Qixin Zhuang

Metal–organic frameworks with inherent recognition sites for selective phosphate sensing through their coordination-induced fluorescence enhancement effect

Luminescent metal–organic frameworks (LMOFs) have attracted significant attention as a unique class of sensing materials. In this work, the intrinsically fluorescent amino derivative of UiO-66 (UiO-66-NH2) was successfully exploited as a fluorescent probe for the sensitive and selective detection of phosphate anions in an aqueous medium. The inorganic Zr–O clusters and organic BDC-NH2 linkers in the elaborated UiO-66-NH2 MOFs were individually designed as phosphate recognition sites and signal reporters. The intrinsic fluorescence of BDC-NH2 was tuned from high to weak emission by ligand-to-metal charge transfer (LMCT) upon its integration into the framework of UiO-66-NH2 MOFs and this weakened fluorescence could be proportionally recovered in correlation with the applied phosphate level through a newly developed competitive coordination effect. The specificity for phosphate recognition of the employed sensory platform was scarcely affected by other possible interfering species. The efficacy of this strategy was demonstrated by a linear phosphate detection range of 5–150 μM and a limit of detection of 1.25 μM, which was far below the detection requirement of phosphate discharge criteria in the water environment. The possible sensing mechanisms for anionic phosphate detection using the currently established fluorescent probe, including host–guest interaction and structure–property correlation, were systematically investigated using XPS, FT-IR, XRD, TEM and N2 sorption techniques.

Preparation and properties of thermostable well-functionalized graphene oxide/polyimide composite films with high dielectric constant, low dielectric loss and high strength via in situ polymerization

This study proposes a novel and facile method to synthesize high-quality NH2-functionalized and carboxyl-functionalized graphene oxide (PPD–CFGO)/polyimide (PI) composite films with high dielectric constant (e), low dielectric loss, high-temperature resistance and outstanding mechanical properties by in situ polymerization. In addition to the partial carboxyl groups located at the edges, the ample hydroxyl and epoxy groups bonded on the basal plane of graphene sheets were exploited to covalently bond to the amines. GO was modified by oxalic acid to obtain carboxyl-functionalized GO (CFGO) before amidation. NH2-functionalized CFGO (PPD–CFGO), dispersing well in dimethylacetamide (DMAc), was the initial platform for polymer grafting to improve the CFGO dispersion in the polymer matrix. Partially reduced graphene nanosheets are formed during the imidization process. The PPD–CFGO/PI composite films exhibit high tensile strength (up to 848 MPa) and Youngs modulus (18.5 GPa). The thermogravimetric analysis results indicate that the PPD–CFGO/PI composites have good thermal stability below 500 °C. The dielectric constant increases up to 36.9 with an increasing amount of PPD–CFGO, higher than that of the pure PI polymer by a factor of 12.5, while the dielectric loss is only 0.0075 and the breakdown strength still remains at a high level (132.5 ± 9.3 MV m−1).

Preparation of thermostable PBO/graphene nanocomposites with high dielectric constant

In this study, poly(p-phenylene benzobisoxazole) (PBO)/graphene composites were prepared using PBO and poly(4,6-dihydroxymetaphenylenediamine terephthalamide) (PHA)-modified graphene oxide (GO-PHA). PHA is the precursor of PBO. GO-PHA was obtained via chemical coupling reaction of amino-terminated PHA and acyl-chloride-functionalized GO. Partially reduced graphene nanosheets and benzoxazole rings were formed after heating. GO-PHA could be stably dispersed in methane sulfonic acid (MSA), which facilitated its uniform distribution in the PBO matrix. The PBO/graphene nanocomposites were obtained by the dissolution of GO-PHA and PBO in MSA. The thermogravimetric analysis results showed that the PBO/graphene composites had good thermal stability below 400 ° C. The dielectric constant of the composites increased as the amount of GO-PHA increased, and the percolation threshold was f(c) = 0.037. The nanocomposite had a dielectric constant of 15.8, which was approximately five times larger than that of pure PBO polymer.

