Novel Formaldehyde Sensor based on Hydrogen Peroxide/Melamine Modulated Photoluminescence of Nitrogen-doped Graphene Quantum Dots

A modulated photoluminescence nanosensor was developed for the quantitative detection of formaldehyde with nitrogen-doped graphene quantum dots and melamine. The sensing system was based on the different activated effects of melamine and hydrogen peroxide on the photoluminescence intensity of nitrogendoped graphene quantum dots. Under the optimal conditions, the modulated photoluminescence sensing system can be used to detect formaldehyde with a good linear relationship between the nitrogen-doped graphene quantum dots photoluminescence difference and the concentration of formaldehyde. The novel sensing system provided new directions for the detection of formaldehyde with high selectivity and quick response.

N-doped graphene quantum dot-decorated N-TiO2/P-doped porous hollow g-C_(3)N_(4) nanotube composite photocatalysts for antibiotic photodegradation and H2 production

Exclusive responsiveness to ultraviolet light (~3.2 eV) and high photogenerated charge recombination rate are the two primary drawbacks of pure TiO_(2). We combined N-doped graphene quantum dots (N-GQDs), morphology regulation, and heterojunction construction strategies to synthesize N-GQD/N-doped TiO_(2)/P-doped porous hollow g-C_(3)N_(4) nanotube (PCN) composite photocatalysts (denoted as G-TPCN). The optimal sample (G-TPCN doped with 0.1wt% N-GQD, denoted as 0.1% G-TPCN) exhibits significantly enhanced photoabsorption, which is attributed to the change in bandgap caused by elemental doping (P and N), the improved light-harvesting resulting from the tube structure, and the upconversion effect of N-GQDs. In addition, the internal charge separation and transfer capability of0.1% G-TPCN are dramatically boosted, and its carrier concentration is 3.7, 2.3, and 1.9 times that of N-TiO_(2), PCN, and N-TiO_(2)/PCN(TPCN-1), respectively. This phenomenon is attributed to the formation of Z-scheme heterojunction between N-TiO_(2) and PCNs, the excellent electron conduction ability of N-GQDs, and the short transfer distance caused by the porous nanotube structure. Compared with those of N-TiO_(2), PCNs, and TPCN-1, the H2 production activity of 0.1%G-TPCN under visible light is enhanced by 12.4, 2.3, and 1.4times, respectively, and its ciprofloxacin (CIP) degradation rate is increased by 7.9, 5.7, and 2.9 times, respectively. The optimized performance benefits from excellent photoresponsiveness and improved carrier separation and migration efficiencies. Finally, the photocatalytic mechanism of 0.1% G-TPCN and five possible degradation pathways of CIP are proposed. This study clarifies the mechanism of multiple modification strategies to synergistically improve the photocatalytic performance of 0.1% G-TPCN and provides a potential strategy for rationally designing novel photocatalysts for environmental remediation and solar energy conversion.

Enhanced formaldehyde sensitivity of two-dimensional mesoporous SnO_(2) by nitrogen-doped graphene quantum dots

Formaldehyde(HCHO) is widely known as an indoor air pollutant,and the monitoring of the gas has significant importance.However,most HCHO sensing materials do not have low detection limits and operate at high temperatures.Herein,two-dimensional(2D) mesoporous ultrathin SnO_(2) modified with nitrogen-doped graphene quantum dots(N-GQDs) was synthesized.The N-GQDs/SnO_(2) nanocomposite demonstrated high efficiency for HCHO detection.With the addition of 1.00 wt%N-GQDs,the response(Ra/Rg) of SnO_(2) gas sensor increased from 120 to 361 at 60℃ for the detection of 10×10^(-6) HCHO.In addition,the corresponding detection limit was as low as 10×10^(-9).Moreover,the sensor exhibited excellent selectivity and stability for the detection of HCHO.The enhanced sensing performance was attributed to both the large specific surface area of SnO_(2) and electron regulation of N-GQDs.Therefore,this study presents a novel HCHO sensor,and it expands the research and application potential of GQDs nanocomposites.

Layered MoS2@graphene functionalized with nitrogen-doped graphene quantum dots as an enhanced electrochemical hydrogen evolution catalyst

A novel three-dimensional (3D) layered MoS2@graphene functionalized with nitrogen-doped graphene quantum dots (MoS2@N-GQDs-GR) composites as an enhanced electrochemical hydrogen evolution catalyst. The few layered MoS2 nanoflowers supported on N-GQDs-GR surface were elaborately fabricated by one-pot hydrothermal method, which MoS2 and N-GQDs-GR exist in a bonding manner of Mo-N. In addition, due to the layered MoS2 sheet edge exposes more hydrogen evolution active sites and N-GQDs-GR have high conductivity, the composites exhibit prominent electrocatalytic activity with a low overpotential 99 mV, a small Tafel slope 49.3 mV/dec. Therefore, that the current work will develop HER catalysts may replace Pt.

