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Pressure-dependent electrolytic conduction of track-etched single conical nanopore
Applied Surface Science 353 (2015) 574–579
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Pressure-dependent electrolytic conduction of track-etched single conical nanopore X.R. Zhu a,b , L. Wang c , C.M. Wang d , Z. Jiao b , W.D. Wang b , G.Y. Qin b , J.M. Xue d,∗ a
Energy-Saving Building Materials Innovative Collaboration Center of Henan Province, Xinyang Normal University, Xinyang 464000, China Henan Key Laboratory of Ion Beam Bioengineering, Physical Engineering College, Zhen Zhou University, Zhengzhou 450052, China Key Laboratory of Nuclear Radiation and Nuclear Technology, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China d State Key Laboratory of Nuclear Physics and Technology, School of Physics, Peking University, Beijing 100871, China b c
a r t i c l e
i n f o
Article history: Received 20 January 2015 Received in revised form 19 June 2015 Accepted 19 June 2015 Available online 25 June 2015 Keywords: Nanopore Track-etched Pressure Rectification Ion current
a b s t r a c t In this paper, a systematic investigation of the influence of pressure on electrolytic conduction of conical nanopores in track-etched polyimide (Kapton) films is described. The effect of pressure is shown to be dependent on the pore orifice size and the rectification ratio of nanopore. For larger Kapton nanopores, the disruption of cation and anion equilibrium distributions within the nanopore caused by pressure resulted in the decrease of the rectification, but the pressure effect on the I–V also exhibited some differences for larger nanopores with different diameter. For smaller nanopores, pressure had a negligible influence on rectification. © 2015 Elsevier B.V. All rights reserved.
1. Introduction The fluid flowing in charged nanopores and nanochannels with asymmetric geometries has many interesting and useful characteristics; two particularly useful characteristics are ion current rectification [1–3] and electro-osmotic flow (EOF) [4–6]. The ion current rectification phenomenon has been widely investigated because of its potential applications in controlling mass transport, detecting biological molecules, and delivering drugs [7,8]. EOF is a well-known mechanism of fluid propulsion in thin capillaries that has previously found applicable in microdevices for fluid manipulation and pumping [9]. In recent years, nanopore-based single molecule detection has attracted great attention of many researchers. Since the size of synthetic pores and channels has been reduced to the dimensions that are comparable to the size of nanoparticles and molecules [10], this analyzing method can provide satisfactory sensitivity for detecting various biological molecules and chemical species. Many works have been carried out on the translocation of nanoparticles by using conically shaped nanopores [11–14]. It has been found that the pressure-driven flow could minimize the broadening of the sample that EOF pumps in many pressure-driven systems, and it
allows for very high resolution separations of ionic species [15,16]. Therefore, it is of great importance to study the pressure-dependent electrolytic conduction in nanopores for single molecule detection. White et al. [17] studied the effect of pressure on the ion current rectification of conically shaped glass nanopores by both experimental methods and finite-element simulations. The experimental results and the finite-element simulation results were well consistent. The results showed that the ion current rectification of the conically shaped glass nanopores in low ionic strength solutions depended on the rate of the pressure-induced flow through the nanopore, i.e. the rectification decreased with increasing the flow rate [17]. In this paper, we use conically shaped Kapton nanopores to investigate the dependence of the electrolytic conduction on the pressure applied at the two ends of the nanopore. We demonstrate that the effect of the pressure depends on the pore orifice size and the electrolyte concentration. The corresponding mechanism is also discussed. Our work would provide important experimental information for the study of the selective transport and sorting of particles in nanofluidic devices. 2. Experimental 2.1. Nanopore fabrication
∗ Corresponding author. E-mail address: [email protected] (X.R. Zhu). http://dx.doi.org/10.1016/j.apsusc.2015.06.116 0169-4332/© 2015 Elsevier B.V. All rights reserved.
