Field-Assisted Alignment of Cellulose Nanofibrils in a Continuous Flow-Focusing System.

The continuous production of macroscale filaments of 17 µm in diameter comprising aligned TEMPO-oxidized cellulose nanofibrils (CNFs) is conducted using a field-assisted flow-focusing process. The effect of an AC external field on the material's structure becomes significant at a certain voltage, beyond which augmentations of the CNF orientation factor up to 16% are obtained. Results indicate that the electric field significantly contributes to improve the CNF ordering in the bulk, while the CNF alignment on the filament surface is only slightly affected by the applied voltage. X-ray diffraction shows that CNFs are densely packed anisotropically in the plane parallel to the filament axis without any preferential out of plane orientation. The improved nanoscale ordering combined with the tight CNF packing yields impressive enhancements in mechanical properties, with stiffness up to 25 GPa and more than 63% (up to 260 MPa), 46% (up to 2.8%), and 120% (up to 4.7 kJ/m3) increase in tensile strength, strain-to-failure, and toughness, respectively. This study demonstrates for the first time the control over the structural ordering of anisotropic nanoparticles in a dynamic system using an electric field, which can have important implications for the development of sustainable alternatives to synthetic textiles.

Uniaxially aligned nanofibrous cylinders by electrospinning.

Aligned nanofibers have drawn increasing interest for applications in biomedical engineering, electronics, and energy storage systems owing to the unique physicochemical properties provided by their anisotropy and high surface-to-volume ratio. Nevertheless, direct fabrication or assembly of aligned nanofibers into a 3-dimensional standalone construct with practically applicable dimensions presents an enormous challenge. We report a facile method to fabricate aligned nanofibrous cylinders, a widely used geometric form, by electrospinning aligned nanofibers across the gap between a pair of pin electrodes placed apart uniaxially. With this approach, cylindrical nanofibrous constructs of several millimeters in diameter and several centimeters in length can be readily produced. The versatility of the approach was demonstrated with several commonly used polymeric and ceramic materials, including polycaprolactone (PCL), chitosan/PCL, polyvinylidene fluoride, and titania. For a model application in tissue engineering, skeletal muscle cells were cultured on nanofibrous cylinders, which effectively produced highly aligned and densely populated myotubes along the nanofiber orientation, favorable for muscle tissue regeneration. With high structural integrity and stability, these can be directly integrated into devices or implanted in vivo as a standalone construct without the support of a substrate, thus increasing the portability, efficiency, and applicability of aligned nanofibers.

Single-step fabrication of patterned gold film array by an engineered multi-functional peptide.

This study constitutes a demonstration of the biological route to controlled nano-fabrication via modular multi-functional inorganic-binding peptides. Specifically, we use gold- and silica-binding peptide sequences, fused into a single molecule via a structural peptide spacer, to assemble pre-synthesized gold nanoparticles on silica surface, as well as to synthesize nanometallic particles in situ on the peptide-patterned regions. The resulting film-like gold nanoparticle arrays with controlled spatial organization are characterized by various microscopy and spectroscopy techniques. The described bio-enabled, single-step synthetic process offers many advantages over conventional approaches for surface modifications, self-assembly and device fabrication due to the peptides' modularity, inherent biocompatibility, material specificity and catalytic activity in aqueous environments. Our results showcase the potential of artificially-derived peptides to play a key role in simplifying the assembly and synthesis of multi-material nano-systems in environmentally benign processes.

Effects of electrode surface modification with chlorotoxin on patterning single glioma cells.

A microchip patterned with arrays of single cancer cells can be an effective platform for the study of tumor biology, medical diagnostics, and drug screening. However, patterning and retaining viable single cancer cells on defined sites of the microarray can be challenging. In this study we used a tumor cell-specific peptide, chlorotoxin (CTX), to mediate glioma cell adhesion on arrays of gold microelectrodes and investigated the effects of three surface modification schemes for conjugation of CTX to the microelectrodes on single cell patterning, which include physical adsorption, covalent bonding mediated by N-hydroxysuccinimide (NHS), and covalent bonding via crosslinking succinimidyl iodoacetate and Traut's (SIA-Traut) reagents. The CTX immobilization to microelectrodes was confirmed by high-resolution X-ray photoelectron spectroscopy. Physically adsorbed CTX showed better support for cell adhesion and is more effective in confining adhered cells on the electrodes than covalently-bound CTX. Furthermore, cell adhesion and spreading on microelectrodes were quantified in real-time by impedance measurements, which revealed an impedance signal from physically adsorbed CTX electrodes four times greater than the signal from covalently-bound CTX electrodes.

Controlling vertical morphology within the active layer of organic photovoltaics using poly(3-hexylthiophene) nanowires and phenyl-C61-butyric acid methyl ester.

In this study, we demonstrate how the vertical morphology of bulk heterojunction solar cells, with an active layer consisting of self-assembled poly(3-hexylthiophene) (P3HT) nanowires and phenyl-C(61)-butyric acid methyl ester (PCBM), can be beneficially influenced. Most device fabrication routes using similar materials employ an annealing step to influence active layer morphology, but this process can create an unfavorable phase migration where P3HT is driven toward the top of the active layer. In contrast, we demonstrate devices that exhibit an increase in relative fullerene concentration at the top of the active layer by introducing the donor phase as a solid nanowire in the active layer solution and altering the pre-spin drying time. X-ray photoelectron spectroscopy and conductive and photoconductive atomic force microscopy provide detailed images of how the surface of the active layer can be influenced; this is done by tracking the concentration and alignment of P3HT and PCBM domains. Using this new procedure, devices are made with power conversion efficiencies surpassing 2%. Additionally, we show that nanowires grown in the presence of the fullerene perform differently than those that are grown and mixed separately; exposure to the nanowire during self-assembly may allow the fullerene to coat nanowire surfaces and influence the photocurrent within the device.

Dielectric surface-controlled low-voltage organic transistors via n-alkyl phosphonic acid self-assembled monolayers on high-k metal oxide.

In this paper, we report on n-alkyl phosphonic acid (PA) self-assembled monolayer (SAM)/hafnium oxide (HfO(2)) hybrid dielectrics utilizing the advantages of SAMs for control over the dielectric/semiconductor interface with those of high-k metal oxides for low-voltage organic thin film transistors (OTFTs). By systematically varying the number of carbon atoms of the n-alkyl PA SAM from six to eighteen on HfO(2) with stable and low leakage current density, we observe how the structural nature of the SAM affects the thin-film crystal structure and morphology, and subsequent device performance of low-voltage pentacene based OTFTs. We find that two primary structural factors of the SAM play a critical role in optimizing the device electrical characteristics, namely, the order/disorder of the SAM and its physical thickness. High saturation-field-effect mobilities result at a balance between disordered SAMs to promote large pentacene grains and thick SAMs to aid in physically buffering the charge carriers in pentacene from the adverse effects of the underlying high-k oxide. Employing the appropriate n-alkyl PA SAM/HfO(2) hybrid dielectrics, pentacene-based OTFTs operate under -2.0 V with low hysteresis, on-off current ratios above 1 x 10(6), threshold voltages below -0.6 V, subthreshold slopes as low as 100 mV dec(-1), and field-effect mobilities as high as 1.8 cm(2) V(-1) s(-1).