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A nanofluidic ion regulation membrane with aligned cellulose nanofibers

A nanofluidic ion regulation membrane with aligned cellulose nanofibers SCIENCE ADVANCES RESEARCH ARTICLE MATERIALS SCIENCE Copyright © 2019 The Authors, some rights reserved; A nanofluidic ion regulation membrane with aligned exclusive licensee American Association cellulose nanofibers for the Advancement of Science. No claim to 1 2 1 1 1 1 1 1 Tian Li , Sylvia Xin Li , Weiqing Kong , Chaoji Chen , Emily Hitz , Chao Jia , Jiaqi Dai , Xin Zhang , original U.S. Government 1 3 4 1 Robert Briber , Zuzanna Siwy , Mark Reed , Liangbing Hu * Works. Distributed under a Creative The advancement of nanofluidic applications will require the identification of materials with high-conductivity Commons Attribution nanoscale channels that can be readily obtained at massive scale. Inspired by the transpiration in mesostruc- NonCommercial tured trees, we report a nanofluidicmembraneconsistingofdensely packed cellulose nanofibers directly License 4.0 (CC BY-NC). derived from wood. Numerous nanochannels are produced among an expansive array of one-dimensional cel- lulose nanofibers. The abundant functional groups of cellulose enable facile tuning of the surface charge den- sity via chemical modification. The nanofiber-nanofiber spacing can also be tuned from ~2 to ~20 nm by structural engineering. The surface-charge-governed ionic transport region shows a high ionic conductivity pla- −1 teau of ~2 mS cm (up to 10 mM). The nanofluidic membrane also exhibits excellent mechanical flexibility, demonstrating stable performance even when the membrane is folded 150°. Combining the inherent advan- tages of cellulose, this novel class of membrane offers an environmentally responsible strategy for flexible and printable nanofluidic applications. INTRODUCTION groups (32), the charged cellulose nanofiber surface can attract layers Ion-regulating nanofluidic membranes have been intensively used in of counterionsadjacenttothe fibers, with an exponentially decaying desalination (1, 2), osmosis energy generation (3, 4), ion/molecular ion concentration toward the center of the channel (Fig. 1D). The separation (5–7), and ionic circuits (8–12). Because of the interactions interface-dominated electrostatic field surrounding the cellulose nano- between solvated ions and the inner channel walls, ion transport in a fibers provides surface charge–governed ion transport along the fiber nanoscale-confined, surface-charged membrane substantially differs direction, enabling desirable ionic separation. from bulk behavior (13–15). High ionic conductivity, which can be The surface charge and geometry of the nanochannels can also be achieved through the nanofluidic effect, is essential for applications such easily tuned to modify the ionic conductivity of the membrane. Owing as ionic circuitry and nanofluidic membranes. Typically, silicon-based to the abundance of the functional groups on the cellulose nanofibers, materials are fabricated via lithography or templating to form aligned the surface charge density can be tuned via chemical stimuli. In this −2 one-dimensional nanoscale channels for nanofluidic membranes, but work, we demonstrate a high surface charge density of −5.7 mC m the material is often brittle or suffers from performance degradation after converting the hydroxyl groups to carboxyl groups, which was upon bending or folding (8, 16), rendering it challenging for flexible greater than previously reported values (8, 13, 20, 21, 33). In addition, applications. Cost-effective nanoporous polymer membranes have also we were able to attain large tunability of the channel size up to an order found great success in the industry, but these materials are not sustain- of magnitude. In this manner, a high surface charge–governed ionic −2 able, and the three-dimensional tortuous nanoporous structure limits conductivity of ~2 mS cm was observed at a KCl concentration of less −2 the ionic conductivity (17, 18). Two-dimensional materials, such as than 10 M. boron nitride, graphene, and MoS , have shown excellent ionic con- Figure 2 (A and B) shows various configurations of the cellulose ductivity and advantageous properties for use in nanofluidic devices, membranes after lignin and hemicellulose removal of the natural wood. but these materials require time-consuming and expensive bottom- When dried in air under ambient temperature, a densified structure can up fabrication methods (14, 19–22). Consequently, advancing tech- be obtained with closely packed cellulose nanofibers, potentially due to nologies that rely on efficient ion regulation necessitate the continued hydrogen bonding and van der Waals forces (34). Scanning electron search for materials with high-performance nanoscale channels that can microscopy (SEM)imagesinFig.2(C andD)showthe parallel-packed be obtained on a massive scale. cellulose fibers aligned with the wood