Surface Modification of Single Track-Etched Nanopores with

Jul 23, 2009 - In this letter, we report a method to modify the surface charge property of single track-etched nanopores with a cationic surfactant (C...
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Surface Modification of Single Track-Etched Nanopores with Surfactant CTAB Yanbo Xie,† Jianming Xue,*,†,‡ Lin Wang,† Xinwei Wang,† Ke Jin,† Long Chen,† and Yugang Wang†,‡ †

State Key Laboratory of Nuclear Physics and Technology, School of Physics and ‡Center for Applied Physics and Technology, Peking University, Beijing 100871, PR China Received May 16, 2009. Revised Manuscript Received July 7, 2009

In this letter, we report a method to modify the surface charge property of single track-etched nanopores with a cationic surfactant (CTAB). The dependence of surface charge density on the surfactant concentration was investigated by measuring the streaming current when the nanopore was immersed in 0.01 M KCl solution with different CTAB concentrations. The results showed that, when the concentration of CTAB was increased gradually, the surface charge of the nanopore was inverted from negative to positive. Our calculation indicated that the surface charge density changed from -9 to 8 mC/m2. We utilized this method to modify the surface property of single conical track-etched nanopores. Its current rectification properties (both the direction and magnitude) were successfully tuned by adjusting the CTAB concentration in the solutions.

Introduction Synthetic nanopores are widely used in many fields, such as ionic transport rectification in nanofluidic diodes,1 DNA detection,2 molecular separation,3 responsive gating,4,5 and energy conversion.6 In all of these applications, the surface property of nanopores, especially the surface electric property, is the key ingredient that influences the outcomes of their applications.7-9 Hence, modifying the surface properties of nanopores is of great interest because the performance of nanodevices could be significantly improved. Currently, chemical surface modification methods have been used to change the surface charge property of track-etched nanopores. In the modification process, the track-etched nanopores are usually covered with a gold film first and then molecular recognition agents, such as DNA, are attached to the gold film surface so that the surface charge property is changed.10-12 However, Ali et al. showed that the surface could be directly modified without depositing a gold film. They attached the *Corresponding author. E-mail: [email protected]. Tel: 86-10-62758494. Fax: 86-10-62751875. (1) Vlassiouk, I.; Siwy, Z. Nano Lett. 2007, 7, 552–556. (2) Harrell, C. C.; Choi, Y.; Horne, L. P.; Baker, L. A.; Siwy, Z.; Martin, C. R. Langmuir 2006, 22, 10837–10843. (3) Ku, J. R.; Stroeve, P. Langmuir 2004, 20, 2030–2032. (4) Xia, F.; Guo, W.; Mao, Y. D.; Hou, X.; Xue, J. M.; Xia, H. W.; Wang, L.; Song, Y. L.; Ji, H.; Ouyang, Q.; Wang, Y. G.; Jiang, L. J. Am. Chem. Soc. 2008, 130, 8345–8350. (5) Hou, X.; Guo, W.; Xia, F.; Nie, F. Q.; Dong, H.; Tian, Y.; Wen, L. P; Wang, L.; Cao, L. X.; Yang, Y.; Xue, J. M.; Song, Y. L.; Wang, Y. G.; Liu, D. S.; Jiang, L. J. Am. Chem. Soc. 2009, 131, 7800–7805. (6) Xie, Y. B.; Wang, X. W.; Xue, J. M.; Jin, K.; Chen, L.; Wang, Y. G. Appl. Phy. Lett. 2008, 93, 163116. (7) Fan, R.; Karnik, R; Yue, M.; Li, D.; Majumdar, A.; Yang, P. Nano Lett. 2005, 5, 1633–1637. (8) Stein, D.; Kruithof, M.; Dekker, C. Phys. Rev. Lett. 2004, 93, 035901. (9) Kim, Y. R.; Min, J.; Lee, I. H.; Kim, S.; Kim, A. G.; Kim, K.; Namkoong, K.; Ko, C. Biosens. Bioelectron. 2007, 22, 2926–2931. (10) Siwy, Z.; Heins, E.; Harrell, C. C.; Kohli, P.; Martin, C. R. J. Am. Chem. Soc. 2004, 126, 10850–10851. (11) Harrell, C. C.; Kohli, P.; Siwy, Z.; Martin, C. R. J. Am. Chem. Soc. 2004, 126, 15646–15647. (12) Siwy, Z.; Trofin, L.; Kohli, P.; Baker, L. A.; Trautmann, C.; Martin, C. R. J. Am. Chem. Soc. 2005, 127, 5000–5001.

