pubs.acs.org/Langmuir © 2009 American Chemical Society
Multiresponsive Hybrid Microgels and Hollow Capsules with a Layered Structure Veronique Lapeyre,†,‡ Natacha Renaudie,†,‡ Jean-Franc-ois Dechezelles,§ Hassan Saadaoui,§ Serge Ravaine,§ and Valerie Ravaine*,†,‡ † Institut des Sciences Mol eculaires, Universit e Bordeaux, ENSCPB, 16 Av. Pey Berland, 33607 Pessac Cedex F33607, France, ‡Institut des Sciences Mol eculaires, CNRS, Talence Cedex F33405, France and §Centre de Recherche Paul Pascal;CNRS, Universit e Bordeaux, 115, Av Albert Schweitzer, Pessac F-33600, France
Received October 3, 2008 Various stimuli-responsive composite particles with a high control of their internal structure and their corresponding hollow capsules are synthesized and characterized by photon correlation spectroscopy, TEM, and AFM. Core-shell particles with a silica core and a thermoresponsive shell are obtained by polymerization of Nisopropylacrylamide (NIPAM) in the presence of silica seeds grafted with a high density of γ-methacryloxypropyltrimethoxysilane (MPS). The influence of the synthesis conditions is studied. The shell thickness increases when the monomer concentration increases in a limited range where uniform composite particles with a single core are obtained. At constant monomer concentration, the shell thickness does not depend on the size of the silica seeds, but the presence of free unbound microgels is observed when the silica surface area decreases. A range of particle diameters and shell thicknesses is thus obtained, which can lead to the corresponding hollow capsules by exposure to hydrofluoric acid solution. The volume phase transition temperature of these materials can be easily tuned by replacing the NIPAM monomer by another N-alkylacrylamide derivative. However, the incorporation of comonomers such as acrylic acid (AA) and a phenylboronic acid (PBA) derivative inhibits the formation of core-shell structures. In order to get pH or glucose responsiveness, these functional groups can be incorporated in the outer shell of a core-double shell structure, with pNIPAM as intermediate shell. pHresponsive and glucose-responsive composite particles are obtained by this method with a high control of their internal structure.
1.
Introduction
Stimuli-responsive submicron particles, commonly known as microgels, have been widely investigated over the past few years and have demonstrated their broad potential applications in various fields such as sensors, drug delivery systems, biomaterials, or even microreactors.1 These cross-linked polymer particles, made of stimuli-responsive polymers, have the ability to swell or shrink upon stimulus application, which triggers the change in their physical properties such as permeability or refractive index. The most studied microgels are made of thermoresponsive polymers such as the poly(N-alkylacrylamide) family and especially the famous poly(N-isopropylacrylamide) (pNIPAM), which exhibits a lower critical solution temperature (LCST) of about 32 °C.2,3 Other comonomers can be combined with NIPAM to provide pH responsiveness or molecular sensitivity. The versatility of these systems reaches its broadest extent when they are associated with another material, which might itself be sensitive to an external stimulus. For example, thermoresponsive magnetic particles have been prepared by incorporating iron oxide materials within microgels.4-8 The presence of inor*Corresponding author. E-mail:
[email protected]. (1) Nayak, S.; Andrew Lyon, L. Angew. Chem., Int. Ed. 2005, 44(47), 7686. (2) Heskins, M.; Guillet, J. E. J. Macromol. Sci., Part A 1968, 2(8), 1441. (3) Schild, H. G. Prog. Polym. Sci. 1992, 17(2), 163. (4) Sauzedde, F.; Elaissari, A.; Pichot, C. Colloid Polym. Sci. 1999, 277 (11), 1041. (5) Sauzedde, F.; Elaissari, A.; Pichot, C. Colloid Polym. Sci. 1999, 277(9), 846. (6) Ding, X. B.; Sun, Z. H.; Wan, G. X.; Jiang, Y. Y. React. Funct. Polym. 1998, 38(1), 11. (7) Deng, Y.; Yang, W.; Wang, C.; Fu, S. Adv. Mater. 2003, 15(20), 1729. (8) Guo, J.; Yang, W.; Deng, Y.; Wang, C.; Fu, S. Small 2005, 1(7), 737.
