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Langmuir 2008, 24, 10414-10420
Polypyrrole Containers Grown on Oil Microdroplets: Encapsulation of Fluorescent Dyes Maciej Mazur Department of Chemistry, UniVersity of Warsaw, Pasteura 1, 02-093 Warsaw, Poland ReceiVed April 15, 2008. ReVised Manuscript ReceiVed July 7, 2008 Polypyrrole microcapsules were synthesized by the chemical deposition of the polymer onto mineral oil droplets adsorbed onto glass or quartz substrates. The droplets act as a kind of a template that directs the polymer growth into the form of three-dimensional containers. The polymer is deposited on both the surface of oil droplets and the glass or quartz substrate entrapping the oil content. The dissolution of chemical species in oil prior to polymer deposition permits the encapsulation of these species within the cavities of the containers. This phenomenon was demonstrated through the entrapment of the fluorescent dyes, pyrene and perylene, within the polymeric capsules. It was shown that the entrapped molecules can be released from the capsules by controlling the hydration of the polymer, which in turn changes the permeability of the oil content through the polypyrrole shells. The polymer growth and encapsulation phenomena were investigated with a range of complementary physicochemical techniques, including microscopic (AFM, SEM, and confocal microscopy) and spectroscopic (steady-state fluorescence and UV-vis absorption) methods. In particular, the use of optical methods was possible due to the deposition of the capsules on transparent substrates (glass, quartz). As a consequence, the optical information on the interior of the capsules was accessible, for example, dye concentration and local polarity.
Introduction Nano- and microstructured conducting polymers have gained considerable attention in recent years due to their remarkable optical, catalytic, and electrical properties. They have been investigated for exciting applications, including novel polymeric batteries, solar cells, electrochromic displays, sensing devices, and antistatic coatings.1-6 The synthesis and applications of polymeric hollow structures are of particular interest for its potential use in drug delivery,7-10 dye encapsulation,11 the protection of proteins,12,13 and heterogeneous catalysis.14,15 These materials are typically synthesized via the so-called hard-template method, where the template directs the polymer growth to form hollow containers. One of the first and most remarkable examples of template synthesis is the preparation of polymeric nanotubes by Martin.16-18 The cylindrical nanotubes can be grown within the pores of polycarbonate or alumina membranes, which then can be released from the (1) Novak, P.; Muller, K.; Santhanam, K. S. V.; Haas, O. Chem. ReV. 1997, 97, 207. (2) Dhawan, S. K.; Singh, N.; Venkatachalam, S. Synth. Met. 2001, 125, 389. (3) Somani, P.; Mandale, A. B.; Radhakrishnan, S. Acta Mater. 2000, 48, 2859. (4) Conroy, K. G.; Breslin, C. B. Electrochim. Acta 2003, 48, 721. (5) Maziarz, E. P.; Lorenz, S. A.; White, T. P.; Wood, T. D. J. Am. Soc. Mass Spectrom. 2000, 11, 659. (6) Huang, J.; Virji, S.; Weiller, B. H.; Kaner, R. B. J. Am. Chem. Soc. 2003, 125, 314. (7) Langer, R. Science 1990, 249, 1527. (8) Donath, E.; Sukhorukov, G. B.; Caruso, F.; Davis, S. A.; Mo¨hwald, H. Angew. Chem., Int. Ed. 1998, 37, 2202. (9) Lu, G.; An, Z.; Tao, C.; Li, J. Langmuir 2004, 20, 8401. (10) Li, J.; Zhang, Y.; Yan, L. Angew. Chem., Int. Ed. 2001, 40, 891. (11) Jang, J.; Oh, J. H. AdV. Mater. 2003, 15, 977. (12) Caruso, F.; Trau, D.; Mo¨hwald, H.; Renneberg, R. Langmuir 2000, 16, 1485. (13) Gill, I.; Ballesteros, A. J. Am. Chem. Soc. 1998, 120, 8587. (14) Morris, C. A.; Anderson, M. L.; Stroud, R. M.; Merzbacher, C. I.; Rolison, D. R. Science 1999, 284, 622. (15) Feilchenfeld, H.; Chumanov, G.; Cotton, T. M. J. Phys. Chem. 1996, 100, 4937. (16) Martin, C. R.; Van Dyke, L. S.; Cai, Z.; Liang, W. J. Am. Chem. Soc. 1990, 112, 8976. (17) Martin, C. R. Acc. Chem. Res. 1995, 28, 61. (18) Martin, C. R. Science 1994, 266, 1961.