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amino-terminated and carboxyl-terminated species to the surface carboxyl groups. After modification, the current rectification direction was reversed, which indicated that the polarity of the nanopore’s surface charge had changed from negative to positive.13 Other researchers immobilized pH-response molecules onto the surface, thus permitting the adjustment of the surface charge density by changing the pH of the solution.14-17 However, these chemical modification methods are complicated and usually have many steps, and the chemical bonding agents usually are expensive, making practical applications problematic. In addition, the surface charge density was not obtained in these studies, with only the I-V curves used to estimate the surface charge property qualitatively. We tried to modify the surface charge property of track-etched nanopores with a physical adsorption method. The cationic surfactant molecules have a strong absorption ability at the aqueous-solid interface, and a surface layer of ionized surfactant molecules can be formed on the solid surface via the self-assembly process.18 It had been reported that surface charge density on the solid surface could be changed from negative to positive gradually by adding different concentrations of cationic surfactant to the electrolyte solution.19 Compared with other methods, it is quite simple, versatile, and reusable. However, the adsorption behavior of surfactant inside nanopores is still unknown. In the present work, we report the experimental results of modifying the surface charge property of single track-etched nanopores by using CTAB (cetyl trimethyl ammonium bromide) molecules. The surface charge density could be calculated by (13) Ali, M.; Schiedt, B.; Healy, K.; Neumann, R.; Ensinger, W. Nanotechnology 2008, 8, 085713. (14) Wanunu, M.; Meller, A. Nano Lett. 2007, 7, 1580–1585. (15) Schepelina, O.; Zharov, I. Langmuir 2008, 24, 14188–14194. (16) Yameen, B.; Ali, M.; Neumann, R.; Ensinger, W.; Knoll, W.; Azzaroni, O. J. Am. Chem. Soc. 2009, 131, 2070–2071. (17) Ali, M.; Yameen, B.; Neumann, R.; Ensinger, W.; Knoll, W.; Azzaroni, O. J. Am. Chem. Soc. 2008, 130 16351–16357. (18) Atkin, R.; Craig, V. S. J.; Wanless, E. J.; Biggs, S. Adv. Colloid Interface Sci. 2003, 103, 219–304. (19) Theodoly, O.; Pereira, L. C.; Bergeron, V.; Radke, C. J. Langmuir 2005, 21, 10127–10139.

Published on Web 07/23/2009

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measuring the kinetic-electric property of changed nanopores, usually the streaming current.20,21 With this method, we obtained the surface charge density as a function of surfactant concentration. We also investigated the ion current rectification of a conical track-etched nanopore influenced by adding CTAB to the electrolyte solution.