Langmuir 2009, 25(8), 4659–4667
ganic particles within the microgels might also be valuable to increase their refractive index. This property is important for optical applications such as photonic crystals.9-13 This strategy has already been applied by incorporating gold nanoparticles within microgels.14 Composite soft materials including silica15-17 or magnetite18 can also be used as emulsifiers to control the demulsification process under the application of a stimulus. The organization of inorganic particles within the microgels may affect their physical properties. Nanoparticles may be simply adsorbed at the surface, randomly19,20 or selectively21,22 distributed within the particle, or located as a (9) Debord, J. D.; Lyon, L. A. J. Phys. Chem. B 2000, 104(27), 6330. (10) Debord, J. D.; Eustis, S.; Debord, S. B.; Lofye, M. T.; Lyon, L. A. Adv. Mater. 2002, 14(9), 658. (11) Lyon, L. A.; Debord, J. D.; Debord, S. B.; Jones, C. D.; McGrath, J. G.; Serpe, M. J. J. Phys. Chem. B 2004, 108(50), 19099. (12) Hellweg, T.; Dewhurst, C. D.; Bruckner, E.; Kratz, K.; Eimer, W. Colloid Polym. Sci. 2000, 278(10), 972. (13) Hellweg, T.; Dewhurst, C. D.; Eimer, W.; Kratz, K. Langmuir 2004, 20(11), 4330. (14) Suzuki, D.; McGrath, J. G.; Kawaguchi, H.; Lyon, L. A. J. Phys. Chem. C 2007, 111(15), 5667. (15) Fujii, S.; Read, E. S.; Binks, B. P.; Armes, S. P. Adv. Mater. 2005, 17 (8), 1014. (16) Binks, B. P.; Murakami, R.; Armes, S. P.; Fujii, S. Langmuir 2006, 22 (5), 2050. (17) Fujii, S.; Armes, S. P.; Binks, B. P.; Murakami, R. Langmuir 2006, 22 (16), 6818. (18) Brugger, B.; Richtering, W. Adv. Mater. 2007, 19(19), 2973. (19) Fujii, S.; Cai, Y.; Weaver, J. V. M.; Armes, S. P. J. Am. Chem. Soc. 2005, 127(20), 7304–7305. (20) Zhang, J.; Xu, S.; Kumacheva, E. J. Am. Chem. Soc. 2004, 126(25), 7908. (21) Suzuki, D.; Kawaguchi, H. Langmuir 2005, 21(25), 12016. (22) Suzuki, D.; Kawaguchi, H. Colloid Polym. Sci. 2006, 284(12), 1443.
Published on Web 3/12/2009
DOI: 10.1021/la9003438
4659
Article
Lapeyre et al.
single core within the particle.23-26 This latter approach has been demonstrated using a “grafting to” method on silica as a core, which presents the advantage to provide a high control of the particle internal structure. Furthermore, hollow hydrogel particles can be easily obtained thanks to the dissolution of silica in the presence of hydrofluoric acid (HF) or sodium hydroxide.27 The variety of materials available with this concept might be broad since silica could contain a fluorescent dye,24 magnetite,7,8 or gold.25 It was shown that the thickness of the hydrogel coating could be controlled.24 However, up to now, the nature of the polymeric part has been essentially limited to pNIPAM, and only silica particles with a diameter below 100 nm were used as cores. In this article, we investigate the synthesis process in more detail and show the routes to build stimuli-responsive core-shell hybrid particles with a wide range of core sizes, shell thicknesses, and various chemistries which can provide (bio)functionality. A strategy to prepare more advanced functional materials is also proposed. Particles with a core-double shell architecture are synthesized. They exhibit a dual responsiveness, which is illustrated by the application to stimuli such as pH or glucose. All these hybrid particles can serve as precursors for hollow hydrogel nanocapsules.
2.
Experimental Section
2.1. Materials. All the reagents were purchased from Sigma-Aldrich unless otherwise noted. Tetraethoxysilane, ammonia (30% in water, SDS), and methacryloxypropyltrimethoxysilane (MPS) were purchased in their reagent grade and used without further purification. Absolute ethanol was purchased from J.T. Baker. N-Isopropylacrylamide (NIPAM) and N-isopropylmethacrylamide (NIPMAM) were recrystallized from hexane (ICS) and dried under vacuum prior to use. N,N0 -Methylenebis(acrylamide) (BIS), potassium persulfate (KPS), and acrylic acid (AA) were used as received as well as hydrofluoric acid. Deionized water, obtained with a Milli-Q system, was used for all synthesis reactions, purification, and solution preparation. 4-(1,6-Dioxo-2,5-diaza-7-oxamyl)phenylboronic acid (DDOPBA) has a pKa of 7.8 and was synthesized according to the procedure described previously by Matsumoto et al.28 Its chemical structure is represented in Scheme 1.
2.2. Particle Synthesis. 2.2.1. Synthesis and Functionalization of Silica Particles. Absolute ethanol (195 mL), water (29 mL), and an aqueous solution of ammonia (11 mL) were introduced in a three-neck round flask of 1 L equipped with a refrigerating system. The mixture was stirred at 300 rpm to homogenize and heated at different temperatures, depending on the desired final particle diameter (140 nm at 70 °C; 230 nm at 40 °C; 350 nm at 27 °C). After stabilization, tetraethoxysilane (16.5 mL) was added quickly, and reaction occurred at the chosen temperature under permanent stirring for 1 h. To graft polymerizable groups onto the silica surface, a given volume of methacryloxypropyltrimethoxysilane was added directly into the particles suspension. The amount of coupling agent was around 5 times higher than the amount necessary to cover the inorganic (23) Zha, L. S.; Zhang, Y.; Yang, W. L.; Fu, S. K. Adv. Mater. 2002, 14 (15), 1090. (24) Gao, H.; Yang, W.; Min, K.; Zha, L.; Wang, C.; Fu, S. Polymer 2005, 46(4 spec. iss.), 1087. (25) Karg, M.; Pastoriza-Santos, I.; Liz-Marzan, L. M.; Hellweg, T. ChemPhysChem 2006, 7(11), 2298. (26) Contreras-C aceres, R.; Sanchez-Iglesias, A.; Karg, M.; PastorizaSantos, I.; Perez-Juste, J.; Pacifico, J.; Hellweg, T.; Fernandez-Barbero, A.; Liz-Marz an, L. M. Adv. Mater. 2008, 20(9), 1666–1670. (27) Gu, J.; Xia, F.; Wu, Y.; Qu, X.; Yang, Z.; Jiang, L. J. Controlled Release 2007, 117(3), 396. (28) Matsumoto, A.; Ikeda, S.; Harada, A.; Kataoka, K. Biomacromolecules 2003, 4(5), 1410.
