pH- and light-regulated ion transport in hourglass shaped Al₂O₃ nanochannels patterned with N719 and APTES

Abstract

Artificial nanochannel systems regulated by diverse stimuli can provide rich ion transport, and are frequently used to mimic intelligent ion channels in biological membranes. Combining shape control and chemical modification of artificial nanochannels is an effective approach in the construction of nanochannel systems with ion transport regulation by multiple stimuli. In this work, symmetric and asymmetric hourglass shaped Al₂O₃ nanochannels were fabricated and the nanochannels were chemically patterned with N719 and APTES at expectant positions. By regulating the pH value, the ionization status of the coated molecules was changed and the charges in the nanochannels were redistributed, which resulted in pH-responsive ion rectification characteristics. When irradiated with light, the charges on the surfaces modified with N719 molecules were increased and new charge distributions in the nanochannels were formed, which led to light-responsive ion transportation behaviour. In the symmetric Al₂O₃ nanochannels, an ion current rectification ratio of about 4.3 was obtained. In the asymmetric Al₂O₃ nanochannels, the light induced current change ratio reached about 1.1. The study of symmetric and asymmetric nanochannels combined with different responsive molecules at expectant positions may provide an innovative approach for the design and fabrication of smart nanochannel systems to simulate biological ion channels.

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1. Introduction

Smart artificial nanochannels, which mimic the intelligent response of biological channels to regulate ion transport through cell membranes, have been developed prosperously in recent years¹ and several results have been achieved in learning from nature. In smart artificial nanochannels, single responses, such as those to specific ions,² light,³ pH,⁴ temperature,⁵ or pressure,⁶ have been demonstrated. However, compared to being regulated by a single stimulus,⁷ biological ion channels that are multi-stimuli responsive achieve more intelligent regulation of ion transport through cell membranes.⁸ Fabrication of complex multi-responsive nanochannel systems, such as pH- and temperature-responsive systems,⁹ and pH- and light-responsive systems,¹⁰ can pave the way to further explore biological smart responses. Considering the structural diversity of biological ion channels, the design of smart artificial nanochannels should not only focus on the chemical modification of nanochannels to respond to ambient stimuli, but should combine structure and surface features together to achieve unusual ion transport characteristics. Various asymmetric shaped nanochannels have been prepared,¹¹ and a nanochannel system based on shape and pH cooperative nanochannels has been reported,¹² but this system only realized a simple pH-response.

Al₂O₃ nanochannels are regarded as a desirable candidate for artificial nanochannels, because of their good chemical stability, high pore densities and the outstanding flexibility in controlling their geometry.¹³ Besides, the inner walls of Al₂O₃ nanochannels carry a large number of hydroxyl groups, which provide a suitable chemical environment for the following modification in the nanochannels. Recently, hourglass shaped Al₂O₃ nanochannels were fabricated, which are generally composed of two segments and a very narrow region (tip section) that is localized inside the nanochannels to effectively control ion transport and provide a desired transmembrane environment. In particular, the position of the tip in the hourglass shaped Al₂O₃ nanochannels could be changed from the center to one segment of the channels by controlling anodizing time and anodizing position, which could lead to a transformation of the channel geometry from a symmetric shape to an asymmetric shape. In this work, symmetric and asymmetric structures of the hourglass shaped Al₂O₃ nanochannels were designed, and for the first time, the ion transport properties of the functionalized nanochannels created through chemical modification were investigated and modulated under pH and light stimuli.

