The Resistance of Polyelectrolyte Multilayers in a Free-Hanging

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The Resistance of Polyelectrolyte Multilayers in a Free-Hanging Configuration Kaori Sugihara,* Ja´nos Vo¨ro¨s, and Tomaso Zambelli Laboratory of Biosensors and Bioelectronics, Institute for Biomedical Engineering, ETH Zurich, Gloriastrasse 35, CH-8092 Zurich, Switzerland ReceiVed: August 5, 2010; ReVised Manuscript ReceiVed: October 4, 2010

The resistivity FPEM of polyelectrolyte multilayers (PEMs), PEI(PSS/PAH)24, PEI(PGA/PAH)12, PEI(HA/ PLL)12 and PEI(PSS/PLL)12, in a free-hanging configuration was estimated combining electrochemical impedance spectroscopy (EIS) and atomic force microscopic (AFM) images. Surprisingly, the obtained value of several kΩcm is at least 6 orders of magnitude lower than that reported previously, where the resistivity was determined in the conventional PEM-on-electrode system. The significant discrepancy indicates the unexpectedly low electrical PEM resistance in the absence of redox-active ions and the sensitivity limitation in the conventional system. Introduction Polyelectrolyte multilayers1 (PEMs) are gaining importance in the bioengineering field for applications such as implant coatings,2 ion-selective membranes,3 drug delivery,4 and cell sheet engineering5 because of their physiological compatibility, robustness, and the versatility of the choice of the polymer couples and substrates. For some of them (e.g., ion or organicsolvent selective membranes, anticorrosion membranes), one of the most important issues is the control of the PEM permeability for certain compounds. Electrochemistry is a straightforward approach to investigate the ion permeability of PEMs. The PEMon-electrode system, where a PEM film is deposited directly on a metal working electrode6–10 (see Figure 1a) has become a model system for monitoring the transport of ions across PEM films using electrochemical impedance spectroscopy (EIS) and/ or cyclic voltammetry (CV). For example, Harris and co-workers have studied the solution-pH and bilayer-number dependence of the permeability of a poly(allylamine hydrochloride)/poly(styrenesulfonate) (PAH/PSS) film to Fe(CN)63- and Ru(NH3)63+.6 Although such redox-active ions are commonly used to increase the signal, the growing PEM applications in the bioengineering field demand studies in physiological buffers with salts such as NaCl and KCl without any multivalent ions, since the charge of ions is known to significantly affect their permeability.11 Cassier and co-workers have estimated the resistivity of a PEM film composed of the same polymer couple PAH/PSS without using redox-active ions on the way to investigate a lipid bilayer-PEM composite;9 however, such a determination of the PEM resistance without redox-active ions is still an open question. On the other hand, polymers such as poly(ethylene oxide)based polymers and polyvinylchlorides in a free-hanging configuration (see Figure 1b) have been fabricated and exploited for secondary batteries12 and for improving the imperfect contact between electrodes and polymer films in air,13 in addition to a variety of works without any electrical focus.3,14–16 Very recently, a free-hanging PEM in a submicrometer-thickness has been fabricated as a support for biomimetic lipid-bilayers toward ionchannel sensing by simply spraying polyelectrolyte solutions on a Si/Si3N4 chip with a single nanopore.17 * Corresponding author. E-mail: [email protected].

Figure 1. Schematics showing the experimental configuration of (a) the conventional PEM-on-electrode system and (b) the free-hangingPEM system, where Ag/AgCl electrodes are neglected since their resistance is much smaller than the rest of the elements (see figure caption in Figure 3).