matrix by dissolving the template. To prepare spherical polymeric hollow structures, solid nano- or microparticles can be used. These particles are coated with a polymer by oxidatively polymerizing the monomer. The particles are then dissolved, leaving cavities within the polymeric containers. The particles used in this process include gold clusters,19-23 silica colloids,24,25 and polystyrene beads.26-32 Polyhedral hollow structures synthesized through the deposition of polymer onto inorganic salts or oxide crystals have been also reported.33-36 As an alternative to the hard template technique, soft-templating strategies have been recently developed. For example, gaseous bubbles were electrochemically generated at the electrode surface and coated with polypyrrole, yielding several microcontainers (19) Marinakos, S. M.; Novak, J. P.; Brousseau, L. C.; House, A. B.; Edeki, E. M.; Feldhaus, J. C.; Feldheim, D. L. J. Am. Chem. Soc. 1999, 121, 8518. (20) Marinakos, S. M.; Anderson, M. F.; Ryan, J. A.; Martin, L. D.; Feldheim, D. L. J. Phys. Chem. B 2001, 105, 8872. (21) Marinakos, S. M.; Shultz, D. A.; Feldheim, D. L. AdV. Mater. 1999, 11, 34. (22) Marinakos, S. M.; Brousseau, L. C.; Jones, A.; Feldheim, D. L. Chem. Mater. 1998, 10, 1214. (23) Wu, M.; O’Neill, S. A.; Brousseau, L. C.; McConnell, W. P.; Shultz, D. A.; Linderman, R. J.; Feldheim, D. L. Chem. Commun. 2000, 775. (24) Hao, L.; Zhu, C.; Chen, C.; Kang, P.; Hu, Y.; Fan, W.; Chen, Z. Synth. Met. 2003, 139, 391. (25) Fu, G. D.; Zhao, J. P.; Sun, Y. M.; Kang, E. T.; Neoh, K. G. Macromolecules 2007, 40, 2271. (26) Feng, X.; Mao, C.; Yang, G.; Hou, W.; Zhu, J.-J. Langmuir 2006, 22, 4384. (27) Park, M. K.; Onishi, K.; Locklin, J.; Caruso, F.; Advincula, R. C. Langmuir 2003, 19, 8550. (28) Yang, L.; Yang, Z.; Cao, W. J. Colloid Interface Sci. 2005, 292, 503. (29) Yang, Y.; Chu, Y.; Yang, F.; Zhang, Y. Mater. Chem. Phys. 2005, 92, 164. (30) Niu, Z.; Yang, Z.; Hu, Z.; Lu, Y.; Han, C. C. AdV. Funct. Mater. 2003, 13, 949. (31) Andreeva, D. V.; Gorin, D. A.; Shchukin, D. G.; Sukhorukov, G. B. Macromol. Rapid Commun. 2006, 27, 931. (32) Shi, X.; Briseno, A. L.; Sanedrin, R. J.; Zhou, F. Macromolecules 2003, 36, 4093. (33) Cheng, D.; Xia, H.; Chan, H. S. O. Nanotechnology 2006, 17, 1661. (34) Zhang, Z.; Sui, J.; Zhang, L.; Wan, M.; Wei, Y.; Yu, L. AdV. Mater. 2005, 17, 2854. (35) Yang, X.; Lu, Y. Polymer 2005, 46, 5324. (36) Zhu, C.-L.; Chou, S.-W.; He, S.-F.; Liao, W.-N.; Chen, C.-C. Nanotechnology 2007, 18, 275604.
