Gelation and large thermoresponse of cranberry-based xyloglucan.

Cranberry waste contains potentially valuable components, such as proanthocyanidins, flavanols, and xyloglucan. Highly-purified xyloglucan (XG) from cranberries were studied through steady and oscillatory shear rheology at various concentrations and temperatures. At room temperature, an apparent yield stress is observed and the storage modulus exceeds the loss modulus ( [Formula: see text] ) for concentrations of 0.5 wt% and higher, indicating that the XG solution has formed a physical hydrogel. Thermoresponsive gelation is observed with a five-order of magnitude increase in shear moduli as it undergoes a weak to strong gel transition around 52 °C. The gelation time was 5 min with an observed storage moduli up to 3500 Pa. Cranberry-based XG exhibits thermoresponsive behavior at concentrations as low as 0.1 wt% (w/v), which is significantly lower than prior gelation studies of XG from other sources. The formation of a weak gel at room temperature and large storage moduli observed at room temperature is likely associated with the low level of impurities and small amount of galactose present in the XG chains.

Enhancing the Hydrophobicity and Antibacterial Properties of SiCN-Coated Surfaces with Quaternization to Address Peri-Implantitis.

Peri-implantitis is a major cause of dental implant failure. This disease is an inflammation of the tissues surrounding the implant, and, while the cause is multi-factorial, bacteria is the main culprit in initiating an inflammatory reaction. Dental implants with silicon carbonitride (SiCN) coatings have several potential advantages over traditional titanium implants, but their antibacterial efficiency has not yet been evaluated. The purpose of this study was to determine the anti-bacterial potential of SiCN by modifying the surface of SiCN-coated implants to have a positive charge on the nitrogen atoms through the quaternization of the surface atoms. The changes in surface chemistry were confirmed using contact angle measurement and XPS analysis. The modified SiCN surfaces were inoculated with Streptococcus mutans (S. mutans) and compared with a silicon control. The cultured bacterial colonies for the experimental group were 80% less than the control silicon surface. Fluorescent microscopy with live bacteria staining demonstrated significantly reduced bacterial coverage after 3 and 7 days of incubation. Scanning electron microscopy (SEM) was used to visualize the coated surfaces after bacterial inoculation, and the mechanism for the antibacterial properties of the quaternized SiCN was confirmed by observing ruptured bacteria membrane along the surface.

Properties of SiCN Films Relevant to Dental Implant Applications.

The application of surface coatings is a popular technique to improve the performance of materials used for medical and dental implants. Ternary silicon carbon nitride (SiCN), obtained by introducing nitrogen into SiC, has attracted significant interest due to its potential advantages. This study investigated the properties of SiCN films deposited via PECVD for dental implant coatings. Chemical composition, optical, and tribological properties were analyzed by adjusting the gas flow rates of NH3, CH4, and SiH4. The results indicated that an increase in the NH3 flow rate led to higher deposition rates, scaling from 5.7 nm/min at an NH3 flow rate of 2 sccm to 7 nm/min at an NH3 flow rate of 8 sccm. Concurrently, the formation of N-Si bonds was observed. The films with a higher nitrogen content exhibited lower refractive indices, diminishing from 2.5 to 2.3 as the NH3 flow rate increased from 2 sccm to 8 sccm. The contact angle of SiCN films had minimal differences, while the corrosion rate was dependent on the pH of the environment. These findings contribute to a better understanding of the properties and potential applications of SiCN films for use in dental implants.

Effects of Electrode Support Structure on Electrode Microstructure, Transport Properties, and Gas Diffusion within the Gas Diffusion Layer.

The effects of gas diffusion layer (GDL) and electrode microstructure, which influence the catalyst layer and catalyst-membrane interface on the performance of a membrane electrode assembly (MEA) for gas-phase electrolysis and the separation of CO2 were experimentally characterized. Several types of GDL materials, with and without a microporous layer (MPL), were characterized using scanning electron microscopy (SEM) and Brunauer-Emmett-Teller (BET) surface area analysis. The diffusion of reactants through the GDL materials was measured to determine the effects on the microstructure and chemical properties on mass transport. The effects on the GDL structure and chemistry were determined through evaluation of Pt-IrO2 MEAs with different GDL materials using constant-current measurements. Increasing the thickness of the MPL and hydrophobicity within the GDL assist with retaining water within the membrane and catalyst layers, which results in greater performance at high current densities.

