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Asymmetric Electrokinetic Proton Transport through 2D Nanofluidic Heterojunctions Xiaopeng Zhang, Qi Wen, Lili Wang, Liping Ding, Jinlei Yang, Danyan Ji, Yanbing Zhang, Lei Jiang, and Wei Guo ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b09285 • Publication Date (Web): 13 Mar 2019 Downloaded from http://pubs.acs.org on March 13, 2019
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Asymmetric Electrokinetic Proton Transport through 2D Nanofluidic Heterojunctions Xiaopeng Zhang,†,$,# Qi Wen,†,# Lili Wang,‡,# Liping Ding,§ Jinlei Yang,†,$ Danyan Ji,†,$ Yanbing Zhang,†,$ Lei Jiang,† and Wei Guo*,† † CAS
Key Laboratory of Bio-inspired Materials and Interfacial Science, Technical Institute of
Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China. ‡ College
of Science, Beijing University of Chemical Technology, Beijing, 100029, P. R. China.
$
University of Chinese Academy of Sciences, Beijing 100049, P. R. China.
§
Center for Physiochemical Analysis and Measurement, Institute of Chemistry, Chinese
Academy of Sciences, Beijing 100190, P. R. China. KEYWORDS (proton transport, nanofluidics, 2D layered materials, ionic rectification, energy conversion)
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ABSTRACT: Nanofluidic ion transport in nacre-like 2D layered materials attracts broad research interest due to sub-nanometer confined space and versatile surface chemistry for precisely ionic sieving and ultrafast water permeation. Currently, most of the 2D-material-based nanofluidic systems are homogenous, and the investigations of proton conduction therein are restricted to symmetric transport behaviors. It remains a great challenge to endow the 2D nanofluidic systems with asymmetric proton transport characteristics and adaptive responsibilities. Herein, we report the asymmetric proton transport phenomena through 2D nanofluidic heterojunction membrane under three different types of electrokinetic driving force, that is, the external electric field, the transmembrane concentration gradient, and the hydraulic pressure difference. The heterogeneous 2D nanofluidic membrane comprises of sequentially stacked negatively and positively charged graphene oxide (n-GO and p-GO) multilayers. We find that the preferential direction for proton transport is opposite under the three types of electrokinetic driving force. The preferential direction for electric-field-driven proton transport is from the n-GO multilayers to the p-GO multilayers, showing rectified behaviors. Intriguingly, when the transmembrane concentration difference and the hydraulic flow are used as the driving force, preferred diffusive and streaming proton current is found in the reversed direction, from the p-GO to the n-GO multilayers. The asymmetric proton transport phenomena are explained in terms of asymmetric proton concentration polarization and difference in proton selectivity. The membrane-scale heterogeneous 2D nanofluidic devices with electrokinetically-controlled asymmetric proton flow provide a facile and general strategy for potential applications in biomimetic energy conversion and chemical sensing.
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Ion transport in nanoscale channels and pores exhibits drastically different properties from that in bulk solution.1, 2 The physical and chemical properties on the channel wall or near the channel orifice begins to dominate the transport behaviors, if at least one dimension of the channel approaches the characteristic length scale of the electrostatic, Van der Waals, or hydrophobic interactions.3, 4 Particularly, in the recent years, laminar nanofluidic channels in nacre-inspired layered membranes further reduce the channel height down to sub-nanometer scale that makes the 2D nanofluidic devices and materials a platform for precisely ionic sieving and ultrafast fluidic permeation.5-7 To date, a variety of 2D nano-building-blocks have been used for constructing functional nanofluidic devices,8-11 such as graphene oxide,12-15 chemically converted graphene,16-18 MoS2,19-21 WS2,22 BN,23 MXene,24, 25 vermiculite,26 and kaolinite.27 Compared to the 1D nanofluidic channels or nanopores, one distinct advantage of the 2D nanofluidic systems is the chemical modification can be conducted in bulk solution prior to the formation of the nanofluidic architectures.28 In this regard, the preassembly modification strategy offers substantially high modification efficiency. Currently, most of 2D-material-based nanofluidic systems are homogenous. Investigations on the transport behaviors in 2D nanofluidic heterostructures are very rare. Directional proton transport plays a crucial role in life process, such as cell pH stabilization,29 generation of membrane potential,30 and energy transduction for ATP synthesis.31 Inspired by this mechanism, proton transport in synthetic nanofluidic systems attracts broad interest.32-35 For example, Fan et al. report surface-governed proton transport in aligned mesoporous silica film, which can be actively modulated by a low gate voltage.36 Recently, proton transport in 2D layered materials receives considerable attention due to the fantastic application in fuel cells.37-39 Sulfonated,40 ozonated,41 and phosphorylated42 graphene oxide membranes (GOMs) are
