Siyuan Rao,†,‡ Kae Jye Si,§ Lim Wei Yap,§ Yan Xiang,*,†,‡ and Wenlong Cheng*,§,^
ARTICLE
Free-Standing Bilayered Nanoparticle Superlattice Nanosheets with Asymmetric Ionic Transport Behaviors †
Beijing Key Laboratory of Bio-inspired Energy Materials and Devices, School of Chemistry and Environment, Beihang University, Beijing 100191, P. R. China, Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry of Education, Beihang University, Beijing 100191, P. R. China, §Department of Chemical Engineering, Monash University, Clayton, Victoria 3800, Australia, and ^The Melbourne Centre for Nanofabrication, 151 Wellington Road, Clayton, Victoria 3800, Australia ‡
ABSTRACT
Natural cell membranes can directionally and selectively regulate the ion transport, which is critical for the functioning of living cells. Here, we report on the fabrication of an artificial membrane based on an asymmetric nanoparticle superlattice bilayered nanosheet, which exhibits similar ion transport characteristics. The superlattice nanosheets were fabricated via a drying-mediated self-assembly of polystyrene-capped gold nanoparticles at the liquidair interface. By adopting a layer-by-layer assembly process, an asymmetric nanomembrane could be obtained consisting of two nanosheets with different nanoparticle size. The resulting nanomembranes exhibit an asymmetric ion transport behavior, and diode-like currentvoltage curves were observed. The asymmetric ion transport is attributed to the cone-like nanochannels formed within the membranes, upon which a simulation map was established to illustrate the relationship between the channel structure and the ionic selectivity, in consistency with our experimental results. Our superlattice nanosheet-based design presents a promising strategy for the fabrication of next-generation smart nanomembranes for rationally and selectively regulating the ion transport even at a large ion flux, with potential applications in a wide range of fields, including biosensor devices, energy conversion, biophotonics, and bioelectronics. KEYWORDS: nanoparticle superlattice . nanosheets . bilayer . nanochannel . ionic selectivity
I
n natural plasma membranes, the lipid bilayers provide a stable biological platform, which allows the embedded functional proteins to perform their biological activities (Figure 1a).1,2 Some embedded biological nanochannel proteins show a unique selectivity toward the transportation of certain types of ions,3,4 which ensures the effectivity of certain cellular activities, e.g., biological homeostasis,4 electric signal transmission,5 and energy conversion.6 In previous studies, the directional ionic selectivity of the plasma membrane has been mimicked by a variety of artificial nanochannel systems with different ionic regulation characteristics,79 for example, a single-ion pump RAO ET AL.
has been demonstrated based on a cooperative pH response double-gate nanochannel,10 and stimuli-responsive moleculemodified solid-state nanochannels have been used to study the effect of several factor on the ion transport.11 It has been discovered that, under the same ionic electrolyte conditions, an asymmetric nanochannel can lead to an asymmetric ion transport, resulting in a diode-like current curve within a certain voltage range.1215 In a conical-shaped nanochannel with a permanent surface charge, its rectifying properties result in a transport of charged ions from the side with the small opening (denoted as nanochannel tip) toward the VOL. XXX
’
NO. XX
’
* Address correspondence to
[email protected],
[email protected]. Received for review August 2, 2015 and accepted October 20, 2015. Published online 10.1021/acsnano.5b04784 C XXXX American Chemical Society
000–000
’
XXXX
A www.acsnano.org
ARTICLE Figure 1. (a) Schematic illustration of bilayered nanoparticle (NP) superlattice nanosheets and their ionic selectivity. In natural plasma membranes, lipid bilayers with embedded functional proteins make the plasma membrane a good platform for many biological reactions. The embedded proteins exhibit different ionic selectivities, which results in a directional ionic selectivity of the plasma membrane. (b) Inspired by the ionic selectivity of plasma membranes, an artificial free-standing bilayered NP nanosheet has been fabricated at the liquidair interface of a water droplet using an evaporation-induced approach. After two fabrication cycles, a free-standing bilayered NP nanosheet was obtained. (c and d) Transmission electron microscopy (TEM) images of the bilayered NP nanosheets. In each monolayer, the NPs are homogeneously distributed, with a uniform particle size. The inset image shows an optical image of the bilayered NP nanosheets. The golden membrane exhibits a good stability at a large scale.
