TEMPO-oxidized Cellulose Nanofibrils Research Articles

Fabrication of photocatalyst film based on TEMPO-oxidized cellulose nanofiber

The powerful performance of TiO 2 and its doped as photocatalyst material initiated the finding of new technique that efficient and effective to degrade the organic pollutant, i.e., azo dyes. This study examined photocatalyst film's photoactivity, TEMPO-oxidized cellulose-containing N-TiO 2 (TOC/N-TiO 2 ), on the degradation of azo dyes. TEMPO-oxidized cellulose, which has a negative charge, was sequentially mixed and stirred in the suspension of TiO 2 and doped TiO 2 . This experiment utilized several instruments to determine the physicochemical properties of the photocatalyst film. The UV-DRS and diffractogram data confirmed the anatase phase as the only phase found in N-TiO 2 , which has a lower bandgap value than the anatase TiO 2 . These data demonstrated the superior photocatalytic of TOC/ N-TiO 2 against azo dyes. Keywords: Doped-TiO 2 ; Film; Photocatalyst; TEMPO-oxidized cellulose nanofiber

TEMPO-oxidized Cellulose Nanofibers/ Alginate Nanocomposite as a Promising nanocomposite Material for the Adsorption of Cationic Dyes

TEMPO-oxidized Cellulose Nanofibers/ Alginate Nanocomposite as a Promising nanocomposite Material for the Adsorption of Cationic Dyes

Adsorption Studies of Ammonia, Protein, and Phytic Acid Using Chitosan Fiber Coated with Oxidized Cellulose Nanofiber

Herein, chitosan (CS) fibers coated with TEMPO-oxidized cellulose nanofibers (CS/TOCN fibers) were successfully prepared using a wet spinning technique; CS was dissolved in acetic acid to obtain a CS doping solution. The tensile strength and ammonia adsorption percentages increased with increasing TOCN concentration. The maximum ammonia adsorption percentage (41.39%, 8.3 mg/g) was obtained when 2% NaOH and 0.01% TOCN were used as the coagulation solution. Additionally, the adsorption of model proteins, including lysozyme (Lz), cytochrome C (Cyt C), and bovine serum albumin (BSA), were studied. In water, the CS/TOCN fibers with negative charges adsorbed more Lz with positive charges than CS fibers containing positive charges. Contrastingly, CS fibers adsorbed more Cyt C and BSA containing negative charges in phosphate-buffered saline solutions than CS/TOCN fibers. Furthermore, the adsorption percentage of phytic acid using the CS/TOCN fibers reached 64.18% (288 mg/g) within 60 min. Thus, TOCNs improved the tensile properties of CS fibers and preferred positively charged materials contaminated in water, such as ammonia, Lz, and phytic acid.

Development of Antifreezing, Printable, Adhesive, Tough, Biocompatible, High-Water Content Hydrogel for Versatile Applications.

Hydrogels with different functionalities such as printability, antifreezing properties, adhesion, biocompatibility, and toughness are being continually developed. However, it has been extremely challenging to design adhesive, antifreezing, tough, and biocompatible multifunctional hydrogels with complex shapes simultaneously and prepare them in a short period. In this paper, novel composite hydrogels, which consist of poly(vinyl alcohol) grafted with styrylpyridinium group (PVA-SbQ) and TEMPO-oxidized cellulose nanofibrils (CNF), were successfully synthesized via UV photo-cross-linking. In addition to UV photo-cross-linking, the PVA-SbQ/CNF hydrogels with different shapes could be rapidly printed by facile visible light-based stereolithography printing and laser direct-writing without any photoinitiators in 3 min and 30 s, respectively. The results show that PVA-SbQ/CNF hydrogels are biocompatible because there are no photoinitiators and cross-linkers required during the printing process under visible light. Moreover, the adhesive, antifreezing, mechanical properties, and water-binding capacity of PVA-SbQ/CNF with high-water contents improved significantly as the CNF contents increased. Such hydrogels, which combine multiple advantages, present great potential for application in wound dressings and portable devices with specific requirements for shapes, adhesion, toughness, and tolerance in extreme environments such as dry environments and low temperatures.