growth direction, while Fig. 2E Cellulose is the most abundant and sustainable material in nature shows a view of the cellulose nanofiber ends. The hydrophilic and highly and is an attractive candidate for a wide range of applications, especially aligned cellulose nanofibers render the membrane highly efficient in those related to eco-friendly membranes (23, 24) and fluidic devices fluidic transport. To demonstrate, one end of a sample 2 cm by 1 cm by (25–30). Here, we demonstrate highly efficient and tunable ion regula- 200 mm in size was immersed into a water solution colored with ink to −1 tion using a cellulose membrane that is composed of aligned nano- measure the liquid uptake. A fluidic rate of >1 mm s was observed channels. These cellulose nanofibers are exposed after extraction of (fig. S1), twice higher than the previously demonstrated values obtained −1 intertwined lignin and hemicellulose from the natural wood (Fig. 1, with randomly oriented cellulose fibers (0.5 mm s )(25, 35). Small- AtoC)(31). Because of the dissociation of the surface functional angle x-ray scattering (SAXS) verified the alignment of the cellulose membrane down to the molecular level, in which an elliptical pattern was observed for the cellulose membranes both without (dry) and Department of Materials Science and Engineering, University of Maryland Col- lege Park, College Park, MD 20742, USA. Department of Physics, Yale University, with (wet) water (Fig. 2F). The dry membrane appears white, which New Haven, CT 06511, USA. Department of Physics and Astronomy, University of is indicative of the complete removal of lignin (fig. S2) (31). Upon California, Irvine, Irvine, CA 92697, USA. Departments of Electrical Engineering immersion in water, a broadband transmittance of >65% was ob- and Applied Physics, Yale University, New Haven, CT 06520, USA. tained from 400 to 1100 nm (Fig. 2G), and letters of the text beneath *Corresponding author. Email: binghu@umd.edu Li et al., Sci. Adv. 2019;5 : eaau4238 22 February 2019 1of6 | SCIENCE ADVANCES RESEARCH ARTICLE Fig. 1. Ion transport within the aligned cellulose nanofibers. (A) A tree trunk containing cellulose nanofibers. (B) Schematic of the removal of intertwined lignin and hemicellulose from natural wood to make the nanofluidic membrane. (C) A nanofluidic membrane that inherits the nanofiber alignment direction from natural wood, as marked. (D) Schematic of the low tortuosity nanofluidic membrane. Photo Credit: T.L., University of Maryland, College Park. Permission granted. Fig. 2. Characterizations of the delignified wood. Photos of various configurations of the (A) nanofluidic cellulose membrane, including (B) a cellulose cable wrapped around a rod. The arrows indicate the nanofiber alignment direction. Scale bars, 1 cm. (C) Side view SEM image of the cellulose membrane and (D) the aligned cellulose nanofibers at higher magnification. (E) Top view SEM image showing the tips of the cellulose nanofibers. (F) The elliptical shape of the diffraction pattern in SAXS for the dry and wet membrane, indicating the molecular level alignment of cellulose. (G) The transmittance of the dry and wet cellulose membrane. Li et al., Sci. Adv. 2019;5 : eaau4238 22 February 2019 2of6 | SCIENCE ADVANCES RESEARCH ARTICLE the membrane became visible. The transparency change is mainly in which s is the surface charge, e is the dielectric constant, e is the attributed to the matched optical refractive indexes of cellulose (1.48) permittivity of vacuum, z is the zeta potential, and l is the Debye −2 (36) and water (typically ~1.50). length, which was −3.2 and −5.7 mC m for the as-made cellulose We investigated the nanofluidic performance of the cellulose and oxidized cellulose, respectively (38). With the estimate of the surface membrane using the ionic conductivity setup shown in Fig. 3A (see charge for these samples, the overall conductivity trend can be fitted Methods). Carboxyl groups have a greater tendency to dissociate into using the following equation negatively charged carboxylates (32), leading to a higher charge den- sity and therefore a higher negative zeta potential in deionized water. k ¼ Zeðm þ m ÞCN þ 2sm =h ð2Þ þ  þ Therefore, using a previously described method (37), we applied TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) oxidation to the cellu- in which Z is the cation valence, m is the cation mobility, m is the anion lose to convert the primary hydroxyl groups on the surface chains of + − mobility, C is the ion concentration, N is the Avogadro’snumber, and the cellulose crystallites into carboxyl groups (37), as illustrated in h is the channel diameter of the nanofluidic cellulose system (39). As Fig. 3B. The resulting oxidized cellulose membrane exhibited a high- shown in Fig. 3C, the fit of Eq. 2 agrees well with the experimental data er zeta potential of −78 mV, compared with −45 mV for just delignified of the ionic conductivity versus KCl concentration and allows us