Formation of Sulfonated Aromatic Ketone Chromophores within

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Formation of Sulfonated Aromatic Ketone Chromophores within Styrene-Acrylic Acid Copolymers and Their pH-Responsive Color Change Wenguang Leng, Shuxue Zhou, Bo You, and Limin Wu* Department of Materials Science and the Key Laboratory of Molecular Engineering of Polymers of MOE, Advanced Materials Laboratory, Fudan University, Shanghai 200433, PR China Received September 24, 2010. Revised Manuscript Received October 17, 2010 Styrene-acrylic acid copolymer was synthesized via solution polymerization and then sulfonated by concentrated sulfuric acid. This sulfonated copolymer displays an obvious pH-responsive color change in aqueous solutions (1 g/L) from yellow (pH 8). This response is as quick as for smallmolecule pH indicators such as methyl orange and phenolphthalein within 1 s and can be reversible. The lowest critical concentration of this pH-responsive copolymer solution is around 0.1 g/L, which is about 50-500 times the necessary amount used for conventional small-molecule pH indicators. An intramolecular cyclization mechanism between a neighboring carboxyl group and a benzene ring to form a sulfonated aromatic ketone is proposed to explain this pHresponsive color change behavior. The molar ratio of 1:1 for styrene to acrylic acid is the most favorable for forming neighboring benzene and carboxyl group pairs in the copolymer chains and subsequently yields sulfonated aromatic ketone chromophores at full capacity.

Introduction Over the past two decades, the development and application of chemical sensors have grown rapidly.1 Among all of these chemical sensors, pH sensors have received the most attention because of the importance of pH measurements in scientific research and practical applications. Conventional pH sensors are prepared by immobilizing pH indicators of small molecules onto/into solid substrates through adsorption, covalent binding, or entrapment. These methods are either complicated or unreliable because the indicators are likely to leach out. To overcome these defects, various sorts of pH-responsive polymers have been developed because they can offer several advantages such as (i) simple fabrication of the sensors, (ii) flexible control over the structure and properties by introducing certain functional groups, (iii) easy processing. Most of these pH-responsive behaviors are based on acidic groups, such as carboxylate, sulfonate, sulfate, and sulfonamide, and a pH-induced conformational transition.2-5 For example, Hong et al.6 successfully attached fluorescence resonance energy transfer (FRET) donors and acceptors to both ends of a pH-sensitive polymeric linker. The polymeric linker showed drastic conformational change with pH variation from the expanded coil state to the collapsed globule state, which resulted in an abrupt on-and-off feature in the FRET efficiency as a result of the change in the distance between FRET donors and acceptors. Benrebouh et al.2 synthesized a copolymer containing N-isopropylacrylamide- and cholic acid-derived methacrylate monomers and found that its solubility and lower critical solution temperature changed with pH. *Corresponding author. E-mail: [email protected].

(1) Lin, J. Trends Anal. Chem. 2000, 19, 541–552. (2) Benrebouh, A.; Avoce, D.; Zhu, X. X. Polymer 2001, 42, 4031–4038. (3) Kathmann, E. E. L.; White, L. A.; McCormick, C. A. Macromolecules 1997, 30, 5297–5304. (4) Hong, S. W.; Kim, K. H.; Huh, J.; Ahn, C.-H.; Jo, W. H. Chem. Mater. 2005, 17, 6213–6215. (5) Hong, S. W.; Jo, W. H. Polymer 2008, 49, 4180–4187. (6) Hong, S. W.; Ahn, C. H.; Huh, J.; Jo, W. H. Macromolecules 2006, 39, 7694– 7700.

