Toluene-Filled Polypyrrole Microvessels: Entrapment and Dynamics of

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Toluene-Filled Polypyrrole Microvessels: Entrapment and Dynamics of Encapsulated Perylene Daria Kubacka,† Paweł Krysin´ski,† Gary J. Blanchard,‡ Jarosław Stolarski,§ and Maciej Mazur*,† Department of Chemistry, UniVersity of Warsaw, Pasteura 1, 02-093 Warsaw, Poland, Department of Chemistry, Michigan State UniVersity, East Lansing, Michigan 48824, United States, and Institute of Paleobiology, Polish Academy of Sciences, Twarda 51/55, 00-818 Warsaw, Poland ReceiVed: August 4, 2010; ReVised Manuscript ReceiVed: October 6, 2010

Solid-supported and free-standing polypyrrole microcapsules were synthesized by deposition of the polymer onto toluene droplets. The polymer forms an encapsulating thin layer on the droplet surface. The encapsulation of the solvent was verified by FTIR measurements. Entrapment of other guest molecules can be achieved by using a solution of the guest molecules to prepare the droplets. This was demonstrated with perylene, a hydrophobic fluorescent molecule with well established spectroscopic properties. The encapsulation of perylene was probed with fluorescence spectroscopic techniques and optical microscopy. Time-resolved measurements allowed determination of relaxation dynamics of the fluorophore trapped in the capsules. It was shown that the rotational diffusion of perylene in toluene droplets is best described as a prolate rotor. The reorientation data suggest an increased solvent viscosity within the capsule. Introduction There has been considerable recent research effort aimed at the synthesis and characterization of hollow polymeric structures because of the inherent utility of such materials. Hollow polymeric structures can exhibit interesting optical, electronic, and catalytic properties and can be used to entrap guest molecules such as enzymes, drugs, dyes, and nanoparticles. For these reasons, hollow polymeric structures have been considered for use in a number of applications, such as drug delivery, protein protection, dye encapsulation, and are used in experiments mimicking controlled biomineralization. This structural motif can also be useful in the construction of novel polymeric batteries, sensing devices, and solar cells.1–4 The synthesis of hollow polymeric structures is typically achieved by a templating method. In the template approach, a colloidal particle is coated with a thin polymer layer which results in the formation of a core-shell structure. Several types of colloidal particle can be used to template the growth of the polymer, e.g. solid, liquid, or gaseous colloids. When solid particles are used as templates, the cores are removed following deposition of the polymer. The removal of the core may be accomplished by etching or dissolution, and a significant challenge can be in the identification of a removal method that does not damage the polymer shell. For gaseous or liquid templates, this step is usually not required, and template removal is more straightforward.3 Nano- and micrometer-sized hollow structures of conjugated polymers are of special interest because these materials possess unique properties that are not readily attainable by other means.5 Conjugated polymer hollow structures can change their permeability or optical properties by changing the protonation or oxidation state of the polymer, thereby providing facile control * Corresponding author. E-mail: [email protected]. † University of Warsaw. ‡ Michigan State University. § Polish Academy of Sciences.

over processes such as release and uptake of encapsulated species, light emission, or conductivity, for example. Spherical or cap-like structures have been prepared through template synthesis using solid,6–24 liquid,25–32 and gaseous33–43 templates. The encapsulation of guest molecules in the hollow structures formed using conjugated polymers has been demonstrated by several authors.6,16,24,33,35 Typically, fluorescent dyes are encapsulated in the hollow polymer structures and they can be detected sensitively using fluorescence spectroscopic methods. The entrapment of enzymes has also been reported, suggesting that hollow polymer structures can be utilized as nanoreactors for selected reaction processes.31 We present in this paper a novel synthetic method for the preparation of micrometer-sized polypyrrole hollow structures. The formation of these microvessels is accomplished by the deposition of the polymer onto toluene droplets, either dispersed in an aqueous medium or adsorbed onto a solid substrate. We demonstrate the entrapment of perylene within the polymer vessel and use the motional dynamics of this chromophore to examine the microscopic properties of the toluene droplet that comprises the template. Our data suggest that the confinement of toluene within the polymeric matrix may alter the fluid properties of the solvent to some extent. Experimental Section Chemicals. All chemicals were of the highest quality available commercially: toluene (Aldrich, 99.9%), pyrrole (Aldrich, 98%), perylene (Aldrich 99%), and iron chloride (III) (Aldrich, 97%). Aqueous solutions were prepared from high purity water (Milli-Q Plus). Instrumentation. A scanning electron microscope (Leo 435 VP) was used to image polymeric capsules. A Nikon Eclipse LV 100 optical microscope was used in reflection mode to visualize the toluene microdroplets adsorbed on the glass substrate. The same microscope operating in fluorescence mode was used to image the emission of perylene entrapped in the microvessels. For fluorescence imaging, the UV-2A excitation/