The nanopores used in this paper were prepared on 12 m thick Kapton foils. The foils were irradiated with single heavy ions of Au
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Fig. 1. Schematic illustration of the ion current measurement.
with an energy of 11.4 MeV nucleon-1 at UNILAC linear accelerator (GSI, Darmstadt, Germany). Then, the foils were mounted in a homemade cell that had one chamber on each side of the foil, separately. Each Kapton foil was etched by NaClO (13% Cl) with an etching stop medium of 1 M KI. The temperature for the etching process was 40 ◦ C [1]. Two platinum electrodes connected with a Keithley 6487 picoammeter were plugged into the solutions in two chambers, respectively. 1 V voltage was supplied across the foil, and the current from the picoammeter was monitored. At the moment when the pore was etched through, a large jump of current was observed, and the stop medium neutralized the etchant
to protect the nanopore from further etching. This one-side etching method produced a single conical nanopore on the foil. We call the larger opening of the conical nanopore the base and call the smaller one the tip. The diameter of the base (Dbase ) was calculated by multiplying the bulk etching rate and the etching time. The diameter of the tip (Dtip ) was estimated by a conductance method: Dtip = 4LI/kUDbase , where L is the length of the nanopore, I is the ion current given an applied voltage U, and k is the conductance of the solution we used for this measurement (typically 1 M KCl), respectively. The inner surface of the nanopore prepared by the above technique is negatively charged [18].
Fig. 2. I–V response curves of Kapton single nanopores in a 0.01 M KCl solution (pH 6.4) at different pressures. (a, b) Nanopore with a tip diameter of 180 nm; (c, d) nanopore with a tip diameter of 30 nm. (a, c) I–V curves at negative applied pressures and (b, d) I–V curves at positive pressures (positive pressures: pore base vs pore tip).
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Fig. 3. Rectification of a Kapton conical-shaped nanopore with a tip diameter of 180 nm in a 0.01 M KCl solution (pH 6.4) at different pressures (positive pressures: pore base vs pore tip).
Fig. 5. Rectification factor of a Kapton conical-shaped nanopore with a tip diameter of 253 nm in a 0.01 M KCl solution (pH 6.4) at different pressures (positive pressures: pore base vs pore tip).
2.2. Ion current measurement
Fig. 2 shows their corresponding I–V curves at the pressure ranging from −0.1 MPa to 0.1 MPa. When there was no applied pressure, two nanopores both exhibited typical asymmetric I–V curves, indicating the presence of ion current rectification effect for both nanopores. It has been demonstrated that the inner surface of a polymer nanopore prepared by the chemical etching method is covered with carboxylate groups, which carry stable negative charge in aqueous solution with neutral pH condition [19]. The asymmetric distribution of the surface charge caused by the asymmetric geometric shape determines the ion current rectification effect of the nanopore in electrolyte solutions [2,20]. In general, nanopores with large tip diameter usually have very weak ion current rectification effect, because the pore diameter is much larger than the thickness of the electric double layer. However, our experimental data showed that the nanopore with the tip diameter of 180 nm still had obvious ion current rectification. This may be related with the shape of the tip side of the nanopore [21,22]. Previous study also demonstrated that, due to the formation of a bullet shaped nanopore in the etching process, the nanopores with tip diameter greater than 100 nm could still display strong ability of rectification in solution with moderate electrolyte concentration [21,23]. When pressure was applied along the larger nanopore (180 nm, Fig. 2a and b), the current at negative potential decreased while the current at positive potential decreased first and then increased with
The measurement system is schematically illustrated in Fig. 1. A foil with a single nanopore was mounted between two cells and immersed in potassium chloride (KCl) solution (pH 6.4). Nitrogen gas was applied at one side (left or right) of the device to drive the solution through the nanopore. Ag/AgCl electrodes were used in the system. The current was recorded directly with a Keithley 6487 picoammeter (for the larger nanopore) or a patch clamp (Axon 200B) operating in the whole cell mode (for the smaller pore), and the scan rate was 50 mV/s. The foil with a single pore was mounted between two cells. Pressure was applied at the left or right side of the device according to different experimental procedures. All pressures and voltages reported in this article were defined as the values in the base vs the values in the tip. 3. Results and discussion To investigate the effect of pressure on the electrolytic conduction of track-etched single conical nanopores, the current–voltage (I–V) characteristics of two Kapton nanopores under different pressures were measured. One nanopore has a larger tip diameter of 180 nm (with a base diameter of 1200 nm), while another one has a smaller tip diameter of 30 nm (with a base diameter of 2913 nm).
Fig. 4. I–V response curves of a Kapton single nanopores with f a tip diameter of 253 nm in a 0.01 M KCl solution (pH 6.4) at different pressures. (a) I–V curves at negative pressures and (b) I–V curves at positive pressures (positive pressures: pore base vs pore tip).
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Fig. 6. I–V curves (a) and rectification ratio (b) of a Kapton single nanopore with a tip diameter of 253 nm in different concentration KCl solutions (pH 6.4).
