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Anomalous temperature dependence of ion transport under osmotic pressure in graphene oxide membranes

Anomalous temperature dependence of ion transport under osmotic pressure in graphene oxide membranes theauthor(s)andthetitle ofthework,journal Grapheneoxide(GO)membraneshaveattractedbroadinterestbecauseoftheiruniquemass citationandDOI. transportproperties.TowardsthecontrollableionictransportinGOmembranes,physicalfieldsor externaldrivingforcesareinducedtocontrolthebehaviorofionicmigrationinsitu.However,the adjustableionictransportregulatedbytemperatureandosmoticpressureinGOmaterialsisstill absent.Herein,wereporttheanomaloustemperaturedependenceofiontransportunderosmotic pressureinGOmembranes.Theionscandiffusespontaneouslyalongtheconcentrationgradient orthetemperaturegradient.Intriguingly,itisfoundthatthereversetemperaturedifferencecan promoteiontransportdrivenbyosmoticpressure.Theoreticalanalysisrevealsthattheanomalous temperaturedependenceofiontransportstemsfromthethermal-diffusion-assistedion concentrationpolarization(ICP).Thehightemperatureinthelow-concentrationsidelargely enhancestheionicthermaldiffusionandsuppressestheICP,whicheventuallystrengthenstheion currentalongtheconcentrationgradient.Thefindingcanbedevelopedintothetemperature sensorforaqueoussolutionsandbringinspirationtotheapplicationinvolvingiontransportunder thermodynamicandosmoticdrivenforces. 1.Introduction between2Dlamella,providingbothexcellentselectiv- ity and fast permeation [23]. The great effort is Synthetic nanoporous membranes have attracted takentocontroltheiontransportinGOmembranes, broad interest because they boost the development including the interlayer spacing, the elemental com- of DNA sequencing [1–3], chemical sensing and positionsuchashydrogenandthepresenceandrelat- delivery [4–6], ion filtration [7, 8], pervaporation iveratioofoxygenfunctionalgroups,thepreparation [9–12], energy conversion [13–16] and nanoelec- method of GO membranes [24–26]. Besides, more tronics [17–19]. The interest stems from the unique materialpropertieshavebeenexploredwidelyasone mass transport properties very different from the of the crucial factors to regulate the ionic transport counterpart of the macro scale [20, 21]. Particularly, properties. It is reported that the broken symmetry the newly emerged 2D materials are considered to of nanofluidic systems, including the geometry [27], be candidates for next generation high-performance surfacechargedistributionorchemicalmodification membrane materials due to their atomic thickness, [28], nanofluidic heterojunction [29, 30] can lead to excellent thermal and chemical stability, outstand- a preferential ionic transport direction through the ing flexibility and solution processability [22]. More nanochannel,whichissimilartotheactivetransport significantly, graphene and its derivatives, includ- acrosstheionchannelsonthecellmembrane[31]. ing graphene oxide (GO) and chemically conver- Although the improvement of the materials tedgraphene,cangeneratehighlyorderedcapillaries endowsabundantpropertyofnanofluidicsystems,it ©2022TheAuthor(s). PublishedbyIOPPublishingLtd 2DMater.9(2022)045010 LLinetal is impossible to change the predefined ionic trans- furtherdevelopedanosmotic-energy-driventemper- port properties once the devices are fabricated [32]. aturesensorbasedonthismechanism.Thesefindings In this regard, several externally tunable approaches are beneficial to the design of advanced 2D nano- havebeenproposedsofar.Mostofthemchangeionic fluidic devices and related applications involving ion transport properties by regulating the nanochannel transportunderthermodynamicandosmoticdriven wall property triggered by external chemical stimuli, forces. such as temperature [33], pH value [34], enzymes [35] and polyvalent cations [36]. Contrary to the chemicalstimuli-responsivestrategies,morephysical 2.Resultsanddiscussion fields or external driving forces are induced to con- troltheionictransportbehaviorsinsitu.Forinstance, GO suspension was fabricated via improved Hum- Guanetalreportedafield-effectreconfigurablenan- mers [49, 50]. The x-ray diffraction (XRD) patterns ofluidicdiodethroughtheapplicationofgatevoltages of the GO membrane indicated that the interlayer which can enhance or diminish the ionic concentra- spacing was 0.92 nm (figure S1). The thickness of tions[32].Thetransmembraneconcentrationgradi- the GO membrane was characterized to be about ent, which results in the electroosmotic flow and 759 nm by field emission scanning electron micro- reversal potential, can regulate the preferred ionic scope (figure S2). Raman spectroscopy of GO mem- transport in both magnitude and direction [37, 38]. brane showed clear D-band and G-band peaks at −1 Thepressure-drivenflowcaneliminateioniccurrent 1328 and 1572 cm , respectively. And the rich rectification under an electrical field [39]. Recently, oxygen-containing functional groups of GO were Yang et al reported a coupled photon–electron–ion identified by Fourier transform infrared spectro- transport. Through asymmetric light illumination, scopy (figureS3). These abundant functional groups anti-gradient transport can be obtained [40]. From rendered the GO negative charge in aqueous solu- then on, the ionic transport driven by light coupled tions. The water contact angle of 36.07 indicated withosmoticpressureorappliedelectricfieldcatches the good hydrophilicity of GO membranes [51, 52] increasingresearchfocus[41–43]. (figure S4). The zeta potential characterization con- Ontheotherhand,temperature,aphysicalpara- firmed the stability of colloidal solutions in the pH meter directly related to the thermal motion of ions, rangeof2–12[53].ThesizeoftheGOsheetsranged can affect ion transport property as well. It is repor- from400to1000nm(figureS5).TheGOmembrane ted the temperature difference can induce the ther- wasmountedintworeservoirswithdifferentconcen- moelectric effect in ion-selective nanochannels or trationsofpotassiumchloridesolution.Acustomized porous materials [44, 45]. Nanofluidic reverse