4660
DOI: 10.1021/la9003438
Scheme 1. Representation of the Structure of DDOPBA
surface with a monolayer (the theoretical amount for such a coverage being nominally 2 molecules nm-2 29). After stirring for 10 h at ambient temperature, the reaction medium was heated to 80 °C for 1 h to promote covalent bonding. When the synthesis was completed, most of the ethanol and ammonia were first removed through evaporation under reduced pressure. All the silica suspensions were then dialyzed against water until neutral pH in order to remove the remaining reactants and replace ethanol with water. The final concentration of the silica suspensions was determined by measuring the mass of a dried extract. Silica size was determined statistically by analyzing TEM images.
2.2.2. Synthesis of Simple Core-Shell SiO2-Microgel Particles. The microgel synthesis was carried out by a conventional precipitation polymerization in the presence of a controlled amount of silica seeds (mSiO2). A typical procedure is the following: a chosen amount of NIPAM, ranging from 20 to 150 mM, and BIS (11 mol % of the total monomer concentration) are introduced in water (7 mL). A certain volume of silica aqueous suspension containing mSiO2 of silica is then added. Water is added to complete the solution volume to 23 mL. This suspension is heated to 70 °C, and oxygen is removed by argon bubbling for 1 h. KPS (5.9 mg) is dissolved in 2 mL of water. This solution is also degassed and rapidly added to the monomersilica suspension. The turbidity of the suspension further intensifies within 10 min, indicating the occurrence of polymerization. The polymerization is allowed to proceed for at least 6 h; afterward, it is allowed to cool. It is finally washed several times by centrifugation. To broaden the variety of functionalities, either all or part of the NIPAM (x mol %) was replaced by another monomer, like NIPMAM, AA, or DDOPBA.
2.2.3. Synthesis of Core-Shell-Shell SiO2-Gel-Gel Particles. The same synthesis procedure was carried out again
replacing the silica seed particles by simple core-shell SiO2microgel seeds. The mixture of monomers contained NIPAM and BIS, or x mol % of NIPAM was replaced by a comonomer, like AA or DDOPBA. 2.2.4. Hollow Microgel Synthesis. The as-prepared coreshell particle suspensions were concentrated 10 times by centrifugation. An equivalent volume of HF solution (40%) were added to this suspension and left for at least 4 h. The suspension was then washed by centrifugation until the continuous phase became neutral.
2.3. Characterizations. 2.3.1. Photon Correlation Spectroscopy. Particle sizes and polydispersities were determined by photon correlation spectroscopy (PCS) using the Zetasizer Nano S90 (Malvern Instruments) operating with a HeNe laser at 90°. The hydrodynamic diameters dH were calculated from diffusion coefficients using the Stokes-Einstein equation. All correlogram analyses were performed with the manufacturer supplied software. The polydispersity index (PDI) is given by the cumulant analysis method. Both material characterization and their swelling behavior upon glucose recognition were investigated (29) Bourgeat-Lami, E.; Lang, J. J. Colloid Interface Sci. 1998, 197(2), 293.
Langmuir 2009, 25(8), 4659–4667
Lapeyre et al.
Article
by this technique. To do so, a droplet of the initial particle suspension (10 μL) was dispersed in a freshly prepared glucose buffered solution (1 mL, phosphate buffer 2 mM, pH 7.5). Before each data collection, the sample was allowed to equilibrate 10 min at the appropriate temperature. Each data point reported is an average of five separate size measurements, which themselves consist of 14 measurements with ∼15 s integration time. Only one measurement per degree was carried out when recording a temperature program. The sample was allowed to equilibrate for 10 min between each temperature. 2.3.2. Transmission Electron Microscopy. The core-shell particles morphology was visualized by transmission electron microscopy (TEM). One drop of the dilute suspension was deposited on a copper grid coated with a carbon membrane. A negative staining procedure using uranyl acetate was used to enhance the contrast after the microgels were deposited on the TEM grid. To see the interface between two hydrogel shells, which have the same electron densities, the contrast could be obtained by a selective staining of the pNIPAM-co-AA core with uranyl acetate. The core-shell microgel suspension (50 μL) was mixed with an uranyl acetate solution (500 μL, 0.75 mM) and shaken for 1 h. The grid was observed with a FEI Tecnai biotwin (120 kV). 2.3.3. Electrophoretic Mobility Measurements. Experiments were carried out using the Zetasizer 3000 HS (from Malvern Instruments, UK) at the appropriate temperature. Particle electrophoretic mobilities were obtained after diluting the microgel suspension in the buffered solution and after allowing the mixture to equilibrate for 10 min. Each value results from at least three measurements. 2.3.4. Atomic Force Microscopy. The structure of the microgels interface was examined by a commercial Nanoscope III multimode AFM (from DI-Veeco, Santa Barbara, CA) equipped with a 12 μm scanner (EV-scanner). The samples were imaged using tapping-mode phase imaging and a standard silicon cantilever (∼40 N/m) to provide topographic and corresponding phase images. The particles were cast on a mica substrate by leaving a drop of a dilute suspension evaporating. Mica was freshly cleaved before each experiment and used as support. Typical imaging scan rates varied between 0.02 and 1 Hz, and proportional and integral gains between 0.8 and 1 were used.
