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Nitrogen-doped graphene quantum dots (N-GQDs) are widely used in biosensing, catalysis, and energy storage due to their excellent conductivity, high specific surface area, unique quantum size effects, and optical properties. In this paper, we successfully synthesized N-GQDs using a facile hydrothermal approach and investigated the effects of different hydrothermal temperatures and times on the morphology and structure of N-GQDs. The results indicated that the size of N-GQDs gradually increased and they eventually aggregated into graphene fragments with increasing temperature or reaction time. Notably, N-GQDs synthesized at 180 °C for 6 h exhibited the most uniform size, with an average diameter of approximately 3.48 nm, a height of 5–6 graphene layers, as well as favorable fluorescence properties. Moreover, the surface of N-GQDs contained abundant oxygen- and nitrogen-containing functional groups, which could provide numerous active sites for electrode reactions. The assembled electrode exhibited typical pseudocapacitive behavior with exceptional electrochemical performance, achieving a specific capacitance of 102 F g−1 at a current density of 1 A g−1. In a 10,000-cycle test, the electrode demonstrated excellent cycling stability with a capacitance retention rate of 78.5%, which laid the foundation for practical application of the electrode. This work successfully applied N-GQDs in supercapacitors, offering new insights into their development for the energy storage field. [ABSTRACT FROM AUTHOR]

N‐graphene quantum dots (N‐GQDs) are prepared using graphite as a carbon source and ammonia as a nitrogen source by a top‐down approach, and further combined with SnO2 to obtain SnO2/N‐GQDs composites by a hydrothermal method. SnO2/N‐GQDs composites are analyzed by morphology and structure characterization, and the composites with different N‐GQDs content are further investigated by instantaneous photocurrent response (I‐t), electrochemical impedance (EIS), linear scanning voltammetry (LSV), and Mott‐Schottky curves (MS). SnO2/N‐GQDs composites exhibit an obvious increment in photocurrent and a decrement in interface charge transfer impedance. When the addition of N‐GQDs is 1.5wt.%, the I‐t value of SnO2/N‐GQDs reaches the optimum of 3.84 × 10−5 A cm−2, which is 2.3 times as large as that of SnO2. Meantime, the carrier density of SnO2/N‐GQDs reaches 2.67 × 1021cm−3, which is 1.5 times as high as that of SnO2. The improvement is attributed to the combination of SnO2 and N‐GQDs, which promotes more charge carriers to be generated and transported under UV irradiation. [ABSTRACT FROM AUTHOR]

In recent years quantum dot (QDs) based resistive switching devices(memristors) have gained a lot of attention. Here we report the resistive switching behavior of nitrogen-doped graphene quantum dots/Polyvinyl alcohol (N-GQDs/PVA) degradable nanocomposite thin film with different weight percentages (wt.%) of N-GQDs. The memristor device was fabricated by a simple spin coating technique. It was found that 1 wt% N-GQDs/PVA device shows a prominent resistive switching phenomenon with good cyclic stability, high on/off ratio of ~102 and retention time of ∼104 s. From a detailed experimental study of band structure, we conclude that memristive behavior originates from the space charge controlled conduction (SCLC) mechanism. Further transient property of built memristive device was studied. Within three minutes of being submerged in distilled water, the fabricated memory device was destroyed. This phenomenon facilitates the usage of fabricated memristor devices to develop memory devices for military and security purposes. [ABSTRACT FROM AUTHOR]

In order to enhance the mechanical properties of ceramic materials and realize the high value utilization of lignin, the nano-lignin precursor was added into the silicon nitride matrix as the second phase to obtain a novel ceramic composite material with excellent mechanical performance in this study. During the preparation, the nano-lignin was transformed into nitrogen-doped graphene quantum dots(N-GQDs) in the ceramic matrix by the pyrolysis-hot pressing sintering coupling process. Aiming to reveal the pyrolysis conversion process of lignin, the segmented products at the different temperature gradient were prepared, and their phase composition and microstructure were deeply analyzed by various test methods. In addition, the continuous pyrolysis process of nano-lignin was further studied by simultaneous thermal analyzer. The results showed that, nano-lignin was transformed into solid products (amorphous carbon) in the temperature range of room temperature to 800 °C and produced a large number of volatiles. Subsequently, in the temperature range of 800∼1400 °C, part of amorphous carbon began to transform into N-GQDs, and part of α-Si 3 N 4 also began to transform into β-Si 3 N 4. When the temperature rose to the temperature of 1700 °C, all amorphous carbon was transformed into N-GQDs through carbon structure rearrangement and nitrogen doping, and meanwhile the most of α-Si 3 N 4 was transformed into β-Si 3 N 4. At the final stage, β-Si 3 N 4 columnar grains were covered by an overlayer composed of N-GQDs, and a novel ceramic composite with a special "core-shell" microstructure was obtained and exhibited excellent mechanical performance. This study presents a promising approach for the preparation of high-performance ceramic materials, and can also intrigue great interests in lignin graphitization and lignin-derived carbon materials. [ABSTRACT FROM AUTHOR]

This study reports the facile synthesis of rationally designed composite materials consisting of nitrogen-doped graphene quantum dots (N-GQDs) and MnCO3/ZnMn2O4 (N/MC/ZM) on Ni foam using a simple hydrothermal method to produce high-performance supercapacitor applications. The N/MC/ZM composite was uniformly synthesized on a Ni foam surface with the hierarchical structure of microparticles and nanosheets, and the uniform deposition of N-GQDs on a MC/ZM surface was observed. The incorporation of N-GQDs with MC/ZM provides good conductivity, charge transfer, and electrolyte diffusion for a better electrochemical performance. The N/MC/ZM composite electrode delivered a high specific capacitance of 960.6 F·g−1 at 1 A·g−1, low internal resistance, and remarkable cycling stability over 10,000 charge–discharge cycles. Additionally, an all-flexible solid-state asymmetric supercapacitor (ASC) device was fabricated using the N/MC/ZM composite electrode. The fabricated ASC device produced a maximum energy density of 58.4 Wh·kg−1 at a power density of 800 W·kg−1 and showed a stable capacitive performance while being bent, with good mechanical stability. These results provide a promising and effective strategy for developing supercapacitor electrodes with a high areal capacitance and high energy density. [ABSTRACT FROM AUTHOR]

Recently, graphene quantum dots (GQDs) with bright fluorescence have emerged as a novel carbon nanomaterial because of their distinctive optical properties and robust chemical inertness. Herein, a simple bottom-up approach is used to prepare nitrogen-doped GQDs (N-GQDs) by using citric acid as the carbon source and tris (hydroxymethyl) aminomethane (Tris) as the nitrogen source. The prepared N-GQDs have a high yield of 61.50% and the quantum yield is 14.42%. Meanwhile, the N-GQDs exhibit clearly fluorescence quenching with the increase of pH value from 3.0 to 12.0. In addition, the N-GQDs possess excellent fluorescence quenching response to tetracycline (TC) due to the inner filter effect. And the fluorescence intensity of N-GQDs exhibits a good linear relationship with the addition of TC in range of 1–50 μM. The detection limit is determined to be 94 nM. Furthermore, the smartphone-based handheld device is developed to track the fluorescence colour changes caused by the variation of pH and TC. Through analysing the RGB values from the fluorescence images, good linearity between RGB values and pH values is obtained (R2 = 0.996), while the detection limit for TC detection is 97 nM. This method has been shown to be effective and reliable along with great promise for real-time visual monitoring of pH and TC values in real samples. [ABSTRACT FROM AUTHOR]

