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Gated Molecular Transport in Highly Ordered Heterogeneous Nanochannel Array Electrode Xingyu Lin, Qian Yang, Fei Yan, Bowen Zhang, and Bin Su* Institute of Analytical Chemistry, Department of Chemistry, Zhejiang University, Hangzhou 310058, P.R. China S Supporting Information *

ABSTRACT: In biology, all protein channels share a common feature of containing narrow pore regions with hydrophobic functional groups and selectivity filter regions abundant with charged residues, which work together to account for fast and selective mass transport in and out of cells. In this work, an ultrathin layer of polydimethylsiloxane (PDMS) was evaporated on the top orifices of charged silica nanochannels (2−3 nm in diameter and 60 nm in length) vertically attached to the electrode surface, and the resulting structure is designated as heterogeneous silica nanochannels (HSNs). As evidenced by voltammetric studies, the transport of ionic species in these HSNs was controlled by both hydrophobic rejection and electrostatic force arising from the top PDMS layer and from the bottom silica nanochannels, respectively. Anionic species encountered both hydrophobic rejection and electrostatic repulsion forces, and thus, their transport was strongly prohibited, while the transport of cationic species was permitted once the electrostatic attraction exceeded the hydrophobic rejection. Moreover, the magnitude of hydrophobic force could be regulated by the PDMS layer thickness, and that of the electrostatic force can be modulated by the salt concentration, solution pH, or applied voltage. It was demonstrated that the HSNs could be activated from an OFF state (no ion can transport) to an ON state (only cation transport occurs) by decreasing the salt concentration, increasing the solution pH, or applying negative voltages. KEYWORDS: nanochannels, heterogeneous, asymmetric wettability, gating, multiforce, electrochemical detection

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been prepared, such as single glass nanopore electrode,39,40 silica colloidal nanopore electrodes,41 and mesoporous silica film electrodes.42−47 After being modified with stimuliresponsive polymers, these electrodes can respond to extra stimuli, such as pH,43,44 salt concentration,39,48 or light.40,49 However, these works mainly focus on nanochannels with a homogeneous structure, through which molecular transport is solely controlled by one kind of force. Recently, nanochannel membranes with asymmetry property on each side (also called Janus membrane) have been fabricated.50 This kind of nanochannel allows opposite properties at each side to work collaboratively, generating many novel behaviors and applications, such as directional oil/water separation,51,52 switchable ion transport,53 and interfacial mass movement.54 In this work, we studied molecular transport in heterogeneous silica nanochannel (HSN) array electrode gated by the counterbalance between hydrophobic and electrostatic forces (as shown in Scheme 1). The HSNs were fabricated by evaporating hydrophobic polydimethylsiloxane (PDMS) oligomers onto the top orifices of silica nanochannels. The upper hydrophobic PDMS layer rejects the passage of all ions, while the negatively charged silica nanochannel exerts an

INTRODUCTION Molecular transport in an ultrasmall nanochannel has received considerable attention in the fields of chemistry and biology. The interaction between molecules and channel surface become dominated when the channel diameter is sufficiently small (comparable to molecule size), resulting in many extraordinary transport phenomena.1−3 Indeed, the generation of nanoscale interfaces enabling the discrimination of molecule transport is ubiquitous in nature. The protein channels embedded in cellular membranes are ultrasmall in size with a heterogeneous hydrophobic/hydrophilic structure,4 such as aquaporins,5 nuclear channels,6 and ion channels.7 They all contain relatively narrow pore regions with hydrophobic residues and selectivity filter regions abundant with charged groups. The cooperation of different parts and various forces leads to precise selectivity, ultrafast flux, and gated ON/OFF function.8−12 Mimicking these behaviors has triggered broad research interests in building up artificial nanochannels because of their potential applications in sensing,13−16 nanofluidics,17,18 molecular separation,19−21 and drug delivery.22,23 So far, a variety of artificial solid-state nanopores/channels have been fabricated,24,25 including SiO2/SiN nanopores,26−28 track-etched polymer nanochannels,29−31 anodic aluminum oxide nanochannels,32−35 and vertically aligned carbon nanotubes.36−38 In addition to these perforative channels, nanochannels/nanopores supported by solid electrodes have also © 2016 American Chemical Society

Received: October 28, 2016 Accepted: November 18, 2016 Published: November 18, 2016 33343

