Showcasing research from Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry of Education, School of Chemistry and Environment, Beihang University, P. R. China
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Ion current behaviors of mesoporous zeolite–polymer composite nanochannels prepared by water-assisted self-assembly We designed novel artificial asymmetry nanochannels based on mesoporous zeolite (MCM-41) and polyimide (PI) by water-assisted self-assembly. Meanwhile, we studied ionic current behaviors and rectifying characteristics of the mesoporous zeolite/polymer composite nanochannels.
a
See Jin Zhai, Liping Heng et al., Chem. Commun., 2014, 50, 3552.
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Ion current behaviors of mesoporous zeolite–polymer composite nanochannels prepared by water-assisted self-assembly†
Received 17th October 2013, Accepted 21st November 2013
Wenjuan Zhang,a Zheyi Meng,a Jin Zhai*a and Liping Heng*b
DOI: 10.1039/c3cc47999d www.rsc.org/chemcomm
Inspired by the asymmetry of biological ion channels in structure and composition, we designed a novel type of artificial asymmetric nanochannels based on mesoporous zeolite (MCM-41) and polyimide (PI) by water-assisted self-assembly. Meanwhile, we studied ionic current behaviors and rectifying characteristics of the mesoporous zeolite–polymer composite nanochannels.
The study of the design and development of biomimetic channels has been receiving a great deal of attention owing to the emergence of novel ion transport phenomena-ion selection, which is one of the most important characteristics of biological ion channels.1–3 Inspired by the asymmetry of biological ion channels in geometry and composition, artificial solid-state nanochannels with asymmetric pore structure and surface charge which can rectify ion transport have been well studied.4–7 These asymmetric artificial nanochannels include glass,8,9 polymer nanopores,10 and so on. Recently, Xia et al.4 reported highly ordered nanochannels of a membrane with abrupt surface charge discontinuity by patterning the nanochannels with surface amine functional groups at designated positions using a twostep anodization process. Hinds et al.11 reported a single-walled carbon nanotube membrane, which can function as a rectifying diode due to ionic steric effects within the nanotubes. Besides, Cheng12 investigates several ion transport behaviors in nanofluidic channels consisting of heterogeneous oxide materials. By utilizing distinct isoelectric points of SiO2 and Al2O3 surfaces and photolithography to define the charge distribution, nanofluidic channels containing positively and negatively charged surfaces are created to form an abrupt junction. Our group has previously reported lightresponsive artificial ion channels based on TiO2 nanotube arrays, whose transport properties can be regulated by external ultraviolet a
Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry of Education, School of Chemistry and Environment, Beihang University, Beijing 100191, P. R. China b Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China. E-mail: [email protected] † Electronic supplementary information (ESI) available: Details of experiments, characterization of nanochannel membrane and molecular sieves, experimental cell setup. See DOI: 10.1039/c3cc47999d
3552 | Chem. Commun., 2014, 50, 3552--3555
light, and light-gating TiO2/Al2O3 heterogeneous nanochannels with ion current rectification characteristics that can be regulated.13,14 More and more artificial solid-state channels are fabricated based on inorganic materials. Mesoporous silicates are widely used for a variety of applications such as gas storage,15 heterogeneous catalysis,16,17 and drug delivery,18,19 e.g. mesoporous zeolite (MCM-41) has been the most popular member of mesoporous silicate materials since its discovery in 1992. It has many remarkable features, such as well-defined pore size and shape, adjustable pore size, hexagonal arrangement of uni-dimensional channels, high specific surface, and high chemical and hydrothermal stability. The surface of MCM-41 consists of mounts of free silicon hydroxyl-SiOH, which can react with siliconalkyls and introduce alkyl, amido, and sulfur hydroxyl groups into MCM-41. The inner surface of MCM-41 acquires a negatively charged SiO-state when in contact with water by the dissociation of silanol groups, and its isoelectric point (pI) is about 2–3.12 Herein, we have combined organic polymers and inorganic mesoporous molecular sieves to design a novel type of artificial asymmetric mesoporous zeolite–polymer composite nanochannels (MZPCN) based on MCM-41 and polyimide (PI) by water-assisted self-assembly, which has attracted considerable attention over recent years due to its facility, speediness, low cost, and the automatic removal of the condensed water droplets.20–24 The fabrication process is shown in Fig. 1. Moist air is blown over the mixed solution of PI and MCM-41 in chloroform, and evaporative cooling leads to the formation of water droplets at the air–solution interface. The monodispersed droplets arrange into a hexagonal array and sink into the polymer solution. Because MCM-41 is hydrophilic and full of hydroxyl radicals on the surface, it gathers around the water droplets. Subsequently, evaporation of the solvent and the water leaves imprints of water droplets. Thus, a large-scale, self-standing, and asymmetric MZPCN membrane (porous film) is formed. We define it as nanochannel-1. For comparison, a symmetric MZPCN membrane (smooth film), which was formed in a dry atmosphere, was also fabricated. We defined it as nanochannel-2. For nanochannel-1, the side of the membrane with a large pore diameter is the base side, which has an average pore diameter of 800–1800 nm
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Fig. 3 The different concentrations of KCl solutions for the current measurements of the nanochannel-1 membrane. (a) I–V characteristics recorded under symmetric electrolyte conditions at 100 mM KCl, 10 mM KCl, 1 mM KCl, 0.1 mM KCl, and MilliQ water (18.2 MO). (b) The current rectification ratio of the nanochannel obtained from the respective I–V curves.