Fig. 7. I–V response curves of a Kapton single nanopores with a tip diameter of 253 nm in 0.005 M (a, b), 0.05 M (c, d) and 0.1 M (e, f) KCl solutions (pH 6.4) at different pressure. (a, c, e) I–V curves at negative pressures and (b, d, f) I–V curves at positive pressures (positive pressures: pore base vs pore tip).
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the increase of the applied pressure, and the I–V curves displayed a more ohmic-like response. The rectification factor, which is defined as the rectification ratio of the ion current magnitude at −0.5 V and +0.5 V, is plotted in Fig. 3. When the applied pressure increased from 0 to ±0.1 MPa, the rectification factor decreased from ∼3 to ∼2.2 and 1.5, respectively. For the smaller nanopore (30 nm, Fig. 2c and d), which had a stronger rectification I–V response, the applied pressure had negligible effect on the current at negative potential. In addition, the I–V curves of the conical Kapton nanopore with a tip diameter of 253 nm and a base diameter of 1500 nm was also measured. This nanopore exhibited almost the same I–V response behavior as the 180-nm nanopore under pressure, that is, the applied pressure along the nanopore resulted in the increase of the ion current at positive potential and decrease at negative potential (Fig. 4a and b). The corresponding rectification factor decreased from ∼6 to ∼2 as the magnitude of the pressure increased from 0 to 0.1 MPa (Fig. 5). Accordingly, we concluded that the increase of the applied pressure can significantly reduce the ion current rectification of the track-etched conical nanopores with large tip diameter. The mechanism of this interesting pressure-dependent ion current rectification phenomenon can be explained as follows. Since the nanopore surface is negatively charged at neutral pH and the radius of the pore orifice is small, the zone at the pore opening is cation-selective. At negative potential, the direction of the K+ flux is from the external bulk solution to the pore interior while Cl− ion move along the opposite direction. As the pore is cation-selective, Cl− 1 ions are rejected by the surface because of the electrostatic repulsion. A consequence of the anion rejection is an increase in the K+ and Cl− concentrations within the pore interior, resulting in a greater conductivity than the value calculated based on the bulk KCl concentration. Oppositely, when a positive potential is applied, the Cl− ions transporting from the external solution to the internal solution are rejected by the surface charge, resulting in the depletion of the Cl− ions within the pore interior and thus decreasing the conductivity of the nanopore [17]. When the pressure is applied along the nanopore, a volumetric flow Q generated through the conically shaped nanopore is estimated by Q = 3r3 P/(8 cot ), where r is the radius of the pore orifice, P is the pressure difference at two ends of the nanopore, is the solution viscosity, and is the half-cone angle of the nanopore, respectively [14,15]. Q is proportional to r3 at constant P. For a smaller nanopore, the flow caused by the applied pressure is too small to change the distribution of the ion concentration inside the nanopore, therefore the change of the current can be ignored and the conductivity of the nanopore is nearly independent of the applied pressure. With the increase of the tip orifice, the pressure-driven volumetric flow increases significantly, and thus it can influence the ion concentration inside nanopore. If the pressure is applied with a positive potential, the pressure-driven flow brings the solution containing K+ and Cl− ions with bulk concentration into the nanopore, causing the increase of the ion concentration inside the nanopore as compared to the case with no pressure. Therefore the conductivity increases and the corresponding ion current rises. On the contrary, under a negative potential, the pressure-driven flow carries the bulk solution into the nanopore, which decreases the ion concentration inside nanopore as compared to the case with no pressure, so the conductivity decreases and the corresponding ion current drops. The rise of the ion current under positive potential together with the drop of ion current under negative potential will lead to a significant decrease of the rectification factor. Thus the decrease phenomenon of the pressure-induced ion current rectification can be obtained. The above analysis is also consistent with the research work by the White’s group [17]. Fig. 5 also shows that, for larger nanopore, the negative pressure also had a greater effect on the rectification ratio, which was slightly more than the positive-pressure
Fig. 8. Rectification factor of a Kapton conical-shaped nanopore with a tip diameter of 253 nm in different concentration KCl solutions (pH 6.4) at different pressure. (a) 0.005 M; (b) 0.05 M; (c) 0.1 M (positive pressures: pore base vs pore tip).
