDs exhibited a relatively high quantum yield of approximately 37%. Prominent blue emission under UV excitation and highly selective PL quenching for Fe were observed. Furthermore, Fe could be detected at concentrations as low as 0.28 M (limit of detection), allowing for high sensitivity toward Fe detection in tap and drinking water samples. In the bio-imaging experiment, the GQDs exhibited a low cytotoxicity for the HeLa cells, and they were clearly illuminated at an excitation wavelength of 405 nm. These results can serve as the basis for developing an environment-friendly, simple, and cost-effective approach of using food waste by converting them into photoluminescent nanomaterials for the detection of metal ions in field water samples and biological cellular studies.

Synthesis and Cell Imaging of Graphene Quantum Dots from Konjac Glucomannan

AbstractGreen photoluminescent graphene quantum dots (GQDs) were synthesized from konjac glucomannan (KGM), which is a natural polysaccharide, by a one‐step hydrothermal method. The resultant GQDs were characterized by various methods such as elemental analysis, X‐ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, atomic force microscopy, high‐resolution transmission electron microscopy, X‐ray diffractometry, and particle size analysis. The photoluminescence properties of the products were evaluated by ultraviolet‐visible spectroscopy, Raman spectroscopy, and fluorescence spectrometry. The lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) of the products were calculated and verified by cyclic voltammetry. A cytotoxicity test indicated that the GQDs prepared from KGM had good biocompatibility. Moreover, the GQDs had excellent cell imaging performance, which suggests that the resultant products are promising candidates for biomedicine.

(Invited) Graphene-Based Materials for Sensing Applications

Graphene-related materials (GRM) such as graphene oxide (GO) and graphene quantum dots (GQDs) display advantageous characteristics with interest for building innovative biosensing platforms and even smart devices such as nano/micromotors for a myriad of uses including sensing. Quenching of the fluorescence induced by GO or photoluminescence of GQDs can easily operate in synergy with various other nanomaterials and platforms opening the way to several unprecedented biosensing strategies and unique nanomotor technologies. Taking advantage of GRMs we are developing simple, sensitive, selective and rapid biosensing platforms that include: a) GO – based microarray & laterals flow technologies taking advantages of high quenching efficiency of GO. A “turn ON by a pathogen” device will be shown as a highly sensitive detection system using plastics or paper/nanopaper substrates; b) GQDs–based sensors for contaminants detection based on the use of multifunctional composite materials that enable rapid, simple and sensitive platforms in connection to smartphone; c) electroluminescent-based approaches d) A water activated GO transfer technology using wax printed membranes for fast patterning of a touch sensitive device with interest for electronic devices including sensing as well as for a cost-efficient nanomotor building technology for several applications. This work is supported by EU (Graphene Flagship), CERCA Programme / Generalitat de Catalunya.

Quenching‐Resistant Solid‐State Photoluminescence of Graphene Quantum Dots: Reduction of π−π Stacking by Surface Functionalization with POSS, PEG, and HDA

AbstractGraphene quantum dots (GQDs) have attracted great attention as next‐generation luminescent nanomaterials due to the advantages of a low‐cost process, low toxicity, and unique photoluminescence (PL). However, in the solid‐state, the strong π−π stacking interactions between the basal planes of GQDs lead to aggregation‐caused PL quenching (ACQ), which impedes practical application to light‐emitting devices. Here, surface functionalized GQDs (F‐GQDs) by polyhedral oligomeric silsesquioxane (POSS), poly(ethylene glycol) (PEG), and hexadecylamine (HDA) to reduce π−π stacking‐induced ACQ is presented. The POSS‐, PEG‐, and HDA‐functionalized GQDs show a significant enhancement in PL intensity compared to bare GQDs by 9.5‐, 9.0‐, and 5.6‐fold in spin‐coated film form and by 8.3‐, 7.2‐, and 3.4‐fold in drop‐casted film form, respectively. Experimental results and molecular dynamics simulations indicate that steric hindrance of the functionalization agent contributes to reducing the π−π stacking between adjacent GQDs and thereby enabling quenching‐resistant PL in the solid‐state. Moreover, the GQD‐based white light‐emitting diodes fabricated by mounting HDA‐GQDs on a UV‐LED chip exhibits efficient down‐conversion for white light emission with a high color rendering index of 86.2 and a correlated‐color temperature of 5612 K at Commission Internationale de l'Éclairage coordinates of (0.333, 0.359).

