Tri Khoa Nguyen

ACTIVATION OF EUKARYOTIC INITIATION FACTOR-2α-KINASES IN OKADAIC ACID-TREATED NEURONS

Phosphorylation of eukaryotic initiation factor-2 alpha (eIF2 alpha) is increased in Alzheimers disease (AD) and this protein can be phosphorylated by several kinases, including double-stranded RNA-dependent protein kinase (PKR), PKR-like endoplasmic reticulum kinase (PERK), amino acids-regulated eIF2 alpha kinase (GCN2) and heme-regulated eIF2 alpha kinase (HRI). PKR and PERK especially are activated in the AD brain, and GCN2 is reported to increase presenilin-1 (PS1) activity. Okadaic acid (OA), a protein phosphatase-2A (PP2A) inhibitor, is known to increase tau phosphorylation, beta-amyloid (A beta) deposition and neuronal death, which are the pathological characteristics of AD. Here, we show that the phosphorylation of eIF2 alpha is increased and its kinases, PKR, PERK and GCN2 are activated in rat neurons by OA. Activating transcription factor (ATF4) which induces apoptosis in response to eIF2 alpha phosphorylation was increased and translocated to nuclei in OA-treated neurons. These results suggest that the successive events of activation of eIF2 alpha kinases and eIF2 alpha phosphorylation leading to ATF4 nuclear translocation may contribute to neuronal death. However, PKR inhibitors did not reduce eIF2 alpha phosphorylation or neuronal toxicity despite inhibiting PKR activity. These results suggest that PKR might not be the most responsible kinase for eIF2 alpha phosphorylation or cell death in PP2A-inhibited conditions such as AD.

Pulsed laser deposition assisted grown continuous monolayer MoSe2

As compared with sulfur, the low reactivity of selenium makes it more challenging to synthesize high quality continuous single layer selenides required for practical applications. Here, we present the synthesis of monolayer MoSe2 by selenization of a pulsed laser deposited MoO3 film on SiO2/Si and sapphire substrates. The laser energy was carefully varied to optimize the growth of highly uniform, continuous monolayer films. Morphological characterization including optical microscopy, field emission scanning electron microscopy and atomic force microscopy results clearly demonstrate that the synthesized film is monolayer, continuous and homogeneous. The Raman and photoluminescence maps of the continuous film exhibit uniform brightness, indicating the uniform and homogeneous nature of the film. Moreover, the full width at half maximum values of A1g and A exciton peaks were found to be 3.7 cm−1 and 24 nm, respectively, showing the excellent optical quality of the grown film.

Tuning optical band gap by electrochemical reduction in TiO2 nanorods for improving photocatalytic activities

In this study, we investigate the tuning of the optical band gap of TiO2 nanorods (TiO2 NRs) by electrochemical methods for improving their photocatalytic activities. A seed layer prepared by RF-magnetron sputtering is employed to increase the adhesion between TiO2 NRs and substrate to prevent the peel-off of TiO2 NRs from substrate during electrochemical reduction process. The morphological study shows the stability of TiO2 NRs structure after reduction process. The electrochemical reduction process increased the amount of Ti3+ (a reduced state of Ti4+) and oxygen vacancy by 2.2% and 3.6%, respectively. The process also tune the optical band of TiO2 NRs from 3.0 eV to 2.84 eV due to the up-shift of valence band to Fermi level. The reduced band gap of NRs improve the photocatalytic activities by 1.48 times due the increase of its absorbance range from UV to visible in solar spectrum. The photocatalytic activities were 24.3 times improved by depositing a ultrathin layer of Pt as electron collector.

Photoreduction route for Cu2O/TiO2 nanotubes junction for enhanced photocatalytic activity

Here, we synthesized copper(I) oxide and titanium dioxide nanotubes (TNTs) heterojunctions (HJs) by a photoreduction method using a low-power UV lamp without involving any additional steps, such as chemical reduction, surfactant, or protection agents. Transmission electron microscopy, X-ray diffraction, Raman scattering, X-ray photoelectron spectroscopy, diffuse reflectance spectra, and photoluminescence spectroscopy were carried out to verify the formation of a HJ between the Cu2O nanoparticles (Cu2O NPs) and TNTs. The efficiency and the rate of methylene blue photo-degradation over the Cu2O/TNTs HJ were found to be nearly double and triple compared to the isolated TNTs. The enhanced efficiency is attributed to the narrow band gap and defect states caused by the oxygen vacancies in the vicinity of HJs. Moreover, the type II band alignment of Cu2O NPs and TNTs naturally separates the photo-generated carriers and constrains the recombination process owing to the internal electric field across the Cu2O/TNTs interface.