Green synthesized graphene quantum dots (GQD) have been doped with nitrogen in an attempt to boost their optical characteristics and application sectors. In the present investigation, the blue luminescent nitrogen-doped GQDs (N-GQDs) were synthesized by single-step hydrothermal synthesis using tamarind shell powder as a precursor. The particle size and zeta potential of N-GQDs were found to be 11.40 nm and be –35.53 mV, respectively. A quantum yield as high as 23.78 % was accomplished at an excitation wavelength of 330 nm at neutral pH. It gets quenched sensitively in the existence of uric acid (UA) combining static quenching, electron transfer, and an inner filter effect mechanism. A linear range was obtained for UA from 10 µM to 100 µM, with a limit of detection (LOD) of 401.72 ± 0.04 pM. Additionally, the N-GQDs were selective toward UA in presence of metal ions and biomolecules that indicated its impending use to monitor UA in clinical samples. In conclusion, this work demonstrates that the N-GQDs as a sensing probe for UA recognition with notable advantages including socioeconomic, simple, and less time-consuming methods as compared to other methods. In the future, it can be potentially explored as a biosensor for UA detection in clinical samples.

Presented in this manuscript are our new research results regarding the fabrication of nitrogen-doped (N-doped) and sulfur-doped (S-doped) graphene quantum dots (GQDs) from citric acid (CA), using the microwave–hydrothermal method. This is a method that uses microwave energy for pyrolysis and carbonization of the precursors of carbon and doped element, followed by a hydrothermal process in NaOH, to yield doped GQDs. Studies using high-resolution transmission electron microscopy (HR-TEM) and Raman spectroscopy have allowed the confirmation of the fabricated products to be GQDs. Detailed studies of absorption, photoluminescence (PL), PL excitation (PLE) spectra, emission–excitation mapping (EEM) images, and PL spectra at different excitation wavelengths have allowed us to find the best excitation wavelength to obtain the highest PL quantum yield (QY) value from the sample, an example being the QY of the S-GQD samples reaching 40.1%. The absorption mechanism, vibration relaxation process, and non-radiative and radiative electron transition are also explained. [ABSTRACT FROM AUTHOR]

The fabrication and improved performance of Z-scheme visible-light-driven Ag3PO4@N-GQDs@g-C3N4 ternary nano/hetero-junctions have been described in this study. Fern-like silver orthophosphate (Ag3PO4) microstructures have been modified using nitrogen-doped graphene quantum dots (N-GQDs) and then have been coated by ultrathin graphitic carbon nitride (g-C3N4) sheets via a combined technique including freeze-drying and refluxing methods. Photocatalytic studies have been conducted through visible-light photo-degradation of standard methylene blue dye in aqueous media. The Ag3PO4@N-GQDs@g-C3N4 ternary nano/hetero-junctions have exhibited the promoted photocatalytic efficiency of 97.91%, which is about 1.07 and 1.34 times higher than that of Ag3PO4@g-C3N4 binary nano/hetero-junctions (91.46%) and pristine-Ag3PO4 microstructures (85.91%), respectively. The excellent recyclability of the Ag3PO4@N-GQDs@g-C3N4 photo-catalyst has been verified in the cycle operations in which the recycling efficiency could have been maintained at 94.92% after five runs of experiments. The quenching effects of scavengers have suggested that the superoxide radicals ( O 2 - · ) and holes ( h + ) are the predominant active species governing the photocatalytic reaction of the Ag3PO4@N-GQDs@g-C3N4 ternary composite due to a Z-scheme junction. The improved photocatalytic activity of Ag3PO4@N-GQDs@g-C3N4 nano/hetero-junctions could have been ascribed to the higher visible-light harvesting capacity, better charge carrier separation and stronger oxidation and reduction ability relevant to the indirect Z-scheme system where N-GQDs act as an efficient electron transfer media. [ABSTRACT FROM AUTHOR]

Z-scheme heterojunction photocatalytic nanomaterial designs have attracted attention due to their high catalytic performance. Deep eutectic solvents (DESs) have been used as green, sustainable media, acting as solvents and structure inducers in the synthesis of nanomaterials. In this work, a novel visible-light-absorption-enhanced bismuth vanadate/bismuth oxychloride/sulfur, nitrogen co-doped graphene quantum dot (BiVO4/BiOCl/S,N-GQDS) heterojunction photocatalyst was prepared in a deep eutectic solvent. The photosynthetic activity of the BiVO4/BiOCl/S,N-GQDS composite was determined by the photocatalytic degradation of rhodamine B (RhB) under visible-light irradiation. The results showed that the highest photocatalytic activity of BiVO4/BiOCl/S,N-GQDS was achieved when the doping amount of S,N-GQDS was 3%, and the degradation rate of RhB reached 70% within 5 h. The kinetic and photocatalytic cycles showed that the degradation of Rhb was in accordance with the quasi-primary degradation kinetic model, and the photocatalytic performance remained stable after four photocatalytic cycles. Ultraviolet–visible diffuse reflectance (UV-DRS) and photoluminescence (PL) experiments confirmed that BiVO4/BiOCl/S,N-GQDS ternary heterojunctions have a narrow band gap energy (2.35 eV), which can effectively improve the separation efficiency of the photogenerated electron–hole pairs and suppress their complexation. This is due to the construction of a Z-scheme charge process between the BiVO4/BiOCl binary heterojunction and S,N-GQDS, which achieves effective carrier separation and thus a strong photocatalytic capability. This work not only provides new insights into the design of catalysts using a green solvent approach but also provides a reference for the study of heterojunction photocatalytic materials based on bismuth vanadate, as well as new ideas for other photocatalytic materials. [ABSTRACT FROM AUTHOR]

A well-established relationship between bandgap energy and Quantum Dots (QDs) dimension, predicted by Effective Mass Approximation (EMA) models, is a cost-effective and reliable method to evaluate the bandgap energy of QDs. The EMA model sheds light on linearity between bandgap energy (E g a p) to their inverse square diameter (1 / d n), as a signature of the quantum confinement, with the n value reflecting the dimension of the quantum confinement. This study systematically investigated the power parameter " n " dependence in the bandgap energy of Nitrogen-functionalized Graphene Quantum Dots (N-GQDs) with distinct C–N configurations, i.e. , Pyridinic-N, Pyrrolic-N, Graphitic-N based on computational and experimental data. The results showed that the calculated bandgap energy values reveal a notable weakening of the size dependence in N-GQDs compared to pristine GQDs, particularly in Pyrrolic-N GQDs, indicative of a unique geometric dimensionality of confinement. This systematic exploration advances our understanding of " n " dependence and bandgap energy in N-GQDs, laying a solid groundwork for modulating the bandgap energy of N-GQDs for optoelectronics and quantum device applications. [Display omitted] • The study explores "n" dependence in N-GQDs' bandgap, revealing quantum confinement insights. • Experimental results align with computational models, validating the adjusted EMA's reliability in N-GQDs' bandgap-linearity. • The study enhances N-GQDs' bandgap modulation understanding, pivotal for optoelectronics and quantum applications. [ABSTRACT FROM AUTHOR]

