Sequential Vapor Infiltration Treatment Enhances the Ionic Current

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Sequential Vapor Infiltration Treatment Enhances the Ionic Current Rectification Performance of Composite Membranes Based on Mesoporous Silica Confined in Anodic Alumina Yanyan Liang and Zhengping Liu* Beijing Key Laboratory of Materials for Energy Conversion and Storage, BNU Key Laboratory of Environmentally Friendly and Functional Polymer Materials, College of Chemistry, Beijing Normal University, Beijing 100875, P. R. China S Supporting Information *

ABSTRACT: Ionic current rectification of nanofluidic diode membranes has been studied widely in recent years because it is analogous to the functionality of biological ion channels in principle. We report a new method to fabricate ionic current rectification membranes based on mesoporous silica confined in anodic aluminum oxide (AAO) membranes. Two types of mesostructured silica nanocomposites, hexagonal structure and nanoparticle stacked structure, were used to asymmetrically fill nanochannels of AAO membranes by a vapor-phase synthesis (VPS) method with aspiration approach and were further modified via sequence vapor infiltration (SVI) treatment. The ionic current measurements indicated that SVI treatment can modulate the asymmetric ionic transport in prepared membranes, which exhibited clear ionic current rectification phenomenon under optimal conditions. The ionic current rectifying behavior is derived from the asymmetry of surface conformations, silica species components, and hydrophobic wettability, which are created by the asymmetrical filling type, silica depositions on the heterogeneous membranes, and the condensation of silanol groups. This article provides a considerable strategy to fabricate composite membranes with obvious ionic current rectification performance via the cooperation of the VPS method and SVI treatment and opens up the potential of mesoporous silica confined in AAO membranes to mimic fluid transport in biological processes.

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INTRODUCTION Nanofluidics studies on the phenomena and applications of fluid transport through nanometer-scale geometries with the smallest dimension being less than 100 nm have been conducted.1 Asymmetric ionic current transport in a nanofluidic device gives rise to ionic current rectification, which is characterized by nonliner, diodelike current−voltage (I−V) curves.2 Ionic current rectifying nanofluidic diodes allow ionic current to be preferentially transported through nanofluidic devices in one direction while prohibiting ionic current transport in the other direction, depending on the polarity of the applied bias,3 which is similar in principle to the gating performance of biological ion channels.4,5 Ionic current rectification usually stems from the broken symmetry of the systems6 such as asymmetric geometry with charged surfaces,7,8 discontinuous charged surfaces,9,10 asymmetric nanofluidic/microfluidic interfaces,11−13 asymmetric wettability,14 and asymmetric electrolytes.15 Research on the manufacture and application of nanofluidic diode membranes with a current rectifying effect provides an avenue to mimic the biological ion channels in a cell membrane using nanofabrication techniques.7−12,14−18 Unique physical and chemical properties such as hardness, self-standing, a uniform pore diameter, and a vertically aligned © XXXX American Chemical Society

nanostructure make AAO membranes a suitable candidate for the fabrication of nanofluidic diode devices.7−12,14−18 The nanofluidic diode membranes with a dimension ranging from 15 to 60 nm were constructed in AAO membranes via the electrochemical anodization technique or the modified anodization process.7,8,10,16−18 On the basis of the asymmetric structure of AAO membranes, nanofluidic diode membranes with branched7,16 and hourglass-type8 architecture were created. Besides these, geometrically symmetric nanochannels of AAO membranes with disconnected charged surfaces derived from a silane agent were constructed to modulate the rectification of ionic current.10 Furthermore, complicated heterostructured Al2O3/SiO2 nanofluidic diode membranes were fabricated successfully with combinations of the modified anodization process and micro/nanofabrication techniques including reactive ion etching,17 atomic layer deposition, and reactive ion etching.18 However, the electrochemical anodization method becomes ineffective when the dimension is less than 10 nm.19,20 Because the natural ion channel possesses a pore entrance of several Received: September 23, 2016 Revised: November 22, 2016 Published: November 23, 2016 A

DOI: 10.1021/acs.langmuir.6b03495 Langmuir XXXX, XXX, XXX−XXX

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explore ion transport in biological process and widens the potential applications in water purification and biosensors.