3.
Results and Discussion
3.1. Synthesis of Core-Shell SiO2-pNIPAM Particles with Various Sizes. The synthesis of hybrid particles using silica as a seeding agent during the polymerization of an organic polymer has been widely studied, mainly with polystyrene as organic part. From these studies, it is well-known that composite materials cannot be obtained using bare silica, unless a compatibilizing agent is added to the synthesis. It can be either a comonomer30-32 or the initiator itself33-35 via electrostatic interactions. These methods are restricted to positively charged agents binding to negatively charged silica and do not allow the formation of core-shell particles. The introduction of cationic comonomer led to a “currant-bun” architecture, whereas “raspberry”-like (30) Barthet, C.; Hickey, A. J.; Cairns, D. B.; Armes, S. P. Adv. Mater. 1999, 11(5), 408. (31) Amalvy, J. I.; Percy, M. J.; Armes, S. P.; Wiese, H. Langmuir 2001, 17 (16), 4770. (32) Percy, M. J.; Barthet, C.; Lobb, J. C.; Khan, M. A.; Lascelles, S. F.; Vamvakaki, M.; Armes, S. P. Langmuir 2000, 16(17), 6913. (33) Luna-Xavier, J. L.; Guyot, A.; Bourgeat-Lami, E. Polym. Int. 2004, 53(5), 609. (34) Luna-Xavier, J. L.; Guyot, A.; Bourgeat-Lami, E. J. Colloid Interface Sci. 2002, 250(1), 82. (35) Luna-Xavier, J. L.; Bourgeat-Lami, E.; Guyot, A. Colloid Polym. Sci. 2001, 279(10), 947.
Langmuir 2009, 25(8), 4659–4667
structures were obtained by heterocoagulation with the cationic initiator. Another method consists in using modified silica bearing reactive double bonds for covalent attachment of the polymer chains on the silica surface. This can be achieved either by the adsorption of a macromonomer, e.g., monomethyl ether monomethyl methacrylate poly (ethylene oxide),36,37 or using a grafted silane such as γ-methacryloxypropyltrimethoxysilane (MPS).29,38 The presence of MPS helps to overcome the hydrophilic character of silica and promotes polymer attachment to the surface. The core-shell morphology has been obtained with a high MPS grafting density.38,39 It has been shown that the nucleation took place through the capture of growing oligomer radicals by the silica particles. Similarly, core-shell microgels with a silica core and a pNIPAM shell were obtained using MPS-modified silica with a high grafting density as seeds.23,25 We chose to work with three types of silica particles having different diameters (140, 230, and 350 nm) but similar high grafting densities, which was calculated as 5 times the theoretical silanol density (i.e., 10 molecules nm-2) and used them as seeds during the polymerization step (Figure 1). We checked that such a high grafting density was necessary to achieve a continuous shell. Silica with lower silanol densities (between 1 and 2.5 times the theoretical silanol density) led to discontinuous shells. First, a series of experiments was carried out at constant silica concentration (CSiO2 = 5 g L-1) and constant initiator concentration (0.9 mM) while varying monomer concentration. The resulting particles were analyzed after a reaction time of at least 6 h, which was sufficient to reach full monomer conversion at this temperature.40 Core-shell microgels with a single silica core were obtained for a range of concentration comprised between 20 and 100 mM, as shown in Figure 2. Less than 10% of the particles have a different morphology such as a dumbbell-like structure. Figure 2a,b has a dark background because a staining procedure was used. In this case, the porous structure of the top hydrogel layer is clearly visualized. The samples presented on other images were stained to a lesser extent. The size of the silica seeds has a strong impact on the formation of core-shell particles. Whereas the smallest silica particles allowed the exclusive formation of core-shell particles, synthesis with bigger silica particles generated a second population of pure microgels, with a diameter smaller than the core-shell one (Figure S1 in Supporting Information). The amount of pure microgels depended on the silica size. For the smallest size (d = 140 nm), no pure microgel could be visualized by TEM. For intermediate size (d = 230 nm), very few isolated microgel particles could be visualized. Their amount was estimated to be less than 20% of the total hydrogel volume. For the biggest size (d = 350 nm), the amount was important and concerned 60-70% of the hydrogel phase. Therefore, it was supposed that this effect was due to the decrease in total silica surface area while increasing its diameter. A deficit of the silica surface, which (36) Reculusa, S.; Poncet-Legrand, C.; Perro, A.; Duguet, E.; BourgeatLami, E.; Mingotaud, C.; Ravaine, S. Chem. Mater. 2005, 17(13), 3338. (37) Reculusa, S.; Poncet-Legrand, C.; Ravaine, S.; Mingotaud, C.; Duguet, E.; Bourgeat-Lami, E. Chem. Mater. 2002, 14(5), 2354. (38) Bourgeat-Lami, E.; Insulaire, M.; Reculusa, S.; Perro, A.; Ravaine, S.; Duguet, E. J. Nanosci. Nanotechnol. 2006, 6(2), 432. (39) Bourgeat-Lami, E.; Herrera, N. N.; Putaux, J. L.; Perro, A.; Reculusa, S.; Ravaine, S.; Duguet, E. Macromol. Symp. 2007, 248, 213. (40) Wu, X.; Pelton, R. H.; Hamielec, A. E.; Woods, D. R.; McPhee, W. Colloid Polym. Sci. 1994, 272(4), 467.