Tailoring color emissions from N-doped graphene quantum dots for bioimaging applications

Unlike inorganic quantum dots,fluorescent graphene quantum dots(GQDs)display excitation-dependent multiple color emission.In this study,we report N-doped GQDs(N-GQDs)with tailored single color emission by tuning p-conjugation degree,which is comparable to the inorganic quantum dot.Starting from citric acid and diethylenetriamine,as prepared N-GQDs display blue,green,and yellow light emission by changing the reaction solvent from water,dimethylformamide(DMF),and solvent free.The X-ray photoelectron spectroscopy,ultraviolet-visible spectra results clearly show the N-GQDs with blue emission(N-GQDs-B)have relatively short effective conjugation length and more carboxyl group because H_(2)O is a polar protic solvent,which tends to donate proton to the reagent to depress the H_(2)O elimination reaction.On the other hand,the polar aprotic solvent(DMF)cannot donate hydrogen,the elimination of H_(2)O is promoted and more nitrogen units enter GQD framework.With the increase of effective p-conjugation length and N content,the emission band of N-GQDS red-shifts to green and yellow.We also demonstrate that N-GQDs could be a potential great biomarker for fluorescent bioimaging.

Nitrogen-doped graphene quantum dots: Optical properties modification and photovoltaic applications

In this work,we utilize a bottom-up approach to synthesize nitrogen self-doped graphene quantum dots (NGQDs) from a single glucosamine precursor via an eco-friendly microwave-assisted hydrothermal method.Structural and optical properties of as-produced NGQDs are further modified using controlled ozone treatment.Ozone-treated NGQDs (Oz-NGQDs) are reduced in size to 5.5 nm with clear changes in the lattice structure and/D//G Raman ratios due to the introduction/alteration of oxygen-containing functional groups detected by Fourier-transform infrared (FTIR) spectrometer and further verified by energy dispersive X-ray spectroscopy (EDX) showing increased atomic/weight percentage of oxygen atoms.Along with structural modifications,GQDs experience decrease in ultraviolet-visible (UV-vis) absorption coupled with progressive enhancement of visible (up to 16 min treatment) and near-infrared (NIR)(up to 45 min treatment) fluorescence.This allows fine-tuning optical properties of NGQDs for solar cell applications yielding controlled emission increase,while controlled emission quenching was achieved by either blue laser or thermal treatment.Optimized Oz-NGQDs were further used to form a photoactive layer of solar calls with a maximum efficiency of 2.64% providing a 6-fold enhancement over untreated NGQD devices and a 3-fold increase in fill factor/current density.This study suggests simple routes to alter and optimize optical properties of scalably produced NGQDs to boost the photovoltaic performance of solar cells.

Graphene quantum dots as sulfiphilic and lithiophilic mediator toward high stability and durable life lithium-sulfur batteries

The development of lithium-sulfur(Li-S) battery as one of the most attractive energy storage systems among lithium metal batteries is seriously hindered by low sulfur utilization, poor cycle stability and uneven redeposition of Li anode. It is necessary to propose strategies to address the problems as well as improve the electrochemical performance. One of the effective solutions is to improve the sulfiphilicity of sulfur cathode and the lithiophilicity of the Li anode. Herein, we reported that a synergistic functional separator(graphene quantum dots(GQDs)-polyacrylonitrile(PAN) @polypropylene(PP) separator)improved the electrochemical activity of sulfur cathode as well as the stability of Li anode. GQDs induced uniform Li^(+)nucleation and deposition, which slowed down the passivation of Li anode and avoided shortcircuit. Further, three-dimensional network constructed by electrospinning nanofibers and the polar functional groups of GQDs could both effectively inhibit the shuttle of LiPSs and improve the sulfur utilization. The stability of Li-S battery was improved by the synergistic effect. In addition, GQDs and electrospinning nanofibers protector increased lifetime of separators. Benefiting from the unique design strategy, Li//Li symmetric battery with GQDs-PAN@PP separators exhibited stably cycling for over 600 h. More importantly, the Li-S full batteries based GQDs-PAN@PP separators enabled high stability and desirable sulfur electrochemistry, including high reversibility of 558.09 mA h g^(-1)for 200 cycles and durable life with a low fading rate of 0.075% per cycle after 500 cycles at 0.5 C. Moreover, an impressive areal capacity of 3.23 mA h cm^(-2)was maintained under high sulfur loading of 5.10 mg cm^(-2). This work provides a new insight for modification separator to improve the electrochemical performance of Li-S/Li metal batteries.

