Enhanced antimicrobial activities of silver-nanoparticle-decorated reduced graphene nanocomposites against oral pathogens.

As a means of capitalizing on the synergistic properties between reduced graphene nanosheets (R-GNs) and silver nanoparticles (AgNPs), an efficient and convenient chemical reduction method was used to prepare silver-nanoparticle-decorated reduced graphene nanocomposites (R-GNs/Ag). The products were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and Raman spectroscopy, which confirmed the loading of well-dispersed silver nanoparticles on reduced graphene sheets. Their antimicrobial activities against oral pathogens such as Candida albicans, Lactobacillus acidophilus, Streptococcus mutans, and Aggregatibacter actinomycetemcomitans were investigated by MIC determination, the counting of colony-forming units (CFU), agar diffusion tests, and growth curve observation. Compared with pure R-GNs and AgNPs, R-GNs/Ag composites exhibited enhanced antimicrobial properties owing to highly dispersed AgNPs on R-GNs.

First-Principles Study of the Transport Properties of Graphene-Hexagonal Boron Nitride Superlattice.

Hexagonal boron nitride (hBN) is a promising material to be integrated with graphene for high-performance graphene based electronics. We investigate the electronic, thermal, and thermoelectric transport properties of graphene-hexagonal boron nitride (G-hBN) superlattice by using the first-principles density functional calculations combined with the non-equilibrium Green's function formalism. The results show that a gap of 0.2 eV is opened in the band structure of the G-hBN superlattice due to the sublattice symmetry broken, the conductance and corresponding electron thermal conductance are both reduced. The phonon thermal conductance is also reduced due to the interlayer interactions, which linearize the flexural phonon modes and reduce the corresponding phonon density of states. Compared with those of graphene, though the electronic and phonon transport are both reduced, while the Seebeck coefficient is greatly enhanced. Finally, the thermoelectric figure of merit ZT of the G-hBN superlattice is enhanced 44% that of graphene. Our findings provide instructional information for future applications of graphene in electronics design.

Visual observation of African trypanosomes during cryopreservation.

Protozoa have been widely used for the study of cryopreservation. The survival rate after cryopreservation has always received the most attention, while the cell viability during the process of freezing and thawing has been much less studied. In the present study, we report successful cryopreservation of Trypanosoma brucei, a parasitic protozoa of human and animals, using controlled-rate freezing at 5°C/min, and real-time observation of activity using a microscope differential scanning calorimeter system during the freezing and thawing process. Trehalose used as a cryoprotective agent at a concentration of 0.4 M allowed the trypanosomes to endure freezing and thawing with >89% survival rate. Results from mechanisms analysis indicate that vitrification by trehalose contributes significantly to the protection of the trypanosomes from damage at low temperature.

On the thermoelectric transport properties of graphyne by the first-principles method.

Graphyne, another two-dimensional carbon allotrope, has received increased attentions in recent years. By using the first-principles density functional calculations combined with the non-equilibrium Green's function formalism, we investigated the electronic, thermal, and thermoelectric transport properties of graphyne systematically and comparatively. It is found that the phonon thermal conductance of graphyne is greatly reduced compared to that of graphene in most temperature regions while larger than that of graphene at low temperatures, which is attributed to the different bond strengths and phonon spectra of graphyne and graphene. Due to the semiconductor property of graphyne, the thermoelectric power (TEP) is found to be one magnitude larger than that of graphene. Besides, distinct peak value regions of TEP in the contour of chemical potential and temperature are displayed for graphyne and graphene. Finally, the thermoelectric figure of merit (ZT) of graphyne is found to be much larger than that of graphene as a result of large TEP and greatly reduced thermal conductance in graphyne, which indicates preferred thermoelectric applications for graphyne.

Cryopreservation of umbilical cord blood-derived mesenchymal stem cells without dimethyl sulfoxide.

Cryopreservation of umbilical cord blood-derived mesenchymal stem cells (UCB-derived MSCs) is crucial step for its clinical applications in cell transplantation therapy. In the cryopreservation of MSCs, dimethyl sulfoxide has been widely used as a cryoprotectant (CPA). However, it has been proved that DMSO has toxic side effects to human body. In this study, DMSO-free CPA solutions which contained ethylene glycol (EG), 1, 2-propylene glycol (PG) and sucrose as basic CPAs, supplemented with polyvinyl alcohol (PVA) as an additive, were developed for the cryopreservation of UCB-derived MSCs. The cryopreservation of UCB-derived MSCs was achieved by vitrification via plunging into liquid nitrogen and by programmed freezing via an optical-DSC system respectively. The viability of thawed UCB-derived MSCs was tested by trypan blue exclusion assay. Results showed that the viability of thawed UCB-derived MSCs was enhanced from 71.2% to 95.4% in the presence of PVA for vitrification, but only < 10% to 45% of viability was found for programmed freezing. These results indicate that PVA exerts a beneficial effect on the cryopreservation of UCB-derived MSCs and suggest the vitrification in combination with the dimethyl sulfoxide free CPA solutions supplemented with PVA would be an efficient protocol for the cryopreservation of UCB-derived MSCs.

Inhibition of nucleation and growth of ice by poly(vinyl alcohol) in vitrification solution.

Control of ice formation is crucial in cryopreservation of biological substances. Successful vitrification using several additives that inhibit ice nucleation in vitrification solutions has previously been reported. Among these additives, here we focused on a synthetic polymer, poly(vinyl alcohol) (PVA), and investigated the effects of PVA on nucleation and growth of ice in 35% (w/w) aqueous 1,2-propanediol solution by using a differential scanning calorimetry (DSC) system equipped with a cryomicroscope. First, the freezing temperature of the solution was measured using the DSC system, and then the change in ice fraction in the solution during cooling was evaluated based on images obtained using the cryomicroscope, at different concentrations of PVA between 0% and 3% (w/w). Based on the ice fraction, the change in residual solution concentration during cooling was also evaluated and then plotted on the state diagram of aqueous 1,2-propanediol solution. Results indicated that, when the partially glassy and partially frozen state was intentionally allowed, the addition of PVA effectively inhibited not only ice nucleation but also ice growth in the vitrification solution. The effect of PVA on ice growth in the vitrification solution was explained based on kinetic limitations mainly due to mass transport. The interfacial kinetics also might limit ice growth in the vitrification solution only when the ice growth rate decreased below a critical value. This coincides with the fact that PVA exhibits a unique antifreeze activity in the same manner as antifreeze proteins when ice growth rate is lower than a critical value.

Glass transition behavior of the vitrification solutions containing propanediol, dimethyl sulfoxide and polyvinyl alcohol.

Knowledge of the glass transition behavior of vitrification solutions is important for research and planning of the cryopreservation of biological materials by vitrification. This brief communication shows the analysis for the glass transition and glass stability of the multi-component vitrification solutions containing propanediol (PE), dimethyl sulfoxide (Me2SO) and polyvinyl alcohol (PVA) by using differential scanning calorimetry (DSC) during the cooling and subsequent warming between 25 and -150 degrees C. The glass formation of the solutions was enhanced by introduction of PVA. Partial glass formed during cooling and the fractions of free water in the partial glass matrix increased with the increasing of PVA concentration, which caused slight decline of glass transition temperature, T(g). Exothermic peaks of devitrification were delayed and broadened, which may result from the inhibition of ice nucleation or recrystallization of PVA.