Scheme 1. Schematic Showing the Setup for Measuring the Streaming Currenta

Experimental Section Nanopore Fabrication and Characterization. Single nanopores were prepared in 12-μm-thick polyethylene terehphalate (PET) foils. The foil was first irradiated with a single U235 particle with an energy of 11.4 MeV/u to form a latent ion track. Because only one ion passes through the foil, a single nanopore could be fabricated in each foil. A single nanopore was more suitable for mechanistic research compared with porous nanopores. The chemical etching rate in the latent ion track region is much faster than that in the bulk material, so a nanometer-sized pore could be fabricated in this region after the chemical etching process. Prior to etching, 1 h of UV irradiation was performed on the foils because it could further enlarge the ratio of etching rates in the ion track region and bulk material. After the etching process, a single nanopore was formed in each PET foil. On the basis of different etching processes, many kinds of nanopores of different shapes can be fabricated. In our experiments, two kinds of track-etched nanopores were prepared (i.e. cylindrical and conical nanopores). The cylindrical nanopores were used in streaming current measurements. They were fabricated by the chemical etching process such that each foil was simultaneously etched from both sides with a 5 M NaOH aqueous solution. The conical nanopores were used to study the effect of surfactant modification on the current rectification behavior. They were fabricated by the etching-stopping method.22 The etchant (9 M NaOH) and the stopping medium (1 M NaCOOH þ 1 M KCl) were placed on each side of a foil. Once the foil had been etched through, the stopping medium could neutralize the etchant inside the nanopore to prevent it from being further etched. With this method, the size of the tip opening of the conical nanopore could be well controlled to several nanometers. The radius at the base opening (Rb) of the conical nanopore was determined by multiplying the bulk etching rate by the etching time. The radius of the tip opening (Rt) was obtained by measuring the conductance of the nanopore in 1 M KCl solution: Rt = LG/(πκRb), where L and G are the length and the conductance of the nanopore, respectively; κ is the conductivity of the 1 M KCl solution. The conical nanopores used in this work had base radii of 200 nm and tip radii of 7 nm. Other details with respect to the nanopore preparation process can be found in previous work.22,23 Streaming Current Measurement. The measurement system is schematically illustrated in Scheme 1a. A foil with a single nanopore was mounted between two cells and immersed in potassium chloride (KCl) solution buffered with phosphorate (pH 7.2). The left cell was airtight, and nitrogen gas was applied to drive the solution through the nanopore. Ag/AgCl electrodes were used in the system. The streaming current (Istr) was recorded directly with a patch clamp (Axon 200B) operating in the whole cell model (β=1). Cetyltrimethylammonium Bromide (CTAB). Surfactant CTAB was added to the salt solution in order to form a selfassembled layer on the nanopore inner surface. The chemical structure of CTAB is shown in Scheme 1b. It has a long hydrophobic carbon tail (approximately 2.4 nm). In aqueous solution, (20) van der Heyden, F. H. J.; Stein, D.; Dekker, C. Phys. Rev. Lett. 2005, 95, 116104. (21) Xue, J. M.; Xie, Y. B.; Yan, Y.; Jin, K.; Wang, Y. G. Biomicrofluidics 2009, 3, 022408. (22) Apel, P. Y.; Korchev, E.; Siwy, Z.; Spohr, R.; Yoshida, M. Nucl. Instrum. Methods Phys. Res., Sect. B 2001, 184, 337–346. (23) Guo, W.; Xue, J. M.; Wang, Y. G.; Liu, Q. Nucl. Instrum. Methods Phys. Res., Sect. B 2008, 266, 3095–3099.

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a (a)The foil with a single pore was mounted between the two cells (the right one is airtight). Current was measured with Ag/AgCl electrodes and recorded with a patch clamp of Axon 200B. (b) Chemical structure of cetyltrimethylammonium bromide (CTAB). CTAB dissociates into a bromide ion (Br-) and a cetyltrimethylammonium ion (CTAþ) in aqueous solution.

Scheme 2. Schematic of Streaming Current Generationa

a The surface charge of the nanopore wall governs the polarity and absolute value of the streaming current.

CTAB dissociates into a bromide ion (Br-) and a cetyltrimethylammonium ion (CTAþ) such that the surface charge property could be modified when CTAþ is adsorbed onto the nanopore surface. The CTAB concentration used in the measurements ranged from 0.025 to 5 cmc (critical micelle concentration, where 1 cmc is equal to 0.89 mM CTAB).24

Results and Discussion Streaming Current and Surface Charge Density as a Function of the CTAB Concentration. The electric field produced by the surface charge could cause cations to accumulate near the surface so that there is a net electrical charge inside the nanopore (Scheme 2). Thus, when the electrolyte solution is driven through the surface-charged nanopore, an electric current will be produced. The polarity of the surface charge and its absolute value could directly influence the polarity and value of the streaming current. Here, we define the pressure gradient pushing the water flow from right to left as the positive direction. If the surface charge were negative, as shown in Scheme 2, then positive charges moving along the flow direction would produce a positive streaming current. On the contrary, when the surface charge is inverted (positive), charges inside the nanopores are (24) R. Atkin, R.; Craig, V. S. J.; Wanless, E. J.; Biggs, S. Adv. Colloid Interface Sci. 2003, 103, 219–304.