case. This phenomenon might be related to the small residual conductivity (decrease or increase based on potential bias) around the tip opening according to the finite-element simulation results by the White’s group [17]. In order to further understand the effect of the applied pressure on the rectification ratio of the single conical track-etched nanopore, we measured the pressure-dependent I–V response curves of the 253 nm nanopore in KCl solutions with different concentrations. First, the I–V curves and rectification factors of the nanopore with different KCl concentration ranging from 1 mM to 1 M are shown in Fig. 6. It is found that the rectification factor of the nanopore reached its maximum value of 5.8 at the concentration of 0.01 M. Beside the concentration of 0.01 M (Figs. 4 and 5), pressuredependent electrolytic conduction of the nanopore was further measured in three other solutions with the KCl concentration of 0.005 M, 0.05 M and 0.1 M, respectively. The I–V curves in Fig. 7 indicated that the nanopore displayed a similar pressure–response behavior when negative or positive pressure was applied in different concentration KCl solutions. However, we observed that the influence of pressure was more efficient if the nanopore had a weaker rectification factor originally. From Fig. 5, the applied pressure of 0.1 MPa was able to suppress the rectification factor to only
X.R. Zhu et al. / Applied Surface Science 353 (2015) 574–579
∼2 in 0.01 M KCl solution. As shown in Fig. 8, when the pressure increased from 0 to ±0.1 MPa, the rectification factor decreased from 4.2 to 1.4 in 0.005 M KCl solution; and when the concentration of KCl was 0.05 M or 0.1 M, the rectification factor decreased from 3.8 or 2.3 to almost 1.0, respectively. Especially for the case of 0.1 M, the ion current rectification disappeared basically at the pressure of 0.08 MPa. Therefore, the influence of pressure on the rectification effect increased with the decrease of the original rectification ratio. Our results have shown that the pressure effect on the ion current rectification also depends on the original rectification of the nanopore. It has been demonstrated that the thickness of the electric double layer is of great importance for the current rectification properties. For a nanopore, smaller rectification ratio generally shows that the proportion of electrical double layer occupying smaller area inside the nanopore. The volumetric flow caused by applied pressure is parabolic laminar flow inside the nanopore. The stream velocity of the layer adjacent to pore wall is slow while the velocity along the central axis is the maximum. If the double electrical layer is thin, mostly of volume flow transports through the nanopore under the applied pressure is bulk electrolyte, the ion depletion and enrichment zone at the pore opening is washed away, thus the ion currents in two opposite directions become nearly equal and the rectification decreases. The smaller the rectification ratio is, the thinner the electrical double layer is, the short ion depletion and enrichment zone is destroyed by the stream more easily, thus the effect of pressure is more obvious. 4. Conclusion In this work, we reported the pressure-dependent electrolytic conduction of the track-etched single conical polymer nanopores prepared on Kapton foils. In general, the effect of the applied pressure is dependent on the tip orifice of the nanopores. For larger nanopores, the rectification ratio decreases with the increase of the applied pressure. For smaller nanopores, the pressure effect is negligible. In addition, we also found that the effect of the pressure
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on nanopore is associated with the rectification ratio: the smaller the rectification ratio is, the more obvious the effect is. Acknowledgment The authors would like to thank the material group in GSI, Darmstadt, Germany for providing the single-ion-irradiated samples. References [1] Z.S. Siwy, P.Y. Apel, D. Dobrev, R. Neumann, R. Spohr, C. Trautmann, K. Voss, Nucl. Instrum. Methods Phys. Res. B. 208 (2003) 143. [2] Z.S. Siwy, Adv. Funct. Mater. 16 (2006) 735. [3] R. Karnik, C. Duan, K. Castelino, H. Daiguji, A. Majumdar, Nano Lett. 7 (2007) 547. [4] J.G. Santiago, Anal. Chem. 73 (2001) 2353. [5] Y.B. Xie, J.M. Xue, L. Wang, X.W. Wang, K. Jin, L. Chen, Y.G. Wang, Langmuir 25 (2009) 8870. [6] Y.B. Xie, X.W. Wang, J.M. Xue, K. Jin, L. Chen, Y.G. Wang, Appl. Phys. Lett. 93 (2008) 163116. [7] C.C. Harrell, Y. Choi, L.P. Horne, L.A. Baker, Z.S. Siwy, C.R. Martin, Langmuir 22 (2006) 10837. [8] V. Rao, J.V. Amar, D.K. Avasthi, N.R. Charyulu, Radiat. Meas. 36 (2003) 585. [9] R. Johann, P. Renaud, Electrophoresis 259 (2004) 3720. [10] D. Branton, D.W. Deamer, A. Marziali, Nat. Biotechnol. 26 (2008) 1146. [11] W.J. Lan, D.A. Holden, J. Liu, H.S. White, J. Phys. Chem. C 115 (2011) 18445. [12] L. Yeh, M. Zhang, S. Qian, Anal. Chem. 85 (2013) 7527. [13] W.J. Lan, C. Kubeil, J.W. Xiong, A. Bund, H.S. White, J. Phys. Chem. C 118 (2014) 2726. [14] M. Schiel, Z.S. Siwy, J. Phys. Chem. C 118 (2014) 19214. [15] G.M. Gusinskii, E.B. Kremer, M.I. Kremer, B.V. Mchedlishvili, J. Eng. Phys. 37 (1979) 1493–1496. [16] M. Whitesides, Nature 442 (2006) 368. [17] W. Lan, D.A. Holden, H.S. White, J. Am. Chem. Soc. 133 (2011) 13300. [18] Z. Siwy, P. Apel, D. Baur, D.D. Dobrev, Y.E. Korchev, R. Neumann, R. Spohr, C. Trautmann, K.