to cal- cellulose (Fig. 3b). The respective ionic conductivity of these materials in culate a channel diameter of ~2 nm for both cellulose membranes. We KCl solutions is shown in Fig. 3C. The ion transport behavior in both attribute the difference in the value of the ionic conductivity plateaus for cellulose membranes exhibited a conductivity plateau orders of mag- each membrane to the difference of the surface charge densities between nitude higher than that of the bulk solution for concentrations below −2 the materials. ~10 M. Within the surface-governed ion transport region, a conduc- −1 tivity as high as ~2 mS cm was obtained for the oxidized membrane We further demonstrated the effect of channel geometry on the ionic −1 compared with 1.1 mS cm for the unmodified counterpart, indicating conductivity by preparing an undensified cellulose membrane (see the effectiveness of modifying the surface functional groups to tune the Methods) and comparing its performance with the densified sample. iontransport behavior.Using Eq.1,weestimated thesurface charge of The undensified samples exhibited an ionic conductivity plateau of −1 the membranes based on the zeta potential 0.2 mS cm , one order lower than that of the densified sample (Fig. 3D). The fit of the undensified cellulose membrane conductivity results indicated a channel diameter of around 20 nm, which is about 10 times s ¼ ee z=l ð1Þ 0 d larger than that of the densified cellulose membrane (channel diameter Fig. 3. Ionic conductivity measurement with chemical modifications and physical densifications. (A) Ionic conductivity measurement setup. (B) Zeta potential of the cellulose fibers and oxidized/surface-charged cellulose under neutral pH with a concentration of cellulose approximately 0.1%. (C) An ionic conductivity test with KCl solution for the cellulose membrane before and after oxidization. The oxidized cellulose exhibits an increased ionic conductivity plateau due to the higher surface charge. (D) Ionic conductivity of the undensified cellulose and densified cellulose membrane in KCl solution. Li et al., Sci. Adv. 2019;5 : eaau4238 22 February 2019 3of6 | SCIENCE ADVANCES RESEARCH ARTICLE ~2 nm). The mechanical properties of the densified and the undensified gate, which will contribute to a large cationic current density. Mean- samples are shown in fig. S3. We obtained the tensile strengths of while, positive gating will repel K and lead to an even lower current 58 ± 5 MPa and 9 ± 2 MPa for the densified and the undensified density than the neutral gating condition (Fig. 4B). Figure 4C shows samples, respectively. the ionic currents measured under different gating potentials from −2 To explore the use of this cellulose nanofiber membrane as an to 2 V. The ion conductivity under V = −2 V was about one order of ion regulation device, we demonstrated the ionic rectification effect magnitude higher than the value under V = 2 V and equivalent to −2 of the material acting as a flexible transistor with electrical gating, in that of 10 M KCl, indicating an efficient accumulation of positive which the cellulose membrane can preferentially accumulate ions that ions with negative gating (Fig. 4D). The device exhibits a negligible have the opposite charge as the channel walls. Silver paste was painted electrical gate leakage current, which was measured to be below the −6 on the membrane to act as the gating metal (Fig. 4A), and a 10 MKCl noise floor of the Keithley 2400. solution was used as the liquid electrolyte. The gating voltage was The cellulose membrane is flexible and even foldable. Figure 4E controlled by a Keithley 2400 power source, while the ionic current– shows a ribbon of the membrane that can be twisted and wrapped voltage characteristics were recorded. When the gating voltage was neg- around a finger. To observe how folding affects the ionic conductivity ative, the local concentration of K should further increase under the performance, we used a membrane 2 cm by 2 mm by 1 mm in size and Fig. 4. A cellulose-based ionic transistor. (A) Schematic of a freestanding cellulose nanofluidic transistor with a painted metal contact for gating. (B) Schematic of the gating effect on the ion distribution within the cellulose membrane. (C) Current-voltage characteristics of the cellulose nanofiber membrane with different gating voltages from −2 to 2 V. (D) Characterization of the transistor using varied gating voltages. Inset: Semi-log plot of ionic current versus gating voltage. (E) Photo image of a flexible and biocompatible cellulose nanofiber ribbon. (F) Ion conductivity shows minimal change upon folding. Li et al., Sci. Adv. 2019;5 : eaau4238 22 February 2019 4of6 | SCIENCE ADVANCES RESEARCH ARTICLE recorded the current under an applied voltage of 0.5 V as we folded the 0.5 M HCl. Last, the cellulose membrane was solvent exchanged to −6 material. The ionic conductivity under a concentration of 10 MKCl acetone and then toluene and allowed to sit in