Dual Network Hydrogel with High Mechanical Properties, Electrical Conductivity, Water Retention and Frost Resistance, Suitable for Wearable Strain Sensors.

With the progress of science and technology, intelligent wearable devices have become more and more popular in our daily life. Hydrogels are widely used in flexible sensors due to their good tensile and electrical conductivity. However, traditional water-based hydrogels are limited by shortcomings of water retention and frost resistance if they are used as the application materials of flexible sensors. In this study, the composite hydrogels formed by polyacrylamide (PAM) and TEMPO-Oxidized Cellulose Nanofibers (TOCNs) are immersed in LiCl/CaCl2/GI solvent to form double network (DN) hydrogel with better mechanical properties. The method of solvent replacement give the hydrogel good water retention and frost resistance, and the weight retention rate of the hydrogel was 80.5% after 15 days. The organic hydrogels still have good electrical and mechanical properties after 10 months, and can work normally at -20 °C, and has excellent transparency. The organic hydrogel show satisfactory sensitivity to tensile deformation, which has great potential in the field of strain sensors.

Rapid in-situ growth of enzyme-mimicking Pd nanoparticles on TEMPO-oxidized nanocellulose for the efficient detection of ascorbic acid

Rapid, efficient and green method of Pd nanoparticles (PdNPs) synthesis on TEMPO-oxidized cellulose nanofibril (TCNF) is demonstrated here. The nanohybrid (PdNPs/TCNF) exhibited peroxidase and oxidase-like activities evident by the oxidation of three chromogenic substrates. Enzyme kinetic studies using 3,3′,5,5′-Tetramethylbenzidine (TMB) oxidation uncovered the excellent kinetic parameters (low Km and high Vmax) and good specific activities of 215 U/g and 107 U/g for peroxidase and oxidase-like activities, respectively. A colorimetric assay for ascorbic acid (AA) detection is proposed based on its ability to reduce oxidized TMB to its colorless form. However, presence of nanozyme caused re-oxidation of TMB to its blue colored form within few minutes resulting in time limitation and inaccurate detection. Thanks to the film forming nature of TCNF; this limitation was overcome by employing PdNPs/TCNF film strips that can be easily removed before AA addition. The assay allowed AA detection in the linear range of 0.25–10 μM with a detection limit of 0.039 μM. The results of AA detection in commercial beverages and vitamin C tablets are matching with the specified values. Further the nanozyme exhibited high tolerance to pH (2−10) and temperature (up to 80 °C) and good recyclability for five cycles.

Valorized soybean hulls as TEMPO-oxidized cellulose nanofibril and polyethylenimine composite hydrogels and their potential removal of water pollutants

Cellulose nanomaterial (CNM) and polyethylenimine (PEI) composites have attracted growing attention due to their multifunctional characteristics, which have been applied in different fields. In this work, soybean hulls were valorized into carboxyl cellulose nanofibrils (COOH-CNFs), and composited into hydrogels with PEI by combining them with cationic chelating and physical adsorption strategies. Cellulose nanofibrils (CNFs) were produced from soybean hulls prior to oxidation by a TEMPO mediated reaction to obtain COOH–CNFs; then drops of zinc chloride were added to 1.5% aqueous COOH–CNF dispersions, which were left for 24 h to form COOH-CNF hydrogels. Finally, the hydrogels were functionalized using different concentration of PEI solutions over a range of pH values. Elemental analysis results showed that 20% aq. PEI at pH 11.6 is the optimum condition to synthesize the COOH–CNF/PEI hydrogels. Additionally, the adsorption efficiency for the removal of anionic methyl blue dyes and Cu(II) ions from water was tested, reaching 82.6% and 69.8%, respectively, after 24 h. These results demonstrate the great potential of COOH–CNF/PEI hydrogels as adsorbent materials for water remediation.Graphical abstract

Responses of Rat Mesenchymal Stromal Cells to Nanocellulose with Different Functional Groups.