10.1021/la801177q CCC: $40.75 2008 American Chemical Society Published on Web 08/14/2008
Polypyrrole Containers Grown on Oil Microdroplets
of various geometries, including bottles, whiskers, and cups.37-40 Chemically generated gas bubbles have also been used to fabricate poly(2-methoxyaniline) microcapsules deposited on glass substrates.41 Liquid droplets can be also used for the template growth of polymeric nano- or microspheres. Emulsion polymerization has been employed to prepare containers of polypyrrole and polyaniline.42,43 The encapsulation of chemical species within cavities of polymeric structures can be realized using three methods. In the first method, a small opening is left within the shell of a container during the container’s synthesis to allow subsequent loading of the empty cavity with guest molecules. After the empty cavity is loaded, the opening is sealed using an additional polymerization step. Using this method, polymeric nanotubes were used to entrap several enzymes. The nanotubes were prepared within a polycarbonate membrane, which was then immersed in a solution of the enzyme, and then finally subjected to a second polymerization step to close the openings of the nanotubes.44,45 Bajpai et al. prepared semiclosed polypyrrole containers in the presence of fluorescein cadaverin in solution. The containers were loaded with the dye molecules, and the openings of the capsules were sealed with a subsequent electropolymerization.40 The second encapsulation method uses the permeation of chemical species through the walls of the container. Using this method, Jang et al. loaded polypyrrole nanospheres with pyrene.46 In the third encapsulation method, the chemical species are entrapped prior to the polymerization step. Feldheim et al. covalently attached rhodamine B to gold nanoparticles, which were then subsequently coated with polypyrrole. The gold cores were then etched in a KCN solution, leaving the dye molecules entrapped in the capsules.19 In this Article, oil microdroplets are used in the template polymerization of pyrrole into semispherical containers deposited on glass or quartz surfaces. The dissolution of fluorescent dyes within the droplets prior to polymer deposition allows their entrapment within the capsules. Because of the use of glass or quartz as substrates, the encapsulated dyes are easily excited from the transparent substrate side, wherein emitted light can be used to track phenomena occurring within the capsules.
Experimental Section Materials and Methods. Chemicals. All chemicals were of the highest quality commercially available: pyrrole (Aldrich, 98%), mineral oil (Aldrich, for IR spectroscopy), pyrene (Aldrich, 99%), perylene (Aldrich, 99.5%), iron(III) chloride (Aldrich, 97%). Pyrrole was distilled prior to use. Aqueous solutions were prepared from high-purity water (Milli-Q Plus). Instrumentation. Reflected light optical microscopy images of the oil droplets adsorbed onto glass and quartz substrates were recorded using a Nanoscope Optical Viewing System OMV-PAL (Veeco). The sheet conductivities of polymeric adlayers were determined using a four-probe technique (Jandel RM3, Jandel Engineering). Contact angle measurements were performed using a homemade instrument equipped with an optical microscope and a MicrOcular 1.3 MP USB camera (Bresser, Germany). (37) Qu, L.; Shi, G. Chem. Commun. 2003, 206. (38) Ma, M.; Qu, L.; Shi, G. J. Appl. Polym. Sci. 2005, 98, 2550. (39) Yuan, J.; Qu, L.; Zhang, D.; Shi, G. Chem. Commun. 2004, 994. (40) Bajpai, V.; He, P.; Dai, L. AdV. Funct. Mater. 2004, 14, 145. (41) Mazur, M. J. Phys. Chem. C, accepted. (42) Gao, Y.; Zhao, L.; Bai, H.; Chen, Q.; Shi, G. J. Electroanal. Chem. 2006, 597, 13. (43) Wei, Z.; Wan, M. AdV. Mater. 2002, 14, 1314. (44) Martin, C. R.; Parthasarathy, R. AdV. Mater. 1995, 7, 487. (45) Parthasarathy, R.; Martin, C. R. J. Appl. Polym. Sci. 1996, 62, 875. (46) Jang, J.; Oh, J. H.; Li, X. L. J. Mater. Chem. 2004, 14, 2872.
