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Light- and electric-field-controlled wetting behavior in nanochannels for regulating nano-confined mass transport Ganhua Xie, Pei Li, Zhiju Zhao, Zhongpeng Zhu, XiangYu Kong, Zhen Zhang, Kai Xiao, Liping Wen, and Lei Jiang J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b13136 • Publication Date (Web): 15 Mar 2018 Downloaded from http://pubs.acs.org on March 15, 2018
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Journal of the American Chemical Society
Light- and electric-field-controlled wetting behavior in nanochannels for regulating nano-confined mass transport Ganhua Xie,1,2,4‡ Pei Li,2,3‡ Zhiju Zhao,5 Zhongpeng Zhu,2,4 Xiang-Yu Kong,2 Zhen Zhang,1,4 Kai Xiao,1,4 Liping Wen2,3,4* and Lei Jiang2,3,4* 1
Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Green Printing, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China 2
Key Laboratory of Bio-inspired Materials and Interfacial Science, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China
3
Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry of Education School of Chemistry and Environment, Beihang University, Beijing 100191, P. R. China 4 5
University of Chinese Academy of Sciences, Beijing 100049, P. R. China
College of Chemical Engineering and Biotechnology, Xingtai University, Xingtai 054001, P. R. China
KEYWORDS: Light, Electric field, nanopores, Supramolecular interaction, Mass transport ABSTRACT: Water wetting behavior in nano-confined environments plays an important role in mass transport and signal transmission of organisms. It is valuable and challengeable to investigate how water behaves in such a nanometer-scale with external stimuli, in particular with electric field and light. Unfortunately, the mechanism of hydrophobic reaction inside the nano-spaces is still obscure and lacks of experimental supporting for the current electric-field- or photoresponsive nanochannels which suffer from fragility or monofunctionality. Here, we design functionalized hydrophobic nanopores to regulate ion transport by light and electric field using azobenzene-derivatives-modified polymer nanochannels. With these addressable ways, we can control the pore surface wetting behavior for switching the nanochannels between non-conducting and conducting states. Furthermore, we found the hydrophobic nanochannels are rough with contact angle of 67.3o, which are extremely different from the familiar ones with smooth pore surface and larger contact angles (>90o). These findings point to new opportunities for studying and manipulating water behavior at nano-confined environments.
INTRODUCTION The behavior of water in nano-confined environments is very different from that of bulk water and plays an essential role in a variety of life activities, such as selective ion transport and water permeation.1 It has been proved theoretically and experimentally that water would stay vaporlike when it is confined between two hydrophobic plates less than 100 nm. 2 In nature, the hydrophobic domain of the biological ion channels contributes to gating water and controlling release of ions and neutral species across membranes for metabolic and signaling purposes.3 Therefore, it is valuable and challengeable to study how water permeates through the hydrophobic nanochannels. Using natural and artificial hydrophobic nanochannels, scientists have made great efforts to investigate water behaviors at the nanometer scale, for example in biological aquaporins4 and carbon nanotubes5. Besides, to mimic biological controllable mass transport, synthetic nanopores with various functions were also developed, which can be regulated by a variety of stimuli, such as
specific ions,6 pressure,7 electric field8 and light.9 Among these addressable systems, photo- and electric-fieldresponsive hydrophobic nanochannels attracted intensive attention since their short response times and reversibility allowed switching between different states rapidly and repeatedly. For example, Smirnov and coworkers fabricated a light-controlled valve by a photochromic spiropyran to control nanopore wetting and acted as an electrical switch.9e To achieve a multi-responsive system, Feringa’s group reported an engineered mechanosensitive channel MscL by attachment of synthetic compounds that undergo pressure- and light-induced switching.9f However, the current photo- and electric-field-responsive ion channels suffer from monofunctionality or inherent instability and restrictions in the environmental conditions they operate in, which limit their practical applications. Also, the mechanism of the hydrophobic reaction inside these nano-spaces is still obscure and lacks of experimental supporting. Therefore, developing light- and electricf i e l d - c o n t r o l l e d
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Figure 1. Characterization of functional molecules and experimental setup. A) Schematic representation of a photo-responsive complex of azobenzene derivative and β-CD. B) Photoisomerization spectra of azobenzene derivatives in the presence of β-CD under UV and Vis irradiation, respectively. C) Schematic illustration of the experimental setup for transmembrane ionic current measurement. Ag/AgCl was used as electrodes. The anode is faced the base sides of the conical nanochannels. D) SEM image of the conical polymer nanochannels from the top view. The base sides of the nanochannels are ~500 nm. E) I-V curves of nanoporous membrane before and after azobenzene derivative modification.