side with the large opening (denoted as nanochannel base). Recent encouraging experimental and theoretical advances regarding the ion transport in basic single nanochannels have stimulated a wide range of interdisciplinary studies involving scientists in the fields of chemistry, materials science, environmental science, and nanotechnology in order to rationally design the next generation of membrane-scale asymmetric nanofluidic systems.16 Such asymmetric nanostructured membranes might be applied in biosensor devices and in the fields of energy conversion, biophotonics and bioelectronics. Previous approaches to fabricate nanochannels include electrochemical etching,17 the anodic oxidation method,18 the electron beam technology,19,20 laser technology,21 ion-track-etching,22,23 and the assembly of mesoporous materials.16 Herein, we report on a new approach to fabricate asymmetric nanochannel arrays using self-assembling nanoparticle (NP) superlattice RAO ET AL.
nanosheets. Such nanosheets have been demonstrated to be highly robust and feature novel, tunable properties. They can be folded into 3D origami,24 doped with additional particles,25 and have been fabricated into binary and ternary nanorod and nanodisk assemblies.26 Despite the exciting progress in the latest research on superlattices, it is still not fully understood how ions can be transported across superlattices. In previous studies on microfluids, the nanointerstices between the assembled particles have been found to play a key role as collective nanochannel networks.2730 However, in our work, the gaps between the NP superlattice nanosheets provide a new possibility to fabricate nanochannels and to regulate their properties on a broader scale. The gap-formed nanochannels (GFNs) feature an intrinsic uniform geometric shape and a homogeneous size, which can be controlled by adjusting the size and surface chemistry of the NPs.31 Our NP nanosheet-based VOL. XXX
’
NO. XX
’
000–000
’
B
XXXX www.acsnano.org
RESULTS AND DISCUSSION Free-standing NP superlattice nanosheets were obtained via a liquidair interface-mediated self-assembly process.24,39,40 When a droplet of the Au NP chloroform solution came in contact with the surface of a convexshape water droplet situated on the porous copper grid, the Au NPs quickly spread as the chloroform evaporated, resulting in the formation of free-floating Au NP sheets (Figure 1b). After the water had completely evaporated, a golden, light-reflecting NP monolayer membrane was obtained with a well-defined ordered structure (see Supporting Information, Figure S1). Utilizing the low surface extension energy of the organic solvent on water, the polymerligand NPs could easily disperse on the surface of the water droplet, resulting in a very thin monolayer after water evaporation. This approach is quite facile for the fabrication of NP monolayer nanosheets and also feasible for preparing a 1D or 3D superlattice architecture with exceptional mechanical robustness.24,40 Repeating this manipulation, bilayered NP superlattice nanosheets could be obtained. As illustrated by the TEM micrographs in Figure 1c,d, the gap between the three adjacent RAO ET AL.