-O. Voss, Surf. Sci. (2003) 532–535. [19] A. Wolf, N. Reber, P.Y. Apel, B.E. Fischer, R. Spohr, Nucl. Instrum. Methods Phys. Res. B 105 (1995) 291. [20] D. Woermann, Phys. Chem. Chem. Phys. 5 (2003) 1853. [21] M.L. Kovarik, K. Zhou, S.C. Jacobson, J. Phys. Chem. B 113 (2009) 15960. [22] P. Ramírez, P.Yu. Apel, J. Cervera, S. Mafé, Nanotechnology 19 (2008) 315707. [23] P.Yu. Apel, I.V. Blonskaya, O.L. Orelovitch, P. Ramirez, B.A. Sartowska, Nanotechnology 22 (2011) 175302.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Pressure-dependent electrolytic conduction of track-etched single conical nanopore X.R. Zhu a,b , L. Wang c , C.M. Wang d , Z. Jiao b , W.D. Wang b , G.Y. Qin b , J.M. Xue d,∗ a
Energy-Saving Building Materials Innovative Collaboration Center of Henan Province, Xinyang Normal University, Xinyang 464000, China Henan Key Laboratory of Ion Beam Bioengineering, Physical Engineering College, Zhen Zhou University, Zhengzhou 450052, China Key Laboratory of Nuclear Radiation and Nuclear Technology, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China d State Key Laboratory of Nuclear Physics and Technology, School of Physics, Peking University, Beijing 100871, China b c
a r t i c l e
i n f o
Article history: Received 20 January 2015 Received in revised form 19 June 2015 Accepted 19 June 2015 Available online 25 June 2015 Keywords: Nanopore Track-etched Pressure Rectification Ion current
a b s t r a c t In this paper, a systematic investigation of the influence of pressure on electrolytic conduction of conical nanopores in track-etched polyimide (Kapton) films is described. The effect of pressure is shown to be dependent on the pore orifice size and the rectification ratio of nanopore. For larger Kapton nanopores, the disruption of cation and anion equilibrium distributions within the nanopore caused by pressure resulted in the decrease of the rectification, but the pressure effect on the I–V also exhibited some differences for larger nanopores with different diameter. For smaller nanopores, pressure had a negligible influence on rectification. © 2015 Elsevier B.V. All rights reserved.
1. Introduction The fluid flowing in charged nanopores and nanochannels with asymmetric geometries has many interesting and useful characteristics; two particularly useful characteristics are ion current rectification [1–3] and electro-osmotic flow (EOF) [4–6]. The ion current rectification phenomenon has been widely investigated because of its potential applications in controlling mass transport, detecting biological molecules, and delivering drugs [7,8]. EOF is a well-known mechanism of fluid propulsion in thin capillaries that has previously found applicable in microdevices for fluid manipulation and pumping [9]. In recent years, nanopore-based single molecule detection has attracted great attention of many researchers. Since the size of synthetic pores and channels has been reduced to the dimensions that are comparable to the size of nanoparticles and molecules [10], this analyzing method can provide satisfactory sensitivity for detecting various biological molecules and chemical species. Many works have been carried out on the translocation of nanoparticles by using conically shaped nanopores [11–14]. It has been found that the pressure-driven flow could minimize the broadening of the sample that EOF pumps in many pressure-driven systems, and it
allows for very high resolution separations of ionic species [15,16]. Therefore, it is of great importance to study the pressure-dependent electrolytic conduction in nanopores for single molecule detection. White et al. [17] studied the effect of pressure on the ion current rectification of conically shaped glass nanopores by both experimental methods and finite-element simulations. The experimental results and the finite-element simulation results were well consistent. The results showed that the ion current rectification of the conically shaped glass nanopores in low ionic strength solutions depended on the rate of the pressure-induced flow through the nanopore, i.e. the rectification decreased with increasing the flow rate [17]. In this paper, we use conically shaped Kapton nanopores to investigate the dependence of the electrolytic conduction on the pressure applied at the two ends of the nanopore. We demonstrate that the effect of the pressure depends on the pore orifice size and the electrolyte concentration. The corresponding mechanism is also discussed. Our work would provide important experimental information for the study of the selective transport and sorting of particles in nanofluidic devices. 2. Experimental 2.1. Nanopore fabrication
∗ Corresponding author. E-mail address: [email protected] (X.R. Zhu). http://dx.doi.org/10.1016/j.apsusc.2015.06.116 0169-4332/© 2015 Elsevier B.V. All rights reserved.