In this study, we report a switchable fluorescence probe based on nitrogen-doped graphene quantum dots (N-GQDs) to detect the presence of tetracycline and L-Cystence. Strong blue emissive N-GQDs were successfully prepared by the hydrothermal treatment of urea and benzophenone, with a quantum yield of 34.9%. Both transmission electron microscopy and atomic force microscope images reveal a nearly monodispersed N-GQDs with spherical morphologies, and provide evidences of 1–3 layered structures of N-GQDs. Meanwhile, the obtained N-GQDs can be utilized for fluorescent labelling due to their superior optical stability and favourable biological compatibility. A strong fluorescence quenching was observed for the presence of tetracycline. The following addition of L-Cysteine can motivate the regeneration of fluorescence. Therefore, a simple strategy for the detection of tetracycline and L-Cystence in natural water was developed due to the relevant sensitivity of N-GQDs nanosensor. [ABSTRACT FROM AUTHOR]

In recent years, multifunctional nanocarriers that provide simultaneous drug delivery and imaging have attracted enormous attention, especially in cancer treatment. In this research, a biocompatible fluorescent multifunctional nanocarrier is designed for the co-delivery of capsaicin (CPS) and nitrogen-doped graphene quantum dots (N-GQDs) using the pH sensitive amphiphilic block copolymer (poly(2-ethyl-2-oxazoline)-b-poly(ε-caprolactone), PEtOx-b-PCL). The effects of the critical formulation parameters (the amount of copolymer, the concentration of poly(vinyl alcohol) (PVA) as a stabilizing agent in the inner aqueous phase, and volume of the inner phase) are evaluated to achieve optimal nanoparticle (NP) properties using Central Composite Design. The optimized NPs demonstrated a desirable size distribution (167.8 ± 1.4 nm) with a negative surface charge (-19.9 ± 0.4) and a suitable loading capacity for CPS (70.80 ± 0.05%). The CPS & N-GQD NPs are found to have remarkable toxicity on human breast adenocarcinoma cell line (MCF-7). The solid fluorescent signal is acquired from cells containing multifunctional NPs, according to the confocal microscope imaging results, confirming the significant cellular uptake. This research illustrates the enormous potential for cellular imaging and enhanced cancer therapy offered by multifunctional nanocarriers that combine drug substances with the novel fluorescent agents., (© 2024 The Author(s). Macromolecular Bioscience published by Wiley‐VCH GmbH.)

In the work, CdS x Se 1-x /N-GQDs composites were fabricated via a simple one-step hydrothermal process and their tunable composition, structure and photoelectric properties were characterized by various techniques. The photoelectric properties of CdS x Se 1-x /N-GQDs could be adjusted by different Se/S ratios and tunable band-gaps. CdS x Se 1-x /N-GQDs composites reached the optimal photocurrent response and the lowest interfacial impedance at the Se/S ratio of 0.75:0.25. Mott-Schottky plots and LSV spectra showed that the n-type CdS 0.25 Se 0.75 /N-GQDs presented a higher carrier density under light illumination. The excellent properties of the composites could be attributed to the mechanism involved in the excitation and electron-transfer process. On one hand, the band-gap of CdS 0.25 Se 0.75 /N-GQDs was narrowed, and more electrons were excited by the lower band-gap energy to promote a superior electron separation and transportation. On the other hand, N-GQDs acted as charge carriers and conductive way providers for electron-transfer instead of electron-hole recombination in the composites. [ABSTRACT FROM AUTHOR]

Z-scheme heterojunction photocatalytic nanomaterial designs have attracted attention due to their high catalytic performance. Deep eutectic solvents (DESs) have been used as green, sustainable media, acting as solvents and structure inducers in the synthesis of nanomaterials. In this work, a novel visible-light-absorption-enhanced bismuth vanadate/bismuth oxychloride/sulfur, nitrogen co-doped graphene quantum dot (BiVO4/BiOCl/S,N-GQDS) heterojunction photocatalyst was prepared in a deep eutectic solvent. The photosynthetic activity of the BiVO4/BiOCl/S,N-GQDS composite was determined by the photocatalytic degradation of rhodamine B (RhB) under visible-light irradiation. The results showed that the highest photocatalytic activity of BiVO4/BiOCl/S,N-GQDS was achieved when the doping amount of S,N-GQDS was 3%, and the degradation rate of RhB reached 70% within 5 h. The kinetic and photocatalytic cycles showed that the degradation of Rhb was in accordance with the quasi-primary degradation kinetic model, and the photocatalytic performance remained stable after four photocatalytic cycles. Ultraviolet–visible diffuse reflectance (UV-DRS) and photoluminescence (PL) experiments confirmed that BiVO4/BiOCl/S,N-GQDS ternary heterojunctions have a narrow band gap energy (2.35 eV), which can effectively improve the separation efficiency of the photogenerated electron–hole pairs and suppress their complexation. This is due to the construction of a Z-scheme charge process between the BiVO4/BiOCl binary heterojunction and S,N-GQDS, which achieves effective carrier separation and thus a strong photocatalytic capability. This work not only provides new insights into the design of catalysts using a green solvent approach but also provides a reference for the study of heterojunction photocatalytic materials based on bismuth vanadate, as well as new ideas for other photocatalytic materials.

Carbon, being a material of the era, has vast usefulness in the synthesis of many novel compounds. One of them is graphene quantum dots (GQDs) have drawn a great deal of attention to the scientific community owing to their low cytotoxicity, higher optical stability, and tremendous photoluminescence (PL) property, which show a new insight in the field of bio-sensing and bio-imaging. Herein, we have demonstrated a facile method to synthesise N-doped GQDs from graphene oxide (GO) using solvothermal treatment with aprotic solvent. GO was synthesised using a modified Hummers' method. The as-prepared N-GQDs showed stronger green emission with 326 nm wavelength. PL study showed that PL characteristics of as-synthesised N-GQDs are mainly due to the surface π→π* and n→π* transitions of N containing functional groups with skeletal carbons of the graphene oxide sheet. The N-GQDs were also examined for bio-imaging of microbial cells including E. coli and Saccharomyces cescerevisiae yeast. The results (blue and green emissions under a fluorescence microscope) of this study showed biocompatibility and the photostability of N-GQDs for their application in the fields ranging from energy to biomedicine. [ABSTRACT FROM AUTHOR]

In the present study, a heterostructure based on Zn 0·1 Cd 0·9 S, N-doped graphene quantum dots (N-GQDs), and graphene was successfully prepared by a simple method. Various analyses are conducted to determine the structure, morphology, and materials performance of the synthesized composite. The results exhibit that the Zn 0·1 Cd 0·9 S/N-GQDs/graphene heterostructure presents excellent photoelectric performance with a high photocurrent of 4.43 × 10−5 A/cm2 and 3.43 × 10−5 A/cm2 under light irradiation of 365 nm and 405 nm, respectively. It demonstrates a two-fold photocurrent enhancement in comparison to blank Zn 0·1 Cd 0.9 S. This remarkable improvement is ascribed to a mechanism in which the N-GQDs act as photosensitizers, enhancing the absorption ability. Concurrently, graphene serves as a carrier mobility substrate, facilitating the separation of the photogenerated electron–hole pairs. The synergetic effect between Zn 0·1 Cd 0·9 S, the N-GQDs, and graphene enhances the photoelectric performance. The Zn 0·1 Cd 0·9 S/N-GQDs/graphene heterostructure provides a new route for the enhancement of the photoelectric performance of a semiconductor under UV–visible light. [ABSTRACT FROM AUTHOR]