Fig. 1 Flow chart for the fabrication of mesoporous zeolite–polymer composite nanochannels (a–d). (a) Evaporation of the solvent cools the solution surface, thus initiating the nucleation and growth of moisture. (b) For the convective currents arising from the evaporation as well as from the airflow across the surface, the water droplets pack into a hexagonal array. (c) The ordered array sinks into the solution. (d) When all of the solvent has evaporated, the film must return to room temperature, thus allowing the water droplets to evaporate as well while leaving behind the porous membrane. (e) The cross section of a single unit of the conceptual nanochannel. (f) The droplets array sink into the solution.
(Fig. S1, ESI†). MCM-41 with the pore size of 3.5 nm (Fig. S2 and S3 ESI†) is the tip side. The hierarchical porous structure can be observed from the cross-sectional images (Fig. S1, ESI†). The ion-transport properties of artificial ion channels are characterized by measuring the ionic current through the channels. In our experiments, the anode faced the base of the nanochannel (Fig. S4, ESI†). We investigated the current–voltage (I–V) characteristics of nanochannel-1 and nanochannel-2. The voltage scan range is from 1 V to 1 V, while the electrolyte is 100 mM KCl at pH 6.52. As shown in Fig. 2a, nanochannel-1 showed high conductivity and high ion current compared with nanochannel-2, which revealed the anion selection of a nanochannel with a negative surface charge. This phenomenon is consistent with those found in previous reports for a negatively charged surface.3,25–27 Moreover, the I–V curve of nanochannel-1 (red curve) shows the current rectification (ICR) property. The ionic current was higher at negative voltage than that at positive voltage, while the I–V curves of channel-2 (black curve) do not demonstrate clear ICR characteristics. The ion current rectification ratio (Rf) of nanochannel-1 increases along with the increasing
Fig. 2 (a) I–V curves of the two kinds of nanochannel membranes record under symmetric electrolyte conditions at 100 mM KCl. Illustration of nanochannel-1 and nanochannel-2 membrane (inset). (b) Ionic rectification ratio obtained from the respective I–V curves (red, nanochannel-1; black, nanochannel-2).
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voltage, but for nanochannel-2, (Rf) remains B1.0 (Fig. 2b). The ICR properties of nanochannel-1 are similar to an asymmetric conically shaped nanochannel28 as we had expected (red curve). Nanochannels with feature sizes compared to the Debye length exhibit unique ionic transport characteristics due to the overlapping electrical double layers.29 The bulk ionic concentration has a great pffiffiffiffiffiffi relationship with the Debye length (lD / 1 Cb , where Cb is the bulk ionic concentration.). To confirm that the concentration of KCl solution can influence the ion transport properties, we investigated the I–V characteristics of nanochannel-1 at different concentrations of KCl solutions. As shown in Fig. 3a, the I–V curves of nanochannel-1 were measured at varying bath KCl concentrations ranging from 0 mM to 100 mM. By comparing the I–V curves obtained with different concentrations of KCl, the ion current dramatically increased with the increase of the concentration of KCl. The Rf value (Fig. 3b) shows that the ionic current Rf increases along with increasing voltage, and when the concentration of KCl is 10 mM (red curve), Rf is the highest. At high KCl concentrations (black curve), Rf is decreased because the amount of mobile ions is high enough to shield the surface charges and hence weakens their impact on the channel conductance. In order to further clarify the ion transport properties, we investigated the I–V characteristics of the nanochannel-1 in three different monovalent inorganic electrolytes: potassium chloride (KCl), sodium chloride (NaCl) and lithium chloride (LiCl) at 10 mM. As shown in Fig. 4a the ion current of KCl is the highest (blue curve), while the ion current of LiCl is the lowest. Since the three inorganic salts share the same anion, the chloride ions, the difference in the transmembrane diffusion current stems from the different cation
Fig. 4 Ion transport properties reveal nanochannel-1 membrane at different electrolytes. (a) I–V response at various electrolytes of 10 mM LiCl (square), 10 mM NaCl (circle) and 10 mM KCl (triangle), respectively. (b) The current rectification ratio of the nanochannel obtained from the respective I–V curves.