DOI: 10.1021/acsami.6b13772 ACS Appl. Mater. Interfaces 2016, 8, 33343−33349

Research Article

ACS Applied Materials & Interfaces

Scheme 1. Schematic Illustration of Selective and Gated Ion Transport in Heterogeneous Silica Nanochannel (HSN) Array Electrode by Modulating the Hydrophobic Rejection and Electrostatic Forces

Figure 1. (a) Cross-sectional SEM picture of silica nanochannel membranes grown on the surface of ITO electrode. (b) Cross-sectional TEM pictures of silica nanochannel membrane. Inset: the high-magnification picture of nanochannels. The scale bar is 10 nm. (c, d) Top-view TEM images of silica nanochannels (c) and PDMS-modified silica nanochannels (d). The insets show the magnified images. The scale bar is 20 nm. nanochannels were removed by immersing the electrode in an ethanol solution containing 0.1 M HCl. PDMS Modification To Prepare HSNs on ITO Electrode. The HSNs on ITO were prepared by the PDMS evaporation method as reported recently.56 Briefly, the prepared nanochannel array was put under a piece of PDMS stamp (their distance was kept at about 1 mm) heated at 100 °C. After 10 h, small PDMS oligomers were evaporated out from the stamp and deposited onto the top orifices of silica nanochannels, forming heterogeneous structures with PDMS layer on the top and bare silica nanochannels on the bottom,56 as shown in Scheme 1. Characterization. Scanning electron microscopy (SEM) images and transmission electron microscopy (TEM) images were captured on a SU8010 scanning electron microscope and an HT7700 transmission electron microscope, respectively. The specimen for the top-view TEM observation was prepared by transferring the silica nanochannel membrane to a 3 mm porous silica nitride window according to our previous work.19 The specimen for the cross-sectional TEM observation was prepared by scraping silica nanochannel membrane fragments from the ITO electrode surface into ethanol, followed by ultrasonic dispersion for 30 min and then deposition onto copper grids. Voltammetry Measurements. Molecular transport in nanochannels was studied by voltammetry using the CHI 920 electrochemical workstation (CHI Instrument, Shanghai). A three-electrode

electrostatic attraction on cation transport and electrostatic rejection on anion transport. As a result, the anion transport was always prohibited and the cation transport occurred as long as the electrostatic attraction exceeded the hydrophobic rejection. In addition, the magnitude of electrostatic interaction can be modulated by the salt concentration, solution pH, or applied voltage. So these HSNs can be activated from an OFF state (no ion can transport) to an ON state (only cation passage occurs) by decreasing the salt concentration, increasing the solution pH, or applying voltages.

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EXPERIMENTAL SECTION

Preparation of Silica Nanochannel Array on Indium Tin Oxide (ITO) Electrode. The silica nanochannel array was prepared on the ITO glass according to the approach reported previously.55 Briefly, the cleaned ITO electrode was immersed in a mixture (140 mL of water, 160 μL of tetraethyl orthosilicate (TEOS), 0.32 g of cetyltrimethylammonium bromide (CTAB), 20 μL of ammonium hydroxide, and 60 mL of ethanol) at 60 °C for 12 h. Under this condition, silicate and CTAB surfactants self-assembled to form silica nanochannels array with CTAB micelles on the ITO electrode surface. The nanochannel electrode was taken out from the solution and washed with water. The remaining CTAB micelles in silica 33344

DOI: 10.1021/acsami.6b13772 ACS Appl. Mater. Interfaces 2016, 8, 33343−33349

Research Article

ACS Applied Materials & Interfaces system was used in this experiment, which consisted of a working electrode (the nanochannel-modified ITO electrodes), a reference electrode (Ag/AgCl with KCl), and a counter electrode (platinum wires). Ru(NH3)63+, Fe(CN)63−, and 2,4,6-trinitrotoluene (TNT) was chosen as the standard probes to study their transport phenomena. The concentration of probe molecules was 200 μM for Ru(NH3)63+ and Fe(CN)63−, and 20 ppm for TNT in all cases. The magnitude of peak current recorded by cyclic voltammetry (CV) or differential pulse voltammetry (DPV) was considered to reflect the molecular transport rate in nanochannels.57,58 The DPV was performed with an increment potential of 0.01 V, amplitude of 0.05 V, pulse width of 0.05 s, and pulse period of 0.5 s. The CV was recorded at a scan rate of 0.05 V s−1. Note that, for experiments with TNT, the solution was degassed for 20 min prior to the measurement.