• High sensitivity with printed graphene electrode modified with N-GQDs. • Improvement of surface area by N-GQDs leads to high electron mobility in electrode. • More efficiency in transferred electrons by catching through the N-GQDs. The improvement of electrochemical properties of graphene compounds can be achieved by using graphene quantum dots (GQDs) due to their high electron mobility, catalytic property and size corresponding to the active site of the enzyme. In this study, we developed a third-generation enzymatic amperometric biosensor based on nitrogen-doped graphene quantum dots (N-GQDs) and printed graphene electrodes (PGE) for the highly sensitive and selective detection of glucose. The PGEs modified with N-GQDs showed a high potential for the precise and sensitive electrochemical detection of glucose. Due to the high electron mobility and electrical conductivity, excellent electrocatalytic activity, large surface area and size compatibility of PGE and N-GQDs with biomolecules, a high sensitivity of 40.76 mA.mM-1.cm-2 and LOD close to 0.098 mM were achieved. The high enzyme surface coverage measured at 3.33 × 10–7 mol.cm-2 further confirmed the excellent enzyme loading capacity and immobilization tendency of the biosensor. Moreover, the N-GQD-based electrochemical biosensors were found to be highly specific and selective even in complex media, especially human blood serum. [Display omitted] [ABSTRACT FROM AUTHOR]

Composite materials comprising nitrogen-doped graphene quantum dots (N-GQDs) modified graphite nitride nanosheets (g-CN) were successfully synthesized via electrostatic adsorption. These N-GQDs not only exhibit uniform dispersion across the g-CN surface but also significantly enhance the number of active sites available. Functioning as co-catalysts, N-GQDs efficiently optimize electron migration pathways, resulting in heightened photocatalytic activity and excellent reproducibility within the composites. Notably, the photocatalytic degradation is attributed to the contributions of ·O 2 , 1O 2 , ·OH, and h+. Furthermore, examining the composite performance under varying pH levels and ionic interferences is pivotal for realizing the practicality of environmental photocatalysis applications. [Display omitted] • The synergistic interaction between N-GQD/CN enhanced the photocatalytic performance for various pollutants and improved light absorption. • N-GQD/CN facilitated charge migration, with the active species identified as ·O 2 –, 1O 2 , ·OH and h+. • The influence of N-GQD/CN on organic pollutant degradation was assessed by simulating a real water environment. Photocatalysis has been prominently featured as a promising technology for solving environmental problems and energy crises. The incorporation of supported non-metallic cocatalysts is paramount in significantly augmenting the efficiency and performance of semiconductor photocatalysis. N-GQD/CN was successfully prepared and showed excellent performance in terms of light-absorbing capacity, carrier separation efficiency and visible-light driving. The findings underscore the exceptional photocatalytic prowess of the 0.02 wt.% N-GQD/CN composites, particularly in the degradation of RhB, HCl-TC, and BPA under visible light irradiation. The 0.02 wt.% N-GQD/CN composite could eliminate 91.2 % RhB in 90 min, remove 72.4 % of the HCl-TC in 180 min, and degrade 47.9 % of the BPA in 4 h. All of the activities of the 0.02 wt.% N-GQD/CN was superior to that of the monomer g-CN. This elevated photocatalytic performance can be predominantly attributed to the N-GQDs acting as co-catalysts. These N-GQDs possess substantial specific surface areas, effectively enhancing light absorption and facilitating synergistic interactions with g-CN nanosheets. Exploring the utility of N-GQD/CN using ionic interference and pH photocatalysis experiments. The present work provides a rational strategy for designing 0D/2D photocatalysts with improved visible light activity for environmental applications. [ABSTRACT FROM AUTHOR]

Graphene quantum dots (GQDs) are well-known for its potential applications for bioimaging, biosensor, and drug carrier in biomedicine. GQDs are well characteristic of intrinsic peroxidase-like catalytic activity, which is proven effective in scavenging the free radicals, such assuperoxide anion, hydrogen peroxide, and hydroxyl radical. GQDs are also well praised for its low in vivo and in vitro toxicity. Here, we found that nitrogen-doped GQDs (N-GQDs) can strongly disturb redox-sensitive system via the selective inhibition of endogenous antioxidant enzyme activities in zebrafish. The enzyme activities or transcription levels of a battery of hemoproteins including catalase (CAT), superoxide dismutase (SOD), respiratory chain complex I, complex Ⅲ, hemoglobin (Hb), and myeloperoxidase (MPO), were significantly suppressed by N-GQDs. We also found that N-GQDs activated the cytochrome P450 monooxygenase (e.g. cyp1a) and the associated aryl-hydrocarbon receptor repressors (ahrr1 and ahrr2) in zebrafish embryos. Compared to the ultrasmall graphene oxide (USGO), N-GQDs exhibited stronger fluorescent permeability and tissue-specific bio-accumulative effects. Taken together, our findings highlighted that exposure to N-GQDs can disrupt endogenous antioxidant enzyme activities, possibly via the competitive inhibition of electron transfer process. Our results in this study provided solid data for biosafety evaluations of various types of GQDs, and created an alert for the future biomedical applications of N-GQDs. [ABSTRACT FROM AUTHOR]

A new label-free aptasensor with high visible-light photoelectrochemical (PEC) activity was fabricated for the detection of AFB1 based on N-doped graphene quantum dots (N-GQDs) sensitized Zn-doped SnS2 (Zn-SnS2) nanocomposites. N-GQDs sensitized Zn-SnS2 showed an eminent PEC performance. Ascorbic acid was utilized as an efficient electron donor for scavenging photogenerated holes and inhibiting light driven electron–hole pair recombination. Under the optimum experimental conditions, the specific binding between AFB1 aptamer and AFB1 resulted in the linear decrease of photocurrent with the increase of logarithm of AFB1 concentration in the range of 0.01–20 ng mL−1 with a detection limit of 3 pg mL−1. The constructed N-GQDs sensitized Zn-SnS2 PEC aptasensor exhibited high sensitivity, low cost, and good reproducibility, which is a promising platform for the detection of other important analytes. [ABSTRACT FROM AUTHOR]

Photocatalysis is one of the most popular organic waste degradation treatments because it can result in the total decomposition of organic pollutants under visible light illumination. Here, we present a metal-free visible-light active catalyst for the effective decomposition of organic aromatic amino derivatives Methylene blue (MB) and Rhodamine B (RB) as model-contaminated dye molecules. Through a one-pot hydrothermal process, blue colour emissive nitrogen-doped graphene quantum dots (N-GQDs) were synthesized from urea and malic acid. The physicochemical characteristic techniques were used to analyze the surface morphology, elemental analysis, surface functional groups, absorption, emission, and band gap of prepared N-GQDs. The effective light absorption properties, transfer of electrons, separation of photoinduced charge carriers, and excellent stability with a band gap of 2.69 eV of synthesized N-GQDs have enabled the degradation of Rh B and MB up to 90 % and 86 % respectively, within 120 min of visible light irradiation. Based on extensive active species trapping experiments, a probable photocatalytic dye degradation mechanism is proposed. These N-GQDs can also be used to make fluorescent ink for imaging and security purposes. [Display omitted] [ABSTRACT FROM AUTHOR]