Langmuir, Vol. 24, No. 18, 2008 10415 UV-vis spectra were recorded using a Lambda 25 (Perkin-Elmer) spectrometer. Fluorescence emission spectra were collected with a Fluorolog 3 spectrometer (Horiba-Jobin Yvon). Atomic force microscopy (Nanoscope V, Veeco), working in contact mode in water (in situ), was used to image the polymeric capsules. The thickness of the polypyrrole films on glass was determined ex situ in air. The films were scratched with a needle and imaged at the scratch edge to determine the adlayer thickness. Scanning electron microscopy images were obtained with a LEO 435 VP microscope on Au-Pd sputter-coated samples. A MultiView 4000 (Nanonics), working in confocal mode, was used to record the fluorescence images of the dyes entrapped within the polymeric containers. The sample was excited from the transparent substrate side with a 442 nm He-Cd laser (Kimmon Koha Co.). The emitted light was collected in the far field with an upright optical microscope (Olympus) equipped with a PMT detector (Perkin-Elmer). The excitation light was filtered using a 442 nm notch filter (Semrock, Inc.) placed in front of the detector. The polymer growth at the oil droplets’ surface was investigated using reflection measurements with essentially the same aforementioned experimental setup. The only difference was that a 442 nm band-pass filter (Edmund Optics) was placed in front of the detector. The reflection signal was recorded as a function of time, while the laser light was focused on the droplet’s oil-water interface. Procedures and Materials. Glass and Quartz Substrates. To support polymeric capsules in ex situ measurements, conventional microscopic coverslips were used. In situ measurements were conducted in glass or quartz cuvettes. Preparation of O/W Emulsions. Oil-in-water emulsions were prepared by extensively shaking a 50 µL volume of mineral oil or mineral oil solution (10 mM pyrene or 0.5 mM perylene) with 5 mL of water for 30 s, and then sonicating with an ultrasonic processor (UP400S, Hielscher Ultrasound Technology) for 3 min at 400 W. Adsorption of Oil Droplets. Glass or quartz slides were immersed in the emulsion for ca. 3 min, and then removed and permitted to dry. For in situ measurements, the emulsions were poured into cuvettes for ca. 3 min to permit the adsorption of droplets onto the inner sides of the cuvettes’ walls. Deposition of Polypyrrole. Polypyrrole was deposited onto the substrates (with or without adsorbed oil droplets) either by immersing the substrate into the polymerization solution or by pouring the solution into the cuvette. In each deposition experiment, a new fresh solution was used. The polymerization bath was prepared by adding 2 mL of aqueous pyrrole (0.14 M) to 2 mL of an aqueous iron(III) chloride solution (10 mM). A typical polymerization time was 20 min. After polymerization, the substrates were immersed in distilled water to remove unbound polymeric material.
Results and Discussion Preparation of Polymeric Capsules. In the following work, polypyrrole microcontainers were deposited on transparent substrates in two stages. First, liquid emulsion microdroplets were adsorbed onto the solid substrate, and then coated with a thin adlayer of polymeric material. To achieve this task, an oilin-water emulsion was prepared by sonication, and then subsequently used as a dipping solution for glass slides. The immersion of the glass slides results in a rapid adsorption of oil droplets onto its surface. The adsorption of oil droplets onto the glass surface is relatively easy because no emulsion stabilizer is used. As a consequence, the emulsion is relatively unstable, permitting fusion of the droplets onto the glass surface. The reflected light optical microscope image of the particles is depicted in Figure 1a. As seen from this image, the droplets are randomly distributed on the surface, with their sizes ranging in several micrometers. The number density of the droplets is ca. 3.9 × 105 droplets/cm2, and their diameter distribution is presented in Figure 1b.
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Figure 2. Dependence of polypyrrole film thickness (9) and sheet conductivity (0) versus polymerization time.
Figure 1. (a) Optical microscopy image of oil droplets adsorbed onto glass (in water). (b) Oil droplet size distribution.
From contact angle measurements, the contact area of the droplets adsorbed on the surface in both the aqueous and the air environments can be estimated. The contact angle of a macroscopic droplet (1 µL) on glass in air is ca. 25°. This value changes considerably when the droplet is immersed in water. Because of the low density of oil, the direct measurement of the contact angle of the oil droplet in water is experimentally difficult; hence, a water droplet in oil was measured as an alternative. A value of 73° (water in oil) corresponds to a 107° contact angle of an oil droplet in water. The considerable difference in contact angles of droplets in water and air significantly influences their respective geometries. The calculated contact area diameter of a 2 µmsized oil droplet in water is ca. 1.9 µm. This value increases to 9.9 µm when the droplet is in an air environment. After oil droplet adsorption, the next step in the microcontainer preparation is the deposition of polypyrrole. Prior to capsule synthesis, the deposition of the polymer onto glass was first examined to better control this process. Polypyrrole can be easily prepared in the form of a thin film adjacent to the substrate by in situ polymerization.47 Polymer layers were grown by immersing glass slides into a polymerization bath containing monomer (pyrrole) and oxidant (FeCl3). On the basis of atomic force microscopy (AFM) measurements, the film thickness was determined as a function of polymerization time (Figure 2). One can see that the thickness increases linearly for polymerization (47) Huang, Z.; Wang, P. C.; MacDiarmid, A. G.; Xia, Y.; Whitesides, G. Langmuir 1997, 13, 6480.
