Co-assembling Polysaccharide Nanocrystals and Nanofibers for Robust Chiral Iridescent Films.

Assembling robust chiral biopolymer structures without compromising vivid optical iridescence is a grand challenge for biocomposite materials. Herein, we report a hierarchical nanocellulose nanostructure with a helicoidal organization co-assembled from chiral rigid cellulose nanocrystals (CNCs) and longer nanofibers isolated from the hydrolyzed wood pulp. This resulting highly iridescent chiral nanocellulose material is much tougher than traditional chiral CNC films. We found that the mixed nanocellulose are composed of needle-like nanocrystals and very long (up to 800 nm) flexible cellulose nanofibers (CNFs). Large-scale molecular simulation indicates that enhanced dynamic hydrogen bonding with labile networking facilitates mechanical reinforcement, owing to increased nanocrystal length, the co-assembly of nanofibrils in mixed bundles, and interchain entanglements. This study provides a novel strategy to transform the wood pulp residues into high-value-added photonic-bound polysaccharide materials. These hierarchical biomaterials can overcome the conflicting trends in designing balanced mechanical and optical performance of chiral biofilms and their conversion to robust chiral photonic materials with enhanced performance.

Flexible conductive Ag nanowire/cellulose nanofibril hybrid nanopaper for strain and temperature sensing applications.

With the rapid development of smart wearable devices, flexible and biodegradable sensors are in urgent needs. In this study, "green" electrically conductive Ag nanowire (AgNW)/cellulose nanofiber (CNF) hybrid nanopaper was fabricated to prepare flexible sensors using the facial solution blending and vacuum filtration technique. The amphiphilic property of cellulose is beneficial for the homogeneous dispersion of AgNW to construct effective electrically conductive networks. Two different types of strain sensors were designed to study their applications in strain sensing. One was the tensile strain sensor where the hybrid nanopaper was sandwiched between two thermoplastic polyurethane (TPU) films through hot compression, and special micro-crack structure was constructed through the pre-strain process to enhance the sensitivity. Interestingly, typical pre-strain dependent strain sensing behavior was observed due to different crack densities constructed under different pre-strains. As a result, it exhibited an ultralow detection limit as low as 0.2%, good reproducibility under different strains and excellent stability and durability during 500 cycles (1% strain, 0.5 mm/min). The other was the bending strain sensor where the hybrid nanopaper was adhered onto TPU film, showing stable and recoverable linearly sensing behavior towards two different bending modes (tension and compression). Importantly, the bending sensor displayed great potential for human motion and physiological signal detection. Furthermore, the hybrid nanopaper also exhibited stable and reproducible negative temperature sensing behavior when it was served as a temperature sensor. This study provides a guideline for fabricating flexible and biodegradable sensors.

Superhydrophobic Electrically Conductive Paper for Ultrasensitive Strain Sensor with Excellent Anticorrosion and Self-Cleaning Property.

Recently, a paper-based (PB) strain sensor has turned out to be an ideal substitute for the polymer-based one because of the merits of renewability, biodegradability, and low cost. However, the hygroexpansion and degradation of the paper after absorbing water are the great challenges for the practical applications of the PB strain sensor. Herein, the superhydrophobic electrically conductive paper was fabricated by simply dip-coating the printing paper into the carbon black (CB)/carbon nanotube (CNT)/methyl cellulose suspension and hydrophobic fumed silica (Hf-SiO2) suspension successively to settle the problem. Because of the existence of ultrasensitive microcrack structures in the electrically conductive CB/CNT layer, the sensor was capable of detecting an ultralow strain as low as 0.1%. During the tension strain range of 0-0.7%, the sensor exhibited a gauge factor of 7.5, almost 3 times higher than that of the conventional metallic-based sensors. In addition, the sensor displayed frequency-independent and excellent durability and reproductivity over 1000 tension cycles. Meanwhile, the superhydrophobic Hf-SiO2 layer with a micro-nano structure and low surface energy endowed the sensor with outstanding waterproof and self-cleaning properties, as well as great sustainability toward cyclic strain and harsh corrosive environment. Finally, the PB strain sensor could effectively monitor human bodily motions such as finger/elbow joint/throat movement and pulse in real time, especially for the wet or rainy conditions. All these pave way for the fabrication of a high-performance PB strain sensor.