spherical NPs in each monolayer can be defined as the conical-shaped nanochannel terminal of the GFN. The gap size can then be described by the areaequivalent circle (Sr = Sgap), with the radius on each side of the GFN denoted as Rtip and Rbase, respectively (Figure S2). The ion transport characteristics of the prepared bilayered superlattice nanosheets were investigated using an electrochemical system operated in potential scanning mode (Figure 2a and Figure S3).41 In our study, Au NPs are equipped with thiol-functionalized polystyrene (PS) to ensure a good mechanical strength of the nanosheets, as well as are modified with negative charge through immersing into mercaptoacetic acid solution. Ions can transport through the gaps between the NPs in the potential-induced direction (Figure 2b). The current measured at the applied scanning potential serves as an indicator for the conductivity of the GFNs. In contrast to the linear current curve observed for the monolayer NP nanosheets, a diode-like current curve was obtained in the potential range from 1 to 1 V for the heterostructure bilayered nanosheets consisting of NPs with an average diameter in the range from 14.69 to 51.64 nm (denoted as 14.69/51.64 nanosheets, Figure S4 and Figure 2c). Considering the diode-like current curve, when the GFNs are negatively charged, potassium cations are the dominating charge carriers traveling through the GFNs.33,42 When the 14.69 nm NP monolayer side was selected as working electrode, a larger current was recorded when a positive scanning potential was applied, which means that the transport of the Kþ ions was facilitated from the monolayer with the larger NPs to the monolayer with the smaller NPs. This is consistent with previous results reported for negatively charged conical nanochannels.7 In addition, the heterostructure NP bilayered nanosheets can exhibit different rectified currents for different ion concentrations. For example, for the 14.69/51.64 nanosheets, a larger transmembrane current was observed at a higher ion concentration. However, the current rectifying ratio (f = I1V/Iþ1V) decreased with increasing electrolyte concentration (Figure 2d). This behavior is related to the ionic Debye length λD,15,43 with λD 1/(cb)1/2, where cb represents the bulk ionic concentration. This dependency on the ion concentration is attributed to the overlapping electrical double layers (EDLs) in nanofluids.15,43 Ceteris paribus, the Kþ ions displayed a larger Debye length and suffered from larger restrictive effects from the EDLs, which would be a reasonable explanation for the trend observed in Figure 2d. More information on the regulatory effect of the ion concentration on the current rectifying ratio was obtained by gradually increasing the KCl concentration from 0.1 mM to 1 M (Figure 2e). For a very low electrolyte concentration, the large Debye length will inhibit the ion movement in the nanochannel, which consequently leads to a lower current rectifying VOL. XXX
’
NO. XX
’
000–000
’
ARTICLE
GFN can exhibit collective nanofluidic phenomena and ion selectivity suitable for practical applications (Figure 1a). Compared with other artificial functional nanochannel systems,32,33 our NP nanosheet-based ion-selective nanochannel arrays can provide a novel platform for ion manipulation in device applications.16,34 This work demonstrates that, when such nanochannels are adopted for membrane applications, an asymmetric ion transport can still be achieved and, at the same time, the current restrictions (nanoampere) observed for previous single-nanochannel systems could be overcome under a high ion flux along with high ion selectivity. We believe that NP nanosheetbased nanochannels represent an exciting new strategy to regulate the ion transport on a nanoscale, which offers many potential applications, e.g., in energytransforming devices and concentration gradient fuel cells, as well as ion detection and biosensor devices. As a model system, we report on the fabrication of NP nanosheet-based GFNs via a liquidair interfacemediated self-assembly of polymer ligand-capped NP superlattice sheets.24,35 We found that the asymmetric ion transport properties, including the rectified current and the rectifying ratio f, could be controlled by varying the size and surface chemistry of the constituent NPs. Furthermore, a simplified model was developed to predict the directional ionic selectivity, which was verified by experiments using NP nanosheet-based GFNs with different interparticle gap sizes. We expect that our model system can be extended to other soft ligands, e.g., functional molecules,24 DNA,36,37 and even a variety of other artificial elements from the periodic table of nanoparticles.38
C
XXXX www.acsnano.org
ARTICLE Figure 2. (a) Schematic illustration of the ion transport measurements performed on the heterostructure bilayered nanosheets in an electrochemical system equipped with a picoammeter. The gap between the two NP monolayers can be simplified as having a conical shape. The gap between the three adjacent spherical NPs in each monolayer can be defined as a conical nanochannel terminal of the gap-formed nanochannel (GFN) and characterized by Rtip and Rbase to restrict the GFN fabrication process. Because the surface of the Au NPs was modified with mercaptoacetic acid, the conical nanochannel can be regarded as having a negative charge under a neutral pH value. A KCl solution was used as electrolyte solution in combination with the Ag/AgCl electrodes. (b) TEM micrograph showing the gap between two NPs. The distance between two adjacent particles is denoted as d and the particle radius as R. (c) Ion transport characteristics obtained for monolayer nanosheets fabricated from 14.69 nm NPs (denoted as 14.69) and heterostructure bilayered nanosheets fabricated from 14.69 and 51.64 nm NPs (denoted as 14.69/51.64). The 14.69/51.64 nanosheets showed asymmetric ion transport characteristics, resulting in a diode-like current curve. (d and e) Ion transport characteristics obtained for the asymmetric 14.69/51.64 nanosheets for different KCl concentrations. The IV curves under 0.1, 1, and 10 mM KCl solution all show that the asymmetric NPs bilayered nanosheets possess asymmetric ionic transport properties; at the same time, the current rectifying ratio varies with the ion concentration. The IV results were obtained under 5 samples with 10 times scanning on each sample.
