Article pubs.acs.org/Langmuir
Hydrogen-Bonded Multilayer Films Based on Poly(N‑vinylamide) Derivatives and Tannic Acid Yukie Takemoto,† Hiroharu Ajiro,†,‡,§,∥,⊥ and Mitsuru Akashi*,†,‡ †
Department of Applied Chemistry, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan ‡ The Center for Advanced Medical Engineering and Informatics, Osaka University, 2-2 Yamada-oka, Suita, Osaka, 565-0871, Japan § Graduate School of Materials Science and ∥Center for Frontier Science and Technology, Nara Institute of Science and Technology, 8916-5 Takayama, Ikoma, Nara 630-0192, Japan ⊥ JST PRESTO, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan S Supporting Information *
ABSTRACT: Layer-by-layer (LbL) assembly based on hydrogen-bonding interactions is generating great interest for biomedical applications because it is composed of neutral polymers, while LbL assembly based on electrostatic interaction requires polycations which may induce toxicity issues. As a neutral polymer, poly(N-vinylamide), which has low toxicity compared to poly(acrylamide), has the potential to fabricate LbL thin films via hydrogen-bonding interactions. Herein we report interpolymer complexes of poly(N-vinylamide)s and natural polyphenol tannic acid to form the multilayered thin film. Poly(N-vinylformamide) and poly(N-vinylacetamide), which are water-soluble and insoluble in acetonitrile, could not form complexes with TA in water. On the other hand, N-alkylated poly(N-vinylamide) such as poly(N-ethyl-N-vinylformamide) and poly(N-methyl-N-vinylacetamide) was soluble in acetonitrile and allowed the LbL assembly to proceed with TA. Furthermore, the QCM frequency shift with films composed of poly(N-ethyl-N-vinylformamide) and TA were stable in water, while those of poly(N-methyl-N-vinylacetamide) and TA were instable in water, possibly because formamide has lower steric hindrance compared to acetamide to allow stronger hydrogen-bonding interactions to take place. Thus, LbL assembly reactions with alkylated poly(N-vinylamide)s and TA were investigated and revealed that poly(N-ethyl-N-formamide) and TA, which are watersoluble, effectively interacted with one another to generate water-stable hydrogen-bonded multilayered films.
1. INTRODUCTION Layer-by-layer (LbL) assembly, which was first reported by Decher,1 is a simple and facile technique for thicknesscontrolled ultrathin polymer films on a substrate by the alternate immersion of substrates into interactive polymer solutions. LbL films have been attracted due to their potential in surface design 2 and have expanded to biomedical applications such as drug delivery devices,3,4 antifouling coating materials,5,6 tissue engineering,7,8 and so on. Until today, there have been many studies based on electrostatic interaction,1,9 van der Waals,10,11 and hydrogen-bond interactions.12−15 In particular, LbL films based on hydrogen bond interactions are attractive for the aforementioned biomedical applications because toxic polycations are not required, in contrast to electrostatic interactions which are widely studied for LbL assembly. As in the first report of hydrogen-bonded LbL films, polyaniline can be assembled with nonionic polymers such as poly(N-vinylpyrollidone), poly(vinyl alcohol), poly(vinylacrylamide), and poly(ethylene glycol) through hydrogen bond interactions.12 Thus, suitable polymers for hydrogenbonded LbL assembly are nonionic polymers, which are less © XXXX American Chemical Society
influenced by the pH or ionic strength in solutions as compared to electrostatic interactions. Tannic acid (TA), a naturally occurring adstringentia, has recently attracted attention because of its unique potential as a coating material. It forms monolayers with ferric ions16,17 and also forms multilayered films with neutral polyamides such as poly(N-vinylpyrrolidone), poly(N-vinyl caprolactam), poly(Nisopropylacrylamide), and poly(2-alkyloxazoline)18−22 by hydrogen bond interactions. These multilayered films are useful for living-cell coating to maintain high viability, possibly because of its low toxicity and appropriate permeability of nutrients or inducer molecules.23−26 For the construction of the hydrogen-bonded multilayered films of TA, an investigation of the chemical structure of polyamides is surely significant. Poly(N-vinylamide)s are one kind of nonionic polymer and have reversed the amide group to poly(acrylamide). NVinylamide monomers exhibit low toxicity as compared to Received: March 14, 2015 Revised: May 28, 2015