Cellulose nanofibrils (CNFs) are multiscale hydrophilic biocompatible polysaccharide materials derived from wood and plants. TEMPO-mediated oxidation of CNFs (TO-CNF) turns some of the primary hydroxyl groups to carboxylate and aldehyde groups. Unlike carboxylic functional groups, there is little or no information about the biological role of the aldehyde groups on the surface of wood-based CNFs. In this work, we replaced the aldehyde groups in the TO-CNF samples with carboxyl groups by another oxidation treatment (TO-O-CNF) or with primary alcohols with terminal hydroxyl groups by a reduction reaction (TO-R-CNF). Rat mesenchymal stem/stromal cells (MSCs) derived from bone marrow were seeded on polystyrene tissue culture plates (TCP) coated with CNFs with and without aldehyde groups. TCP and TCP coated with bacterial nanocellulose (BNC) were used as control groups. Protein adsorption measurements demonstrated that more proteins were adsorbed from cell culture media on all CNF surfaces compared to BNC. Live/dead and lactate dehydrogenase assays confirmed that all nanocellulose biomaterials supported excellent cell viability. Interestingly, TO-R-CNF samples, which have no aldehyde groups, showed better cell spreading than BNC and comparable results to TCP. Unlike TO-O-CNF surfaces, which have no aldehyde groups either, TO-R-CNF stimulated cells, in osteogenic medium, to have higher alkaline phosphatase activity and to form more biomineralization than TCP and TO-CNF groups. These findings indicate that the presence of aldehyde groups (280 ± 14 μmol/g) on the surface of TEMPO-oxidized CNFs might have little or no effect on attachment, proliferation, and osteogenic differentiation of MSCs.

Fluoride ion adsorption isotherms, kinetics, and thermodynamics on iron(III) oxyhydroxide powders containing cellulose nanofibrils

The adsorption isotherms, kinetics, and thermodynamics of fluoride ions (F−) on FeOOH powders in water were investigated to obtain fundamental information on FeOOH powders, which are used as F− adsorbents in drinking and industrial water, and industrial wastewater. FeOOH powders were prepared as precipitates by mixing aqueous FeCl3 and NaOH solutions (1:3 mol/mol) in the presence of 2,2,6,6,-tetramethylpiperidine-1-oxyl radical (TEMPO)-oxidized cellulose nanofibrils (TOCNs), carboxymethylcellulose (CMC), or TEMPO-oxidized cellulose (TOC) fibers (without nanofibrillation), and subsequent drying and pulverizing. The FeOOH:TOCN, FeOOH:CMC, and FeOOH:TOC dry mass ratios were controlled at 87:13. The amount of F− adsorbed by the FeOOH/TOCN powder per FeOOH mass was higher than those adsorbed by FeOOH, FeOOH/CMC, or FeOOH/TOC. The F− adsorption isotherms on the FeOOH-containing powders showed higher correlation coefficients with the Langmuir model than with the Freundlich model. This indicates that F− adsorbed on FeOOH initially formed a monolayer, predominantly via physical adsorption. Pseudo-second-order kinetics fitted well to the time-dependent F− adsorption behaviors on the FeOOH-containing powders. Thermodynamic analysis of F− adsorption on the FeOOH-containing powders showed that the ΔG values were negative, which indicates that F− adsorption on the FeOOH-containing powders proceeded spontaneously in water. The negative ΔG value for FeOOH/TOCN was higher than those for FeOOH, FeOOH/CMC, and FeOOH/TOC at the same temperature. This shows that the FeOOH/TOCN powder can be used as an excellent and efficient F− adsorbent in water.Graphical abstract

Investigating the thermal conductivity and flame-retardant properties of BN/MoS2/PCNF composite film containing low BN and MoS2 nanosheets loading