In this work, we exploit the free-hanging PEM as a system to investigate the PEM resistance RPEM in a physiological environment without the usage of redox-active ions. Four kinds of PEMs;PEI(PSS/PAH)24, PEI(PGA/PAH)12, PEI(HA/PLL)12, and PEI(PSS/PLL)12;were characterized by EIS. The obtained values were compared to the one reported in the PEM-onelectrode system, and the found major differences were discussed. Materials and Methods The Si/Si3N4 chips were fabricated by Leister (CH) according to a procedure already published.18 The central region of the chips has a 300-nm thick Si3N4 membrane with a single 800nm pore or an array of 800-nm pores with a separation of 2 µm. The chips were cleaned for 15 min by an oxygen-plasma cleaner (PDC-32G, Harrick, U.S.A.) just before the experiments. All the experiments were performed in a HEPES buffer solution prepared with 10 mM 4-(2-hydroxyethyl)piperazine-1-ethanesulfonic acid purchased from Fluka (Buchs, Switzerland) and 0.15 M sodium chloride (Fluka, Buchs, Switzerland) in ultrapure water filtered through Milli-Q Gradient A10 filters (Millipore AG, Switzerland). The pH was adjusted to 7.4 using 6 M NaOH (Fluka, Buchs, Switzerland). Polyethyleneimine (PEI, MW ) 25 000 g/mol, branched, 408727), poly(allylamine hydrochloride) (PAH, MW ) 70,000 g/mol, 283223), poly(sodium 4-styrenesulfonate) (PSS, MW ) 70 000 g/mol, 243051), polyL-lysine hydrobromide (PLL, MW ) 15 000-30 000 g/mol, P7890), poly-L-glutamic acid sodium salt (PGA, MW ) 15 000-50 000 g/mol, P4761) were purchased from SigmaAldrich Chemie GmbH (Switzerland), and sodium hyaluronate (HA, MW ) 357 kDa) was purchased from Lifecore Biomedical, LLC (USA). PEI, PAH, PSS, PLL, PGA, and HA were

10.1021/jp107362y  2010 American Chemical Society Published on Web 10/20/2010

PEM Resistivity in Free-Hanging Configuration dissolved at a concentration of 1 mg/mL in HEPES. All the solutions were sterile filtrated through 0.22 µm filters. For the atomic force microscopic (AFM) imaging, we used the Nanowizard I BioAFM (JPK Instruments, Germany) and the Mikromasch CSC38/noAl cantilevers in contact mode (set point: ∼1 nN, scan rate: 0.6-1.0 Hz). For the EIS, we used the Autolab PGSTAT12 Instrument (Ecochemie, Utrecht, The Netherlands), equipped with a FRA-module. EIS spectra were recorded at 0 V offset potential applying 10 mV signal amplitude between two Ag/AgCl electrodes purchased from Lot-Oriel AG (WPI reference Electrode for EC-QCM Module QSP 020) for the experiments in the free-hanging configuration. For the three electrodes measurement in the PEM-on-electrode system, a gold electrode with a surface area of ∼1 cm2 as the working electrode, a platinum wire as a counter electrode, and Ag/AgCl as a reference electrode were used. PEM films were deposited with a custom-made automated spraying system reported previously.17 A polyelectrolyte solution with PEI was first sprayed as an anchor layer, followed by the alternate spraying of the polyanion solution, physiological buffer (HEPES) as a rinsing step, and polycation solution. After the deposition of PEI(polyanion/ polycation)n on the chip, the sample was carefully dried with a nitrogen gun and placed either on a glass slide for AFM measurements or in an electrochemical cell for EIS measurements and rehydrated with HEPES. AFM measurements confirmed that the process of dehydration and rehydration of PEMs does not cause a major change in the conformation of PEM films on a flat glass slide. Results and Discussion We studied four types of PEM films with different polyelectrolyte couples [PEI(PSS/PAH)24, PEI(PGA/PAH)12, PEI(HA/ PLL)12, and PEI(PSS/PLL)12], selected because of their relevance for biomedical applications.19–21 The PSS/PAH system is known to grow linearly with the number of the layers n, while the other couples (PGA/PAH, HA/PLL, and PSS/PLL) are reported to grow exponentially.22–25 We exploited the spray method for the deposition of the PEM films.26,27 Twelve bilayers were deposited for the three exponentially growing PEM, while 24 bilayers were deposited for the linearly growing PEM to get a comparable PEM amount in the pore. First, the AFM topological images of the samples were taken after the PEM deposition onto the chips with pore arrays to confirm that the pores are completely closed with each PEM. We emphasize that before the deposition of PEM films in the pores, AFM scanning was impractical because of the lack of the force feedback over the empty pores. Therefore, a successful AFM imaging itself partially supports the existence of a PEM in the pores. Pore arrays instead of a single pore were used to facilitate the pore localization. Figure 2a,b shows the AFM images taken from the sprayed side (Figure 2a) and the back side (Figure 2b) after the deposition of PEI(HA/PLL)12. On the sprayed side, a smooth surface was imaged, while a pattern of bumps with 400-nm height, corresponding to the location of the pore pattern, was found on the back side. These AFM images prove that the pores are completely closed with the PEM film. The thickness of the PEM film on the nonpore area was determined by the scratch method on the sprayed side as 100 nm (Figure 2c). The shape of the PEM was roughly derived as illustrated in a schematic in Figure 2d, which we use later when we estimate the resistivity. Such AFM images from the sprayed side, from the back side, and with a scratch were also taken for other PEM films [PEI(PGA/PAH)12, PEI(PSS/PLL)12, PEI(PSS/ PAH)24], and shown in Figure 2e-p. For PEI/(PGA/PHA)12,

