A Combined Physical–Chemical Polymerization Process for

Article pubs.acs.org/Macromolecules

A Combined Physical−Chemical Polymerization Process for Fabrication of Nanoparticle−Hydrogel Sensing Materials Qingzhou Cui, Wei Wang,* Baohua Gu, and Liyuan Liang Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6036, United States ABSTRACT: A new type of light diffracting hydrogel composite film consisting of nanoparticles and polymer has been fabricated through a combined physical−chemical polymerization process. In this fabrication approach, polymers with ionic functional groups for molecular recognition were successfully incorporated into a hydrogel of colloidal photonic crystals. We first embedded crystalline colloidal arrays into a nonionic polymer hydrogel by a physical freeze−thaw process using a formulation that contained poly(vinyl alcohol), dimethyl sulfoxide, water, and monodispersed polystyrene nanoparticles. Then, a second ionic polymer network, which interpenetrates into the first polymer network, was formed through a photochemical polymerization process. During the formation of the second polymer network, functional groups for molecular recognition were introduced. Finally, the physically cross-linked nonionic polymer network was removed by hydrothermal dissolution, leaving the chemically cross-linked ionic polymer network intact to support the nanoparticle arrays and to perform chemical sensing. This approach significantly reduces the complexity of established methods, in which multiple-step chemical reactions are required to incorporate functional groups into the polymer hydrogel. A new acrylic acid hydrogel with embedded colloidal photonic crystals was made as an example to demonstrate pH sensing based on diffraction induced by changes of lattice parameter of the crystalline colloidal arrays.

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INTRODUCTION Soft colloidal photonic crystals, also known as crystalline colloidal arrays (CCAs), are formed by non-close-packed, three-dimensional arrays of colloidal nanoparticles in suspensions, where the lattice spacing is much greater than the particle diameter itself.1−5 When immobilized in polymer network and functionalized with molecular recognition groups (MRGs), CCA thin films are used as chemical sensing materials.6,7 The colloidal photonic crystal, which has been embedded in functionalized polymer network, can change its crystal lattice parameters, i.e., distance between colloidal nanoparticles, in response to external stimulus such as presence of an analyte molecule, leading to diffraction wavelength shift. As demonstrated in previous studies, the wavelength shift is proportional to the analyte molecule concentration; thus, the chemical sensors have been developed as quantitative tools.8−13 However, due to high sensitivity to ionic species, the surfacecharge-stabilized CCAs can be embedded into only a few noncharged polymer networks. It remains a challenge to directly immobilize the CCA into polymers with charged functional groups because charged species can create disorder in the nanoparticle arrays. In sensor fabrication, incorporating MRGs is a critical step for enhanced selectivity and sensitivity. For the currently reported sensor materials, noncharged polymer networks often need to be functionalized through post-multiple-step chemical treatment to generate active MRGs.9,11−14 Because of the © 2012 American Chemical Society

complexity of molecular coupling process, it is difficult to quantitatively control the MRGs in postchemical reactions, thus the reproducibility is poor. As a result, individual sensor calibration is often necessary, which greatly compromises the simplicity of this technology. Currently, poor processing reproducibility and method complicity limit broader applications of the sensing materials. Directly using polymer with charged MRGs to fabricate the CCA hydrogel materials without creating disorders in CCA can essentially solve these problems. Recently, a method was presented to copolymerize charged MRGs into two-dimentional CCA hydrogels,15−17 but a toxic mecury subphase is needed for assembling close-packed colloidal particle arrays. In this research, we present a new approach to fabricate the CCA hydrogel sensor materials. The CCA is first immobilized in a physically cross-linked hydrothermal dissolvable polymer gel.18,19 An interpenetrating network (IPN) polymerization method20−22 is then used to form the second polymer structure with charged monomers in the cross-linked hydrothermal dissolvable hydrogel. By this synthesis strategy, universal monomers with various charged MRGs can be used to construct a variety of photonic hydrogels, which will extensively broaden applications of the material. As proof-of-concept, Received: June 1, 2012 Revised: September 26, 2012 Published: October 11, 2012 8382

dx.doi.org/10.1021/ma301119f | Macromolecules 2012, 45, 8382−8386

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Diffraction Measurement. Diffraction measurement was performed with an Ocean Optics USB 2000-UV−vis spectrometer with a deuterium tungsten halogen source and a fiber-optic reflection probe. The setup was described in detail elsewhere.24 Free floating hydrogel was held by an O-ring at the bottom of a Petri dish. The pH titration for the acrylic acid hydrogel was performed in 30 mL of 150 mM NaCl solution in a Petri dish. Solution pHs were measured by a Ross Ultra Semimicro pH electrode (Orion, Thermo Electron Corp.), and different pH values were adjusted by adding diluted HCl (J.T. Baker) or NaOH (J.T. Baker) solution (in 150 mM NaCl) to the Petri dish.

charged acrylic acid is used to fabricate hydrogel with embedded nanoparticle arrays. We demonstrate that this sensor material sensitively responses to external pH changes upon protonation/deprotonation process.