elec- rubberheatingsheetwasusedtocontrolthesolution trodialysisprocessisfoundtobethermaldependence temperature on one side to build a temperature dif- [46,47].Longetalpredictedtheup-diffusionthermal ferenceacrossthemembrane(figureS6). gradient can enhance the nanofluidic salinity- Theiondiffusiondrivenbytheconcentrationdif- gradientenergyharvestingbyanumericalsimulation ference or the temperature difference was along the approach [48]. It can be expected that temperature gradient direction (figures 1(a) and (b)). It is expec- can be applied in 2D materials to control the inter- ted that the osmotic ion transport can be promoted lamellarionictransport.However,relatedresearchis by applying a temperature gradient in the same dir- stillabsent. ection as the concentration gradient. Intriguingly, Herein, we report anomalous temperature- an unexpected enhancement of ion diffusion was dependent ionic transport under osmotic pressure observed when the reverse temperature difference in GO membranes. In contrast to the conventional (RTD) was applied together with the concentration viewpoint that the concentration gradient and the difference (figures 1(c) and (d)). For instance, the temperature gradient in the same direction will syn- diffusion current under the concentration difference ergistically increase the ion transport, we found that of 1 mM|300 mM KCl was 7.13 µA. When the low- thereversetemperaturegradientcangreatlypromote concentration (LC) solution was heated to 348 K, the osmotic-driven ionic transport in GO mem- the diffusion current increased to 11.30 µA, with branes. In particular, ion transport increases with an increased magnitude of 58.5%. In sharp con- the reverse temperature gradient. Through tuning trast,whenthehigh-concentration(HC)solutionwas themagnitudeanddirectionofappliedthermalcon- heated to 348 K, the diffusion current decreased to ditions, the transmembrane ionic transport can be 6.32 µA,withadropmagnitudeof11.4%.Themem- continuously regulated. This phenomenon can be branepotential(V )sharesasimilartrendwiththe mem observed under different concentration conditions, diffusion current (figure 1(e)). Accordingly, the high pHvaluesandelectrolytesolutions.Theoreticalana- outputpowercanbeachievedundertheRTD. lysis based on finite element numerical calculation The anomalous temperature dependence of ion reveals the mechanism of the anomalous temperat- transport can be observed under various concen- uredependenceofiontransportinnanochannels.We tration difference conditions of 1 mM|10 mM to 2 2DMater.9(2022)045010 LLinetal Figure1.Anomaloustemperaturedependenceofion transportunderosmoticpressureinGOmembranes.The diffusioncurrentandmembranepotentialdrivenbythe concentrationdifferenceandtemperaturedifferenceare (a)and(b),respectively.(c)Schematicillustrationof forwardtemperaturedifference(FTD),notemperature difference(NTD)andreversetemperaturedifference (RTD).Theconcentrationconditionwas1mM|300mM KCl.TheRTDenhancesdiffusioncurrent(d)and membranepotential.(e)Thecorrespondingtemperature conditionsareT =T =298K,T =348Kand HC LC HC T =298K,T =298KandT =348K,respectively. LC HC LC 1 mM|1000 mM. As shown in figure 2(a), the diffu- sion current increases with the concentration differ- ence under the conditions of no temperature differ- ence (NTD), forward temperature difference (FTD) and RTD. At all concentration gradients tested, RTD increasesthediffusioncurrentwhileFTDdecreasesit. Theeffectoftemperaturegradientonthemembrane potential is similar (figure 2(b)). The enhanced dif- fusion current and membrane potential in the RTD condition eventually promote the output power of Figure2.Theanomaloustemperaturedependenceofion transportcanbeobservedundervariousconcentration osmoticenergyconversion(figure2(c)). conditions,includingthediffusioncurrent(a),the Theperformanceofiontransportunderosmotic membranepotential(b),andtheoutputpower(c).The temperatureconditionsofNTD,FTDandRTDarethe pressure can be regulated by the direction and mag- sameasfigure1.KClsolutionconcentrationconditions nitudeoftemperaturedifference.Thetemperatureof rangefrom1mM|10mMto1mM|1000mM. one side was fixed to 298 K while the other side var- ied from 298 K to 348 K. As shown in figure 3(a), thehigherRTDcanpromoteiontransportdrivenby osmotic pressure, resulting in greater diffusion cur- The competition between temperature gradient rent and membrane potential. On the other hand, andconcentrationgradientleadstotheshiftbetween FTD weakens the diffusion current and membrane normal ionic transport and the anomalous temper- potential. In this regard, the output power can be aturedependence(figureS7).Forinstance,whenthe tuned by the direction and magnitude of the applied concentration difference is high (10 mM|300 mM), temperaturedifference.Figure3(b)illustratesthatthe theioniccurrentdecreaseswiththetemperaturedif- RTD of −50 K can improve the output power from ference obviously, which corresponds to the anom- 0.16 to 0.31 µW, with the magnitude up to 51.6%, alous temperature dependence. When the ion con- comparedwithNTD. centration difference reduces to 230 mM|300 mM, 3 2DMater.9(2022)045010 LLinetal Figure3.Theperformanceofiontransportunderosmotic pressurecanberegulatedbythedirectionandmagnitudeof theappliedtemperaturedifference,including(a)the diffusioncurrent,membranepotential,and(b)output power.Theappliedsolutionsconcentrationwas 1mM|300mMKCl.Thelow-temperaturesidewasfixedto 298K,whiletheothersidevariedfrom298Kto348K. Figure4.Numericalcalculation.