J. Phys. Chem. B, Vol. 114, No. 44, 2010 13983 the image from the sprayed side (Figure 2e) also shows a relatively flat surface, which confirms that the pores are closed. On the backside, a pattern of hollows, corresponding to the pore position, was imaged (Figure 2f). These are PEM-clogged pores but not open pores, since the depth of the hollows is not more than 150 nm, while AFM imaging over pores was impossible without PEM as mentioned previously. For PEI/(PLL/PSS)12, the images from the sprayed side (Figure 2i) show a pattern of bumps corresponding to the pore position, which also indicates the complete closing of the pores with the PEM film. The image from the back side shows a pattern that consists of both bumps and hollows (Figure 2j). The depth of the hollow is not more than 100 nm, thus we can conclude that the PEMs are deposited in the pores for the same reason as mentioned above. The inhomogeneity of the surface was considered in the error bar when we estimated the shape of the PEM inside of the pores (Figure 2l). For PEI/(PSS/PHA)24, the image from the sprayed side (Figure 2m) shows a smooth surface with a slight pitch of less than 50 nm, corresponding the pore position. The image from the back side shows a pattern that consists of a bump and small hollows not deeper than 50 nm (Figure 2n). They also prove the complete pore clogging by the PEM film, since AFM imaging is impossible if the pore is open as mentioned previously. The inhomogeneity was also taken into account for the error bar (Figure 2p). The thicknesses of the PEM films on the nonpore area were determined by the scratch method as 200, 100, and 100 nm for PEI(PGA/PAH)12, PEI(PSS/PLL)12, and PEI(PSS/PAH)24, respectively (Figure 2g,k,o), and the shapes of the PEM in the pore were derived as illustrated in the schematics in Figure 2h,l,p. The PEM films closed the pores more rapidly than on substrates (e.g., linear-growing system PSS/PAH closed the 800-nm pores at n ) 24, even though it typically has a thickness of only ∼100 nm on flat glass substrates). Such a rapid PEM growth in nanopores has been extensively reported and studied,16,28–32 where the mechanism of the growth has been explained with the “gel model” linked to the polyelectrolyte complexation in the nanopores. Next, the same PEM films were deposited on the chip with a single 800-nm pore, and EIS spectra were taken. Figure 3a shows the impedance Z of a bare chip with a single pore (CSP, +) and of the CSP after the deposition of PEI(PSS/PAH)24 (]), PEI(PGA/PAH)12 ([), PEI(HA/PLL)12 (0), and PEI(PSS/PLL)12 (9), respectively. A clear increase in the impedance Z from CSP (∼1 MΩ) to CSP + PEM (10-100 MΩ) is observed at the lower frequencies ( f < 1 Hz) for all PEMs. The data points of CSP (+) were fitted with the equivalent circuit Rbuffer (RSP, Cchip) as schematized in Figure 3b, where Rbuffer, RSP, and Cchip correspond to the buffer resistance and the resistance and capacitance of the chip with a single pore, respectively. Corresponding fitting results are RSP ) 2.2 ( 0.1 MΩ, Cchip ) 1.9 ( 0.1 nF, and Rbuffer ) 4.6 ( 0.3 kΩ, where the error is associated with the fitting errors. Both values RSP and Cchip are in agreement with those that have been measured in previous works,33,17 where the same type of chips were used. As a rough approximation, RSP is the resistance of the buffer solution confined in the cylindrical shape of the pore (800-nm diameter and 300-nm height). Nevertheless, the resistivity calculated from RSP and the shape of the pore is still in the same order, but 5 times larger than the resistivity of HEPES buffer (FHEPES ) 73.5 Ωcm). This overestimation occurred because we approximated that RSP originates only from inside of the pore, neglecting the detailed current distribution around the pore. In other words, even if we neglect the detailed current distribution, the order of the magnitude of the resistivity calculated with the shape of