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Xie et al. Scheme 3. Schematic Diagram of CTAB Adsorption on the LiquidSolid Interface at Different Concentrationsa

Figure 1. Electric-kinetic properties of a track-etched nanopore in 0.01 M KCl with different concentrations of CTAB (Cs). (a) The streaming current responds linearly to an external pressure difference; the dashed line is a linear fit to the experimental data (dots); different colors stand for different concentrations of CTAB in 0.01 M KCl. (b) The streaming conductance (Sstr represents the streaming current per bar) drops quickly with increasing Cs; when Cs passes 1 cmc, Sstr reaches a saturating value. (c) The calculated surface charge density has also been obtained from measurements of streaming current.

dominated by negative charges, which reverse the polarity of the streaming current . If other experimental conditions are fixed, then the streaming current is determined only by the polarity and absolute value of the surface charge density. In other words, we can estimate the sign of the surface charge and calculate its absolute value by measurement of the streaming current only.21 We measured the streaming current (Istr) of single track-etched nanopores in aqueous solutions with 0.01 M KCl and a CTAB concentration (Cs) ranging from 0.025 to 5 cmc. We used cylindrical nanopores with effective radii of 105 nm to perform the experiments. The measured Istr values behave as a function of the external pressure difference (ΔP) as shown in Figure 1a. Clearly, Istr has a linear relation with respect to ΔP in the experiment. By linearly fitting the Istr-ΔP curves (dashed line), we obtained the streaming conductance (Sstr), which represents the streaming current per unit pressure difference (slope of the fitted curve). The streaming conductance (Sstr) with respect to surfactant concentrations (Cs) is shown in Figure 1b. As Cs increased from 0 to 5 cmc, Sstr changed from positive to neutral and then to negative. Sstr changed quickly with initial Cs increments and reached its maximum negative value when Cs was 0.5 cmc. Then its absolute 8872 DOI: 10.1021/la9017213

a The adsorption process can be divided into four steps. With increasing Cs, the surface charge density varied quickly. When the bilayer has formed, the surface charge reaches a saturation value. For a better illustration of the adsorption process, the length of the surfactant is not shown in its real proportions with respect the nanopore. The length of CTAB was actually much smaller than the radius of the nanopore. See the main text for a description of parts a-d.

value dropped slightly and finally stabilized when Cs was higher than 2 cmc. According to the adsorption process of CTAB in macroscale systems,25 we proposed an adsorption model for CTAB inside nanopores to explain the experimental results mentioned above. In aqueous solution, CTAþ has adsorbes strongly to the solid surface so that a surfactant molecular layer can be formed on the surface. As a result, the adsorption quantities of surfactant dominate the surface charge density. The adsorption process is divided into four steps as surfactant concentration (Cs) increases. In the beginning, the surfactant is absorbing via electrostatic interactions with the polymer substrate (Scheme 3a). The adsorbed CTAþ ions lower the value of σs, which decreases the streaming conductance (Sstr). In region II, the substrate surface charge has been neutralized, which leads to lower Istr (Scheme 3b). With increasing Cs in solution, the adsorption quantities continue to increase, and surface charge becomes positive so that Istr is inverted (Scheme 3c). Finally, the bilayer can be formed when Cs passes a critical concentration and Istr also obtains a saturated value (Scheme 3d). In addition, we noticed that there was a small peak at Cs=0.5 cmc and thereafter Sstr decreased a little. Actually, this does not mean a smaller quantity of adsorbed molecules, but because of the degree of ionization, the quantity of adsorbed surfactant molecules decreased in higher-Cs solutions.25 When Cs is higher than 0.5 cmc, the adsorption quantities of the inner surface were almost saturated, and most of the surfactant molecules absorb along with their neutralizing counterion; therefore, the degree of ionization of absorbed surfactant decreases.25 (25) Paria, S.; Kartic, C.; Khilar Adv. Colloid Interface Sci. 2004, 110, 75–95.

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Figure 2. (a) Streaming current of about 3 pA without any surfactant adsorbed on the surface. (b) When adding 2 cmc to the solution after 2 min, the streaming current was quickly reversed but a little unstable. (c) With a bilayer of CTAB molecules formed on the surface, the streaming current was about -3 pA. (d) After 10 min of flushing with water, the polarity of streaming current was still negative but the absolute value decreased. (e) After about 50 min, the streaming current almost reached the original value.