A straightforward methodology for obtaining Nitrogen-doped Graphene Quantum Dots (N -GQDs) with aggregation-induced emission (AIE) using salting-out purification processes is presented in this work. Various salts, including NaCl at concentrations of 50 mM, were found to be effective for stratification of graphene-based materials (GBMs) by salting-out treatments. These salts were shown to be safer, more cost-effective, and environmentally sustainable alternatives to (NH 4) 2 SO 4 reported in previous works. Remarkably, this method demonstrated the ability to promote the self-assembly of N -GQDs into stable J-aggregates-like arrays with an average fluorescence decay time of ca. 4 ns and absolute quantum yield with values of 3.5 % and 6.9 % for 100 mM and 2 M, respectively. In contrast to a typical quenching effect caused by π-stacking, these arrays exhibited aggregation-induced emissions, as evidenced by the presence of new excitation and emission bands at 532 and 600 nm, respectively. The scalability of the process, along with the absence of additional purification steps, renders these results of great scientific and technological interest. Furthermore, the modulable fluorescence, size, and stability of these GBMs suggest their potential application in energy cells, sensors, and biomedical imaging. [Display omitted] [ABSTRACT FROM AUTHOR]

Abstract An efficient photocatalyst consisting of nitrogen-doped graphene quantum dots (N-GQDs) dispersed on the surface of the three-dimensional hierarchical bismuth oxyhalide (BiOX, X = Br, Cl) was synthesized via a simple solvothermal method. The catalysts were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), UV–vis diffuse reflectance spectrophotometer (DRS), and photoluminescence spectroscopy (PL). The N-GQDs play an important role in the improvement of photocatalytic performance because it significantly increases BiOX absorption of visible light and effectively traps photogenerated electrons to accelerate the separation of photogenerated electron-hole pairs. The superior activity of N-GQDs/BiOX was exhibited to the organic contaminant degradation under visible light. The 7 wt% N-GQDs/BiOBr and 7 wt% N-GQDs/BiOCl exhibited strong catalytic activity, in which RhB was almost completely removed after irradiation for 60 and 75 min, respectively. In our work, we provide a strategy that for improving the photocatalytic activity of BiOX with semiconductor quantum dots. [ABSTRACT FROM AUTHOR]

In this paper, a test strip-based sensor was developed for thiacloprid quantitative detection based on PDA molecularly imprinted polymer (MIP) and nitrogen-doped graphene quantum dots (N-GQDs). Thiacloprid is a new type of nicotine insecticide, which can block the normal neurotransmitter delivery process in insects. In the sensing system, N-GQDs were immersed into filter paper at first. Then, dopamine (DA) with thiacloprid can be self-polymerized on test strip surface to form the uniform PDA film. After removed thiacloprid template, the established poly dopamine (PDA) MIP can selectively recognize thiacloprid. As a result, captured thiacloprid can enhance the fluorescence intensity of N-GQDs into the test strip. As a result, the fluorescence intensity of N-GQDs can be linearly related within a certain range of thiacloprid concentration. Under the optimum conditions, the proposed sensor for thiacloprid detection exhibited a linear ranging from 0.1 mg/L to 10 mg/L with a low detection limit of 0.03 mg/L. The N-GQDs based test strip-based sensor for thiaclopridis reported for the first time. The sensing system has high selectivity to thiacloprid and provides new opportunities in the pesticide detection. [ABSTRACT FROM AUTHOR]

In this paper, we have investigated a resistive ammonia (NH3) gas sensor fabricated with a combination of polyaniline (PANI) as a conductive polymer and different weight percentages of nitrogen-doped graphene quantum dots (N-GQDs). We used two different metals of silver (Ag) and aluminium (Al) as electrodes to the sensing films. It has been found that Ag contact showed an ohmic behaviour and Al electrode exhibited a Schottky junction. This paper presented here shows that the sensor with Ag electrode has much higher response to NH3 than that of the sensor with Al electrode. The sensor of (50wt%)N-GQDs/PANI with Ag contact exhibited the best response of 110.92 towards 1500 ppm NH3 at room temperature. However, a much response of 86.91 was measured for the sensor with Al electrode. Fourier transform infrared spectroscopy of the synthesized structure was investigated to analyze the differences between pure PANI and N-GQDs/PANI. Further, field emissions scanning electron microscopy was used to characterize sensing samples. [ABSTRACT FROM PUBLISHER]

[Display omitted] • Nitrogen doped graphene quantum dots (N-GQDs) have been prepared. • N-GQDs has been used as passivation layer for the protection of AgNWs. • AgNWs based flexible electrochromic device was fabricated. • Flexible electrochromic device exhibited excellent stability of more than 500 cycles. • Flexible stability of 1200 cycles has been achieved. The design and development of cost-effective and highly stable flexible transparent conductive electrodes (f-TCEs) is crucial for various optoelectronic applications. Herein, we report the fabrication of nitrogen-doped graphene quantum dot (N-GQD)-protected silver nanowire (AgNWs)-modified polyethylene terephthalate (PET) electrodes as f-TCEs (N-GQDs/AgNWs/PET). Furthermore, we fabricated flexible electrochromic devices (ECDs) for smart window applications using N-GQD/AgNW/PET as the f-TCE. The presence of N-GQDs on the AgNW/PET electrode improved the stability of the developed N-GQD/AgNW/PET-based flexible ECDs. The N-GQDs/AgNWs/PET-based flexible ECDs demonstrated an excellent coloration efficiency (Ƞ CE) of 42.7 cm2C −1 with an optical contrast of 45.4 %. Additionally, they demonstrated fast coloring and bleaching times of 6.9, and 5.7 s, respectively. [ABSTRACT FROM AUTHOR]

In this paper, a suitable and high-performance electrode material for practical applications was developed using a simple two-step approach. Here, we report that nitrogen-doped graphene quantum dots with co-doped polyaniline (N-GQDs-PANI) were fabricated by in situ polymerization as supercapacitor electrodes and that the effect of the amount of N-GQDs in the electrode material on the electrochemical performance was investigated. The integration of N-GQDs into co-doped PANI structure prevented the agglomeration of PANI and enabled the fabrication of N-GQDs as thin film on the surface. This modification improved the charge transfer of the N-GQDs-PANI electrode compared to N-GQDs and pure PANI electrodes, allowing it to exhibit superior electrochemical performance. The N-GQDs-PANI electrode has a specific capacitance of 503 F/g at a current density of 5 A/g with a capacitance retention of about 91.9 % after 10000 charge/discharge cycles. • Binary N-GQDs-PANI was prepared with co-doped PANI for the first time. • The electrode has a high capacitance (503 F/g) at a current density of 5 A g−1. • N-GQDs-PANI electrode exhibits superior rate capability 97 % (5–100 A g−1). • N-GQDs-PANI electrode has a capacitance of 750 F/g at a scan rate of 20 mV/s. • The nanocomposite exhibits long-cycle stability (91.9 % after 10000 cycles). [ABSTRACT FROM AUTHOR]

Nitrogen doped graphene quantum dots (N-GQDs) have been fabricated using lemon juice by a simple and cost effective thermal pyrolysis technique, without the consumption of any harmful chemicals or solvents. N-GQDs displayed excellent water solubility, good stability and excitation dependent down conversion fluorescence (FL). The obtained N-GQDs displayed a strong FL emission at wavelength around 440 nm at optimum excitation wavelength of 340 nm. FL spectrum of N-GQDs showed highly selective and sensitive quenching with the addition of 2,4,6-trinitrophenol (TNP). The calibration curve from Stern − Volmer equation showed a linear range from 0 − 4 μM with obtained detection limit of 420 nM. This work demonstrated simple, green and facile synthesis of N-GQDs and the obtained N-GQDs were successfully applied for the detection of mutagenic water pollutant, TNP. [ABSTRACT FROM AUTHOR]