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Figure 2. AFM topographical images of the chip with a 800-nm pore array after the deposition of PEI(HA/PLL)12, PEI(PGA/PAH)12, PEI(PSS/ PLL)12, and PEI(PSS/PAH)24 taken from (a,e,i,m) the sprayed side and (b,f,j,n) the back side, and the corresponding cross section along the white lines. (c,g,k,o) Images of the scratch on the sprayed side of the chip. (d,h,l,p) Schematics of the shape of the PEM film confined in the pore according to the AFM images.

the material in the pore is reliable; therefore, we focus only on it in the discussion. The curves of CSP + PEM were fitted with the equivalent circuit Rbuffer (RPEM, Cchip+PEM) described in Figure 3c. RPEM represents the resistance of the PEM in the pore, and Cchip+PEM represents the sum of the capacitance of CSP with the PEM on it and the capacitance of the free-hanging PEM, which are in parallel. By fixing Rbuffer ) 4.6 kΩ, we obtained RPEM ) 15 ( 1 MΩ and Cchip+PEM ) 1.3 ( 0.1 nF for PEI(HA/ PLL)12, RPEM ) 26 ( 1 MΩ and Cchip+PEM ) 2.3 ( 0.1 nF for PEI(PSS/PAH)24, RPEM ) 13 ( 1 MΩ and Cchip+PEM ) 1.9 (

0.1 nF for PEI(PGA/PAH)12, and RPEM ) 86 ( 5 MΩ and Cchip+PEM ) 3.9 ( 0.2 nF for PEI(PSS/PLL)12. Compared to the value without PEM films (RSP ) 2.2 ( 0.1 MΩ), we found a remarkable increase of the resistance in the pore due to the presence of the PEM films for all four types. Although the resistance differs for each PEM film, they are all in the same order, i.e., tens of megaohms. On the other hand, a major increase of the capacitances was not observed, although the capacitances slightly increased for some types of PEM films by a few nanofarads. Detection of the capacitance of the PEM

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Figure 3. (a) Impedance Z of a bare chip with a single pore (CSP) (+) and of CSP after the deposition of (PSS/PAH)24 (]), (PGA/PAH)12 ([), (HA/PLL)12 (0), and (PSS/PLL)12 (9). The schematics show the equivalent circuits used for the data analysis before (b) and after (c) the PEM deposition. The capacitance of the pore is neglected because it is negligible compared to that of the Si3N4 membrane. The resistance and the capacitance of the Ag/AgCl electrodes, attached in series to the circuit, are also neglected because they are much smaller than the rest of the elements in the circuit (RAg/AgCl el ∼ 3 kΩ and CAg/AgCl el < 100 pF, as confirmed by EIS measurements only with the electrodes).