angstroms, nanofluidics with a smaller size are beneficial to mimicking the transport behavior of the biological ion channel.21,22 Compared to the AAO membrane, a mesoporous silica film prepared via the self-assembly method can control the pore size to between 2 and 50 nm. Artificial ion channel architectures had been built on the silica film with 3D cubic mesopores smaller than 10 nm and exhibited proton selectivity.23,24 The ionic mesochannels’ silica film constructed by columnar mesopores with a pore diameter smaller than 10 nm and further modified with phosphate-bearing polymer brushes showed excellent proton and calcium gating performance.25 The silica film with mesoporous architecture often grows on the silicon substrate because of its poor mechanical strength, which makes it difficult to determine the transmembrane ionic currents across the membranes directly.26 The connected mesoporous silica embedded in self-supported porous membranes by template synthesis probably can overcome this limitation.27 The AAO membrane is not only a suitable candidate for fabricating nanofluidic devices as mentioned before but also an excellent platform in template synthesis and charge transport.19 Three methods, sol−gel synthesis, evaporation-induced selfassembly synthesis, and VPS, are commonly used for the preparation of mesostructured silica within AAO membrane.28 The VPS method is the simplest one among them, and the mesostructured silica confined in AAO membranes obtained by this method is robust and less contracted because of the relatively high temperature and concentration of the structuredirecting agent (SDA) during the VPS process.29 But some defects still existed in the prepared composite membranes, such as silica nanorods detaching from the AAO membrane and incomplete filling in the AAO membrane, leading to a current leakage.30 SVI treatment is a powerful approach to preparing a defect-free silica film.31 In addition, SVI treatment had been confirmed to improve the electrical properties32 and the proton conductivity33 and decrease the dielectric constants34 and refractive indexes.35 The reaction of tetraethyl orthosilicate (TEOS) vapor with silanol groups on mesoporous silica walls enables the repair of cracks in the synthesis membranes, enhances the stability of the framework, and increases the hydrophobicity of the framework after SVI treatment.36,37 Herein, we employ the VPS process with an aspiration approach to confine the mesoporous silica-SDA nanocomposites in nanochannels of porous AAO membranes and then utilize SVI treatment to modify synthesized membranes to fabricate nanofluidic diode devices. The inherently asymmetric filling of the mesostructured silica composite in the VPS process38 results in the distinction of surface conformations, wettability, and silica species components of two opposite surfaces of as-synthesized membranes. On the basis of the asymmetric filling of the VPS method, besides the asymmetric surface conformations and silica species components, the hydrophobic surfaces and the silica species deposited on the heterogeneous surfaces of membranes after SVI treatment changed the pore entrance property and improved the gating performance of the prepared membranes, which were contributed to rectify ionic current transport across the transmembrane.12 I−V curves of the membranes prepared under suitable conditions clearly exhibit the diodelike ionic current rectifying characteristic. The study on ion transport in asymmetric membranes fabricated with mesoporous structure silica-imbedded AAO membranes provides an avenue to

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EXPERIMENTAL SECTION

Materials. Porous AAO membranes (Anodic 25) with a diameter of 25 mm, thickness of 60 μm, and pore diameter of 200 nm were purchased from Whatman International Limited. TEOS and cationic surfactant hexadecyltrimethylammonium bromide (CTAB) were purchased from Aladdin Industrial Corporation. Nonionic surfactant poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (EO20-PO70-EO20, Pluronic P123, Mn ≈ 5800 g/ mol) was purchased from Sigma-Aldrich Corporation. Hydrochloric acid aqueous solution (36 wt %), hydrogen peroxide aqueous solution (30 wt %), phosphoric acid solution (85 wt %), and ethanol were purchased from Beijing Chemical Works. The deionized water (resistance 18.2 MΩ/cm) used in the experiment was purified with a Millipore Simplicity ultrapure water system (Millipore Corporation). Fabrication of Composite Mesostructured Silica/Alumina Membranes. The schematic diagram of the fabrication process is illustrated in Figure 1. As shown in Figure 1a, a piece of 200 nm AAO