Relationship between Ethane and Ethylene Diffusion inside ZIF-11 Crystals Confined in Polymers to Form Mixed-Matrix Membranes.

Self-diffusivities of ethane were measured by multinuclear pulsed field gradient (PFG) NMR inside zeolitic imidazolate framework-11 (ZIF-11) crystals dispersed in several selected polymers to form mixed-matrix membranes (MMMs). These diffusivities were compared with the corresponding intracrystalline self-diffusivities in ZIF-11 crystal beds. It was observed that the confinement of ZIF-11 crystals in ZIF-11 / Torlon MMM can lead to a decrease in the ethane intracrystalline self-diffusivity. Such diffusivity decrease was observed at different temperatures used in this work. PFG NMR measurements of the temperature dependence of the intracrystalline self-diffusivity of ethylene in the same ZIF-11 / Torlon MMM revealed similar diffusivity decrease as well as an increase in the diffusion activation energy in comparison to those in unconfined ZIF-11 crystals in a crystal bed. These observations for ethane and ethylene were attributed to the reduction of the flexibility of the ZIF-11 framework due to the confinement in Torlon leading to a smaller effective aperture size of ZIF-11 crystals. Surprisingly, the intra-ZIF diffusion selectivity for ethane and ethylene was not changed appreciably by the confinement of ZIF-11 crystals in Torlon in comparison to the selectivity in a bed of ZIF-11 crystals. No ZIF-11 confinement effects leading to a reduction in the intracrystalline self-diffusivity of ethane and ethylene were observed for the other two studied MMM systems: ZIF-11 / Matrimid and ZIF-11 / 6FDA-DAM. The absence of the confinement effect in the latter MMMs can be related to the lower values of the polymer bulk modulus in these MMMs in comparison to that in ZIF-11 / Torlon MMM. In addition, there may be a contribution from possible differences in the ZIF-11/polymer adhesion in different MMM types.

Radio frequency heating of metallic and semiconducting single-walled carbon nanotubes.

Here we report the effect of metallic (m-) and semiconducting (s-) properties of single-walled carbon nanotubes (SWCNTs) on the response of SWCNT films to radio frequency (RF) heating. We separated high-purity m- and s-SWCNTs from an initial SWCNTs mixture and prepared thin films using vacuum filtration method. The areal density of the films is 9.6 µg cm-2, and the DC conductivities are in the range of 7800-49 000 S m-1. We show rapid and non-contact Joule heating of films using a fringing-field RF applicator, and we observe maximum heating rates in the frequency range of 60-70 MHz. We determine that the more conductive m-SWCNT films reflect RF fields and heat at a maximum rate of 1.51 °C s-1 compared to maximum heating rate of 25.6 °C s-1 for s-SWCNT films. However, m-SWCNTs heat up faster than s-SWCNTs when dispersed in a dielectric medium. Our results confirm the non-monotonic relationship between RF heating rate and conductivity for CNT-based materials such that conductivity is required for heating but high values are correlated with reflections. Our findings also suggest that RF heating could be a possible metric for evaluating film purity because impurities in the films affect the conductivity and thus RF heating rate. We anticipate that RF heating may occur in SWCNT-based electronics and affect their performance.

Boiling and quenching heat transfer advancement by nanoscale surface modification.

All power production, refrigeration, and advanced electronic systems depend on efficient heat transfer mechanisms for achieving high power density and best system efficiency. Breakthrough advancement in boiling and quenching phase-change heat transfer processes by nanoscale surface texturing can lead to higher energy transfer efficiencies, substantial energy savings, and global reduction in greenhouse gas emissions. This paper reports breakthrough advancements on both fronts of boiling and quenching. The critical heat flux (CHF) in boiling and the Leidenfrost point temperature (LPT) in quenching are the bottlenecks to the heat transfer advancements. As compared to a conventional aluminum surface, the current research reports a substantial enhancement of the CHF by 112% and an increase of the LPT by 40 K using an aluminum surface with anodized aluminum oxide (AAO) nanoporous texture finish. These heat transfer enhancements imply that the power density would increase by more than 100% and the quenching efficiency would be raised by 33%. A theory that links the nucleation potential of the surface to heat transfer rates has been developed and it successfully explains the current finding by revealing that the heat transfer modification and enhancement are mainly attributed to the superhydrophilic surface property and excessive nanoscale nucleation sites created by the nanoporous surface.

Strongly Bound Sodium Dodecyl Sulfate Surrounding Single-Wall Carbon Nanotubes.

