William Z. Xu

CdS and CdTeS quantum dot decorated TiO2 nanowires. Synthesis and photoefficiency

An easy process was developed to synthesize TiO(2) nanowires sensitized with CdS and CdTeS quantum dots (QDs) requiring no pretreatment of the TiO(2) nanowires prior to nanoparticle generation. CdS and CdTeS nanoparticles were firstly grown by an in situ colloidal method directly onto the TiO(2) surface, hence not requiring subsequent functionalization of the QDs. The resulting nanostructure assembly and composition was confirmed by transmission electron microscopy (TEM), x-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. Successful decoration of the TiO(2) nanowires by the QDs was observed by TEM, while XPS spectra provided clear evidence for the coexistence of CdS and CdTeS QDs and TiO(2) nanowires. The electronic structure of the TiO(2) nanowires was preserved as indicated by Raman spectroscopy. Preliminary photocurrent measurements showed that inclusion of Te in CdS QDs improved the photocurrent efficiency. Compared to bare TiO(2) nanowires, CdS/TiO(2) nanoassemblies showed an enhancement in photocurrent efficiency of 300% while CdTeS/TiO(2) presented an improvement of 350%. This study indicates that the generation of strongly anchored CdS and CdTeS QDs on a TiO(2) nanowire surface is achievable without introduction of a linker molecule, whose presence is known to decrease the electron injection efficiency.

A novel approach to the synthesis of SiO2–PVAc nanocomposites using a one-pot synthesis in supercritical CO2

Inorganic–polymer nanocomposites are of significant interest for emerging materials due to their improved properties and unique combination of properties. A novel one-step synthesis route has been developed for making the polymer nanocomposites silica–poly(vinyl acetate) (SiO2–PVAc) in supercritical CO2 (scCO2), wherein all raw chemicals, tetraethoxysilane (TEOS)/tetramethoxysilane (TMOS), vinyltrimethoxysilane (VTMO), vinyl acetate, initiator, and hydrolysis agent were introduced into one autoclave. In-situ ATR-FT-IR was applied to monitor the process in scCO2, and the parallel reactions of free radical polymerization, hydrolysis/condensation, and linkage to the polymer matrix, were found to take place. The nanocomposites were also studied by transmission electron microscopy (TEM) and EDX element Si-mapping. Well-dispersed nanoparticles of 10–50 nm were formed. This process provides a significant improvement by providing a one-step synthesis route where the potentially recyclable scCO2 works as a solvent, a modification agent, and a drying agent. This green process has potentially many advantages in producing new and unique materials, along with waste-reduction and energy-saving properties. Production of metal-oxide–polymer nanocomposites from non-inhalable liquid precursors also has significant potential for non-toxicity in biomedical and other fields.

Hydroxyapatite−TiO 2 ‑based Nanocomposites Synthesized in Supercritical CO 2 for Bone Tissue Engineering: Physical and Mechanical Properties

Calcium phosphate-based nanocomposites offer a unique solution toward producing scaffolds for orthopedic and dental implants. However, despite attractive bioactivity and biocompatibility, hydroxyapatite (HAp) has been limited in heavy load-bearing applications due to its intrinsically low mechanical strength. In this work, to improve the mechanical properties of HAp, we grew HAp nanoplates from the surface of one-dimensional titania nanorod structures by combining a coprecipitation and sol-gel methodology using supercritical fluid processing with carbon dioxide (scCO2). The effects of metal alkoxide concentration (1.1-1.5 mol/L), reaction temperature (60-80 °C), and pressure (6000-8000 psi) on the morphology, crystallinity, and surface area of the resulting nanostructured composites were examined using scanning electron microscopy (SEM), transmission electron microscopy (TEM), powder X-ray diffraction (XRD), and Brunauer-Emmet-Teller (BET) method. Chemical composition of the products was characterized using Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and X-ray absorption near-edge structure (XANES) analyses. HAp nanoplates and HAp-TiO2 nanocomposites were homogeneously mixed within poly(ε-caprolactone) (PCL) to develop scaffolds with enhanced physical and mechanical properties for bone regeneration. Mechanical behavior analysis demonstrated that the Youngs and flexural moduli of the PCL/HAp-TiO2 composites were substantially higher than the PCL/HAp composites. Therefore, this new synthesis methodology in scCO2 holds promise for bone tissue engineering with improved mechanical properties.