DOI: 10.1021/la9003438
4661
Article
Lapeyre et al.
Figure 1. Schematic representation of the preparation of hybrid core-shell microgels.
Figure 2. TEM images (120 kV) of core-shell particles synthesized under different experimental conditions: (a) dSiO2 = 140 nm, [NIPAM] = 35 mM, scale bar 200 nm; (b) dSiO2 = 140 nm, [NIPAM] = 70 mM, scale bar 100 nm; (c) dSiO2 = 230 nm, [NIPAM] = 35 mM, scale bar 200 nm; (d) dSiO2 = 350 nm, [NIPAM] = 35 mM, after removing pure microgels by centrifugation, scale bar 200 nm. could be offered to the growing polymer chains, could explain the occurrence of pure microgels. Experiments were carried out with increasing the silica concentration. The amount of introduced large silica (d = 350 nm) was calculated in order to keep the surface area constant compared to the small silica (d = 140 nm). A concentration of 14 g L-1 was used instead of 5 g L-1 initially. The amount of free microgels significantly decreased (below 30%), meaning that the polymerization occurred preferentially at the silica surface. This result supports the idea that enough silica surface area is necessary to ensure an efficient capture of the reactive species at the beginning of the polymerization, i.e., growing oligomers. A similar result was already reported for polystyrene-silica hybrid particles.38 A practical consequence of this finding is the limited range of size of silica seeds that can be used with no bulk nucleation. The larger the seeds, the higher concentration of silica required. Since the solid content cannot be indefinitely extended, this means that the preparation of core-shell particles in the micrometer size range cannot be achieved without the occurrence of free microgels. However, it should be noted that in most cases the pure microgels could be easily removed by centrifugation, as proved by Figure 2d. This was possible when the size of pure microgels was smaller than the size of core-shell particles. The influence of monomer concentration on the shell thickness was systematically investigated for each size of silica. The two examples of core-shell particles obtained at a given size of silica (Figure 2a,b) show that the thickness of the hydrogel shell can be tuned by simply changing the monomer concentration in the polymerization process. The size estimation was carried out both by PCS and by TEM in the dried 4662
DOI: 10.1021/la9003438
state. The size distribution was very narrow in the case of small silica seeds (d = 140 and 230 nm), as indicated by PDI below 0.1. However, in the case of large silica seeds (d = 350 nm), the size distribution was quite large, with a PDI above 0.1. This could be due to the presence of remaining pure microgels. Therefore, the values were not representative of the core-shell particles. The evolution of the core-shell hydrodynamic diameters as a function of NIPAM concentrations was plotted for the two smallest silica sizes only, i.e., 140 and 230 nm (Figure 3a). It shows that the thickness of the shell increases when the monomer concentration increases. This feature was already reported by Gao et al. for silica with a diameter of 100 nm,24 but it is of interest to determine the influence of the size of the silica seeds on the thickness of the shell. The two silica sizes display a parallel evolution, indicating that the shell thickness is the same at a given monomer concentration. For comparison, the core-shell diameters were estimated by TEM in the dried state (Figure 3b). In general, TEM is not considered as a reliable method for the size determination of microgels because microgels can dry into different collapsed states. However, in our case, the objects were rather homogeneous over large scale (see Figure S2 in Supporting Information). TEM gave complementary information since only the core-shell diameters were measured, and the average value was not perturbed by the potential presence of pure microgels. Figure 3b confirms the tendency observed by PCS and extends the results to the largest silica size. It can be seen that the thickness evolution is the same for the three sizes. More precisely, the thickness of the shell is identical whatever the size of the core for a given monomer concentration. This result indicates that the polymerization rate is independent of the size of the seeds. The polymerization rate was reported to depend on the grafting density,38 which is identical for the three studied silica. Shell thicknesses between 20 and 100 nm could be obtained in this range of monomer concentration. Therefore, the ratio between the core radius and the thickness could be tuned between 0.08 (thin shell, large core) to 1.4 (thick shell, small core). This wide range of morphologies offers interesting perspectives for future capsule formation upon core dissolution. The synthesis of thicker shells was attempted by increasing further the monomer concentration. However, above 100 mM, no simple core-shell structures could be obtained. In the case of small silica particles (140 nm), large irregular microgels of more than 500 nm in size with multiple cores were observed, whereas in the case of bigger silica particles (230 and 350 nm), polymerization led to large pieces of hydrogel entrapping the seeds. The obtained core-shell particles were further imaged by atomic force microscopy, yielding a topographic height image as well as a phase image. Phase images are able to reveal differences in mechanical properties due to interactions between the tip and the particles. The topographic image showed a regular array of ordered particles, which confirmed the good uniformity of the sample (Figure 4a). The height of the particles could be determined by imaging a section across three aligned particles (Figure 4b). The height was ∼170 nm (with a core diameter of 140 nm) and remained almost constant from one particle to the other. The phase image obtained under mild tapping conditions revealed the presence of a central part with a different chemical nature, evidenced by different interactions with the tip (Figure 4d). This feature could be clearly related to the presence of the Langmuir 2009, 25(8), 4659–4667
Lapeyre et al.
