Influence of Transmembrane Ionic Current Based on PNIPAM

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C: Physical Processes in Nanomaterials and Nanostructures

Influence of Transmembrane Ionic Current Based on PNIPAM Modified Nanochannels Mengfei Liu, Jinzheng Zhang, Yujuan Qiao, Tingyan Ye, Nannan Liu, Xiangju Xu, Chao Zou, and Shaoming Huang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b02152 • Publication Date (Web): 24 Apr 2019 Downloaded from http://pubs.acs.org on April 24, 2019

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The Journal of Physical Chemistry

Influence of Transmembrane Ionic Current Based on PNIPAM Modified Nanochannels Mengfei Liu,†,║ Jinzheng Zhang, †,║ Yujuan Qiao,† Tingyan Ye,† Nannan Liu,*,† Xiangju Xu,† Chao Zou† and Shaoming Huang*,†,‡ Key Laboratory of Carbon Materials of Zhejiang Province, College of Chemistry & Materials

†

Engineering, Wenzhou University, Wenzhou 325027, P. R. China. School of Material and Energy, Guangdong University of Technology, Guangzhou 510006, P. R. China.

‡

ABSTRACT: With the inspiration of biological ion channels from nature, the scientific community started to build up biomimetic nanochannels/nanopores. Especially for the functional modification of the nanochannels/nanopores. However, there is no detailed explanation of the dominant reason that influences transmembrane ionic current in modification process. Here, we studied the poly (Nisopropylacrylamide) (PNIPAM) by the atom transfer radical polymerization (ATRP) method into Anodic Aluminum Oxide (AAO) membrane. There are detailed explanation of the dominant reason including the Steric effect and Hydrophilic-Hydrophobic effect that influence transmembrane ionic current in modification process. The findings point to the modified membrane provide a general and convenient method for the migration behavior of hydrophilic and hydrophobic ions, which has potential application prospect.

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Inspired by the important physiological functions of biological ion channels, much attention has been paid on mimicking these properties and structures to build up biomimetic nanochannels/nanopores.1-4 Recently, the biomimetic nanochannels have fascinating prospects based on the development of nanometer materials, which are common materials including polyethylene terephthalate (PET) film, graphene, porous polyimide (PI) membrane, mesoporous silica, and anodized alumina oxide (AAO) membrane.5-8 One of the significant methods to characterize the shapes and properties of various nanochannels is transmembrane ionic current.9-11 Scanning transmembrane electrical potential was applied to measure the resulting ionic current. In view of the remarkable performance of nanometer materials, much attention has been further paid on designing novel devices for ultrafiltration,12 desalination,13 DNA-sequencing,14 energy conversion,15 biosensing,16 functional properties17-19 and many other applications. For instance, Jiang et al. utilized highly-efficient gating based on AAO nanochannels containing ATP aptamers that achieved the IMPLICATION logic operations within the nanofluidic structures.20 A multifunctional peptide-conjugated AIEgen for efficient and sequential targeted gene delivery into the nucleus as a therapeutic approach to serious is reported by Xia et al.21-22 Jiang et al. employed bio-inspired photoelectric conversion based on nanochannels of PET membranes, contributing to the potential applications of environmentally friendly energy conversion.23

Surface modification is the most widely used strategy to improve the surface properties of biomimetic nanochannels.24-26 Su et al. developed a flexible, superhydrophobic, and conductive tungsten disulfide (WS2) nanosheets-wrapped sponge (SCWS) for the high-sensitivity detection of tiny vibration from the water surfaces and from the grounds.27 Hence, finding a suitable modifier which can improve the properties has become the key issue in this field of research. Because the poly (N-isopropylacrylamide) (PNIPAM) undergoes such a sharp property change in response to a moderate thermal stimulus near physiological temperatures (37°C),28 so it has attracted great interest in the biomaterials community. For 2 ACS Paragon Plus Environment

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example, Azzaroni et al. fabricated a thermally actuated macromolecular gates onto the nanopore to control the ion flux by chemical modification with PNIPAM brushes.29 Xia et al. explored the influence of long and short polymer chains on the gating properties of nanochannel systems modified with a poly (N-isopropylacrylamide-co-acrylamidophenylboronic acid) (NIPAAm-co-PBA) on the inner surface of the solid-state nanochannels.30 However, there is no detailed explanation of the dominant reason that influences transmembrane ionic current in modification process.