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mobilities, which is scaled by their diffusion coefficients (D+), as listed in Table S2 (ESI†). The diffusion coefficients of Li, Na, and K cations (D+) are incremented sequentially, which has the same trend as the ion current shown in the I–V curves. The corresponding voltage-dependent Rf value (Fig. 4b) shows that the ICR increases along with increasing of voltage, and similar rectification is observed for all of these inorganic salts. The ion current transport can be clearly observed at a higher transmembrane potential and the ion current rectification ratio (Rf) can reach up to B1.6. These properties are attributed to the cooperative function of the asymmetric structure and the surface charge of the nanochannels. Several mechanisms have been suggested to explain the ICR effect observed in synthetic nanoporous systems.1,28 Referring to the mechanism of Woermann,1,30 we proposed the possible mechanism as follows: the pore can be divided into three microscale regions, the tip region (at the small opening of the pore), a transition zone, and the bulk region (at the base of the nanochannel). The pH value of the solution (>pI) makes the H+ deprotonated, resulting in the nanochannel with a negative charge and the nanochannel is cationselective. The nanochannel-1 with asymmetric pore structure and surface charge resulted in different ion concentrations at different regions. At the tip region, the ion concentrations are the highest and stem from the high surface charge density. This zone is also characterized by high cation mobility due to the inner surface of mesoporous zeolite with a negative charge. At the bulk region, the concentrations of cations are the same as in the bulk solution. The situation at the transition zone is more complex because its characteristics depend on the polarity of the applied voltage. With the cathode at the tip of the nanochannel, as shown in Fig. 5a, the cations flow from bulk region to tip region (arrows represent the flow of cations), which decreases the ion concentration in the transition zone. And the interior concentration-gradient-dependent ions flowing in the reverse direction will decrease the conductance of the ion
Fig. 5 Mechanism of the ion current rectification in nanochannels. (a) At a positive bias, the ions flow from bulk region to tip region (arrows represent the flow of cations), concentration-gradient-dependent ions flow in the reverse direction, which decrease the conductance of ion current. (b) At a negative bias, the cations flow from tip region to bulk region. Concentration-gradientdependent ions flow in the same direction, which increase the conductance of ion current. (Plus or minus sign represents cathode and anode, respectively.)
3554 | Chem. Commun., 2014, 50, 3552--3555
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current. In this case, the ion concentration in the nanochannel decreases and consequently a lower ion current is measured. When the cathode is at the base of the nanochannel, as shown in Fig. 5b, the cations flowing from the tip-to-base direction increase the ion concentration in the transition zone and close to the tip of the nanochannel. The interior concentration-gradient-dependent ion flow is in the same direction, which facilitates the conductance of the ion current. Therefore, the ion concentration in the nanochannel increases and consequently a high ion current is measured. Similarly I–V curves of nanochannel-1 show that the ionic current is higher at a negative voltage than at a positive voltage. Thus, the nanochannel we have designed here performs ionic current rectification. In summary, inspired by the asymmetry of biological ion channels, we have described a new type of artificial nanochannels based on MCM-41 and PI by water-assisted self-assembly. The asymmetric geometry in combination with the negatively charged surface contributes to the ion current rectification characteristics. Our present strategy realizes the facile large-scale creation of artificial solid-state nanochannels with ion current rectification characteristics, which may show some applications as nanofluidic diode, sensors, and separation materials. This work was supported by the National Research Fund for Fundamental Key Projects (2011CB935704), National Natural Science Foundation (21271016) and Specialized Research Fund for the Doctoral Program of Higher Education (30400002011127001).