NMR techniques have been widely used to infer molecular structure, including surfactant aggregation. A combination of optical spectroscopy, proton NMR spectroscopy, and pulsed field gradient NMR (PFG NMR) is used to study the adsorption number for sodium dodecyl sulfate (SDS) with single-wall carbon nanotubes (SWCNTs). Distinct transitions in the NMR chemical shift of SDS are observed in the presence of SWCNTs. These transitions demonstrate that micelle formation is delayed by SWCNTs due to the adsorption of SDS on the nanotube surface. Once the nanotube surface is saturated, the free SDS concentration increases until micelle formation is observed. Therefore, the adsorption number of SDS on SWCNTs can be determined by the changes to the apparent critical micelle concentration (CMC). PFG NMR found that SDS remains strongly bound onto the nanotube. Quantitative analysis of the diffusivity of SDS allowed calculation of the adsorption number of strongly bound SDS on SWCNTs. The adsorption numbers from these techniques give the same values within experimental error, indicating that a significant fraction of the SDS interacting with nanotubes remains strongly bound for as long as 0.5 s, which is the maximum diffusion time used in the PFG NMR measurements.

Controlling the Geometries of Si Nanowires through Tunable Nanosphere Lithography.

A tunable nanosphere lithography (NSL) technique is combined with metal-assisted etching of silicon (Si) to fabricate ordered, high-aspect-ratio Si nanowires. Non-close-packed structures are directly prepared via shear-induced ordering of the nanospheres. The spacing between the nanospheres is independent of their diameters and tuned by changing the loading of nanospheres. Nanowires with spacings between 110 and 850 nm are easily achieved with diameters between 100 and 550 nm. By eliminating plasma or heat treatment of the nanospheres, the diameter of the nanowires fabricated is nearly identical to the nanosphere diameter in the suspension. The elimination of this step helps avoid common drawbacks of traditional NSL approaches, leading to the high-fidelity, large-scale fabrication of highly crystalline, nonporous Si nanowires in ordered hexagonal patterns. The ability to simultaneously control the diameter and spacing makes the NSL technique more versatile and expands the range of geometries that can be fabricated by top-down approaches.

Fabricating vertically aligned sub-20 nm Si nanowire arrays by chemical etching and thermal oxidation.

Silicon nanowires (SiNWs) are appealing building blocks in various applications, including photovoltaics, photonics, and sensors. Fabricating SiNW arrays with diameters <100 nm remains challenging through conventional top-down approaches. In this work, chemical etching and thermal oxidation are combined to fabricate vertically aligned, sub-20 nm SiNW arrays. Defect-free SiNWs with diameters between 95 and 200 nm are first fabricated by nanosphere (NS) lithography and chemical etching. The key aspects for defect-free SiNW fabrication are identified as: (1) achieving a high etching selectivity during NS size reduction; (2) retaining the circular NS shape with smooth sidewalls; and (3) using a directional metal deposition technique. SiNWs with identical spacing but variable diameters are demonstrated by changing the reactive ion etching power. The diameter of the SiNWs is reduced by thermal oxidation, where self-limiting oxidation is encountered after oxidizing the SiNWs at 950 °C for 1 h. A second oxidation is performed to achieve vertically aligned, sub-20 nm SiNW arrays. Si/SiO2 core/shell NWs are obtained before removing the oxidized shell. HRTEM imaging shows that the SiNWs have excellent crystallinity.

Selective desorption of high-purity (6,5) SWCNTs from hydrogels through surfactant modulation.

Selective desorption of (6,5) single-wall carbon nanotubes from hydrogels only occurs at specific co-surfactant ratios. High-purity fractions are obtained at this ratio even with long elution times and different total co-surfactant concentrations. These results suggest that each (n,m) type forms a thermodynamically-stable surfactant structure in the co-surfactant solution, enabling high-fidelity separations in a single column.

Relationship between single-file diffusion of mixed and pure gases in dipeptide nanochannels by high field diffusion NMR.

High field NMR diffusometry reveals single-file diffusion of CO/CH4 mixture in dipeptide nanochannels with a coincident mobility for CO and CH4. In contrast to the relationship commonly observed for normal diffusion, this mixture mobility is only slightly smaller than that of pure CO which diffuses much faster than pure CH4.

Improving Performance via Blocking Layers in Dye-Sensitized Solar Cells Based on Nanowire Photoanodes.

