Search

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]

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]

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]

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. [ABSTRACT FROM AUTHOR]

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]

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]

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 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]

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]

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]

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]

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]