film6–9 is difficult in our system, since the measured capacitance originates mostly from the capacitance of the Si3N4 membrane, because the area of the thin Si3N4 membrane in the chip is 106 times larger than that of the pore, therefore we focus the rest of our discussions only on the resistance. To compare the obtained RPEM with that determined by the conventional PEM-on-electrode system, we roughly estimate the order of magnitude of the PEM resistivity FPEM using RPEM and the shape of the PEM in the pore estimated by AFM previously. Although the precise estimation of FPEM requires the accurate current distribution in and around the pore, we use a simple model, which is adequate to estimate the order of magnitude of the value. When the current flows through the pore, the cross section of the current is restricted to S ) π × (400 nm)2 only in the pore area, while outside of the pore it can diffuse through much larger cross section (maximum S ) π × (400 nm)2 × 106 since the whole chip area is 106 times larger than the pore area as mentioned), thus the resistance of the system mainly originates from the resistivity of the material confined in the pore. Therefore, using the shape of the PEM film confined in the pore (e.g., φ ) 800 nm and l ) 300 nm for PEI(HA/PLL)12 from Figure 2d), the obtained value of RPEM (e.g., RPEM ) 15 ( 1 MΩ for PEI(HA/PLL)12) and the simple equation R ) Fl/S (l is the height, and S is the bottom area of the cylinder, e.g., l ) 300 nm and S ) π × (400 nm)2 for PEI(HA/PLL)12), the resistivity of the free-hanging PEM FPEM was calculated (e.g., FPEM ∼ 2.5 ( 0.2 kΩcm for PEI(HA/ PLL)12; see note).34 The values for each PEM film are shown in Table 1. For all four types of PEM films, FPEM is on the order of several kΩcm. Although there is a slight tendency that larger values were obtained for PEM films with higher densities, here we just focus on the order of the magnitude because of the large error bars. Finally we compare the obtained order of FPEM to that determined in the conventional PEM-on-electrode configuration. Cassier and co-workers have estimated an aerial resistivity of 143 kΩcm2 for (PSS/PAH)4 on a gold electrode in a physiological buffer (note that different definitions of the PEM resistivity are reported in the literature depending on the dimensions). This corresponds to FPEM ∼ 70 GΩcm assuming a typical layer thickness of ∼20 nm. The FPEM obtained by the free-hanging PEM for the same polyelectrolyte couple (FPEM )

TABLE 1: Parameters Obtained by Free-Hanging PEMs in a Buffer Solution with 10 mM HEPES and 150 mM NaCla (HA/PLL)12 (PSS/PAH)24 (PGA/PAH)12 (PSS/PLL)12

RPEM (MΩ)

l (nm)

FPEM (kΩcm)

15 ( 1 23.5 ( 6.5* 13 ( 1 86 ( 5

300 275 ( 25 150 ( 5 250 ( 50

2.5 ( 0.2 4.4 ( 1.6 4.3 ( 0.5 18 ( 4.6

a

The errors in RPEM come from the fitting error except for (PSS/ PAH)24 (see * in table), where the error includes values from four individual measurements additionally. The errors in l originate from the inhomogeneity of the shape of PEM film estimated by AFM images.

4.4 ( 1.6 kΩcm from Table 1) is surprisingly more than 6 orders of magnitude smaller. Furthermore, Table 1 shows that not only PSS/PAH but all four types of PEM films have a FPEM on the order of several kΩcm. This suggests that the significant disagreement in the value of FPEM obtained by the PEM-on-electrode configuration and the free-hanging PEM is not attributed to the specific polyelectrolyte couple but to the difference in the measurement methodology. In the following discussion, we shed a light onto this significant contradiction. First, one may assume that the difference in the morphological structure between the PEM in the free-hanging configuration and the one in the PEM-onelectrode system may have caused this contradiction, since they are not identical due to the difference in the growth mechanism as mentioned previously. However, the resistivity difference in the six orders of the magnitude cannot be explained unless the density of the PEM differs by 6 orders of magnitude, which is unlikely. The small differences in the chemical conditions of the PEM fabrication such as the concentration of salt, polymer solution, and the kind of the buffer solution may have caused the difference in the value, nevertheless they hardly explain the discrepancy of 6 orders of magnitude. The most significant difference in the two systems is that the PEM is directly deposited on the working electrode in the PEM-on-electrode system, while electrodes are untouched in the free-hanging configuration. This direct contact of the PEM to the electrode induces an additional change in the resistance, the change in the charge transfer resistance ∆Rct, which originates from the electron transfer across the electrode surface, apart from the

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Figure 4. (a) EIS spectra measured in PEM-on-electrode system with gold working electrode before and after the deposition of PEI(PSS/ PAH)n. The EIS results were fit with Rbuffer (Rct, Cdl), and Rct was plotted against the number of the PEM bilayer n in (b).