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Certain conductive polymers, such as polyaniline,7,8 substituted polyaniline,9,10 and polypyrrole,11 can change color as a function of pH and thus can also be used for pH sensing. For example, Aquino-Binag et al.12 prepared hydroquinone-functionalized polypyrrole thin-film pH sensor with a good potentiometric response of 46.0 mV 3 pH-1. However, these conductive pHresponsive polymers also suffer from some shortcomings such as (i) interference from other ions and reducing and oxidizing agents and (ii) the need for pre/reconditioning (e.g., in 0.1 M HCl) before each measurement to overcome the hysteresis due to the conformational change in polymer chains occurring with pH changes. Recently, Malik13 synthesized sulfonated styrene-divinylbenzene macroporous resins with pH-dependent color changes. An acidcatalyzed acylation of phenyl rings with -COOH groups and sulfonation of the phenyl rings was proposed to explain this phenomenon. However, the color change from a darker color (in neutral water) to a brighter color (in 0.1 M NaOH solution) in this case was too feeble to be applied as a pH indicator. In this letter, we report a much simpler pH-responsive polymer and its preparation method. In this approach, a styrene-acrylic acid copolymer was first synthesized via solution polymerization and then evaporated to remove the solvent, followed by sulfonation in excess concentrated sulfuric acid. It was very interesting to find that this sulfonated hydrophilic copolymer exhibited an obvious pH-responsive color change in aqueous solutions from yellow (pH 8) and that this response was as quick as for conventional small-molecule pH indicators such as methyl orange (7) Ge, Z.; Brown, C. W.; Sun, L.; Yang, S. C. Anal. Chem. 1993, 65, 2335–2338. (8) Grummt, U. W.; Pron, A.; Zagorska, M.; Lefrant, S. Anal. Chim. Acta 1997, 357, 253–259. (9) Sotomayor, M. D. P. T.; de Paoli, M. A.; Oliveira, W. A. D. Anal. Chim. Acta 1997, 353, 275–280. (10) Pringsheim, E.; Terpetschnig, E.; Wolfbeis, O. S. Anal. Chim. Acta 1997, 357, 247–252. (11) de Marcos, S.; Wolfbeis, O. S. Anal. Chim. Acta 1996, 334, 149–153. (12) Aquino-Binag, C. N.; Kumar, N.; Lamb, R. N. Chem. Mater. 1996, 8, 2579–2585. (13) Malik, M. A. Ind. Eng. Chem. Res. 2009, 48, 6961–6965.

Published on Web 10/29/2010

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and phenolphthalein and could be reversible. Furthermore, this response was not interfered with by reducing or oxidizing agents as was the case for polyaniline. An intramolecular cyclization mechanism between neighboring carboxyl group and benzene ring to form a sulfonated aromatic ketone was proposed to explain this phenomenon.

Experimental Section Styrene, acrylic acid, concentrated sulfuric acid (98% mass fraction), fuming nitric acid, hydrochloric acid, acetic acid, sodium hydroxide, sodium carbonate, ammonia, and tetrahydrofuran (THF) were purchased from Shanghai Chemical Reagent Co. (China) and were used as received. Azobisisobutyronitrile (AIBN) and butanone were purchased from Aladdin Reagent Co. (China) and were used as received. Anthrone (an aromatic ketone derivative with a 97% mass fraction) was purchased from Aldrich and used as received. Deionized water was applied for all treatment processes. The styrene-acrylic acid copolymer with a molecular weight of around 16 000 g/mol as determined by GPC was synthesized by solution polymerization. In a typical procedure, 10 g of styrene, 7.0 g of acrylic acid, 80 g of butanone, and 0.2 g of AIBN were placed into a 250 mL four-necked flask equipped with a mechanical stirrer, a thermometer with a temperature controller, an N2 inlet, a Graham condenser, and a heating mantle. This mixture was deoxygenated via bubbling nitrogen gas at room temperature for 30 min and then stirred at 70 C for 8 h. Ten grams of the as-synthesized copolymer solution (∼20 wt % solid content) was treated with a rotary evaporation meter at 65 C to remove the solvent. For the sulfonation reaction, the copolymer gel was then mixed with 20 mL of concentrated sulfuric acid under constant stirring at room temperature for 40 h to obtain a dark-red slurry. The UV-vis spectra were obtained in a UV-vis spectrophotometer (Mapada, UV-1800PC spectrophotometer, China) in the 200-1000 nm range. The samples were rinsed with deionized water three times beforehand and then dispersed in deionized water and adjusted to the desired pH value before measurement. GPC (Waters 1515/Waters 2414, Milford, MA) was run using an RI detector at 35 C and THF as the eluent at a flow rate of 1.0 mL/min and polystyrene as the calibration standard. The series column (HR5: WAT054460; HR4: WAT044225; HR3: WAT044222) was kept at a pressure of 565 psi. An FTIR scan was performed on a Nicolet Nexus 470FTIR spectrometer with powder-pressed KBr pellets. The samples were centrifuged and rinsed with deionized water three times and dried before being scanned. A solid-state 13C NMR instrument with an Infinity Plus model 300WB was used to clarify the chain structures of the copolymers. The samples were rinsed with deionized water three times and dried in an oven at 50 C for 48 h. The dried copolymers were ground into powder before being used to conduct 13C measurements.