(a)Simulationmodel.The lengthofnanochannelwas20nm,thediameterwas8nm, −2 andthesurfacechargedensitywas −0.06Cm .The the competition between temperature gradient and settingofthesolutionconcentrationwas1mM|300mM KCl.Thelow-temperaturesidewasfixedto298K,whilethe concentration gradient almost reaches balance. The othersidevariedfrom298Kto348K.Thecalculated ioniccurrentkeepsstablewiththeincrementoftem- diffusioncurrent(b),membranepotential(c)andoutput perature difference. Under the single driven force of power(d)sharethesametrendwiththeexperimental observations. temperaturedifference(300mM|300mM),ioniccur- rentsincreasewiththetemperaturedifference,corres- ponding to the normal ion diffusion driven by tem- peraturedifference. The anomalous temperature dependence of ion evidently promote ion transport driven by osmotic transportcanbeobservedindifferentpHvaluesand pressure. Meanwhile, FTD suppresses the ion trans- electrolyte solutions (NaCl and LiCl), as shown in port along the concentration gradient. These are in figuresS8andS9,respectively.Inaddition,theanom- agreement with the experimental results. Addition- alous temperature dependence of ion transport can ally, the slope of the curve of the diffusion cur- also be found in polymer membrane (figure S10). rent, membrane potential and output power imply Owingtotheexcellentselectivityandfastpermeation that the promoting effect of RTD is greater than stemmedfromhighlyorderedcapillariesinGOmem- the suppressing effect of FTD, which is also con- branes, the ionic current tested in GO membranes sistent with the experimental observations. The cal- is much larger than that of polyethylene terephthal- culation of the channel length (L) of 1000 nm also ate(PET)membranes.GOmembranesprovidegreat confirms this anomalous temperature dependence advantages for the possible applications to harvest (figureS12). the low-grade heat from the environment or indus- Thelocaldistributionofionconcentrationindic- trialwasteheatthroughcooperatingwiththeosmotic atesthatthethermal-diffusion-assistedconcentration energyconversionprocess. polarizationistheoriginoftheanomaloustemperat- We carried out the theoretical analysis based on uredependenceiniontransportunderosmoticpres- thecontinuitymodeltorevealtheoriginoftheanom- sure. As shown in figure 5(a), the ion concentration alous temperature dependence in osmotic ion trans- polarization (ICP) in the LC side is obviously differ- port. As shown in figure 4(a), the calculation model entfromtheHCside.TheRTDcansignificantlysup- includes two bulk reservoirs connected with a single presstheICPattheorificeoftheLCside.Thelocalion cylindrical nanochannel. The length of nanochannel concentrationsnearthelow-andHCentrancequant- (L)is20nm,thediameterofnanochannel(D)is8nm itatively illustrate the differences between the NTD, −2 and the surface charge density (σ) is −0.06 C m , FTD and RTD conditions (figures 5(b) and (c)). In consistent with the literature [21]. The setting of the particular, the ion concentrations of NTD, FTD and concentrationandthetemperatureconditionsinthe RTD in the HC sides are respectively 536, 546 and model is consistent with the experimental installa- 533 mM, with a difference of only 2%. For compar- tions. The detailed boundary conditions are shown ison, the difference is much more obvious on the LC infigureS11andtableS1.Thefiniteelementmethod side. Compared with NTD, the local ion concentra- was employed to solve the coupled Poisson–Nernst– tionintheLCsidewasfoundtobeenhancedby10.6% Planck (PNP), Navier–Stokes (NS) and fluid heat underFTDandsuppressedby13%underRTD.Com- transfer equations with appropriate boundary con- pared with the high temperature in the HC side, the ditions, yielding the current, electric potential and high temperature significantly reduces the ion con- ionconcentrationinthesystem.Thecalculateddiffu- centrationintheLCside. sion current, membrane potential and output power TheICPassistedbythermaldiffusionleadstodif- are shown in figures 4(b)–(d). The applied RTD can ferent effects when the high temperature is applied 4 2DMater.9(2022)045010 LLinetal Figure5.Thethermal-diffusion-assistedICP.(a)Thetotal ionconcentrationdistributionofthenanochannelunder theconditionsofNTD(∆T =0K),FTD(∆T =40K)and RTD(∆T = −40K).Thetotalionconcentrationsatthe LCside(b)andHCside(c)revealthatRTDcan significantlysuppresstheICPattheorificeoftheLCside. Theconcentrationgradientfixedat1mM|300mM. in the LC and HC sides. For one thing, the ion transport driven by temperature difference is much smaller than that driven by concentration difference (figures 1(a) and (b)). Thus, when the concentra- tion difference and temperature difference exist at the same time, the ion migration is dominated by thechemicalpotentialgeneratedfromconcentration gradient. For another thing, since the diffusion coef- ficient of ions is positively correlated with temperat- ure, the high temperature condition can largely pro- mote the local ionic diffusion. By this way, when the hightemperatureisappliedintheLC,wheretheICP issignificantlystrongerthanthatofHCside,thepro- motionofioniccurrentishighlyapparent. During the process of ion transport, the thermal Figure6.Thetemperaturesensorforaqueoussolutions. effects have two aspects. On the one hand, it can (a)Theschematicillustrationforthetemperaturesensor. (b)Therelationshipamongtheconcentrationgradient, drive the ions to move along the temperature gradi- diffusioncurrentandthetemperaturedifference.TheLC ent.Ontheotherhand,thethermal-inducediondif- sidewasfixedat1mM,theHCsiderangesfrom10to fusion can reduce concentration polarization. Espe- 1000mM.Thelow-temperaturesidewasfixedat298K, andthehigh-temperaturesidewasincreasedfrom298Kto cially,whenthehightemperatureisapplieduponthe 348K.(c)Forthegivenconcentrationconditionswhenthe LCside,wherethesevereICPoccurs,thethermaldif- solutiontemperatureofonesideisfixedat298K,the resultingioniccurrenthasaone-to-onecorrespondence fusion effectively suppressed the ICP. As shown in withthetemperatureofthesolutionontheothersideof figureS13(a),when ∆T = −40K,theionconcentra- thesystem. tionintheLCsideisreducedbyabout22%compared with ∆T =40K.Forcomparison,theconcentration differencebetween +40Kand −40Kismerely2.4% in the HC side. Therefore, the RTD can enhance the commonlyexistinawiderrangeoftemperatureand ion diffusion driven by osmotic pressure. Moreover, gradientdifferences(figureS14). the magnitude regulated by RTD is more evident This anomalous temperature dependence of ion thanthatbyforwardstemperaturedifference.Forthis transport under osmotic pressure can be developed reason, the curve in the negative range has a larger to osmotic-energy-driven temperature sensors slope than that in the positive range (figure S13(b)). (figure 6(a)). The current is determined by the tem- It is also in accord with the data in figures 3 and 4. perature gradient and the concentration gradient Thecalculationresultsshowthecompetitionbetween across the GO membrane. Under the fixed concen- temperaturegradientandconcentrationgradientcan tration condition and the temperature on one side, 5 2DMater.9(2022)045010 LLinetal thetemperatureontheothersidecanbepredictedby The interlayer spacing of the dried GO mem- measuringthetransmembranecurrent(figure6(b)). brane was measured by a polycrystalline x-ray dif- For example, when the solution temperature of one fractometer (XRD) with a Cu Kα Radiation source side is fixed to 298 K, the resulting ionic current (Rigaku Ultima IV). The microstructures of the sur- has a one-to-one correspondence with the temper- face and cross-section of the GO membrane were ature of the solution on the other side of the system obtainedbyafieldemissionscanningelectronmicro- (figure6(c)).Thisdeviceprovidesanewapproachto scope (SUPRA 55 SAP-PHIRE). The Raman spec- directlydetectthesolutiontemperaturethroughelec- tra was tested on an IDSpec ARCTIC Raman spec- trical signals. This self-driven process does not rely trometer. Fourier transform infrared spectrometer on additional energy sources. The numerical calcu- (Nicolet Is5) was used to characterize the chemical lation results imply this phenomenon may also exist bondsonthesurfaceofGOsheets.Thezetapotential −1 inother2Dmaterialswithchargedinnerwalls.Itcan ofGOdispersion(0.1mgml )andparticlesizedis- be excepted that the 2D materials with high surface tribution of GO sheets was surveyed with a Malvern density and low flow resistance will have excellent ZetasizerNanoZS90. detectionperformance. 4.2.Electricalmeasurements The diffusion current and membrane potential were 3.Conclusions measured by a Keithley 2400 picoammeter. The GO membrane was mounted between the two reservoirs In summary, we report the anomalous temperature ofpolytetrafluoroethylene.Bothreservoirswerefilled dependence of ion transport under osmotic pres- with KCl solution and all solutions were prepared sure in GO membranes. When the RTD is applied using ultrapure water (18.2 MΩ cm). Ag/AgCl elec- coupled with the concentration difference, the ion trodes were used to apply a transmembrane elec- transport is enhanced. Through controlling the dir- trical potential and remained stable during the test- ection and magnitude of temperature difference, the ing process. The output power (P) is calculated by ion diffusion driven by osmotic pressure can be reg- P =I ×V /4.Inthecircumstancesofaseriesof diff mem ulated. Theoretical analysis reveals the anomalous concentration gradients and the temperature differ- temperature dependence of ion transport origins ence,electrodepotentialgeneratedbytheelectrodeis from the thermal-diffusion-assisted concentration deducted. polarization. Compared with the high concentration side, the high temperature in the low concentra- 4.3.Numericalcalculations tion side enhances the ion diffusion and suppresses Theiondiffusionacrossthenanochannelwasinvest- the ICP, which eventually strengthens the ion trans- igated by the finite element numerical simulation port. The findings can bring inspiration to the methodbasedonthecoupledPNP,NSandfluidheat high-performance nanofluidic systems involving ion transferequations[54].Thecoupledpartialdifferen- transportunderthermodynamicandosmoticdriven tial equations with appropriate boundary conditions forces. were solved, obtaining the diffusion current, electric potential and ion concentration in the system [55]. For gaining an affordable computation scale, we use 4.Experimental a two-dimensional axisymmetric model to calculate a single cylindrical channel. The settings of the con- 4.1.Materialpreparationandcharacterization centrationgradientandthetemperaturegradientare GO was synthesized from natural graphite. Graph- consistent with the experiments. More details can be ite powders (98.5%), sodium nitrate (NaNO , 99%) obtainedinsupportinginformation. and concentrated sulfuric acid (H SO , 98%) were 2 4 mixed and leave to set for 12 h at 263.2 K. After- Dataavailabilitystatement ward, potassium permanganate (KMnO , 99%) was added slowly to the mixture. Then the mixture was All data that support the findings of this study are stirredfor1hat273.2K.Itwasheatedto308.3Kand included within the article (and any supplementary reacted fully for 12 h. Keeping the stirring rate and files). fixing the temperature at 273.2 K, deionized water (DI, 18.2 MΩ cm) was added slowly to the mixture, and then added a mixture of DI and hydrogen per- Acknowledgments oxide (H O , 30%). After the reaction, the mixture 2 2 was kept in solution for 12 h. Then, the precipit- This work was supported by the National Nat- ated acidic suspension was centrifuged several times ural Science Foundation of China (12175188), −1 (12000rmin )untilthepHofthesupernatantwas FujianKeyLaboratoryofFunctionalMarineSensing approximately7.Finally,theresultingGOdispersion Materials, Minjiang University (Grant No. MJUKF- was freeze-dried to obtain fluffy GO and stored in FMSM202009), the Fundamental Research Funds darkness. fortheCentralUniversitiesofChina(20720210051), 6 2DMater.9(2022)045010 LLinetal theNaturalScienceFoundationofFujianProvinceof [17] LinZ,McCrearyA,BriggsN,SubramanianS,ZhangK, SunY,LiX,BorysNJ,YuanHandFullerton-ShireySK China(No.2019J05015)andXMUTrainingProgram 20162Dmaterialsadvances:fromlargescalesynthesisand ofInnovationandEntrepreneurshipforUndergradu- controlledheterostructurestoimprovedcharacterization ates(2020Y1600). techniques,defectsandapplications2DMater.3042001 [18] LiH,DongZ,ZhangY,LiL,WangZ,WangC,ZhangKand ZhangH2021Recentprogressandstrategiesin ORCIDiDs photodetectorsbasedon2Dinorganic/organic heterostructures2DMater.8012001 LingxinLin https://orcid.org/0000-0003-3627- [19] MittaSB,ChoiMS,NipaneA,AliF,KimC,TeheraniJT, 1382 HoneJandYooWJ2021Electricalcharacterizationof2D materials-basedfield-effecttransistors2DMater.8012002 LiuxuanCao https://orcid.org/0000-0002-7788- [20] QuR,ZengX,LinL,ZhangG,LiuF,WangC,MaS,LiuC, MiaoHandCaoL2020Vertically-orientedTi C T MXene 3 2 x membranesforhighperformanceofelectrokineticenergy References conversionACSNano1416654–62 [21] ZhangZ,WangC,LinL,XuM,WuYandCaoL2020 [1] GarajS,HubbardW,ReinaA,KongJ,BrantonDand Rectifiediontransportinultra-thinmembranegovernedby GolovchenkoJA2010Grapheneasasubnanometre outermembraneelectricdoublelayerChin.J.Chem. trans-electrodemembraneNature467190–3 381757–61 [2] HeeremaSJandDekkerC2016Graphenenanodevicesfor [22] WangL,BoutilierMSH,KidambiPR,JangD, DNAsequencingNat.Nanotechnol.11127–36 HadjiconstantinouNGandKarnikR2017Fundamental [3] TraversiF,RaillonC,BenameurSM,LiuK,KhlybovS, transportmechanisms,fabricationandpotential TosunM,KrasnozhonD,KisAandRadenovicA2013 applicationsofnanoporousatomicallythinmembranesNat. 