resistance of the PEM RPEM, which originates from the PEM matrix that hinders the diffusion of ions (see the schematic in Figure 1a). Therefore, in the PEM-on-electrode systems, some works have assumed RPEM ) 06 or ∆Rct ) 0,9 neglecting one of them, while others have attempted to extract each value from the obtained EIS data by fitting with an equivalent circuit composed of both RPEM and ∆Rct.7,8 On the contrary, in the freehanging configuration, the resistance increase due to the PEM is certainly RPEM, since the charge transfer resistance does not change (see the schematic in Figure 1b). Thus we hypothesize that the value taken in the free-hanging system is RPEM, while the one that has been taken in the PEM-on-electrode system was not RPEM but ∆Rct, and the significant contradiction occurred because two completely different physical quantities, RPEM and ∆Rct, were compared. To investigate the hypothesis, we examined which between RPEM or ∆Rct was dominantly detected in the conventional PEMon-electrode system by studying the layer-number n dependence of the resistance in the PAH/PSS system. If RPEM was dominantly detected in the system, it should increase linearly with n since it is simply proportional to the thickness of the PEM film,10 which grows linearly in the PSS/PAH system as mentioned previously. EIS before and after the deposition of PEI(PSS/PAH)n in 0 < n < 20 was monitored with three electrodes with a gold working electrode (see Figure 4a,b). The same kind of buffer solution as well as the salt and the polymer concentration as the former experiment in the free-hanging configuration were used to eliminate the suspicion that the difference in the value came from them. Before the PEM deposition (gold ]), in the lowest frequency (f ) 0.1 Hz) where Z ∼ R since Vac ∼ Vdc, Z is 32 kΩ, indicating the original high Rct of the gold surface (see the black circle in Figure 4a). After the deposition of one PEI layer ((), unexpectedly Z decreased to 21 kΩ (see the red circle in Figure 4a), which already implies that the change in the impedance detected in the system is not RPEM but ∆Rct, since the value of RPEM cannot be negative by definition. After the deposition of 10 PEM bilayers (0), Z increased to 42 kΩ (see the blue circle in Figure 4a), while the value remained the same at n ) 20 (also see the blue circle in Figure 4a), which also indicates that ∆Rct is detected in the system because RPEM should keep increasing linearly with n.35

Sugihara et al. Since it is now clear that ∆Rct is detected in the PEM-onelectrode system as we expected, the data curves were fitted with a simple equivalent circuit Rbuffer (Rct, Cdl), where Cdl is the double layer capacitance, by fixing Rbuffer ) 13.6 Ω, and Rct was plotted against n in Figure 4b. As we saw in the EIS spectra, Rct decreased after the deposition of PEI by 60 kΩ, implying that the deposition of the PEI layer promoted the electron transfer between the solution and the gold electrode. Subsequent deposition of PSS and PAH increased and decreased Rct respectively in n < 2, while the continuous increase of the value was observed in 2 < n < 15, and finally Rct was saturated at n ) 20. First, the increase in the resistance by tens of kilohms (several GΩcm in resistivity) from that of the original gold electrode was indeed observed at n ) 10 (see the red arrow in Figure 4b) as reported even with the same buffer solution, polymer, and salt concentration, which suggests the contradiction is not due to those chemical parameters. However, the n dependence of the resistance in Figure 4b clearly shows that the measured resistance is not RPEM but Rct. Furthermore, importantly, the fact that the simple linear increase in the resistance that we expect for the feature of RPEM was not observed implies that RPEM was not detected in this system at all, or in other words, the order of RPEM is much smaller than that of ∆Rct. Although the thickness-dependent increase of the resistance has been observed with the presence of redox-active ions,6,7,10 where the original Rct of the electrode is significantly reduced and RPEM is enhanced by the redox-active ions, it cannot be detected without them, because now the largest resistance in the system becomes not RPEM but Rct. Additionally, the oscillation of Rct, depending on the topmost layer (positively charged polyelectrolytes, PEI and PAH, or negatively charged polyelectrolytes, PSS), was observed while n < 2, which distinguished after two bilayers. This phenomenon has been reported and explained by Ruan and co-workers previously,10 where they linked it to the alternation of the attraction and the repulsion between the interface and the redox probe.36 The distinguishment of the oscillation has been also explained by the same authors associated with the relation between the Debye length and the thickness of the film. As a result, we can state that the obtained value in the PEM-on-electrode system was ∆Rct, while the value detected in the free-hanging configuration was RPEM. The discrepancy arose because we compared ∆Rct in the PEM-on-electrode system to RPEM in the free-standing configuration. It has to be emphasized that the fact that the RPEM was not even detected in the PEM-on-electrode system indicates that RPEM , ∆Rct, which is in an agreement with the obtained value of FPEM (tens of kΩcm) in the free-hanging configuration. Here we note that our data indicated FPEM , GΩcm in both configurations, however, the precise determination of FPEM below the order of magnitude is an open question. Conclusion In conclusion, we demonstrated a resistance measurement of PEM films deposited over a Si/Si3N4 chip with an 800-nm pore without the presence of redox-active ions. The resistivity FPEM of four types of PEM films [PEI(PSS/PAH)24, PEI(PGA/PAH)12, PEI(HA/PLL)12, and PEI(PSS/PLL)12] was estimated combining EIS results and AFM images. We found values of FPEM on the order of several kΩcm for all PEM types, which are at least 6 orders of magnitude lower than those determined in the conventional PEM-on-electrode system. By a close comparison between the free-hanging configuration and the PEM-onelectrode system, we found that the value we obtained in the free-hanging configuration was the resistance of the PEM film

