Hongbo Xu

Fabrication of a nitrogen-doped graphene quantum dot from MOF-derived porous carbon and its application for highly selective fluorescence detection of Fe3+

Nitrogen doping of carbon quantum dots results in improved fluorescence performance and a wider range of applications in photocatalysis, sensors, bioimaging, etc. Herein, a water-soluble and well-crystallized nitrogen-doped graphene quantum dot (N-GQD) has been obtained by using a MOF-derived carbon (ZIF-8C) as a new source of graphitic sheets. The preparation is based on a rapid, eco-friendly and efficient acid vapour cutting strategy, which is different from previously reported solution chemistry routes. The as-prepared N-GQD is photoluminescent and exhibits an excitation-independent behaviour. Because of the presence of O-functional groups on the surface, the obtained N-GQD can serve as a fluorescent sensing probe for highly selective detection of Fe3+ ions with a detection limit of 0.08 μM (at a signal-to-noise ratio of 3). This work would enable new opportunities for the wider use of MOF-based materials and also contribute to the fluorescent analysis of Fe3+.

Graphene quantum dots: recent progress in preparation and fluorescence sensing applications

Fluorescent graphene quantum dots (GQDs) have attracted tremendous attention because of their unique 2D layered structure, large surface area, good water solubility, tunable fluorescence, high photostability, excellent biocompatibility and low toxicity, which make them promising candidates for applications in various fields. In this review, we summarize the latest progress in research on GQDs, focusing on their preparation via both top-down and bottom-up routes and application in fluorescence sensing of inorganic ions, organic molecules and biomaterials. This review provides insight into GQDs to inspire their further development, including their controllable preparation and use in a wider range of sensing applications, by the large community of researchers focusing on graphene.

One-step synthesis of isoreticular metal–organic framework-8 derived hierarchical porous carbon and its application in differential pulse anodic stripping voltammetric determination of Pb( ii )

A novel nanoporous carbon material (NPC) was prepared by simple pyrolysis of an isoreticular metal–organic framework-8 (IRMOF-8) at 1000 °C. The obtained NPC possessed a three-dimensional hierarchy of micro-, meso-, and macropores, a high surface area of 1715 m2 g−1 and a large pore volume of 1.70 cm3 g−1. By using Nafion and bismuth films as co-modifiers, the obtained NPC was further used for the fabrication of a sensitive NPC–Nafion/bismuth composite modified glassy carbon electrode (NPC–Nafion/Bi/GCE) for electrochemical determination of Pb(II) in aqueous solution. The synergistic effect of the Nafion and Bi films, as well as the high activated surface area, hierarchical porous texture, and good electrical conductivity of the NPC contributed to the enhanced electrochemical response of Pb(II) at the NPC–Nafion/Bi/GCE during differential pulse anodic stripping voltammetry. Under the optimum conditions, the fabricated electrode exhibited two linear ranges varying from 1–7 μg L−1 and 7–70 μg L−1, respectively. The detection limit was estimated to be 0.8 μg L−1 (at a signal-to-noise ratio of 3) for Pb(II), which was about 12 times lower than the guideline value set by the World Health Organization (WHO) for drinking water. The NPC–Nafion/Bi/GCE showed high reproducibility, with relative standard deviations of 6.2% and 3.4% for multiple successive measurements of 10 and 50 μg L−1 Pb(II), respectively. The composite electrode was successfully used in the analyses of Pb(II) in tap-water samples, and good recoveries were obtained for various spiked values.