Herein, we report the PNIPAM-modified AAO membrane can be obtained by the atom transfer radical polymerization (ATRP) method.31 By adjusting the modification time of PNIPAM brushes, the influence of transmembrane ionic current based on PNIPAM modified nanochannels had been explored in different modification time. Analyzing the data of transmembrane current of each phase, two factors affecting current change has been proposed: Steric effect and Hydrophilic-Hydrophobic effect (Figure 1). Here, the Steric effect are related to effective size of nanochannels. And the HydrophilicHydrophobic effect means conversion of hydrophilic and hydrophobic groups of PNIPAM on the nanochannels wall.32 When it is Steric effect, the hydrophobic -CH(CH3)2 coiled in the wall leading to an increase of the effective nanochannels size of the nanochannel. So the Steric effect make transmembrane current increase. On the other hand, PNIPAM brushes swollen in HydrophilicHydrophobic effect, thus decreasing the effective cross section of the nanochannels. The hydrophilic and hydrophobic group conversion makes transmembrane ionic current uncertain, in which remain challenge in nanochannels research.

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Figure 1. Illustration of two factors affecting transmembrane ionic current. The two factors affecting transmembrane ionic current change of the AAO membrane that PNIPAM modified: Steric effect and Hydrophilic-Hydrophobic effect. When it is Steric effect, the hydrophobic −CH(CH3)2 coiled in the wall leading to an increase of the effective nanochannels size of the nanochannel. So the Steric effect make transmembrane current increase. On the other hand, PNIPAM brushes swollen in HydrophilicHydrophobic effect, thus decreasing the effective cross section of the nanochannels.

METHODS The compound devices are constructed on nanopores with the pore diameters of 70±10 nm embedded in AAO membranes. To realize the functions with nanopores, we chemically modified the nanochannels with PNIPAM brush by ATRP (Figure S1). In brief, we first modified the membrane having aminated nanochannels (5%, acetone solution of APTMS, 10h, room temperature) with the initiator groups (mixture solution of DCM solution, BIBB solution and Et3N solution, 12h, room temperature). At 20°C, then we proceed to the PNIPAM growth by immersing the initiator-modified membrane into mixture solution of ethanol and purified water (volume ratio: 1:1) of NIPAAm, PBA, CuBr, and PMDETA under 30min, 60min, 120min, 240min and 720min respectively.30 The copolymer brushes are in situ grown from the nanochannels wall through a conventional surface-initiated radical polymerization process.


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Figure 2. SEM (Scanning electron microscope) and transmembrane ionic current change of the PNIPAM unmodification and modification on the AAO membrane. (a, b) Cross-sectional and top-view SEM images of unmodified AAO membranepore (diameters is 70±10 nm). (c, d) After 120min of modified AAO membrane, cross-sectional and top-view SEM images at 20°C. It is clear that a mass of floccules are grafted onto the inner wall of the nanochannels after treatment with PNIPAM. (e) At 20°C, the resulting transmembrane ionic current effectively change at different modification time.

It is the copolymer brush attachment onto the nanochannels wall that obvious confirmed by monitoring the cross-sectional and top-view scanning electron microscope (SEM) images before and after AAO membrane modification. Figure 2 shows the results in the observation of bare membrane and modified membrane. It can clearly be seen that the original membrane has a characteristic ideally anodic porous alumina structure (Figure 2a-b). After modified 120min with PNIPAM polymer brush at 20°C, one can clearly see that polymer chains densely pack on the membrane surface (Figure 2c-d). And these



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indicate that a mass of floccules are grafted onto the inner wall of the nanochannels after treatment with PNIPAM for 120min.

Similarly, the transmembrane ionic current change of nanochannels also confirms that AAO membrane was modified by PNIPAM. In all measurement, ion currents were measured by a Keithley 6487 picoammeter (Keithley Instruments, Cleveland, OH) (Figure S2). Ag/AgCl electrodes are used to apply a transmembrane potential across the film. The AAO membrane is placed between the two chambers of the conductance cell, and both half of the cell are filled with 0.1M KCl. The transmembrane potential used in this work is a scanning voltage, which varied from -0.2 to +0.2V. Each test is repeated 3 times to obtain the average conductance value at same voltage. As shown in Figure 2e, as a mass of floccules are gradually grafted onto the inner nanochannels wall, the effective nanochannels size smaller result in transmembrane ionic current decreases. It is 78.8% down that transmembrane current change during the 0 min (diamond, dark) and 60min (triangle, blue) (Figure S3). After 120min, the ionic current was almost unchanged, which is evidenced as nanochannels almost filled with PNIPAM chain.

