Research Article www.acsami.org
Redox Properties of Polyvinylamine‑g‑TEMPO in Multilayer Films with Sodium Poly(styrenesulfonate) Qiang Fu,† Igor Zoudanov,† Emil Gustafsson,† Dong Yang,† Leyla Soleymani,‡ and Robert H. Pelton*,† †
Department of Chemical Engineering and ‡Department of Engineering Physics, McMaster University, 1280 Main Street West, Hamilton, Ontario Canada, L8S 4L7 S Supporting Information *
ABSTRACT: Layer-by-layer (LbL) assemblies of polyvinylamine with grafted TEMPO moieties (PVAm-T) with sodium polystyrenesulfonate (PSS) were prepared on gold-sulfonate surfaces, and the redox properties were measured by cyclic voltammetry. LbL compositions were probed by quartz crystal microbalance (wet) and ellipsometric (dry) film measurements. Approximately 30% of the TEMPO moieties in the LbL assemblies were redox-active when the total TEMPO coverage was varied up to 6 μmol/m2, by either varying the TEMPO content in PVAm-T or by varying the number of LbL bilayers. Three non-redox-active PVAm/PSS blocking bilayers were required to prevent the electrode from oxidizing PVAm-T in the exterior LbL layer. This suggests significant intermixing between the layers in the LbL film. In addition to contributing to the small but growing body of work on redox polymers based on grafted TEMPO, this work serves as a reference point for understanding the redox properties of colloidal PVAm-T-laccase complexes in future work. KEYWORDS: redox polymers, polyelectrolyte multilayers, cyclic voltammetry, EQCM-D, TEMPO
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INTRODUCTION Cellulose is an important platform in the push to more sustainable products and processes. However, cellulose must be modified for most applications. TEMPO mediated oxidation is one of the most widely used approaches to functionalizing cellulose by virtue of the selective oxidation of C6 primary hydroxyls to aldehydes and acids.1 Our interests involve using TEMPO to oxidize cellulose to facilitate the covalent coupling of polyvinylamine (PVAm) to cellulose surfaces. PVAm grafted “primer” coatings on cellulose2 provide amine-rich surfaces that can be readily modified, providing a wide variety of functions, including bioactive surfaces, redox-active surfaces, and surfaces giving exceptionally high cellulose/cellulose wet adhesion. Although TEMPO chemistry offers many advantages, its use in some industrial operations is inhibited by the cost, the environmental impact of TEMPO, and the difficulty in recycling TEMPO.3 For our applications, we have solved these limitations by grafting TEMPO onto PVAm to give polyvinylamine-g-TEMPO (PVAm-T), see structure in Figure 1. In water, this very cationic polymer spontaneously adsorbs onto cellulose. In the presence of a primary oxidant such as bleach, the cellulose is oxidized to aldehydes which subsequently can react with amine groups to form imine and aminal bonds after drying.4 This is a unique technology because PVAm-T is both the cellulose oxidation catalyst and the polymer to be grafted to cellulose. More recently, we have shown that ∼500 nm colloidal complexes formed between PVAm-T and the redox enzyme laccase also oxidize cellulose surfaces.5 We explained this by hypothesizing that TEMPO-to-TEMPO electron © 2017 American Chemical Society
Figure 1. PVAm-T structure. The degree of hydrolysis (DH), the degree of TEMPO substitution based on amines (DS), and the degree of ionization (α) are functions of w, x, y, and z. The polymer properties are summarized in Table 1.