times above 10 min, reaching values of up to ca. 100 nm (for 30 min). A similar curve is obtained from conductivity measurements. The sheet conductivity was determined using a standard four-probe method and was observed to increase monotonically with polymerization times ranging from 0 to ca. 350 µS/0. The low value at 5 min (close to 0) is likely due to discontinuity in the polymeric film (discontinuities are also seen on AFM images, not shown). A bulk conductivity of 27 S/cm for the polymer film can be recovered by considering the linear parts of both the thickness and the conductivity plots (above 10 min). This value agrees well with the bulk conductivity of polypyrrole reported in the literature.48 Using this information, polypyrrole was deposited onto the droplet-decorated surfaces. The oil droplets were deposited onto the quartz cuvette’s walls, which was then filled with the polymerization solution and used to obtain the UV-vis absorption spectra of the growing polypyrrole. The consecutive spectra for various polymerization times are depicted in Figure 3a. The absorbance spectra measured by the spectrometer include the contribution from the polymeric film deposited on the quartz surface and oil droplet interface, as well as from the bulky polymer precipitated in the solution. These spectra show a poorly resolved absorption band at ca. 470 nm and a very broad band with a maximum likely above the higher limit of the investigated spectral range. As the polymerization time increased, both bands and the background were observed to gradually increase. Confocal microscopy was used to directly probe the formation of polymer at the droplets’ oil-water interface. The monochromatic, 442 nm light of a He-Cd laser was coupled into the microscope and focused close to the oil/water interface of the droplet deposited on the inner side of the quartz cuvette’s wall. The sample was illuminated from the quartz window side, thus permitting the collection of reflected light by the microscope objective. The intensity of the light reflected from the droplet’s interface was recorded as a function of polymerization time. The deposition curve is shown in Figure 3b. It can be seen that the intensity rapidly decreases to a constant value after ca. 8 min. The decrease in the reflected light intensity is most likely due to the absorption of the polymeric film formed at the oil-water interface. Comparison with AFM and conductivity data (Figure 2) may suggest that polypyrrole deposits faster on the oil surface than on glass, which is reasonable because polypyrrole is known to preferentially deposit on hydrophobic substrates;47 however, it seems more likely that the initial rapid decrease in the reflection (48) Maddison, D. S.; Unsworth, J.; Roberts, R. B. Synth. Met. 1988, 26, 99.
Polypyrrole Containers Grown on Oil Microdroplets
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Figure 4. In situ (aqueous environment) AFM image of a polypyrrole capsule deposited on glass.
Figure 3. (a) Consecutive UV-vis absorption spectra of pyrrole polymerized in glass cuvettes (cuvette thickness: 1 mm). (b) Time dependence of light intensity (442 nm) reflected from the oil droplet-water interface during polypyrrole deposition.
signal is primarily associated with changes in the film compactness rather than its thickness. As long as the film formed at the oil-water interface is not continuous, the reflected light signal decreases. Once the oil surface is coated with a compact polypyrrole adlayer, further increases in the film thickness do not decrease the light intensity. The polypyrrole grown onto the oil droplets was further studied with in situ AFM. The AFM image of a typical polypyrrole capsule immersed in water is depicted in Figure 4. The capsule appears as a hemispherical object adjacent to the substrate surface. The diameter of the capsule is ca. 5 µm, and its height is ca. 350 nm. The capsule is surrounded by a polymeric film covering the glass surface (the topography of the glass is considerably different, the AFM image not shown). The diameter of the capsule is significantly larger than the average diameter of the droplets adsorbed onto the glass in water (compare Figure 1). This is likely due to changes in the oil-water interfacial tension associated with polymer deposition. Assuming a constant volume of oil and the dimensions of the capsule as determined by AFM, one can calculate the initial observable droplet diameter to be ca. 2.1 µm. This shows that the geometry of the oil droplet changes considerably during polymer deposition. Additional microscopic studies were performed using scanning electron microscopy (SEM). Because this is an ex situ technique, the substrate was dried and sputter-coated with a thin Pd-Au film to facilitate SEM imaging. The image of the polymeric
Figure 5. SEM image of polypyrrole grown on oil droplet decorated glass.