A
DOI: 10.1021/acs.langmuir.5b00767 Langmuir XXXX, XXX, XXX−XXX
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Langmuir acrylamide;27 therefore, poly(N-vinylamide)s have potential applications as functional materials in biomedical fields. The synthesis approach of monomers or polymers and their polymerization reactivity were reported in relatively recent years.28,29 Our research group has focused on poly(Nvinylamide) for decades for its potential application to biomedical materials and have reported the synthesis route of monomers,30 the polymerization of N-vinylamide monomers,31 and drug release materials.32,33 It is noteworthy that poly(Nvinyl amide)s can be modified to other structures or properties due to their structure, such as cationic poly(N-vinylamine) by hydrolysis34 or N-alkylated polymers.35 N-alkylated polymers are especially attractive in that the chemical structure around the amide group will influence hydrogen bond interactions, while well-known poly(N-vinylpyrrolidone) has less structural variety. In this study, we aimed to construct LbL-assembled films by hydrogen bonds between various TA and alkylated poly(Nvinylamide)s. Poly(N-vinylamide) has the potential to be optionally modified with an alkyl group to control the hydrophobicity. Poly(N-vinylformamide) (PNVF) and poly(N-vinylacetamide) (PNVA) are water-soluble and insoluble to acetonitrile. We modified PNVF and PNVA with alkyl groups to suppress interpolymer interactions between poly(Nvinylamide)s themselves and to make them more hydrophobic in nature, keeping the water-soluble characteristics. N-alkylated poly(N-vinylamide) such as poly(N-ethyl-N-vinylformamide) (PNEF) and poly(N-methyl-N-vinylacetamide) (PNMA) effectively formed hydrogen-bonded complexes with TA in acetonitrile. Furthermore, prepared multilayered films composed of PNEF and TA were stable in water, while those of PNMA and TA were unstable in water, possibly because formamide groups have lower steric hindrance compared to acetamide groups to allow for stronger hydrogen bond interactions. These prepared LbL-assembled films are composed of nonionic and low toxicity poly(N-vinylamide) derivatives and natural polyphenol and are expected to be useful to the cell-surface coating for the preservation of livingcell functionalities.
NaCl(aq) (0.2 mol/L) was used as an eluent at 1.0 mL/min. MS were measured with a JMS-700 system (JEOL Ltd., Japan). 2.2. Synthesis of N-Ethyl-N-vinylformamide (NEF). In a glass flask, NaH (7.2 g, 180 mmol) was placed and washed with anhydrous THF (20 mL) three times under nitrogen, and 40 mL of anhydrous DMF was introduced. NVF (10.7 g, 150 mmol) in anhydrous DMF (20 mL) was slowly added at 0 °C. After stirring for 6 h at room temperature, iodoethane (6 mL, 150 mmol) was slowly added with a syringe at 0 °C. The reactor was warmed up to room temperature to stir another 12 h. Water was introduced into the reaction mixture to terminate the reaction, and the mixture was extracted in diethyl ether and water, washing the organic layer successively with brine solution. The organic layer was combined and dried with anhydrous MgSO4 and then further purified with a silica gel column using hexane/diethyl ether as the eluent. The mixture of NEF and hexane was obtained as a liquid (15.2 g, purity 34.1%, 69.7% yield). Rf = 0.3 (hexane/ethyl acetate = 1/2). 1H NMR (CD3CN, 400 MHz) δ: 1.08 (t, 3H, −CH3), 3.52−3.60 (m, 2H, N−CH2−), 4.37−4.71 (m, 2H, CH2CH−N), 6.63−6.69 (dd, J = 8 and 16 Hz, 0.75H, CH2CH−N, cis), 7.06− 7.12 (dd, J = 8 and 16 Hz, 0.25H, CH2CH−N, trans), 8.13, 8.25 (s, 1 H, (CO)H). EI-MS: [M]+ = 99. 2.3. Polymerization of NVF, NVA, NEF, and NMA. Poly(Nvinylformamide) (PNVF), poly(N-vinylacetamide) (PNVA), PNEF, and PNMA were obtained by free-radical polymerization. The procedure of typical radical polymerization was as follows. NEA or NMA, toluene, and AIBN were combined in a 20 mL glass tube. The reactor was capped, and then N2 bubbling was achieved for 2 min. The reaction mixture was heated to 60 °C to start the polymerization. After the required time, it was cooled to room temperature, and the reaction product was solvated with ethanol and was poured into 400 mL of diethyl ether. The polymer was recovered by decantation. The obtained polymer was dried under vacuum at 25 °C for 3 h. The number-average molecular weights of PNVF, PNVA, PNEF, and PNMA were 10 300, 92 200, 3200, and 30 000, respectively. 