transfer occurred in PVAm-T, even when only 10% of the amine repeat units bore a grafted TEMPO. Our bleachactivated, cellulose autografting PVAm-T is a novel technology that has the potential to greatly expand the property space cellulosic materials. Thus, it is important to characterize the electron transport mechanisms in adsorbed PVAm-T layers. The goals of the work described herein include confirming that PVAm-T polymers with relatively low TEMPO contents are able to support TEMPO-to-TEMPO electron transport. Received: November 29, 2016 Accepted: January 19, 2017 Published: January 20, 2017 5622
DOI: 10.1021/acsami.6b15319 ACS Appl. Mater. Interfaces 2017, 9, 5622−5628
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ACS Applied Materials & Interfaces Table 1. Properties of PVAm-T Copolymersa designation
PVAm DH (mol %)
PVAm MW (KDa)
TEMPO DS (mol %)
EWT (kDa)
EWN (Da)
PVAm-T4L PVAm-T10L PVAm-T13L PVAm-T25L PVAm-T5H PVAm-T12H PVAm-T22H PVAm-T35H
75 75 75 75 100 100 100 100
45 45 45 45 340 340 340 340
4 10 13 25 5 12 22 35
2.26 1.00 0.809 0.500 1.36 0.665 0.438 0.337
94.0 111 121 167 71.8 90.7 123 181
a
See structure in Figure 1. DH is the percentage of N-vinylformamide groups hydrolyzed to amine groups in the parent PVAm. DS is the mole fraction of PVAm amine groups bearing pendant TEMPO. EWT is the TEMPO equivalent weight at pH 7.5, and EWN is the amine equivalent weight at pH 7.5. Preparation of PVAm-T. A series of PVAm-T was synthesized as previously reported.5 In a typical experiment, 55 mg of 4-carboxyTEMPO was dissolved in 100 mL of deionized water, and 157 mg of EDC was reacted with the 4-carboxy-TEMPO. A 170 mg portion of sulfo-NHS was added, and the reaction was stirred for 20 min at pH 5.5 at room temperature. PVAm solution (100 mg PVAm in 40 mL deionized water) was slowly added dropwise into the 4-carboxyTEMPO solution, and the solution pH was maintained at 7.0 for 4 h with 0.1 M HCl and 0.1 M NaOH. The resulting solution was dialyzed (MWCO 3 kDa) against deionized water for 2 weeks. Purified and dried PVAm-T samples were obtained through lyophilization and stored in a desiccator. The amine content of the PVAm-T samples was determined by potentiometric and conductometric titration. The amine contents of PVAm and its copolymers were measured by conductometric titration. The degree of TEMPO substitution was calculated from the change in amine contents compared to the unmodified parent PVAm; more details were given previously.4 Every TEMPO conjugated to PVAm converts a titratable amine to a nontitratable amide; see Figure S1 in the Supporting Information. Table 1 summarizes the properties of the synthesized PVAm-T copolymers. Electrochemical Quartz Crystal Microbalance with Dissipation (EQCM-D). EQCM-D measurements were carried out with an electrochemical QCM-D cell (E401 model from Q-Sense, Sweden). A QSX301 gold sensor served as the working electrode; the counter electrode was platinum, and a low leakage “Dri-Ref 2SH” Ag/AgCl (3 M KCl) electrode (World Precision Instruments) was the reference electrode. The QSX301 gold sensor and the platinum electrode were cleaned with piranha solution at 75 °C for 5 min and then thoroughly rinsed with deionized water, and dried by N2. Note that piranha is dangerous, and appropriate precautions must be taken when working with it. Sulfonate groups were grafted to the gold surface by immersing the electrode into 3 mM sodium 3-mercapto-1-propanesulfonate (MPS) ethanol solution for 24 h at room temperature, followed by rinsing with ethanol, and finally by deionized water, and dried with N2.14 Using the traditional LbL approach, alternating layers of cationic PVAm-T and anionic PSS were adsorbed on the gold-sulfonate electrode. PVAm-T solution (0.1 g/L in 0.1 M NaCl with pH 7.5) was pumped through the EQCM-D cell at a flow rate of 0.100 mL/min for 15 min. The cell was then rinsed with NaCl solution (0.1 M with pH 7.5) for another 15 min, followed by PSS (0.1 g/L in 0.1 M NaCl with pH 7.5) for 15 min again followed by rinsing with salt solution. All measurements were performed at 23 °C. The buildup of polymer layers was continuously monitored by QCM-D; after each rinsing step following a completed polymer bilayer, the flow was stopped, and the electrochemical properties of the film were measured by cyclic voltammetry performed with a Palmsens potentiostat (PalmSens, Amsterdam, Netherlands) in a conventional three-electrode configuration. In most experiments, the potential was varied between 0.3 and 0.8 V at scan rates between 5 and 100 mV/s. The measurement time for each condition was 20 min. In addition to voltammograms, the instruments software PSTrace 4.7 was used to extract peak voltages and the areas under the oxidation curves. The errors associated with the area measurements were estimated from the variation over three