Graphene quantum dots doped poly(vinyl alcohol)hybrid membranes for desalination via pervaporation

Pervaporation desalination by highly hydrophilic materials such as poly(vinyl alcohol)(PVA)based separation membrane is a burgeoning technology of late years.However,the improvement of membrane flux in pervaporation desalination has been a difficult task.Here,a novel hybrid membrane with doped graphene oxide quantum dots(GOQDs)which is rich in hydrophilic groups and small size into the matrix of PVA was prepared to improve the membrane flux.The membranes structures were described by field emission scanning electron microscopy(FESEM),atomic force microscopy(AFM),Fourier transform infrared(FT-IR),differential scanning calorimetry(DSC),thermogravimetric analysis(TGA)and X-ray diffraction(XRD).And more,Water contact angle,swelling degree,and pervaporation properties were carried out to explore the effect of GOQDs in PVA matrix.In addition,GOQDs content in the hybrid membrane,NaCl concentration,and feed temperature were investigated accordingly.Moreover,the hydrogen bonds between PVA chains were weakened by the interaction between GOQDs and PVA chains.Significantly,the hybrid membrane with optimized doped GOQDs content,200 mg·L^(-1),displays a high membrane flux of 17.09 kg·m^(-2)·h^(-1)and the salt rejection is consistently greater than 99.6%.

Synergistically coupling of graphene quantum dots with Zn-intercalated MnO2 cathode for high-performance aqueous Zn-ion batteries

Cost-effective,safe,and highly performing energy storage devices require rechargeable batteries,and among various options,aqueous zinc-ion batteries(ZIBs)have shown high promise in this regard.As a cathode material for the aqueous ZIBs,manganese dioxide(MnO_(2))has been found to be promising,but certain drawbacks of this cathode material are slow charge-transfer capability and poor cycling performance.Herein,a novel design of graphene quantum dots(GQDs)integrated with Zn-intercalated MnO_(2)nanosheets is put forward to construct a 3D nanoflower-like GQDs@ZnxMnO_(2)composite cathode for aqueous ZIBs.The synergistic coupling of GQDs modification with Zn intercalation provides abundant active sites and conductive medium to facilitate the ion/electron transmission,as well as ensure the GQDs@ZnxMnO_(2)composite cathode with enhanced charge-transfer capability and high electrochemical reversibility,which are elucidated by experiment results and in-situ Raman investigation.These impressive properties endow the GQDs@ZnxMnO_(2)composite cathode with superior aqueous Zn^(2+) storage capacity(~403.6 mAh·g^(−1)),excellent electrochemical kinetics,and good structural stability.For actual applications,the fabricated aqueous ZIBs can deliver a substantial energy density(226.8 W·h·kg^(−1)),a remarkable power density(650 W·kg^(−1)),and long-term cycle performance,further stimulating their potential application as efficient electrochemical storage devices for various energy-related fields.

Graphene Quantum Dot‑Mediated Atom‑Layer Semiconductor Electrocatalyst for Hydrogen Evolution

The hydrogen evolution reaction performance of semiconducting 2H-phase molybdenum disulfide(2H-MoS_(2))presents a significant hurdle in realizing its full potential applications.Here,we utilize theoretical calculations to predict possible functionalized graphene quantum dots(GQDs),which can enhance HER activity of bulk MoS_(2).Subsequently,we design a functionalized GQD-induced in-situ bottom-up strategy to fabricate near atom-layer 2H-MoS_(2) nanosheets mediated with GQDs(ALQD)by modulating the concentration of electron withdrawing/donating functional groups.Experimental results reveal that the introduction of a series of functionalized GQDs during the synthesis of ALQD plays a crucial role.Notably,the higher the concentration and strength of electron-withdrawing functional groups on GQDs,the thinner and more active the resulting ALQD are.Remarkably,the synthesized near atom-layer ALQD-SO_(3)demonstrate significantly improved HER performance.Our GQD-induced strategy provides a simple and efficient approach for expanding the catalytic application of MoS_(2).Furthermore,it holds substantial potential for developing nanosheets in other transition-metal dichalcogenide materials.