As shown in Scheme 1, the pH responsive molecule (3-aminopropyl)triethoxysilane (APTES) and light sensitive molecule di-tetrabutylammonium cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato) ruthenium(II) (N719) were used to functionalize the hourglass shaped Al₂O₃ nanochannels, including those with a symmetric shape and an asymmetric shape. First, APTES was used to modify the inner walls of the whole nanochannel and then N719 was used to asymmetrically modify one segment of each nanochannel using a well-developed diffusion-limited patterning (DLP) modification method,¹⁴ which is an effective approach that enabled the patterning of functional molecules in the solution phase onto the nanochannel surface. Combined with the narrowed tip region of the hourglass shaped nanochannels, the N719 molecules could be coated onto one segment of the nanochannels when the N719 molecules diffused along the nanochannel axis, by controlling the concentration of the N719 solution and the diffusion time (Scheme 1a and b). Therefore, siloxane-tethered amine groups and carboxylate groups were separately created on the inner surface of the Al₂O₃ nanochannels in opposite segments with a corresponding pK_a of about 6.2 (ref. 4c) and 5.5–5.9,¹⁵ respectively. The pK_a of our fabricated siloxane-tethered amine groups in the Al₂O₃ nanochannels was lower than that of the amino groups in aqueous solution due to the confined effect from the amino groups at its neighboring position and the opposite direction of the nanochannels in the amino protonation process as previously reported works have shown.¹⁶ The surface charges along the APTES- and N719-modified segments of the nanochannels, which were caused by the protonation/deprotonation of functionalized groups upon pH variation, contributed to the pH-modulated ionic transportation behaviour (Scheme 1c and d). When the nanochannels are irradiated with visible light at a certain pH value, the N719 molecule becomes excited and electron transition from metal to ligand in the molecules occurs.¹⁷ The surface charge density of the N719 segment increases, which provides light-modulated ion transport properties.

Scheme 1 (a) Schematic illustration of the symmetric and asymmetric hourglass shaped Al₂O₃ nanochannels patterned with N719 (red) and APTES (yellow). (b) An enlarged drawing of a single functionalized symmetric Al₂O₃ nanochannel. The siloxane-tethered amine groups and carboxylate groups were separately created on the inner wall of the nanochannel. (c and d) The pH and light controlled surface charge change of the amine groups and carboxylate groups on the inner walls of the channels causes a fluctuation of the ionic transportation in our prepared functionalized nanochannels. (e) An enlarged drawing of the two single functionalized asymmetric Al₂O₃ nanochannels patterned with APTES and N719 with length ratios of about 1 : 2 and 2 : 1, respectively, which provide controllable ion transport characteristics by light stimulus, as well as the combined effect of chemical modification and the asymmetric shape.

Asymmetric hourglass shaped Al₂O₃ nanochannels with a length ratio of the two segments of about 1 : 2 were fabricated. Based on the N719 modification position, two kinds of functionalized asymmetric nanochannels were designed. The terms LSN and SSN mean that N719 was coated onto the long or short segment of the nanochannels, respectively (Scheme 1e). This intelligent nanochannel array system demonstrates a combined effect from the asymmetric shape, designed chemical modification, pH and light.

2. Experimental

2.1 Fabrication of symmetric and asymmetric hourglass shaped Al₂O₃ nanochannels

The hourglass shaped alumina nanochannels were fabricated using a double-sided anodization method with an in situ pore opening process, as shown in Fig. S1 (ESI†). Al foil (99.999% pure, 0.1 mm thick) was cleaned using acetone, ethanol, and MilliQ water (18.2 MΩ) in sequence. The Al foil was used as the anode with a stainless plate as the counter electrode. Then, the cleaned Al foil was electrically polished in a mixed solution of HClO₄ and ethanol (1 : 4 in volume ratio) under 17 V at 5 °C. The two sides of the electropolished Al foil were separately anodized for 1 h in a 0.3 M oxalic acid solution at 5 °C, as shown by step (1) in Fig. S1.† The resulting porous oxide layer was removed in a solution containing 6 wt% phosphoric acid and 3.5 wt% chromic acid at 90 °C for 2 h (step (2) in Fig. S1†). In order to prepare the symmetric hourglass shaped alumina nanochannels, the etched Al foil with highly ordered hemispherical concaves on each side was further anodized under the same conditions as the first anodization procedure. When the anodized current was decreased to zero, the Al foil was totally anodized, and two layers of Al₂O₃ nanotube arrays, which were not conducting, were fabricated. Following this, an in situ pore opening procedure was carried out for 30 min to form the conducting symmetric hourglass shaped alumina nanochannels (step (3) and (4) in Fig. S1†).