By combining quartz crystal microbalance measurements with cyclic voltammetry (EQCM-D) and with ellipsometric characterization of the dried layer-by-layer (LbL) multilayer films, we were able to estimate the fraction of TEMPO moieties that could be oxidized by a gold electrode. Polymers with grafted TEMPO moieties are not new. TEMPO has been grafted to water-insoluble polymers giving a supported catalyst for the oxidation of soluble molecules.6 Grafted TEMPO probes have also been widely used for electron spin resonance studies of polymers.7 There appears to be a resurgence in polymer-grafted TEMPO based on potential applications in energy storage.8−10 Many of the studies involve redox polymers deposited on electrode surfaces using LbL assembly. In one of the earlier LbL redox studies, Laurent and Schlenoff showed that the extent of interpenetration between polymer layers in LbL constructs could be probed by placing non-redox-active pairs of blocking layers between the electrode and the redox polymer layers.11 They reported that four pairs of blocking layers were required to prevent electron transfer to the redox polymers; we report herein similar results. Closest to our work is a publication by Anzai’s group12 that describes LbL films based on alternating layers of anionic poly(acrylic acid) with 46 mol % of the carboxyls bearing grafted TEMPO substitution and cationic polyethylenimine. They reported that their multilayer films decomposed when the TEMPOs were oxidized. By contrast, our work involves lower levels of TEMPO grafted on the cationic polymer, PVAm, and we have used polystyrenesulfonate as the anionic polymer. Our polymers were chemically stable, and the multilayers remained intact during cyclic voltammetry. Finally, our work is focused on PVAm-T oxidation of, and subsequent grafting to, cellulose for adhesion applications. However, the ability of PVAm-T to adsorb spontaneously onto most surfaces in water provides a simple route to TEMPO-rich surfaces for other applications.
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EXPERIMENTAL SECTION
Materials. Two different polyvinylamine samples were obtained from BASF Canada: Lupamin 5095, Mw 45 kDa, hydrolysis degree 75%, and Lupamin 9095, Mw 340 kDa, hydrolysis degree 95%. We completely hydrolyzed the Lupamin 9095 following Gu et al.’s method.13 Type 1 water (18.2 MΩ cm−1, Barnstead Nanopure Diamond) was used for all experiments. The PVAm was dialyzed against deionized water and freeze-dried before use. 4-CarboxyTEMPO, sodium 3-mercapto-1-propanesulfonate (MPS), sodium polystyrenesulfonate (PSS), Mw 70 kDa (density 0.801 g/mL), 1-ethyl-3-(3-(dimethylamino)propyl) carbodiimide (EDC), and N-hydroxysulfosuccinimide sodium salt (sulfo-NHS) were purchased from Sigma-Aldrich and used as received. 5623
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ACS Applied Materials & Interfaces measurements. Finally, the density of redox-active TEMPO moieties was calculated from the CV results; see example calculations in the Supporting Information. Ellipsometry. The quantity of TEMPO moieties in the multilayer films was estimated from ellipsometric characterization of dried multilayer films. The desired number of polymer layers was deposited on the sensor in the EQCM-D after which the chip was taken out and air-dried. The thickness of the dry films was measured using a Nanofilm EP3W imaging ellipsometer (Accurion Inc., Germany) fitted with a wavelength 658 nm solid state laser. The ellipsometry data was analyzed using the modeling software EP4 (Accurion Inc.). The refractive index of the multilayer films, needed to determine the film thickness, was calculated as the average refractive index of PVAmT and PSS. The refractive index of PSS, 1.484, was taken from the literature,15 whereas the refractive indices of the synthesized PVAm-Ts were measured from spin coated films. These films were spin coated from 0.1 wt % aqueous PVAm-T solutions at 3000 rpm on piranhacleaned silicon wafers. The thick (∼100 nm) PVAm-T polymer films were annealed at 50 °C overnight after which the refractive index was determined by ellipsometry. The refractive index of PVAm-T25L was 1.462 ± 0.021, where the error estimate was based on three measurements. Finally, the coverage (dry mass/area) of PVAm-T in each layer was estimated from the incremental ellipsometric thickness and the density of PVAm-T (1.08 g/mL, assumed to be the same as PVAm16).