Graphene Quantum Dots from Partially Unzipped Multi-Walled Carbon Nanotubes: Promising Materials for Oxygen Electrodes

Photoluminescent graphene quantum dots (GQDs) the one- or two-layer graphene nanoparticles 1.5–3 nm in size with a blue emission have been obtained from electrochemically synthesized partially unzipped multi-walled carbon nanotubes ultrasonically treated for one hour. Various physicochemical methods including photoluminescence spectroscopy, XRD, TEM, and Raman spectroscopy, have been used to characterize the electrochemically synthesized partially unzipped multi-walled carbon nanotubes and GQDs. Two-layer oxygen electrodes were fabricated, where GQDs served as an active layer. The investigations of electrocatalytic characteristics of the oxygen electrodes fabricated of the obtained materials were carried out in a fuel half-cell with an alkaline electrolyte. The fabricated oxygen electrodes were stable for six months at a discharge current density of 200 mA cm−2. The obtained GQDs are promising materials as a new type of catalyst carriers for oxygen electrodes of fuel cells.

One-step synthesis of fluorescent graphene quantum dots as an effective fluorescence probe for vanillin detection

This study proposes an easy bottom-up method for the synthesis of photoluminescent (PL) graphene quantum dots (GQDs) using citric acid as the carbon source. The obtained GQDs were characterized by high-resolution transmission electron microscopy (HRTEM), UV-vis absorption spectroscopy, fluorescence spectroscopy, and Fourier transform infrared spectroscopy (FT-IR). The synthesised GQDs have an average diameter of 4.76 ± 0.96 nm, with a lattice spacing of 0.24 nm. The GQDs exhibit excitation-independent PL emission. The surface of the GQDs has a variety of functional groups (hydroxyl, carboxyl, and ether groups etc.) to enhance its stability and water solubility. In this study, a fluorescent “on–off” sensor is developed for the selective detection of vanillin in chocolates using GQDs as a fluorescent probe. Under optimal conditions, fluorescence intensity of the GQDs has a good linear relationship with the vanillin concentration (0.0–2.1 × 10−5 mol L−1), with a limit of detection of 2.5 × 10−8 mol L−1. For detection in real samples, the percent recovery of vanillin and the relative standard deviation were 88.0–108.9% and 0.90–5.4%, respectively. Thus, this GQDs-based method has good accuracy and precision and can be used for vanillin detection in practical applications.

Simple Method of Graphene Quantum Dots Preparation from Partially Unzipped Multi-Walled Carbon Nanotubes

A method for producing graphene quantum dots (GQDs) by ultrasonic grinding of e1lectrochemically obtained partially unzipped multi-walled carbon nanotubes is proposed. As a result, photoluminescent GQDs with a blue glow have been obtained from partially unzipped multi-walled carbon nanotubes processed two hours by ultrasonic grinding (UZ-CNTs). Various physicochemical methods including photoluminescence spectroscopy have been used to study the electrochemically synthesized UZ-CNTs GQD. After ultrasonic grinding of electrochemically synthesized UZ-CNTs the GQDs were obtained as a result of the exfoliation from the stress-bent MWCNTs walls one- or two-layer cup-shaped graphene nanoparticles 1.5-3 nm in size. The obtained GQDs are promising materials as a new type of catalyst carriers for oxygen electrodes of fuel cells.

Temperature-dependent Raman investigation and photoluminescence of graphene quantum dots with and without nitrogen-doping

The study of the temperature dependence of Raman spectrum of graphene quantum dots is important for further understanding of the fine structure and properties of the material such as atomic bonds, thermal expansion, and thermal conductivity. In this work, we present new results on the temperature dependence of the frequency of G peak in the Raman spectra of bare GQDs and nitrogen-doped graphene quantum dots (N-GQDs), respectively. The nature of these GQDs is investigated using high-resolution transmission electron microscopy together with Raman, absorption, and photoluminescence spectra. We monitored a shift in G peak frequency toward lower frequencies with the temperature ranging from 81 to 663 K. In addition, a slight decrease in G peak intensity and an increase in D peak intensity have been observed, which is especially clear for Raman spectra of N-GQD sample. From the linear relationship of G peak frequency with temperature, we determine for the first time the value of the temperature coefficient χ of the G mode. Values of − 0.0222 ± 0.001 cm−1 K−1 for GQDs, and − 0.0243 ± 0.0013 cm−1 K−1 for N-GQDs were deduced. These results show a clear heat expansion in the graphene lattice, although small in size, when sample temperature is increased from 81 to 663 K. Our result is important to further understand physical effects induced by anharmonic phonons in QGDs and should be taken to account when designing efficient nanoelectronic devices employing GQDs.