2.4. DLS Measurements. The size distributions of the polymer solutions were measured by dynamic light scattering (DLS) using a Zetasizer Nano ZS (Malvern Instruments, Malvern, U.K.). 2.5. LbL Assembly on a Quartz Crystal Microbalance (QCM). The quantitative analysis of LbL assembly using a QCM was performed essentially as reported in our previous studies. An AT-cut quartz crystal with a parent frequency of 9 MHz was purchased from USI (Fukuoka, Japan). The frequency was monitored by an Iwatsu frequency counter (model 53131 A). The quartz crystal (9 mm diameter) was coated on both sides with mirrorlike polished gold electrodes (4.5 mm diameter). The amount of polymer adsorbed, Δm, could be calculated by measuring the frequency decrease in the QCM, ΔF, using Sauerbrey’s equation as follows
2. EXPERIMENTAL SECTION 2.1. Materials and Measurements. N-Vinylformamide (NVF) was purchased from Tokyo Chemical Industry Co. Ltd. (Japan) and was used after distillation. N-Vinylacetamide (NVA) was purchased from Showa Denko (Japan) and was recrystallized from toluene/ hexane (1/1) and dried under vacuum at room temperature. Iodoethane and anhydrous tetrahydrofuran (THF) were purchased from Tokyo Chemical Industry Co. Ltd. (Japan) and used as received. Anhydrous N,N-dimethylformamide (DMF) and N-methylacetamide were purchased from Sigma-Aldrich Japan. Tannic acid (TA), sodium hydride (NaH) 60% in oil, diethyl ether, and 2,2′-azobis(isobutyronitrile) (AIBN) were purchased from Wako Pure Chemical Industries, Ltd. (Japan). Poly(N-vinylpyrrolidone) (PVP, Mw = 55 000) was purchased from Sigma-Aldrich Japan. 1 H NMR spectra were recorded with a JEOL JNM-GMX400 system. Attenuated total reflection (ATR) IR spectra were obtained with a Spectrum 100 FT-IR spectrometer (PerkinElmer, Japan). The interferograms were coadded 16 times and Fourier transformed at a resolution of 4 cm−1. The number-average molecular weights and their distribution were measured by size exclusion chromatography (SEC). A ChromNAV system (JASCO Corporation, Japan) using AS-2055 and RI-2031 was employed with poly(ethylene oxide) (PEO) standards at room temperature. Two commercial columns (TSK gel G3000PW and TSK gel G5000PW) were connected in series, and
−ΔF =
2F0 2 Δm A ρq μq
where F0 is the parent frequency of the QCM (9 × 106 Hz), A is the electrode area (0.159 cm2), ρq is the density of the quartz (2.65 g cm−3), and μq is the shear modulus (2.95 × 1011 dyn cm−2). Before the assembly measurements, the QCM electrodes were treated with piranha solution (H2SO4/40% H2O2 aqueous solution = 3:1 by volume) for 1 min, followed by rinsing with pure water and drying with N2 gas in order to clean the surface. The cleaned QCM chip was then immersed in a polymer solution (10 unit mM, acetonitrile) for 5 min, rinsed with acetonitrile, and then dried under N2 gas. The frequency decrease was measured. The QCM chip was next immersed into TA solution (10 unit mM of hydroxyl group, acetonitrile). This alternative deposition was repeated until the desired assembly layers were built up. Aqueous solutions at pH 2, 7, and 12 were prepared with 0.01 N HCl(aq) and 0.01 N NaOH(aq) for pHdepending LbL assembly. A polymer aqueous solution (10 unit mM; pH 2, 7, 12) and TA aqueous solutions (10 unit mM of the hydroxyl group; pH 2, 7, 12) were prepared, and then the LbL assembly was conducted in the same manner. B
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3. RESULTS AND DISCUSSION Chemical structures of polymers used in this study are shown in Figure 1. In order to avoid intrapolymer hydrogen bond
PNVF aqueous solution or PNVA aqueous solution was added to the TA aqueous solution to obtain the aqueous mixtures. The appearances of these aqueous mixtures were transparent. The mean size of the polymers was not dependent on the polymer concentration in both cases of PNVF and TA (Figure 2a) and PNVA and TA (Figure 2b). It is considered that there was almost no complexation between poly(Nvinylamide) and TA in water. In order to build the complex thorough hydrogen bonding interaction, poly(N-alkyl-N-vinylamide) such as PNEF and PNMA was investigated. Because PNEF and PNMA have additional hydrophobic moieties for PNVF and PNVA, the hydrogen-bonding interaction between polymers and