air to evaporate all exhibited minimal changes upon folding, with no notable performance solvent. degradation for a folding angle of up to 150° (Fig. 4F). We have also evaluated the membrane’s stability in conductivity and mechanical Characterization strength (figs. S4 and S5). After several wet-dry cycles, the membrane An SEM (Hitachi SU-70) was used to characterize the morphologies of shows no notable degradation of the performance. the samples. Compositional analysis of the cellulose membrane and original wood was carried out using high-performance liquid chroma- tography (Ultimate 3000; Thermo Fisher Scientific, USA). The optical DISCUSSION transmittance from 400 to 1100 nm was characterized by ultraviolet- In this work, we report a permselective nanofluidic membrane featuring visible absorption spectroscopy (Lambda 35; PerkinElmer, USA) with aligned cellulose nanofibers directly derived from wood. The removal of an integrating sphere to collect all transmitted light. To determine the lignin and hemicellulose introduces numerous open nanochannels zeta potential of the original and TEMPO-oxidized white wood, the among the cellulose nanofibers. Via structural engineering, the diameter samples were dispersed in deionized water by ultrasonication treatment of the nanochannels can be tuned from 2 to 20 nm. A high surface (100% amplitude, 10 min, Fisher Scientific FS 110D) to produce a 0.1 wt −2 charge density of −5.7 mC m was obtained after chemical modifica- % (weight %) concentration. Zeta potential (z) of the dispersed suspen- tion of the cellulose functional groups. The long-range ordered arrays sion was measured using a Zetasizer Nano S90 (Malvern Instrument) of the charged nanofibers led to highly efficient ion transport in the without adjusting the ionic strength. The z value was calculated from −1 centimeter-long devices. A high ionic conductivity plateau of ~2 mS cm the electrophoretic mobility using the Henry equation and Huckel ap- was obtained, which is orders of magnitude higher than that of the bulk proximation. The tensile strength of the wood was measured with a TA KCl solution at the same concentrations. In addition, we used electrostatic Q800 DMA system in controlled-force mode. The force increased from −6 gating to further increase the ionic conductivity of 10 M KCl electrolyte 1 up to 18 N at 0.01 N/min. −2 to a value that is equivalent to the ionic conductivity of 10 MKCl.This work demonstrates the use of wood-derived cellulose membranes toward Current voltage measurement foldable, scalable, and high-performance nanofluidic devices. A cellulose membrane was embedded into polydimethylsiloxane elastomer with two wells carved on either side to serve as reservoirs for the electrolyte solution. The cellulose nanofluidic membrane MATERIALS AND METHODS was immersed in electrolyte for at least 1 day before measurement. Materials and chemicals Two homemade Ag/AgCl electrodes were inserted into the reser- American Basswood was purchased from Walnut Hollow Company. voirs. BioLogic from Science Instrument was used to record the Sodium hydroxide (NaOH) and sodium sulfite (Na SO ) (both pur- current signal while providing the voltage source at a scanning rate 2 3 −1 chased from Carolina Biological Supply Company) and hydrogen per- of 10 mV s . oxide (H O , 30% solution; EMD Millipore Corporation) were used for 2 2 lignin extraction of the Basswood. The deionized water (type I deionized water; purity, >18 megohm·cm) from ChemWorld was used to further SUPPLEMENTARY MATERIALS remove the residual ions from the delignified wood. Supplementary material for this article is available at http://advances.sciencemag.org/cgi/ content/full/5/2/eaau4238/DC1 Cellulose membrane preparation Fig. S1. Highly hydrophilic cellulose demonstrating directional fluidic transport. Fig. S2. Composition results of the natural wood and the wood membrane after step I and II Basswood was cut along the growth direction at various dimensions treatments. (typical thickness, <5 mm; length/width, <10 cm). NaOH and Na SO 2 3 Fig. S3. The mechanical tensile strength for the densified and undensified samples. were dissolved in deionized water at concentrations of 2.5 and 0.4 M, −5 Fig. S4. Current-voltage characteristics of the membrane infiltrated multiple times with 10 M respectively. The wood slices were boiled in the solution for 10 hours. KCl solution. Then, the wood slices were immersed in boiling H O solution (30%) Fig. S5. Tensile strength of the membranes that underwent different dry-wet cycles. 2 2 until completely white. The resulting wood membrane was then rinsed in deionized water to remove the residual ions and chemicals. The ef- fective removal of residual chemicals was verified by obtaining a low −1 REFERENCES AND NOTES ionic conductivity of the solution (<20 mScm ) that had been used 1. S. J. Kim, S. H. Ko, K. H. Kang, J. Han, Direct seawater desalination by ion concentration to wash the sample. 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A nanofluidic ion regulation membrane with aligned cellulose nanofibers