Electron recombination in dye-sensitized solar cells (DSSCs) results in significant electron loss and performance degradation. However, the reduction of electron recombination via blocking layers in nanowire-based DSSCs has rarely been investigated. In this study, HfO2 or TiO2 blocking layers are deposited on nanowire surfaces via atomic layer deposition (ALD) to reduce electron recombination in nanowire-based DSSCs. The control cell consisting of ITO nanowires coated with a porous shell of TiO2 by TiCl4 treatment yields an efficiency of 2.82%. The efficiency increases dramatically to 5.38% upon the insertion of a 1.3 nm TiO2 compact layer between the nanowire surface and porous TiO2 shell. This efficiency enhancement implies that porous sol-gel coatings on nanowires (e.g., via TiCl4 treatment) result in significant electron recombination in nanowire-based DSSCs, while compact coatings formed by ALD are more advantageous because of their ability to act as a blocking layer. By comparing nanowire-based DSSCs with their nanoparticle-based counterparts, we find that the nanowire-based DSSCs suffer more severe electron recombination from ITO due to the much higher surface area exposed to the electrolyte. While the insertion of a high band gap compact layer of HfO2 between the interface of the conductive nanowire and TiO2 shell improves performance, a comparison of the cell performance between TiO2 and HfO2 compact layers indicates that charge collection is suppressed by the difference in energy states. Consequently, the use of high band gap materials at the interface of conductive nanowires and TiO2 is not recommended.

Unique toxicological behavior from single-wall carbon nanotubes separated via selective adsorption on hydrogels.

Over the past decade, extensive research has been completed on the potential threats of single-wall carbon nanotubes (SWCNTs) to living organisms upon release to aquatic systems. However, these studies have focused primarily on the link between adverse biological effects in exposed test organisms on the length, diameter, and metallic impurity content of SWCNTs. In contrast, few studies have focused on the bioeffects of the different SWCNTs in the as-produced mixture, which contain both metallic (m-SWCNT) and semiconducting (s-SWCNT) species. Using selective adsorption onto hydrogels, high purity m-SWCNT and s-SWCNT fractions were produced and their biological impacts determined in dose-response studies with Pseudokirchneriella subcapitata as test organism. The results show significant differences in the biological responses of P. subcapitata exposed to high purity m- and s-SWCNT fractions. Contrary to the biological response observed using SWCNTs separated by density gradient ultracentrifugation, it is found that the high-pressure CO conversion (HiPco) s-SWCNT fraction separated by selective adsorption causes increased biological impact. These findings suggest that s-SWCNTs are the primary factor driving the adverse biological responses observed from P. subcapitata cells exposed to our as-produced suspensions. Finally, the toxicity of the s-SWCNT fraction is mitigated by increasing the concentration of biocompatible surfactant in the suspensions, likely altering the nature of surfactant coverage along SWCNT sidewalls, thereby reducing potential physical interaction with algal cells. These findings highlight the need to couple sample processing and toxicity response studies.

Comparing electron recombination via interfacial modifications in dye-sensitized solar cells.

Establishing a blocking layer between the interfaces of the photoanode is an effective approach to improve the performance of dye-sensitized solar cells (DSSCs). In this work, HfO2 blocking layers are deposited via atomic layer deposition (ALD) onto tin-doped indium oxide (ITO) and TiO2. In both cases, addition of the blocking layer increases cell efficiencies to greater than 7%. The improved performance for a HfO2 layer inserted between the ITO/TiO2 interface is associated with an energy barrier that reduces electron recombination. HfO2 blocking layers between the TiO2/dye interface show more complex behavior and are more sensitive to the number of ALD cycles. For thin blocking layers on TiO2, the improved device performance is attributed to the passivation of surface states in TiO2. A distinct transition in dark current and electron lifetime are observed after 4 ALD cycles. These changes to performance indicate thick HfO2 layers on TiO2 formed an energy barrier that significantly hinders cell performance.

Interactive forces between sodium dodecyl sulfate-suspended single-walled carbon nanotubes and agarose gels.