deposit is depicted in Figure 5. One can see a number of dark circles randomly distributed in the continuous polymeric film. The number density of the spots is ca. 6 × 104 cm-2 (determined from the SEM images by counting the dark spots and dividing the value by the area of the surface), which is significantly lower than the number density of droplets on glass. This shows that a considerable fraction of droplets was removed from the surface during transferring the sample from one solution (emulsion) to another (polymerization bath). The dark spots are the areas of lowered secondary electron emission from the sample. It is apparent that during drying, the polymeric capsules collapsed and the dark circles observed on the SEM images correspond to oil that was flown away from the capsules through their shells during the drying and metal sputtering processes. The oil-soaked spots reveal reduced conductivity; hence, the amount of emitted secondary electrons (collected by SEM detector) is relatively lower as compared to other measured areas, accounting for the appearance of these areas as a dark color on the image. Encapsulation of Fluorescent Dyes within Polymeric Capsules. The potential for guest molecule encapsulation inside the cavities of polymeric structures grown on droplet templates seems to be one of the primary advantages of the presented approach. When molecules are dissolved in the oil droplets prior to polymer deposition, they become entrapped after the formation of the structure. To demonstrate the encapsulation of molecules within polypyrrole capsules, two fluorescent dyes, pyrene and perylene, were selected. Their presence inside the capsules can
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Figure 6. (a) Consecutive in situ emission spectra of pyrene being encapsulated in polypyrrole containers (excitation at 320 nm from the quartz window side). (b) Dependence of calculated pyrene concentration (9) and intensity of the emission band at 393 nm (0) versus polymerization time. Inset: Calibration curve - dependence of the 465 to 393 nm band ratio as a function of pyrene concentration in mineral oil.
be easily detected with spectrofluorometry, provided the structures are deposited on transparent and nonfluorescent substrates, for example, quartz. The molecules can be excited from the transparent window side, and the emitted light can be effectively collected with the spectrometer. The growth of polymeric capsules and the encapsulation process have been investigated in situ by registering the emission spectra of pyrene dissolved in the oil droplets. The droplets were adsorbed on the inner side of the wall of a quartz cuvette filled with a polymerization solution. The excitation beam was directed onto the cuvette wall at ca. 45°, and the emitted light was collected using the front-face option of the spectrometer. Figure 6a shows the consecutive emission spectra of pyrene being entrapped in polymeric capsules recorded at varying polymerization times. Characteristic bands at ca. 393 and 468 nm, corresponding to pyrene, can be observed.49 The band at 468 is due to excimer (excited dimers) emission. The band at ca. 393 nm, which is associated with pyrene monomer emission, exhibits a fine vibronic structure. The ratio of the 372 and 383 nm bands is known to be dependent on the polarity of pyrene local environment.50,51 The calculated ratio of these bands is equal to ca. 0.56 and does not change significantly during the polymer deposition. The global intensity of the spectra diminishes with polymerization time. The dependence of the emission band at 393 nm (49) Winnik, F. M. Chem. ReV. 1993, 93, 587. (50) Dong, D. C.; Winnik, F. Can. J. Chem. 1984, 62, 2560. (51) Mazur, M.; Blanchard, G. J. J. Phys. Chem. B 2005, 109, 4076.