In order to prepare the asymmetric hourglass shaped Al₂O₃ nanochannels, the anodization process of the Al foil was strictly controlled with different anodization times on the two sides, as shown from step (5) to (7) in Fig. S1.† The etched Al foil with the hemispherical concaves was mounted in a tailor-made groove, and only one side of the Al surface made contact with the electrolyte. The etched Al foil was anodized at 50 V (5 °C) for 4 h with vigorous stirring and the Al₂O₃ nanotube arrays grew on only one side of the Al foil. Then the resulting Al foil with the Al₂O₃ nanotube arrays was taken out from the groove, and was further anodized under the same conditions as the first anodization procedure for the symmetric nanochannels. Because of the different anodization times for the two sides, asymmetric nanotubes with a length ratio of 1 : 2 with a barrier oxide layer at the bottom of each nanotube were successfully fabricated. After an in situ pore opening process was conducted for 30 min, the asymmetric hourglass shaped Al₂O₃ nanochannels were built, which possessed two segments with a length ratio of about 1 : 2. The in situ pore opening process did not change the length ratio of the channels. The microstructures and morphology of symmetric and asymmetric hourglass shaped Al₂O₃ nanochannels were characterized with a scanning electron microscope (SEM, Hitachi, S4300).

2.2 Functionalization with APTES and N719

The surface of the hourglass shaped Al₂O₃ nanochannels was modified with APTES, and the reaction process is schematically shown in Fig. S2 (ESI†). Firstly, the as-prepared hourglass shaped Al₂O₃ nanochannels were boiled in hydrogen peroxide (30% H₂O₂) for 1 h to introduce the hydroxylate onto the surface of the nanochannels and then were dried completely using nitrogen. Next, each side of the nanochannels was treated in a mixed solution of APTES and acetone (1 : 4 in volume ratio) for 5 min, and then was washed with acetone and dried. So, the Al₂O₃ nanochannels were successfully modified with APTES, which resulted in siloxane-tethered amino groups coated on the surface of the nanochannels.

Using the diffusion-limited patterning (DLP) method as in previously reported work,¹⁴ the modification of the nanochannels with N719 molecules was carried out in the designated position of the nanochannels. The APTES modified hourglass shaped Al₂O₃ nanochannels were fixed between two chambers of the home-made electrochemical cell, as shown in Fig. S3 (ESI†). One chamber was filled with a 0.3 mM ethanol solution of N719 dye and the other chamber was filled with only ethanol. Therefore, the N719 modification was performed only on one side of the hourglass shaped Al₂O₃ nanochannels. A series of functionalized symmetric hourglass shaped Al₂O₃ nanochannels were obtained with N719 modification times of 10 min, 20 min, 30 min, 1 h, 2 h and 3 h, respectively. As for the asymmetric hourglass shaped Al₂O₃ nanochannels with a length ratio of the two segments of about 1 : 2, the N719 modification time for the long segment was carried out for 4 h, while that for the short segment was 2.5 h. Upon completion of washing with ethanol (99%) and drying under a stream of nitrogen, hourglass shaped Al₂O₃ nanochannels patterned with N719 only in the expected position (long segment or short segment) were finally prepared. Our fabricated Al₂O₃ nanochannels before and after modification with APTES and N719 on the inner walls were then characterized using a fluorescent microscope (Vision Engineering Co., UK).

2.3 Ion transport measurement under pH and light regulation

The pH- and light-modulated ion transportation properties of the hourglass shaped Al₂O₃ nanochannels were characterized by measuring the ionic current through the channels. The nanochannels were mounted between the two chambers of the electrochemical cell, which were filled with 0.1 mM KCl solution. The ionic current was measured by a Keithley 6487 picoammeter. Ag/AgCl electrodes were settled in each cell to apply the desired potential and to measure the resulting ionic current, and a scanning voltage was applied from −2 V to +2 V. The current–voltage (I–V) measurement was operated by alternating light in an on or off state under pH values changing from 4 to 9. The pH of the electrolyte was adjusted with 1 M hydrochloric acid or potassium hydroxide solution. For the ionic current measurements of the modified symmetric hourglass shaped Al₂O₃ nanochannels, the anode was always facing the APTES segment while the cathode was facing the N719 segment, as shown in Fig. S4 (ESI†). Visible light was used to irradiate the N719 side of the channels with an irradiance of 90 mW cm⁻². For the ionic current measurements of the original or functionalized asymmetric hourglass shaped Al₂O₃ nanochannels, the cathode was always facing towards the short segment of the nanochannels.