Results and Discussion The sulfonated copolymer slurry has a dark-red color, as indicated in Figure 1a. When a certain amount of sulfonated copolymer slurry was added to 20 mL of deionized water under agitation, the copolymer suspension turned yellow at pH values below 6 (Figure 1b). When this copolymer suspension was neutralized with sodium hydroxide solution to a pH value of 6 to 7, the color suddenly changed from yellow to khaki (Figure 1c). Adding another small amount of alkali would make the color change from khaki to red at a pH value of 7 to 8 (Figure 1d). When the pH value of this suspension was adjusted to above 8, it became purple (Figure 1e). This color response is as quick as for conventional small-molecule pH indicators such as methyl orange and phenolphthalein within 1 s and can be reversible when sodium Langmuir 2010, 26(23), 17836–17839

Figure 1. pH-responsive color changes of sulfonated copolymer solutions and solids at various pH values. (a) Sulfonated copolymer slurry, (b) pH 8, (f)-(i) solid states of the compounds in b-e. (j) UV-vis spectra of sulfonated copolymer solutions with various pH values from 4 to 10. Deionized water is used as the baseline.

hydroxide and hydrochloric acid solutions are alternately used to adjust the pH of the system. When these samples are rinsed with deionized water three times to remove the possible disturbance from sulfuric acid or metallic ions or when other kinds of strong or weak acids or bases (e.g., nitric acid, acetic acid, sodium carbonate, and ammonia) are used to adjust the pH of the system, the same pH-responsive behavior is observed. Besides, this response is not interfered with by reducing or oxidizing agents as is the case for polyaniline, and these copolymer solids centrifuged from aqueous solutions retain their corresponding colors (Figure 1f-i). These results suggest that this pH-responsive behavior should be attributed to the intrinsic properties of sulfonated copolymers rather than the conformational change reported for previous pH-responsive polymers.2-5 The UV-vis spectra of the sulfonated copolymer solutions as a function of pH are demonstrated in Figure 1j. As the pH value increases from 4 to 10, there exists an obvious red shift for the absorption peak from 474 to 570 nm, corresponding to the absorbance of blue and green-yellow light, respectively. Thus, their complementary colors are yellow and purple, consistent with the observed colors. It should be noticed that the titration jump occurred in the range of pH 7 to 8, which is quite similar to conventional pH indicators, suggesting an analogous pH-responsive mechanism (formation of conjugated acid/base pairs). However, the lowest critical concentration of this pH-responsive copolymer solution is around 0.1 g/L (Supporting Information, Figure S1), which is about 50-500 times the necessary amount used for conventional small-molecule pH indicators such as methyl orange and phenolphthalein. To understand the pH-responsive mechanism of this sulfonated styrene-acrylic acid copolymer, a series of control experiments were conducted as shown in Table S1. Compared with the obvious color change of the sulfonated styrene-acrylic acid copolymer (Table S1, run 1), sulfonated polystyrene homopolymer (run 2) shows only a slight color change, as in the case reported by Malik,13 but the sulfonated poly(acrylic acid) homopolymer (run 3) displays no color change at all. This means that it DOI: 10.1021/la103833x