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EXPERIMENTAL SECTION

Physically Cross-Linked Hydrogel Preparation. Highly charged monodispersed polystyrene (PS) nanoparticles were synthesized by emulsion polymerization using styrene as monomer, 1allyloxy-2-hydroxypropanesulfonate as comonomer for charge enhancer, divinylbenzene as cross-linker, and ammonium persulfate as initiator.23 The synthesized PS nanoparticles were purified by dialysis with dialysis tubing (Thermo Scientific SnakeSkin, 10K MWCO) in deionized H2O (18.2 MΩ cm−1, Barnstead Nanopure Water Purification System) to form CCA. For hydrogel formation, a poly(vinyl alcohol) (PVA) solution was first made by dissolving 4.2 g of PVA (99.7 mol % hydrolyzed, MW ≈ 78 000, Polysciences, Inc.) in 39.1 g of dimethyl sulfoxide (DMSO) (VWR International). Upon heating and stirring on a hot plate, the PVA was completely dissolved. After the PVA dissolution, 5.0 mL of deionized H2O was added and mixed well with the PVA solution. When the stock PVA solution in DMSO−H2O mixture was cooled to ambient temperature, it formed gelation. To make the physically cross-linked hydrogel, in a typical preparation, 1.0 g of PVA stock solution (in gelation form) was mixed with 0.675 g of PS nanoparticle suspension (particle size of 117.5 nm, concentration of ∼20 wt %), 0.30 g of ion-exchange resin (AG 501-X8 (D) Resin, 20−50 mesh, Bio-Rad Laboratories, Inc.), and 1.0 mL of deionized H2O in a glass vial. The suspension was thoroughly mixed by alternating vortexing and heating on a hot plate (at 50 °C). After a few minutes, the suspension sample exhibited strong diffraction, and it was centrifuged for 5 min (SpeedVac, SC2000, Savant). After the centrifugation, the ion-exchange resin settled to the bottom of the vial and thus was separated from nanoparticle array. The suspension was taken out with a 1 mL syringe and injected into a quartz cell with 125 μm thickness, created by a parafilm spacer. The cell was put in a freezer (−20 °C) for 2 h. After the 2 h period, the cell was taken out of the freezer and placed in a water bath at ambient temperature for 16 h to allow the crystalline structure to be fully developed. Subsequently, the hydrogel was taken out of the quartz cell and rinsed with deionized water thoroughly. The hydrogel was then transferred to a 150 mM NaCl solution for storage before further processing. Preparation of IPN Using Charged Monomers. First, a stock monomer solution was made by placing 0.55 g of acrylic acid (99.5% low water content, stabilized with ca. 200 ppm methoxypheno, Alfa Aesar) and 0.027 g of N, N′-methylenebis(acrylamide) (99+%, Alfa Aesar) in 10.05 g of 150 mM NaCl (J.T. Baker) solution. The acrylic acid was destabilized by Al2O3 particles right before use. About 2 mL of the stock solution (2.02 g was weighed) was mixed with 20 μL of photoinitiator (2,2-diethoxyacetophenone (DEAP), 10 wt % in DMSO, Acros Organics) in a dark glass vial. The mixture was mixed by vortexing and stored in the vial. A piece of physically cross-linked hydrogel (1 cm × 1 cm) was immersed in the solution in dark. The vial was kept in dark for 2 h with occasional shaking. After the 2 h period, the hydrogel was carefully placed flat in the center of a dry quartz plate and covered by another quartz plate. Parafilm of a layer thickness was used as spacer surrounding the hydrogel between the two plates in the quartz cell. Additional monomers solution (the same solution for hydrogel immersion) was injected into the new cell to get rid of possible air voids. Let the cell sit and further equilibrate for 1 h so that the monomer formulation can uniformly diffuse into the hydrogel. Then the cell was placed between two UV lamps (Black Ray, 100 W mercury lamp, 365 nm) for photopolymerization for 45 min. Upon completion of photopolymerization, the cell was opened in deionized water. The resulting hydrogel was rinsed with large quantities of 150 mM NaCl and then stored in the 150 mM NaCl solution. For PVA dissolution to remove the first polymer network, the hydrogel in the 150 mM NaCl solution was heated to 55 °C for 5 min. Then the resulting hydrogel was rinsed again and stored in 150 mM NaCl solution.