The nanopores used in this paper were prepared on 12 m thick Kapton foils. The foils were irradiated with single heavy ions of Au
X.R. Zhu et al. / Applied Surface Science 353 (2015) 574–579
575
Fig. 1. Schematic illustration of the ion current measurement.
with an energy of 11.4 MeV nucleon-1 at UNILAC linear accelerator (GSI, Darmstadt, Germany). Then, the foils were mounted in a homemade cell that had one chamber on each side of the foil, separately. Each Kapton foil was etched by NaClO (13% Cl) with an etching stop medium of 1 M KI. The temperature for the etching process was 40 ◦ C [1]. Two platinum electrodes connected with a Keithley 6487 picoammeter were plugged into the solutions in two chambers, respectively. 1 V voltage was supplied across the foil, and the current from the picoammeter was monitored. At the moment when the pore was etched through, a large jump of current was observed, and the stop medium neutralized the etchant
to protect the nanopore from further etching. This one-side etching method produced a single conical nanopore on the foil. We call the larger opening of the conical nanopore the base and call the smaller one the tip. The diameter of the base (Dbase ) was calculated by multiplying the bulk etching rate and the etching time. The diameter of the tip (Dtip ) was estimated by a conductance method: Dtip = 4LI/kUDbase , where L is the length of the nanopore, I is the ion current given an applied voltage U, and k is the conductance of the solution we used for this measurement (typically 1 M KCl), respectively. The inner surface of the nanopore prepared by the above technique is negatively charged [18].
Fig. 2. I–V response curves of Kapton single nanopores in a 0.01 M KCl solution (pH 6.4) at different pressures. (a, b) Nanopore with a tip diameter of 180 nm; (c, d) nanopore with a tip diameter of 30 nm. (a, c) I–V curves at negative applied pressures and (b, d) I–V curves at positive pressures (positive pressures: pore base vs pore tip).
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Fig. 3. Rectification of a Kapton conical-shaped nanopore with a tip diameter of 180 nm in a 0.01 M KCl solution (pH 6.4) at different pressures (positive pressures: pore base vs pore tip).
Fig. 5. Rectification factor of a Kapton conical-shaped nanopore with a tip diameter of 253 nm in a 0.01 M KCl solution (pH 6.4) at different pressures (positive pressures: pore base vs pore tip).