PEM Resistivity in Free-Hanging Configuration RPEM, while that which had been observed in the PEM-onelectrode system was the change in the charge transfer resistance ∆Rct due to the coverage of the electrode by the PEM. Thus, the significant discrepancy occurred mainly because completely different two physical quantities, RPEM and ∆Rct, were compared. It revealed not only the unexpectedly low electrical resistance of the PEM in the absence of the redox-active ions, but also the potential difficulty in measuring RPEM in the conventional PEM-on-electrode system without the usage of redox-active ions, since the system does not have the sensitivity to detect RPEM due to the much higher Rct of the electrode. Our work indicated the importance of evaluating “which value between Rct or RPEM the system dominantly detects” in the PEM-onelectrode system. The key point is the relation between Rct and RPEM (Rct . RPEM or Rct , RPEM). When Rct . RPEM, the detected resistance is dominantly Rct, while when Rct , RPEM, it is RPEM. Importantly, the relation depends on the conditions such as presence/absence of redox-active ions and the material of the working electrode. Acknowledgment. This work was supported by the EU Seventh Research Framework Program (FP7) (ASMENA Project). We are grateful to Marco di Berardino and Patrick Surbled (Leister, CH) for the chip fabrication, Stephen Wheeler (LBB) for the electrochemical cell, and Dominik Textor (LBB) for the construction of the spray robot. References and Notes (1) Decher, G. Science 1997, 277 (5330), 1232–1237. (2) Souto, R. M.; Laz, M. M.; Reis, R. L. Biomaterials 2003, 24 (23), 4213–4221. (3) Hollman, A. M.; Bhattacharyya, D. Langmuir 2004, 20 (13), 5418– 5424. (4) Kakizawa, Y.; Kataoka, K. AdV. Drug DeliVery ReV. 2002, 54 (2), 203–222. (5) Guillaume-Gentil, O.; Akiyama, Y.; Schuler, M.; Tang, C.; Textor, M.; Yamato, M.; Okano, T.; Voros, J. AdV. Mater. 2008, 20 (3), 560–+. (6) Harris, J. J.; Bruening, M. L. Langmuir 2000, 16 (4), 2006–2013. (7) Barreira, S. V. P.; Garcia-Morales, V.; Pereira, C. M.; Manzanares, J. A.; Silva, F. J. Phys. Chem. B 2004, 108 (46), 17973–17982. (8) Chur-Min, C.; Hsuan-Jung, H. Anal. Chim. Acta 1995, 300 (1-3), 15–23. (9) Cassier, T.; Sinner, A.; Offenha¨user, A.; Mo¨hwald, H. Colloids Surf., B 1999, 15 (3-4), 215–225. (10) Ruan, Q.; Zhu, Y.; Li, F.; Xiao, J.; Zeng, Y.; Xu, F. J. Colloid Interface Sci. 2009, 333 (2), 725–733. (11) Krasemann, L.; Tieke, B. Langmuir 1999, 16 (2), 287–290. (12) Jeon, J.-D.; Kwak, S.-Y. Macromolecules 2006, 39 (23), 8027– 8034. (13) Bai, T.; Bradshaw, R. D.; Ramanathan, T.; Nunalee, F. N.; Shull, K. R.; Mason, T. O.; Brinson, L. C. Polym. Eng. Sci. 2009, 49 (3), 441– 453.

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