10.1021/jp107316u  2010 American Chemical Society Published on Web 10/25/2010

Toluene-Filled Polypyrrole Microvessels emission filter was used. Contact angle measurements were performed using a homemade instrument equipped with an optical stereoscopic microscope and a MicrOcular 1.3 MP USB camera (Bresser, Germany). Infrared spectra were recorded in reflection mode using a Nicolet 6700 Continuum FTIR microscope (Thermo Electron Corporation). Raman spectra were recorded with LabRAM HR Raman confocal microscope (Horiba Jobin Yvon). For the Raman measurements, the excitation source was a LION semiconductor laser (Sacher Lasertechnik) operating at 784.7 nm. A microscope (Multi View 4000, Nanonics) operating in confocal mode, using 442 nm He-Cd laser light source (Kimmon Koha Co.), was used to investigate polymer growth on the surface of the toluene droplets. Optical emission spectra were collected with a Fluorolog FL3-2-IHR320 spectrometer equipped with a TBX-04 PMT detector (Horiba-Jobin Yvon). Fluorescence lifetime measurements of perylene were made using a time-correlated single photon counting (TCSPC) instrument that has been described in detail previously,44 and we recap only its salient features here. The source laser is a passively mode locked CW Nd:YVO4 laser (Spectra Physics Vanguard), that produces 13 ps 1064 nm pulses at 80 MHz repetition rate. The third harmonic (355 nm, 2.5 W average power) of this laser is used to pump synchronously a Coherent 702-2 cavity dumped dye laser. The dye laser produces 430 nm 5 ps pulses at a 4 MHz repetition rate using Stilbene 420 laser dye (Exciton). Fluorescence transient signals were detected using Hamamatsu R3809U microchannel plate photomultiplier tubes, with wavelength selection provided by Spectral Products CM112 subtractive double monochromators. The detection electronics are a Becker and Hickl SPC-132 system, with the constant fraction discrimination, time-to-amplitude conversion, data binning and digital delay functions housed in a single system. Emission transients are collected simultaneously at polarizations parallel and perpendicular with respect to the vertically polarized excitation pulse. The instrument response function for this system is typically 35 ps fwhm. Preparation of Toluene/Water Emulsion. Toluene/water emulsions were prepared by the addition of toluene (100 µL) to water (3 mL), shaking, and then sonicating for 30 s at 400 W using an ultrasonic processor (UP400S, Hielscher Ultrasound Technology). Preparation of Nonsupported Polypyrrole Capsules. The polymerization bath was prepared by adding 1 mL of aqueous pyrrole (0.14 M) to 1 mL of an aqueous iron(III) chloride solution (0.07 M). The resulting solution was added immediately to the toluene/water emulsion (3 mL) and allowed to react for 30 min. The resulting polypyrrole capsules were separated from the supernatant liquid by centrifugation. Preparation of Glass-Supported Polypyrrole Capsules. To prepare supported polypyrrole vessels, a glass slide was immersed in the toluene/water emulsion, positioned parallel to the emulsion/air interface. This arrangement allowed for the accumulation of toluene droplets at the lower surface of the glass slide. Alternatively, the emulsion was poured into a glass cuvette and toluene droplets adsorbed onto its inner walls. For either deposition method, following 5 min of exposure to the emulsion, the solution was exchanged for the polymerization bath. The polymerization solution for these experiments was prepared by mixing 2 mL of aqueous 0.1 M pyrrole with 2 mL of aqueous 0.05 M FeCl3 solution. After 30 min the reaction mixture was separated from the solid support and the support rinsed with deionized water and dried.

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Figure 1. SEM image nonsupported polypyrrole capsules.

