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Biomimetic Voltage-Gated Ultra-Sensitive PotassiumActivated Nanofluidic based on Solid-State Nanochannel Kai Wu, Kai Xiao, Lu Chen, Ru Zhou, Bo Niu, Yuqi Zhang, and Liping Wen Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b01705 • Publication Date (Web): 08 Aug 2017 Downloaded from http://pubs.acs.org on August 13, 2017
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Biomimetic Voltage-Gated Ultra-Sensitive Potassium-Activated Nanofluidic
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based on Solid-State Nanochannel
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Kai Wu,1,† Kai Xiao,2,4, † Lu Chen, 3 Ru Zhou,1 Bo Niu,3,4 Yuqi Zhang,1,* and Liping Wen3,* 1
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College of Chemistry and Chemical Engineering, Shaanxi Key Laboratory of Chemical Reaction Engineering, Yan’an University, Yan’an, Shaanxi Province, 716000, P. R. China
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ABSTRACT
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In living organism, voltage-gated potassium channels play a crucial role and are largely
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responsible for various vital movements. For life science, it is significant and challenging to
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imitate and control the potassium ions transportation with a convenient artificial system. Here,
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we reported a voltage-gated ultra-sensitive potassium-activated nanofluidic system using 4′-
19
Aminobenzo-18-crown-6 molecules functionalized funnel-shaped solid-state nanochannel. The
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switch-like property between open and close states can be tuned freely by reversible
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immobilization and release of potassium ions. By virtue of good reversibility and excellent
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stability, this system can potentially be applied in controlled drug release and biosensors.
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KEYWORDS: Nanochannel, Nanopore, Nanofluidic, Ionic gating, Ion transportation
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Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Green Printing, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China. 3
Laboratory of Bioinspired Smart Interfacial Science, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China.
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University of Chinese Academy of Sciences, Beijing 100049, P. R. China.
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Potassium channels are the most widely distributed type of ion channels and can be found in
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all living organisms. They are largely responsible for shaping the electrical behavior of cell
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membranes, for example, controlling the action potential duration, the rate of action potential
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firing, the spread of excitation and Ca2+ influx, and providing active opposition to excitation.1
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Due to the different gating mechanisms in the conduction pathway to control current flow,
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potassium channels have been classified as many different types. Among them, the voltage-gated
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potassium channel is a type of nanochannel whose opening or closing is responsive to changes in
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the cell membrane voltage.2 The specific voltage-responsiveness endow the voltage-gated
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potassium channels broad applications not only in the effective drug targets for diseases such as
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myotonic muscular dystrophy and sickle cell anemia in vivo, but also in the biosensors,
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nanofluidic devices and controlled nano-valves in vitro. However, these protein channels,
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embedded in fragile lipid bilayers, are hard to be applied in changing external environments,
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which have limited their practical applications.1
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To address this fundamental gap, we reported a biomimetic voltage-gated ultra-sensitive
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potassium-activated nanofluidic system based on solid-state nanochannel. The solid-state
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nanochannels are inspired by the biological protein channels and have similar structure and
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functions with them.3-5 In recent years, solid-state nanochannels have been well developed for
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the purpose of constructing smart materials with various applications in biosensors,
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micro/nanofluidic devices, and clinical medicine settings due to their numerous advantages, for
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example, stability, easy modification and flexible geometry.6,
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nanopores/nanochannels based on multifarious materials have been constructed, such as
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biomaterials nanopores,8 inorganic nanochannels,9 polymer nanochannels,10 and heterogeneous
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nanochannels.11
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nanopores/nanochannels in the field of ionic regulation, many functional groups which are
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responsive to environmental stimuli have been introduced into solid-state nanochannels, and
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realized various responsive systems, including pH,12 light,13 temperature,14 and pressure.15
Meanwhile,
to
further
expand
the
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Many kinds of solid-state
application
of
solid-state
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Figure 1. From biological potassium channels to biomimetic system. (a) The schematic
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demonstration of the simplified voltage-gated potassium channels in living systems and the
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artificial voltage-responsive potassium-activated ionic gating. (b) The cross section SEM image
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of the funnel-shaped nanochannel composed of a long nano-sized cylindrical segment and a
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conical segment. The scale bar is 1 µm. (c) The schematic representation of the modification and
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voltage-gated potassium-activated process: i) The carboxyl groups in the inner surface of PET
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nanochannel after etching. ii) The electroneutral channel surface after modification of 4-
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AB18C6. iii) The embedding of K+ into the cavity of 4-AB18C6 molecules, which increased
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space steric (close state). iv) The electroneutral channel surface after releasing of K+ under
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external voltage.