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Anomalous temperature dependence of ion transport under osmotic pressure in graphene oxide membranes

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© 2022 The Author(s). Published by IOP Publishing Ltd
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2053-1583
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10.1088/2053-1583/ac7ecd
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Abstract

theauthor(s)andthetitle ofthework,journal Grapheneoxide(GO)membraneshaveattractedbroadinterestbecauseoftheiruniquemass citationandDOI. transportproperties.TowardsthecontrollableionictransportinGOmembranes,physicalfieldsor externaldrivingforcesareinducedtocontrolthebehaviorofionicmigrationinsitu.However,the adjustableionictransportregulatedbytemperatureandosmoticpressureinGOmaterialsisstill absent.Herein,wereporttheanomaloustemperaturedependenceofiontransportunderosmotic pressureinGOmembranes.Theionscandiffusespontaneouslyalongtheconcentrationgradient orthetemperaturegradient.Intriguingly,itisfoundthatthereversetemperaturedifferencecan promoteiontransportdrivenbyosmoticpressure.Theoreticalanalysisrevealsthattheanomalous temperaturedependenceofiontransportstemsfromthethermal-diffusion-assistedion concentrationpolarization(ICP).Thehightemperatureinthelow-concentrationsidelargely enhancestheionicthermaldiffusionandsuppressestheICP,whicheventuallystrengthenstheion currentalongtheconcentrationgradient.Thefindingcanbedevelopedintothetemperature sensorforaqueoussolutionsandbringinspirationtotheapplicationinvolvingiontransportunder thermodynamicandosmoticdrivenforces. 1.Introduction between2Dlamella,providingbothexcellentselectiv- ity and fast permeation [23]. The great effort is Synthetic nanoporous membranes have attracted takentocontroltheiontransportinGOmembranes, broad interest because they boost the development including the interlayer spacing, the elemental com- of DNA sequencing [1–3], chemical sensing and positionsuchashydrogenandthepresenceandrelat- delivery [4–6], ion filtration [7, 8], pervaporation iveratioofoxygenfunctionalgroups,thepreparation [9–12], energy conversion [13–16] and nanoelec- method of GO membranes [24–26]. Besides, more tronics [17–19]. The interest stems from the unique materialpropertieshavebeenexploredwidelyasone mass transport properties very different from the of the crucial factors to regulate the ionic transport counterpart of the macro scale [20, 21]. Particularly, properties. It is reported that the broken symmetry the newly emerged 2D materials are considered to of nanofluidic systems, including the geometry [27], be candidates for next generation high-performance surfacechargedistributionorchemicalmodification membrane materials due to their atomic thickness, [28], nanofluidic heterojunction [29, 30] can lead to excellent thermal and chemical stability, outstand- a preferential ionic transport direction through the ing flexibility and solution processability [22]. More nanochannel,whichissimilartotheactivetransport significantly, graphene and its derivatives, includ- acrosstheionchannelsonthecellmembrane[31]. ing graphene oxide (GO) and chemically conver- Although the improvement of the materials tedgraphene,cangeneratehighlyorderedcapillaries endowsabundantpropertyofnanofluidicsystems,it ©2022TheAuthor(s). PublishedbyIOPPublishingLtd 2DMater.9(2022)045010 LLinetal is impossible to change the predefined ionic trans- furtherdevelopedanosmotic-energy-driventemper- port properties once the devices are fabricated [32]. aturesensorbasedonthismechanism.Thesefindings In this regard, several externally tunable approaches are beneficial to the design of advanced 2D nano- havebeenproposedsofar.Mostofthemchangeionic fluidic devices and related applications involving ion transport properties by regulating the nanochannel transportunderthermodynamicandosmoticdriven wall property triggered by external chemical stimuli, forces. such as temperature [33], pH value [34], enzymes [35] and polyvalent cations [36]. Contrary to the chemicalstimuli-responsivestrategies,morephysical 2.Resultsanddiscussion fields or external driving forces are induced to con- troltheionictransportbehaviorsinsitu.Forinstance, GO suspension was fabricated via improved Hum- Guanetalreportedafield-effectreconfigurablenan- mers [49, 50]. The x-ray diffraction (XRD) patterns ofluidicdiodethroughtheapplicationofgatevoltages of the GO membrane indicated that the interlayer which can enhance or diminish the ionic concentra- spacing was 0.92 nm (figure S1). The thickness of tions[32].Thetransmembraneconcentrationgradi- the GO membrane was characterized to be about ent, which results in the electroosmotic flow and 759 nm by field emission scanning electron micro- reversal potential, can regulate the preferred ionic scope (figure S2). Raman spectroscopy of GO mem- transport in both magnitude and direction [37, 38]. brane showed clear D-band and G-band peaks at −1 Thepressure-drivenflowcaneliminateioniccurrent 1328 and 1572 cm , respectively. And the rich rectification under an electrical field [39]. Recently, oxygen-containing functional groups of GO were Yang et al reported a coupled photon–electron–ion identified by Fourier transform infrared