PVAm-T/PSS Layer-by-Layer Films. LbL layers of PVAm-T and polystyrenesulfonate (PSS) were formed on EQCM-D gold sensor surfaces. To facilitate adsorption of the first PVAm-T layer, the gold was treated with sodium 3-mercapto-1-propanesulfonate to give surface sulfonate groups.14 Figure 2 shows the EQCM-D frequency change
Figure 2. QCM-D frequency change per adsorbed layer for (PVAmT25L/PSS)n and the corresponding dry film thicknesses measured by ellipsometry.
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RESULTS AND DISCUSSION PVAm-T Compositions. A series of PVAm-T copolymers with varying TEMPO contents was prepared, and the generic PVAm-T structure is shown in Figure 1. In water, PVAm-T consists of a mixture of four types of repeating units, Figure 1. Polyvinylamine is prepared by the hydrolysis of poly(Nvinylformamide), and most commercial PVAms are not completely hydrolyzed. We define PVAm-T compositions by DH, the degree of hydrolysis of the parent PVAm; DS, the fraction of amine groups coupled to TEMPO; and α, the degree of ionization of amine groups. The properties of the PVAm-T polymers used herein are summarized in Table 1. The copolymers are based on two parent PVAms, one with DH = 0.75, a value given by the supplier and one fully hydrolyzed by us to give DH = 1. The DS values were determined from the difference in amine contents, measured by conductometric titration, before and after TEMPO grafting. Finally, α values were estimated by the Katchalsky model17,18 that predicts α = 0.46 for PVAm in 0.1 M NaCl at pH 7.5, the main conditions used in this work. Two other compositional parameters, useful for data manipulations, are EWT, the TEMPO equivalent weight (Da), and EWN, the equivalent weight of amine groups, both ionized and nonionized. These equivalent weights were calculated from DS, DH, and α values, and are also tabulated in Table 1. Calculations were performed with Mathcad Prime 3.1 and are given in the SI. PVAm-T Solution Properties. PVAm-T is water-soluble from pH 2 to 12. However, the TEMPO moieties in PVAm-T are more hydrophobic than PVAm alone. Therefore, TEMPO substitution has two impacts: it lowers the surface tension (see Figure S3), and TEMPO substituents cause the polymer coils in aqueous solutions to shrink (see Figure S5). On the basis of these observations, many of the properties of PVAm-T are dominated by amine groups over the range of TEMPO contents investigated. For example, PVAm-T coverage (mass/area) in LbL films, described next, was not sensitive to the TEMPO content; see Table S2.
