1Department of Materials Science and Engineering, University of Maryland Col-lege Park, College Park, MD 20742, USA. 2Department of Physics, Yale University,New Haven, CT 06511, USA. 3Department of Physics and Astronomy, University ofCalifornia, Irvine, Irvine, CA 92697, USA. 4Departments of Electrical Engineeringand Applied Physics, Yale University, New Haven, CT 06520, USA.*Corresponding author. Email: [email protected]
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A nanofluidic ion regulation membrane with alignedcellulose nanofibersTian Li1, Sylvia Xin Li2, Weiqing Kong1, Chaoji Chen1, Emily Hitz1, Chao Jia1, Jiaqi Dai1, Xin Zhang1,Robert Briber1, Zuzanna Siwy3, Mark Reed4, Liangbing Hu1*
The advancement of nanofluidic applications will require the identification of materials with high-conductivitynanoscale channels that can be readily obtained at massive scale. Inspired by the transpiration in mesostruc-tured trees, we report a nanofluidic membrane consisting of densely packed cellulose nanofibers directlyderived 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 bystructural engineering. The surface-charge-governed ionic transport region shows a high ionic conductivity pla-teau of ~2 mS cm−1 (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 andprintable nanofluidic applications.
INTRODUCTIONIon-regulating nanofluidic membranes have been intensively used indesalination (1, 2), osmosis energy generation (3, 4), ion/molecularseparation (5–7), and ionic circuits (8–12). Because of the interactionsbetween solvated ions and the inner channel walls, ion transport in ananoscale-confined, surface-charged membrane substantially differsfrom bulk behavior (13–15). High ionic conductivity, which can beachieved through the nanofluidic effect, is essential for applications suchas ionic circuitry and nanofluidic membranes. Typically, silicon-basedmaterials are fabricated via lithography or templating to form alignedone-dimensional nanoscale channels for nanofluidic membranes, butthe material is often brittle or suffers from performance degradationupon bending or folding (8, 16), rendering it challenging for flexibleapplications. Cost-effective nanoporous polymer membranes have alsofound great success in the industry, but these materials are not sustain-able, and the three-dimensional tortuous nanoporous structure limitsthe ionic conductivity (17, 18). Two-dimensional materials, such asboron nitride, graphene, and MoS2, have shown excellent ionic con-ductivity and advantageous properties for use in nanofluidic devices,but these materials require time-consuming and expensive bottom-up fabrication methods (14, 19–22). Consequently, advancing tech-nologies that rely on efficient ion regulation necessitate the continuedsearch formaterials with high-performance nanoscale channels that canbe obtained on a massive scale.
Cellulose is the most abundant and sustainable material in natureand is an attractive candidate for a wide range of applications, especiallythose related to eco-friendly membranes (23, 24) and fluidic devices(25–30). Here, we demonstrate highly efficient and tunable ion regula-tion using a cellulose membrane that is composed of aligned nano-channels. These cellulose nanofibers are exposed after extraction ofintertwined lignin and hemicellulose from the natural wood (Fig. 1,A to C) (31). Because of the dissociation of the surface functional
groups (32), the charged cellulose nanofiber surface can attract layersof counter ions adjacent to the fibers, with an exponentially decayingion concentration toward the center of the channel (Fig. 1D). Theinterface-dominated electrostatic field surrounding the cellulose nano-fibers provides surface charge–governed ion transport along the fiberdirection, enabling desirable ionic separation.
The surface charge and geometry of the nanochannels can also beeasily tuned to modify the ionic conductivity of the membrane. Owingto the abundance of the functional groups on the cellulose nanofibers,the surface charge density can be tuned via chemical stimuli. In thiswork, we demonstrate a high surface charge density of −5.7 mC m−2
after converting the hydroxyl groups to carboxyl groups, which wasgreater than previously reported values (8, 13, 20, 21, 33). In addition,we were able to attain large tunability of the channel size up to an orderof magnitude. In this manner, a high surface charge–governed ionicconductivity of ~2mS cm−2 was observed at a KCl concentration of lessthan 10−2 M.
