Letter pubs.acs.org/Langmuir
H2O2‑Induced Decomposition of Layer-by-Layer Films Consisting of Phenylboronic Acid-Bearing Poly(allylamine) and Poly(vinyl alcohol) Katsuhiko Sato, Mao Takahashi, Megumi Ito, Eiichi Abe, and Jun-ichi Anzai* Graduate School of Pharmaceutical Sciences, Tohoku University, Aramaki, Aoba-ku, Sendai 980-8578, Japan S Supporting Information *
ABSTRACT: Layer-by-layer (LbL) films were prepared by an alternate deposition of phenylboronic acid-bearing poly(allylamine hydrochloride) (PBA−PAH) and poly(vinyl alcohol) (PVA) on the surface of a quartz slide to develop thin films that can be decomposed in response to hydrogen peroxide (H2O2). The PBA−PAH/PVA films decomposed in the presence of H 2 O 2 ; the degree and kinetics of decomposition depend on the concentration of H2O2 and on the pH of the solution. For example, the film decomposition completely occurred in 30 min in 1.0 mM H2O2 solution at pH 7.4, while it took 180 min in 0.1 mM H2O2 solution. The H2O2-induced decomposition of the film can be explained in terms of the oxidative scission of the carbon− boron bond in PBA residues in the PBA−PAH chains. A potential use of the PBA−PAH/PVA films in developing H2O2-sensitive delivery systems was suggested.
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INTRODUCTION
Stimuli-sensitive layer-by-layer (LbL) films have been widely used to build sensors1 and delivery systems.2−5 To date, several studies focused on LbL films that are sensitive to temperature changes,6 electric potential,7,8 pH changes,9−11 and specific ions and molecules.12,13 Besides these, phenylboronic acid (PBA)modified LbL films have recently received much attention for the preparation of thin films and microcapsules that decompose in the presence of glucose.14−18 In particular, PBA derivatives selectively bind 1,2- and 1,3-diol compounds to form cyclic boronate esters;19−23 thus, this reaction can be exploited to form boronate ester-linked films. For instance, LbL films were successfully built from poly(acrylate) and poly(acrylamide) by coupling the PBA side chains with polysaccharide and poly(vinyl alcohol) (PVA), respectively.15−18 The glucoseinduced decomposition of these LbL films was studied, suggesting a potential use of the LbL films in the development of glucose-triggered delivery systems. In this work, we have found that the LbL films prepared with PBA-modified poly(allylamine hydrochloride) (PBA−PAH) and PVA decompose in response to H2O2 at physiological pH (Figure 1). The H2O2-induced decomposition of the LbL films was found to be rather rapid, e.g., the film decomposition was nearly complete in 30 min upon exposure to 1.0 mM H2O2 at pH 7.4. According to our data, the film decomposition is caused by the oxidative scission of the carbon−boron bond promoted by H2O2. The present communication reports the construction of PBA−PAH/PVA multilayer films and their H2O2-induced decomposition. © 2014 American Chemical Society
Figure 1. Schematic illustration of the H2O2-induced decomposition of LbL films composed of PBA−PAH and PVA.
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EXPERIMENTAL SECTION
PBA−PAH was synthesized by reacting poly(allylamine hydrochloride) (PAH) and 4-carboxyphenylboronic acid (4CPBA) in a H2O/dimethyformamide mixture (1:1 by volume) in the presence of 1-ethyl-3-(3-(dimethylamino)propyl)carbodiimide and N-hydroxysuccinimide. The reaction mixture was dialyzed in H2O and afforded PBA−PAH with 26% PBA residues (e.g., 26% of amino groups in PAH were substituted with 4CPBA). LbL films were prepared by an alternate deposition of PBA−PAH and PVA on the surface of a quartz slide for UV spectroscopic analysis of the film deposition. A quartz crystal microbalance (QCM) was also employed for the gravimetric analysis of the films. H2O2-induced decomposition of the films was studied by recording UV spectra of the films or by monitoring QCM frequency changes of the film-coated Received: May 6, 2014 Revised: July 21, 2014 Published: July 28, 2014 9247
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Letter
quartz resonators. Further details on the entire procedure of the synthesis of PBA−PAH, the preparation of films, and the evaluation of H2O2-induced film decomposition can be found in the Supporting Information.