with each additional adsorbed layer. The average frequency shift of a PVAm-T25L layer was −20 Hz, and the average frequency shift of a PSS layer was −5 Hz. The total dissipation shift was less than 2 × 10−6 (see Supporting Information Figure S6). The curves under different overtones overlapped well, and the dissipation shifts were less than 10−7 of the frequency shifts, indicating that the (PVAm-T25L/PSS)4 multilayer film was rigid, and the absorbed mass can be calculated from the Sauerbrey equation.19,20 Figure 2 also shows the dry thickness of PVAm-T25L/PSS multilayer films measured by ellipsometry. The average PVAmT25L layer thickness is 0.8 ± 0.2 nm, and the average thickness of a PSS layer is 0.6 ± 0.2 nm. The mass of PVAm-T25L/PSS multilayer films was calculated from both EQCM-D and ellipsometry results. EQCM-D gave the wet mass of the adsorbed polymer whereas ellipsometry gave dry film mass. By comparing these results, we estimated the water contents of the multilayer films, and the results (see Figure 3 and tabulated
Figure 3. Water content of PVAm-T25L/PSS multilayer films as a function of the number of polymer layers.
data in Table S1) show that about 75 wt % of the multilayer films is water. Redox Properties of PVAm-T/PSS Multilayer Films. Cyclic voltammetry (CV) was used to measure the content of 5624
DOI: 10.1021/acsami.6b15319 ACS Appl. Mater. Interfaces 2017, 9, 5622−5628
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ACS Applied Materials & Interfaces redox-active TEMPO moieties in the multilayer films. In this technique, the electrical potential on a gold-sulfonate EQCM sensor was cycled, and the resulting current was measured. Figure 4A shows the cyclic voltammograms of (PVAm-T25L/
The CV oxidation curve shows an increase in the oxidation current as the potential approaches 0.8 V. Published CV curves for immobilized TEMPO do not show this behavior,12 whereas our control experiments with unmodified gold (Figure S8) and with TEMPO-free PVAm (Figure S9) did. Therefore, we conclude that the high voltage oxidation current is not associated with a TEMPO redox process. We suspect this behavior may be due to oxidation of the gold surface; similar peaks have been reported for gold surfaces.21 Finally, the TEMPO oxidation and reduction peaks are similar to other TEMPO results in the literature; see Table 2. The ratio of oxidative (positive) peak currents to the reductive (negative) peak currents ranged from 1 for the lowest two scan rates, down to 0.74 for the highest two scan rates in Figure 4; therefore, the redox process was not completely reversible at high scan rates. Table 1 compares our results to those from other types of TEMPO-grafted polymers. Both our peak values and the difference between the oxidation and reduction peak voltages are within the range of the published results. Another characteristic of the cyclic voltammograms in Figure 4 is that both the oxidation and the reduction peak currents increased linearly with the scan rate (see Figure 4B). This linear relationship is indicative of immobilized redox-active species whereas, for mobile TEMPO, the peak current would be proportional to the square root of the scan rate.10 TEMPO oxidation converts a neutral stable radical to the corresponding cationic oxoammonium ion (see structure in Figure 4A). Being a hydrogel with about 75% water (Figure 3), the multilayer films respond by shrinking, decreasing the water content in the film by approximately 10%. Furthermore, the changes were reversible; see Figure S7. Compared to the influence of supporting electrolyte on polyelectrolyte multilayer swelling,25 or to 800% changes typically observed with thermal sensitive hydrogels,26 10% fluctuations seem small. We do not know the influence of these volume changes on the redox properties. Figure 5 shows the cyclic voltammograms of PVAm-T25L/ PSS multilayer films as a function of the number of bilayers, n. Both the maximum oxidative and reductive peak currents increased with n. This is an important observation because it indicates that TEMPO moieties far from the electrode surface are redox-active. Redox-Active TEMPO Measurements. The areas under the oxidation curves were used to estimate the coverage (μmol/m2) of redox-active TEMPO moieties in the multilayer films. Example calculations are shown in the Supporting Information together with tabulated results in Table S1. Figure 6 shows the coverage of redox-active TEMPO moieties as a function of the
Figure 4. (a) Cyclic voltammograms of (PVAm-T25L/PSS)4 at various scan rates. (b) Peak current as a function of on scan rate for the (PVAm-T25L/PSS)4 multilayer film.