Figure 2 (A and B) shows various configurations of the cellulosemembranes after lignin and hemicellulose removal of the natural wood.When dried in air under ambient temperature, a densified structure canbe obtained with closely packed cellulose nanofibers, potentially due tohydrogen bonding and van der Waals forces (34). Scanning electronmicroscopy (SEM) images in Fig. 2 (C and D) show the parallel-packedcellulose fibers aligned with the wood growth direction, while Fig. 2Eshows a viewof the cellulose nanofiber ends. The hydrophilic and highlyaligned cellulose nanofibers render the membrane highly efficient influidic transport. To demonstrate, one end of a sample 2 cm by 1 cm by200 mm in size was immersed into a water solution colored with ink tomeasure the liquid uptake. A fluidic rate of >1 mm s−1 was observed(fig. S1), twice higher than the previously demonstrated values obtainedwith randomly oriented cellulose fibers (0.5 mm s−1) (25, 35). Small-angle x-ray scattering (SAXS) verified the alignment of the cellulosemembrane down to themolecular level, in which an elliptical patternwas observed for the cellulose membranes both without (dry) andwith (wet) water (Fig. 2F). The dry membrane appears white, whichis indicative of the complete removal of lignin (fig. S2) (31). Uponimmersion in water, a broadband transmittance of >65% was ob-tained from 400 to 1100 nm (Fig. 2G), and letters of the text beneath
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Fig. 1. Ion transport within the aligned cellulose nanofibers. (A) A tree trunk containing cellulose nanofibers. (B) Schematic of the removal of intertwined lignin andhemicellulose from natural wood to make the nanofluidic membrane. (C) A nanofluidic membrane that inherits the nanofiber alignment direction from natural wood, asmarked. (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 cablewrapped 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) thealigned cellulose nanofibers at higher magnification. (E) Top view SEM image showing the tips of the cellulose nanofibers. (F) The elliptical shape of the diffractionpattern 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.
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the membrane became visible. The transparency change is mainlyattributed to the matched optical refractive indexes of cellulose (1.48)(36) and water (typically ~1.50).
We investigated the nanofluidic performance of the cellulosemembrane using the ionic conductivity setup shown in Fig. 3A (seeMethods). Carboxyl groups have a greater tendency to dissociate intonegatively charged carboxylates (32), leading to a higher charge den-sity and therefore a higher negative zeta potential in deionized water.Therefore, using a previously described method (37), we appliedTEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) oxidation to the cellu-lose to convert the primary hydroxyl groups on the surface chains ofthe cellulose crystallites into carboxyl groups (37), as illustrated inFig. 3B. The resulting oxidized cellulose membrane exhibited a high-er zeta potential of −78mV, comparedwith−45mV for just delignifiedcellulose (Fig. 3b). The respective ionic conductivity of thesematerials inKCl solutions is shown in Fig. 3C. The ion transport behavior in bothcellulose membranes exhibited a conductivity plateau orders of mag-nitude higher than that of the bulk solution for concentrations below~10−2 M. Within the surface-governed ion transport region, a conduc-tivity as high as ~2 mS cm−1 was obtained for the oxidized membranecompared with 1.1 mS cm−1 for the unmodified counterpart, indicatingthe effectiveness of modifying the surface functional groups to tune theion transport behavior. Using Eq. 1, we estimated the surface charge ofthe membranes based on the zeta potential
s ¼ ee0z=ld ð1Þ
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in which s is the surface charge, e is the dielectric constant, e0 is thepermittivity of vacuum, z is the zeta potential, and ld is the Debyelength, which was −3.2 and −5.7 mC m−2 for the as-made celluloseandoxidized cellulose, respectively (38).With the estimate of the surfacecharge for these samples, the overall conductivity trend can be fittedusing the following equation
k ¼ Zeðmþþ m�ÞCNA þ 2smþ=h ð2Þ
in whichZ is the cation valence, m+ is the cationmobility, m− is the anionmobility, C is the ion concentration,NA is the Avogadro’s number, andh is the channel diameter of the nanofluidic cellulose system (39). Asshown in Fig. 3C, the fit of Eq. 2 agrees well with the experimental dataof the ionic conductivity versus KCl concentration and allows us to cal-culate a channel diameter of ~2 nm for both cellulose membranes. Weattribute the difference in the value of the ionic conductivity plateaus foreachmembrane to the difference of the surface charge densities betweenthe materials.
We further demonstrated the effect of channel geometry on the ionicconductivity by preparing an undensified cellulose membrane (seeMethods) and comparing its performance with the densified sample.The undensified samples exhibited an ionic conductivity plateau of0.2 mS cm−1, one order lower than that of the densified sample (Fig.3D). The fit of the undensified cellulose membrane conductivity resultsindicated a channel diameter of around 20 nm, which is about 10 timeslarger 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 ofthe cellulose fibers and oxidized/surface-charged cellulose under neutral pH with a concentration of cellulose approximately 0.1%. (C) An ionic conductivity test withKCl solution for the cellulose membrane before and after oxidization. The oxidized cellulose exhibits an increased ionic conductivity plateau due to the higher surfacecharge. (D) Ionic conductivity of the undensified cellulose and densified cellulose membrane in KCl solution.