Figure 1. (a) Schematic of the fabrication procedure for mesostructured silica composites embedded in an AAO membrane with the VPS method and SVI treatment, step by step. (b) Illustration of the prepared membrane after the VPS process and SVI treatment. membrane was fixed on the filtering apparatus, a 2 mL SDA/0.2 M HCl aqueous solution was dropped onto the AAO membrane, and then a water pump was used to penetrate the channels of the AAO membrane with the mixture under reduced pressure for ∼10 min until the color of the membrane changed from transparent to white. Finally, the AAO membrane with adsorbed surfactants was dried in air for 12 h. After that, 2 mL of TEOS was dropped into the bottom of the Teflon liner, and then the AAO membrane with adsorbed surfactants was set on the homemade wire support frame to isolate it from the TEOS liquid. The stainless steel autoclave equipped with a Teflon liner was closed tightly and placed in an oven to carry out the VPS process at a predetermined temperature for a period of time. The denomination of the samples was determined by the VPS conditions, with sample names AAO-C18-80-2h and AAO-P35-120-12h representing 200 nm AAO as a hard template. CTAB or P123 was used as the SDA, the mass ratio of SDA/0.2 M HCl was 18 or 35 wt %, the reaction temperature was 80 or 120 °C, and the reaction time was 2 or 12 h. Then the SVI treatment was used to modify the synthesized membranes with silica-SDA nanocomposites confined in AAO channels by the VPS method. First, 2 mL of TEOS was added to the bottom of a 50 mL Teflon liner and isolated from the synthesized membrane by a homemade wire support frame. Then the stainless steel autoclave with the Teflon liner was closed and placed in a preheated oven to carry out the SVI treatment with a heating program. The SVI treatment program consisted of holding the sample at 80 °C for 12 h twice, 100 °C for 12 h twice, 120 °C for 12 h twice, and 140 B

DOI: 10.1021/acs.langmuir.6b03495 Langmuir XXXX, XXX, XXX−XXX

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Langmuir °C for 12 h twice, step by step. When CTAB was used as the SDA and the SVI treatment was carried out at 120 °C for 12 h twice, the obtained sample was named AAO-C18-80-2h-SVI. Similarly, with P123 as the SDA and the program was carried out at 100 °C for 12 h twice, the prepared sample was named AAO-P35-120-12h-SVI. Ionic Current Measurements. Ionic current transport through the prepared membranes was investigated with a model 6487 picoammeter/voltage source (Keithley Instruments Inc., USA). A membrane was clamped between two Teflon cells and screwed down tightly. Two homemade Ag/AgCl electrodes were used as working electrodes to measure the transmembrane ionic currents by operating the applied potential. The applied voltage was scanned from −1 V to +1 V with a scan rate of 100 mV/60 s. Deionized water was used as the electrolyte solution. The samples were immersed in the electrolyte solution for at least 10 min to ensure the membranes were wetted completely before measurements. Characterization. Scanning electron microscopy (SEM) images were taken from a Hitachi S-4800 field emission electron microscope (Hitachi, Japan). The cross-sectional images of membranes were taken after slightly etching with a 10 wt % phosphoric acid solution for 15 min. The images of silica nanorods were acquired after the complete etching of AAO membranes with a 10 wt % phosphoric acid solution for 24 h. All samples were inspected after the Pt layer was deposited. Transmission electron microscopy (TEM) images were taken with a JEM-2100 LaB6 transmission electron microscope (JEOL, Japan) at an accelerating voltage of 200 kV. To obtain the plane view images of mesostructued silica nanorods, the residual powders after the complete etching of AAO membranes were dispersed in ethanol with an ultrasonic bath and then dropped onto the carbon membrane support copper grid and dried in air. For a cross-sectional view of the prepared membrane, a small piece of membrane was adhered to the copper ring by epoxy resin glue and then ground with 800, 1000, and 2000 mesh sand papers subsequently and finally polished with a Gantan precision ion polishing system model 691 until the specimen was electron transparent. Nitrogen adsorption and desorption isotherms were measured with a Quadrasorb SI automated surface area and pore size analyzer (Quantachrome Instruments, USA) at 77 K. SDAs were extracted by Soxhlet extraction for 24 h. Eight pieces of extracted membranes were ground into powder and degassed at 423 K for 5 h before measurements. The specific areas were calculated by the multipoint Brunauer−Emmett−Teller (BET) equation. The pore diameter distributions were analyzed with the Barrett−Joyner−Halenda (BJH) method according to the branch of desorption isotherms. Contact angles were acquired via an OCA 15EC Dataphysics optical contact angle measurement system (Dataphysics, Germany) using the sessile-drop technique with a dosing volume of 2.0 μL and a dosing rate of 2.0 μL/s. The measurements were repeated on five different locations of the same specimen, and the contact angle values were obtained by taking an average of the five measured values. Fouriertransform infrared (FT-IR) spectra were collected on a Nexus 670 (Nicolet, USA) infrared spectrometer using the attenuated total reflectance (ATR) attachment in the wavelength range of 400−4000 cm−1 with a resolution of 2 cm−1, and both sides of the specimens were scanned 64 times. X-ray photoelectron spectra (XPS) were recorded on an ESCSLAB 250Xi Elecreon spectroscope (Thermo Fisher, U.K.). Chemical composites of the specimens were calculated according to the spectra.