water is reduced and that interaction between polymers and TA will be induced in aqueous media. In addition to the additional hydrophobic circumstance, the solubility of polymers also changes: PNVF and PNVA are not soluble to acetonitrile, while PNEF and PNMA are soluble to acetonitrile. Thus, the complexation can be investigated not only in the aqueous phase but also in the acetonitrile phase. The aqueous mixture of PNEF and TA was transparent, and the mean size was almost the same as that of PNVF and TA (Figures 3a and 2c). It indicates that the alkylation of PNVF is not as effective for complex formation with TA in aqueous media. The complex formation of PNEF and TA effectively occurred in acetonitrile media because the mean size became larger according to the polymer concentration (Figure 3b−d). When the polymer concentration is 1 (Figure 3b), 5 (Figure 3c), and 10 unit mM (Figure 3d), the mean size became 50, 109, and 181 nm, respectively. The mixture of PNMA and TA was also tested in the aqueous media and in the acetonitrile media (Figure 3b). In the case of PNMA, the aqueous mixture of PNMA and TA was somewhat turbid and the mean size (Figure 3e) was also larger compared to the case of PNVA (Figure 2d). PNMA probably has a larger molecular weight, and thus complexation easily occurred compared to PNEF. The mixture of PNMA and TA in acetonitrile was obviously turbid, and the mean size was highly off-scale (Figure 3f). De Geest and Hoogenboom reported that poly(2-oxazoline)s and TA form a complex even in water.22,26 Sukhishvili et al. also reported that poly(N-vinylpyrrolidone) and TA form a complex in water.20 It is clearly assumed that the hydrogen -bonding interaction between poly(N-alkylated vinylamide) and TA is weak compared to that of poly(2-oxazoline) and TA and that of poly(N-vinylpyrrolidone) and TA. FT-IR spectra were investigated to confirm the hydrogenbonding interaction between poly(N-vinylamide)s and TA
Figure 1. Chemical structures of PNVF (a), PNVA (b), PNEF (c), PNMA (d), and TA (e).
formation of PNVF or PNVA and in order to introduce interpolymer hydrogen bond complexation composed of poly(N-vinyl alkylamide) and TA, protons linked to nitrogen atoms in NVA and NVF were alkylated with methyl or ethyl groups. N-alkylated N-vinylamides would be hydrophobic compared to PNVA and PNVF; therefore, it would be advantageous for hydrogen bond formation with TA. The results from the radical polymerization of N-vinylamides are listed in Table S1. The average molecular weights of PNEF (Table S1, entry 4) and PNMA (Table S1, entry 3) were 3200 and 30 000, respectively, which is lower than for PNVF and PNVA whose molecular weights are 10 300 (Table S1, entry 1) and 92 200 (Table S1, entry 2), respectively. This is possibly because the vinyl moiety is sterically hindered by the alkyl moiety linked to the nitrogen atom during radical polymerization. N-Ethyl-N-vinylacetamide (Table S1, entry 5) was not polymerized because of steric hindrance. It is well known that complex formation in the solution relates to multilayered assembly in the case of the polyelectrolyte complex.36,37 The strong polycation and polyanion form a polyelectrolyte complex in solution, as well as multilayered deposition. On the other hand, the weak polycation and polyanion do not form multilayered films although they form complexes in solution. Also in the hydrogen-bonded multilayered deposition of poly(N-vinylamide) and TA, it is surely significant to understand the complex formation behavior in the solution and the assembly behavior on the substrates. At first, the complexation of poly(Nvinylamide)s and TA in the solution was confirmed by DLS.38
Figure 2. DLS measurements of the mixture solution of PNVF, PNVA, and TA in water with various concentrations. The mixtures of PNVF and TA in water with 1 (a), 5, (b), and 10 unit mM (c). Mixture of PNVA and TA in water with 1 (d), 5 (e), and 10 unit mM (f). C
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Figure 3. DLS measurements of mixtures composed of PNEF, PNMA, and TA in water or in acetonitrile. Ten unit mM PNEF and 10 unit mM TA in water (a), 1 unit mM PNEF and 1 unit mM TA in acetonitrile (b), 5 unit mM PNEF and 5 unit mM TA in acetonitrile (c), 10 unit mM PNEF and 10 unit mM TA in acetonitrile (d), 1 unit mM PNMA and 1 unit mM TA in water (e), and 1 unit mM PNMA and 1 unit mM TA in acetonitrile (f).