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Abstract

SCIENCE ADVANCES RESEARCH ARTICLE MATERIALS SCIENCE Copyright © 2019 The Authors, some rights reserved; A nanofluidic ion regulation membrane with aligned exclusive licensee American Association cellulose nanofibers for the Advancement of Science. No claim to 1 2 1 1 1 1 1 1 Tian Li , Sylvia Xin Li , Weiqing Kong , Chaoji Chen , Emily Hitz , Chao Jia , Jiaqi Dai , Xin Zhang , original U.S. Government 1 3 4 1 Robert Briber , Zuzanna Siwy , Mark Reed , Liangbing Hu * Works. Distributed under a Creative The advancement of nanofluidic applications will require the identification of materials with high-conductivity Commons Attribution nanoscale channels that can be readily obtained at massive scale. Inspired by the transpiration in mesostruc- NonCommercial tured trees, we report a nanofluidicmembraneconsistingofdensely packed cellulose nanofibers directly License 4.0 (CC BY-NC). derived from wood. Numerous nanochannels are produced among an expansive array of one-dimensional cel- lulose nanofibers. The abundant functional groups of cellulose enable facile tuning of the surface charge den- sity via chemical modification. The nanofiber-nanofiber spacing can also be tuned from ~2 to ~20 nm by structural engineering. The surface-charge-governed ionic transport region shows a high ionic conductivity pla- −1 teau of ~2 mS cm (up to 10 mM). The nanofluidic membrane also exhibits excellent mechanical flexibility, demonstrating stable performance even when the membrane is folded 150°. Combining the inherent advan- tages of cellulose, this novel class of membrane offers an environmentally responsible strategy for flexible and printable nanofluidic applications. INTRODUCTION groups (32), the charged cellulose nanofiber surface can attract layers Ion-regulating nanofluidic membranes have been intensively used in of counterionsadjacenttothe fibers, with an exponentially decaying desalination (1, 2), osmosis energy generation (3, 4), ion/molecular ion concentration toward the center of the channel (Fig. 1D). The separation (5–7), and ionic circuits (8–12). Because of the interactions interface-dominated electrostatic field surrounding the cellulose nano- between solvated ions and the inner channel walls, ion transport in a fibers provides surface charge–governed ion transport along the fiber nanoscale-confined, surface-charged membrane substantially differs direction, enabling desirable ionic separation. from bulk behavior (13–15). High ionic conductivity, which can be The surface charge and geometry of the nanochannels can also be achieved through the nanofluidic effect, is essential for applications such easily tuned to modify the ionic conductivity of the membrane. Owing as ionic circuitry and nanofluidic membranes. Typically, silicon-based to the abundance of the functional groups on the cellulose nanofibers, materials are fabricated via lithography or templating to form aligned the surface charge density can be tuned via chemical stimuli. In this −2 one-dimensional nanoscale channels for nanofluidic membranes, but work, we demonstrate a high surface charge density of −5.7 mC m the material is often brittle or suffers from performance degradation after converting the hydroxyl groups to carboxyl groups, which was upon bending or folding (8, 16), rendering it challenging for flexible greater than previously reported values (8, 13, 20, 21, 33). In addition, applications. Cost-effective nanoporous polymer membranes have also we were able to attain large tunability of the channel size up to an order found great success in the industry, but these materials are not sustain- of magnitude. In this manner, a high surface charge–governed ionic −2 able, and the three-dimensional tortuous nanoporous structure limits conductivity of ~2 mS cm was observed at a KCl concentration of less −2 the ionic conductivity (17, 18). Two-dimensional materials, such as than 10 M. boron nitride, graphene, and MoS , have shown excellent ionic con- Figure 2 (A and B) shows various configurations of the cellulose ductivity and advantageous properties for use in nanofluidic devices, membranes after lignin and hemicellulose removal of the natural wood. but these materials require time-consuming and expensive bottom- When dried in air under ambient temperature, a densified structure can up fabrication methods (14, 19–22). Consequently, advancing tech- be obtained with closely packed cellulose nanofibers, potentially due to nologies that rely on efficient ion regulation necessitate the continued hydrogen bonding and van der Waals forces (34). Scanning electron search for materials with high-performance nanoscale channels that can microscopy (SEM)imagesinFig.2(C andD)showthe parallel-packed be obtained on a massive scale. cellulose fibers aligned with the wood growth direction, while Fig. 