Synthesis and characterization of novel TiO2-poly(propylene fumarate) nanocomposites for bone cementation

This work reports on a new methodology for synthesizing poly(propylene fumarate) (PPF)/titania nanowire composites which would be beneficial in tissue engineering for orthopaedic bone cements. The synthetic procedure reacted PPF with maleic anhydride to create HOOC-PPF-COOH species in a ring-opening reaction at room temperature. These species were then coordinated to titania nanowires by metal carboxylate bonding through the end -COOH groups. These PPF-grafted nano-TiO2 assemblies were then further polymerized and crosslinked in the presence of N-vinylpyrrolidone to produce the bone cements. The synthesis and modification of PPF was confirmed by NMR (1H and 13C) and XPS, while the reaction chemistry of the functionalized PPF and nano-TiO2 was also investigated by XPS and FTIR. Mechanical testing of the resulting composites demonstrated a significant reinforcement of the tensile and flexural properties, showing the utility of this synthetic approach for bone tissue engineering.

Quantum dots/silica/polymer nanocomposite films with high visible light transmission and UV shielding properties

The dispersion of light-absorbing inorganic nanomaterials in transparent plastics such as poly(ethylene-co-vinyl acetate) (PEVA) is of enormous current interest in emerging solar materials, including photovoltaic (PV) modules and commercial greenhouse films. Nanocrystalline semiconductor or quantum dots (QDs) have the potential to absorb UV light and selectively emit visible light, which can control plant growth in greenhouses or enhance PV panel efficiencies. This work provides a new and simple approach for loading mesoporous silica-encapsulated QDs into PEVA. Highly luminescent CdS and CdS-ZnS core-shell QDs with 5 nm size were synthesized using a modified facile approach based on pyrolysis of the single-molecule precursors and capping the CdS QDs with a thin layer of ZnS. To make both the bare and core-shell structure QDs more resistant against photochemical reactions, a mesoporous silica layer was grown on the QDs through a reverse microemulsion technique based on hydrophobic interactions. By careful experimental tuning, this encapsulation technique enhanced the quantum yield (∼65%) and photostability compared to the bare QDs. Both the encapsulated bare and core-shell QDs were then melt-mixed with EVA pellets using a mini twin-screw extruder and pressed into thin films with controlled thickness. The results demonstrated for the first time that mesoporous silica not only enhanced the quantum yield and photostability of the QDs but also improved the compatibility and dispersibility of QDs throughout the PEVA films. The novel light selective films show high visible light transmission (∼90%) and decreased UV transmission (∼75%).

Construction and enhanced photocatalytic activities of a hydrogenated TiO2 nanobelt coated with CDs/MoS2 nanosheets

A few-layered CDs (carbon dots)/MoS2 nanosheet-coated hydrogenated TiO2 (H-TiO2) nanobelt heterostructures—referred to as CDs/MoS2@H-TiO2—with a flexible three-dimensional (3D) hierarchical configuration were prepared via a facial hydrothermal reaction. Note that the visible photocatalytic activity of H-TiO2 was improved compared with that of pristine rutile TiO2, which can be mainly attributed to the optical absorption and charge carrier trapping of oxygen vacancies and Ti3+ ions in TiO2 nanobelts created by the hydrogenation. The CDs/MoS2@H-TiO2 ternary photocatalysts exhibit excellent UV and visible photocatalytic property. Via optimizing the proportion of each component, the CDs/MoS2@H-TiO2 composite showed the highest photocatalytic degradation activity when the content of the CDs/MoS2 co-catalyst was 5.0 wt% and the content of CDs in this cocatalyst was 25%. Further study revealed that the considerable photodegradation rate under UV irradiation and a large promotion of the photocatalytic activity in both the visible and near-infrared (NIR) region originated from the synergistic effect of oxygen vacancies, interfacial modification, and the vectorial charge-transfer channel design. Our study provides a desired strategy to understand and realize a rationally designed electronic transition between a semiconductor and cocatalysts, which is of great importance for the enhancement of charge separation and obtaining improved photocatalytic performance.

Isolation and immunosuppressive effects of 6″-O-acetylginsenoside Rb1 extracted from North American ginseng.