Photoluminescence of graphene quantum dots enhanced by microwave post-treatment

Graphene quantum dots (GQDs) have a great potential in sensors and bioimaging for their lower toxicity, perfect water solubility and bright luminescence. However, considerable efforts still need to devote to enhance the photoluminescence quantum yields (PLQYs) of GQDs. At first, high PLQYs are guaranteed by confined synthesizing GQDs/layered double hydroxides (LDHs) with well-defined structure. Followed by a convenient microwave post-treatment, PLQYs of GQDs are further improved from 21.0% to 34.6%. After microwave post-treatment, the integrity of π-conjugated system has been enhanced and non-radiative processes also have been reduced inside GQDs, which both confirmed to be responsible for the improvement of PL properties. This work may point out a new direction for enhancing the PLQYs of GQDs. Moreover, owing to the stability and well PL performance of GQDs/LDHs, it has a great potential application in the outdoor anti-counterfeiting inks.

Room temperature manufacturing photoluminescent graphene quantum dots based on MXene

Carbide–derived graphene quantum dot (GQDs) have high potential to the photoluminescent field owing to its non-toxicity, strong light harvesting capability and large surface area. In order to cost-effectively and stably produce GQDs, lowering synthesis temperature of GQDs is one of the great candidates, but it has been still a big challenge. Herein, the photoluminescent GQDs were obtained by KOH (aq) of MXene at room temperature controlling the reaction time and concentration. This facile and cost-effective method can produce GQDs/TiO2 nanoparticles which had the average particle size about 1.5 nm with minor portion of amorphous carbon. The functionalized and defects in GQDs/TiO2 crystalline nanoparticles were characterized according to fabricating conditions and photoluminescent properties were precisely detected. Although the quantum yield of fabricated GQDs/TiO2 nanoparticles is 2.06%, it could be improved by optimizing the conditions of KOH activation. It was the meaningful result using facile synthesis for GQDs/TiO2 nanoparticles at room temperature and it could encompass key factor to manufacturing quantum dots. The technique can be considered general enough to expand to various research fields such as information security, display, biocompatible multicolor cellular imaging and sensors.

Hydroxyl rich graphene quantum dots for the determination of Hg(II) in the presence of large concentration of major interferents and in living cells

Sensitive determination of Hg(II) by photoluminescence (PL) quenching of graphene quantum dots (GQDs) has been investigated extensively in the past. However, its selective determination was reported only in the presence of same concentration of other interferences. The present study reports the sensitive and selective determination of Hg(II) at pH 7 in the presence of 500- and 1000-fold higher concentration of Fe(III) and other metal ions, respectively using hydroxyl rich GQDs for the first time. The GQDs prepared from citric acid exhibit a strong blue luminescence with excitation independent emission properties. The PL of GQDs was completely vanished after the addition of Hg(II) whereas it was noticeably affected while adding Fe(III). However, the PL remains unaltered while adding other heavy and transition metal ions. Thus, the present fluorescent probe was used for the selective and sensitive determination of Hg(II). As the concentration of Hg(II) increases from 0 to 20 µM, the emission intensity linearly decreases with the correlation coefficient of 0.995 and the limit of detection (LOD) was found to be 9.87 nM (S/N=3). Among the different metal ions, Fe(III) alone interferes on the determination of Hg(II). However, the GQDs successfully determine Hg(II) in the presence of 500- and 1000-fold higher concentration of Fe(III) and other metal ions, respectively. The results obtained from the different spectroscopic and microscopic studies indicated that the quenching of GQDs was due to both complexation and formation of metallic Hg. The SEM studies showed that Hg(0) was formed on GQDs surface with size 10-20 nm. Finally, the present fluorescent probe was used to determine Hg(II) in different water samples and lung cancer cells.