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proposed as solid electrolyte, and exhibit excellent proton conductivity. Shao et al. further use clay nanosheets to build layered membranes for proton conduction, and greatly enhance the thermal stability of the membrane materials.26 So far, these existing 2D nanofluidic systems display symmetric proton transport properties. Several attempts are made to realize rectified transport of metal ions in 2D nanofluidic systems.43,
44
For example, we fabricate a photo-switchable ionic diode membrane based on
spiropyran modified GO multi-layers that shows ionic current rectification in response to light illumination.45 Gao et al. propose the concept of “kirigami 2D nanofluidics”, in which asymmetric ion transport properties can be achieved by simply tailoring the geometric shape of the GOM.46 However, it still remains a great challenge to endow the 2D nanofluidic systems with asymmetric proton transport characteristics and adaptive responsibilities. Herein, we report asymmetric proton transport phenomena through 2D nanofluidic heterojunctions under three different types of electrokinetic driving force, that is, the electric field, the transmembrane concentration gradient, and the hydraulic pressure difference. The heterogeneous nanofluidic membrane comprises of sequentially stacked negatively and positively charged graphene oxide (n-GO and p-GO) multi-layers. We find that the preferential direction for proton conduction is opposite under the three kinds of electrokinetic driving force. Driven by the electric field, protons are preferred to transport from the n-GO to the p-GO multilayers, showing pronounced rectification. Intriguingly, when the concentration difference and the hydraulic flow are used as driving force, preferred diffusive and streaming proton current are found in the reversed direction. The asymmetric proton transport phenomena are explained in terms of asymmetric proton concentration polarization and difference in proton selectivity. The electrokinetically-controlled preferential proton transport in heterogeneous 2D layered materials
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provides a facile and general strategy for potential applications in biomimetic energy conversion and chemical sensing. RESULTS AND DISCUSSION To obtain positively charged GO nanosheets, branched polyethyleneimine (PEI) was covalently coupled with the carboxylic acid groups on the n-GO nanosheets through a two-step chemical reaction (Figure 1a and Supporting Information). The n-GO and p-GO dispersions can be stable for months (Figure 1b). Atomic force microscopic (AFM) characterization confirms that the thickness of n-GO and p-GO flakes is about 0.84 and 1.24 nm, respectively (Figure 1c and 1d). The increased thickness in p-GO is attributed to the conjugation of PEI, in agreement with previous report.47 In HCl solution, the n-GO and p-GO nanosheets keep their charge properties revealed by zeta potential measurements (Figure 1e). The bi-layered heterogeneous GOMs (BHGOMs) were fabricated by sequentially depositing negatively and positively charged GO colloids on a polymeric filter membrane (Figure 1f). Via the two-step flow-directed self-assembly process, the BHGOM is self-supporting and flexible (Figure 1g). Scanning electron microscopic (SEM) characterization on the cross section of the BHGOM shows laminar microstructure, but no clear boundary between the n-GO and p-GO multi-layers is found (Figure 1h), suggesting the bi-layered membrane is integrate. The BHGOM is hydrophilic on both n-GO and p-GO sides (Figure 1i and 1j). After a thermal annealing process,15 the BHGOMs show excellent stability in water and HCl solution (Figure S2). X-ray diffraction (XRD) tests on hydrated BHGOMs show two independent diffraction peaks at 7.91° and 6.39°, indicative of an interlayer spacing of 1.12 and 1.38 nm, respectively. The two diffraction peaks suggest the presence of n-GO (7.86°) and p-GO multi-layers (7.12°) in the