This paper investigated a room temperature resistive ammonia gas sensor based on a conductive polymer and N-doped graphene quantum dots (N-GQDs) dopant made on a transparent substrate with electrodes. The sensor fabricated with conductive polymer showed a good sensing response that increases considerably with the addition of N-GQDs. The sensing response of the poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT-PSS) to NH3 increased from 30.13% to 212.32% at 1500 ppm with the addition of 50 wt% N-GQDs. The response time of the N-GQDs doped sensor decreased to 6.8 min when compared with the sensor without N-GQDs and the stability of the sensor having combined N-GQDs and PEDOT-PSS was higher than that of the PEDOT-PSS sensor. Meanwhile, the structure and morphology of the sensing film are characterized by Fourier transform infrared spectroscopy and field emissions scanning electron microscopy. [ABSTRACT FROM PUBLISHER]

Currently, graphene quantum dots (GQDs) are widely used as antibacterial agents, and their effects are dependent on the reactive oxygen species (ROS) generated by photodynamic and peroxidase activities. Nevertheless, the supply of substrates or light greatly limits GQDs application. Besides, due to compensatory mechanisms in bacteria, comprehensive analysis of the molecular mechanism underlying the effects of GQDs based on cellular-level experiments is insufficient. Therefore, N-GQDs with inherent excellent, broad-spectrum antibacterial efficacy under acidic conditions were successfully synthesized. Then, via multi-omics analyses, the antibacterial mechanisms of the N-GQDs were found to not only involve generation ROS but also be associated with changes in osmotic pressure, interference with nucleic acid synthesis and inhibition of energy metabolism. More surprisingly, the N-GQDs could destroy intracellular acid-base homeostasis, causing bacterial cell death. In conclusion, this study provides important insights into the antibacterial mechanism of GQDs, offering a basis for the engineering design of antibacterial nanomaterials. [Display omitted] • N-GQDs with inherently antibacterial efficacy under acidic conditions were prepared. • Germicidal efficacy of N-GQDs was related to -COOH form and -C O content. • N-GQDs antibacterial mechanisms were studied based on multi-omics technology. • N-GQDs could generate ROS, destroy osmotic pressure, and interfere with DNA synthesis. • N-GQDs could damage energy metabolism and acid-base homeostasis causing bacteria death. [ABSTRACT FROM AUTHOR]

Here we report a remarkable transformation of nitrogen-doped multiwalled carbon nanotubes (MWCNTs) to size selective nitrogen-doped graphene quantum dots (N-GQDs) by a two-step electrochemical method. The sizes of the N-GQDs strongly depend on the applied anodic potential, moreover increasing potential resulted in a smaller size of N-GQDs. These N-GQDs display many unusual size-dependant optoelectronic (blue emission) and electrocatalytic (oxygen reduction) properties. The presence of N dopants in the carbon framework not only causes faster unzipping of MWCNTs but also provides more low activation energy site for enhancing the electrocatalytic activity for technologically daunting reactions like oxygen reduction. The smaller size of N-GQDs has shown better performance as compared to the large N-GQDs. Interestingly, N-GQDs-3 (size = 2.5±0.3 nm, onset potential = 0.75 V) show a 30-mV higher positive onset potential shift compared to that of N-GQDs-2 (size = 4.7±0.3 nm, onset potential = 0.72 V) and 70 mV than that of N-GQDs-1 (size = 7.2±0.3, onset potential = 0.68 V) for oxygen reduction reaction (ORR) in a liquid phase. These result in the size-dependent electrocatalytic activity of N-GQDs for ORR as illustrated by the smaller sized N-GQDs (2.5±0.3 nm) undoubtedly promising metal-free electrocatalysts for fuel cell applications. [ABSTRACT FROM AUTHOR]

Fluorescent nanomaterials with multiple anti-counterfeiting play a vital role in economic and social fields in our daily lives. In this work, a hybridized composite of boron and nitrogen co-doped graphene quantum dots (B, N-GQDs) and lanthanide-doped up-conversion NaYF 4 was prepare for a double anti-counterfeiting application. The composite was obtained by one-pot hydrothermal method and it was commixed with glycerol in a ratio of 4 : 1 to prepare a fluorescence ink, which exhibited independent blue and green fluorescence emissions under ultraviolet light and near-infrared laser excitation. Therefore, it had a considerable application in confidential communications, logo designing and file encryptions. • The nanocomposite of NaYF 4 : Yb, Ho/B, N-GQDs was synthesized by one-pot hydrothermal method. • Independent blue and green fluorescence are observed in ultraviolet and infrared excitation for double anti-counterfeiting. • NaYF 4 : Yb, Ho/B, N-GQDs was dispersable in glycerol to prepare for ink application. [ABSTRACT FROM AUTHOR]

The design of a fluorescent system for tracking valine remains a challenge till date. This work demonstrated the first use of the metal complex of dopamine-functionalized nitrogen doped graphene quantum dots (N-GQDs-DA) as coordination-driven fluorescent platform for selective detection of valine in human serum and real food samples. The fluorescence of the N-GQDs-DA was quenched after the addition of Cr3+, which acts as both a fluorescence quencher and a receptor for target molecule. The quenched fluorescence of the N-GQDs-DA/Cr3+ sensing system was linearly restored as the valine concentration increased from 1.33 × 10−4 M to 62.5 × 10−4 M. Different coordination states of Cr3+ opened or blocked a PET course, which constructed "turn-off or turn-on" fluorescence switching. The "INHIBIT" logic gate operation can be carried out using the Cr3+ and valine as inputs and the emission bands EM 315 nm and EM 440 nm as outputs, indicating that they have promising applications in intelligent dual analyte sensing systems. MALDI-TOF mass spectra elucidated the formation of a low-fluorescent species [C 55 H 32 Cl 3 Cr 2 N 4 O 10 ]+ and a high-fluorescent species [C 70 H 58 Cr 2 N 7 O 14 ]+ without and with valine, respectively. This methodology provides a new concept for the design and development of highly selective optical system for single amino acid. [Display omitted] • The complex of N-GQDs-DA/Cr3+ was used to detect valine selectively. • Coordination-driven fluorescence spectra constructed a fluorescence switching. • The sensing system can detect valine in living cells and real foods. [ABSTRACT FROM AUTHOR]

Nitrogen-doped graphene quantum dots modified SnO 2 composites (SnO 2 /N-GQDs) were prepared by a solvothermal method, where N-GQDs were synthesized through a hydrothermal process with citric acid and urea as sources. The lattice stripes of 0.33 nm, 0.26 nm and 0.24 nm were ascribed to (110) and (101) crystal planes of SnO 2 and (1120) crystal plane of N-GQDs, respectively. The photoelectric properties of SnO 2 /N-GQDs composites with different N-GQDs contents were characterized using I-t, LSV, EIS, and Mott-Schottky. Among them, SnO 2 /N-GQDs-2 composites reached the smallest electrochemical impedance and achieved the optimal transient photocurrent value of 1.880 × 10−4 A/cm2. Compared with pure SnO 2 , SnO 2 /N-GQDs-2 presented a 5-fold increase in the photocurrent density. LSV showed that the incorporation of N-GQDs could enhance the photocurrent intensity of SnO 2. The Mott-Schottky slope of SnO 2 /N-GQDs-2 in light was smaller than that of SnO 2 and SnO 2 /N-GQDs-2 in dark, indicating that more carriers were generated under UV illumination and transferred with the doping of N-GQDs. [ABSTRACT FROM AUTHOR]