1656 cm−1 after complexation with TA (Table 1). These results fit to the reported data39 that PVP in chloroform is observed at
(Figure 4). Carbonyl groups (CO) in PNEF and PNMA, with an absorption band around 1620−1640 cm−1, are sensitive
Table 1. Absorption Peaks of CO Stretching CO stretching (cm−1) entry
conditions
PNEF
PNMA
PVP
1 2 3
solid state solution state in acetonitrile complex with TA in acetonitrile
1657 1672 1654
1629 1642 1611
1667 1680 1656
1674 cm−1 and then that in ethylene glycol it is observed at 1661 cm−1 because the carbonyl group interacts with the hydroxyl group of ethylene glycol. This tendency is almost the same as PNEF and PNMA. The absorption band of hydroxyl groups of tannic acid is observed at 3334 cm−1 in the solid state and 3344 cm−1 in acetonitrile solution. The hydroxyl groups of tannic acid are possibly bound to the carbonyl group of TA in the dried state. In acetonitrile, the binding becomes loose and the absorption band is shifted to higher wavenumbers. After complex formation with PNEF or PNMA, the absorption band was too weak but weakly observed at 3261 and 3243 cm−1 (Figure S1, S2). This result supports the hydrogen bond formation between PNEF and TA or PNMA and TA. Next, we prepared thin films using the above-mentioned hydrogen bonding interaction. Figure S3 shows the LbL assembly of poly(N-alkyl-N-vinylamide) and TA in water. The frequency shift of the QCM substrate immersed in PNEF and TA did not change during LbL assembly, as implied in the DLS analysis. It also did not change in the case of PNMA and TA, although complex formation was observed in the aqueous phase in the DLS analysis. It is considered that interactions between these molecules are not enough for their deposition onto substrates. On the other hand, PVP as a well-known poly(Nvinylamide) is reported to form a complex with TA even in water. This result indicates that multilayered films composed of PNEF or PNMA and TA are formed via weaker hydrogen bond interactions compared to that of PVP and TA. Then, the LbL assembly of poly(N-alkylated-N-vinylamide) and TA in acetonitrile was measured (Figure 5). Both systems showed supralinear growth. The deposition amounts of poly(N-alkylated-N-vinylamide) and TA per unit area were estimated from Sauerbrey’s equation to 3.6 and 2.1 μg/cm2 for PNEF and TA and 4.4 and 2.0 μg/cm2 for PNMA and TA, respectively. Therefore, the weight ratios assembled from poly(N-vinylamide) and TA on substrates were estimated to be
Figure 4. FT-IR spectra of polymers and TA. (a) TA in the dried state, (b) TA in acetonitrile, (c) PNEF in the dried state, (d) PNEF in acetonitrile, (e) complex of PNEF and TA in acetonitrile, (f) PNMA in the dried state, (g) PNMA in acetonitrile, and (h) complex of PNMA and TA in acetonitrile.