RESULTS AND DISCUSSION Because the transmembrane ionic current occurred great change during 120min, so our group further exploited the dominant factors that influence transmembrane ionic current in modification process. According the Hydrophilic-Hydrophobic effect, at 50°C, hydrophilic -CONH2 self-aggregation on the PNIPAM chain, structure collapse, exposure of the hydrophobic -CH(CH3)2 group, and the whole chain is in a relatively hydrophobic state. Under 20°C conditions, the hydrophilic -CONH2 of the PNIPAM chain and the hydrophobic -CH(CH3)2 simultaneously exist. In a similar vein, as the chain length of


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PNIPAM gradually increase, the effective size of nanochannels is smaller that results in transmembrane ionic current decreases according the principle of Steric effect.

Figure 3. The nanochannels of PNIPAM-Modified AAO membrane were detected its ionic current at 20°C (the blue line) and 50°C (the red line) respectively. In order to make the description of ionic current change more obvious, five point-in-time of AAO membrane modified PNIPAM were selected for illustration including 0min, 10min, 30min, 60min and 120min. The process of AAO membrane modified PNIPAM can be divided into three parts just like a scales: (a) The period of 0 - 30min is first part. (b) About 30min as an important node. (c) The last part is after 30min.

As Figure 3 reveals, the AAO membrane having PNIPAM-Modified nanochannels is respectively detected at 20°C (the blue line) and 50°C (the red line). In order to make the description of ionic current change more obvious, five point-in-time of AAO membrane modified PNIPAM were selected for illustration including 0min, 10min, 30min, 60min and 120min. And the transmembrane ion conductance was used to describe the phenomenon. Based on the above two theories, the process of AAO membrane modified PNIPAM can be divided into three parts just like a scales. (a) The period of 0 (the bare channel with BIBB modified and the channel without the NIPAAm monomer added) - 30min is first part. In the first thirty minutes, the conductance was changed under different temperature. And the conductance value of 20°C less than under the condition of 50°C, the conductance ratio (50°C /20°C) of 7 ACS Paragon Plus Environment


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0min, 10min is 3.70, 2.25 individually (Figure S4). During this period, NIPAAm gradually added and the crystals of micro-nano scale fill the pores, so the main influencing factor of the transmembrane ionic current is the effective nanochannels size. (b) About 30min is an important node. When time is 30min, the NIPAAm monomer already take up most of nanochannels. The conductance of detection at 20°C (31±0.09μS) about equal to detection at 50°C (30±0.96μS) suggests that the two effects (Steric effect and Hydrophilic-Hydrophobic effect) are balanced against each other. (c) The last part is after 30min. During this period, the conductance of 20°C greater than 50°C. Keep modificating for 120 minutes, the conductance value of the PNIPAM-modified AAO membrane was no change more as before. So, these results further verify that effective size of nanochannels was no change and the main factor affecting the ionic current of the nanochannels is Hydrophilic-Hydrophobic effect after modified 30min.

Therefore, the ionic current through nanochannels is mainly controlled by two influencing factors: Steric effect and Hydrophilic-Hydrophobic effect. The three phases (a), (b), (c) are commonly controlled by two effects, in which make different transmembrane ionic current.

After modificated 120 minutes, the main factor affecting the ionic current of the nanochannels is Hydrophilic-Hydrophobic effect. To better understand the ionic current response of PNIPAM-modified AAO membrane under different electrolytes, modification time for 120min was determined and put forward four kinds of electrolyte (Figure S5): KCl (Potassium chloride), DDBAC (Dodecyl dimethyl benzyl ammonium), DTAC (Dodecyl trimethyl ammonium chloride), EDTA-2K (Dipotassium salt) as a model for the analysis of penetration through PNIPAM-modified AAO membrane. In all electrolyte solution, the transmembrane ionic conductance of unmodified nanopores increases with temperature (Figure S6). 8 ACS Paragon Plus Environment

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Figure 4. Ionic conductance of KCl, DDBAC, DTAC, EDTA-2K modulated by converting temperature on nanopores modified with PNIPAM chains at 20°C and 50°C respectively.