It has been reported that PBA derivatives are converted to phenols through oxidative breakage of the carbon−boron bond promoted by H2O2.25 Fluorescence sensors sensitive to H2O226,27 and H2O2-induced delivery systems28 have been studied on the basis of the H2O2-induced conversion of PBA to phenols. These studies strongly suggest that the PBA−PAH/ PVA LbL films are decomposed by H2O2 because PBA−PAH and PVA are connected to each other through boronate ester bonds in the film. H2O2-promoted breakage of the carbon− boron bond of PBA moieties in the film would result in the decomposition of the film. In order to evaluate the response of the (PBA−PAH/PVA)10 film to H2O2, the film was immersed in H2O2 solutions of different concentrations (between 0.05 and 1.0 mM), and the film decomposition was monitored using UV spectra. Figure 3 shows the decomposition kinetics of the
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RESULTS AND DISCUSSION The UV spectra of the PBA−PAH/PVA films exhibited an absorption maximum at 245 nm, originating from PBA residues in PBA−PAH (Figure 2). While the absorbance at 245 nm
Figure 2. UV absorption spectra of (PBA−PAH/PVA)n films (n = 1− 10) prepared at pH 7.4. (Inset) Plots of absorbance of the LbL films at 245 nm as a function of the number of bilayers. Films were prepared using PVA solution with (■) and without (●) 10 mM fructose. The average values of three measurements are plotted. Figure 3. Decomposition of the (PBA−PAH/PVA)10 film as a function of time in the presence of H2O2 in (a−e) 10 mM HEPES buffer at pH 7.4, (f) 10 mM MES buffer at pH 6.0, and (g) 10 mM CHES buffer at pH 9.0. Concentrations of H2O2 are (a) 0, (b) 0.05, (c) 0.1, (d) 0.5, and (e−g) 1.0 mM.
increased with increasing deposition number, the absorbance followed the opposite trend upon deposition of PVA (Figure 2, inset) probably due to a partial desorption of PBA−PAH. The results displayed in Figure 2 thus suggest that boronate ester bonds between PBA−PAH and PVA promote the formation of PBA−PAH/PVA films. To further explore this point, we repeated the procedure to prepare PBA−PAH/PVA films using a PVA solution containing 10 mM fructose. Fructose, which contains 1,2- and 1,3-diol groups, is known to compete with PVA to bind the PBA moieties of PBA−PAH, with the binding constant of fructose to PBA at pH 8.5 being 560 M−1.19 Indeed, the formation of boronate ester-linked LbL films is rarely successful in the presence of fructose.16 Thus, as expected, deposition of the PBA−PAH/PVA film was severely suppressed by fructose (Figure 2, inset). Furthermore, the deposition of the PBA−PAH/PVA film was suppressed at acidic pH values (Figure S1). These results confirm that film deposition occurs through the formation of boronate ester bonds between PBA− PAH and PVA. In addition, LbL films composed of PBA−PAH and alginic acid (AGA) were prepared to be used as a reference film that is cross-linked with electrostatic bonds between positive charges in PAH chains and negatively charged AGA (Figure S2). The film deposition was studied using a quartz crystal microbalance (QCM) to estimate the amount of polymeric materials loaded in the films. Figure S3 plots the changes in resonance frequency (ΔF) of the dry films as a function of the number of bilayers. On the basis of these data, the weights of the (PBA−PAH/PVA)10 and (PBA−PAH/AGA)10 films were calculated to be 1.2 × 10−5 and 9.6 × 10−6 g cm−2, respectively. Thus, assuming that the density of the film is 1.2 g cm−3 in the dry state,24 the average thickness of the films can be estimated to be 100 and 80 nm for (PBA−PAH/PVA)10 and (PBA− PAH/AGA)10, respectively.