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RESULTS AND DISCUSSION

Membrane Characterization. As-prepared ultrasmall silica nanochannel array electrode was characterized by SEM and TEM. As seen from the cross-sectional SEM images (Figure 1a), the prepared nanochannel membrane on the ITO glass are flat and continuous over a large area without any cracks. The cross-sectional TEM image in Figure 1b suggests that the membrane contains perpendicular nanochannels arrays with a uniform channel size. The top-view TEM image (Figure 1c) reveals a highly ordered, hexagonal packing of pores with a uniform diameter of 2−3 nm. After PDMS modification, the volatile small PDMS chains were deposited only on the top orifices of silica nanochannels, thus generating a hydrophobic thin layer on the top orifices of hydrophilic silica channels. The surface of pristine channels on the bottom remains hydrophilic and negatively charged, as illustrated in Scheme 1. As shown in Figure 1d, the morphology of silica nanochannels did not change significantly after modification. However, the image was obscured slightly by the nonconductive PDMS layer. More characterization of PDMS oligomers can be found in our recently published work.56 Gated by the Salt Concentration. Ion transport in silica nanochannels was measured by CV on the underlying ITO electrode using cationic and anionic electroactive probes, Ru(NH3)63+ and Fe(CN)63−, respectively. In the case of unmodified silica nanochannels without PDMS coating, ion transport is solely controlled by the electrostatic interaction between ion and channel surface. Figure 2a shows the influence of salt concentration on the transport process. Clearly, at a low salt concentration (1 mM NaCl, pH 6), the silica nanochannels were negatively charged and electrical double layer (EDL) at the channel wall surface strongly overlapped. Thus, only cationic Ru(NH3)63+ was able to transport. However, at a high salt concentration (0.5 M NaCl), the EDL is thin so that both cationic Ru(NH3)63+ and anionic Fe(CN)63− can transport through the nanochannels. In contrast, ion transport in HSNs is governed by the balance of hydrophobic and electrostatic forces exerted by the PDMS layer and the silica nanochannel surface beneath, respectively. The hydrophobic force rejects the passage of all hydrophilic molecules, whether they are cations or anions. The electrostatic force operates with an apparent dependence on the salt concentration. At a high salt concentration, the electrostatic force is largely shielded by the salt and the PDMS layer; thus, no ion transport is allowed even for cations. Indeed, as shown in Figure 2b, redox current was observed for neither Ru(NH3)63+ nor Fe(CN)63− (see the blue curves), meaning the HSNs were completely “OFF”. At a low salt concentration, the EDL became thicker and extended to the bulk solution59 so

Figure 2. (a, b) CV studies of ion transport in unmodified silica nanochannels (a) and HSNs (b) at low and high salt concentrations using Ru(NH3)63+ and Fe(CN)63− as probes, respectively. (c) Illustration of selective ion transport in HSNs electrode by the salt concentration. At a low salt concentration, the electrostatic attraction to cations becomes stronger than hydrophobic rejection. By contrast, both forces reject the transport of anions.

that the ultrathin PDMS layer can no longer screen completely the electrostatic interaction anymore (as exemplified in Figure 2c). In this case, cationic Ru(NH3)63+ is able to transport through HSNs because the electrostatic attraction becomes stronger than the hydrophobic rejection, while the transport of anionic Fe(CN)63− was still completely suppressed due to the rejection of both hydrophobic and electrostatic forces. The CV curves of Ru(NH3)63+ and Fe(CN)63− at other salt concentrations can also be found in Figure S1, Supporting Information. The results indicate that HSNs can be activated from OFF to ON state for cation transport by decreasing the salt concentration, which is akin to that of the transient receptor potential V1 (TRPV1) cation channel.9,12,60 To the best of our knowledge, activating an artificial nanochannel at a low salt concentration has not been reported yet. Gated by pH. HSNs can also be switched ON/OFF by the solution pH. First, the influence of pH on the mass transport in unmodified silica nanochannels was also comparatively investigated, as shown in Figure 3a. The reported pKa of silanol on the silica nanochannel surface is about 3.61 By changing the solution pH from 3 to 10, the channel surface varied from uncharged to negatively charged state, thus allowing the transport of both cationic Ru(NH3)63+ and anionic Fe(CN)63− at low pH but that of only cationic 33345