Time-efficient syntheses of nitrogen and sulfur co-doped graphene quantum dots with tunable luminescence and their sensing applications

Heteroatom doped graphene quantum dots (GQDs) are particularly promising in bioimaging and fluorescent sensing because of their better photoluminescence tunability compared to pristine GQDs. Herein, two nitrogen and sulfur co-doped GQDs (N,S-GQDs) with varied fluorescence emission wavelength were synthesized via HNO3 vapour cutting route, in which a porous polythiophene-derived carbon served as the sulfur source while the HNO3 vapour was presented as the scissor and the nitrogen source. The as-prepared N,S-GQDs exhibited blue and yellow-green coloured fluorescence, owing to their varied morphologies and surface states resulted from varied reaction temperature. Compared to the typical top-down syntheses via hydrothermal or solvothermal routes, the present HNO3 vapour cutting method is prominently efficient in time expense and product separation. An application of the obtained greenish-yellow N,S-GQDs for highly selective and sensitive fluorescent detection of Fe3+ was demonstrated, with a linear range of 0–130 μM and a detection limit of 0.07 μM. The protocol reported here can also be readily applied for facial synthesis of other heteroatom doped GQDs.

Luminescent properties and sensing performance of a carbon quantum dot encapsulated mesoporous silica/polyacrylonitrile electrospun nanofibrous membrane

A novel turn-off and label-free fluorescent sensor based on a carbon quantum dot encapsulated mesoporous silica/polyacrylonitrile (CDs/mesoSiO2/PAN) electrospun nanofibrous membrane for the determination of Fe(III) was evaluated. Compared with that of the free-state CDs, the fluorescent behavior of the embedded CDs was altered because of the involvement of mesoSiO2/PAN electrospun nanofibers as a supporting matrix. On the basis of previously reported understandings, the photoluminescent (PL) mechanism of the CDs had been further studied. It is evident that the fixed blue emission of CDs occurs from the carbon defect states, whereas the excitation-dependent emission at longer wavelengths is related to confinement effects of the CDs. Moreover, compared with the free-state CDs, the CDs/mesoSiO2/PAN nanofibrous membrane exhibits a more stable PL signal, improved anti-photobleaching performance, and better pH adaptability because of the possible hydrogen bonding between the SiOH groups in nanofibers and the surface oxygen-containing groups of the CDs, which create an adequate chemical environment for the preservation of the carbon defect states in luminescence generation as well as the active sites for Fe(III) binding. Applying the CDs/mesoSiO2/PAN membrane for the determination of Fe(III) in tap water samples indicates that it can work efficiently as a PL sensor.

N-Doped Ordered Mesoporous Carbon Originated from a Green Biological Dye for Electrochemical Sensing and High-Pressure CO2 Storage

Herein, a series of nitrogen-doped ordered mesoporous carbons (NOMCs) with tunable porous structure were synthesized via a hard-template method with a green biological dye as precursor, under various carbonization temperatures (700-1100 °C). Compared with the ordered mesoporous silica-modified and unmodified electrodes, the use of electrodes coated by NOMCs (NOMC-700-NOMC-1100) resulted in enhanced signals and well-resolved oxidation peaks in electrocatalytic sensing of catechol and hydroquinone isomers, attributable to NOMCs open porous structures and increased edge-plane defect sites on the N-doped carbon skeleton. Electrochemical sensors using NOMC-1000-modified electrode were fabricated and proved feasible in tap water sample analyses. The NOMCs were also used as sorbents for high-pressure CO2 storage. The NOMC with the highest N content exhibits the best CO2 absorption capacities of 800.8 and 387.6 mg/g at 273 and 298 K (30 bar), respectively, which is better than those of other NOMC materials and some recently reported CO2 sorbents with well-ordered 3D porous structures. Moreover, this NOMC shows higher affinity for CO2 than for N2, a benefit of its higher nitrogen content in the porous carbon framework.