3. Results and discussion

As shown in Fig. 1, SEM images of the two opening sides and cross-section view reveal the symmetric shape of our prepared hourglass Al₂O₃ nanochannels and the narrow tips inside of the nanochannels can be seen clearly. The two opening sides had relatively large diameters depending on the anodization voltage, while the tip channels etched from the barrier layers had small diameters determined by the in situ pore opening process time. In detail, both the top and bottom sides of the nanochannels had nearly the same pore diameters of about 35 ± 5 nm (Fig. 1a1 and a2). The thickness of our as-prepared Al₂O₃ nanochannels was about 100 ± 10 μm and a weak boundary between two layers of nanochannels could be observed in the SEM image at a low magnitude (Fig. 1a3). The tip channel diameters inside the Al₂O₃ nanochannels were observed as 15 ± 5 nm (Fig. 1a4).

Fig. 1 SEM images (a1: top; a2: bottom; a3: cross-section; a4: tip-section at high magnitude) of our fabricated symmetric hourglass shaped Al₂O₃ nanochannels. The two opening sides had large diameters, while the tip channels had small diameters. The symmetric shape was revealed from a weak boundary between two segments of the channels. (b and c) Schematic diagram of a single unmodified and functionalized nanochannel patterned with APTES (yellow) and N719 (purple). In the cross-sectional optical (b1 and c1) and fluorescence images (b2 and c2), strong contrasts between the two segments of the modified nanochannels were observed, which were more clear than that of the unmodified channels. N719 was asymmetrically coated onto the inner walls of the symmetric hourglass shaped Al₂O₃ nanochannels after 3 h of modification, which provided the same length of N719 segment as that of the APTES segment. Scale bar: 20 μm.

As a result of chemical modification, the N719 modified area of the channel membrane turned purple as shown in Fig. S5 (ESI†), while the APTES coated area experienced no color change compared with the unmodified channels. From the cross-sectional optical (Fig. 1b1 and b2) and fluorescence (Fig. 1c1 and c2) characterizations, a stronger contrast between the two segments of the functionalized Al₂O₃ nanochannels was observed than that of the unmodified nanochannels (Fig. 1b), which was deduced from the different scattering and/or absorbing effects between APTES and N719. By increasing the N719 modification time, the length of the N719 segment could be increased, as shown in Fig. S6 (ESI†), which illustrated that N719 could be successfully coated onto the inner walls of the nanochannels using the DLP method. In our work, the N719 molecules could reach a boundary that was half the length of the nanochannels when the N719 solution diffusion time was 1 h. However, the concentration of the N719 solution at the tip region of the channels was less than that of the original modification solution. So, the coated layer of N719 molecules on the inner walls was inhomogeneous at this time. When the diffusion time was 3 h (Fig. 1c), more N719 was coated onto the inner walls, which caused the purple color to darken. It is necessary to note that the N719 molecules could not pass through the narrow tip region of the channels in our experiment even though the diffusion time had been further extended. However, the tip channels became slightly blocked when the diffusion time was more than 3 h. As a result, N719 could be asymmetrically coated onto one segment of the symmetric hourglass shaped Al₂O₃ nanochannels, which provided nearly the same length as the APTES segment in the opposite side of the channels.