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RESULTS AND DISCUSSION Formation of a Robust, Hydrothermal Dissolvable Hydrogel. The freeze−thaw process had been used to fabricate PVA hydrogels in previous studies.25−27 Asher et al. have incorporated monodispersed PS nanoparticles into PVA polymer matrix to fabricate thermal reversible, light diffracting hydrogel film.28 However, the nanocomposite materials need to be chemically cross-linked in water. In our study, we directly immobilized PS nanoparticles into PVA hydrogels without need for any chemical cross-linkers as follows. First, highly hydrolyzed PVA was used to make PVA stock solution, which can form gelation before freeze−thaw processing. Second, longer thawing time of 16 h at ambient temperature was used to let crystalline structure to fully develop in a confined quartz cell. Figure 1 shows examples of hydrogels that have been produced by the method above (without any molecular cross-linker) and their corresponding visible light diffraction spectra. In the fabrication, concentration and size of the nanoparticles can be adjusted to obtain diffraction at different wavelengths. For example, in Figure 1, the three spectra were obtained from preparation with (a) 0.69 g (117.5 nm, ∼20 wt %), (b) 0.58 g (117.5 nm, ∼20 wt %), and (c) 0.71 g (133.4 nm, ∼20 wt %) PS nanoparticle suspensions in water and 1.0 g of PVA stock solution, respectively. By reducing nanoparticle concentration, thus increasing particle−particle distance, the hydrogel diffraction spectrum is shifted from blue to green as shown in the Figure 1a, b. By increasing the nanoparticles size, we can further shift the diffraction color to longer wavelength of yellow as shown in Figure 1c. During the freeze−thaw process, crystallite sites were formed in the bulk of the polymer, and they functioned as self-crosslink for the PVA polymeric network.18,29 These cross-links are critical to the formation of three-dimensional polymer network, which restricts the movement of the nanoparticles and therefore sustains the ordered nanoparticles array. The existence of ordered nanoparticles arrays ensured the integrity of the hydrogels exhibiting visible colors (as shown in Figure 1), and the diffracted wavelength obeys Bragg’s law (mλ = 2nd sin θ). The physical nature of the cross-link, however, is weak and can be destroyed by heating. When the light-diffracting composite hydrogels were heated in water, the crystallites could be broken and PVA molecules redissolve into solution. Therefore, the hydrogel structure collapses and the locked CCA is released. The physically cross-linked hydrogel can be completely dissolved within a couple of minutes when the temperature is above 50 °C as shown in Figure 2. There was no noticeable difference for the hydrothermal dissolution of hydrogels with and without embedded PS nanoparticles. Interpenetrating Network (IPN) To Make a Second Polymer Network To Lock the Nanoparticles Array. The hydrothermal dissolvable polymer network can be used as a 8383

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method,20,21 we incorporated the CCA which remained in hydrogel after the hydrothermal dissolvable polymer template was removed. This process is often associated with hydrogel volume change as illustrated schematically in Figure 3. In previous studies, only nonionic monomer molecules in pure water were used to make the hydrogels with embedded CCAs, because CCAs in suspension are extremely sensitive to ionic species. Ionic molecules can screen the electric double layer of colloidal nanoparticles and cause disorders in the CCAs, and thus it is difficult to directly introduce ionic MRGs into the polymer network without complex postchemical treatment. Here we fabricated the light-diffracting hydrogels directly from monomer molecules with various charged MRGs in solutions in the presence of ionic impurities. As proof-ofconcept, we made a hydrogel by photopolymerization from acrylic acid, which is a charged monomer with −COO− functional group. First, a piece of PVA hydrogel was immersed in a solution containing an acrylic acid monomer and a crosslinker of N,N′-methylenebis(acrylamide). In this formulation, both charged monomer molecules and 150 mM NaCl were used which could destroy the polystyrene colloidal photonic crystal structure if used directly with CCA. Second, the immersed hydrogel was placed in the center of a quartz plate cell with a parafilm spacer. After the cell was sealed, additional monomer solution was injected to fill the void space and let the cell sit and the solution completely diffuse into the polymer network. Last, the cell was placed under UV light for photopolymeization. When the cell was opened after structure development in a water bath, the hydrogel swelled significantly compared to the hydrogel before IPN which can be observed by the diffraction wavelength red shift as shown in Figure 4. This is due to a second polymer network was incorporated into the hydrogel. The free energy of mixing causes the hydrogel to swell.8,30 At the medium pH condition, the acrylic acid is deprotonated and a sodium ion occupy the proton position and the free energy of mixing causes the hydrogel swell, leading to red-shifted diffraction wavelength. The hydrogel further swells after PVA dissolution in the 150 mM NaCl solution. This is because the number of cross-links decreases with crystallite (the physically formed cross-linkers) dissolution. According to the Flory theory, hydrogel volume is related to the free energy of elasticity, which is related to the number of cross-links within the hydrogel as shown in eq 1.30−32

Figure 1. Diffraction spectra of hydrogels in 150 mM NaCl solution developed from the physical freeze-thaw process. The blue, green, and yellow hydrogels were prepared in 1.0 g of PVA stock solution with (a) 0.69 g (117.5 nm, ∼20 wt %), (b) 0.58 g (117.5 nm, 20 wt %), and (c) 0.71 g (133.4 nm, ∼20 wt %) of PS nanoparticle suspensions, respectively. Images show the polymer/nanoparticles composite materials suspended in solution.