to hydrogen bonds. Also, TA has carbonyl groups, with absorption bands at 1699 cm−1 in the dried state (Figure 4a) and at 1732 cm−1 in acetonitrile solution (Figure 4b). The absorption band at 1607 cm−1 is derived from the phenol group in TA. The absorption bands of PNEF in the solid state were observed at 1657 cm−1 (Figure 4c), while in acetonitrile they were observed at 1672 cm−1 (Figure 4d) because the carbonyl group is free without interpolymer interaction. After forming a complex with TA in acetonitrile, the absorption band of CO is lowered to 1654 cm−1 (Figure 4e), indicating that the CO group is engaged in the hydrogen-bonding interaction. In the same manner, PNMA in the solid state showed absorption bands at 1629 cm−1 (Figure 4f), shifted to 1642 cm−1 in acetonitrile (Figure 4g), and then shifted to 1611 cm−1 after complex formation with TA in acetonitrile (Figure 4h). When compared to the case of poly(N-vinylpyrrolidone) (PVP), the absorption band of CO was observed at 1667 cm−1 in the dried state, at 1680 cm−1 in acetonitrile, and at D
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Figure 5. QCM frequency shift during LbL assembly (n = 3). The PNEF (a) and PNMA (b) in acetonitrile and TA in acetonitrile were alternately assembled on a QCM substrate for a 12-step assembly at a concentration of 10 unit mM at 25 °C. The odd-numbered step is a PNEF or PNMA step (opened circle), and the even-numbered step is a TA step (closed circle). The QCM substrate was rinsed with water for 5 min (13th step), 5 min (14th step), 10 min (15th step), and 20 min (16th step) after 12-step assembly (gray circle). The error bar shows the standard deviation of the frequency shift of each assembling step.
Figure 6. QCM frequency shift during LbL assembly (n = 2, the average was shown). QCM substrates were immersed in PNEF or TA in pH 2 (a), PNEF or TA in pH 7 (b), PNEF or TA in pH 12 (c), PNMA or TA in pH 2 (d), PNMA or TA in pH 7 (e), and PNMA or TA in pH 12 (f) for a 12step assembly at a concentration of 10 units mM at 25 °C. Aqueous solutions at various pH values were prepared with 0.01 N NaCl(aq) and 0.01 N HCl(aq). The odd-numbered step is PNEF or PNMA (open squares, circles, and triangles), and the even-numbered step is TA (closed squares, circles, and triangles).
compared to that in Figure 6, in which measurements were conducted in distilled water, because the ionic strength of these aqueous solutions under various pH conditions is higher than in distilled water. At pH 12, there was almost no frequency change. This is because TA is deprotonated and does not interact with the carbonyl group of PNEF. LbL assembly of PNMA and TA showed the same behavior (Figure 6d−f), although the assembled amount is lower than that of PNEF and TA possibly because the carbonyl group derived from the acetamide moiety is at a disadvantage for the hydrogen-bonding interaction. As another possible interaction for LbL assembly, the dipole−dipole interaction should be taken into account. The carbonyl group of poly(N-vinylamide) and that of tannic acid may interact thorough the dipole−dipole interaction. The C O stretching band will be shifted to lower wavenumber through the dipole−dipole interaction, which is the same results as in Figure 4. Furthermore, this interaction is enhanced under the pH 2 condition because of the protonated carbonyl oxygen and the cationized carbonyl carbon, which agree with Figure 6. However, this is probably not the main driving force in this case. As shown in Figures S1 and S2, the absorbed band of O− H stretching of TA was slightly shifted to lower wavenumber, which suggests the association of the OH group in complex formation. Furthermore, the poly(N-vinylamide)s themselves did not generate multilayers as well as tannic acid itself. The
66/34 (PNEF/TA) and 65/35 (PNMA/TA), which means that the ratios of carbonyl groups/hydroxyl groups were 57/43 and 56/44. Thus, almost the same number of carbonyl groups of poly(N-vinylamide) and hydroxyl groups of TA would have interacted with each other. The LbL assembly was also observed by UV−vis absorption spectra (Figure S4). The absorbances at 210 nm derived from poly(N-vinylamide) and 275 nm derived from tannic acid were gradually increased during LbL assembly. Furthermore, the obtained multilayered film of PNEF and TA was stable after a 40 min immersion in water. This result implies strong polymer−polymer interactions. Although the hydrogen-bonding interaction between PNMA and TA was energetically stable compared to that between PNEF and TA, PNMA has sterically hindered carbonyl groups compared to PNEF. It is valuable that components PNEF, PNMA, and TA could be assembled in acetonitrile to make poorly stable films in water, although they could not be assembled in water. Next, the pH-dependent assembly of PNEF and TA was investigated in aqueous media. It is certain that the LbL assembly of PNEF and TA will be sensitive to pH because TA is the weak acid (pKa ≈ 10). Figure 6 shows the LbL assembly of PNEF and TA under various pH conditions. Even in the aqueous media, LbL assembly of PNEF and TA showed linear growth at pH 2 (Figure 6a). At pH 7, the amount of the assembled multilayered film was smaller than at pH 2. The amount of the assembled multilayered film at pH 7 was larger E