hydrophobic nanochannels to regulate mass transport at the nanometer scale is an interesting and valuable challenge. Here, we present hydrophobic nanochannels whose wettability can be controlled by light and electric field. By modifying the inner surface with hydrophobic azobenzene derivatives, the solid-state nanochannels switched between wetted conductive state and dewetted nonconductive state. These processes can be regulated by ultraviolet (UV)/visible (Vis) light and different voltages, which is potential to be used for regulating ions and water transmembrane transport. Azobenzene derivatives (Azo) are well-known photochromes that can be reversibly isomerized between trans and cis conformations and experienced d-spacing changes by UV (365 nm) and Vis lights (430 nm).10 The photoisomerization of azobenzene derivatives can be used to control the inclusion and exclusion with β-cyclodextrins (CD) whose inner cavities are hydrophobic and outer surfaces are hydrophilic due to the hydrophobic and van der Waals interactions of this host-guest reaction (Figure 1A). These machinelike switching motions have been applied in molecular motors and information storage.11 As shown in Figure 1B, the absorption of trans-Azo at 346 nm is enhanced after treating with β-CD and the corresponding molar extinction coefficients increase from 3.16×10-3 M-1cm-1 to 3.72×10-3 M-1cm-1. Therefore, the enhanced absorption should be caused by the enhanced molar extinction coefficient after inclusion.
In other words, the enhanced absorption suggests the formation of an inclusion complex between Azo and β-CD after Vis irradiation; Meanwhile, the absorption of cis-Azo in the presence of β-CD under UV irradiation is almost identical to pure cis-Azo, which suggested that the cisAzo hardly interact with β-CD.12 Besides, the hydrophilic surface of the β-CD facilitates the Azo solubility in water after forming the inclusion complex (Figure S1). Therefore, combining with photo-stimuli, β-CD can be utilized to control the wettability of Azo-modified surface. RESULTS AND DISCUSSION The solid-state nanochannels were fabricated via iontrack-etching in 12-μm-thick polyimide (PI) membranes (pore density: 1x106 cm-2, as shown in Figure S2). After ~1 h asymmetric etching, nanochannels with conical shape were obtained and characterized by I-V measurement as shown in Figure 1C. The base sides (large opening) of the studied pores were ~500 nm in diameter (Figure 1D), and the tip sides (small opening) were calculated with a parallelly etched single nanochannel (Figure S3). The resulted carboxyl groups on the pore surface render the pores hydrophilic and allow electrolyte solutions to go through, exhibiting excellent ion-current rectification (Figure 1E). After grafting these carboxyl groups with azobenzene derivatives (phenyldiazenyl)phenoxy)propan-1-amine via a two-step method (Figure S4, S5 and S6), the pore surface became hydrophobic, which was confirmed by the new peak
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Figure 2. Light-responsive properties of the Azo-modified nanochannels. (A) Schematic of light-responsive host-guest reaction between azobenzene derivative and β-CD in nano-confined environments. (B) I-V characterization of the reversible Azo/β-CD reaction controlled by light in the nanochannels. (C) Current recordings at -1 V under UV, Vis, β-CD, UV/Vis, UV/β-CD and Vis/β-CD, respectively. (D) Optical reversibility of the opening and closing in these Azo-functionalized nanochannels system by recording currents at -1 V.