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Scheme 1. pH-Responsive Mechanism of the Sulfonated StyreneAcrylic Acid Copolymer

is the sulfonated benzene group itself that constitutes the pHresponsive infrastructure whereas the incorporation of a carboxyl group can intensify the originally weak conjugated structure to a larger extent. Such an obvious pH-responsive behavior did not take place when polystyrene was mixed with either poly(acrylic acid) or acrylic acid and then sulfonated (runs 4 and 5), which suggests that only the chemical interaction of acrylic acid with the sulfonated benzene ring in the same chain can really enhance the signal for the color change. Even the sulfonated styrenemethacrylic acid copolymer never revealed a pH-responsive color change (run 6); this was possibly because the methyl group could block the chemical interaction between the carboxyl group and the sulfonated benzene ring as a result of steric hindrance. When styrene was copolymerized with maleic anhydride and sulfonated (run 7), obvious pH-responsive behavior was not observed because of the protection of the carboxyl group by acid anhydride in the nonaqueous system. On the basis of these experiments, it can be deduced that both the sulfonation of the benzene ring and the chemical reaction of the carboxyl group with the benzene ring are the two key parameters for the pH-responsive color change. Thus, we would propose a pH-responsive mechanism of the sulfonated styreneacrylic acid copolymer described by Scheme 1 as follows: The original styrene-acrylic acid copolymer is represented by structure (I). During the sulfonation period, sulfonic groups are preferentially grafted onto the para position of the benzene ring14 and the neighboring carboxyl group in the polymer chain can acylate and graft onto the meta position of the sulfonic group to form an occlusive hexagon ring,13 as indicated by structure II. Under acidic conditions, the carbonyl group of this sulfonated aromatic ketone combines with a proton to form an enolconjugated structure (III or resonance structure IV). This structure can easily turn into a structure with carbonyl groups and sulfate anions (V) under basic condition, and structures III and V are reversible, corresponding to yellow and purple, respectively. However, if basic conditions are applied to structure V for several days, then it would gradually hydrolyze to open the ring and form structure VI, with its color fading gradually. In fact, if nitration is employed to replace sulfonation, then the nitrated styreneacrylic acid copolymer can also reveal a pH-responsive color change from colorless in an acidic medium to yellow in an alkaline medium (Figure S2). To verify the above mechanism, FTIR was used to characterize the chain structure of the styrene-acrylic acid copolymer before and after sulfonation treatment as illustrated in Figure 2. Before (14) Canovas, M. J.; Sobrados, I.; Sanz, J.; Acosta, J. L.; Linares, A. J. Membr. Sci. 2006, 280, 461–469.

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Figure 2. FTIR analysis of the styrene-acrylic acid copolymer (a) before sulfonation and after sulfonation for (b) 4, (c) 20, and (d) 40 h.

13 C NMR analysis of the styrene-acrylic acid copolymer before and after sulfonation treatment. (a) Before sulfonation, (b) sulfonation for 4 h, and (c) sulfonation for 40 h.

Figure 3. Solid-state

sulfonation, the strong carbonyl group absorption peak appears at 1705 cm-1 and the broad stretching vibration band for the hydroxyl group is located in the range of 3200-2500 cm-1 (Figure 2 a), both of which confirm the existence of the carboxyl group. The peaks at 1492, 1450, 759, and 698 cm-1 are the characteristic absorptions of styrene.15 As the sulfonation time is extended from 4 to 40 h, a new peak appears at 1177 cm-1 corresponding to the absorption of the sulfonic group grafted onto the para position of the benzene ring16 (Figure 2b-d). Meanwhile, the absorption of the carbonyl group red shifts to 1675 cm-1 and the intensity of the CdC stretching vibration of the benzene ring at 1594 cm-1 increases. This indicates the formation of an aromatic ketone. Also, the peaks at 885 and 850 cm-1 are the characteristic fingerprint positions of the 1,2,4-trisubstituted benzene, further confirming structure V as illustrated in Scheme 1. Figure 3 further reveals the solid-state 13C NMR spectra of the copolymer powders before and after sulfonation treatment. Before sulfonation (Figure 3a), the copolymer shows chemical shifts at 128.7 (C-2) and 144.6 (C-3) ppm for the carbons at the benzene ring and at 183.8 (C-4) and 41.0 (C-1) ppm for the carbon of the carboxyl group and the carbon of the trunk chain, respectively.14 After sulfonation for 4 h (Figure 3b), the peak of the carboxyl groups becomes weakened and a new peak for the carbonyl groups appears at 203.4 (C-40 ) ppm. Extending the (15) Leng, W.; Chen, M.; Zhou, S.; Wu, L. Langmuir 2010, 26, 14271–14275. (16) Deng, Z.; Chen, M.; Wu, L.; Zhou, S.; You, B. Langmuir 2006, 22, 6403– 6407.