Article
Figure 3. Evolution of the core-shell diameters as a function of NIPAM concentration, at constant silica concentration (CSiO = 5 g L-1): (a) hydrodynamic diameter measured by DLS; (b) diameter measured by TEM. The lines are guides for the eyes.
2
Figure 4. AFM images of core-shell particles (dSiO = 140 nm, CSiO = 5 g L-1, [NIPAM] = 70 mM): (a) height image and (b) its corresponding 2
2
cross section; (c) height image with a height range of 400 nm; (d) phase image with a phase range of 130°.
silica core, as assessed by the presence of dumbbell particle having two cores. Finally, the thermoresponsive behavior of the core-shell particles was investigated (Figure 5). The hydrodynamic diameters were found to be bigger at low temperature than at high temperature, which indicated that at least part of the particle is thermoresponsive. The volume phase transition was found to be around 35 °C, which is close to the LCST of pNIPAM. The transition was not sharp, in agreement with Langmuir 2009, 25(8), 4659–4667
the high cross-linking ratio of the hydrogel shell (11 mol %). The thermal response was identical whatever the shell thickness. The presence of underlying silica did not seem to affect the volume phase transition. 3.2. Synthesis and Characterization of Capsules. The core dissolution was achieved by incubating the core-shell particles in HF solution (20%) for at least 4 h. After washing by centrifugation, the suspension was observed by TEM and AFM in the dried state. TEM studies confirmed that the core DOI: 10.1021/la9003438
4663
Article
Lapeyre et al.
Figure 5. Hydrodynamic diameter as a function of temperature for three core-shell particles having a core size of 140 nm (as measured by TEM) and different shell thicknesses of 55, 80, and 105 nm. dissolution was complete (Figure 6). These objects appear to be softer and more deformable than the corresponding core-shell particles, as their shape is less spherical. Capsules with different thicknesses could be obtained from the corresponding mother particles. Shorter exposures to HF led to incomplete dissolution of the silica core, as shown in Figure 6c. AFM characterization confirmed the dissolution of the core, as can be deduced from the height of the cross section (Figure 7). The maximum height was found to be ∼80 nm. 3.3. Application of the Concept to Other Monomers. To extend the range of available applications, the above technology was applied to other monomers. Other N-alkylacrylamide derivatives were tested, and comonomers were introduced. The thermoresponsive NIPAM monomer was replaced by N-isopropylmethacrylamide (NIPMAM). pNIPMAM is known to have a higher LCST than pNIPAM (around 45 °C). The seeded polymerization succeeded in providing core-shell particles, with a shell thickness similar to that of the corresponding pNIPAM shell and with the expected volume phase transition temperature (Figure 8). A slightly higher swelling ratio was observed at room temperature, which may be due to a different cross-linking ratio over the shell. Indeed, it is well-known that the polymerization rate of BIS is higher than NIPAM,41,42 which is itself higher than NIPMAM.43 Therefore, it is expected that the crosslinking degree is not uniform across the shell thickness but decreases from the center of the particle to the periphery, with a more pronounced effect for NIPMAM. Besides the temperature responsiveness, other stimuli could be of interest for biological applications. In particular, the response to variations of pH or to the concentration of a target molecule is of major interest. pH-responsive particles can be obtained by incorporating a pH-sensitive monomer (41) McPhee, W.; Tam, K. C.; Pelton, R. J. Colloid Interface Sci. 1993, 156 (1), 24. (42) Guillermo, A.; Cohen Addad, J. P.; Bazile, J. P.; Duracher, D.; Elaissari, A.; Pichot, C. J. Polym. Sci., Part B: Polym. Phys. 2000, 38(6), 889. (43) Duracher, D.; Elaissari, A.; Pichot, C. J. Polym. Sci., Part A: Polym. Chem. 1999, 37(12), 1823.