In order to investigate the combined effect of shape and chemical modification on ionic transportation, asymmetric hourglass shaped Al₂O₃ nanochannels were fabricated. Here, the two opening sides of the asymmetric channels had nearly the same diameter of about 35 ± 5 nm (Fig. 2a1 and a2), which is different to previously reported asymmetric hourglass shaped polymer nanochannels.^11a,12 The thickness of the asymmetric nanochannels was about 103 ± 10 μm (Fig. 2a3), which was well controlled in our work. The length ratio between two asymmetric segments was about 1 : 2. The narrow tip diameter of the hourglass shaped Al₂O₃ nanochannels was estimated to be about 15 ± 5 nm (Fig. 2a4).

Cross-sectional optical and fluorescence images also revealed the asymmetric characteristics of the fabricated nanochannels before (Fig. 2b) and after modification. According to the results obtained from coating N719 onto the symmetric Al₂O₃ nanochannels, diffusion process times of 2.5 and 4 h using N719 solution were applied and N719 molecules were correspondingly immobilized onto the short segment (Fig. 2c) or the long segment (Fig. 2d) of the asymmetric nanochannels, respectively. Therefore, two kinds of surface functionalized asymmetric hourglass shaped Al₂O₃ nanochannels have been represented with the length ratios between the N719 and APTES segments being modulated from 1 : 2 to 2 : 1.

Fig. 2 SEM images (a1: top; a2: bottom; a3: cross-section; a4: tip-section at high magnitude) of our fabricated asymmetric hourglass shaped Al₂O₃ nanochannels. The two opening sides had nearly the same diameter, which were larger than that of the tip channels. The length ratio between two asymmetric segments was about 1 : 2. (b, c and d) Schematic diagrams of the single unmodified (b) and functionalized (c and d) nanochannel patterned with APTES (yellow) and N719 (purple). As shown in cross-sectional optical (b1, c1, and d1) and fluorescence images (b2, c2 and d2), N719 molecules were coated onto the short segment or the long segment of the channels after 2.5 or 4 h of employing the DLP modification process, respectively. Scale bar: 20 μm.

To certify the light regulated ion transport characteristics of the N719 modified channels, the current–voltage (I–V) properties of the symmetric hourglass shaped Al₂O₃ nanochannels before and after APTES and N719 modification were investigated, which were measured using 1 mM KCl electrolyte solution at pH 6 with visible light in an on/off state. As shown in Fig. S7 (ESI†), nonlinear I–V curves were observed for the N719 modified nanochannels and the ion current at negative voltage was higher than that at positive voltage, while the original and APTES modified nanochannels exhibited linearly symmetric I–V curves because both the shape and the surface charge of these two kinds of channels were symmetric and were not able to show any ion current rectification (ICR) effects regardless of whether the light irradiation was on or off. Here, the ICR ratio is defined as |I_{−2 V}/I_{+2 V}|, which means the ratio of the absolute values of ionic currents are recorded at ± 2 V. Upon light irradiation, the nanochannels after N719 modification exhibited an increased ion current at negative voltage, and demonstrated increased ionic conductance of the KCl solution that was in contrast to the dark state. Therefore, we further investigated the light-modulated I–V properties of the N719 modified nanochannels under different modification times, as shown in Fig. 3a and b. By increasing the modification time from 1 h to 3 h, the ionic currents of the N719 and APTES patterned symmetric nanochannels increased under negative voltage, while they decreased under positive voltage because the amount of N719 molecules on the inner walls of the channels increased and the tip channels became narrowed. The corresponding ICR ratios gradually increased as shown in Fig. 3c and the ICR ratios in the light state were higher than that of in the dark state. However, when the N719 modification time was over 3 h, the conductive tip-region of the channels were slightly blocked because of excessive coating. The best ICR ratio was obtained under a 3 h patterning procedure with N719 molecules and the ionic transportation current of our fabricated functionalized Al₂O₃ nanochannels could be enhanced with light irradiation. Therefore, these functionalized symmetric hourglass shaped Al₂O₃ nanochannels were chosen as the appropriate candidate to carry out the following investigation on the pH- and light-regulated ionic transportation properties.

Fig. 3 (a and b) I–V properties of the N719 and APTES modified symmetric hourglass shaped Al₂O₃ nanochannels obtained by adjusting the N719 coating time from 1 h to 3 h, recorded using light as an on/off switch. The measurement was obtained using a 0.1 mM KCl electrolyte at pH 6. (c) ICR ratio change with N719 coating time.