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versus time is presented in Figure 6b. There are likely three main reasons for this phenomenon. When the oil droplet becomes coated with a polymeric film, the multiply reflected excitation light inside the droplet is absorbed by the polymer. As a consequence, the intensity of the emitted light is lowered. The second reason for a fluorescence signal decrease is the oxidation of pyrene by FeCl3 that is present in the polymerization mixture.52 Finally, the pyrene emission may be quenched through the interaction of the dye molecules with polypyrrole.53,54 The polymer absorption effect, as shown above (Figure 3b), is dominant in the initial stages of polypyrrole growth (until ca. 8 min). To quantify the pyrene degradation effect, the concentration of pyrene inside the droplets/capsules was determined. The oxidation reaction likely occurs at the water-oil interface, yielding several quinone and hydroquinone pyrene derivatives, which are less or nonfluorescent.52 Pyrene dye is known to reveal excimer emission with an excimer band that increases with molar concentration.49 The excimer band (468 nm) intensity was measured and normalized with respect to the monomer emission band at 393 nm. To calculate the concentration inside the capsules, a calibration curve was prepared. The spectra of several pyrene solutions in mineral oil were recorded, and the ratio of the bands at 468 and 393 nm was plotted versus the molar dye concentration (inset to Figure 6b). Using this information, it was possible to determine the pyrene concentration within the oil droplets during the polypyrrole deposition simply by determining the excimer to monomer band ratio. The dependence of pyrene concentration on the polymerization time is depicted in Figure 6b. One can see that the pyrene concentration rapidly decreases at the initial stages of polymer deposition (within ca. 3 min). The rate of decrease slows until the concentration reaches a constant value of ca. 8.7 mM after ca. 15 min (this corresponds to 87% of the initial concentration). On the basis of the above experiments, one can conclude that the diminution of emission intensity below 15 min can be explained in terms of light absorption and dye degradation; however, because the emission intensity also decreases after 15 min, this effect must also be associated with the quenching of the pyrene fluorescence. The fluorescence signal (after 15 min) decreases linearly at a rate of 1.9%/min of the initial (extrapolated) value. Let us assume a model of a droplet as a cylinder with a 5 µm diameter and 200 nm height (having a volume comparable to that of the capsule imaged by AFM, Figure 4). If the light intensity is proportional to the oil volume, a decrease of 1.9%/ min would correspond to a 3.8 nm/min decrease in the droplet height. From the physical point of view, such a scenario could be interpreted as growth of polypyrrole film toward the interior of the droplet. Because of the quenching of the dye fluorescence by polypyrrole, only the pyrene/oil residing within the capsule that was not absorbed by the polymer emits light. The polymeric layer becomes swollen with oil/pyrene at a rate of 3.8 nm/min, which produces a decrease in the fluorescence signal at a rate of 1.9%/min. This coincides with a polymeric film growth at ca. 3.3 nm/min, as determined from AFM measurements (compare Figure 2a). If these simplified calculations are correct, the following scenario can be drawn. The polypyrrole is initiated at the oil-water interface, and its growth is directed toward the oil phase. Nearly the entire polymeric film becomes soaked in oil, likely with the exception of a thin hydrated layer of polymer at the aqueous phase side. (52) Mazur, M.; Blanchard, G. J. J. Phys. Chem. B 2004, 108, 1038. (53) Ramanavicius, A.; Kurilcik, N.; Jursenas, S.; Finkelsteinas, A.; Ramanaviciene, A. Biosens. Bioelectron. 2007, 23, 499. (54) Song, X.; Wang, H.; Shi, J.; Park, J.-W.; Swanson, B. I. Chem. Mater. 2002, 14, 2342.
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Figure 8. Time dependence of the emission band at 393 nm of pyrene encapsulated in polypyrrole containers. The measurement is performed ex situ in air using the front face option of the spectrometer: the sample excited at 320 nm from the quartz window side (9), the sample excited from the polymer side (0).
Figure 7. (a) Consecutive in situ emission spectra of perylene being encapsulated in polypyrrole containers (excitation at 415 nm from the quartz window side). (b) Confocal fluorescence microscopy image of perylene encapsulated in polypyrrole containers (excitation at 442 nm from the glass window side; the capsules are immersed in water).
We also studied the encapsulation of another fluorescent dye, perylene. The consecutive emission spectra of perylene dissolved in oil droplets during polypyrrole deposition are shown in Figure 7a. The spectrum of perylene reveals three characteristic emission bands at 440, 468, and 500 nm.55 Similar to the case with pyrene, the intensity of the spectrum diminishes with polymerization time, but decreases much faster. This is likely due to the lower oxidation potential of perylene with respect to pyrene.56 The Fe3+ ions present in the polymerization solution oxidize dye molecules to several quinone and hydroquinone derivatives, which are nonfluorescent.55 Unfortunately, the molar concentration of perylene in the capsules cannot be easily estimated, because perylene does not reveal excimer emission. The entrapment of perylene in the polypyrrole capsules was also confirmed by confocal fluorescence microscopy. The sample was scanned with laser light at 442 nm from the glass window side, and the emitted light at wavelengths above 442 nm was collected with a microscope objective. A typical confocal microscopy image of perylene entrapped in polypyrrole capsules is shown in Figure 7b. One can see several bright circular spots corresponding to fluorescence light emitted by perylene molecules. The size of the spots is in the range of several micrometers, which agrees well with the size of capsules previously determined by AFM (compare Figure 4). (55) Mazur, M.; Blanchard, G. J. Langmuir 2005, 21, 1038. (56) Mazur, M.; Krysinski, P.; Blanchard, G. J. Langmuir 2005, 21, 8802.