Polyol-mediated synthesis of ZnO nanoparticle-assembled hollow spheres/nanorods and their photoanode performances

ZnO nanoparticle-assembled hollow spheres (raspberry-like) and elliptical nanorods (rice-like) were synthesized via a facile polyol process. Employing ethylene glycol as a polyol led to a ZnO nanoparticle-assembled hollow sphere structure, while diethylene glycol resulted in an elliptical nanorod structure. The ZnO hollow spheres had a higher Brunauer-Emmett-Teller (BET) surface area, better dye adsorption, more incident light trapping, and lower defect density than the ZnO elliptical nanorods. The ZnO hollow sphere-based dye-sensitized solar cells (DSSCs) exhibited a three-times higher current density than the ZnO elliptical nanorod-based DSSCs.

MoS2 monolayers on Si and SiO2 nanocone arrays: influences of 3D dielectric material refractive index on 2D MoS2 optical absorption

Heterostructures enable the control of transport and recombination of charge carriers, which are either injected through electrodes, or created by light illumination. Instead of full 2D-material-heterostructures in device applications, using hybrid heterostructures consisting of 2D and 3D materials is an alternative approach to take advantage of the unique physical properties of 2D materials. In addition, 3D dielectric nanostructures exhibit useful optical properties such as broadband omnidirectional antireflection effects and strongly concentrated light near the surface. In this work, the optical properties of 2D MoS2 monolayers conformally coated on 3D Si-based nanocone (NC) arrays are investigated. Numerical calculations show that the absorption in MoS2 monolayers on SiO2 NC is significantly enhanced, compared with that for MoS2 monolayers on Si NC. The weak light confinement in low refractive index SiO2 NC leads to greater absorption in the MoS2 monolayers. The measured photoluminescence and Raman intensities of the MoS2 monolayers on SiO2 NC are much greater than those on Si NC, which supports the calculation results. This work demonstrates that 2D MoS2-3D Si nano-heterostructures are promising candidates for use in high-performance integrated optoelectronic device applications.

Ultrasonic-Assisted Spin-Coating: Improved Junction by Enhanced Permeation of a Coating Material within Nanostructures

Over the last decades, the spin-coating (SC) technique has been widely used to prepare thin films of various materials in the liquid phase on arbitrary substrates. The technique simply relies on the centrifugal force to spread a coating solution radially outward over the substrate. This mechanism works fairly well for solutions with low surface tension to form thin films of reasonable junctions on smooth substrates. Here, we present a modified SC technique, namely, ultrasonic-assisted spin-coating (UASC), to form thin films of coating solution having high surface tension on rough substrates with excellent junctions. The UASC technique couples SC with an external ultrasonic wave generator to provide external perturbation to locally break down big drops of the coating material into smaller droplets via Rayleigh instability. Because of their lower mass, these tiny droplets gain low momenta and move slowly both in radial and azimuthal directions, giving them an enough time to effectively permeate within pores, thereby yielding excellent junctions. Furthermore, we also investigated the effect of junction improvement on conventional and inverted bulk heterojunction organic solar cells. Intriguingly, the organic solar cells fabricated by the UASC method showed an improved efficiency compared to typical SC owing to efficient charge transfer across the junction. These results clearly imply that UASC is a simple and powerful technique which can significantly enhance the device performance by improving the junction. Moreover, we believe that UASC can be more effective for the preparation of devices composed of multilayers of different materials having complicated nanostructures.

Electrocatalytic hydrogen evolution reaction based on reduced graphene oxide:Pt nanocomposite

We fabricated the composite of reduced graphene oxide (rGO) nanosheets and platinum nanoparticels (Pt NPs) using a pyrolysis spray coating method to investigate their electrocatalytic hydrogen evolution reaction (HER) performance. The atomic force microscopy was used to characterize surface morphology of rGO:Pt nanohybrids. Pt is mostly decorated on rGO nanosheets in form of NPs with diameter sizes in range of 5–30 nm. The HER performance in 0.25 M H2SO4 of the rGO:Pt nanohybrids showed a yielding Tafel slope of 123 mV/decade which is better than that of Pt (166 mV/decade). Even the rGO itself exhibits a poor HER activity, it work as an excellent supporting base in improving catalytic active site, electrical conductivity, and accepting the spillover hydrogen atoms from Pt NP surface in electrocatalytic HER activities.