The pH- and light-regulated ionic transport characteristics of the as-prepared APTES and N719 modified symmetric hourglass shaped Al₂O₃ nanochannels were experimentally investigated using I–V measurements, as shown in Fig. 4a and b. The measurements were recorded under a symmetric electrolyte solution of 0.1 mM KCl by gradually changing the pH values from 4 to 9 under light in an on or off state, and the anode was faced towards the APTES segment while the cathode was faced towards the N719 segment. The nonlinear I–V curves were obviously observed and the ionic transportation currents of the channels at negative voltage were higher than those at positive voltage, which meant that ionic selective transport and rectifying occurred at each pH value regardless of whether the light irradiation was on or off. Increasing the pH from 4 to 8 led to a gradual decrease of the recorded ionic current, which indicated that the degree of ionization of amino groups and carboxyl groups was modulated by changing the pH of the solution. When the pH value was continuously increased to 9, the channels exhibited nearly the same ionic transportation properties to that at pH 8 because nearly all carboxylate groups were deprotonated in this case. Upon light irradiation, the ionic currents of the channels increased at each pH value condition (Fig. S8, ESI†). In particular, at pH 6 the ionic conductivity of the channels increased remarkably under negative voltage (Fig. 4b). Here, an electronic transition from metal to ligand in the N719 molecule was triggered by light irradiation. With the redistribution of charge on the ligand, the charge density on the carboxyl groups became higher in the excited state compared to the ground state.¹⁵ So, ion transport behavior was regulated using light irradiation due to the enhanced interaction between ions and surface charges.

Fig. 4 I–V properties of APTES and N719 modified symmetric hourglass shaped Al₂O₃ nanochannels were measured by gradually changing the pH values from 4 to 9 in the dark state (a) and the light state (b). (c) Schematic diagram illustrating the pH-modulated ionic transport properties of the channels. At negative voltage, ions accumulated inside the tip region of the channels.

Fig. 4c illustrates how the rectification of the APTES and N719 functionalized symmetric Al₂O₃ nanochannels occurred with a pH controllable change, which can be explained using the enrichment-depletion theory as detailed in previously reported work.¹⁸ At a pH value 4 or 5, the APTES segment contained protonated amine groups (–NH₃⁺) and carried a positive charge, while the N719 segment was electrically neutral (–COOH), which resulted in the inhomogeneous distribution of the chemical composition and the surface charge in the symmetric nanochannels. The Cl⁻ concentration was higher than the K⁺ concentration in the APTES segment because of the strong interaction between the positively charged walls and Cl⁻ in the solution. At a negative voltage, both K⁺ and Cl⁻ were driven toward the tip region and accumulated inside the tip channels, while at a positive voltage, the depletion of ions induced a low ionic current. As a result, ion current rectification in the nanochannels was established. The nanochannels at pH 5 exhibited weaker ICR properties than that at pH 4 because the positive charge density in the APTES segment was reduced. At pH 6, the hourglass shaped nanochannels exhibited heterogeneous surface charges, composed of the positively charged APTES segment and the negatively charged N719 segment, which enhanced the ionic accumulation/depletion in the tip regions of the channels. So, the best ICR properties were obtained at pH 6 in our work. When the pH value was above 6, the carboxyl groups on the inner wall surface of the N719 segment were deprotonated (–COO⁻) and the APTES segment was neutral (–NH₂). Depending on the cooperative effect that the surface charge polarity and distribution were simultaneously reversed, and that the cations (K⁺) were the majority carriers, the rectification direction in this case had no change compared with other conditions.