Molecular interactions at the hexadecane/water interface in the presence of surfactants studied with second harmonic generation

It is important to investigate the influence of surfactants on structures and physical/chemical properties of oil/water interfaces. This work reports a second harmonic generation study of the adsorption of malachite green (MG) on the surfaces of oil droplets in a hexadecane/water emulsion in the presence of surfactants including sodium dodecyl sulfate, polyoxyethylene-sorbitan monooleate (Tween80), and cetyltrimethyl ammonium bromide. It is revealed that surfactants with micromolar concentrations notably influence the adsorption of MG at the oil/water interface. Both competition adsorption and charge-charge interactions played very important roles in affecting the adsorption free energy and the surface density of MG at the oil/water interface. The sensitive detection of the changing oil/water interface with the adsorption of surfactants at such low concentrations provides more information for understanding the behavior of these surfactants at the oil/water interface.

Temperature dependent 2D self-assembled motif transition of copper–phthalocyanine derivates at air/HOPG interface: an STM study

It is known that copper(II) 2,3,9,10,16,17,23,24-octakis(octyloxy)-29H,31H-phthalocyanine (CuPcOC8) molecules can self-organize on highly oriented pyrolytic graphite and form both quartic symmetry structures with relatively low molecular packing density and hexagonal symmetry structures with high packing density. However, the mechanism of the formation of the two types of molecular adlayers, and the tunability of this metal phthalocyanine film in ambient atmosphere has not been discussed. In this work, by increasing the temperature during the self-assembly process, or applying an annealing treatment to the self-assembled film, the transition of the self-assembled structure from quartic to hexagonal symmetry was observed. With images obtained with scanning tunnelling microscopy under different conditions and statistical analysis of the ratios of the quartic to hexagonal domain areas, this work revealed the existence of an energy barrier between the quartic and the hexagonal structures. The hexagonal phase corresponds to a thermodynamically stable state while the quartic phase is kinetically favorable but thermodynamically metastable. It was also found that with high enough temperature and given enough time, uniform ordered assembly motifs of CuPcOC8 molecules on a large scale can be obtained.

A protocol of self-assembled monolayers of fluorescent block molecules for trace Zn(II) sensing: structures and mechanisms

2D epitaxial nanostructures formed by functional molecules at interfaces/surfaces have attracted considerable research interest because of their promising applications in catalysis, photonics, electronic devices, and so on. In this study, we demonstrate for the first time that a monolayer of functional molecules may be used as a fluorescence indicator for heavy metal ions. Self-assembly of bisligand TPE-C4-L2 on highly oriented pyrolytic graphite was performed and the self-assembled monolayer (SAM) was used for trace amounts of Zn(II) sensing with an output signal of fluorescence. In contrast to the Zn(II)-induced fluorescence enhancement of the TPE-C4-L2 block molecule in solution, the self-assembled TPE-C4-L2 film showed quenched fluorescence after Zn(II) was introduced. Using a high-resolution scanning tunneling microscope, the SAM structures of TPE-C4-L2 and its zinc complex were determined. The correlation between their optical properties and structures at the molecular level was also revealed.

Metal dependent motif transition in a self-assembled monolayer of bipyridine derivatives via coordination: An STM study

Low-dimensional molecular motifs with diversity developed via the on-surface chemistry are attracting growing interest for their potential in advanced nanofabrication. In this work, scanning tunneling microscopy was employed to investigate the in situ and ex situ metal coordinations between 4,4-ditetradecyl-2,2-bipyridine (bpy) and Zn(ii) or Cu(ii) ions at a highly oriented pyrolytic graphite (HOPG)/1-phenyloctane interface under ambient conditions. The results demonstrate that the bpy adopts a flat-lying orientation with its substituted alkyl chains in a tail-to-tail arrangement in a bpy monolayer. For the in situ coordination, the bpy/Zn(ii) and bpy/Cu(ii) complexes are aligned in edge-on fashions, wherein the bpy stands vertically on the HOPG surface and interdigitates at the alkyl chains. In the two-dimensional arrays of ex situ coordinated complexes, metal dependent motifs have been observed with Zn(ii) and Cu(ii), wherein the bipyridine moieties are parallel to the graphite surface. These results suggest that the desired on-surface coordination architectures may be achieved by the intentional selection of the metal centers.