2E Cellulose is the most abundant and sustainable material in nature shows a view of the cellulose nanofiber ends. The hydrophilic and highly and is an attractive candidate for a wide range of applications, especially aligned cellulose nanofibers render the membrane highly efficient in those related to eco-friendly membranes (23, 24) and fluidic devices fluidic transport. To demonstrate, one end of a sample 2 cm by 1 cm by (25–30). Here, we demonstrate highly efficient and tunable ion regula- 200 mm in size was immersed into a water solution colored with ink to −1 tion using a cellulose membrane that is composed of aligned nano- measure the liquid uptake. A fluidic rate of >1 mm s was observed channels. These cellulose nanofibers are exposed after extraction of (fig. S1), twice higher than the previously demonstrated values obtained −1 intertwined lignin and hemicellulose from the natural wood (Fig. 1, with randomly oriented cellulose fibers (0.5 mm s )(25, 35). Small- AtoC)(31). Because of the dissociation of the surface functional angle x-ray scattering (SAXS) verified the alignment of the cellulose membrane down to the molecular level, in which an elliptical pattern was observed for the cellulose membranes both without (dry) and Department of Materials Science and Engineering, University of Maryland Col- lege Park, College Park, MD 20742, USA. Department of Physics, Yale University, with (wet) water (Fig. 2F). The dry membrane appears white, which New Haven, CT 06511, USA. Department of Physics and Astronomy, University of is indicative of the complete removal of lignin (fig. S2) (31). Upon California, Irvine, Irvine, CA 92697, USA. Departments of Electrical Engineering immersion in water, a broadband transmittance of >65% was ob- and Applied Physics, Yale University, New Haven, CT 06520, USA. tained from 400 to 1100 nm (Fig. 2G), and letters of the text beneath *Corresponding author. Email: binghu@umd.edu Li et al., Sci. Adv. 2019;5 : eaau4238 22 February 2019 1of6 | SCIENCE ADVANCES RESEARCH ARTICLE Fig. 1. Ion transport within the aligned cellulose nanofibers. (A) A tree trunk containing cellulose nanofibers. (B) Schematic of the removal of intertwined lignin and hemicellulose from natural wood to make the nanofluidic membrane. (C) A nanofluidic membrane that inherits the nanofiber alignment direction from natural wood, as marked. (D) Schematic of the low tortuosity nanofluidic membrane. Photo Credit: T.L., University of Maryland, College Park. Permission granted. Fig. 2. Characterizations of the delignified wood. Photos of various configurations of the (A) nanofluidic cellulose membrane, including (B) a cellulose cable wrapped around a rod. The arrows indicate the nanofiber alignment direction. Scale bars, 1 cm. (C) Side view SEM image of the cellulose membrane and (D) the aligned cellulose nanofibers at higher magnification. (E) Top view SEM image showing the tips of the cellulose nanofibers. (F) The elliptical shape of the diffraction pattern in SAXS for the dry and wet membrane, indicating the molecular level alignment of cellulose. (G) The transmittance of the dry and wet cellulose membrane. Li et al., Sci. Adv. 2019;5 : eaau4238 22 February 2019 2of6 | SCIENCE ADVANCES RESEARCH ARTICLE the membrane became visible. The transparency change is mainly in which s is the surface charge, e is the dielectric constant, e is the attributed to the matched optical refractive indexes of cellulose (1.48) permittivity of vacuum, z is the zeta potential, and l is the Debye −2 (36) and water (typically ~1.50). length, which was −3.2 and −5.7 mC m for the as-made cellulose We investigated the nanofluidic performance of the cellulose and oxidized cellulose, respectively (38). With the estimate of the surface membrane using the ionic conductivity setup shown in Fig. 3A (see charge for these samples, the overall conductivity trend can be fitted Methods). Carboxyl groups have a greater tendency to dissociate into using the following equation negatively charged carboxylates (32), leading to a higher charge den- sity and therefore a higher negative zeta potential in deionized water. k ¼ Zeðm þ m ÞCN þ 2sm =h ð2Þ þ  þ Therefore, using a previously described method (37), we applied TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) oxidation to the cellu- in which Z is the cation valence, m is the cation mobility, m is the anion lose to convert the primary hydroxyl groups on the surface chains of + − mobility, C is the ion concentration, N is the Avogadro’snumber, and the cellulose crystallites into carboxyl groups (37), as illustrated in h is the channel diameter of the nanofluidic cellulose system (39). As Fig. 3B. The resulting oxidized cellulose membrane exhibited a high- shown in Fig. 3C, the fit of Eq. 2 agrees well with the experimental data er zeta potential of −78 mV, compared with −45 mV for just delignified of the ionic conductivity versus KCl concentration and allows us to cal- cellulose (Fig. 3b). The