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In this work, the funnel-shaped poly(ethylene terephthalate) (PET) solid-state nanochannels
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were modified with 4′-Aminobenzo-18-crown-6 (4-AB18C6) to realize smart potassium
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responsive gating mimicking protein potassium channel. This artificial gating system is similar
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but not identical with biological protein channel. To the protein voltage-gated potassium channel,
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it is sensitive to transmembrane voltage, which can open or close the channels to allow
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potassium ions transportation. But for the biomimetic artificial system, the potassium ion can
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bind to 4-AB18C6 molecules in the channel firstly, and then can be driven out of the channel by
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external voltage (Figure 1a). As a result, the switchable and controllable states between opening
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and closing of the channel can be realized. Particularly, the recyclable voltage-gated potassium-
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activated nanochannel was based on the biomimetic funnel-shaped nanochannel, which was
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fabricated from an asymmetric two-step track-etching technique in a 12-µm-thick PET
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membrane (Supporting Information, Figure S1).16,
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The cross-section scanning electron
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microscope (SEM) image (Figure 1b) showed that the prepared funnel-shaped nanochannel was
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composed of a conical segment about 7.5 µm and a cylindrical segment about 4.5 µm. The base
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(large) opening was about 600 nm while the tip (small) opening was about 15 nm (Supporting
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Information, Figure S2). The funnel-shaped channel is more stable and sensitive compared with
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other symmetric or asymmetric channels because of the long nano-sized cylindrical segment.16
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For the etched funnel-shaped PET nanochannel, carboxyl groups were formed in the etching
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process (Figure 1c i).18, 19 The 4-AB18C6 molecules were immobilized in the PET channel by a
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conventional
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sodium salt (EDC/NHSS) coupling reaction with the carboxyl groups. The channel surface
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changed from electronegative state to electroneutral state after the modification process and the
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gating in this condition was defined as open state (Figure 1c ii). Then, the addition of K+ ions
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closed the gating because K+ ions entered into the cavity of 4-AB18C6 molecules (Figure 1c iii),
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which changed the surface charge properties and increased the space steric.20, 21 After applying a
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constant external voltage, the gating opened again because K+ ions were released from the
1-ethyl-3-(3-dimethyllaminopropyl)
carbodiimide/N-hydroxysulfosuccinimide
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nanochannel (Figure 1c iv) and transported to the other side of the channel. A reversible ionic
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gating, therefore, was realized.
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Figure 2. The ionic transport properties of the K+ activated ionic gating. (a) The I-V curves of
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the unmodified nanochannel before (dark cyan square) and after K+ activation (dark yellow
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square). (b) The I-V curves of the nanochannel modified with 4-AB18C6 before (dark cyan
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square) and after K+ activation (dark yellow square). (c) The ionic current ratios (IK+/I) of the
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unmodified and 4-AB18C6-modified nanochannels after activation with 10 µM K+ at +1.4. V
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(light gray) and -1.4 V (dark gary), respectively.
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Current-voltage (I-V) curves of the funnel-shaped nanochannel before and after modification
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with 4-AB18C6 and in the absence or presence of K+ were used to characterize open/close states
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of the ionic gating, which were measured in 0.1 M Tris-HCl (pH 4.5) solution. As shown in
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Figure 2a, unmodified funnel-shaped nanochannel exhibited an ionic diode property with -25 nA
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under -1.4 V and 5 nA under +1.4 V owing to its asymmetric structure and negative surface
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charge.22, 23 Unsurprisingly, the I-V curve had no obvious change after addition of 10-6 M K+ ions
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to unmodified nanochannel. After modification with 4-AB18C6 molecules, the ionic current
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decreased markedly because the channel diameter and surface charge decreased (Figure 2b,
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Supporting Information, Figure S3). The X-ray photoelectron spectra (XPS) characterization
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(Supporting Information, Figure S4, Table S1, and Table S2) and contact angle measurements
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(Supporting Information, Figure S5) of PET membrane surface before and after modification of
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4-AB18C6 also confirmed the binding behavior. When K+ ions with concentration of 10-6 M
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were added to the system, the current decreased obviously from 6 nA to about 2 nA both under -
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1.4 V and +1.4 V (Figure 2b). This phenomenon can be mainly attributed to the decreased
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surface charge density.24, 25 To the 4-AB18C6 modified nanochannel, some residual negative
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charge was still existed in the channel because of incomplete modification. When the
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nanochannel was activated with K+, part of negative charge on the inner surface of our system
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was neutralized (Supporting Information, Figure S6 and S7). Ultimately, the nanochannel
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was maintained electroneutral state and the gating changed from open to close state.