4664
DOI: 10.1021/la9003438
such as acrylic acid (AA). These microgels swell when the pH increases at a pH above the pKa of acrylic acid because the presence of carboxylate groups increases the charge density of the polymer chain and drives mobile counterions, which exert an osmotic pressure within the gel. The synthesis of thermoresponsive core-shell particles was repeated but substituting 10 mol % of the NIPAM by acrylic acid. The synthesis yielded three types of particles: a few core-shell particles with shells thicker than that of the corresponding pure pNIPAM analogue (dCS-NIPAM-AA = 350 nm compared to dCS-NIPAM = 300 nm), a few bare silica particles, and a large number of pure microgels (d = 700 nm). Because of the large size of pure microgels, it was not possible to isolate core-shell particles by centrifugation. Glucose-responsive microgels can be obtained by introducing phenylboronic acid (PBA) functions, which can be achieved either by copolymerization44,45 or by grafting carboxylate microgels.46-48 PBA group is known to make a complex with glucose49,50 and shift the ionization equilibrium toward the charged form when complexation occurs around the pKa,51 which causes gel swelling upon an increase of the glucose concentration.52,53 Again, 10 mol % of the NIPAM was substituted by a PBA derivative monomer, 4(1,6-dioxo-2,5-diaza-7-oxamyl)phenylboronic acid (DDOPBA). The synthesis of core-shell particles did not succeed. Immediately after initiation, aggregation of the silica occurred in the seeded monomer solution. Substantial secondary nucleation was also observed. With regard to the results presented above, it appears that the introduction of a comonomer drastically changes the interactions between the growing oligomers and the surface of silica. The reasons for that are not understood at this point. A complete understanding of the mechanism would require further experiments including kinetics studies, which is out of the scope of this article. Besides, core-shell microgels bearing a gel interior and a gel shell were obtained with various monomers.54-57 Therefore, it is possible to achieve different chemical functions when polymerizing at a microgel surface, instead of modified silica surface. This hypothesis will be assessed by using the silica corepNIPAM shell particles as seeds for a second polymerization step (Figure 9). Core-double shell particles should therefore be prepared. 3.4. Synthesis of Core-Shell-Shell Particles with Various Chemistries. 3.4.1. SiO2@pNIPAM@pNIPAM Particles. A proof of principle of this method has been first attempted by polymerizing a second shell around the core-shell particles described above. Silica-pNIPAM core-shell seeds were in(44) Lapeyre, V.; Gosse, I.; Chevreux, S.; Ravaine, V. Biomacromolecules 2006, 7(12), 3356. (45) Lapeyre, V.; Ancla, C.; Catargi, B.; Ravaine, V. J. Colloid Interface Sci. 2008, 327(2), 316. (46) Hoare, T.; Pelton, R. Macromolecules 2007, 40(3), 670–678. (47) Hoare, T.; Pelton, R. Biomacromolecules 2008, 9(2), 733–740. (48) Zhang, Y.; Guan, Y.; Zhou, S. Biomacromolecules 2006, 7(11), 3196. (49) Boeseken, J. Carbohydr. Chem. 1947, 4, 189–210. (50) Aronoff, S.; Chen, T.; Cheveldayoff, M. Carbohydr. Res. 1975, 40, 299. (51) Lorand, J. P.; Edwards, J. O. J. Org. Chem. 1959, 24(6), 769. (52) Kataoka, K.; Miyazaki, H.; Bunya, M.; Okano, T.; Sakurai, Y. J. Am. Chem. Soc. 1998, 120(48), 12694. (53) Matsumoto, A.; Yoshida, R.; Kataoka, K. Biomacromolecules 2004, 5(3), 1038. (54) Jones, C. D.; Lyon, L. A. Macromolecules 2000, 33(22), 8301. (55) Jones, C. D.; Lyon, L. A. Langmuir 2003, 19(11), 4544. (56) Jones, C. D.; Lyon, L. A. Macromolecules 2003, 36(6), 1988. (57) Berndt, I.; Richtering, W. Macromolecules 2003, 36(23), 8780.
Langmuir 2009, 25(8), 4659–4667
Lapeyre et al.
Article
Figure 6. TEM image of the capsules obtained after HF exposure of the core-shell particles obtained with dSiO = 140 nm, CSiO = 5 g L-1, and 2
2
varying monomer concentrations: (a) [NIPAM] = 35 mM; (b) [NIPAM] = 70 mM ; (c) [NIPAM] = 70 mM, incomplete dissolution. Scale bar is 200 nm.
Figure 7. AFM image of a capsule obtained from core dissolution of the same core-shell particle as in Figure 6a: (a) height image and (b) its corresponding cross section.
Figure 8. Evolution of the hydrodynamic diameter of the core-shell particles as a function of temperature for pNIPAM (a) and pNIPMAM (b) shell. The insets represent TEM images of both samples at the same scale (scale bar is 100 nm) in the dried state. troduced in the second polymerization step. One third of the particles obtained after the first polymerization are washed by centrifugation and incorporated in the second polymerization medium. The obtained particles (Figure 10) are found to be much bigger (d = 340 nm) than the core-shell seeds (d = 185 nm). However, the interface between the two polymer shells could not be visualized by TEM due to the identical chemical nature of the polymer. 3.4.2. SiO2@pNIPAM@pNIPAM-AA Particles. The same polymerization procedure was repeated, but 10 mol % of the NIPAM was replaced by AA. The success of the second shell polymerization was estimated by TEM (Figure 10). Again, the thickness of the total shell increased Langmuir 2009, 25(8), 4659–4667
Figure 9. Schematic representation of the preparation of hybrid core-double shell microgels. compared to that of the seeds (d = 420 nm instead of d = 200 nm for the seeds). Since carboxylic groups were expected to be present in the outer shell, it was attempted to visualize the DOI: 10.1021/la9003438
4665
Article
Lapeyre et al.
Figure 10. TEM images of the core-shell seeds (a) and the core-shell-shell particles (b) and (c). The second polymerization was performed with [NIPAM] = 35 mM and 1.6 g L-1 of silica seeds. Image c shows a core-double shell particle containing acrylic acid in the outer shell. The white circle is placed at the frontier between the two shells. The scale bar is 200 nm for all the images.
Figure 12. Representation of the complexation equilibrium between the (alkylamido)phenylboronic acid and glucose in aqueous solution.