The fluorescence signal of dyes entrapped within polymeric capsules is stable after removal from the polymerization solution, provided the capsules stay immersed in an aqueous medium. When the samples are allowed to dry, the fluorescence signals disappear. It seems this is associated with the collapse of the capsules, which was already observed in the SEM measurements (compare Figure 5). To clarify the cause of signal quenching, fluorescence measurements of samples in a dry environment were recorded immediately after their removal from the polymerization solution. Figure 8 depicts pyrene fluorescence signals collected at 393 nm, recorded when the sample was excited from the quartz window side and polymer side, respectively. When the pyrene entrapped within the capsules was excited from the quartz window side, a drying-induced lowering of the fluorescence signal is observed (Figure 8, 9). On the other hand, when the sample is excited from the polymer-deposited side (Figure 8, 0), an initially low fluorescence signal is observed. With drying, the fluorescence signal increases, reaching a plateau after ca. 20 min. The explanation of this phenomenon is as follows. When the polypyrrole is wet, due to hydrophobicity, the oil stays entrapped within the capsules. When the polymer becomes dry, the oil with the fluorophore flows away from the capsules through the polypyrrole walls. This behavior is associated with the increase of the emission signal recorded by the spectrometer when the sample is excited from the polymer side. A similar result is obtained when the fluorescence of an individual capsule containing perylene is measured. The excitation light (442 nm) was focused onto the capsule from the polymer side using a confocal microscope, and the fluorescence signal was measured as a function of time (see Figure I, Supporting Information). The observable increase of the fluorescence is due to perylene release from the capsule. It was also confirmed that subsequent rehydration of the sample in water does not result in uptake of the oil content back into the capsule as the fluorescence signal does not decrease during the drying of the rehydrated sample.
Conclusions Polypyrrole capsules were prepared by depositing polymer onto glass (or quartz) substrates decorated with oil microdroplets. When guest molecules are dissolved in the oil droplets prior to polymer deposition, they become entrapped within the formed polymer capsules. The fate of the molecules encapsulated within
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the polymeric containers can be tracked using fluorescence characterization methods. Encapsulated dyes can be excited from the transparent window side, while the emitted light can be collected in front-face geometry. This configuration permits information to be collected on the concentration and microenvironment of the dye directly from the interior of the container. The release of the microcapsule content can be achieved by controlling the hydration of the polypyrrole shell. When the containers are in an aqueous or humid environment, the dye molecules stay entrapped. When the polypyrrole is dried, the oil with the fluorophore permeates through the polymeric walls, flowing away from the containers. In this Article, it was shown that the encapsulation of chemical species involves several phenomena, for example, degradation of encapsulated molecules or their interaction with polymeric shell. Gaining that information was possible due to the use of fluorescent dyes that allow tracking their behavior within the capsules. These findings provide basis for further work on preparation of polymeric structures. For example, to prevent the capsule collapse, “solidified” rather than liquid droplets can be used. We are currently extensively working on this issue in our laboratory.
Mazur
Acknowledgment. The funding for this work was provided by the Ministry of Science and Higher Education within project N204 117 32/3116 for years 2007-2010. Confocal microscopy and spectrofluorometry measurements were conducted in the Structural Research Laboratory (SRL) at the Department of Chemistry, University of Warsaw, Poland. SRL has been established with financial support of the European Regional Development Fund in the Sectoral Operational Programme “Improvement of the Competitiveness of Enterprises, years 2004-2006” project no: WKP_1/1.4.3./1/2004/72/72/165/ 2005/U. Supporting Information Available: Figure I: Time dependence of perylene fluorescence signal intensity versus time. The measurement is performed ex situ in air using a confocal microscope. The excitation light of the He-Cd laser (442 nm) is focused onto the individual polymeric capsule from the polymer side, and the emitted light is collected with the microscope objective. This material is available free of charge via the Internet at http://pubs.acs.org. LA801177Q