Selective adsorption onto agarose gels has become a powerful method to separate single-walled carbon nanotubes (SWCNTs). A better understanding of the nature of the interactive forces and specific sites responsible for adsorption should lead to significant improvements in the selectivity and yield of these separations. A combination of nonequilibrium and equilibrium studies are conducted to explore the potential role that van der Waals, ionic, hydrophobic, π-π, and ion-dipole interactions have on the selective adsorption between agarose and SWCNTs suspended with sodium dodecyl sulfate (SDS). The results demonstrate that any modification to the agarose gel surface and, consequently, the permanent dipole moments of agarose drastically reduces the retention of SWCNTs. Because these permanent dipoles are critical to retention and the fact that SDS-SWCNTs function as macro-ions, it is proposed that ion-dipole forces are the primary interaction responsible for adsorption. The selectivity of adsorption may be attributed to variations in polarizability between nanotube types, which create differences in both the structure and mobility of surfactant. These differences affect the enthalpy and entropy of adsorption, and both play an integral part in the selectivity of adsorption. The overall adsorption process shows a complex behavior that is not well represented by the Langmuir model; therefore, calorimetric data should be used to extract thermodynamic information.

Mitigation of the impact of single-walled carbon nanotubes on a freshwater green algae: Pseudokirchneriella subcapitata.

This study investigates the biological response of Pseudokirchneriella subcapitata to single-walled carbon nanotubes (SWNTs) suspended in gum Arabic (GA), using typical 96-hour algal bioassays and long-term growth studies. Changes in algal biomass and cell morphology associated with specific SWNT-treatments were monitored and the mechanisms of observed biological responses investigated through a combination of biochemical and spectroscopic methods. Results from short-term bioassays showed a growth inhibition in culture media containing >0.5 mg SWNT/L and a final GA concentration of 0.023% (v/v). Interestingly, the observed toxicity disappears when GA concentrations are brought to levels ≥ 0.046%. Long-term experiments based on toxic combination of SWNTs and GA showed that P. subcapitata would easily recover from an initial growth inhibition effect. Overall, these findings point to the possibility of GA to mitigate the toxicity of SWNTs, making it an ideal surfactant if SWNT suspension in GA does not alter the performance sought from these nanotubes.

Aqueous suspension methods of carbon-based nanomaterials and biological effects on model aquatic organisms.

The preparation of aqueous suspensions of carbon-based nanomaterials (NMs) requires the use of dispersing agents to overcome their hydrophobic character. Although studies on the toxicity of NMs have focused primarily on linking the characteristics of particles to biological responses, the role of dispersing agents has been overlooked. This study assessed the biological effects of a number of commonly used dispersing agents on Pseudokirchneriella subcapitata and Ceriodaphnia dubia as model test organisms. The results show that for a given organism, NM toxicity can be mitigated by use of nontoxic surfactants, and that a multispecies approach is necessary to account for the sensitivity of different organisms. In addition to the intrinsic physicochemical properties of NMs, exposure studies should take into account the effects of used dispersing fluids.

An interfacial and bulk charge transport model for dye-sensitized solar cells based on photoanodes consisting of core-shell nanowire arrays.

Dye-sensitized solar cells (DSSCs) based on ordered photoanode morphologies, such as nanotubes and nanowires, are widely gaining attention because these geometries are believed to enhance interfacial charge transfer and bulk charge transport. Unfortunately, experimental results have yet to show substantial improvement to conversion efficiency over nanoparticle-based DSSCs. A model is developed to characterize the performance of an idealized photoanode based on an ordered array of transparent conductive nanowires coated with an anatase titania shell. The role of the interfacial electric field in nanowire-based DSSCs is explored computationally by turning electron migration ON or OFF. The results show that back-reaction rates are most strongly influenced by the electric field. These electron loss mechanisms can be reduced by several orders of magnitude, leading to improvements in short-circuit current, open-circuit voltage, and fill factor.

Swelling the hydrophobic core of surfactant-suspended single-walled carbon nanotubes: a SANS study.

Localized solvent environments form around single-wall carbon nanotubes (SWCNTs) because of the ability of surfactant molecules to solubilize immiscible organic solvents. Although these microenvironments around SWCNTs have already been used for fundamental and applied studies, small-angle neutron scattering (SANS) was used here to assess the size and shape of the solvent domains, their uniformity and distribution on the sidewalls, and the effect of solvent swelling on the aggregation state of the suspension. SANS measurements confirm both the formation of local solvent environments and that no irreversible aggregation of the nanotube suspension occurs after the SDS molecules are swollen in solvent. The results also corroborate prior conclusions based on photoluminescence that the structure formed is dependent of the nature of the solvent-surfactant combination; SWCNTs suspended with SDS and swelled with benzene have a more uniform coating on the sidewall than those swelled with o-dichlorobenzene. These differences can be important to understanding the effect of the local environment on the photoluminescence properties and the interaction of SWCNTs with interfaces.