spectro- transport. Through asymmetric light illumination, scopy (figureS3). These abundant functional groups anti-gradient transport can be obtained [40]. From rendered the GO negative charge in aqueous solu- then on, the ionic transport driven by light coupled tions. The water contact angle of 36.07 indicated withosmoticpressureorappliedelectricfieldcatches the good hydrophilicity of GO membranes [51, 52] increasingresearchfocus[41–43]. (figure S4). The zeta potential characterization con- Ontheotherhand,temperature,aphysicalpara- firmed the stability of colloidal solutions in the pH meter directly related to the thermal motion of ions, rangeof2–12[53].ThesizeoftheGOsheetsranged can affect ion transport property as well. It is repor- from400to1000nm(figureS5).TheGOmembrane ted the temperature difference can induce the ther- wasmountedintworeservoirswithdifferentconcen- moelectric effect in ion-selective nanochannels or trationsofpotassiumchloridesolution.Acustomized porous materials [44, 45]. Nanofluidic reverse elec- rubberheatingsheetwasusedtocontrolthesolution trodialysisprocessisfoundtobethermaldependence temperature on one side to build a temperature dif- [46,47].Longetalpredictedtheup-diffusionthermal ferenceacrossthemembrane(figureS6). gradient can enhance the nanofluidic salinity- Theiondiffusiondrivenbytheconcentrationdif- gradientenergyharvestingbyanumericalsimulation ference or the temperature difference was along the approach [48]. It can be expected that temperature gradient direction (figures 1(a) and (b)). It is expec- can be applied in 2D materials to control the inter- ted that the osmotic ion transport can be promoted lamellarionictransport.However,relatedresearchis by applying a temperature gradient in the same dir- stillabsent. ection as the concentration gradient. Intriguingly, Herein, we report anomalous temperature- an unexpected enhancement of ion diffusion was dependent ionic transport under osmotic pressure observed when the reverse temperature difference in GO membranes. In contrast to the conventional (RTD) was applied together with the concentration viewpoint that the concentration gradient and the difference (figures 1(c) and (d)). For instance, the temperature gradient in the same direction will syn- diffusion current under the concentration difference ergistically increase the ion transport, we found that of 1 mM|300 mM KCl was 7.13 µA. When the low- thereversetemperaturegradientcangreatlypromote concentration (LC) solution was heated to 348 K, the osmotic-driven ionic transport in GO mem- the diffusion current increased to 11.30 µA, with branes. In particular, ion transport increases with an increased magnitude of 58.5%. In sharp con- the reverse temperature gradient. Through tuning trast,whenthehigh-concentration(HC)solutionwas themagnitudeanddirectionofappliedthermalcon- heated to 348 K, the diffusion current decreased to ditions, the transmembrane ionic transport can be 6.32 µA,withadropmagnitudeof11.4%.Themem- continuously regulated. This phenomenon can be branepotential(V )sharesasimilartrendwiththe mem observed under different concentration conditions, diffusion current (figure 1(e)). Accordingly, the high pHvaluesandelectrolytesolutions.Theoreticalana- outputpowercanbeachievedundertheRTD. lysis based on finite element numerical calculation The anomalous temperature dependence of ion reveals the mechanism of the anomalous temperat- transport can be observed under various concen- uredependenceofiontransportinnanochannels.We tration difference conditions of 1 mM|10 mM to 2 2DMater.9(2022)045010 LLinetal Figure1.Anomaloustemperaturedependenceofion transportunderosmoticpressureinGOmembranes.The diffusioncurrentandmembranepotentialdrivenbythe concentrationdifferenceandtemperaturedifferenceare (a)and(b),respectively.(c)Schematicillustrationof forwardtemperaturedifference(FTD),notemperature difference(NTD)andreversetemperaturedifference (RTD).Theconcentrationconditionwas1mM|300mM KCl.TheRTDenhancesdiffusioncurrent(d)and membranepotential.(e)Thecorrespondingtemperature conditionsareT =T =298K,T =348Kand HC LC HC T =298K,T =298KandT =348K,respectively. LC HC LC 1 mM|1000 mM. As shown in figure 2(a), the diffu- sion current increases with the concentration differ- ence under the conditions of no temperature differ- ence (NTD), forward temperature difference (FTD) and RTD. At all concentration gradients tested, RTD increasesthediffusioncurrentwhileFTDdecreasesit. Theeffectoftemperaturegradientonthemembrane potential is similar (figure 2(b)). The enhanced dif- fusion current and membrane potential in the RTD condition eventually promote the output power of Figure2.Theanomaloustemperaturedependenceofion transportcanbeobservedundervariousconcentration osmoticenergyconversion(figure2(c)). conditions,includingthediffusioncurrent(a),the Theperformanceofiontransportunderosmotic membranepotential(b),andtheoutputpower(c).The temperatureconditionsofNTD,FTDandRTDarethe pressure can be regulated by the direction and mag- sameasfigure1.KClsolutionconcentrationconditions nitudeoftemperaturedifference.Thetemperatureof rangefrom1mM|10mMto1mM|1000mM. one side was fixed to 298 K while the other side var- ied from 298 K to 348 K. As shown in figure 3(a), thehigherRTDcanpromoteiontransportdrivenby osmotic pressure, resulting in greater diffusion cur- The competition between temperature gradient rent and membrane potential. On the other hand, andconcentrationgradientleadstotheshiftbetween FTD weakens the diffusion current and membrane normal ionic transport and the anomalous temper- potential. In this regard, the output power can be aturedependence(figureS7).Forinstance,whenthe tuned by the direction and magnitude of the applied concentration difference is high (10 mM|300 mM), temperaturedifference.Figure3(b)illustratesthatthe theioniccurrentdecreaseswiththetemperaturedif- RTD of −50 K can improve the output power from ference obviously, which corresponds to the anom- 0.16 to 0.31 µW, with the magnitude up to 51.6%, alous temperature dependence. When the ion con- comparedwithNTD. centration difference reduces to 230 mM|300 mM, 3 2DMater.9(2022)045010 LLinetal Figure3.Theperformanceofiontransportunderosmotic pressurecanberegulatedbythedirectionandmagnitudeof theappliedtemperaturedifference,including(a)the diffusioncurrent,membranepotential,and(b)output power.Theappliedsolutionsconcentrationwas 1mM|300mMKCl.Thelow-temperaturesidewasfixedto 298K,whiletheothersidevariedfrom298Kto348K. Figure4.Numericalcalculation.