Ultrasensitive and Highly Compressible Piezoresistive Sensor Based on Polyurethane Sponge Coated with a Cracked Cellulose Nanofibril/Silver Nanowire Layer.

With the rapid development of flexible wearable electronics, a piezoresistive sensor with low detection limit and wide strain sensing range turns out to be a great challenge for its application in this field. Here, a cracked cellulose nanofibril/silver nanowire (CA) layer-coated polyurethane (PU) sponge was acquired through a simple dip-coating process followed by precompression treatment. The electrical conductivity and mechanical property of the conductive CA@PU sponge could be effectively tuned through changing the dip-coating number. As a piezoresistive sensor, the sponge exhibited the capability of detecting both small and large motions over a wide compression strain range of 0-80%. Based on the "crack effect", the sensor possessed a detection limit as low as 0.2% and the gauge factor [GF, GF = (Δ R/ R0)/ε, where Δ R, R0, and ε represent the instantaneous resistance change, original resistance, and strain applied, respectively] was as high as 26.07 in the strain range of 0-0.6%. Moreover, the "contact effect" enabled the sensor to be applicable for larger strain, and the GF decreased first and then became stable with increasing compression strain. In addition, frequency- and strain-dependent sensing performances were observed, demonstrating that the sensor can respond reliably to different applied frequencies and strains. Furthermore, the sensor displayed exceptional stability, repeatability, and durability over 500 cycles. Finally, the sensor could be applicable for the detection of various human bodily motions, such as phonation, stamping, knee bending, and wrist bending. Most importantly, the sponge also exhibited great potential for the fabrication of artificial electronic skin. Herein, the conductive CA@PU sponge will undoubtedly promote the development of high-performance flexible wearable electronics.

Dual-Excitation Nanocellulose Plasmonic Membranes for Molecular and Cellular SERS Detection.

We demonstrate that cellulose nanofiber (CNF) biomaterials with high transparency and mechanical robustness can be combined with gold nanorods to form a multifunctional porous membrane for dual-mode surface-enhanced Raman scattering (SERS) detection of both small molecules and cells. The nanoporous nature of the nanofiber membranes allows for effective molecular filtration and preconcentration of the analytes, further boosting the SERS performance. Specifically, because of the low fluorescence and Raman background of the CNF matrix, extremely low loading density of gold nanorods can be used. The nanorod assemblies within the CNF network can be resonantly driven by a 532 nm laser (transverse plasmonic mode) and near resonantly driven at by a 785 nm laser (longitudinal mode), facilitating dual operational modes at two excitation wavelengths. The shorter wavelength excitation mode yields better Raman scattering efficiency and has been demonstrated to be capable of detecting rhodamine 6G (R6G) dyes down to picomolar concentrations. On the other hand, the longer wavelength excitation mode provides autofluorescence suppression for the better detection of microorganisms such as Escherichia coli, shortening the required integration time from hours to minutes. Upon drastically lowering the spectral background noise and utilizing nanofiltration, the plasmonic CNF membranes reported here show significantly improved SERS sensitivity and detection fidelity as compared to traditional metal, metal oxide, synthetic polymer, and paper SERS substrates.

Wrapping Nanocellulose Nets around Graphene Oxide Sheets.

Constructing advanced functional nanomaterials with pre-designed organized morphologies from low-dimension synthetic and biological components is challenging. Herein, we report an efficient and universal amphiphilicity-driven assembly strategy to construct "hairy" flexible hybrid nanosheets with a 1D cellulose nanofibers (CNFs) net conformally wrapped around 2D graphene oxide (GO) monolayers. This interface-driven bio-synthetic assembly is facilitated by tailoring the surface chemistry of flexible GO sheets, resulting in individual sheets tightly surrounded by dense conformal nanocellulose network. The mechanical stability of the products far exceeds the compressive instability limits of both individual components. Additionally, the CNF network significantly enhances the wetting ability of initial hydrophobic reduced GO nanosheets, allowing fast water transport combined with high filtration efficiency.