PSS)4 multilayer films at various scan rates. At a scan rate of 100 mV/s, the positive peak was located at 0.61 V, which corresponds to oxidizing TEMPO to oxoammonium, and the negative current peak was located at 0.56 V, which corresponds to reducing oxoammonium to TEMPO. For the ideal case of surface-immobilized redox-active groups, the CV curves should appear as two parabolic curves, one the mirror image of the other around the zero-current line. However, for the multilayer film results in Figure 4A, the curves are distorted with peak oxidation voltages greater than the corresponding reduction peaks.
Table 2. Comparison of our Cyclic Voltammetry Results with Published Results for Polymer-Grafted TEMPO system
scan rate mV/s
oxidation peak V vs Ag|AgCl
100 100
0.61 0.52
0.56 0.46
22
50
0.63
0.60
12
10
0.51
0.46
23
10
0.50
0.43
23
1
0.68
this work, Figure 4 TEMPO, not grafted to polymer, pH 7 in 80 mM tetraethyl ammonium perchlorate aqueous solution polyacrylic acid-g-TEMPO acid LbL with polyethylenimine, pH 7 in 10 mM HEPES buffer polyacetylene-g-TEMPO, amide linkage, 1 mM polymer in CH2Cl2 with 0.1 M tetrabutylammonium perchlorate polyacetylene-g-TEMPO, ester linkage, 1 mM polymer in CH2Cl2 with 0.1 M tetrabutylammonium perchlorate polyacrylamide-g-TEMPO, 0.5 M NaBF4 pH 4.3 5625
reduction peak V vs Ag|AgCl
∼0.6
ref
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DOI: 10.1021/acsami.6b15319 ACS Appl. Mater. Interfaces 2017, 9, 5622−5628
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ACS Applied Materials & Interfaces
to trains would be redox-active. The dashed line labeled “first layer only” in Figure 6 was calculated assuming the mass coverage of PVAm-T in the first layer was 0.5 mg/m2 and that only 20% of the first layer was redox-active, i.e., p = 0.2. The other limiting case is when the TEMPO DS is so high that, through TEMPO-to-TEMPO hopping, all TEMPO moieties are redox-active. The dashed line labeled “All active” was calculated assuming the coverage of each PVAm-T layer was 0.5 mg/m2 and that all TEMPO moieties were redoxactive. We emphasize that the dashed curves are not meant to fit the results, but instead illustrate the shape of the curves for the two limiting cases. The results in Figure 5, Figure 6, and Table S1 show that the surface coverage of redox-active TEMPOs increases with the number of polyelectrolyte bilayers. Another way to vary the coverage of redox-active TEMPO is to vary the TEMPO DS in PVAm-T. Figure 7 (data are tabulated in Table S2) shows the
Figure 5. Cyclic voltammograms of (PVAm-T25L/PSS)n multilayer films in 0.1 M NaCl, pH 7.5. Scan rate 20 mV/s.
Figure 6. Coverage of redox-active TEMPO in (PVAm-T25L/PSS)n multilayer films as a function of number of bilayers. The two limiting cases were estimated assuming p = 0.2, Γ1 = 0.5 mg/m2.