Figure 2. SEM images of (a, b) AAO-C18-80-2h and (c, d) AAO-P35120-2h. (a, c) Plane view of silica nanorods. (b, d) Cross-sectional view of prepared membranes.

the form of intact nanorods. SEM images of AAO-P35-120-12h are shown in Figure 2c,d. P123-directed silica nanocomposites filling nanochannels of AAO membranes are constructed to produce rod-shaped stacking with silica nanoparticles. The mesoporous structures, asymmetric properties, and ionic current rectification performance of prepared membranes before and after SVI treatment are discussed as follows. Mesoporous Structures of Composite Membranes after SVI Treatment. The mesoporous structures of prepared membranes are revealed by TEM and the corresponding Fast Fourier Transform (FFT) patterns. The structure parameters of prepared membranes are calculated from the determined nitrogen adsorption and desorption isotherms. TEM images of AAO-C18-80-2h in Figure 3a,b show that the mesostructure of silica nanorods filling in nanochannels of AAO membranes are dominated by the hexagonal phase. The columnar hexagonal phase and circular hexagonal phase coexist in AAO-C18-80-2h, which was also confirmed by the inset FFT patterns in Figure 3b. The nitrogen adsorption and desorption isotherm of 200 nm AAO and the structure parameters of all

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RESULTS AND DISCUSSION Figure 1b illustrates the scheme of membrane structures after the VPS process and SVI treatment. Figure 2 reveals the morphologies of silica−SDA nanocomposites confined in AAO membranes by the VPS method with CTAB or P123 as the SDA. Figure 2a,b shows plane-view SEM images of confined silica nanocomposites and cross-sectional-view SEM images of AAO-C18-80-2h, respectively. CTAB-directed silica nanocomposites filling nanochannels of AAO membranes are in

Figure 3. TEM images of AAO-C18-80-2h (a, b) before and (c, d) after SVI treatment. (a, c) Plane view of silica nanorods. (b, d) Crosssectional view of prepared membranes. C