The transmembrane ionic current of KCl, DDBAC, DTAC, EDTA-2K change by transition temperature on nanopores that modified with PNIPAM chains at 20°C and 50°C respectively (Figure 4, the black bar and the red bar). According the Hydrophilic-Hydrophobic effect, the dominant factor of the transmission properties is the hydrophilic group under 20°C conditions. In the condition of 20°C (the black bar), to analyse KCl and EDTA-2K firstly. Because EDTA-2K contains amidogen which is one kind of hydrophobic group, so the penetration of EDTA-2K (11.82±0.05μS) less than KCl (19.20±0.11μS). Secondly, to compare DDBAC and DTAC. Because the DDBAC have one more aromatic ring (belong to hydrophobic group) than DTAC, so the penetration of DTAC (8.75±0.02μS) better than DDBAC (6.84±0.02μS) in PNIPAM-modified AAO membrane. Thirdly, to compare EDTA2K and DDBAC. EDTA-2K have more hydrophilic group and DDBAC containing linear carbon is major hydrophobic group. Hence, the penetration of EDTA-2K greater than DDBAC. In short, PNIPAM-modified AAO membrane can clearly identify different penetration capabilities (KCl> EDTA-2K>DTAC>DDBAC) at 20°C. Conversely, the whole PNIPAM chain in a relatively hydrophobic state at 50℃. Because KCl and EDTA-2K have hydrophilic abilities, so the conductance value decrease after heating up. In contrast, DDBAC and DTAC have more hydrophobic group and 9 ACS Paragon Plus Environment


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increase conductance values at 50°C. So the penetration capabilities of four kinds of electrolyte is DDBAC > DTAC > KCl > EDTA-2K (the red bar). Therefore, the PNIPAM-modified AAO membrane can distinguish electrolytes with different hydrophilic and hydrophobic abilities.

Another important feature of the PNIPAM-modified AAO membrane is reversible variation. The valid device that distinguishing ions must show excellent robustness, stability and controllable properties. Figure 5 presents the reversible variation of the normalized conductance of KCl and DDBAC in the presence of consecutive temperature change. These measurements confirm that the the thermosensitive ions-discriminating nanochannels has excellent transport reversibility for hydrophobic and hydrophilic ions. And reversible variation of DTAC and EDTA-2K are obtained to further confirm the reversibility of this temperature-responsive system (Figure S7).


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Figure 5. Reversible variation of the normalized conductance of different electrolyte solution through PNIPAM brushes modified cylindrical nanopores device upon alternating the solution temperature between 20°C and 50°C. Ionic conductance cycle curves are measured in (a) 0.1M KCl (b) 0.1M DDBAC.

CONCLUSION In summary, we explain the perforated current of the PNIPAM-modified AAO membrane. The effective polymerization time of in-situ free radical polymerization is about 0 - 120min. The two main factors play a crucial role in process of modification: the Steric effect and Hydrophilic-Hydrophobic effect. The PNIPAM-modified AAO membrane also satisfies the permeability of the nanochannels to four model ions. It indicate that the modified membrane provide a general and convenient method for the migration behavior of hydrophilic and hydrophobic ions, which has potential application prospect. ASSOCIATED CONTENT Supporting Information Available The Supporting Information is available free of charge on the ACS Publications website at DOI:



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Chemical reagent; principle of chemically modified PNIPAM in alumina nanochannels membranes; experimental setup for transmembrane ionic current measurements; the molecular structures of the four electrolytes; transmembrane ionic conductance of unmodified nanopores in four electrolyte solution which was detected at 20°C and 50°C respectively; and reversibility of the temperature-activated nanofluidic gating device. (PDF) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected] Author Contributions These authors contributed equally.

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Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (21505101), and Zhejiang Provincial Natural Science Foundation of China (LQ16B050003). REFERENCES (1) Hou, X.; Guo, W.; Jiang, L. Biomimetic Smart Nanopores and Nanochannels. Chem. Soc. Rev. 2011, 40, 2385-401. (2) Zhang, H. C.; Tian, Y.; Jiang, L. Fundamental Studies and Practical Applications of Bio-Inspired Smart Solid-State Nanopores and Nanochannels. Nano Today. 2016, 11, 61-81. (3) Long, Z.; Zhan, S.; Gao, P.; Wang, Y.; Lou, X.; Xia, F. Recent Advances in Solid Nanopore/Channel Analysis. Anal. Chem. 2018, 90, 577-588. (4) Su, B.; Tian, Y.; Jiang, L. Bioinspired Interfaces with Superwettability: From Materials to Chemistry. J. Am. Chem. Soc. 2016, 138, 1727-48. 12 ACS Paragon Plus Environment

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Influence of Transmembrane Ionic Current Based on PNIPAM

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