respective ionic conductivity of these materials in culate a channel diameter of ~2 nm for both cellulose membranes. We KCl solutions is shown in Fig. 3C. The ion transport behavior in both attribute the difference in the value of the ionic conductivity plateaus for cellulose membranes exhibited a conductivity plateau orders of mag- each membrane to the difference of the surface charge densities between nitude higher than that of the bulk solution for concentrations below −2 the materials. ~10 M. Within the surface-governed ion transport region, a conduc- −1 tivity as high as ~2 mS cm was obtained for the oxidized membrane We further demonstrated the effect of channel geometry on the ionic −1 compared with 1.1 mS cm for the unmodified counterpart, indicating conductivity by preparing an undensified cellulose membrane (see the effectiveness of modifying the surface functional groups to tune the Methods) and comparing its performance with the densified sample. iontransport behavior.Using Eq.1,weestimated thesurface charge of The undensified samples exhibited an ionic conductivity plateau of −1 the membranes based on the zeta potential 0.2 mS cm , one order lower than that of the densified sample (Fig. 3D). The fit of the undensified cellulose membrane conductivity results indicated a channel diameter of around 20 nm, which is about 10 times s ¼ ee z=l ð1Þ 0 d larger than that of the densified cellulose membrane (channel diameter Fig. 3. Ionic conductivity measurement with chemical modifications and physical densifications. (A) Ionic conductivity measurement setup. (B) Zeta potential of the cellulose fibers and oxidized/surface-charged cellulose under neutral pH with a concentration of cellulose approximately 0.1%. (C) An ionic conductivity test with KCl solution for the cellulose membrane before and after oxidization. The oxidized cellulose exhibits an increased ionic conductivity plateau due to the higher surface charge. (D) Ionic conductivity of the undensified cellulose and densified cellulose membrane in KCl solution. Li et al., Sci. Adv. 2019;5 : eaau4238 22 February 2019 3of6 | SCIENCE ADVANCES RESEARCH ARTICLE ~2 nm). The mechanical properties of the densified and the undensified gate, which will contribute to a large cationic current density. Mean- samples are shown in fig. S3. We obtained the tensile strengths of while, positive gating will repel K and lead to an even lower current 58 ± 5 MPa and 9 ± 2 MPa for the densified and the undensified density than the neutral gating condition (Fig. 4B). Figure 4C shows samples, respectively. the ionic currents measured under different gating potentials from −2 To explore the use of this cellulose nanofiber membrane as an to 2 V. The ion conductivity under V = −2 V was about one order of ion regulation device, we demonstrated the ionic rectification effect magnitude higher than the value under V = 2 V and equivalent to −2 of the material acting as a flexible transistor with electrical gating, in that of 10 M KCl, indicating an efficient accumulation of positive which the cellulose membrane can preferentially accumulate ions that ions with negative gating (Fig. 4D). The device exhibits a negligible have the opposite charge as the channel walls. Silver paste was painted electrical gate leakage current, which was measured to be below the −6 on the membrane to act as the gating metal (Fig. 4A), and a 10 MKCl noise floor of the Keithley 2400. solution was used as the liquid electrolyte. The gating voltage was The cellulose membrane is flexible and even foldable. Figure 4E controlled by a Keithley 2400 power source, while the ionic current– shows a ribbon of the membrane that can be twisted and wrapped voltage characteristics were recorded. When the gating voltage was neg- around a finger. To observe how folding affects the ionic conductivity ative, the local concentration of K should further increase under the performance, we used a membrane 2 cm by 2 mm by 1 mm in size and Fig. 4. A cellulose-based ionic transistor. (A) Schematic of a freestanding cellulose nanofluidic transistor with a painted metal contact for gating. (B) Schematic of the gating effect on the ion distribution within the cellulose membrane. (C) Current-voltage characteristics of the cellulose nanofiber membrane with different gating voltages from −2 to 2 V. (D) Characterization of the transistor using varied gating voltages. Inset: Semi-log plot of ionic current versus gating voltage. (E) Photo image of a flexible and biocompatible cellulose nanofiber ribbon. (F) Ion conductivity shows minimal change upon folding. Li et al., Sci. Adv. 2019;5 : eaau4238 22 February 2019 4of6 | SCIENCE ADVANCES RESEARCH ARTICLE recorded the current under an applied voltage of 0.5 V as we folded the 0.5 M HCl. Last, the cellulose membrane was solvent exchanged to −6 material. The ionic conductivity under a concentration of 10 MKCl acetone and then toluene and allowed to sit in air to evaporate all exhibited