[Display omitted] • Elaborated procedure is designed to decoration of N-GQDs and NCS on the carbon sphere core support. • Conductive N-GQDs used to serve as second shell and provide the electrochemical interface. • Physicochemical properties of NCS contribute the surface effects allowing fast ion diffusion. • Discharge capacity of 1244 mAhg−1 for N-GQDs/NCS/CS electrodes is more than NCS/CS and NCS. Porous carbon spheres (CSs) decorated with sand-rose shaped NiCo 2 S 4 /Co 9 S 8 /CoS and N- doped graphene quantum dots (N-GQDs) were prepared via one-step free-surfactant hydrothermal route. Ni-Co sulfide based materials (NCS) were created in the presence of Tioglycolic acid (TGA) as both sulfur source and capping agent. Altering of synthesis parameters involving metal salt type, TGA and NaOH amount help to control the sulfide-based materials features. The obtained 3D hierarchical N-GQD/NCS/CS core-double shell spheres were considered in terms of morphology, dimension and porosity. Different formed Ni-Co sulfide structures were evaluated for hydrogen storage capacity. The achieved 3D microspheres could be utilized as a favorable candidate in electrochemical hydrogen storage, incorporating the prominent electro-conductivity of N-GQDs and CSs carbonic materials and high capacity of sand-rose shaped NCS nanostructures. Carbon microspheres formed through rapid sonochemical approach and various linear amines were applied as conductive support. It was discovered that the presenting of N-GQDs prominently improves the discharge efficiency of the core-double shell spheres from 930 to 1244 mAh g−1 because of trap positions formed by both Nitrogen dopants and edge conditions which can adsorb charge transporters to rise the storage proficiency. [ABSTRACT FROM AUTHOR]

A series of nitrogen-doped graphene quantum dots decorated iodine bismuth oxide (BiOI/x%N-GQDs, x = 0,1,3,5,7,9,10) was prepared by a facial precipitation method at room temperature. Several characterizations indicated that 1–9 nm N-GQDs were evenly decorated on the surface of BiOI nanosheets. The visible light driven photocatalytic experiments showed that BiOI/9%N-GQDs possessed an excellent degradation property towards 20 mg/L methyl orange (MO), and its degradation efficiency obtained 93.11%. The results of free radical capture experiments demonstrated that h+ played a major role during the process of degrading MO. The possible photocatalytic mechanism had been proposed. Besides, BiOI/9%N-GQDs catalyst effectively degraded other organic pollutants, such as rhodamine B (RhB), methylene blue (MB), bisphenol A (BPA), and inorganic pollutant of Cr(Ⅵ). The catalytic property of BiOI/9%N-GQDs was better than those of the similar reported systems. The catalytic system of BiOI/9%N-GQDs was predicted to have good prospects in further practical applications. [Display omitted] • Nitrogen-doped graphene quantum dots (N-GQDs) with 1–9 nm were successfully prepared. • N-GQDs were evenly decorated on the surface of BiOI nanosheets (BiOI/x%N-GQDs). • BiOI/x%N-GQDs were in-situ composited by a facial precipitation method. • BiOI/9%N-GQDs showed the most obvious enhancement to visible light degrading MO. • The BiOI/9%N-GQDs could effectively degrade MO, RhB, MB, BPA or Cr(VI). [ABSTRACT FROM AUTHOR]

• A new ECL-RET donor-receptor was proposed and fabricated for ultrasensitive detection of PCT. • IRMOF-3 not only acted as a carrier for N-GQDs by internal encapsulation and external decoration, but also a coreaction accelerator. • ZnO@Au nanorod was firstly applied as a dual-quenching prober of ECL signal. • The designed immunosensor exhibited low detection limit of 12.58 fg mL−1 for PCT. Herein a novel electrochemiluminescence resonance energy transfer (ECL-RET) pair of isoreticular metal organic framework-3 (IRMOF-3) enriched nitrogen-doped graphene quantum dots (N-GQDs) luminescent material (N-GQDs@IRMOF-3@N-GQDs, donor) and Au nanoparticles modified zinc oxide nanorod (ZnO@Au, acceptor) were firstly designed to fabricate a dual-quenching sandwich-type ECL immunosensor for ultrasensitive analysis of procalcitonin (PCT). IRMOF-3 not only acted as a carrier for abundantly enriching luminescent N-GQDs by internal encapsulation and external decoration, but also a coreaction accelerator which could boost the transformation of persulfate ion S 2 O 8 2− to generate more anion radical SO 4 −, so as to promote the ECL emission of N-GQDs. Additionally, Au nanoparticles were loaded onto the ZnO nanorods through in-situ modification to obtain ZnO@Au, which was firstly applied as a dual-quencher of ECL signal. In the best test environment, a wide detection range from 0.0001 to 100 ng mL−1 was obtained and the limit of detection is 12.58 fg mL−1 (S/N = 3). With excellent reproducibility and sensitivity, the practical test consequence of PCT in human serum samples was satisfactory. These results suggested that this sensing system could provide a reference model for effective detection of disease markers in clinical medicine. [ABSTRACT FROM AUTHOR]

Nitrogen dioxide (NO 2), as a toxic gas, seriously harms the environment and human health. Semiconductor metal oxide (MOS) nanocomposites modified by N-doped graphene quantum dots (N-GQDs) have attracted extensive attention as sensing materials for NO 2. Here, the N-GQDs modified ZnO composite material was successfully prepared by the hydrothermal method. Compared with pure ZnO, G-Z-2 (N-GQD S doping amount of 2 mL) exhibits excellent sensing performance for NO 2. The G-Z-2 based sensor reduces the working temperature from 160 °C to 100 °C. The G-Z-2 is shown to be sensitive to 5 ppm NO 2 and has a massively enhanced response of about 11.6 times. The detection limit was as low as 0.1 ppm. Moreover, it shows excellent reproducibility, selectivity and stability for the detection of NO 2. The highly active N atom doping enhances the electron transfer of ZnO to N-GQD S and the adsorption of NO 2 molecules. The heterojunction between ZnO and N-GQD S interface expands the resistance modulation, which improves the sensor sensitivity. This work can provide a promising strategy for improving the NO 2 gas sensing performance based on semiconductor metal oxide. [ABSTRACT FROM AUTHOR]