Cellulose has attracted considerable attention as a potential substitute for plastics. However, the flammability and high thermal insulation properties of cellulose contradict the unique requirements for highly integrated and miniaturized electronics i.e., rapid thermal dissipation and efficient flame retardancy. In this work, cellulose was first phosphorylated to achieve intrinsic flame-retardant properties, and subsequently treated with MoS2 and BN, ensuring efficient dispersion throughout the material. Via chemical crosslinking, a sandwich-like unit was formed, in the order of BN, MoS2, and phosphorylated cellulose nanofibers (PCNF). The sandwich-like units were further self-assembled, layer-by-layer, to successfully create BN/MoS2/PCNF composite films exhibiting excellent thermal conductivity and flame retardancy, and comprised a low MoS2 and BN loading. The thermal conductivity of the BN/MoS2/PCNF composite film containing 5 wt% BN nanosheets was higher than that of neat PCNF film. The combustion characterization of BN/MoS2/PCNF composite films revealed highly desirable properties that were far more superior than the BN/MoS2/TCNF (TCNF, TEMPO-oxidized cellulose nanofibers) composite films. Moreover, the toxic volatiles that escaped from flaming BN/MoS2/PCNF composite films were significantly reduced compared to that of the BN/MoS2/TCNF composite film alternative. The thermal conductivity and flame retardancy of BN/MoS2/PCNF composite films have promising application prospects in highly integrated and eco-friendly electronics.

The Use of Corn Stover-Derived Nanocellulose as a Stabilizer of Oil-in-Water Emulsion.

Agricultural byproducts such as corn stover are widely available sources for preparation of nanocellulose, which is an emerging green chemical with versatile applications. In this study, corn stover-derived nanocellulose was prepared via bleaching, alkaline treatment, 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) oxidation, and ultrasonication. The as-prepared TEMPO-oxidized cellulose nanofibril (TEMPO-CNF) was characterized by transmission electron microscopy, UV-Vis spectrophotometry, rheometry, and zeta potential measurement. Droplet size, phase behavior, and thermodynamic stability of TEMPO-CNF stabilized oil-in-water emulsions were investigated. Results show that TEMPO-CNF with a width of 4 nm, length of 353 nm, and surface charge of 1.48 mmol/g COO- can be prepared from corn stover. In addition, TEMPO-CNF can be used as an emulsion stabilizer for lemongrass essential oil loaded oil-in-water emulsion. This study is among the first to report that TEMPO-CNF improved the freeze-thaw stability of oil-in-water emulsions stabilized by small molecular weight surfactants (e.g., Tween 80).

L-lysine biodetector based on a TOCNFs-coated Quartz Crystal Microbalance (QCM)

Developing a simple, cost effective and accurate detection method for L-lysine (Lys), L-leucine (Leu) and glycine (Gly) as the important analytes in clinical diagnostics, biological processes and food industries is of great significance. Herein, we prepared spin-coated TEMPO-oxidized cellulose nanofibrils (TOCNFs) on Quartz Crystal Microbalance (QCM) chip to achieve QCM biodetectors. The coated QCMs were carefully characterized before and after interaction with amino acids (AA) using water contact angle (WCA), Fourier transform infrared spectrophotometry-attenuated total reflectance (FTIR-ATR), Raman spectroscopy, and scanning electron microscopy (SEM). In addition, to study the response of the fully covered QCM chips to the aqueous AA, the frequency reductions of the detectors in proportion to different AA concentrations (10–1000 μg/mL) were recorded. The data showed a linear range of detection R2 = 0.9987, 0.9933, 0.9833 and a calculated limit of detection (LOD) equal to 49, 113 and 178 µg/ml for Lys, Gly and Leu, respectively. We observed the highest frequency shifts (the strongest interaction with TOCNFs) for Lys detection while response to Leu was negligible.

Surfactant induced gelation of TEMPO-oxidized cellulose nanofibril dispersions probed using small angle neutron scattering.