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~2 nm). Themechanical properties of the densified and the undensifiedsamples are shown in fig. S3. We obtained the tensile strengths of58 ± 5 MPa and 9 ± 2 MPa for the densified and the undensifiedsamples, respectively.
To explore the use of this cellulose nanofiber membrane as anion regulation device, we demonstrated the ionic rectification effectof the material acting as a flexible transistor with electrical gating, inwhich the cellulose membrane can preferentially accumulate ions thathave the opposite charge as the channel walls. Silver paste was paintedon themembrane to act as the gatingmetal (Fig. 4A), and a 10−6MKClsolution was used as the liquid electrolyte. The gating voltage wascontrolled by a Keithley 2400 power source, while the ionic current–voltage characteristics were recorded.When the gating voltage was neg-ative, the local concentration of K+ should further increase under the
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gate, which will contribute to a large cationic current density. Mean-while, positive gating will repel K+ and lead to an even lower currentdensity than the neutral gating condition (Fig. 4B). Figure 4C showsthe ionic currents measured under different gating potentials from −2to 2 V. The ion conductivity under Vg = −2 V was about one order ofmagnitude higher than the value under Vg = 2 V and equivalent tothat of 10−2 M KCl, indicating an efficient accumulation of positiveions with negative gating (Fig. 4D). The device exhibits a negligibleelectrical gate leakage current, which was measured to be below thenoise floor of the Keithley 2400.
The cellulose membrane is flexible and even foldable. Figure 4Eshows a ribbon of the membrane that can be twisted and wrappedaround a finger. To observe how folding affects the ionic conductivityperformance, 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 thegating effect on the ion distribution within the cellulose membrane. (C) Current-voltage characteristics of the cellulose nanofiber membrane with different gatingvoltages 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 imageof a flexible and biocompatible cellulose nanofiber ribbon. (F) Ion conductivity shows minimal change upon folding.
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recorded the current under an applied voltage of 0.5 V as we folded thematerial. The ionic conductivity under a concentration of 10−6 M KClexhibited minimal changes upon folding, with no notable performancedegradation for a folding angle of up to 150° (Fig. 4F). We have alsoevaluated the membrane’s stability in conductivity and mechanicalstrength (figs. S4 and S5). After several wet-dry cycles, the membraneshows no notable degradation of the performance.
DISCUSSIONIn thiswork, we report a permselective nanofluidicmembrane featuringaligned cellulose nanofibers directly derived fromwood. The removal oflignin and hemicellulose introduces numerous open nanochannelsamong the cellulose nanofibers. Via structural engineering, the diameterof the nanochannels can be tuned from 2 to 20 nm. A high surfacecharge density of −5.7 mC m−2 was obtained after chemical modifica-tion of the cellulose functional groups. The long-range ordered arraysof the charged nanofibers led to highly efficient ion transport in thecentimeter-long devices. Ahigh ionic conductivity plateau of ~2mS cm−1
was obtained, which is orders of magnitude higher than that of the bulkKCl solution at the sameconcentrations. In addition,weused electrostaticgating to further increase the ionic conductivity of 10−6MKCl electrolyteto a value that is equivalent to the ionic conductivity of 10−2MKCl. Thiswork demonstrates the use of wood-derived cellulosemembranes towardfoldable, scalable, and high-performance nanofluidic devices.
MATERIALS AND METHODSMaterials and chemicalsAmerican Basswood was purchased from Walnut Hollow Company.Sodium hydroxide (NaOH) and sodium sulfite (Na2SO3) (both pur-chased from Carolina Biological Supply Company) and hydrogen per-oxide (H2O2, 30% solution; EMDMillipore Corporation) were used forlignin extraction of theBasswood. The deionizedwater (type I deionizedwater; purity, >18 megohm·cm) from ChemWorld was used to furtherremove the residual ions from the delignified wood.