minimal changes upon folding, with no notable performance solvent. degradation for a folding angle of up to 150° (Fig. 4F). We have also evaluated the membrane’s stability in conductivity and mechanical Characterization strength (figs. S4 and S5). After several wet-dry cycles, the membrane An SEM (Hitachi SU-70) was used to characterize the morphologies of shows no notable degradation of the performance. the samples. Compositional analysis of the cellulose membrane and original wood was carried out using high-performance liquid chroma- tography (Ultimate 3000; Thermo Fisher Scientific, USA). The optical DISCUSSION transmittance from 400 to 1100 nm was characterized by ultraviolet- In this work, we report a permselective nanofluidic membrane featuring visible absorption spectroscopy (Lambda 35; PerkinElmer, USA) with aligned cellulose nanofibers directly derived from wood. The removal of an integrating sphere to collect all transmitted light. To determine the lignin and hemicellulose introduces numerous open nanochannels zeta potential of the original and TEMPO-oxidized white wood, the among the cellulose nanofibers. Via structural engineering, the diameter samples were dispersed in deionized water by ultrasonication treatment of the nanochannels can be tuned from 2 to 20 nm. A high surface (100% amplitude, 10 min, Fisher Scientific FS 110D) to produce a 0.1 wt −2 charge density of −5.7 mC m was obtained after chemical modifica- % (weight %) concentration. Zeta potential (z) of the dispersed suspen- tion of the cellulose functional groups. The long-range ordered arrays sion was measured using a Zetasizer Nano S90 (Malvern Instrument) of the charged nanofibers led to highly efficient ion transport in the without adjusting the ionic strength. The z value was calculated from −1 centimeter-long devices. A high ionic conductivity plateau of ~2 mS cm the electrophoretic mobility using the Henry equation and Huckel ap- was obtained, which is orders of magnitude higher than that of the bulk proximation. The tensile strength of the wood was measured with a TA KCl solution at the same concentrations. In addition, we used electrostatic Q800 DMA system in controlled-force mode. The force increased from −6 gating to further increase the ionic conductivity of 10 M KCl electrolyte 1 up to 18 N at 0.01 N/min. −2 to a value that is equivalent to the ionic conductivity of 10 MKCl.This work demonstrates the use of wood-derived cellulose membranes toward Current voltage measurement foldable, scalable, and high-performance nanofluidic devices. A cellulose membrane was embedded into polydimethylsiloxane elastomer with two wells carved on either side to serve as reservoirs for the electrolyte solution. The cellulose nanofluidic membrane MATERIALS AND METHODS was immersed in electrolyte for at least 1 day before measurement. Materials and chemicals Two homemade Ag/AgCl electrodes were inserted into the reser- American Basswood was purchased from Walnut Hollow Company. voirs. BioLogic from Science Instrument was used to record the Sodium hydroxide (NaOH) and sodium sulfite (Na SO ) (both pur- current signal while providing the voltage source at a scanning rate 2 3 −1 chased from Carolina Biological Supply Company) and hydrogen per- of 10 mV s . oxide (H O , 30% solution; EMD Millipore Corporation) were used for 2 2 lignin extraction of the Basswood. The deionized water (type I deionized water; purity, >18 megohm·cm) from ChemWorld was used to further SUPPLEMENTARY MATERIALS remove the residual ions from the delignified wood. Supplementary material for this article is available at http://advances.sciencemag.org/cgi/ content/full/5/2/eaau4238/DC1 Cellulose membrane preparation Fig. S1. Highly hydrophilic cellulose demonstrating directional fluidic transport. Fig. S2. Composition results of the natural wood and the wood membrane after step I and II Basswood was cut along the growth direction at various dimensions treatments. (typical thickness, <5 mm; length/width, <10 cm). NaOH and Na SO 2 3 Fig. S3. The mechanical tensile strength for the densified and undensified samples. were dissolved in deionized water at concentrations of 2.5 and 0.4 M, −5 Fig. S4. Current-voltage characteristics of the membrane infiltrated multiple times with 10 M respectively. The wood slices were boiled in the solution for 10 hours. KCl solution. Then, the wood slices were immersed in boiling H O solution (30%) Fig. S5. Tensile strength of the membranes that underwent different dry-wet cycles. 2 2 until completely white. The resulting wood membrane was then rinsed in deionized water to remove the residual ions and chemicals. The ef- fective removal of residual chemicals was verified by obtaining a low −1 REFERENCES AND NOTES ionic conductivity of the solution (<20 mScm ) that had been used 1. S. J. Kim, S. H. Ko, K. H. Kang, J. Han, Direct seawater desalination by ion concentration to wash the sample. 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