Facile Synthesis of Water-Soluble, Highly-Fluorescent Graphene Quantum Dots from Graphene Oxide Reduction for Efficient Cell Labelling

In this work, we report a simple, one-step, green procedure to fabricate strong blue and yellow photoluminescent graphene quantum dots (GQDs) as by-product of the synthesis of mesoporous graphene hydrogel (GHs). The graphene hydrogel was obtained by chemical reduction of graphene oxide using ascorbic acid at mild temperature. As a consequence of the network formation, small fluorescent GQDs can be isolated from the residual solvent, purified from the by-products and finally concentrated to produce GQDs. The GQDs chemistry and morphology were characterized by X-ray Photoelectron Spectroscopy (XPS) and Transmission Electron Microscopy (TEM). The GQDs mean diameter was about 5–10 nm and they exhibited an intense luminescence in the visible range with an excitation wavelength-dependent fluorescence. Our experiments showed that GQDs were easily internalized in living cells and furthermore, such internalization did not adversely affect cell viability.

Calcium‐Induced Photoluminescence Quenching of Graphene Quantum Dots in Hard Water: A Quick Turn‐Off Sensing Approach

AbstractThe photoluminescence of graphene quantum dots (GQD) is widely explored for sensory applications. Among various metal ions to be detected in aqueous bodies, Ca2+ ions are often neglected which is evident from the lack of literature. However, owing to booming industrialization, the available potable water is mostly crossing the prescribed hardness limits. Presented report summarize the synthesis of GQD via modified hydrothermal cutting of graphene oxide, which was further employed for sensing Ca2+ ions in hard water samples. The as synthesized GQD exhibited its characteristic blue photoluminescence on excitation with longwave UV (λ ≈ 365 nm) which was suppressed on the introduction of Ca2+ ions. The GQD as photoluminescent probe determined the concentration of Ca2+ ion in water with an accuracy of 96.74% ± 3.26%. The LOD of the reported method was found to be 5 μM and it was sensitive towards Ca2+ ions in the presence of interfering ions such as Al3+, Mg2+, and K+ which are commonly found in hard water samples. Therefore, the determination of hardness of water w.r.t Ca2+ using GQD based PL turn off sensing is a quick, viable and economical method.

Facile design and development of photoluminescent graphene quantum dots grafted dextran/glycol-polymeric hydrogel for thermoresponsive triggered delivery of buprenorphine on pain management in tissue implantation

A novel nano-formulations of biocompatible, biodegradable and thermo-responsive graphene quantum dots (GQDs) loaded dextran/poly(N-isopropylacrylamide) (Dex/PNIPAM) copolymeric matrix was synthesized and analyzed the materials characterization, sustained drug delivery system, tissue feasibility in the tissue implantation site. This research report was aimed to grafting and functionalizing thermo-responsive (Dex/PNIPAM) copolymeric composite with presence of graphene quantum dots to achieve thermal responsive drug delivery (TrDD) with no harm effect in the implantation site. The synthesized GQD by using ionic liquid were evaluated by spectroscopic (DLS, PL, XRD and Raman spectroscopy) and Transmission electron microscopic analysis (TEM). The ultra-small GQDs loaded Dex/PNIPAM and was appeared to be asymmetric and open uniform porous structure, which can be significantly favorable for cell uptake and greatly influenced to be an effective drug carrier into the cellular compartment with good fluid flow. The PNIPAM polymeric composite were exhibited sustained and enhanced drug release percentages with increasing temperature at above low critical solution temperature (LCST) is 39 °C comparable to the cumulative drug release profile of below LCST (32 °C), which demonstrated that thermo-responsive polymer was played a significant role in the delivery system. The treated group of GQDs-Dex/PNIPAM was observed that no inflammation and shows noteworthy stromal cell infiltration, demonstrating that the synthesized drug carriers did not harm to the nerves and tissues and only was responsible for the pain management.

N-Doped Graphene Quantum Dots Using Different Bases

Photoluminescent graphene quantum dots (GQDs) have received tremendous attention due to their sui generis chemical, electronic and optical properties but fabricating the pristine quality of GQD is extremely challenging. Herein, we have reported the pyrolysis of citric acid which in the presence of different bases viz. triethylamine, ammonium hydroxide and urea, produced N-doped GQDs at different pH. The effect of different pH has been studied in detail to optimize the formation conditions of the GQD. Ultraviolet–visible (UV–Vis) spectroscopy and normalized fluorescence spectra were applied to analyze the optical properties of the GQD. The mean particle size was analyzed by a particle size analyzer (dynamic light dispersion).