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in Figure 2c,d. The nitrogen adsorption−desorption isotherm of AAO-P35-120-12h (Figure 4c) has a step at around P/P0 = 0.6−0.8 and a hysteresis loop indicating the existence of mesopores.29 The mesoporous structure originates from the stacking of nanoparticles in the channels of an AAO membrane. The BET surface area is 26.0 m2/g, the BJH pore diameter is 2.2 nm, and the pore volume is 0.057 cm3/g, which all are smaller than those of AAO-C18-80-2h. These results show different mesostructured silica nanorods confined in nanochannels of AAO membranes depending on the variation of VPS conditions. Differences between CTABdirected and P123-directed silica nanorods in AAO membranes stem from the self-assembly micelle networks of SDAs adsorbed on the pore walls of the AAO membrane and the forms of SDA-TEOS-silica species during the vapor-deposition hydrolysis process.29,39 Because of the charge−charge interaction, the cationic heads of CTAB adsorb on the negative surface of the AAO membrane to form a lamellar structure of the micelle network. TEOS vapor infiltrated and hydrolyzed in the hydrophilic part of the micelle network, causing the lamellar micelle structure to transform to a hexagonal structure.39 Using the nonionic P123 block copolymer as the SDA, polymer P123 attaches to the channel walls of the AAO membrane through the hydrogen-bonding interaction to form an inverse hexagonal phase structure easily at high concentration.40,41 Because P123 chains tend to shrink at high temperature (120 °C), the inverse hexagonal structure changes to closely stacked silica nanoparticles while the TEOS vapor diffuses and hydrolyzes in the AAO membrane.29 Figure 3c,d shows TEM images of AAO-C18-80-2h-SVI and the corresponding FFT patterns. The results suggest that hexagonal columnar and circular mesostructured silica nanorods become less ordered after SVI treatment. The nitrogen adsorption−desorption isotherm of AAO-C18-80-2h-SVI (Figure 4b) has a step at around P/P0 = 0.6 and a hysteresis loop. The shapes of isotherm and hysteresis loop have been changed, whereas the mesoporous structure still exists after SVI treatment. The BET surface area (48.9 m2/g) is similar to the one before SVI treatment, but the pore volume decreases significantly (0.058 cm3/g) after SVI treatment. The pore diameter distribution evaluated by the BJH method is centered at 3.6 nm, indicating that the SVI treatment leads to a uniform pore diameter of less than 10 nm; however, the larger pores are almost unchanged (as shown in the inset of Figure 4b). The reason for the structure evolution after SVI treatment is ascribed to the deposited silica species generated from TEOS infiltration and hydrolysis in mesoporous silica nanorods, resulting in a loss of ordering in hexagonal mesostructures and a decrease in pore volume after SVI treatment. TEM images in Figure 5c,d reveal the structure of AAO-P35120-12h-SVI. After SVI treatment, two packing types of mesostructured silica are found in this sample. Besides the reserved silica nanoparticle packing, coil-type stacked filaments are also observed, indicating that phase transformation occurs during SVI treatment. The nitrogen adsorption−desorption isotherm of AAO-P35-120-12h-SVI (Figure 4d) has a step at around P/P0 = 0.6−0.8 and a hysteresis loop, which is characteristic of mesopores. There is little change in the isotherms and the hysteresis loop compared to those of AAOP35-120-12h. The BET surface area (23.8 m2/g), the BJHbased pore diameter (1.9 nm), and the pore volume (0.055 cm3/g) are slightly reduced after SVI treatment. The results

samples are given in Figure S1 and Table S1, respectively. The nitrogen adsorption−desorption isotherm of AAO-C18-80-2h (shown in Figure 4a) is classified as type IV with a step around

Figure 4. Nitrogen adsorption and desorption isotherms of prepared membranes before and after SVI treatment. (a) AAO-C18-80-2h, (b) AAO-C18-80-2h-SVI, (c) AAO-P35-120-12h, and (d) AAO-P35-12012h-SVI.

0.4P/P0 and a hysteresis loop due to the condensation of nitrogen in mesopores.39 The specific surface area of AAOC18-80-2h evaluated by the BET method is 49.4 m2/g, over 3 times that of a blank AAO membrane (14.8 m2/g), because of the filled hexagonal mesostructured silica. The pore diameter distribution calculated with the BJH method indicates that the pore diameters are 3.2 and 3.7 nm in accordance with the coexistence of two hexagonal mesostructures in AAO-C18-802h, with a pore volume of 0.083 cm3/g. Figure 5a,b shows the TEM images of mesostructured silica filled with AAO-P35-120-12h. The silica nanorods consisted of packed silica nanoparticles, which agrees with the SEM images

Figure 5. TEM images of AAO-P35-120-12h (a, b) before and (c, d) after SVI treatment. (a, c) Plane view of silica nanorods. (b, d) Crosssectional view of prepared membranes. D

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Langmuir show that the phase transformation during SVI treatment changes the texture of AAO-P35-120-12h. Thus, the mesostructure in prepared membranes is maintained after SVI treatment. The mesoporous structure in AAO-C80-2h-SVI is derived from hexagonal packing with a diameter of 3.6 nm, and the mesostructure in AAO-P35-12012h-SVI originates from the stacking of silica nanoparticles with a diameter of 1.9 nm. Models of hexagonal-packed mesostructured silica nanochannels in nanoconfinement space42−44 or silica nanoparticle crystal packed architecture45−47 in the fabrication and application of nanofluidic devices have been reported and exhibit good performance. From the viewpoint of nanofluidics, AAO-C18-80-2h-SVI and AAO-P35-120-12h-SVI can be considered to be promising candidates for fabricating nanofluidic membranes. Asymmetric Properties of Composite Membranes after SVI Treatment. The surface morphologies, wettability, and silica components of prepared membranes before and after SVI treatment are investigated by SEM, contact angle measurements, XPS, and ATR FT-IR spectrometry, respectively. Figure S2 presents SEM images of both surfaces and the cross section of a blank 200 nm AAO membrane. The opening pore diameters of both surfaces are different from each other: the wide pore was defined as the base, and the small pore was defined as the tip. From the cross-sectional SEM images, a thin layer (about 1−2 μm) with small pores is located on the upper part of the AAO membrane. The results show that 200 nm AAO is intrinsically asymmetric. SEM images of tip and base surfaces of AAO-C18-80-2h and AAO-P35-120-12h are presented in Figures 6a,b and 7a,b, respectively. There are