Notes and references 1 Z. S. Siwy, Adv. Funct. Mater., 2006, 16, 735. 2 L. Wen, Q. Liu, J. Ma, Y. Tian, C. Li, Z. Bo and L. Jiang, Adv. Mater., 2012, 24, 6193. 3 X. Hou, H. Dong, D. Zhu and L. Jiang, Small, 2010, 6, 361. 4 C. Y. Li, F. X. Ma, Z. Q. Wu, H. L. Gao, W. T. Shao, K. Wang and X. H. Xia, Adv. Funct. Mater., 2013, 23, 3836. 5 M. H. Zhang, Z. Y. Meng, J. Zhai and L. Jiang, Chem. Commun., 2013, 49, 2284. 6 M. H. Zhang, X. Hou, J. T. Wang, Y. Tian, X. Fan, J. Zhai and L. Jiang, Adv. Mater., 2012, 24, 2424. 7 X. Hou, H. Zhang and L. Jiang, Angew. Chem., Int. Ed., 2012, 51, 5296. 8 A. E. Abelow, O. Schepelina, R. J. White, A. V. Belisle, K. W. Plaxco and I. Zharov, Chem. Commun., 2010, 46, 7984. 9 C. Reichenbach, G. Kalies, D. Enke and D. Klank, Langmuir, 2011, 27, 10699. 10 E. B. Kalman, O. Sudre, I. Vlassiouk and Z. S. Siwy, Anal. Bioanal. Chem., 2009, 394, 413. 11 J. Wu, K. Gerstandt, H. B. Zhang, J. Liu and B. J. Hinds, Nat. Nanotechnol., 2012, 7. 12 L.-J. Cheng and L. J. Guo, ACS Nano, 2009, 3, 10. 13 Q. Zhang, Z. Liu, X. Hou, X. Fan, J. Zhai and L. Jiang, Chem. Commun., 2012, 48, 5901. 14 Q. Zhang, Z. Hu, Z. Liu, J. Zhai and L. Jiang, Adv. Funct. Mater., 2013, DOI: 10.1002/adfm.201301426. 15 S. Ramachandran, J. H. Ha and D. K. Kim, Catal. Commun., 2007, 8, 1934. 16 K. Iwanami, T. Sakakura and H. Yasuda, Catal. Commun., 2009, 10, 1990. 17 F. Yao, J. Peng, W. Hao and M. Cai, Catal. Lett., 2012, 142, 803. 18 Y. J. Yang, X. Tao, Q. Hou, Y. Ma, X. L. Chen and J. F. Chen, Acta Biomater., 2010, 6, 3092. 19 S. Wang, Microporous Mesoporous Mater., 2009, 117, 1. 20 J. H. Kim, M. Seo and S. Y. Kim, Adv. Mater., 2009, 21, 4130. 21 M. L. Hoa, M. Lu and Y. Zhang, Adv. Colloid Interface Sci., 2006, 121, 9. 22 T. Zhang, Y. Wu, L. Gao, Z. Song, L. Zhao, Y. Zhang and J. Tao, Soft Matter, 2013, 9, 638. 23 G. O. Lloyd, J. L. Atwood and L. J. Barbour, Chem. Commun., 2005, 1845.
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27 Z. Siwy, E. Heins, C. C. Harrell, P. Kohli and C. R. Martin, J. Am. Chem. Soc., 2004, 126, 10850–10851. 28 Q. Liu, Y. Wang, W. Guo, H. Ji, J. Xue and Q. Ouyang, Phys. Rev. E, 2007, 75, 051201–051206. 29 Y. Lei, W. Wang, W. Wu and Z. Li, Appl. Phys. Lett., 2010, 96, 263102. 30 D. Woermann, Phys. Chem. Chem. Phys., 2003, 5, 1853.
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24 M. H. Guerrero and M. H. Stenzel, Polym. Chem., 2012, 3, 563. 25 P. Y. Apel, I. V. Blonskaya, O. L. Orelovitch, P. Ramirez and B. A. Sartowska, Nanotechnology, 2011, 22, 175302. 26 L. T. Sexton, L. P. Horne and C. R. Martin, Mol. BioSyst., 2007, 3, 667.