(a)Simulationmodel.The lengthofnanochannelwas20nm,thediameterwas8nm, −2 andthesurfacechargedensitywas −0.06Cm .The the competition between temperature gradient and settingofthesolutionconcentrationwas1mM|300mM KCl.Thelow-temperaturesidewasfixedto298K,whilethe concentration gradient almost reaches balance. The othersidevariedfrom298Kto348K.Thecalculated ioniccurrentkeepsstablewiththeincrementoftem- diffusioncurrent(b),membranepotential(c)andoutput perature difference. Under the single driven force of power(d)sharethesametrendwiththeexperimental observations. temperaturedifference(300mM|300mM),ioniccur- rentsincreasewiththetemperaturedifference,corres- ponding to the normal ion diffusion driven by tem- peraturedifference. The anomalous temperature dependence of ion evidently promote ion transport driven by osmotic transportcanbeobservedindifferentpHvaluesand pressure. Meanwhile, FTD suppresses the ion trans- electrolyte solutions (NaCl and LiCl), as shown in port along the concentration gradient. These are in figuresS8andS9,respectively.Inaddition,theanom- agreement with the experimental results. Addition- alous temperature dependence of ion transport can ally, the slope of the curve of the diffusion cur- also be found in polymer membrane (figure S10). rent, membrane potential and output power imply Owingtotheexcellentselectivityandfastpermeation that the promoting effect of RTD is greater than stemmedfromhighlyorderedcapillariesinGOmem- the suppressing effect of FTD, which is also con- branes, the ionic current tested in GO membranes sistent with the experimental observations. The cal- is much larger than that of polyethylene terephthal- culation of the channel length (L) of 1000 nm also ate(PET)membranes.GOmembranesprovidegreat confirms this anomalous temperature dependence advantages for the possible applications to harvest (figureS12). the low-grade heat from the environment or indus- Thelocaldistributionofionconcentrationindic- trialwasteheatthroughcooperatingwiththeosmotic atesthatthethermal-diffusion-assistedconcentration energyconversionprocess. polarizationistheoriginoftheanomaloustemperat- We carried out the theoretical analysis based on uredependenceiniontransportunderosmoticpres- thecontinuitymodeltorevealtheoriginoftheanom- sure. As shown in figure 5(a), the ion concentration alous temperature dependence in osmotic ion trans- polarization (ICP) in the LC side is obviously differ- port. As shown in figure 4(a), the calculation model entfromtheHCside.TheRTDcansignificantlysup- includes two bulk reservoirs connected with a single presstheICPattheorificeoftheLCside.Thelocalion cylindrical nanochannel. The length of nanochannel concentrationsnearthelow-andHCentrancequant- (L)is20nm,thediameterofnanochannel(D)is8nm itatively illustrate the differences between the NTD, −2 and the surface charge density (σ) is −0.06 C m , FTD and RTD conditions (figures 5(b) and (c)). In consistent with the literature [21]. The setting of the particular, the ion concentrations of NTD, FTD and concentrationandthetemperatureconditionsinthe RTD in the HC sides are respectively 536, 546 and model is consistent with the experimental installa- 533 mM, with a difference of only 2%. For compar- tions. The detailed boundary conditions are shown ison, the difference is much more obvious on the LC infigureS11andtableS1.Thefiniteelementmethod side. Compared with NTD, the local ion concentra- was employed to solve the coupled Poisson–Nernst– tionintheLCsidewasfoundtobeenhancedby10.6% Planck (PNP), Navier–Stokes (NS) and fluid heat underFTDandsuppressedby13%underRTD.Com- transfer equations with appropriate boundary con- pared with the high temperature in the HC side, the ditions, yielding the current, electric potential and high temperature significantly reduces the ion con- ionconcentrationinthesystem.Thecalculateddiffu- centrationintheLCside. sion current, membrane potential and output power TheICPassistedbythermaldiffusionleadstodif- are shown in figures 4(b)–(d). The applied RTD can ferent effects when the high temperature is applied 4 2DMater.9(2022)045010 LLinetal Figure5.Thethermal-diffusion-assistedICP.(a)Thetotal ionconcentrationdistributionofthenanochannelunder theconditionsofNTD(∆T =0K),FTD(∆T =40K)and RTD(∆T = −40K).Thetotalionconcentrationsatthe LCside(b)andHCside(c)revealthatRTDcan significantlysuppresstheICPattheorificeoftheLCside. Theconcentrationgradientfixedat1mM|300mM. in the LC and HC sides. For one thing, the ion transport driven by temperature difference is much smaller than that driven by concentration difference (figures 1(a) and (b)). Thus, when the concentra- tion difference and temperature difference exist at the same time, the ion migration is dominated by thechemicalpotentialgeneratedfromconcentration gradient. For another thing, since the diffusion coef- ficient of ions is positively correlated with temperat- ure, the high temperature condition can largely pro- mote the local ionic diffusion. By this way, when the hightemperatureisappliedintheLC,wheretheICP issignificantlystrongerthanthatofHCside,thepro- motionofioniccurrentishighlyapparent. During the process of ion transport, the thermal Figure6.Thetemperaturesensorforaqueoussolutions. effects have two aspects. On the one hand, it can (a)Theschematicillustrationforthetemperaturesensor. (b)Therelationshipamongtheconcentrationgradient, drive the ions to move along the temperature gradi- diffusioncurrentandthetemperaturedifference.TheLC ent.Ontheotherhand,thethermal-inducediondif- sidewasfixedat1mM,theHCsiderangesfrom10to fusion can reduce concentration polarization. Espe- 1000mM.Thelow-temperaturesidewasfixedat298K, andthehigh-temperaturesidewasincreasedfrom298Kto cially,whenthehightemperatureisapplieduponthe 348K.