The pH and light tunable current rectification based on the APTES- and N719-modified symmetric Al₂O₃ nanochannels was quantified by the ICR ratio (Fig. 5a). As shown by the I–V curves, the rectification ratios were obviously different under different pH values. The ICR ratios reached a maximum value at pH 6 because the surface charge polarity and distribution were the most asymmetric in this case than that at other pH values. In the dark state, the ICR ratios in acidic solutions were higher than those in natural or alkaline solutions, indicating that there were less carboxyl groups than amino groups on the inner walls of the channels because the N719 molecule is larger than the APTES molecule. As mentioned above, increasing the pH led to a gradual decrease of the positive charge density and an increase of the negative charge density. Upon light irradiation, the ICR ratios increased under each pH value because the light induced electrons increased the asymmetry of the surface charge polarity at low pH values or surface charge distribution at high pH values. In particular, when the pH value was above 6, the ICR ratios remarkably increased, which can be attributed to the decrease of the effective diameters of the tip channels due to the enhanced electrostatic repulsion between N719 and anions on the inner walls of the channels. The effect of light stimulus on the ionic current is quantitatively defined as |(I_light − I_dark)/I_dark|_{−2 V}, which means that the light current change (LCC) ratio between the responsive current (I_light − I_dark) and the dark ionic current at a voltage of −2 V under a certain pH value. As shown in Fig. 5b, changing the pH value from 4 to 9, resulted in a similar variation in the trend of LCC ratios to that of the ICR ratios by light irradiation. The LCC ratio at pH 6 reached a maximum value of about 0.9. The nanochannels exhibited nearly the same LCC ratios at high and low pH values except pH 6, which indicated that the cooperative effect of pH and light stimuli played the leading role in our light-regulated ion transportation experiment. These results reveal that the ICR and LCC ratios can be modulated easily by simply changing the solution pH values and making use of light stimulus. Therefore, tunable ionic rectification characteristics can be realized by asymmetric chemical modification with APTES and N719 molecules on symmetric hourglass shaped Al₂O₃ nanochannels.

Fig. 5 Quantitative investigation of the ionic transportation properties of the APTES and N719 modified symmetric hourglass shaped Al₂O₃ nanochannels. (a) Ion current rectification ratios versus the pH values in the dark state (black) and the light state (red). ICR ratio = |I_{−2 V}/I_{+2 V}|. (b) The light current change ratios under certain pH values, meaning the ratio between the light responsive current and the dark ionic current. LCC ratio = |(I_light − I_dark)/I_dark|_{−2 V}. The light responsive current is defined as the difference between the dark ionic current and the ionic current under light irradiation. The ionic current was recorded at a voltage of −2 V.

Because the two functionalized segments of the Al₂O₃ nanochannels possessed opposite charges only at pH 6, and exhibited outstanding ion transport properties in the symmetric nanochannels, in our experiments I–V curves based on asymmetric hourglass shaped Al₂O₃ nanochannels were measured at pH 6 with the cathode facing toward the short segment of the nanochannels. Fig. 6 reveals the ion transport performance of the asymmetric nanochannels before and after modification with visible light in an on/off state. The unmodified alumina nanochannels showed a nonlinear I–V curve exhibiting rectification behaviour because of the asymmetric shape of the channels (Fig. 6a), as well that the fact that the isoelectric point (pI) of alumina is about 8.5,¹⁹ which provided the positively charged surface with Al–OH₂⁺ terminals. After the hydroxyl groups on the alumina surface were reacted with APTES, the ionic transport current decreased greatly, while slight rectification behaviour was also observed. Besides, no light responsive ionic current was detected in these two cases. When N719 molecules were coated onto the short segment of the channels (SSNs), the nanochannels showed the same direction of ionic selective transportation as the unmodified channels, which demonstrated a high conductance at negative voltage and a low conductance at positive voltage, as shown in Fig. 6b. Here, the ion transport properties were controlled by the combined effect of the channel shape and the channel surface charge distribution. When N719 molecules were coated onto the long segment of the channels (LSNs), the nanochannels showed reversed rectification properties, so that the high and low conducting states appeared at positive voltage and negative voltage, respectively. In this case, the reversed surface charge polarity was the major effect determining the rectification direction of the channels. Upon visible light irradiation, the ionic current of these two kinds of N719 modified asymmetric channels increased but in different manners. The ionic current of the SSNs dramatically increased at negative voltage, which was the opposite response to that of the LSNs. The asymmetric channels functionalized by chemically patterned APTES and N719 at designated positions provided the light-modulated and controlled ionic transportation properties.