ratio.4345 All these features of our prepared heterostructured NPs nanosheets indicate obvious ionic selectivity and qualify this monolayer fabrication with possible applications for ionic gating. Another important parameter affecting the ion selectivity is the nanochannel terminal size. In our system, the NP gap size of each monolayer depends on Rtip and Rbase in the GFN conical model. As defined above, the largest circle that can be inscribed into the gap between the three adjacent NPs of each monolayer can be used to represent the nanochannel terminal, whose size can be calculated from the distance d between two adjacent NPs and the size R of the NPs (Figure 2b). As illustrated by the TEM investigations, the area-equivalent circle radius, i.e., Rtip and Rbase on each side of the GFN, can be obtained through
Rtip=base
ffi sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffi 1 3 (dþ2R)2 R2 ¼ 2 2π
in which R can be adjusted by varying the NP diameter, and d is related to the polymer chain length of PS (Figure S2). A statistical calculation was conducted using the TEM micrographs, and the results are shown in Figure S4. RAO ET AL.
To systematically analyze the asymmetric ion transport characteristics of the NP bilayered nanosheets, a simulation map has been created based on the conical nanochannel model to illustrate the relationship between the ionic current rectification and the Rtip/Rbase ratio. The qualitative analysis can be supported by considering a theoretical model consisting of coupled Poisson and NernstPlanck (PNP) equations,15,46,47 R which is illustrated as Ii ∼ 2πe rJi dr, i = þ, . Considering the nanochannel ion transport behavior for each terminal, the ionic current rectification relationship can be simplified to f ∼ (R2base/R2tip).48 The color gradient map shown in Figure S5 displays the variation of the ionic current rectification. For Rtip < Rbase, the analysis and discussion focuses on the section beneath the dotted line. When Rtip is larger than the ionic Debye length (triangular pink zone on the top-right), the ionic transport is not inhibited due to the lack of restrictions from the EDLs, demonstrating that the nanochannel is in an on-state.49 In contrast, when the nanochannel size is too small for a complete trans-channel movement of the ions under the effect of the EDLs in the nanochannel, the situation was denoted as the offstate (gray sections on the left and bottom). According to the simulation, a larger current rectification can be VOL. XXX
’
NO. XX
’
000–000
’
D
XXXX www.acsnano.org
CONCLUSIONS
Figure 3. Current rectifying ratio of the bilayered NP nanosheets can be regulated by varying the NP size. The black dots show the experimentally obtained current rectification ratios for different Rtip/Rbase ratios (14.69/14.69, 14.69/27.19, 14.69/51.64, and 14.69/76.57, respectively). In a certain range, the current rectifying ratio significantly increased with channel base sizes in a linear relationship. Each current rectifying result was obtained under 5 samples with 10 times scanning on each sample.
expected in the dark red zone, which corresponds to a larger Rtip/Rbase ratio. This map therefore provides a guideline for optimizing the ionic selectivity for a desired device application. Utilizing the simulation, we fabricated a series of nanosheets with different Rbase values (14.69, 27.19, 51.64, and 76.57 nm, Figure S6) with Rtip fixed to 14.69 nm. For each Rtip/Rbase pair shown in the TEM micrographs, the current rectifying ratio f is indicated by a dot in Figure 3. With increasing Rbase, the current rectifying ratio significantly increased, with a maximum rectification ratio of up to 13 for the 14.69/76.57 nanosheets. This trend indicates that a more effective
EXPERIMENTAL SECTION Preparation of the Au Nanoparticles. Gold(III) chloride trihydrate (HAuCl4 3 3H2O, g99.9%) was purchased from Sigma-Aldrich. Thiol-functionalized polystyrene (Mn = 50 000 g/mol, Mw/Mn = 1.06, for the preparation of NPs with a diameter >45 nm; Mn = 12 000 g/mol, Mw/Mn = 1.09, for the preparation of NPs with a diameter