of nitrogen element (Figure S7) and the nonconductive state of the functional membrane with pore sizes ranged from 10 nm to 15 nm (Figure S8).8b The coverage of Azo groups on the surface is estimated to be ~90% (Figure S9, Table S1 and S2). Unless otherwise mentioned, the pore size of the used PI membrane in this work is 10-15 nm. The transport properties of the Azo-modified nanochannels were studied under photo-stimuli. It has been shown previously that long hydrophobic nanopores possess non-conducting, closed state characterized by no currents, which are extremely different from those of biomimetic nanopores gating by physical conformation changes or charge changes.8b Figure 2A shows the ion current through nanopores that were modified with Azo molecules. After modification with hydrophobic azobenzene derivatives, the nanochannels became closed and the transmembrane ionic current became constant at 0 when we studied the membrane with scanning voltage from -2 V to +2 V (Figure 2B). The non-conducting behavior with no current through the nanopores was extremely different from the rectified I-V response of unmodified nanochannels with large currents in forward direction and small currents in backward directions (for example, 356 nA at -2 V, as shown in Figure 1E).13 When being treated with 10 mM β-CD solution under irradiation of Vis
light, the nanopores were recovered to be open and ionic currents were observed but smaller than those of the unmodified nanopores (-12 nA at -2 V, as shown in Figure 2B). The reduced ionic currents were attributed to the modified layer of Azo molecules which not only covered some surface charges but also reduced the effective pore size to some extent. The rectified ionic current of the Vislight-responsive nanopores indicated there are some residual negative charges.8b, 14 We studied the photo-responsive wettability changes in nano-confined environments with different irradiation conditions by recording 5-min-long ion-current time series at forward direction (-1 V), which facilitates the cations transport trough the conical nanochannels. For treatments with UV, Vis, β-CD, UV/Vis and UV/β-CD, the pores were completely closed and no current were recorded. When being exposed to Vis light and 10 mM β-CD solution,12d burst of large-amplitude current signals of several nanoamperes could be observed, which means that the pores became open and conductive. Under this condition, the pore can stay in the open state (Figure 2C). The results were in agreement with corresponding I-V curves, in which a rectified line was found only in Vis/βC D a n d t h e o t h e r s
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Figure 3. Electric-field-responsive properties of the hydrophobic nanochannels. (A) Current recordings under various voltage ranged from 0 to -2.8 V in the step of 0.2 V. There is a burst of currents after applying -2.6 V for ~4 min. (B) I-V scans under -2 V to 2 V, -3 V to 3 V, -4 V to 4 V and -5 V to 5 V, sequentially. At the scan 4 (-5 V to 5 V), a typical rectified I-V curve was observed. The inserted picture showed the top-ten scanning cycles of scan 4. (C) Reversibility of the opening and closing states in these hydrophobic nanochannels with different voltages.
coincided with x-axises (Figure 2B and S10). Unless otherwise mentioned, the intensities of the UV and Vis light are 30 mW cm-2 and 200 mW cm-2, respectively. In addition, the Azo-functionalized nanopores can be switched between conducting and non-conducting states by light. After opening with Vis/β-CD, the pores can be switched to be closed by irradiating with UV light and recover to a nonconductive state. The photoresponsive switching process can be achieved reversibly when the pores were treated with Vis/β-CD and UV, alternately (Figure 2D). We believed this photo-reversibility was ascribed to hydrophobic host-guest interaction between Azo and β-CD, which controlled the surface wettability of pores at the nanoscale. The photo-reversibility of this this system makes it extremely different from the other amphiphile ‘soaps’ (Figure S11). It is worth noting that it is relatively easy to open the nanopores and make the membrane conductive, but it takes several hours to recover its non-conducting state using UV irradiation. Such a hysteresis also occurs in hydrophobic filters of biological ion channels which contributed to gating the flow of matter through the channels.3b We believe that the activation barrier for dewetting the nanochannels is so large that it is hard to dry the hydrophobic pores by capillary evaporation, especially in such long nanopores.3a,3c Therefore, we cannot switch the pore to be closed by UV as easily as the opening process by Vis/β-CD.