Fig. 6 I–V properties of the asymmetric hourglass shaped Al₂O₃ nanochannels before and after modification were measured at pH 6 using light as an on/off switch. The insets are schematic diagrams to illustrate that the cathode was faced toward the short segment of the channels. (a) Current rectification of unmodified nanochannels and APTES modified nanochannels. (b) The current rectification effect could be converted by altering the N719 modification position in the channels. Upon light irradiation, the ionic currents of the APTES and N719 functionalized asymmetric channels increased.

To investigate the combined ionic transportation controlled by shape and chemical modification, the ICR ratios and the LCC ratios of the asymmetric Al₂O₃ nanochannels before and after modification were determined and are shown in Fig. 7. Note that the ICR ratio of the LSNs is defined as |I_{+2 V}/I_{−2 V}| due to the reversed current, and the LCC ratios were calculated at a voltage of ±2 V. As mentioned previously, the unmodified and the APTES modified asymmetric nanochannels showed rectification characteristics with low ICR ratios, and could not be modulated by light irradiation (Fig. 7a). In the dark, the two kinds of N719 modified asymmetric nanochannels had nearly the same ICR ratios because the point where the surface charge polarity changed was nearly at the same position.²⁰ Under light irradiation, the ICR ratio of the LSNs dramatically increased, which resulted in the asymmetric charge distribution of LSNs becoming more obvious than that of the SSNs. Furthermore, the LCC ratios of the LSNs with the ionic current recorded at +2 V was higher than that of the SSNs with the ionic current recorded at −2 V, which demonstrated that the effect of light stimulus on the ionic current for LSNs was higher than that for SSNs because of the longer N719 segments (Fig. 7b). Additionally, these two kinds of functionalized asymmetric nanochannels offered lower ICR ratios than that of the symmetric nanochannels patterned with APTES and N719, which revealed the decisive role of the channel shape in determining the rectification direction and magnitude as previously predicted by theoretical calculations.²¹ It has been reported that to obtain the maximum degree of rectification, the surface charge transition position should be located exactly in the middle of the channel length. However, the LSNs exhibited the best LCC ratios in our experiments due to the fact that they possessed the longest N719 segment. Therefore, the ICR ratio and LCC ratio could be modulated easily and the rectification polarity could be reversed by the combination of effectively controlling the channel shape and the N719 coating conditions.

Fig. 7 (a) ICR ratios and (b) LCC ratios of the asymmetric hourglass shaped Al₂O₃ nanochannels before and after modification. The ionic current was measured at pH 6 under light in an on and off state.

4. Conclusions

In conclusion, symmetric and asymmetric hourglass shaped Al₂O₃ nanochannels were chemically patterned with APTES and N719 at designated positions. The pH- and light-regulated ion current rectification ratios and light induced ion current change ratios have been demonstrated. The ICR and LCC of the symmetric nanochannels were regulated mainly by pH and light stimuli, and the ICR and LCC approached peaks of about 4.3 and 0.9 at pH 6. In the asymmetrical nanochannels, the ion transportation behaviour was revealed to be extremely sensitive to the length of the N719 coated section. Two reversed rectification directions, ionic rectification behaviour and light current changes were observed by changing the position of the APTES and N719 coating. The maximum ICR and LCC in the asymmetrical nanochannels were about 3.7 and 1.1, respectively. The influences of the asymmetrical structure, N719 coating position and light on the ion transportation behaviour have been analyzed in detail. This work reveals that tunable ion transportation properties of artificial nanochannels can be obtained not only by multiple stimuli, but also by effectively combining the control of the structure and chemical modification at designated positions.

Acknowledgements

We thank the National Natural Science Foundation of China (Grant No. 21471012 and Grant No. 21101009). The work was also supported by the Fundamental Research Funds for the Central Universities.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Fabricating process, I–V curves and visible light and fluorescence images. Detailed kinetic information in the form of additional schematic illustration and figures. See DOI: 10.1039/c6ra09490b

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