Template-Guided Assembly of Silk Fibroin on Cellulose Nanofibers for Robust Nanostructures with Ultrafast Water Transport.

The construction of multilength scaled hierarchical nanostructures from diverse natural components is critical in the progress toward all-natural nanocomposites with structural robustness and versatile added functionalities. Here, we report a spontaneous formation of peculiar "shish kebab" nanostructures with the periodic arrangement of silk fibroin domains along straight segments of cellulose nanofibers. We suggest that the formation of these shish kebab nanostructures is facilitated by the preferential organization of heterogeneous (ß-sheets and amorphous silk) domains along the cellulose nanofiber driven by modulated axial distribution of crystalline planes, hydrogen bonding, and hydrophobic interactions as suggested by all-atom molecular dynamic simulations. Such shish kebab nanostructures enable the ultrathin membrane to possess open, transparent, mechanically robust interlocked networks with high mechanical performance with up to 30 GPa in stiffness and 260 MPa in strength. These nanoporous robust membranes allow for the extremely high water flux, up to 3.5 × 104 L h-1 m-2 bar-1 combined with high rejection rate for various organic molecules, capability of capturing heavy metal ions and their further reduction into metal nanoparticles for added SERS detection capability and catalytic functionalities.

Synthesis, Assembly, and Applications of Hybrid Nanostructures for Biosensing.

The robust, sensitive, and selective detection of targeted biomolecules in their native environment by prospective nanostructures holds much promise for real-time, accurate, and high throughput biosensing. However, in order to be competitive, current biosensor nanotechnologies need significant improvements, especially in specificity, integration, throughput rate, and long-term stability in complex bioenvironments. Advancing biosensing nanotechnologies in chemically "noisy" bioenvironments require careful engineering of nanoscale components that are highly sensitive, biorecognition ligands that are capable of exquisite selective binding, and seamless integration at a level current devices have yet to achieve. This review summarizes recent advances in the synthesis, assembly, and applications of nanoengineered reporting and transducing components critical for efficient biosensing. First, major classes of nanostructured components, both inorganic reporters and organic transducers, are discussed in the context of the synthetic control of their individual compositions, shapes, and properties. Second, the design of surface functionalities and transducing path, the characterization of interfacial architectures, and the integration of multiple nanoscale components into multifunctional ordered nanostructures are extensively examined. Third, examples of current biosensing structures created from hybrid nanomaterials are reviewed, with a distinct emphasis on the need to tailor nanosensor designs to specific operating environments. Finally, we offer a perspective on the future developments of nanohybrid materials and future nanosensors, outline possible directions to be pursued that may yield breakthrough results, and envision the exciting potential of high-performance nanomaterials that will cause disruptive improvements in the field of biosensing.

Interfacial Shear Strength and Adhesive Behavior of Silk Ionomer Surfaces.

The interfacial shear strength between different layers in multilayered structures of layer-by-layer (LbL) microcapsules is a crucial mechanical property to ensure their robustness. In this work, we investigated the interfacial shear strength of modified silk fibroin ionomers utilized in LbL shells, an ionic-cationic pair with complementary ionic pairing, (SF)-poly-l-glutamic acid (Glu) and SF-poly-l-lysine (Lys), and a complementary pair with partially screened Coulombic interactions due to the presence of poly(ethylene glycol) (PEG) segments and SF-Glu/SF-Lys[PEG] pair. Shearing and adhesive behavior between these silk ionomer surfaces in the swollen state were probed at different spatial scales and pressure ranges by using functionalized atomic force microscopy (AFM) tips as well as functionalized colloidal probes. The results show that both approaches were consistent in analyzing the interfacial shear strength of LbL silk ionomers at different spatial scales from a nanoscale to a fraction of a micron. Surprisingly, the interfacial shear strength between SF-Glu and SF-Lys[PEG] pair with partially screened ionic pairing was greater than the interfacial shear strength of the SF-Glu and SF-Lys pair with a high density of complementary ionic groups. The difference in interfacial shear strength and adhesive strength is suggested to be predominantly facilitated by the interlayer hydrogen bonding of complementary amino acids and overlap of highly swollen PEG segments.