Since the Azo-modified nanopores were hydrophobic, we studied the electric-field-induced wetting and dewetting behavior at such a hydrophobic nano-confined environment. To investigate the voltage-dependent ionic transport, we monitored the transmembrane ion current by recording ion-current time series with different voltages in step of 0.2 V from 0 to -2.8 V. As shown in Figure 3A, for voltages below -2.6 V, the pore was totally closed and no current was recorded. At -2.6 V, the current initially remains constant at 0 for ~4 min and then suddenly burst with distinct current values. When increasing the applied voltages above -2.8 V, the transmembrane currents enlarge with increasing voltages and become more and more stable to almost remain constant (Figure S12). This behavior can also be found in previously reported electric-field-induced hydrophobic nanopores but hardly discussed.8b We propose that the process of voltage-induced wetting in the long nanopore must last for enough time to fill the channel with electrolyte solution. Therefore, the hydrophobic nanopores can be completely opened for enough time at critical voltage. The data shown in Figure 3B were obtained by a series of sweeps from -2 V to +2 V, from -3 V to +3V, from -4 V to +4 V and from -5 V to +5 V, with step of 0.2 V/s. No current was recorded in the former three series scans, and their I-V curves coincided with x-axises, which means the hydrophobic nanopores were still closed even above the critical voltage for tens of seconds. While in the scan from -5 V to
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Figure 4. Mechanism speculation about the photo- and electric-field-responsive nanochannels. Contact angles of Azo-modified o o PI surface by treating with Vis/β-CD (A: 16.5±1.8 ) and UV/drying (B: 67.3±0.2 ), respectively. (C) An AFM image of the etched PI film. The roughness (Ra) is 1.53 nm. (D and E) Lateral Force Microscope (LFM) images of the Azo-modified PI film by treating with Vis/β-CD and UV/drying, respectively. Fluorescence microscope images of the PI surface before (F) and after (G) Azo modification.
+5 V, we can observe a rectified I-V curve, which was an averaging result of 20 parallel scans. In fact, in the first scan, the membrane is still nonconductive. From the second scan, burst of current signals were observed and the current fluctuate a lot until the tenth scan. During this fluctuation process, the I-V curves have the tendency to be a typical and stable rectified line. After ten scans, the IV curves become stable (Inserted picture in Figure 3C). The process of dynamical opening means that the voltage-dependent wetting process in hydrophobic nanopore is correlated to the applied time and this result is in agreement with our previous proposal. To confirm the electric-field-responsive reversibility of the hydrophobic nanopores, we continued to scan the membrane by sweeps from -5 V to +5 V, from -4 V to +4 V, from -3 V to +3 V and from -2 V to +2 V, sequentially (Figure S13). Different from the opening process, the I-V curves of all scans were rectified and stable. We believe the reason of this result is in agreement with the hysteresis behavior of UV-induced closing process that it is hard to dry the hydrophobic nanopores once the pore wetted. After 1 h drying treatment with N2, the transmembrane ion current recovered to be 0 and the membrane recovered to be nonconductive. This voltage-induced switching between conductive and nonconductive states can be totally repeated without damping under critical voltage (2.6 V) or higher voltages (-3.8 V, for example). The observed phenomena can be associated with the wettability switching of the pore surface, which resulted in the changes of the interface force in the nanochannels. The equilibrium of forces at a water-gas interface in a nanopore can be described by balancing the pressure difference with the capillary force9f
P P
∆γ
(1)
where PE is the external pressure outside the membrane, PI is the pressure inside the pore, and R is the radius of curvature of meniscus at the pore mouth. The surface difference Δγ is calculated via the Young equation15 Δγ ≡ γ γ γ cos
(2)
where γwl is the surface tension of the wall/liquid interface, γwv is the surface tension of the wall/vapor interface, and γ is the free surface tension of the liquid/vapor interface. Here, it is necessary to clarify the definition of hydrophobility. At nanometer scale, the structural density of confined water molecules was variable, allowing attractive or repulsive forces determining the wettability or the surface energy of immersed surfaces. Long-range attractive forces could be detected when the surface exhibits a water contact angle > 65°. In contrast, repulsive forces could be found between surfaces with contact angle < 65°. In other words, from the view of the chemical and structural states of water drops, the division of hydrophilicity and hydrophobility is 65o (critical contact angle, θc) rather than the generally accepted 90o as in the mathematical concept.16 Besides, we believe the etched surface of the PI nanochannel is rough at the nanometer scale as shown in Figure 4A and S14, which is extremely different from the reported smooth interface in the pores. Considering the enhancement effect of surface roughness,17 Equation 1 can be modified as
∙γ
(3)
where θe is the apparent contact angle and f is correction factor which is a positive number unless θc