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The K+ ions activated gating also can be characterized by the current ratios (IK+/I), in which I
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and IK+ are the ionic currents measured before and after the addition of K+ at -1.4 V and +1.4 V,
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respectively. For unmodified channel, the current ratios (IK+/I) were about 1.0, while the ratios of
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4-AB18C6-modified channel were about 0.25 both under positive and negative voltage (Figure
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2c). It demonstrated that embedding of K+ into 4-AB18C6 can activate the ionic gating and alter
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the ion transportation in the channel effectively.
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The activation process, the gating changing from open state to close state, could be gradually
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realized by introducing K+ ions. The transformation process was monitored by immersing
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nanochannel in different concentrations KCl solutions for same time and recording conductance
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of the channel when applied voltage was -1 V and +1 V, respectively (Figure 3a and Figures S8).
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The conductance of the gating without K+ ions activation was about 3.4 nS under -1 V and 2.2 nS
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under +1 V, which represented that the gating was in open state. Then we immersed 4-AB18C6-
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modified nanochannel in 10-15 M KCl solution for 2h, and tested the conductance. After
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introduction of K+ ions, the conductance of the gating decreased to 2.7 nS under -1 V and 2.0 nS
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under +1 V. Along with the increase of K+ ions concentration, the gating was closed gradually.
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When the concentration of K+ ions increased to 10-9 M, the conductance decreased to a minimum
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value about 1.5 nS under -1 V and 0.7 nS under +1 V, and then leveled off with sequentially
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increasing the concentration of K+ ions. This is because all the 4-AB18C6 molecules modified in
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the channel have combined with K+ ions in this condition.
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Figure 3. The dependence of the K+ activated ionic gating on the potassium ions concentrations
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and external voltage. (a) The conductance of the 4-AB18C6-modified channel before and after
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activation with different concentrations of K+ under +1.0 V (dark yellow square) and -1.0 V
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(dark cyan square). (b) Ionic currents before (black line) and after (green line) activation with 10-
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before and after applying an external voltage of 4V.
M K+ at different constant voltage. (c) The K+ ions concentration in the Tris-HCl solution
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Furthermore, the ultra-sensitive potassium-activated ionic gate was sensitive to external
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voltage. The voltage-gated transport properties can be demonstrated by measuring the ionic
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currents under constant voltage. As shown in Figure 3b, the ionic current before activation with
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potassium ions stabilized around 2.3 nA under +1 V, in which condition the gating kept open.
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After addition of K+ ions with 10-5 M, the ionic current decreased to about 0.7 nA under +1 V. It
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was a stable close state. However, the ionic current increased remarkably rather than stabilized in
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a certain value when the applied voltage was +2.0 V according to the electric-field wetting
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effect.26, 27 It can be clearly found that the ionic current continuously and sharply increased from
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about 14 nA to 28 nA at +3.0 V. Subsequently, the current across the nanochannel under +1 V
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was tested and a stable ionic current about 2.3 nA was obtained again, which indicated that the
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K+ activated gating can be recovered to the initial open state by applying a constant voltage. This
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phenomenon can be well explained by the corresponding bonding and releasing process of K+
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ions in the 4-AB18C6-modified nanochannel driven by voltage.28, 29 Before activation with K+
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ions (State 1, Figure 3b), the 4-AB18C6-modified nanochannel was hydrophilic and the ionic
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current was high (open state). After 4-AB18C6 binded K+ ions, the ionic current decreased (close
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state) due to change in surface charge properties and wettability (Figure S5). Subsequently, the
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K+ ions bound in the nanochannel can be removed from the channel to the bulk Tris-HCl
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solutions by applying a higher constant voltage (State 2, Figure 3b), and then the channel return
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to the open state (open state).