Ligand-Exchange Dynamics on Gold Nanocrystals: Direct Monitoring of Nanoscale Polyvinylpyrrolidone-Thiol Domain Surface Morphology.

We report direct high-resolution monitoring of an evolving mixed nanodomain surface morphology during thiol adsorption on polyvinylpyrrolidone (PVP)-stabilized single crystal gold nanocrystals. The thiol adsorption and replacement dynamics are much more complex than a simple complete substitution of the initial polymer ligand. We observed that during ligand exchange with linear thiol, the nanocrystal surface evolved from an initial 1 nm uniform PVP coating into a remarkably stable network of globular PVP domains 20-100 nm in size and ∼4 nm in height surrounded by thiol self-assembled monolayers. The final stability of such a mixed thiol-PVP surface morphology can possibly be attributed to the interfacial energy reduction from partially solvophilic surfaces and the entropic gain from mixed ligand surface layers. The ligand-exchange dynamics and the unusual equilibrium morphology revealed here provide important insights into both displacement dynamics of surface-bound molecules and the nanoscale peculiarities of surface functionalization of colloidal metal substrates.

Ultrastrong Freestanding Graphene Oxide Nanomembranes with Surface-Enhanced Raman Scattering Functionality by Solvent-Assisted Single-Component Layer-by-Layer Assembly.

We report single-component ultrathin reduced graphene oxide (rGO) nanomembranes fabricated via nonconventional layer-by-layer assembly (LbL) of graphene oxide flakes, using organic solvent instead of water to provide strong complementary interactions and to ensure the uniform layered growth. This unique approach does not require regular polymeric from the assembly process or intermediate surface chemical modification. The resulting ultrastrong freestanding graphene oxide (rGO) LbL nanomembranes with a very low thickness of 3 nm (three GO monolayers) can be transferred over a large surface area across tens of square centimeters by using a facile surface-tension-assisted release technique. These uniform and ultrasmooth nanomembranes with high transparency (up to 93% at 550 nm) and high electrical conductivity (up to 3000 S/m) also exhibit outstanding mechanical strength of 0.5 GPa and a Young's modulus of 120 GPa, which are several times higher than that of other reported regular rGO films. Furthermore, up to 94 wt % of silver nanoplates can be sandwiched between 5 nm GO layers to construct a flexible freestanding protected noble metal monolayer with surface-enhanced Raman scattering properties. These flexible rGO/Ag/rGO nanomembranes can be transferred and conformally coat complex surfaces and show a cleaner Raman signature, enhanced wet stability, and lower oxidation compared to bare Ag nanostructures.

Self-Powered Electronic Skin with Biotactile Selectivity.

Power-generating flexible thin films for facile detection of biotactile events are fabricated from patterned metal-graphene oxide biopaper. These tactile materials are mechanically robust with a consistent output of 1 V and high response rate of 20 Hz. It is demonstrated that the simple quadruple electronic skin sensitively and selectively recognizes nine spatial biotactile positions and can readily be expanded.

Size effect on order-disorder transition kinetics of FePt nanoparticles.

The kinetics of order-disorder transition of FePt nanoparticles during high temperature annealing is theoretically investigated. A model is developed to address the influence of large surface to volume ratio of nanoparticles on both the thermodynamic and kinetic aspect of the ordering process; specifically, the nucleation and growth of L10 ordered domain within disordered nanoparticles. The size- and shape-dependence of transition kinetics are quantitatively addressed by a revised Johnson-Mehl-Avrami equation that included corrections for deviations caused by the domination of surface nucleation in nanoscale systems and the non-negligible size of the ordered nuclei. Calculation results based on the model suggested that smaller nanoparticles are kinetically more active but thermodynamically less transformable. The major obstacle in obtaining completely ordered nanoparticles is the elimination of antiphase boundaries. The results also quantitatively confirmed the existence of a size-limit in ordering, beyond which, inducing order-disorder transitions through annealing is impossible. A good agreement is observed between theory, experiment, and computer simulation results.