Figure 7. Coverage of redox-active TEMPO moieties as a function of PVAm-T TEMPO DS in (PVAm-TxL/PSS)4 multilayers. The polymers were based on the parent 45 kDa PVAm, 75% hydrolyzed.
number of PVAm-T layers. The coverage of redox-active TEMPO increased with the number of PVAm-T/PSS layer pairs, with 3 and 4 generating a more redox-active TEMPO than bilayers 1 and 2. The ellipsometry results in Figure 2 show that the first layer dry thickness was less than one-half the subsequent PVAm-T layer values, meaning that the first layer has fewer TEMPOs. It seems reasonable that the configuration of adsorbed PVAm-T onto gold-sulfonate will differ compared to adsorption onto polystyrenesulfonate surfaces in the thicker assemblies. Also, it is well-known in the LbL literature that the amount of adsorbed polymer is lower in the first and possibly second bilayers than in subsequent layers, especially for low charge density substrates.27 The dashed lines in Figure 6 show two limiting cases that are now explained. There are two relevant electron transport pathways for PVAm-T in LbL assemblies: (1) transport between the gold-sulfonate surface and adsorbed TEMPOs and (2) transport between neighboring TEMPO moieties. In both cases the electron transfer partners must be within ∼0.6 nm for efficient electron hopping.8 Consider first the PVAm-T adsorbed directly on the gold-sulfonate surface. Fleer et al. have discussed the configuration of adsorbed polymers in detail, and the relevant parameter is p, the fractions of polymer segments present as adsorbed trains; the remainder of the polymer chain is present as loops and tails, not in contact with the surface (0 < p ≤ 1).28 It seems reasonable to propose that, for PVAm-T copolymers with low DS values, only those TEMPOs attached
coverage of redox-active TEMPO for (PVAm-TxL/PSS)4 for a series of PVAm-T where the DS varied from 4% to 25%. The results show a linear relationship between PVAm-T TEMPO DS and the redox-active TEMPO coverage in an assembly of four bilayers. The dashed curves show the corresponding limiting cases. Note that the two dashed lines in Figure 7 are not linear because, for a fixed mass of PVAm-T, the mass fraction of TEMPO moieties is not a linear function of DS. We anticipated that, at very low TEMPO DS values, the results would be close to the “first layer only” case since the TEMPO groups were predicted to be too sparse for TEMPO-to-TEMPO electron transfer. At high TEMPO DS, we expected a sloped line similar to the “all active” case, with the switch between the two cases happening at a percolation threshold. However, we saw no evidence of threshold behavior, possibly due to few data points and noisier results at low TEMPO contents compared to the high TEMPO content experiments. In the above figures, the limiting cases were estimated with the assumption that each PVAm-T layer had a coverage of 0.5 mg/m2 and that the train fraction, p, was 20%. However, our ellipsometric analysis of dried films gave a direct measure of the PVAm-T coverage. Figure 8 compares the redox-active TEMPO coverage to the overall TEMPO coverage for the two data sets, varying the number of bilayers (Figure 6) and varying the PVAm-T TEMPO DS (Figure 7). Most of the data are close to the red line corresponding to 30% of the TEMPOs 5626
DOI: 10.1021/acsami.6b15319 ACS Appl. Mater. Interfaces 2017, 9, 5622−5628
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Figure 9. Effect of blocking bilayers on oxidation peak current of (PVAm-T25L/PSS)2 multilayers. Conditions: 0.1 M NaCl, pH 7.5, scan rate 50 mV/s.
Figure 8. Comparison of the redox-reactive TEMPO coverage, measured by cyclic voltammetry, and the overall TEMPO contents in LbL films calculated from ellipsometry results. Approximately 30% of the TEMPOs are redox-reactive over the whole data set.
laccase, TEMPO mediated oxidation converts surface cellulose C6 hydroxyls to aldehyde groups.4,5 Therefore, PVAm-T has promise as primer layer for enhancing adhesion to cellulose surfaces. The main goal of the present work was to employ cyclic voltammetry to determine the influence of the TEMPO content in PVAm-T on the fraction of redox-active TEMPO in relatively well-defined LbL assemblies. This information serves as a reference point in the future analysis of the electrochemical properties of colloidal complexes formed by mixing PVAm-T with laccase. Figure 8 shows that the redox-active TEMPO content is about 30% of total TEMPO coverage in the multilayer assemblies. This work also contributes to a small body of published work involving redox-active layer-by-layer assemblies based on TEMPO. Our assemblies were stable and contained about 75% water. The swelling decreased reversibly with TEMPO oxidation. Like other LbL systems, our blocking experiments (Figure 9) indicated significant mixing of PVAm-T between layers in the LbL assemblies. Finally, since PVAm-T is a relatively new polymer, we determined some solution properties. PVAm-T is slightly more hydrophobic than PVAm, as evidenced by decreasing surface tension with increasing TEMPO substitution. However, unlike TEMPO derivatives of poly(acrylic acid),33 PVAm-T with DS values up to 35% is water-soluble over the pH range 1−9.