Light-emitting graphene quantum dots (GQDs) are widely investigated due to their distinct merits. However, GQDs generally suffer from aggregation-induced luminescence quenching, which means they are highly emissive in a solution state with uniform dispersion but dramatically quenched in a solid or aggregated state. This problem significantly limits the application of GQDs, partially in the solid-state light-emitting devices. In this report, we successfully developed a simple and efficient hydrothermal method for the production of nitrogen doped graphene quantum dots (N-GQDs) with strong solid-state fluorescence (SSF) by using citric acid and o-Phenylenediamine as precursors. Under the 365 nm UV light illumination, the produced N-GQDs in an aqueous state exhibit blue color, with a quantum yield of 58%. As the concentration of N-GQDs increases, the photoluminescence exhibits an obvious red-shift from blue to yellow. For the N-GQDs in solid state, yellow luminescence with a high photoluminescence quantum yield (PLQY) of 28% is achieved under the 405 nm excitation. Finally, via the simple adjustment of thickness and the concentration of N-GQDs in blue emitting InGaN chips, color converter is enabled by constructing white light-emitting diode (WLED) device with improved color rendering index (CRI) and correlated color temperature (CCT). • A simple and efficient hydrothermal method for the production of N-GQDs with strong solid-state fluorescence. • The as-prepared N-GQDs in an aqueous state exhibit blue color, with a quantum yield of 58%. • As the concentration of N-GQDs increases, the photoluminescence exhibits an obvious red-shift from blue to yellow. • Yellow emitted N-GQDs in solid state exhibit excitation-independent behavior with a quantum yield of 28%. • The color converter is enabled by constructing white light-emitting diode device with improved performance. [ABSTRACT FROM AUTHOR]

In this study, Zn x Cd 1-x S/N-GQDs composites with different Zn/Cd ratios were synthesized via a facile solvothermal process. A series of techniques were introduced to investigate the effect of Zn/Cd ratios and N-GQDs on the performances of Zn x Cd 1-x S/N-GQDs. The results show that the crystal structure and photoelectric performance of the composites can be tuned via adjusting the Zn/Cd ratios. Zn x Cd 1-x S/N-GQDs composites transform from a hexagonal structure to a cubic structure with the increase of Zn content, and the diffraction peaks are located between hexagonal phase CdS and cubic phase ZnS. Meanwhile, the photocurrent responses and electrochemical impedance present good tunability via adjusting the Zn/Cd ratios. Zn 0.9 Cd 0.1 S/N-GQDs composites with the higher photocurrent value of 3.79 μA cm−2 and lower interfacial impedance are superior to Zn x Cd 1-x S/N-GQDs composites with other Zn/Cd ratios. Moreover, the introduction of N-GQDs obviously enhances the photoelectric properties of the semiconductor. • We successfully synthesized Zn x Cd 1-x S/N-GQDs composites by facile solvothermal process. • Zn x Cd 1-x S/N-GQDs composites exhibit alloyed Zn x Cd 1-x S structure instead of the independent ZnS and CdS. • The photoelectric properties of Zn x Cd 1-x S/N-GQDs present a good tunability via adjusting the Zn/Cd ratios. • The introduction of N-GQDs enhanced the photoelectric properties of Zn 0.9 Cd 0.1 S with the higher photocurrent intensity. [ABSTRACT FROM AUTHOR]

A Z-scheme I-BiOCl/N-GQD (i.e., nitrogen-doped graphene quantum dot) heterojunction was prepared by a one-pot precipitation method at room temperature. The doped iodine decreased the band gap of BiOCl, the introduced N-GQDs enhanced light harvesting and prolonged the photogenerated electron lifetime, and the resultant Z-scheme heterojunction promoted the spatial separation of interfacial charges. Thus, the composite showed high photoelectrochemical activity and a big cathodic photocurrent signal. On the basis of the coordination of chlorpyrifos with surface Bi(III) of the composite, a cathodic photoelectrochemical sensor was constructed for the selective detection of chlorpyrifos. In this case, chlorpyrifos decreased the lifetime of photogenerated electrons, so the photocurrent became small. Furthermore, the photocurrent changed and the logarithm of chlorpyrifos concentration presented a linear relationship. The linear range was 0.3-80 ng mL -1 , and the limit of detection was estimated to be 0.01 ng mL -1 (defined as S/N = 3). The present strategy can also be used for the design and fabrication of other PEC sensors suitable for different analytes.

The development of a fast-response sensing technique for detection of cysteine can provide an analytical platform for prescreening of disease. Herein, we have developed a fluorescence turn off-on fluorescence sensing platform by combining nitrogen-doped graphene quantum dots (N-GQDs) with V 2 O 5 nanosheets for the sensitive and selective detection of cysteine in human serum samples. V 2 O 5 nanosheets with 2-4 layers are successfully synthesized via a simple and scalable liquid exfoliation method and then deposited with 2-8 nm of N-GQDs as the fluorescence turn off-on nanoprobe for effective detection of cysteine in human serum samples. The V 2 O 5 nanosheets serve as both fluorescence quencher and cysteine recognizer in the sensing platform. The fluorescence intensity of N-GQDs with quantum yield of 0.34 can be quenched after attachment onto V 2 O 5 nanosheets. The addition of cysteine triggers the reduction of V 2 O 5 to V 4+ as well as the release of N-GQDs within 4 min, resulting in the recovery of fluorescence intensity for the turn off-on detection of cysteine. The sensing platform exhibits a two-stage linear response to cysteine in the concentration range of 0.1-15 and 15-125 μM at pH 6.5, and the limit of detection is 50 nM. The fluorescence response of N-GQD@V 2 O 5 exhibits high selectivity toward cysteine over other 22 electrolytes and biomolecules. Moreover, this promising platform is successfully applied in detection of cysteine in human serum samples with excellent recovery of (95 ± 3.8) - (108 ± 2.4)%. These results clearly demonstrate a newly developed redox reaction-based nanosensing platform using N-GQD@V 2 O 5 nanocomposites as the sensing probe for cysteine-associated disease monitoring and diagnosis in biomedical applications, which can open an avenue for the development of high performance and robust sensing probes to detect organic metabolites.

The efficient separation of photogenerated electron-hole pairs in photoactive materials is highly desired, allowing their transfer to specific sites for undergoing redox reaction in various applications. The construction of ternary heterojunctions is a practical strategy to enhance the migration of photogenerated electron that realizes the synergistic effect of multicomponents rather than the simple overlay of single component. Here, we demonstrate an available way to fabricate new BiOCl/BiVO 4 /nitrogen-doped graphene quantum dot (N-GQD) ternary heterojunctions that exhibit higher efficiency in charge separation than any binary heterojunction or pure material under visible-light irradiation. UV-vis diffuse reflectance spectroscopy demonstrated that the proposed BiOCl/BiVO 4 /N-GQD ternary heterojunctions possess the narrower band gap energy. More importantly, the ternary heterojunctions reveal the prolonged lifetime of photogenerated charges and enhanced the separation efficiency of photogenerated electron-hole pairs, which may be ascribed to sensitization based on an internal Z-scheme charge transfer at the interface of N-GQDs with oxygen functional groups. Furthermore, we examine the photoactive performance of proposed ternary heterojunctions in aqueous solution by using the photodegradation of bisphenol A as a model system and BiOCl/BiVO 4 /N-GQD ternary heterojunctions also display a dramatically enhanced photodegradation rate. The proposed charge separation and transfer process of BiOCl/BiVO 4 /N-GQD ternary heterojunctions for the enhanced photoactivity were deduced by electrochemical measurements, photoluminescence, and electron spin resonance. The results demonstrate that a Z-scheme charge process was formed between BiOCl/BiVO 4 binary heterojunctions and N-GQDs, leading to an efficient charge carrier separation and strong photocatalytic ability. Notably, this work may assist in a better understanding of the role of N-GQDs in kinds of heterojunctions. Conceivably, it can be extended to fabricate other photocatalytic systems and photoelectrochemical platforms associated with photoactive materials.