In this work, we studied TEMPO-oxidized cellulose nanofibril (OCNF) suspensions in the presence of diverse surfactants. Using a combination of small angle neutron scattering (SANS) and rheology, we compared the physical properties of the suspensions with their structural behavior. Four surfactants were studied, all with the same hydrophobic tail length but different headgroups: hexaethylene glycol mono-n-dodecyl ether (C12EO6, nonionic), sodium dodecyl sulfate (SDS, anionic), cocamidopropyl betaine (CapB, zwitterionic), and dodecyltrimethylammonium bromide (DTAB, cationic). Contrast variation SANS studies using deuterated version of C12EO6 or SDS, or by varying the D2O/H2O ratio of the suspensions (with CapB), allowed focusing only on the structural properties of OCNFs or surfactant micelles. We showed that, in the concentration range studied, for C12EO6, although the nanofibrils are concentrated thanks to an excluded volume effect observed in SANS, the rheological properties of the suspensions are not affected. Addition of SDS or CapB induces gelation for surfactant concentrations superior to the critical micellar concentration (CMC). SANS results show that attractive interactions between OCNFs arise in the presence of these anionic or zwitterionic surfactants, hinting at depletion attraction as the main mechanism of gelation. Finally, addition of small amounts of DTAB (below the CMC) allows formation of a tough gel by adsorbing onto the OCNF surface.

Multiple Structure Reconstruction by Dual Dynamic Crosslinking Strategy Inducing Self‐Reinforcing and Toughening the Polyurethane/Nanocellulose Elastomers

AbstractHigh‐performance elastomers are expected to possess excellent healing and recycling ability, damage resistance in conjunction with high strength and toughness. Herein, a dual dynamic crosslinking strategy is implemented by multiple hydrogen and disulfide bonds to obtain a novel amorphous and transparent polyurethane/nanocellulose elastomer with excellent self‐healing, self‐reinforcing and toughening performance. First, hydrogen bonds are introduced in TEMPO‐oxidized cellulose nanofibers (TCNF) by modification with 2‐ureido‐4[1H]‐pyrimidone (UTCNF), while disulfide bonds (SS) are introduced in the polyurethane (PU) main chain, leading to the formation of dual dynamic cross‐linking networks. The PU‐SS‐UTCNF elastomer can fully self‐heal within 4.0 h at 50 °C. Surprisingly, for the first time, the PU‐SS‐UTCNF elastomer also self‐strengthens and self‐toughens after multiple hot‐pressing, with tensile strength and toughness that increase by up to 401% and 257% compared to original elastomer samples, up to 50.0 MPa and 132.5 MJ m‐3. The self‐strength and self‐toughening effects are attributed to 1) reconstruction of dual dynamic networks that increase the cross‐linking degree during the multiple hot‐pressing processes; 2) multiple hydrogen bonds in the system are beneficial to the orientation of highly crystallized UTCNF, as a replacement of stress‐induced process in deformation under external tensile force.

Preparation of spherical microparticles composed of cellulose nanofiber and cellulose diacetate via Pickering emulsion templating

Cellulose nanofiber (CNF) derived from woody bioresources is a fascinating natural nanomaterial. In this work, spherical microparticles were fabricated by using TEMPO-oxidized CNF (TOCNF) and cellulose diacetate (CDA) via Pickering emulsion templating. The CDA-dissolved organic solvents were emulsified stably with TOCNF, followed by removing the solvents to form microspheres with core-shell structures, where the CDA cores were covered with the TOCNF shells. The prepared spherical microparticles possessed an average diameter and sphericity index of 6.4 μm and 0.96, respectively. The zeta-potential value was approximately -48 mV, indicating the stable colloidal system in water. The CDA/TOCNF microparticles were stained with toluidine blue dye for negatively-charged TOCNF. Besides, furry nanofiber-like morphology was observed on the particle surface by scanning electron microscopy. Wood-derived CDA/TOCNF microspheres are a promising alternative to fossil resource-derived, non-biodegradable microbeads in cosmetic applications.