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Langmuir alternative layer-by-layer is necessary to form films, suggesting hydrogen bonding to be the main driving force. The morphologies of multilayered film on QCM substrate were observed by SEM. Both PNEF/TA LbL and PNMA/TA LbL seemed to be constructed thorough their aggregation with each other (Figure S5). Especially PNEF/TA LbL was relatively smooth surface compared to PNMA/TA LbL. Those results agreed with DLS data that is the mixture of PNMA and TA are in acetonitrile has the larger diameter compared to that of PNEF and TA in acetonitrile. These multilayered films would be valuable for the detection of Fe(III) ions in water because it is known that TA and Fe form a complex to show black color. Recently, the loading capacity of Fe(III) ions in LBL films composed of poly(ethylenimine) and tannic acid are studied, which we aim to apply to the detection of Fe3+ ions with polymer gels.40 It is surely expected that the multilayered film in this study also would be effective for the detection of Fe(III) ions. When a multilayered film of poly(N-vinylamide)s and TA was immersed in an Fe3(SO)4 aqueous solution (1 mM), the film became relatively black (Figure S6c−e), although it could not be measured by the UV−vis absorption spectrum (Figure S6a,b). It is considered that the hydroxyl groups of TA which are unused for film formation would capture the Fe(III) ion to show the black color. The multilayered film would be modified and optimized for the sensing system in the near future.
Present Addresses
(Y.T.) Health & Crop Sciences Research Laboratory, Sumitomo Chemical Company Limited, 2-1 Takatsukasa, 4Chome, Takarazuka, Hyogo 665-8555, Japan. (M.A.) Graduate School of Frontier Biosciences, Osaka University, 2-1 Yamada-oka, Suita 565-0871, Japan. Funding
This work was partially supported by a Grant-in-Aid for Scientific Research (S) (23225004) and a Grant-in-Aid for Challenging Exploratory Research (26620182) from the Ministry of Education, Culture, Sports, Science and Technology. This work was also supported in part by the MEXT project, “Creating Hybrid Organs of the Future” at Osaka University. This work was partially supported by JST PRESTO “Molecular Technology” with Prof. Takashi Kato. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are grateful for fruitful discussions with Drs. T. Kida, M. Matsusaki, and T. Akagi.
4. CONCLUSIONS We first prepared ultrathin films by LbL assembly composed of poly(N-alkyl-N-vinyl alkylamide) and natural polyphenol TA, based on hydrogen bonds. The complexations of PNEF and TA and PNMA and TA in acetonitrile were observed by DLS measurements. FTIR analyses revealed that complex formation is conducted by hydrogen bond interactions between carboxyl groups of PNEF or PNMA and hydroxyl groups of TA. LbL assemblies of PNEF and TA and PNMA and TA successfully progressed on QCM substrates. The stability of films in water was completely different depending on the structure of poly(Nalkyl-N-vinylamide). From the QCM frequency shift, the multilayered film composed of PNEF and TA was stable in water, while that of PNMA and TA was unstable in water, although every component was soluble in water. These results indicate that PNEF has an advantage in hydrogen bond formation compared to PNMA. These multilayered films would be hopeful as film sensors for the detection of Fe(III) ions or the coating materials of living cells.
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ASSOCIATED CONTENT
S Supporting Information *
Polymerization of N-vinylamides. Full-range FT-IR spectra of poly(N-vinylamide)s and tannic acid. LbL assembly of poly(Nvinylamide)s and tannic acid in distilled water. UV-vis absorption spectra during LbL assembly of poly(N-alkyl-Nvinylamide)s and tannic acid. SEM images of multilayered films on QCM substrates. Capturing Fe(III) ions with multilayered films composed of poly(N-alkyl-N-vinylamide)s and tannic acid. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b00767.
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DOI: 10.1021/acs.langmuir.5b00767 Langmuir XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.langmuir.5b00767 Langmuir XXXX, XXX, XXX−XXX