2.2. Ion current measurement
Fig. 2 shows their corresponding I–V curves at the pressure ranging from −0.1 MPa to 0.1 MPa. When there was no applied pressure, two nanopores both exhibited typical asymmetric I–V curves, indicating the presence of ion current rectification effect for both nanopores. It has been demonstrated that the inner surface of a polymer nanopore prepared by the chemical etching method is covered with carboxylate groups, which carry stable negative charge in aqueous solution with neutral pH condition [19]. The asymmetric distribution of the surface charge caused by the asymmetric geometric shape determines the ion current rectification effect of the nanopore in electrolyte solutions [2,20]. In general, nanopores with large tip diameter usually have very weak ion current rectification effect, because the pore diameter is much larger than the thickness of the electric double layer. However, our experimental data showed that the nanopore with the tip diameter of 180 nm still had obvious ion current rectification. This may be related with the shape of the tip side of the nanopore [21,22]. Previous study also demonstrated that, due to the formation of a bullet shaped nanopore in the etching process, the nanopores with tip diameter greater than 100 nm could still display strong ability of rectification in solution with moderate electrolyte concentration [21,23]. When pressure was applied along the larger nanopore (180 nm, Fig. 2a and b), the current at negative potential decreased while the current at positive potential decreased first and then increased with
The measurement system is schematically illustrated in Fig. 1. A foil with a single nanopore was mounted between two cells and immersed in potassium chloride (KCl) solution (pH 6.4). Nitrogen gas was applied at one side (left or right) of the device to drive the solution through the nanopore. Ag/AgCl electrodes were used in the system. The current was recorded directly with a Keithley 6487 picoammeter (for the larger nanopore) or a patch clamp (Axon 200B) operating in the whole cell mode (for the smaller pore), and the scan rate was 50 mV/s. The foil with a single pore was mounted between two cells. Pressure was applied at the left or right side of the device according to different experimental procedures. All pressures and voltages reported in this article were defined as the values in the base vs the values in the tip. 3. Results and discussion To investigate the effect of pressure on the electrolytic conduction of track-etched single conical nanopores, the current–voltage (I–V) characteristics of two Kapton nanopores under different pressures were measured. One nanopore has a larger tip diameter of 180 nm (with a base diameter of 1200 nm), while another one has a smaller tip diameter of 30 nm (with a base diameter of 2913 nm).
Fig. 4. I–V response curves of a Kapton single nanopores with f a tip diameter of 253 nm in a 0.01 M KCl solution (pH 6.4) at different pressures. (a) I–V curves at negative pressures and (b) I–V curves at positive pressures (positive pressures: pore base vs pore tip).
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Fig. 6. I–V curves (a) and rectification ratio (b) of a Kapton single nanopore with a tip diameter of 253 nm in different concentration KCl solutions (pH 6.4).
Fig. 7. I–V response curves of a Kapton single nanopores with a tip diameter of 253 nm in 0.005 M (a, b), 0.05 M (c, d) and 0.1 M (e, f) KCl solutions (pH 6.4) at different pressure. (a, c, e) I–V curves at negative pressures and (b, d, f) I–V curves at positive pressures (positive pressures: pore base vs pore tip).
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the increase of the applied pressure, and the I–V curves displayed a more ohmic-like response. The rectification factor, which is defined as the rectification ratio of the ion current magnitude at −0.5 V and +0.5 V, is plotted in Fig. 3. When the applied pressure increased from 0 to ±0.1 MPa, the rectification factor decreased from ∼3 to ∼2.2 and 1.5, respectively. For the smaller nanopore (30 nm, Fig. 2c and d), which had a stronger rectification I–V response, the applied pressure had negligible effect on the current at negative potential. In addition, the I–V curves of the conical Kapton nanopore with a tip diameter of 253 nm and a base diameter of 1500 nm was also measured. This nanopore exhibited almost the same I–V response behavior as the 180-nm nanopore under pressure, that is, the applied pressure along the nanopore resulted in the increase of the ion current at positive potential and decrease at negative potential (Fig. 4a and b). The corresponding rectification factor decreased from ∼6 to ∼2 as the magnitude of the pressure increased from 0 to 0.1 MPa (Fig. 5). Accordingly, we concluded that the increase of the applied pressure can significantly reduce the ion current rectification of the track-etched conical nanopores with large tip diameter. The mechanism of this interesting pressure-dependent ion current rectification phenomenon can be explained as follows. Since the nanopore surface is negatively charged at neutral pH and the radius of the pore orifice is small, the zone at the pore opening is cation-selective. At negative potential, the direction of the K+ flux is from the external bulk solution to the pore interior while Cl− ion move along the opposite direction. As the pore is cation-selective, Cl− 1 ions are rejected by the surface because of the electrostatic repulsion. A consequence of the anion rejection is an increase in the K+ and Cl− concentrations within the pore interior, resulting in a greater conductivity than the value calculated based on the bulk KCl concentration. Oppositely, when a positive potential is applied, the Cl− ions transporting from the external solution to the internal solution are rejected by the surface charge, resulting in the depletion of the Cl− ions within the pore interior and thus decreasing the conductivity