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composite membrane (Figure 1k). Considering the thickness of monolayer graphene (~0.34 nm), the effective height of the laminar nanochannels should be in the range of 0.78-1.04 nm, which allows the permeation of most hydrated simple ions.48 We further use a Methylene blue permeation experiment to confirm that there are no connected larger voids in the BHGOM (Figure S3). To investigate proton transport through the heterogeneous GOM, a piece of BHGOM was mounted in between a two-compartment electrochemical cell (Figure 2a and Figure S4a).15 HCl solution (1 mM) was filled in the two reservoirs (Figure S5). Under a scanning voltage between 2 and +2 V, we find a rectified proton current through the BHGOM (Figure 2b). The preferential direction is from the n-GO multi-layers to the p-GO multi-layers. The degree of the asymmetric proton transport depends on the HCl concentration (Figure 2c). By increasing the HCl concentration from 100 µM to 0.1 M, the rectification ratio of the proton current (I(np)/I(pn)) gradually enhances from 1.5 to 5.9. For comparison, we also test the proton transport behavior through homogeneously charged GOMs. Both the n-GOM and p-GOM exhibit linear current-voltage response to the scanning voltage (Figure S6), and the non-rectified property keeps in the whole tested concentration range (Figure 2c). Interestingly, by tuning the relative proportion of the thickness of the n-GO and p-GO multilayers, the degree of the asymmetric proton transport in BHGOM can be further modulated. For example, we kept the total thickness of the BHGOMs to about 20 μm. With the proportion of the n-GO multi-layers varied from 10% to 90%, we find that all the heterogeneous GOMs exhibit asymmetric proton transport behaviors (Figure 2d and Figure S7), and the highest proton current rectification ratio is found when the thickness of n-GO and p-GO multi-layers get relatively balanced (the proportion of n-GO multi-layers approaching 50%). This trend is in accord with
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previous theoretical predictions on ionic current rectification through bipolar 1D nanofluidic diodes.49 These experimental evidences clearly show that the asymmetric proton transport properties in BHGOM arise from the 2D nanofluidic heterostructures. The opposite charge of nGO and p-GO nanosheets, as well as the precise difference in channel height, may account for the asymmetric proton transport. This claim is also supported by previous empirical regularities found in 1D nanofluidic systems.2 To further understanding the mechanism, homogeneous n-GOMs and p-GOMs were separately soaked in HCl solution with different concentrations ranging from 10-5-10-2 M for about 1 hour. Excess HCl was washed out with deionized water. Afterwards, the membranes were stained in the solution of 8-hydroxy-1, 3, 6-pyrenetrisulfonic acid trisodium salt (HA, 0.1 mg/mL) for 1 hour (Figure 3a). HA is a pH-sensitive fluorescence probe, whose fluorescent intensity reflects the environmental proton concentration.35 A laser scanning confocal microscope (Olympus, IX81-FV 1000) was used to characterize the surface fluorescent intensity of the stained GOMs. Typical fluorescent images were shown in Figure 3a (the GOMs were soaked in 10-5 M HCl). Under identical imaging conditions, the fluorescent intensity from both n-GOMs and p-GOMs goes down with the rising HCl concentration (Figure 3b), in accord with previous results using HA as pH indicators.35 The fluorescent intensity from the n-GOMs is lower than that from the p-GOMs in all the tested HCl concentration, indicating higher proton concentration in n-GOMs than that in p-GOMs. This observation is in agreement with the conductance measurement (Figure 3c). Based on these considerations, we propose the mechanism shown in Figure 3d. Proton transport in GOMs follows a Grotthuss mechanism that the protons hop through the continuous hydrogen-bonded networks contributed by the hydrophilic functional groups on GO and the
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interlayer water molecules.38 Proton conductivity in GOM is thus determined by the carrier (proton) density in the membrane.4 As indicated by the XRD results (Figure 1k), the interlayer spacing of n-GOM is smaller than p-GOM. But, the conductance measurements show that the proton conductivity in n-GOM is typically 2-5 times of that in p-GOM at HCl concentrations from 10-5-10-2 M (Figure 3c). These evidences suggest that proton transport in GOMs is surfacegoverned,13 and the proton concentration in n-GOM is notably higher than that in p-GOM. Due to electrostatic interactions between protons and GO multi-layers, when the surface charge of GO were reversed from negative to positive, part of the absorbed protons are expelled from the membrane.50 Likewise, in the composite BHGOM, the proton concentration in n-GO part should be higher than that in the p-GO part. The asymmetric proton concentration polarization results in an intramembrane proton concentration gradient (Δcin[H+], Figure 3d). For V>0, the proton transport is from the n-GO to the p-GO multi-layers (I(np)), parallel to the Δcin[H+]. Therefore, it becomes the preferential direction for proton transport. On the contrary, for V0, the proton transport is from the n-GO to the p-GO multi-layers (I(np)), parallel to the Δcin[H+]. Thus, it becomes the preferential direction. For V