Effects of Coal Rank and High Organic Sulfur on the Structure and Optical Properties of Coal-based Graphene Quantum Dots

Coal-based graphene quantum dots(GQDs) were successfully produced via a one-step chemical synthesis from six different coal ranks, from which two superhigh organic sulfur(SHOS) coals were selected as natural S-doped carbon sources for the preparation of S-doped GQDs. The effects of coal properties on coal-based GQDs were analyzed by means of high-resolution transmission electron microscopy(HRTEM), X-ray diffraction(XRD), Fourier transform infrared(FTIR) spectroscopy, X-ray photoelectron spectroscopy(XPS), ultraviolet-visible(UV-Vis) absorption spectroscopy, and fluorescence emission spectra. It was shown that all coal samples can be used to prepare GQDs, which emit bluegreen and blue fluorescence under ultraviolet light. Anthracite-based GQDs have a hexagonal crystal structure without defects, the largest size, and densely arranged carbon rings in their lamellae; the highrank bituminous coal-based GQDs are relatively reduced in size, with their hexagonal crystal structure being only faintly visible; the low-rank bituminous coal-based GQDs are the smallest, with sparse lattice fringes and visible internal defects. As the metamorphism of raw coals increases, the yield decreases and the fluorescence quantum yield(QY) initially increases and then decreases. Additionally, the surface of GQDs that were prepared using high-rank SHOS coal(high-rank bituminous coal) preserves rich sulfur content even after strong oxidation, which effectively adjusts the bandgap and improves the fluorescence QY. Thus, high-rank bituminous coal with SHOS content can be used as a natural S-doped carbon source to prepare S-doped GQDs, extending the clean utilization of low-grade coal.

Toxicity of Graphene Quantum Dots in Zebrafish Embryo

Objective To evaluate the bio-safety of graphene quantum dots （GQDs）, we studied its effects on the embryonic development of zebrafish. Methods In vivo, biodistribution and the developmental toxicity of GQDs were investigated in embryonic zebrafish at exposure concentrations ranging from 12.5-200μg/mL for 4-96 h post-fertilization （hpf）. The mortality, hatch rate, malformation, heart rate, GQDs uptake, spontaneous movement, and larval behavior were examined. Results The fluorescence of GQDs was mainly localized in the intestines and heart. As the exposure concentration increased, the hatch and heart rate decreased, accompanied by an increase in mortality. Exposure to a high level of GQDs （200μg/mL） resulted in various embryonic malformations including pericardial edema, vitelline cyst, bent spine, and bent tail. The spontaneous movement significantly decreased after exposure to GQDs at concentrations of 50, 100, and 200μg/mL. The larval behavior testing （visible light test） showed that the total swimming distance and speed decreased dose-dependently. Embryos exposed to 12.5 μg/mL showed hyperactivity while exposure to higher concentrations （25, 50, 100, and 200μg/mL） caused remarkable hypoactivity in the light-dark test. Conclusion Low concentrations of GODs were relatively non-toxic. However, GQDs disrupt the progression of embryonic development at concentrations exceeding 50 μg/mL.

High-Performance Photo-Modulated Thin-Film Transistor Based on Quantum dots/Reduced Graphene Oxide Fragment-Decorated ZnO Nanowires

In this paper, a photo-modulated transistor based on the thin-film transistor structure was fabricated on the flexible substrate by spin-coating and magnetron sputtering. A novel hybrid material that composed of Cd Se quantum dots and reduced graphene oxide(RGO) fragment-decorated ZnO nanowires was synthesized to overcome the narrow optical sensitive waveband and enhance the photo-responsivity. Due to the enrichment of the interface and heterostructure by RGO fragments being utilized, the photo-responsivity of the transistor was improved to 2000 AW^(-1) and the photo-sensitive wavelength was extended from ultraviolet to visible. In addition, a positive back-gate voltage was employed to reduce the Schottky barrier width of RGO fragments and ZnO nanowires. As a result, the amount of carriers was increased by 10 folds via the modulation of back-gate voltage. With these inherent properties, such as integrated circuit capability and wide optical sensitive waveband, the transistor will manifest great potential in the future applications in photodetectors.