(PBA−PAH/PVA)10 film estimated from changes in the UV absorbance of the H2O2 solutions. The absorbance increased when the film was immersed in the solutions, suggesting that the LbL film decomposed and dissolved in the solution. The percentage of film decomposition increased with the concentration of H2O2 in the range of 0.05−1.0 mM. In contrast, in the solution with no H2O2, the film was stable. Thus, the film decomposition was faster in the higher-concentration H2O2 solutions. For example, the decomposition was completed within 30 min in the presence of 0.5 and 1.0 mM H2O2, while it took 180 min to complete the decomposition in 0.1 mM H2O2 solution. These results clearly showed that the (PBA−PAH/ PVA)10 film is sensitive to H2O2 and therefore decomposes in H2O2 solutions of pH 7.4. In contrast, the (PBA−PAH/AGA)10 film was found to be stable even at large concentrations of H2O2, such as 1.0 mM. This is not surprising because the formation of the (PBA−PAH/AGA)10 film is based on electrostatic interactions between PBA−PAH and AGA, which further supports the decomposition mechanism of the (PBA−PAH/PVA)10 film. The H2O2-induced decomposition of the (PBA−PAH/ PVA)10 film also occurred at different pH values, such as 9.0 and 6.0, although in the latter case the decomposition was slightly slower (Figure 3). In addition, we ascertained that the (PBA−PAH/PVA)10 film was stable in pH 3.0−9.0 solutions unless H2O2 was added, i.e., only 3 to 4% decomposition was observed without H2O2 at pH 9.0. Thus, these findings confirmed that H2O2 is the main cause of the decomposition of the (PBA−PAH/PVA)10 film and that pH changes play a minor 9248
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Figure 4. UV spectra of (a) (PBA−PAH/PVA)10 and (b) (PBA−PAH/AGA)10 films (A) before and (B) after immersion in a 1.0 mM H2O2 solution at pH 7.4 for 30 min. The UV spectra marked with C are those for the H2O2 solution in which the films were immersed.
role. It should be noted here that the (PBA−PAH/PVA)10 film was stable in acidic solutions even though the film could not be prepared at pH 3.0 and 5.0 (Figure S1), suggesting that the (PBA−PAH/PVA)10 films prepared at higher pH are kinetically stable to acidic decomposition. These results show the versatility of the films toward exposure to diverse environments. UV spectra of the (PBA−PAH/PVA)10 film and the H2O2 solution before and after film decomposition were recorded to investigate the mechanism of the process (Figure 4a). The (PBA−PAH/PVA)10 film-coated quartz slide exhibited the absorption maximum at 245 nm associated with PBA residues; the absorption band fully disappeared after immersion of the quartz slide in 1.0 mM H2O2 solution. In addition, the H2O2 solution, in which the film had been immersed, exhibited UV absorption bands at longer wavelengths (λ > 250 nm). This may be due to the formation of phenol-substituted PAH from PBA−PAH in the presence of H2O2.29 Figure 4b shows the absorption spectra of the (PBA−PAH/AGA)10 film before and after treatment in 1.0 mM H2O2 solution. As mentioned before, the (PBA−PAH/AGA)10 film did not decompose in the presence of H2O2, but the absorption maximum of the film shifted to longer wavelengths after the film had been immersed in 1.0 mM H2O2 solution. This finding suggested that PBA residues in the film were converted to phenolic derivatives by H2O2. QCM results provided further support for the H2O2-induced decomposition of the LbL films. Figure 5 shows a typical QCM result for the deposition and decomposition of solutions(PBA− PAH/PVA)5 film, i.e., ΔF first decreased upon alternate
depositions of PBA−PAH and PVA and, upon exposing the film to H2O2, follows an opposite trend. Thus, the QCM results also demonstrate the H2O2-induced decomposition of the (PBA−PAH/PVA)5 film.
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CONCLUSIONS We have demonstrated that the PBA−PAH/PVA films decompose in the presence of H2O2 at physiological pH. Our results also showed that the percentage of film decomposition and the decomposition kinetics mainly depend on the concentration of H2O2 and on the pH of the solution. We are confident that the PBA−PAH/PVA films presented in this contribution can be useful in building H2O2-sensitive delivery vehicles, such as microparticles and microcapsules. Furthermore, the PBA−PAH/PVA films could be employed together with enzymes, such as glucose oxidase (GOx) which generates H2O2 as a reaction product, in order to build glucose-triggered insulin delivery systems. GOx-modified polymer films have been successfully used for the in vivo delivery of insulin, in which the biological activity of insulin is not impaired by H2O2.30
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ASSOCIATED CONTENT
* Supporting Information S
Ddetailed experimental section and UV and QCM results. This material is available free of charge via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was partially supported by the Japan Society for the Promotion of Science Grants-in-Aid for Scientific Research (grant numbers 24659013 and 25460031).
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REFERENCES
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Figure 5. Typical QCM response for the deposition and decomposition of PBA−PAH/PVA LbL films. The concentration of H2O2 (ranging from 0.01 to 10 mM) is also shown. Measurements were carried out using a flow-through cell in 10 mM HEPES buffer at pH 7.4. 9249
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