Coupling Particle Ordering and Spherulitic Growth for Long-Term Performance of Nanocellulose/Poly(ethylene oxide) Electrolytes

Development of lithium-ion batteries with composite solid polymer electrolytes (CPSEs) has attracted attention due to their higher energy density and improved safety compared to systems utilizing liquid electrolytes. While it is well known that the microstructure of CPSEs affects the ionic conductivity, thermal stability, and mechanical integrity/long-term stability, the bridge between the microscopic and macroscopic scales is still unclear. Herein, we present a systematic investigation of the distribution of TEMPO-oxidized cellulose nanofibrils (t-CNFs) in two different molecular weights of poly(ethylene oxide) (PEO) and its effect on Li+ ion mobility, bulk conductivity, and long-term stability. For the first time, we link local Li-ion mobility at the nanoscale level to the morphology of CPSEs defined by PEO spherulitic growth in the presence of t-CNF. In a low-MW PEO system, spherulites occupy a whole volume of the derived CPSE with t-CNF being incorporated in between lamellas, while their nuclei remain particle-free. In a high-MW PEO system, spherulites are scarce and their growth is arrested in a non-equilibrium cubic shape due to the strong t-CNF network surrounding them. Electrochemical strain microscopy and solid-state 7Li nuclear magnetic resonance spectroscopy confirm that t-CNF does not partake in Li+ ion transport regardless of its distribution within the polymer matrix. Free-standing CSPE films with low-MW PEO have higher conductivity but lack long-term stability due to the existence of uniformly distributed, particle-free, spherulite nuclei, which have very little resistance to Li dendrite growth. On the other hand, high-MW PEO has lower conductivity but demonstrates a highly stable Li cycling response for more than 1000 h at 0.2 mA/cm2 and 65 °C and more than 100 h at 85 °C. The study provides a direct link between the microscopic dynamic, Li-ion transport, bulk mechanical properties and long-term stability of the derived CPSE and, and as such, offers a pathway towards design of robust all-solid-state Li-metal batteries.

Highly magnetic hybrid foams based on aligned tannic acid-coated iron oxide nanoparticles and TEMPO-oxidized cellulose nanofibers.

Lightweight iron oxide nanoparticle (IONP)/TEMPO-oxidized cellulose nanofibril (TOCNF) hybrid foams with an anisotropic structure and a high IONP content were produced using magnetic field-enhanced unidirectional ice-templating. Coating the IONP with tannic acid (TA) improved the processability, the mechanical performance, and the thermal stability of the hybrid foams. Increasing the IONP content (and density) increased the Young's modulus and toughness probed in compression, and hybrid foams with the highest IONP content were relatively flexible and could recover 14% axial compression. Application of a magnetic field in the freezing direction resulted in the formation of IONP chains that decorated the foam walls and the foams displayed a higher magnetization saturation, remanence, and coercivity compared to the ice-templated hybrid foams. The hybrid foam with an IONP content of 87% displayed a saturation magnetization of 83.2 emu g-1, which is 95% of the value for bulk magnetite. Highly magnetic hybrid foams are of potential interest for environmental remediation, energy storage, and electromagnetic interference shielding.

Toughening of poly(ionic liquid)-based ion gels with cellulose nanofibers as a sacrificial network.

Ion gels have the potential to be used in a broad range of applications, such as in carbon dioxide separation membranes and soft electronics. However, their low mechanical strength limits their practical applications. In this study, we developed double-network (DN) ion gels composed of TEMPO-oxidized cellulose nanofibers with hydrophobic groups (TOCNF) and cross-linked poly[1-ethyl-3-vinylimidazolium bis(trifluoromethanesulfonyl)imide] (PC2im-TFSI) networks. The mechanical strength of the gel increased as the amount of TOCNF in the gels increased up to 6 wt%. Moreover, the fracture energy of the DN ion gels with 6 wt% TOCNF was found to be 19 times higher than that of the PC2im-TFSI single network (SN) ion gels. Cyclic stress-strain measurements of the DN gels showed that the loading energy on the gels dissipates owing to the destruction of the physically cross-linked TOCNF network in the gels. The DN ion gels also exhibited a high decomposition temperature of approximately 400 °C because of the thermal stability of all components. Additionally, the fracture energy of the TOCNF/poly(ionic liquid) (PIL) DN ion gel was two times higher than that of the silica nanoparticles/PIL DN ion gel developed in our previous study [Watanabe et al., Soft Matter, 2020, 16, 1572-1581]. This suggests that fiber-shaped nanomaterials are more effective than spherical nanomaterials in enhancing the mechanical properties of ion gels. These results show that TOCNF can be used to toughen PIL-based ion gels and hence broaden their applications.