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to ¥, the color gradually shoals. This is because the ratio of 1:1 for styrene to acrylic acid is the most favorable for forming neighboring benzene and carboxyl group pairs in the copolymer chains in spite of the possible random copolymerization. Thus, the acidcatalyzed acylation between neighboring carboxyl groups and the benzene ring easily yields sulfonated aromatic ketone chromophores at full capacity on the basis of the above mechanism. Figure 4. Changing color of the sulfonated styrene-acrylic acid copolymer in alkaline solution with various molar ratios of styrene to acrylic acid: (a) 0, (b) 1:9, (c) 2:8, (d) 1:1, (e) 8:2, (f) 9:1, and (g) ¥. The concentration of copolymer solutions is kept at 1 g/mL for all samples.

sulfonation time to 40 h would further decrease the peak for the carboxyl groups (Figure 3c), leaving a stronger peak at 202.9 ppm for the carbonyl groups. Besides, the C atom (C-5) in the para position shifts to 138.5 ppm after sulfonation, which is also consistent with previously reported conclusions.14 These results can confirm that the neighboring carboxyl group in the copolymer chain has indeed acylated in the meta position of the sulfonic group to form an occlusive hexagon ring, making the carboxyl groups disappear but carbonyl groups appear. The ChemDraw Ultra NMR C-13 estimation also shows consistent chemical shifts with experimental results (Figure S3). Thus, both solid-state 13C NMR analysis and theoretical anticipation can validate the rationality of the above mechanism. In addition, an aromatic ketone molecule called anthrone with a similar structure to our proposed intermediates was sulfonated; it shows a similar pH-responsive color change (Figure S4). This can also further prove that the sulfonated aromatic ketone plays a crucial role in the pH-responsive behavior. However, this small molecule has a pH-responsive color change that is similar to that of the nitrated styrene-acrylic acid copolymer solution but a little different from that of the sulfonated copolymer. This is probably because adjacent groups such as benzene and the benzene ring substituted with sulfonic groups also affect the conjugated extent of sulfonated aromatic ketones, thus causing a difference in absorption properties. Figure 4 illustrates the influence of the molar ratio of styrene against acrylic acid monomers for copolymerization within this pH-responsive color change. When the molar ratio increases from 0 to 1:1, the color of the copolymer in alkaline solution becomes deeper. However, as the molar ratio further increases from 1:1

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Conclusions On the basis of this study, a very simple pH-responsive polymer has been synthesized from the solution copolymerization between styrene and acrylic acid and the sulfonation of this copolymer. This copolymer exhibits obvious, quick pH-responsive color changes from yellow in an acidic medium to khaki/red in a neutral medium to purple in a basic medium, and this pH-based color change is reversible. This pH-responsive property should be attributed to two key parameters: the sulfonation of the benzene ring and the intramolecular cyclization between neighboring carboxyl group and the benzene ring to form a sulfonated aromatic ketone. We believe that this pH-responsive property could be extended by designing similar structural copolymer derivatives using other kinds of aromatic monomers, and these kinds of hydrophilic copolymers may find applications in various biological and chemical sensors. Acknowledgment. The financial support of this research by the National Natural Science Foundation of China (no. 20774023), the National “863” Foundation, the Science & Technology Foundation of Shanghai (0952 nm01000), and the innovative team of the Ministry of Education of China (IRT0911) is appreciated. Supporting Information Available: Sulfonation of various polymers and their colors in acidic and alkaline solutions. Color changes of the sulfonated styrene-acrylic acid copolymer in alkaline solution at various concentrations. Color of the nitrated copolymer solution in an acidic solution and an alkaline solution. ChemDraw Ultra NMR C-13 estimation. Color of a sulfonated anthrone suspension in an acidic solution and an alkaline solution. This material is available free of charge via the Internet at http://pubs.acs.org.

DOI: 10.1021/la103833x

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