Figure 11. Evolution of the hydrodynamic diameter and the electrophoretic mobility of core-shell-shell particles containing AA in the outer shell as a function of pH at 25 °C.
shell interface by specific staining. Uranyl acetate is known to interact with carboxylic groups.54 The core-double shell particles were incubated with uranyl acetate for 60 min under agitation. The resultant image is shown in Figure 10c. Each particle is divided into three areas of different darkness. The deep inner part is the silica. The outer shell is divided in two regions which can be attributed to different chemical interaction with the staining molecule. Electrophoretic mobility measurements were carried out to confirm the presence of acrylic acid in the outer part of the particles. Indeed, electrophoretic measurements can provide information about the surface charge density. Results are presented in Figure 11. It is shown that the electrophoretic mobility is always negative and increases in absolute value as the pH increases. This is in agreement with the presence of carboxylic acid groups which become carboxylate anions when the pH increases. The profile of ionization of polyions is expected to be broad, over more than 3.5 pH units, as reported by Pelton.58 The evolution of the hydrodynamic diameters as a function of pH has been measured in parallel. It is found to change from 630 nm at pH = 3 to 900 nm at pH = 7 at T = 25 °C. The swelling of the microgels is thus related to the variation of the ionization degree with the pH. This is another indirect proof of the presence of carboxylates in the hydrogel shell. 3.4.3. SiO2@pNIPAM@pNIPAM-PBA Particles. Finally, the second shell synthesis was successfully carried out in the presence of PBA monomer derivative, namely DDOPBA (Figure 12). The diameter in collapsed state was 350 nm instead (58) Hoare, T.; Pelton, R. Macromolecules 2004, 37(7), 2544.
4666
DOI: 10.1021/la9003438
Figure 13. Evolution of the hydrodynamic diameter and the electrophoretic mobility of core-shell-shell particles containing PBA in the outer shell as a function of glucose concentration at pH 7.5 (phosphate buffer, 2 mM), T = 25 °C.
of 200 nm for the seeds. As stated previously and illustrated in Figure 12, the PBA group can be in its neutral or charged form depending on the pH. The presence of glucose at a pH around the pKa of the PBA group also increases the presence of charged groups. Indeed, the PBA-glucose complex exists only in its charged form,51 which results in a shift of the ionization equilibrium around the pKa of the PBA toward the charged form. Again, electrophoretic mobility measurements were conducted to assess the presence of acid-base groups. The PBA derivative has a pKa of 7.8. The electrophoretic mobilities were found to reach a plateau below pH = 7 and was close to zero (-0.6 10-8 m2/(V s)) (see Figure S3 in Supporting Information). The remaining value is attributed to the residual charge conferred by the initiator fragments. Above pH 7, the absolute value of the electrophoretic mobility Langmuir 2009, 25(8), 4659–4667
Lapeyre et al.
Article
increased continuously up to -1.8 10-8 m2/(V s). This is in agreement with the presence of PBA groups on the polymer chain. At a pH of 7.5, the electrophoretic mobility of the microgels progressively increased when the glucose concentration increased (Figure 13). This indicated that the complex between PBA group and glucose occurred. In parallel, the hydrodynamic diameter was estimated in the same conditions. It was found to be proportional to the glucose concentration. These experiments confirm that hybrid glucose-sensitive microgels have been synthesized. These particles can be precursors for future glucose-responsive hollow nanocapsules or can be used as functional hybrid materials for optical devices.
4.
Conclusion
The synthesis of multiresponsive core-shell particles bearing a silica core and various shell chemical compositions has been reported. Thermoresponsive core-shell silica-pNIPAM particles have been used as models to systematically investigate the influence of the synthesis conditions. The shell thickness can be controlled by the monomer concentration. At a given monomer concentration, it remains constant regardless of the silica diameter. In contrast, the seed diameter influences the amount of unbound microgels due to the variation of the silica surface area. The range of conditions suitable to obtain coreshell microgels has been identified, which offers a wide range of core sizes and shell thicknesses, as well as their corresponding capsules obtained by exposure to HF. The responsive properties of the particles depend on the nature of the polymer chain constituting the shell. The thermoresponsive properties can be easily tuned by replacing
Langmuir 2009, 25(8), 4659–4667
NIPAM by another alkylacrylamide derivative because their chemical structure is similar. However, the synthesis of pH- or glucose-responsive particles is not trivial because the required comonomers present a different affinity for the silica surface. Simple core-shell particles could not be obtained with these comonomers, but core-double shell bearing functional monomers in the outer shell have been successfully synthesized. Their pH-responsive and glucose-responsive behavior is presented. The glucose responsivess of such highly structured particles offers interesting perspective in terms of biomedical applications, as either sensors or drug delivery systems. Glucose sensors can be achieved by assembly of these uniform particles packed into colloidal crystals. Their optical properties will be improved by the higher refractive index due to the presence of the silica core. Moreover, the dissolution of the core will lead to glucose-responsive hollow capsules, which is particularly attractive for insulin closed-loop delivery.59 Further improvements are currently under study to synthesize glucose-responsive materials operating at physiological conditions. Acknowledgment. The authors thank Etienne Gontier and Sabrina Lacomme from Sercomi, University of Bordeaux 2, for their technical support concerning TEM imaging. Supporting Information Available: Complementary TEM views (Figures S1 and S2); and electrophoretic measurements concerning the pH dependence of PBA microgels (Figure S3). This material is available free of charge via the Internet at http://pubs.acs.org. (59) Ravaine, V.; Ancla, C.; Catargi, B. J. Controlled Release 2008, 132(1), 2.
DOI: 10.1021/la9003438
4667