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Chem. Commun., 2014, 50, 3552--3555 | 3555
As featured in:
Ion current behaviors of mesoporous zeolite–polymer composite nanochannels prepared by water-assisted self-assembly We designed novel artificial asymmetry nanochannels based on mesoporous zeolite (MCM-41) and polyimide (PI) by water-assisted self-assembly. Meanwhile, we studied ionic current behaviors and rectifying characteristics of the mesoporous zeolite/polymer composite nanochannels.
a
See Jin Zhai, Liping Heng et al., Chem. Commun., 2014, 50, 3552.
www.rsc.org/chemcomm Registered charity number: 207890
ChemComm View Article Online
Published on 22 November 2013. Downloaded by University of Newcastle on 03/10/2014 07:55:07.
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Cite this: Chem. Commun., 2014, 50, 3552
Ion current behaviors of mesoporous zeolite–polymer composite nanochannels prepared by water-assisted self-assembly†
Received 17th October 2013, Accepted 21st November 2013
Wenjuan Zhang,a Zheyi Meng,a Jin Zhai*a and Liping Heng*b
DOI: 10.1039/c3cc47999d www.rsc.org/chemcomm
Inspired by the asymmetry of biological ion channels in structure and composition, we designed a novel type of artificial asymmetric nanochannels based on mesoporous zeolite (MCM-41) and polyimide (PI) by water-assisted self-assembly. Meanwhile, we studied ionic current behaviors and rectifying characteristics of the mesoporous zeolite–polymer composite nanochannels.
The study of the design and development of biomimetic channels has been receiving a great deal of attention owing to the emergence of novel ion transport phenomena-ion selection, which is one of the most important characteristics of biological ion channels.1–3 Inspired by the asymmetry of biological ion channels in geometry and composition, artificial solid-state nanochannels with asymmetric pore structure and surface charge which can rectify ion transport have been well studied.4–7 These asymmetric artificial nanochannels include glass,8,9 polymer nanopores,10 and so on. Recently, Xia et al.4 reported highly ordered nanochannels of a membrane with abrupt surface charge discontinuity by patterning the nanochannels with surface amine functional groups at designated positions using a twostep anodization process. Hinds et al.11 reported a single-walled carbon nanotube membrane, which can function as a rectifying diode due to ionic steric effects within the nanotubes. Besides, Cheng12 investigates several ion transport behaviors in nanofluidic channels consisting of heterogeneous oxide materials. By utilizing distinct isoelectric points of SiO2 and Al2O3 surfaces and photolithography to define the charge distribution, nanofluidic channels containing positively and negatively charged surfaces are created to form an abrupt junction. Our group has previously reported lightresponsive artificial ion channels based on TiO2 nanotube arrays, whose transport properties can be regulated by external ultraviolet a
Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry of Education, School of Chemistry and Environment, Beihang University, Beijing 100191, P. R. China b Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China. E-mail: [email protected] † Electronic supplementary information (ESI) available: Details of experiments, characterization of nanochannel membrane and molecular sieves, experimental cell setup. See DOI: 10.1039/c3cc47999d
3552 | Chem. Commun., 2014, 50, 3552--3555
light, and light-gating TiO2/Al2O3 heterogeneous nanochannels with ion current rectification characteristics that can be regulated.13,14 More and more artificial solid-state channels are fabricated based on inorganic materials. Mesoporous silicates are widely used for a variety of applications such as gas storage,15 heterogeneous catalysis,16,17 and drug delivery,18,19 e.g. mesoporous zeolite (MCM-41) has been the most popular member of mesoporous silicate materials since its discovery in 1992. It has many remarkable features, such as well-defined pore size and shape, adjustable pore size, hexagonal arrangement of uni-dimensional channels, high specific surface, and high chemical and hydrothermal stability. The surface of MCM-41 consists of mounts of free silicon hydroxyl-SiOH, which can react with siliconalkyls and introduce alkyl, amido, and sulfur hydroxyl groups into MCM-41. The inner surface of MCM-41 acquires a negatively charged SiO-state when in contact with water by the dissociation of silanol groups, and its isoelectric point (pI) is about 2–3.12 Herein, we have combined organic polymers and inorganic mesoporous molecular sieves to design a novel type of artificial asymmetric mesoporous zeolite–polymer composite nanochannels (MZPCN) based on MCM-41 and polyimide (PI) by water-assisted self-assembly, which has attracted considerable attention over recent years due to its facility, speediness, low cost, and the automatic removal of the condensed water droplets.20–24 The fabrication process is shown in Fig. 1. Moist air is blown over the mixed solution of PI and MCM-41 in chloroform, and evaporative cooling leads to the formation of water droplets at the air–solution interface. The monodispersed droplets arrange into a hexagonal array and sink into the polymer solution. Because MCM-41 is hydrophilic and full of hydroxyl radicals on the surface, it gathers around the water droplets. Subsequently, evaporation of the solvent and the water leaves imprints of water droplets. Thus, a large-scale, self-standing, and asymmetric MZPCN membrane (porous film) is formed. We define it as nanochannel-1. For comparison, a symmetric MZPCN membrane (smooth film), which was formed in a dry atmosphere, was also fabricated. We defined it as nanochannel-2. For nanochannel-1, the side of the membrane with a large pore diameter is the base side, which has an average pore diameter of 800–1800 nm
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Fig. 3 The different concentrations of KCl solutions for the current measurements of the nanochannel-1 membrane. (a) I–V characteristics recorded under symmetric electrolyte conditions at 100 mM KCl, 10 mM KCl, 1 mM KCl, 0.1 mM KCl, and MilliQ water (18.2 MO). (b) The current rectification ratio of the nanochannel obtained from the respective I–V curves.