Because the equivalent surface charge density is the product of the absorption quantity and degree of ionization (discussed later), the surface charge density would decrease a little, which also causes the streaming conductance to decrease at high Cs. On the basis of the Sstr-Cs relation, we were able to calculate the surface charge densities (σs) at different surfactant concentrations. For a cylindrical nanopore, the streaming current obeys the RR equationIstr ¼ 0 2πruðrÞ½Fþ ðrÞ -F - ðrÞ drwhere u(r) and F(r) are the velocity distribution and ion concentration profile at position r in the radial direction, respectively; R is the pore radius. Because F(r) depends on the surface charge density, the values of σs can be obtained by analyzing the experimental data with the Poisson-Boltzmann (PB) and Navier-Stokes (NS) theories. Details of the calculation are shown in the Supporting Information and our previous work.21 Figure 1c shows the calculation results of the σs-Cs relation. Obviously, the surface charge density strongly depended on the CTAB concentrations in the electrolyte solution. Without CTAB in solution, σs was -9 mC/m2, which is similar to the value of -12 mC/m2 obtained by Dejardin et al. in multitrack nanopores.26 When Cs was increased, σs was reversed and reached a maximum value of 8 mC/m2 when Cs was 0.5 cmc, and then it decreased a little to 5 mC/m2. It has been reported that with adsorption on the millimeter-scale channel the adsorption quantity was 1.6 mg/m2 and the degree of ionization was about 10% when the bilayer has been formed.19 Then σs could be calculated as σs = ReΓ þ σo,19 where R and Γ are the degree of ionization and adsorption density, respectively; e is the unit charge and σo is the surface charge density without any CTAB in solution. The first part of the equation is due to the adsorption of CTAB, and the second part was generated by danglings;the original surface charge density produced by chemical etching. With the CTAB bilayer formed on the aqueous-solution interface, the estimated value of σs was around 30 mC/m2, which was much higher than (26) Dejardin, P.; Vasina, E. N.; Berezkin, V. V.; Sobolev, D.; Volkov, V. I. Langmuir 2005, 21, 4680–4685.

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our measured value of 5 mC/m2. Inside the nanopore, the concentration of Kþ is much higher than the bulk value; this will decrease the critical micelle concentration of CTAB.18 As a result, the adsorption quantity of CTAB on the pore wall would be lower than that on the solid surface and the surface charge density was not as high as expected. However, additional experiments are needed to clarify this problem. We also found that the CTAþ layer on the nanopore surface was very robust; even under a high pressure gradient (∼600 bar/cm) the streaming current was still stable. Moreover, this kind of physical modification could be easily removed via water flushing. Figure 2a shows that the streaming current was about 3 pA under a 0.5 bar pressure difference in 0.01 M KCl solution without any CTAB in solution. The streaming current could be quickly reversed with 2 cmc surfactant in solution, although Istr is not stable after 2 min (Figure 2b). When CTAB surfactant adsorption reached equilibrium on the surface, the polarity of the streaming current reversed (about -3 pA, as shown in Figure 2c). Then we use high pressure to drive 0.01 M KCl through the nanopore to remove the absorbed surfactant. Ten minutes later, the experimental results show that the polarity of Istr was still negative but much less than before (Figure 2d). This indicated that part of surfactant molecules were removed from the nanopore. After 50 min of flushing, Istr becomes positive again and almost recovers to its original value (Figure 2e). Our experiment proved that this kind of surface modification could be easily removed. Current Rectification of a Single Track-Etched Nanopore Tuned by the CTAB Modification Method. We tried to apply this surface modification method to adjust the ionic current rectification behavior of a conical track-etched nanopore, which had been studied widely as the most important characteristic of nanofluidic diodes. Current rectification means that currents recorded with a certain external voltage are quite different than that recorded with a voltage of the same absolutely value but opposite polarity. This phenomenon is caused by the ion enrichment/depletion inside the conical nanopore when an external DOI: 10.1021/la9017213