Extraction of medicinally active components from natural health products has become an emerging source for drug discovery. Of particular interest for this work was the finding and testing of a new ginsenoside from North American ginseng (Panax quinquefolius). In the present study, a large amount of 6″-O-acetylginsenoside Rb1, compound 7, was found using ultrasonic extraction of North American ginseng with DMSO aqueous solution. This new ginsenoside was well identified with MS, FTIR, and 1D (1H and 13C) and 2D (gCOSY, gHSQC, and gHMBC) NMR. Subsequent bioassay experiments confirmed that compound 7 demonstrated an additional immunosuppressive activity towards inhibiting the production of nitric oxide and tumor necrosis factor alpha in lipopolysaccharide-induced macrophage cells in a dose-dependent manner using murine macrophages. This new ginsenoside is encouraging for the further exploration and development of novel drugs.

The effect of silica thickness on nano TiO2 particles for functional polyurethane nanocomposites

In order to help reduce the agglomeration of TiO2 nanoparticles in polyurethane coatings while enhancing their photoactivity and mechanical/physical properties, this work examined encapsulating TiO2 nanoparticles in a thin layer of SiO2, prior to their nanocomposite polymerization. By applying a Stöber process, varying thicknesses of SiO2 were successfully coated onto the surface of anatase and rutile TiO2 nanoparticles. The methylene blue results showed that different loadings of SiO2 onto the TiO2 surface significantly influenced their photocatalytic activity. When the loading weight of SiO2 was lower than 3.25 wt%, the photocatalytic activity was enhanced, while with higher loadings, it gave lower photocatalytic activity. When the rutile phase TiO2 surface was fully covered with SiO2, an enhanced photocatalytic activity was observed. When these silica coated nanoparticles were applied in polyurethane coatings, increasing the amount of SiO2 on the titania surface increased the coatings contact angle from 75° to 87° for anatase phase and 70°-78° for rutile phase. The Youngs modulus was also increased from 1.06 GPa to 2.77 GMPa for anatase phase and 1.06-2.17 GPa for rutile phase, attributed to the silica layer giving better integration. The thermal conductivity of the polyurethane coatings was also successfully decreased by encapsulating SiO2 on the titania surface for next generation high performance coatings.

Angiogenic Rg1 /Sr-Doped TiO2 Nanowire/Poly(Propylene Fumarate) Bone Cement Composites.

A new approach is provided for preparing radiopaque and angiogenic poly(propylene fumarate) (PPF) bone cements by integrating Sr-doped n-TiO2 nanowires and ginsenoside Rg1 suitable for treating osteonecrosis. High aspect ratio radiopaque TiO2 -nanowires are synthesized by strontium doping in supercritical CO2 for the first time, showing a new phase, SrTiO3 . PPF is synthesized using a transesterification method by reacting diethyl fumarate and propylene glycol, then functionalized using maleic anhydride to produce terminal carboxyl groups, which are subsequently linked to the nanowires. The strong interfacial adhesion between functionalized PPF and nanowires is examined by scanning electron microscopy, Fourier transform infrared, X-ray photoelectron spectroscopy, thermal analysis, and mechanical testing. An angiogenic modulator, ginsenoside Rg1 , is integrated into the bone cement formulation with the mechanical properties, radiopacity, drug release, and angiogenesis behavior of the formed composites explored. The results show superior radiopacity and excellent release of ginsenoside Rg1 in vitro, as well as a dose-dependent increase in the branching point numbers. The present study suggests this new methodology provides sufficient mechanical properties, radiopacity, and angiogenic activity to be suitable for cementation of necrotic bone.

Tuning the optical properties of silicon quantum dots via surface functionalization with conjugated aromatic fluorophores

Silicon Quantum Dots (SQDs) have recently attracted great interest due to their excellent optical properties, low cytotoxicity, and ease of surface modification. The size of SQDs and type of ligand on their surface has a great influence on their optical properties which is still poorly understood. Here we report the synthesis and spectroscopic studies of three families of unreported SQDs functionalized by covalently linking to the aromatic fluorophores, 9-vinylphenanthrene, 1-vinylpyrene, and 3-vinylperylene. The results showed that the prepared functionalized SQDs had a highly-controlled diameter by HR-TEM, ranging from 1.7–2.1 nm. The photophysical measurements of the assemblies provided clear evidence for efficient energy transfer from the fluorophore to the SQD core. Fӧrster energy transfer is the likely mechanism in these assemblies. As a result of the photogenerated energy transfer process, the emission color of the SQD core could be efficiently tuned and its emission quantum efficiency enhanced. To demonstrate the potential application of the synthesized SQDs for bioimaging of cancer cells, the water-soluble perylene- and pyrene-capped SQDs were examined for fluorescent imaging of HeLa cells. The SQDs were shown to be of low cytotoxicity