DOI: 10.1021/acsami.6b13772 ACS Appl. Mater. Interfaces 2016, 8, 33343−33349

Research Article

ACS Applied Materials & Interfaces

transport of Ru(NH3)63+ became more and more significant with increasing the solution pH. In a high ionic strength environment, because of the shielding effect of the salt, the transport of Ru(NH3)63+ was observed only at a pH higher than 7 (see Figure 3d). Thus, the pHopen point (the threshold pH at which the nanochannels are open) can be altered from 3 to 7 by increasing the solution ionic strength. Note that, in the entire pH range, the transport of Fe(CN)63− remained completely suppressed even in high ionic strength solutions, showing the precise cation permselectivity. This excellent performance essentially arises from the heterogeneous nanochannel structure. Only those ions attracted by the bottom part of the nanochannel can pass through the top hydrophobic barrier, while others are always repelled, thus resulting in an excellent selectivity toward cations (as exemplified in Figure 3e). Moreover, the activated channel can be easily switched OFF again by decreasing the solution pH (see Figure S2), as the electrostatic attraction turned weaker than the hydrophobic rejection. It should be noted that the intensity of hydrophobic rejection force can also be modulated by the PDMS evaporation time. With the PDMS evaporation time increased, the PDMS layer will become thicker and denser, resulting in a higher blocking barrier for ion transport. Thus, the pHopen point will become larger and even disappear completely (see Figure S3). These results prove that the balance between hydrophobic rejection and electrostatic attraction is responsible for the excellent selective and gated molecular transport, which is known to play an important role for cargo passage through the nuclear pore complex (NPC).6 Indeed, the NPC has a heterogeneous chemical composition, containing both positively charged parts and hydrophobic ones in the channel. Only those highly negatively charged karyopherins can pass through the hydrophobic energy barrier in NPC due to the strong electrostatic attraction force. All these results demonstrate that HSNs can be switched between a complete OFF state and an ON state by changing the solution pH, similar to that of TRPV1, which becomes open at pH higher than 7.8.10 Meanwhile, the pHopen point can be controlled by the hydrophobic force (PDMS evaporation time) and electrostatic force (solution ionic strength). Gated by Voltages. Ionic transport in HSNs electrode can also be activated by applying voltages. We measured the transport of Ru(NH3)63+, Fe(CN)63−, and TNT in a mixture solution, as shown in Figure 4a. When the potential was first swept from +0.5 to −1.0 V (the blue line in Figure 4a), no transport of Ru(NH3)63+ or Fe(CN)63− was observed, whereas only characteristic reduction peaks for TNT were displayed. However, the transport of Ru(NH3)63+ appeared in the second scanning cycle and became more and more remarkable in the following cycles (the red curves), while the transport of Fe(CN)63− was blocked all the time. Corresponding DPV results also confirmed the selective transport of cation if a negative enough voltage was biased (see Figure S4). Moreover, no ion transport occurred when only sweeping potential to −0.4 V in the same solution or sweeping to −1.0 V in the solution without TNT (see Figure S5). This means that the imposition of a negative enough potential to reduce TNT is critical for activating channel “ON” for Ru(NH3)63+ transport. Similar result was also obtained by reducing dissolved O2 present in solution instead of TNT (see Figure S6). TNT and O2 could transport through the PDMS layer and be reduced at a negative enough potential to generate OH− by

Figure 3. (a, b) CV studies of ion transport in silica nanochannels at different pH values in the low (a) and high (b) ionic strength environments using Ru(NH3)63+ and Fe(CN)63− as probes. The vertical coordinates show voltammetric peak currents. The low and high ionic strength environments refer to 1 mM NaCl and 0.5 M NaCl. (c, d) CV studies of ion transport in HSNs at different pH in the low (c) and high (d) ionic strength environments using Ru(NH3)63+ and Fe(CN)63− as probes. (e) Schematic illustration of pH-gated ion transport in HSNs electrode. By increasing the solution pH, the electrostatic attraction exceeds the hydrophobic rejection, leading to the selective transport of cations. By contrast, both forces reject the transport of anions.

Ru(NH3)63+ at high pH. However, in a high ionic strength environment, the charge permselectivity disappeared so that both cation and anion could transport in HSNs at all pH values (Figure 3b). The weak permselective performance in a high ionic strength environment is also widely observed in other pHgated systems. All these results show that unmodified silica nanochannels with a homogeneous surface are lacking the pHgated ON/OFF functionality.39,48 In the case of HSNs, the electrostatic interaction between ionic species and the underlying part of the channel was weak at pH 3 so that all ionic species were rejected by the hydrophobic PDMS layer even in the low ionic strength environment (as shown in Figure 3c), meaning the channel was “OFF”. When the solution pH was increased, the silica channel surface became negatively charged, thus exerting a strong electrostatic attraction on the mass transport of Ru(NH3)63+ to compete against the hydrophobic rejection from the PDMS layer. Channels thus turned open at pH larger than 3 and the 33346

DOI: 10.1021/acsami.6b13772 ACS Appl. Mater. Interfaces 2016, 8, 33343−33349

Research Article

ACS Applied Materials & Interfaces

demonstrate that the voltage-induced redox reaction in the nanochannels could modulate the relative magnitude of electrostatic and hydrophobic forces, leading to an ON or OFF state (Figure 4c).