Fig. 1 Flow chart for the fabrication of mesoporous zeolite–polymer composite nanochannels (a–d). (a) Evaporation of the solvent cools the solution surface, thus initiating the nucleation and growth of moisture. (b) For the convective currents arising from the evaporation as well as from the airflow across the surface, the water droplets pack into a hexagonal array. (c) The ordered array sinks into the solution. (d) When all of the solvent has evaporated, the film must return to room temperature, thus allowing the water droplets to evaporate as well while leaving behind the porous membrane. (e) The cross section of a single unit of the conceptual nanochannel. (f) The droplets array sink into the solution.
(Fig. S1, ESI†). MCM-41 with the pore size of 3.5 nm (Fig. S2 and S3 ESI†) is the tip side. The hierarchical porous structure can be observed from the cross-sectional images (Fig. S1, ESI†). The ion-transport properties of artificial ion channels are characterized by measuring the ionic current through the channels. In our experiments, the anode faced the base of the nanochannel (Fig. S4, ESI†). We investigated the current–voltage (I–V) characteristics of nanochannel-1 and nanochannel-2. The voltage scan range is from 1 V to 1 V, while the electrolyte is 100 mM KCl at pH 6.52. As shown in Fig. 2a, nanochannel-1 showed high conductivity and high ion current compared with nanochannel-2, which revealed the anion selection of a nanochannel with a negative surface charge. This phenomenon is consistent with those found in previous reports for a negatively charged surface.3,25–27 Moreover, the I–V curve of nanochannel-1 (red curve) shows the current rectification (ICR) property. The ionic current was higher at negative voltage than that at positive voltage, while the I–V curves of channel-2 (black curve) do not demonstrate clear ICR characteristics. The ion current rectification ratio (Rf) of nanochannel-1 increases along with the increasing
Fig. 2 (a) I–V curves of the two kinds of nanochannel membranes record under symmetric electrolyte conditions at 100 mM KCl. Illustration of nanochannel-1 and nanochannel-2 membrane (inset). (b) Ionic rectification ratio obtained from the respective I–V curves (red, nanochannel-1; black, nanochannel-2).
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voltage, but for nanochannel-2, (Rf) remains B1.0 (Fig. 2b). The ICR properties of nanochannel-1 are similar to an asymmetric conically shaped nanochannel28 as we had expected (red curve). Nanochannels with feature sizes compared to the Debye length exhibit unique ionic transport characteristics due to the overlapping electrical double layers.29 The bulk ionic concentration has a great pffiffiffiffiffiffi relationship with the Debye length (lD / 1 Cb , where Cb is the bulk ionic concentration.). To confirm that the concentration of KCl solution can influence the ion transport properties, we investigated the I–V characteristics of nanochannel-1 at different concentrations of KCl solutions. As shown in Fig. 3a, the I–V curves of nanochannel-1 were measured at varying bath KCl concentrations ranging from 0 mM to 100 mM. By comparing the I–V curves obtained with different concentrations of KCl, the ion current dramatically increased with the increase of the concentration of KCl. The Rf value (Fig. 3b) shows that the ionic current Rf increases along with increasing voltage, and when the concentration of KCl is 10 mM (red curve), Rf is the highest. At high KCl concentrations (black curve), Rf is decreased because the amount of mobile ions is high enough to shield the surface charges and hence weakens their impact on the channel conductance. In order to further clarify the ion transport properties, we investigated the I–V characteristics of the nanochannel-1 in three different monovalent inorganic electrolytes: potassium chloride (KCl), sodium chloride (NaCl) and lithium chloride (LiCl) at 10 mM. As shown in Fig. 4a the ion current of KCl is the highest (blue curve), while the ion current of LiCl is the lowest. Since the three inorganic salts share the same anion, the chloride ions, the difference in the transmembrane diffusion current stems from the different cation
Fig. 4 Ion transport properties reveal nanochannel-1 membrane at different electrolytes. (a) I–V response at various electrolytes of 10 mM LiCl (square), 10 mM NaCl (circle) and 10 mM KCl (triangle), respectively. (b) The current rectification ratio of the nanochannel obtained from the respective I–V curves.