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This process can be confirmed by comparing the K+ ions concentration in the Tris-HCl
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solutions before and after applying constant voltage by inductively coupled plasma mass
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spectroscopy (ICP-MS).30 Figure 3c showed that the K+ ions concentration in the Tris-HCl
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solution before applying voltage was about 18.5 ppb, which may be ascribed to the instrument
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itself because the K+ ions concentration was as low as the detection line. After applying a
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constant voltage, the K+ ions concentration in the Tris-HCl solutions significantly increased to
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about 130 ppb, which indicated that the majority of K+ ions were released into the solution and
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the gating was recovered to the initial open state. Through the measurement, the surface charge
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density is estimated to be about 0.12e nm-2 (supporting information S9). In this way, the
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estimation might be seen as an alternate experimental measure of surface charges and effective
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area.31
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Figure 4. The reversible I-V curves of the 4-AB18C6-modified nanochannel after alternately
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adding K+ ions and applying a voltage of 4 V.
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The biomimetic voltage-gated potassium-activated nanochannel has also been endowed
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excellent reversibility and stability when alternately exposed to K+ ions and applied an external
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voltage. Figure 4 exhibited the cycling performance of the nanochannel, which can be illustrated
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by the I-V curves measured under the condition of adding K+ ions and applying voltage,
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respectively. The ionic current of the 4-AB18C6-modified nanochannel activated by K+ ions was
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about 2 nA under +1.4 V and -2.5 nA under -1.4 V, in which the gate was in a close state. After
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applying an external voltage of 4 V, the ionic current under +1.4 V increased to about 5 nA while
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the current under -1.4 V decreased to about -7.8 nA due to the release of K+ ions, in which the
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gate was in an open state. The responsive switchability from close state to open state remained
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stable and the ionic current had no obvious attenuation after several cycles. Therefore, the 4-
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AB18C6-modified nanochannel system can be used as a multifunctional ionic gate with strong
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stability and good reversibility.
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In summary, we described here the construction of voltage-gated ultra-sensitive potassium-
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activated gating system based on solid-state funnel-shaped nanochannel. Construction of such a
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gating system required the modification of the inner surface with 4′-Aminobenzo-18-crown-6
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molecules. The 4-AB18C6 molecules could combine with K+ ions, even concentration low to 10-
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transmembrane ionic currents. Meanwhile, the release of K+ ions can be realized by applying an
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external constant voltage. In this way, the switchability between open state and close state can be
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easily obtained. The gating is similar to the protein voltage-gated potassium channel and exhibits
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excellent stability compared to the biological nanochannel, which has potential application in the
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field of controlled drug release and biosensors.
M, to change the surface charge and wettability of the channel, and then regulate
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ASSOCIATED CONTENT
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Supporting Information. Details of the experimental setup fabrication and characterization, X-
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ray photoelectron spectra characterization, contact angle measurement, and the dependence of
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the K+ ions concentrations. These materials are available free of charge via the Internet at
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http://pubs.acs.org.
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AUTHOR INFORMATION
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Corresponding Author
17 18 19
[email protected] (L.W.) and
[email protected] (Y. Z.) Author Contributions
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[†]
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Notes
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The authors declare no competing financial interest.
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ACKNOWLEDGMENT
These authors contributed equally to this work.
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The authors thank the Material Science Group of GSI (Darmstadt, Germany) for providing the
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ion-irradiated samples. This work was supported by the National Key Research and
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Development Program of China (2017YFA0206904, 2017YFA0206900), the National Natural
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Science Foundation (21625303, 51673206, 21434003, 91427303, 21421061), the Key Research
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Program of the Chinese Academy of Sciences (KJZD-EW-M03), the Natural Science
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Fundamental Research Program Key Projects of Shaanxi Province (2016JZ005) and the Key
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Laboratory Research Program of the Education Department of Shaanxi Provincial Government
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(15JS121).
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We report a biomimetic voltage-gated ultra-sensitive potassium-activated nanofluidic system.
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The states between open and close can be switched freely when the prepared gating was
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alternately exposed to K+ ions and applied an external voltage. This system can potentially be
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applied in controlled drug release and biosensors.
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