ϑ1/3 =

G′ υeRT

(1)

Assuming a fixed shear storage modulus (G′) during the hydrogel swelling, the hydrogel volume (ϑ) is determined by the cross-link density (υe). When the crystallite sites dissolve, cross-link number decreases and hydrogel swells, leading to diffraction wavelength red-shift. From Figure 4, the diffraction peak width broadened during the IPN and PVA dissolution process, and this may be caused by particle disordering to certain degree within the colloidal crystal array. Acrylic Acid Hydrogel pH Sensor. The acrylic acid hydrogel contains carboxyl groups and can be readily used as a pH sensor. A pH titration curve in Figure 5 shows that the poly(acrylic acid) hydrogel swells significantly with an increase of pH. The diffraction wavelength shift is so significant that the visible diffraction color change can be visually identified for the hydrogel as shown by inserted images in Figure 5.

Figure 2. Dissolution-induced weight loss (W/W0, residue weight/ original weight) of physically cross-linked PVA hydrogel in 150 mM NaCl at different temperatures.

template to make new hydrogels through the interpenetrating network (IPN) method. Different from the previous IPN 8384

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Figure 3. Schematic illustration of the interpenetrating network (IPN) method for preparation of hydrogels with embedded crystalline colloidal arrays.

Figure 5. The pH titration curve for the acrylic acid composite hydrogel. Inserted images show the color changes of the hydrogel at different pH values.

groups. Therefore, proton was replaced by sodium ions with increasing pH. The replacement process causes the free energy of mixing change, leading to red-shifted diffraction wavelength as shown in the titration curve in Figure 5. One advantage of this new IPN method is that we are able to make hydrogels with finely tuned, specific amount of acrylic acid groups. We can now copolymerize carboxyl groups containing acrylic acid monomers with acrylamide monomers. By changing acrylic acid/acrylamide molar ratio in a mixture, we can tune the carboxylic group amount in the hydrogel, which is directly related to how many molecular recognition groups are incorporated into the sensor material. By eliminating the postchemical treatment such as hydrolysis process, sensor manufacturing complexity is significantly decreased and reproducibility is greatly improved. This may help to increase the reproducibility of the technology through quantitative control of MRGs. Another advantage for making composite hydrogels through the IPN method is that we can now extend the types of light diffracting hydrogels to various charged polymer systems, which may find many new applications in making new CCA sensor devices and other new functional devices.

Figure 4. Diffraction spectra of hydrogels during the IPN process: (a) before IPN; (b) after IPN; (c) after PVA dissolution. The inserted images are the photos of the hydrogels in 150 mM NaCl.

The titration is a deprotonation process as shown in reaction 1. The titration was performed in 150 mM NaCl solution to

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CONCLUSION Polymer interpenetration was used tomake polymerized CCAs with charged monomers. The method provides a new strategy to attach MRGs without the need for a postchemical modification process. As a result, charged monomer molecules and ionic species can be readily incorporated into the photonic

maintain a constant ionic strength during the process. In the 150 mM NaCl solution, there are abundant sodium ions which can diffuse into the hydrogel and bind to the carboxylate 8385

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(27) Ricciardi, R.; Mangiapia, G.; Lo Celso, F.; Paduano, L.; Triolo, R.; Auriemma, F.; De Rosa, C.; Laupretre, F. Chem. Mater. 2005, 17, 1183. (28) Muscatello, M. M. W.; Asher, S. A. Adv. Funct. Mater. 2008, 18, 1186. (29) Takeshita, H.; Kanaya, T.; Nishida, K.; Kaji, K. Macromolecules 2001, 34, 7894. (30) Flory, P. J. Principles of Polymer Chemistry; Cornell University: Ithaca, NY, 1953. (31) Anseth, K. S.; Bowman, C. N.; BrannonPeppas, L. Biomaterials 1996, 17, 1647. (32) Erman, B.; Mark, J. E. Structure and Properites of Rubberlike Networks; Oxford University Press: New York, 1997.

hydrogel materials. By eliminating the postchemical treatment for introducing MRGs quantitatively, the new approach significantly reduces the complexity for polymerization of CCAs. We demonstrated the formation of robust acrylic acid hydrogel, which can be used as a visual pH sensor.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected], Ph (865) 241-5245, Fax (865) 5763989. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by Laboratory Directed Research and Development Funds at Oak Ridge National Laboratory (ORNL). ORNL is managed by UT-Battelle, for U.S. Department of Energy, under Contract DE-AC05-00OR22725.

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REFERENCES

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