of the nanopore [17]. When the pressure is applied along the nanopore, a volumetric flow Q generated through the conically shaped nanopore is estimated by Q = 3r3 P/(8 cot ), where r is the radius of the pore orifice, P is the pressure difference at two ends of the nanopore, is the solution viscosity, and is the half-cone angle of the nanopore, respectively [14,15]. Q is proportional to r3 at constant P. For a smaller nanopore, the flow caused by the applied pressure is too small to change the distribution of the ion concentration inside the nanopore, therefore the change of the current can be ignored and the conductivity of the nanopore is nearly independent of the applied pressure. With the increase of the tip orifice, the pressure-driven volumetric flow increases significantly, and thus it can influence the ion concentration inside nanopore. If the pressure is applied with a positive potential, the pressure-driven flow brings the solution containing K+ and Cl− ions with bulk concentration into the nanopore, causing the increase of the ion concentration inside the nanopore as compared to the case with no pressure. Therefore the conductivity increases and the corresponding ion current rises. On the contrary, under a negative potential, the pressure-driven flow carries the bulk solution into the nanopore, which decreases the ion concentration inside nanopore as compared to the case with no pressure, so the conductivity decreases and the corresponding ion current drops. The rise of the ion current under positive potential together with the drop of ion current under negative potential will lead to a significant decrease of the rectification factor. Thus the decrease phenomenon of the pressure-induced ion current rectification can be obtained. The above analysis is also consistent with the research work by the White’s group [17]. Fig. 5 also shows that, for larger nanopore, the negative pressure also had a greater effect on the rectification ratio, which was slightly more than the positive-pressure
Fig. 8. Rectification factor of a Kapton conical-shaped nanopore with a tip diameter of 253 nm in different concentration KCl solutions (pH 6.4) at different pressure. (a) 0.005 M; (b) 0.05 M; (c) 0.1 M (positive pressures: pore base vs pore tip).
case. This phenomenon might be related to the small residual conductivity (decrease or increase based on potential bias) around the tip opening according to the finite-element simulation results by the White’s group [17]. In order to further understand the effect of the applied pressure on the rectification ratio of the single conical track-etched nanopore, we measured the pressure-dependent I–V response curves of the 253 nm nanopore in KCl solutions with different concentrations. First, the I–V curves and rectification factors of the nanopore with different KCl concentration ranging from 1 mM to 1 M are shown in Fig. 6. It is found that the rectification factor of the nanopore reached its maximum value of 5.8 at the concentration of 0.01 M. Beside the concentration of 0.01 M (Figs. 4 and 5), pressuredependent electrolytic conduction of the nanopore was further measured in three other solutions with the KCl concentration of 0.005 M, 0.05 M and 0.1 M, respectively. The I–V curves in Fig. 7 indicated that the nanopore displayed a similar pressure–response behavior when negative or positive pressure was applied in different concentration KCl solutions. However, we observed that the influence of pressure was more efficient if the nanopore had a weaker rectification factor originally. From Fig. 5, the applied pressure of 0.1 MPa was able to suppress the rectification factor to only
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∼2 in 0.01 M KCl solution. As shown in Fig. 8, when the pressure increased from 0 to ±0.1 MPa, the rectification factor decreased from 4.2 to 1.4 in 0.005 M KCl solution; and when the concentration of KCl was 0.05 M or 0.1 M, the rectification factor decreased from 3.8 or 2.3 to almost 1.0, respectively. Especially for the case of 0.1 M, the ion current rectification disappeared basically at the pressure of 0.08 MPa. Therefore, the influence of pressure on the rectification effect increased with the decrease of the original rectification ratio. Our results have shown that the pressure effect on the ion current rectification also depends on the original rectification of the nanopore. It has been demonstrated that the thickness of the electric double layer is of great importance for the current rectification properties. For a nanopore, smaller rectification ratio generally shows that the proportion of electrical double layer occupying smaller area inside the nanopore. The volumetric flow caused by applied pressure is parabolic laminar flow inside the nanopore. The stream velocity of the layer adjacent to pore wall is slow while the velocity along the central axis is the maximum. If the double electrical layer is thin, mostly of volume flow transports through the nanopore under the applied pressure is bulk electrolyte, the ion depletion and enrichment zone at the pore opening is washed away, thus the ion currents in two opposite directions become nearly equal and the rectification decreases. The smaller the rectification ratio is, the thinner the electrical double layer is, the short ion depletion and enrichment zone is destroyed by the stream more easily, thus the effect of pressure is more obvious. 4. Conclusion In this work, we reported the pressure-dependent electrolytic conduction of the track-etched single conical polymer nanopores prepared on Kapton foils. In general, the effect of the applied pressure is dependent on the tip orifice of the nanopores. For larger nanopores, the rectification ratio decreases with the increase of the applied pressure. For smaller nanopores, the pressure effect is negligible. In addition, we also found that the effect of the pressure
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