Hydroxylated graphene quantum dots as fluorescent probes for sensitive detection of metal ions

Highly sensitive methods are important for monitoring the concentration of metal ions in industrial wastewater.Here,we developed a new probe for the determination of metal ions by fluorescence quenching.The probe consists of hydroxylated graphene quantum dots(H-GQDs),prepared from GQDs by electrochemical method followed by surface hydroxylation.It is a non-reactive indicator with high sensitivity and detection limits of 0.01μM for Cu2+,0.005μM for Al3+,0.04μM for Fe3+,and 0.02μM for Cr3+.In addition,the low biotoxicity and excellent solubility of H-GQDs make them promising for application in wastewater metal ion detection.

Modulation of magnetic and electrical properties of bilayer graphene quantum dots using rotational stacking faults

Bilayer graphene quantum dots with rotational stacking faults(RSFs) having different rotational angles were studied.Using the first-principles calculation, we determined that these stacking faults could quantitatively modulate the magnetism and the distribution of spin and energy levels in the electronic structures of the dots.In addition, by examining the spatial distribution of unpaired spins and Bader charge analysis, we found that the main source of magnetic moment originated from the edge atoms of the quantum dots.Our research results can potentially provide a new path for producing all-carbon nanodevices with different electrical and magnetic properties.

Quantum computation with two-dimensional graphene quantum dots

We study an array of graphene nano sheets that form a two-dimensional S = 1/2 Kagome spin lattice used for quantum computation. The edge states of the graphene nano sheets are used to form quantum dots to confine electrons and perform the computation. We propose two schemes of bang-bang control to combat decoherence and realize gate operations on this array of quantum dots. It is shown that both schemes contain a great amount of information for quantum computation. The corresponding gate operations are also proposed.

Recent Advances on Graphene Quantum Dots for Electrochemical Energy Storage Devices

Graphene quantum dots(GQDs)which are nanofragments of graphene with an average size between 2 and 50 nm have attracted much attention due to their outstanding properties such as high conductivity,high surface area,and good solubility in various solvents.GQDs combine the quantum confinement and edges effects and the properties of graphene.Therefore,GQDs offers a broad range of applications in various fields(medicine,energy conversion,and energy storage devices).This review will present the recent research based on the introduction of GQDs in batteries,supercapacitors,and microsupercapacitors as electrodes materials or mixed with an active material as an auxiliary agent.Tables,discussed on selected examples,summarize the electrochemical performances and finally,challenges and perspectives are recalled for the subsequent optimization strategy of electrode materials.This review is expected to appeal a broad interest on functional GQDs materials and promote the further development of high-performance energy storage device.

An approach to controlling the fluorescence of graphene quantum dots: From surface oxidation to fluorescent mechanism

We report a facile method of synthesizing graphene quantum dots（GQDs） with tunable emission. The as-prepared GQDs each with a uniform lateral dimension of ca. 6 nm have fine solubility and high stability. The photoluminescence mechanism is further investigated based on the surfacestructure and the photoluminescence behaviors. Based on our discussion, the green fluorescence emission can be attributed to the oxygen functional groups, which could possess broad emission bands within the π –π ＊ gap. This work is helpful to explain the vague fluorescent mechanism of GQDs, and the reported synthetic method is useful to prepare GQDs with controllable fluorescent colors.

Graphene quantum dots for tumor optical diagnosis and treatment

Graphene quantum dots(GQDs)are two-dimensional carbon nano-material with excellent physicochemical properties,including biocompatibility,high photoluminescence,water-solubility,hypotoxicity and so on.Owing to bigπ-conjugated structure and oxygen-containing functional groups,GQDs have the ability to absorb various chemical drugs viaπ-πstacking and electrostatic interaction.On the other hand,owning to photoluminescence,GQDs also have potential to serve as fluorescence probe for bioimaging,especially for optical diagnostics.In addition,GQDs have the ability to generate reactive oxygen species(ROS)including singlet oxygen upon photoexcitation.Therefore,GQDs are likely to be used for photodynamic therapy.This article aims to review the frontier researches about GQDs in the field of fluorescence probe,photodynamic therapy and anti-tumor drug delivery system.

Graphene Quantum Dots Derived from Carbon Fibers for Oxidation of Dopamine

We demonstrated a facile method to prepare photoluminescent graphene quantum dots using commercial polyacrylonitrile（PAN） based carbon fibers（CFs） as the raw material by facile chemical oxidation and exfoliation method. The as-prepared GQDs with uniform size exhibit an excitation-independent photoluminescence behavior, which is similar to other semiconductor quantum dots. Moreover, when acting as catalyst the uniform GQDs have better activity for electrochemical oxidation of dopamine（DA） than graphene oxides（GOs）. The square wave voltammogram（SWV） peak values of GQDs are in good correspondence with DA concentrations and can act as a sensor of DA.