Figure 7. SEM images of (a, c) the tip surfaces and (b, d) the base surfaces of AAO-P35-120-12h membranes (a, b) before and (c, d) after SVI treatment.

nanocomposites located on the upper part of the AAO membranes. All of these impart original asymmetry to the membranes prepared via the VPS method. Figures 6c,d and 7c,d show the SEM images of both surface conformations of AAO-C18-80-2h-SVI and AAO-P35-120-12hSVI. After SVI treatment, some bulk silica species precipitate on the tip of the AAO-C18-80-2h-SVI sample, whereas the flocculation of silica species almost covers the tip of the AAO-P35-120-12h-SVI sample. Correspondingly, the base surfaces of two samples are more blocked than before SVI treatment. The SEM images show that the surface conformations of both samples became more asymmetric after SVI treatment than before,48 and the degree of asymmetry of AAOP35-120-12h-SVI is higher than that of AAO-C18-80-2h-SVI. The tip surface compositions of AAO-C18-80-2h and AAOP35-120-12 before and after SVI treatment determined by XPS are summarized in Table 1. The ratio of Si/Al in AAO-C18-80Table 1. Surface Composition of 200 nm AAO, AAO-C1880-2h, and AAO-P35-120-12h Samples before and after SVI Treatment As Determined by XPS element content (atom %) entry 200 nm AAO AAO-C18-80-2h AAO-C18-80-2h-SVI AAO-P35-120-12h AAO-P35-120-12h-SVI

Figure 6. SEM images of (a, c) the tip surfaces and (b, d) the base surfaces of AAO-C18-80-2h membranes (a, b) before and (c, d) after SVI treatment.

a

Al 20.7 12.5 2.3 2.9 1.3

Si a

7.4 19.9 23.5 17.9

C

O

Si/Al

36.6 40.2 27.8 11.8 35.9

42.7 39.9 50.0 61.7 44.8

0.6 8.7 8.1 13.8

a

Contents are too low to be determined.

2h changes from 0.6 to 8.7, and the ratio of Si/Al in AAO-P35120-12h changes from 8.1 to 13.8. The atomic contents of silicon increase while the aluminum contents decrease after SVI treatment for both samples. The XPS results confirmed the deposition of silica species on the membranes after SVI treatment. The contact angles of composite membrane surfaces are displayed in the insets of Figures 6 and 7. Contact angles of both surfaces of 200 nm AAO membrane are shown in the inset of Figure S2a,b. Contact angles of tip and base surfaces of 200 nm AAO are 14.8 ± 1.0 and 11.4 ± 2.9°, respectively. Because of the capillary effect, water impregnates the channels of the

clusters but no layers on all of the surfaces of both prepared membranes. The number of residuals on the surface of AAOP35-120-12h is more than that on AAO-C18-80-2h because the molecular size of triblock copolymer P123 is larger than that of CTAB. The approximate fill rate of the tip side is about 80%, and that of the base side is about 20%. The surface morphologies observed from SEM images suggest that the vacuum pump aspiration technique is intended to obtain clean surfaces. And the pressure gradient generated by the aspiration technique causes the tip surfaces to be more blocked than the base surfaces in both samples, which results in the silica−SDA E

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Langmuir 200 nm AAO membrane, leading to the hydrophilic state.49 Compared to the blank AAO membrane, contact angles of both surfaces of AAO-C18-80-2h and AAO-P35-120-12h increase abruptly after the VPS process, and the increment of the contact angle of the tip surfaces is more than that of the base surfaces. The tip surfaces of AAO-C18-80-2h and AAO-P35120-12h become hydrophobic (contact angle >90°), and the contact angles are 104.2 ± 1.9 and 123.4 ± 2.9°, respectively. The base surfaces of AAO-C18-80-2h and AAO-P35-120-12h are still hydrophilic (contact angle