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The results indicated that the I-V curves were very sensitive to the concentration of CTAB. When the I-V relation was measured in pure 0.1 M KCl solution, the rectification value (Q) was approximately 4. With 0.1 cmc CTAB added to solution (0.1 M KCl), the original surface charges were almost neutralized and were even positive according to the analysis via the streaming current. As a result, low surface charge induces a weak enrichment/depletion effect inside the nanopore, and current rectification almost disappeared. With a higher Cs, the surface charge reversed from negative to positive; therefore, the preferential direction of ionic transport also reversed. As a result, Q became lower than 1. When Cs was higher than the saturation concentration, σs became stable, and Q reached a constant value of 0.3. In total, Q changed more than 10 times as Cs varied. Therefore, this method is a good candidate for adjusting the current rectification of the track-etched conical nanopores. The method of using surfactant molecules to control the surface properties of the nanopore has several advantages for practical application; the modification is very simple and the nanopores could be used repeatedly. Besides, a surfactant such as CTAB was much cheaper than a chemical agent. Hence, this method has many potential applications, such as the abovediscussed application in tuning the current rectification of nanofluidic diodes. Moreover, it can also be used in molecule separation29 and nanofluidic batteries, which can convert kinetic energy into electric power via the kinetic-electric phenomenon with surface-charged nanopores.6 Figure 3. Current rectification of a conical nanopore in solutions with different CTAB concentrations. (a) I-V curves with different concentrations of CTAB. (b) Current rectification value Q (Q = I(1 V)/I(-1 V)) regulated by CTAB; the rectification effect could be adjusted with the surfactant modification method.

electrical field is applied.23,27,28 The surface charge is the most important ingredient for the ion enrichment/depletion effect, and a higher surface charge density would cause a stronger ionic current rectification phenomenon. Besides, the polarity of the surface charge determines the species of enriched and depleted ions and controls the preferential direction of ionic transport. Thus, the preferential ionic transport direction is expected to be reversed if the surface charge polarity changes from negative to positive.29 Quantitatively, the magnitude of current rectification is evaluated in terms of rectification factor Q, which is the ratio of the magnitude of currents at (1 V (Q = |I(þ1 V)/I(-1 V)|). As discussed above, the surface charge density could be controlled as a function of surfactant concentration. Therefore, it is reasonable to expect that the specified current rectification value could be achieved by adding suitable concentrations of CTAB to solutions. The I-V curves of single conical nanopores were measured in 0.1 M KCl aqueous solutions with different concentrations of CTAB. The nanopores were immersed in the solutions for at least 2 h before recording data in order to make sure that the system was in equilibrium. The measured results are shown in Figure 3a, and the rectification factors (Q) are shown in Figure 3b. (27) Wang, X. W.; Xue, J. M.; Wang, L.; Guo, W.; Zhang, W. M.; Wang, Y. G.; Liu, Q.; Ji, H.; Ouyang, Q. J. Phys. D 2007, 40, 7077–7084. (28) Cervera, J.; Schiedt, B.; Neumann, R.; Mafe, S.; Ramı´ rez, P. J. Chem. Phys. 2006, 124, 104706. (29) Savariar, E. N.; Krishnamoorthy, K.; Thayumanavan, S. Nat. Nanotech. 3 2008, 112–117.

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Conclusions By measuring the streaming current through a track-etched nanopore, the dependence of surface charge density on the surfactant concentration was investigated. The results indicated that CTAB molecules could enter the nanopore and form selfassembled layers on the inner surface, which can change the surface charge property of the track-etched nanopore in PET foils. By controlling the CTAB concentration, the surface charge density varied from -9 to 8 mC/m2. This method provides a simple method for adjusting the surface charge density as needed. With this method, the ion current rectification of a single conical track-etched nanopore can be designed as expected with respect to both the preferential direction of ionic transport and the magnitude of rectification. The above results show that it is possible to form self-assembled surfactant layers on the inner surface of a track-etched nanopore in PET foils. The surface charge density could be quantitatively adjusted by controlling the CTAB concentration. Compared with other irreversible surface modification methods, surface modification with surfactant is simple, versatile, and reversible. Acknowledgment. Financial support from NSFC (Grant No 10675009) is gratefully acknowledged. The authors want to thank Prof. Jianbin Huang of Peking University for providing purified CTAB samples and useful discussion, and Prof. Neumann of GSI for providing the single ion irradiated PET samples. Supporting Information Available: Calculation of the surface charge density. This material is available free of charge via the Internet at http://pubs.acs.org.

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