being redox-active. Again, there is no evidence of a percolation threshold when going from low to high TEMPO coverages. Published theoretical studies suggest that for electron hopping (transfer) between TEMPO groups to occur, the distance between the TEMPO moieties must be less than ∼0.6 nm.8 One published estimate is that at least 60% of polymer repeat units must bear TEMPO groups for effective transfer in dilute polymer solutions.9 By contrast, our results show that 30% of the TEMPO groups are redox-active in multilayer films (Figure 8). This is remarkable because, in our experiments, the PVAm-T should be rather immobile in multilayer films. In addition, we have used very short tethers between TEMPO and PVAm (see structure in Figure 1). Nevertheless, our results seem consistent with other redox-active LbL systems; see examples in Table 3. Table 3. Examples of Redox Polymer LbL Assemblies Described in the Literaturea LBL systems PAA-T/PEI PAH-Fc/DNA PEI-Fc/DNA PAH-Fc/PVS PGA/PAH
redox species DS (%) 46 7.2 2.3 15 10 mM ferrocyanide solution
fraction redox-active (%)
ref
“most” 20−25 40−60 56 ∼50%
12 29 29 30 31
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a
The following abbreviations are used: PEI is polyethyleneimine; Fc is ferrocene; PVS is polyvinylsulfate; PAH is polyallylamine; and PGA is poly(L-glutamic acid).
ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b15319. Details regarding synthetic scheme, PVAm-T surface tension, PVAm-T intrinsic viscosity, QCM-D measurements, CV plots, and LbL properties (PDF)
Blocking Electron Transport in Polyelectrolyte Multilayers. A series of polyelectrolyte multilayer films was prepared where between 0 and 3 nonredox “blocking” bilayers of PVAm/ PSS were deposited on gold sulfate sensor surfaces, followed by two redox-active PVAm-T25L bilayers. Figure 9 shows the oxidative peak currents as a function of the number of blocking layers. Three pairs of nonredox blocking layers were required to completely prevent electron transport to the outermost two PVAm-T25L bilayers. This result is most likely a reflection of significant interpenetration of polymer chains between layers. A similar conclusion was made by earlier researchers working with other redox-active polyelectrolyte multilayers.11,32
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Robert H. Pelton: 0000-0002-8006-0745 Notes
The authors declare no competing financial interest.
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CONCLUSIONS PVAm-T spontaneously adsorbs onto aqueous cellulose surfaces, and in the presence of a primary oxidant such as bleach or
ACKNOWLEDGMENTS BASF Canada is acknowledged for funding this project through a grant to R.H.P. entitled “Understanding Cellulose Interactions 5627
DOI: 10.1021/acsami.6b15319 ACS Appl. Mater. Interfaces 2017, 9, 5622−5628
Research Article
ACS Applied Materials & Interfaces
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with Reactive Polyvinylamines”. Jie Yang is acknowledged for useful discussions of cyclic voltammetry experiments. Some measurements were performed in the McMaster Biointerfaces Institute funded by the Canadian Foundation for Innovation. R.H.P. holds the Canada Research Chair in Interfacial Technologies. L.S. holds the Canada Research Chair in Miniaturized Biomedical Devices.
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DOI: 10.1021/acsami.6b15319 ACS Appl. Mater. Interfaces 2017, 9, 5622−5628