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CONCLUSIONS In summary, ion transport in ultrasmall HSNs was investigated in this work. The hydrophobic PDMS layer on the top orifices of silica nanochannels rejects the transport of all ions, while the negatively charged silica nanochannel exerts an electrostatic attraction on cation transport and electrostatic rejection on anion transport. Therefore, anion transport was always prohibited, while the cation could be selectively transported once the electrostatic attraction force surpassed the hydrophobic rejection force. The magnitude of former force can be regulated by the salt concentration, solution pH and applied voltage while the latter can be modulated by PDMS evaporation time. Thus, the HSNs can be activated from an OFF state (no ion can transport) to an ON state (only allow cation passage) by decreasing the salt concentration, increasing solution pH, and applying voltage. In future work, more complex channels can be fabricated and molecular transport in these sophisticated channels can be controlled by various interactions, leading to many new transport phenomena. It should be noted that the HSN membrane could also be detached from the ITO electrode to attain perforated channels.19 In this case, the transport of nonredox active small ions, such as K+, Na+, and Cl−, can also be studied to examine the applicability of HSN in biological systems. We believe that both electrode-supported and free-standing HSNs with ultrasmall heterogeneous compositions are potentially useful in ion transport study, drug delivery, sensing, nanofluidics, and molecular separation.

Figure 4. (a) Successive CVs of ion transport in HSNs in the solution consisting of Ru(NH3)63+, Fe(CN)63−, and TNT. The blue curve represents the first potential scan segment. The red ones are the following scan segments. The electrolyte used was 0.5 M NaCl. (b) Switchability of the voltage-gated HSNs electrode. The CV peak current of Ru(NH3)63+ was measured after −1 or +1 V was applied to electrode alternatively for 20 and 60 s, respectively. The electrolyte used was 0.5 M NaCl. (c) Schematic illustration of voltage-gated ion transport in HSNs electrode. At −1.0 V, the electrochemically generated OH− locally brings more negative charges to the channel surface, resulting in a strong electrostatic attraction to cation and thus allowing its transport. −

R−NO2 + 6e + 4H 2O → R−NH 2 + 6OH

O2 + 4e + 2H 2O → 4OH

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−

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ASSOCIATED CONTENT

S Supporting Information *

(1)

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b13772. Reagents and more CV and DPV results (PDF)

(2) −

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We believe that electrochemically generated OH , although its amount may be very small, can alter the local pH effectively in nanochannels to a much higher level. Once the solution pH in nanochannels reaches “pHopen” (as shown in Figure 3d), where the electrostatic attraction becomes stronger than the hydrophobic rejection due to more negative charges on the channel surface, the channel will be “ON” and the selective transport of cation appears. The increased pH in nanochannels can be observed from the negative shift of TNT peak potential in the second potential scanning cycle.62 This assumption was also confirmed by the results obtained in a phosphate buffer (PB) or HCl solution in which the generated OH− was consumed immediately and the transport of Ru(NH3)63+ was suppressed all the time (see Figure S7). After a positive voltage (e.g., +1.0 V) was applied, the channel was switched OFF again because H+ was generated in the nanochannel to decrease the local pH (see Figure S8). In this case, the HSNs could be switched between ON and OFF state repeatedly by changing voltage polarity without changing the solution (as displayed in Figure 4b). Moreover, the ON/OFF flux ratio is calculated to be 174 ± 15, showing an excellent voltage-gated ON/OFF switching ability. The salt concentration used here was 0.5 M NaCl. Even in such high ionic strength solution, the channel still worked well and exhibited precise cation selectivity. All these results

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Web site: http://mypage.zju.edu. cn/binsu. ORCID

Bin Su: 0000-0003-0115-2279 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Nature Science Foundation of China (21335001 and 21575126) and the Nature Science Foundation of Zhejiang Province (LR14B050001).

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REFERENCES

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DOI: 10.1021/acsami.6b13772 ACS Appl. Mater. Interfaces 2016, 8, 33343−33349

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DOI: 10.1021/acsami.6b13772 ACS Appl. Mater. Interfaces 2016, 8, 33343−33349