Synthesis and copolymerization of benzoxazines with low-dielectric constants and high thermal stability

Two novel bridged benzoxazole-based benzoxazine monomers, HOH-a and FOH-a, were synthesized using a non-solvent method. The structure of HOH-a and FOH-a was confirmed by Fourier transform infrared spectroscopy (FTIR), 1H NMR, and elemental analysis. The curing behaviors of these two monomers were studied by differential scanning calorimetry and FTIR. The corresponding polybenzoxazines, poly(HOH-a) and poly(FOH-a), display higher thermal stability than the polybenzoxazine from common bisphenol-A-based benzoxazine (BA-a), especially the char yields at 800 °C, which are 61% and 62% for poly(HOH-a) and poly(FOH-a), respectively. Moreover, the dielectric constants and dielectric loss of these polybenzoxazines are low and change slightly at room temperature in the frequency range of 0.1 Hz to 1 MHz. The dielectric constant of poly(FOH-a) in 1 MHz is only 2.21. Furthermore, poly(HOH-a/BA-a) and poly(FOH-a/BA-a) copolymers were prepared, and the thermal stability and dielectric and mechanical properties of these copolymers were studied. The flexural modulus and flexural strength of the poly(FOH-a/BA-a) copolymer can be increased to 66% and 41%, respectively, after the addition of 20 wt.% FOH-a.

In situ synthesis of ternary BaTiO3/MWNT/PBO electromagnetic microwave absorption composites with excellent mechanical properties and thermostabilities

Different from traditional microwave absorbing nanoparticles as fillers in non-mechanical coatings, the BaTiO3/MWNT/PBO ternary composites can be potentially used as a structural microwave absorption device in broader applications, especially in the aerospace industry. BaTiO3 particles with a diameter of 5–15 nm were immobilized onto the surface of MWNTs via a solvothermal synthesis method. BaTiO3/MWNT/PBO ternary composites with varied compositions were then prepared via the in situ polymerization of p-phenylenebenzobisoxazole (PBO) with a uniform dispersion of BaTiO3/MWNT nanocomposites in the polymer-poly p-phenylenebenzobisoxazole (PBO)-matrix (a conjugated polymer with splendid mechanical properties and thermal stability). The BaTiO3/MWNT/PBO composites possessed outstanding microwave absorbing performances in addition to desirable mechanical properties and thermal stability.

Porphyrinic MOFs for reversible fluorescent and colorimetric sensing of mercury(II) ions in aqueous phase

Developing an effective method that could provide simple, rapid and visual determination of Hg2+ in water has attracted great attention. In this work, a novel chemical sensor with a porphyrin-based luminescent metal–organic framework (LMOFs) which exhibited a dual-mode response to trace amounts of mercury ions (Hg2+) has been successfully constructed. The porous LMOF probe was fabricated using a simple solvothermal reaction, and assembled with Zr–O clusters as inorganic nodes and meso-tetra(4-carboxyphenyl)porphyrin (TCPP) ligands as organic bridging struts. The sensing activity was realized by using the inherent TCPP struts as recognition sites and signal reporter, which presented a specific interaction with Hg2+ over other potentially interfering metallic cations. The LMOFs which were developed exhibited a visible fluorescent quenching (bright red-dark red) and a colorimetric response (purple-light green) in the presence of Hg2+ with a response rate as rapid as 2 min. Additionally, the fluorescence quenching showed a linear correlation in the Hg2+ concentration range from 0.1 to 10 μM and the limit of detection was calculated to be 6 nM, which was in agreement with the acceptable level of Hg2+ in drinking water mandated by the United States Environmental Protection Agency. Furthermore, the sensor for Hg2+ detection was reversible after the treatment with potassium iodide solution, which suggested that it has great potential as an economical alternative for practical Hg2+ quantification in water. The easily constructed sensory probe was also applied to determine the concentration of Hg2+ in tap water to demonstrate its practical applications.

Synthesis of acid-soluble graphene and its use in producing a reduced graphene oxide–poly(benzobisoxazole) composite

A new chemically modified graphene was prepared using carboxylated graphene oxide (GO) and fluorinated poly(hydroxyamide) (6FPHA), the latter is the precursor of fluorinated poly(benzobisoxazole) (6FPBO). The results showed that, unlike common GO, which can only be dispersed in water under neutral or basic conditions, the RGO–6FPBO (reduced graphene oxide–6FPBO) could be stably dispersed in Lewis acids. The RGO–6FPBO–poly(benzobisoxazole) (PBO) composite was achieved by simple solution blending of RGO–6FPBO and PBO in methanesulfonic acid. The obtained RGO–6FPBO–PBO composite films exhibited a dramatic increase in electrical conductivity, by 8 orders of magnitude at the RGO–6FPBO content of 4 wt%, without significantly sacrificing optical transparency. When the content of RGO–6FPBO was higher than 4 wt%, the dielectric constant of the composite film also increased remarkably. The mechanical properties and thermal stability of the composite were also improved by the incorporation of RGO–6FPBO.