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mobilities, which is scaled by their diffusion coefficients (D+), as listed in Table S2 (ESI†). The diffusion coefficients of Li, Na, and K cations (D+) are incremented sequentially, which has the same trend as the ion current shown in the I–V curves. The corresponding voltage-dependent Rf value (Fig. 4b) shows that the ICR increases along with increasing of voltage, and similar rectification is observed for all of these inorganic salts. The ion current transport can be clearly observed at a higher transmembrane potential and the ion current rectification ratio (Rf) can reach up to B1.6. These properties are attributed to the cooperative function of the asymmetric structure and the surface charge of the nanochannels. Several mechanisms have been suggested to explain the ICR effect observed in synthetic nanoporous systems.1,28 Referring to the mechanism of Woermann,1,30 we proposed the possible mechanism as follows: the pore can be divided into three microscale regions, the tip region (at the small opening of the pore), a transition zone, and the bulk region (at the base of the nanochannel). The pH value of the solution (>pI) makes the H+ deprotonated, resulting in the nanochannel with a negative charge and the nanochannel is cationselective. The nanochannel-1 with asymmetric pore structure and surface charge resulted in different ion concentrations at different regions. At the tip region, the ion concentrations are the highest and stem from the high surface charge density. This zone is also characterized by high cation mobility due to the inner surface of mesoporous zeolite with a negative charge. At the bulk region, the concentrations of cations are the same as in the bulk solution. The situation at the transition zone is more complex because its characteristics depend on the polarity of the applied voltage. With the cathode at the tip of the nanochannel, as shown in Fig. 5a, the cations flow from bulk region to tip region (arrows represent the flow of cations), which decreases the ion concentration in the transition zone. And the interior concentration-gradient-dependent ions flowing in the reverse direction will decrease the conductance of the ion
Fig. 5 Mechanism of the ion current rectification in nanochannels. (a) At a positive bias, the ions flow from bulk region to tip region (arrows represent the flow of cations), concentration-gradient-dependent ions flow in the reverse direction, which decrease the conductance of ion current. (b) At a negative bias, the cations flow from tip region to bulk region. Concentration-gradientdependent ions flow in the same direction, which increase the conductance of ion current. (Plus or minus sign represents cathode and anode, respectively.)
3554 | Chem. Commun., 2014, 50, 3552--3555
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current. In this case, the ion concentration in the nanochannel decreases and consequently a lower ion current is measured. When the cathode is at the base of the nanochannel, as shown in Fig. 5b, the cations flowing from the tip-to-base direction increase the ion concentration in the transition zone and close to the tip of the nanochannel. The interior concentration-gradient-dependent ion flow is in the same direction, which facilitates the conductance of the ion current. Therefore, the ion concentration in the nanochannel increases and consequently a high ion current is measured. Similarly I–V curves of nanochannel-1 show that the ionic current is higher at a negative voltage than at a positive voltage. Thus, the nanochannel we have designed here performs ionic current rectification. In summary, inspired by the asymmetry of biological ion channels, we have described a new type of artificial nanochannels based on MCM-41 and PI by water-assisted self-assembly. The asymmetric geometry in combination with the negatively charged surface contributes to the ion current rectification characteristics. Our present strategy realizes the facile large-scale creation of artificial solid-state nanochannels with ion current rectification characteristics, which may show some applications as nanofluidic diode, sensors, and separation materials. This work was supported by the National Research Fund for Fundamental Key Projects (2011CB935704), National Natural Science Foundation (21271016) and Specialized Research Fund for the Doctoral Program of Higher Education (30400002011127001).
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