(c)Forthegivenconcentrationconditionswhenthe LCside,wherethesevereICPoccurs,thethermaldif- solutiontemperatureofonesideisfixedat298K,the resultingioniccurrenthasaone-to-onecorrespondence fusion effectively suppressed the ICP. As shown in withthetemperatureofthesolutionontheothersideof figureS13(a),when ∆T = −40K,theionconcentra- thesystem. tionintheLCsideisreducedbyabout22%compared with ∆T =40K.Forcomparison,theconcentration differencebetween +40Kand −40Kismerely2.4% in the HC side. Therefore, the RTD can enhance the commonlyexistinawiderrangeoftemperatureand ion diffusion driven by osmotic pressure. Moreover, gradientdifferences(figureS14). the magnitude regulated by RTD is more evident This anomalous temperature dependence of ion thanthatbyforwardstemperaturedifference.Forthis transport under osmotic pressure can be developed reason, the curve in the negative range has a larger to osmotic-energy-driven temperature sensors slope than that in the positive range (figure S13(b)). (figure 6(a)). The current is determined by the tem- It is also in accord with the data in figures 3 and 4. perature gradient and the concentration gradient Thecalculationresultsshowthecompetitionbetween across the GO membrane. Under the fixed concen- temperaturegradientandconcentrationgradientcan tration condition and the temperature on one side, 5 2DMater.9(2022)045010 LLinetal thetemperatureontheothersidecanbepredictedby The interlayer spacing of the dried GO mem- measuringthetransmembranecurrent(figure6(b)). brane was measured by a polycrystalline x-ray dif- For example, when the solution temperature of one fractometer (XRD) with a Cu Kα Radiation source side is fixed to 298 K, the resulting ionic current (Rigaku Ultima IV). The microstructures of the sur- has a one-to-one correspondence with the temper- face and cross-section of the GO membrane were ature of the solution on the other side of the system obtainedbyafieldemissionscanningelectronmicro- (figure6(c)).Thisdeviceprovidesanewapproachto scope (SUPRA 55 SAP-PHIRE). The Raman spec- directlydetectthesolutiontemperaturethroughelec- tra was tested on an IDSpec ARCTIC Raman spec- trical signals. This self-driven process does not rely trometer. Fourier transform infrared spectrometer on additional energy sources. The numerical calcu- (Nicolet Is5) was used to characterize the chemical lation results imply this phenomenon may also exist bondsonthesurfaceofGOsheets.Thezetapotential −1 inother2Dmaterialswithchargedinnerwalls.Itcan ofGOdispersion(0.1mgml )andparticlesizedis- be excepted that the 2D materials with high surface tribution of GO sheets was surveyed with a Malvern density and low flow resistance will have excellent ZetasizerNanoZS90. detectionperformance. 4.2.Electricalmeasurements The diffusion current and membrane potential were 3.Conclusions measured by a Keithley 2400 picoammeter. The GO membrane was mounted between the two reservoirs In summary, we report the anomalous temperature ofpolytetrafluoroethylene.Bothreservoirswerefilled dependence of ion transport under osmotic pres- with KCl solution and all solutions were prepared sure in GO membranes. When the RTD is applied using ultrapure water (18.2 MΩ cm). Ag/AgCl elec- coupled with the concentration difference, the ion trodes were used to apply a transmembrane elec- transport is enhanced. Through controlling the dir- trical potential and remained stable during the test- ection and magnitude of temperature difference, the ing process. The output power (P) is calculated by ion diffusion driven by osmotic pressure can be reg- P =I ×V /4.Inthecircumstancesofaseriesof diff mem ulated. Theoretical analysis reveals the anomalous concentration gradients and the temperature differ- temperature dependence of ion transport origins ence,electrodepotentialgeneratedbytheelectrodeis from the thermal-diffusion-assisted concentration deducted. polarization. Compared with the high concentration side, the high temperature in the low concentra- 4.3.Numericalcalculations tion side enhances the ion diffusion and suppresses Theiondiffusionacrossthenanochannelwasinvest- the ICP, which eventually strengthens the ion trans- igated by the finite element numerical simulation port. The findings can bring inspiration to the methodbasedonthecoupledPNP,NSandfluidheat high-performance nanofluidic systems involving ion transferequations[54].Thecoupledpartialdifferen- transportunderthermodynamicandosmoticdriven tial equations with appropriate boundary conditions forces. were solved, obtaining the diffusion current, electric potential and ion concentration in the system [55]. For gaining an affordable computation scale, we use 4.Experimental a two-dimensional axisymmetric model to calculate a single cylindrical channel. The settings of the con- 4.1.Materialpreparationandcharacterization centrationgradientandthetemperaturegradientare GO was synthesized from natural graphite. Graph- consistent with the experiments. More details can be ite powders (98.5%), sodium nitrate (NaNO , 99%) obtainedinsupportinginformation. and concentrated sulfuric acid (H SO , 98%) were 2 4 mixed and leave to set for 12 h at 263.2 K. After- Dataavailabilitystatement ward, potassium permanganate (KMnO , 99%) was added slowly to the mixture. Then the mixture was All data that support the findings of this study are stirredfor1hat273.2K.Itwasheatedto308.3Kand included within the article (and any supplementary reacted fully for 12 h. Keeping the stirring rate and files). fixing the temperature at 273.2 K, deionized water (DI, 18.2 MΩ cm) was added slowly to the mixture, and then added a mixture of DI and hydrogen per- Acknowledgments oxide (H O , 30%). After the reaction, the mixture 2 2 was kept in solution for 12 h. 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Journal

2D MaterialsIOP Publishing

Published: Oct 1, 2022

Keywords: graphene oxide membrane; ion transport; anomalous temperature dependence; ion concentration polarization; osmotic pressure

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