Cellulose membrane preparationBasswood was cut along the growth direction at various dimensions(typical thickness, <5 mm; length/width, <10 cm). NaOH and Na2SO3
were dissolved in deionized water at concentrations of 2.5 and 0.4 M,respectively. The wood slices were boiled in the solution for 10 hours.Then, the wood slices were immersed in boiling H2O2 solution (30%)until completely white. The resulting wood membrane was then rinsedin deionized water to remove the residual ions and chemicals. The ef-fective removal of residual chemicals was verified by obtaining a lowionic conductivity of the solution (<20 mS cm−1) that had been usedto wash the sample. To make the undensified membrane, the samplewas subsequently treated by freeze drying. To make a densified mem-brane, the sample was dried in air under ambient temperature whilebeing pressed for 5 hours. For typical TEMPO oxidation, the cellulosemembrane (1 g) was soaked in 100 ml of water containing 0.016 g ofTEMPO and 0.1 g of NaBr, with the pH value being adjusted to 10 byadding 0.5 M NaOH solution. Approximately 5 mmol NaClO wasadded to the solution to initiate the oxidation reaction. The pH valueof the solution was closely monitored using a pHmeter and was main-tained at 10 by continuously adding 0.5 M NaOH. The reaction wasterminated after 2.5 hours by adding 0.5 M HCl to lower the pH to 7.The membrane was then taken out of the solution and washed with
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0.5 M HCl. Last, the cellulose membrane was solvent exchanged toacetone and then toluene and allowed to sit in air to evaporate allsolvent.
CharacterizationAn SEM (Hitachi SU-70) was used to characterize the morphologies ofthe samples. Compositional analysis of the cellulose membrane andoriginal wood was carried out using high-performance liquid chroma-tography (Ultimate 3000; Thermo Fisher Scientific, USA). The opticaltransmittance from 400 to 1100 nm was characterized by ultraviolet-visible absorption spectroscopy (Lambda 35; PerkinElmer, USA) withan integrating sphere to collect all transmitted light. To determine thezeta potential of the original and TEMPO-oxidized white wood, thesamples were dispersed in deionized water by ultrasonication treatment(100% amplitude, 10min, Fisher Scientific FS 110D) to produce a 0.1wt% (weight %) concentration. Zeta potential (z) of the dispersed suspen-sion was measured using a Zetasizer Nano S90 (Malvern Instrument)without adjusting the ionic strength. The z value was calculated fromthe electrophoretic mobility using the Henry equation and Huckel ap-proximation. The tensile strength of the wood was measured with a TAQ800DMA system in controlled-force mode. The force increased from1 up to 18 N at 0.01 N/min.
Current voltage measurementA cellulose membrane was embedded into polydimethylsiloxaneelastomer with two wells carved on either side to serve as reservoirsfor the electrolyte solution. The cellulose nanofluidic membranewas immersed in electrolyte for at least 1 day before measurement.Two homemade Ag/AgCl electrodes were inserted into the reser-voirs. BioLogic from Science Instrument was used to record thecurrent signal while providing the voltage source at a scanning rateof 10 mV s−1.
SUPPLEMENTARY MATERIALSSupplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/2/eaau4238/DC1Fig. S1. Highly hydrophilic cellulose demonstrating directional fluidic transport.Fig. S2. Composition results of the natural wood and the wood membrane after step I and IItreatments.Fig. S3. The mechanical tensile strength for the densified and undensified samples.Fig. S4. Current-voltage characteristics of the membrane infiltrated multiple times with 10−5 MKCl solution.Fig. S5. Tensile strength of the membranes that underwent different dry-wet cycles.
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Acknowledgments: We acknowledge the support of the Maryland NanoCenter and itsAIMLab. We acknowledge the help from J. Y. Zhu at the U.S. Forest Products Lab on thecompositional analysis of original and delignified wood. Funding: This work was supportedas part of the Nanostructures for Electrical Energy Storage (NEES), an Energy FrontierResearch Center funded by the U.S. Department of Energy, Office of Science, and Basic EnergySciences under award number DESC0001160. Author contributions: T.L. and L.H. designedthe experiments. T.L., S.X.L., Z.S., and M.R. carried out the data analysis of ionic conductivity.T.L., W.K., C.C., E.H., C.J., and J.D. carried out the performance measurements and materialcharacterization. X.Z. and R.B. carried out the mechanical tests and structural characterization.All authors contributed to the writing of the manuscript. Competing interests: T.L. andL.H. are the inventors on a patent currently pending with the University of Maryland (no. 62/789325, filed 7 January 2019). All the other authors declare that they have no competinginterests. Data and materials availability: All data needed to evaluate the conclusions in thepaper are present in the paper and/or the Supplementary Materials. Additional data related tothis paper may be requested from the authors.
Submitted 11 June 2018Accepted 11 January 2019Published 22 February 201910.1126/sciadv.aau4238
Citation: T. Li, S. X. Li, W. Kong, C. Chen, E. Hitz, C. Jia, J. Dai, X. Zhang, R. Briber, Z. Siwy,M. Reed, L. Hu, A nanofluidic ion regulation membrane with aligned cellulose nanofibers.Sci. Adv. 5, eaau4238 (2019).