3-D Printable Flexible Hydrogel-Based Sensors With Gradient Porous Structure

Hydrogel-based flexible pressure sensors have been intensively studied to detect human physiological activities. Here, a kind of hydrogel with excellent 3-D printability has been developed. The gelation method based on the freeze-thaw cycles of polyvinyl alcohol (PVA) and the addition of (TEMPO)-oxidized cellulose nanofibrils (TOCNF) as rheological modifier rendered the printed hydrogel patterns great morphological fidelity. In addition, the conductivity of the hydrogels is as high as 3.8 S/m with the promotion of TOCNF. Also, the prepared hydrogels have gratifying mechanical properties under the salting-out effect (tensile and compressive stresses increased by 527% and 460%, respectively). The printed hydrogels with porous structure were applied to the electrode and dielectric layer of capacitance sensors (the dielectric layer was wrapped with ultrathin polyethylene (PE) film). The porous structure effectively improved the sensitivity of the pressure sensors (from 0.023 and 0.182 to 0.045 and 0.689 kPa <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{-1}$ </tex-math></inline-formula> ) when used as electrode and dielectric layer, respectively), while the gradient porous structure further broadened the linearity range of high sensitivity (electrode, from 2.92 to 15.35 kPa; dielectric layer, from 11.21 to 39.51 kPa). The results indicated that gradient porous structure is a simple and effective strategy to enhance both the sensitivity and linearity range of hydrogel-based sensors. The hydrogel-based sensors with printed structure have stable and sensitive capacitance and resistance response, and can also be used for motion monitoring of body parts, such as finger and arm.

Flexible cellulose-based piezoelectric composite membrane involving PVDF and BaTiO3 synthesized with the assistance of TEMPO-oxidized cellulose nanofibrils.

High-performance flexible barium titanate (BaTiO3)-based piezoelectric devices have gained much attention. However, it is still a challenge to prepare flexible polymer/BaTiO3-based composite materials with uniform distribution and high performance due to the high viscosity of polymers. In this study, novel hybrid BaTiO3 particles were synthesized with assistance of TEMPO-oxidized cellulose nanofibrils (CNFs) via a low-temperature hydrothermal method and explored for their application in piezoelectric composites. Specifically, Ba2+ was adsorbed on uniformly dispersed CNFs with a large amount of negative charge on their surface, which nucleated, resulting in the synthesis of evenly dispersed CNF-BaTiO3. The obtained CNF-BaTiO3 possessed a uniform particle size, few impurities, high crystallinity and dispersity, high compatibility with the polymer substrate and surface activity due to the existence of CNFs. Subsequently, both polyvinylidene fluoride (PVDF) and TEMPO-oxidized CNFs were employed as piezoelectric substrates for the fabrication of a CNF/PVDF/CNF-BaTiO3 composite membrane with a compact structure, displaying the tensile strength of 18.61 ± 3.75 MPa and elongation at break of 3.06 ± 1.33%. Finally, a thin piezoelectric generator (PEG) was assembled, which output a considerable open-circuit voltage (4.4 V) and short-circuit current (200 nA), and could also power a light-emitting diode and charge a 1 μF capacitor to 3.66 V in 500 s. Its longitudinal piezoelectric constant (d 33) was 5.25 ± 1.04 pC N-1 even with a small thickness. It also exhibited high sensitivity to human movement, outputting a voltage of about 9 V and current of 739 nA for only a footstep. Thus, it exhibited good sensing property and energy harvesting property, presenting practical application prospects. This work provides a new idea for the preparation of hybrid BaTiO3 and cellulose-based piezoelectric composite materials.