Core/shell-structured hyperbranched aromatic polyamide functionalized graphene nanosheets-poly(p-phenylene benzobisoxazole) nanocomposite films with improved dielectric properties and thermostability

This study reports the synthesis of core/shell-structured hyperbranched aromatic polyamide functionalized graphene nanosheets-poly(p-phenylene benzobisoxazole) (GNs-HAP-PBO) nanocomposite films with improved dielectric properties and thermostability. PBO precursor polymer chains were grafted onto the ample amino-terminated GNs-HAP via in situ polymerization, and then the reduction of GNs-HAP and the intramolecular cyclization of PBO precursors were achieved through thermal treatment. The unique core/shell-structure is effective to prevent the aggregation of GNs and improves the dispersion of GNs in the GNs-HAP-PBO nanocomposites, forming microcapacitor networks in the matrix. The GNs-HAP-PBO nanocomposite films exhibit lower dielectric loss in comparison with solvothermally reduced graphene oxide/PBO nanocomposite films. At 1 kHz and 200 °C, a dielectric constant of 66.27 and a dielectric loss of 0.045 are observed in the GNs-HAP-PBO nanocomposite films with 2 wt% GNs-HAP. Moreover, the maximum energy density of the GNs-HAP-PBO nanocomposite films is up to 6 J cm−3 owing to the high breakdown strength (132.5 ± 9.3 kV mm−1). The GNs-HAP-PBO nanocomposite films with 2 wt% GNs-HAP also exhibit excellent tensile strength (125 MPa), Youngs modulus (6.4 GPa), and high thermal stability (temperature of 5 wt% loss = 643 °C). This work demonstrates a promising strategic approach to fabricating high dielectric materials under extreme environments.

Synthesis and photoluminescence properties of polybenzoxazoles containing perylenebisimide functionalized graphene nanosheets via stacking interactions

The stable acidic dispersion of reduced graphene oxide (RGO) is prepared by a non-covalent functionalization method using perylenebisimide-modified fluorinated poly(hydroxyamide) (PTCDA-6FPHA). Then a series of RGO–polybenzoxazole (PBO) composites were prepared by a solution blending method in MSA. The change of perylenebisimide absorption peaks in the UV-Vis absorption spectra indicates that π–π stacking indeed occurs between graphene and perylene rings. The conjugation between functionalized RGO and PBO molecular chains increases the conjugation degree of the composite, since the PTCDA-6FPHA-containing perylene rings have a high π-electron delocalization degree. The energy gap of RGO–PBO composite is 2.72 eV with the absorption edge at 456 nm. The corresponding data of pure PBO film are 2.78 eV and 446 nm, respectively. Moreover, the intensity of the two emission peaks at 442 and 467 nm in the PL emission spectra of RGO–PBO decreases rapidly along with the increase of RGO–PBO ratio. The intensity of the weak emission peak at 622 nm increases with the increase of the RGO–PBO ratio. This behavior can be attributed to the energy transfer between benzoxazole chromophore units and functionalized RGO. Meanwhile, the RGO–PBO composite has outstanding thermal stability.

NH2-functionalized carbon-coated Fe3O4 core–shell nanoparticles for in situ preparation of robust polyimide composite films with high dielectric constant, low dielectric loss, and high breakdown strength

Novel Fe3O4/polyimide (PI) composite films with extraordinary dielectric properties were prepared via in situ polymerization of PI in the presence of 4,4′-diaminodiphenyl ether (ODA), pyromellitic dianhydride (PMDA) and compatible amine-functionalized carbon-coated Fe3O4 nanoparticles (Fe3O4@C–NH2). Fe3O4 nanoparticles prepared via solvothermal reaction were coated with crosslinked glucose and then surface-grafted via carbodiimide coupling. The core–shell structure of Fe3O4@C–NH2 was thereafter obtained with amine functionalities on the carbonized-glucose shell. Homogeneous dispersion of Fe3O4@C–NH2 and strong interfacial covalent bonding on the shell surface contributed to high dielectric constants (e) and extremely low dielectric losses (tan δ) in the composite films. The highest dielectric constant of the composite films was 58.6 at 1 kHz when the Fe3O4@C–NH2 composition was 1.13 vol%, as well as a low dielectric loss of 0.0091 and a high breakdown strength of 126.5 ± 11.3 MV m−1 were observed without considerable sacrifice of the thermal stability and mechanical properties.