Encapsulation of Perylene in Polypyrrole Capsules. The encapsulation procedures were identical to those reported above except neat toluene was replaced by a 0.85 mM solution of perylene in toluene. Results and Discussion The first stage in the synthesis of polypyrrole microcointainers involves the preparation of the toluene/water emulsion. The emulsion droplets are subsequently used as soft templates that direct the growth of polypyrrole onto their surface. Two types of droplets can be used in such a synthesis; droplets suspended in the bulk medium and the droplets that are adsorbed onto a solid substrate. Depending on the type of the template used, either a nonsupported spherical capsule or supported capsules in the form of spherical caps can be formed. We consider these two types of microvessels separately. Nonsupported Polymeric Capsules. To prepare the polymeric capsules, the toluene/water emulsion was formed by sonication, followed by the addition of polymerization solution to the emulsion. This reaction resulted in the deposition of polypyrrole onto the surface of the toluene droplets suspended in the emulsion. The oxidant to monomer ratio was carefully chosen to achieve the required kinetics of polymerization and the appropriate rate of polymer deposition at the toluene/water interface (the microscopic images of polymer deposits produced at varying oxidant to monomer ratios can be seen in the Supporting Information). After separation of the polymeric product from the supernatant liquid (containing any unreacted monomer or oligomers), the resulting polymer structures were imaged using scanning electron microscopy (SEM; Figure 1). The deposited filled polymer structures consist of spheres with an average diameter of 2.7 ( 0.6 µm (1σ). Some of the capsules are slightly collapsed, likely due to evaporation of the solvent entrapped within the structures. The wall thickness of the vessels can be estimated at ca. 0.1-0.2 µm. One can also observe some amorphous polymeric product, which is attributable to the polymerization of pyrrole in the bulk aqueous phase. It seems that the amount of this undesirable product could be significantly reduced by modifying the monomer structure (e.g., by attaching hydrophobic and/or hydrophilic side groups that would allow self-assembly of the monomer at the water/toluene interface) to induce polymer formation exclusively at the droplets’ surface. Alternatively, the use of anionic stabilizers to prepare the W/O emulsion could result in preorganization of oxidized monomers (due to Coulombic interactions with the surfactant) thus

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Figure 2. FTIR spectra in reflection mode of (a) polypyrrole capsules, (b) polypyrrole, and (c) toluene.

promoting polymer formation at the interface. The chemical composition of the structures was studied using FTIR spectroscopy (Figure 2). We show in Figure 2a the IR spectrum of polypyrrole capsules prepared in bulk emulsion. Several bands can be attributed to polypyrrole based on a comparison of the data (Figure 2a) to a polypyrrole reference spectrum (Figure 2b).45,46 The bands centered at 1542 and 1130 cm-1 are assigned to the pyrrole ring CdC/C-C stretching mode and breathing mode, respectively. The 1450 cm-1 band is likely associated with the pyrrole C-N stretch, and possibly overlaps the C-H asymmetric bending vibration of the toluene methyl group. A broad band at 1294 cm-1 is the C-H in-plane bending mode. A set of bands associated with toluene encapsulated in the microvessels can be assigned based on the toluene reference spectrum (Figure 2c).47 The most characteristic peaks appear in the 1700-2000 cm-1 range, and these bands are associated with overtones and combination modes. The bands in the 3000 cm-1 region are C-H stretching modes, and the bands at 1605, 1495, and 1451 cm-1 are attributable to CdC stretching modes in the phenyl ring. A broad band at 3522 cm-1 that is attributable neither to the polymer nor to toluene is seen in the spectrum (Figure 2a). This band can be assigned to the N-H stretching mode of monomeric pyrrole48 incorporated in the microvessels during the polymerization process. Since the pyrrole is soluble in toluene, after addition of the polymerization solution to the emulsion, the monomer may partition into the toluene phase. As a consequence, some of the unreacted monomer likely becomes encapsulated inside the microvessels. Surface-Supported Polymeric Capsules. The processing and investigation of nonsupported polymeric capsules is relatively difficult due to problems associated with control over polymerization time, separation of the products following the reaction, and the handling of individual microvessels for physicochemical measurements. To overcome these limitations, we have synthesized polypyrrole microvessels directly on a planar solid surface. This means of microvessel deposition allows better control over the reaction process and facilitates their characterization. The idea behind this approach to mi-

crovessel formation is to adsorb liquid droplets (toluene) onto a solid substrate (glass slide) and then to deposit polypyrrole onto the droplet-decorated surface. The polymer is deposited onto the surface of toluene droplets as well as the solid supporting substrate. To adsorb the toluene droplets, the emulsion was poured into a glass cuvette or a glass slide was immersed into the emulsion. The average size of the adsorbed toluene droplets in our experiments is ca. 15 µm (Figure 3a). The observed diameter of adsorbed droplets is significantly larger than the diameter of the droplets in the bulk emulsion (compare Figures 1 and 3a). This finding may be partly due to changes in droplet shape resulting from adsorption onto the glass surface. The contact angle of a macroscopic toluene droplet immersed in water and adsorbed on glass is 141°, and based on this value, the size of nonadsorbed droplets should be 14.8 µm, assuming no volume change upon adsorption. This simple calculation shows clearly that the reason for the larger size of the adsorbed droplets must not be simply due to the adsorption process. It is likely that droplet fusion and the resulting surface coverage is determined mainly by the buoyant force driving the droplets to the surface in our experimental geometry. For our experiments, each droplet is pressed to the surface with a buoyant force that is proportional to the third power of the particle radius, resulting in an experimental bias in favor of large droplets. After the adsorption of toluene droplets, the monomer and oxidant were added to initiate the polymerization. Confocal microscopy in reflection mode was used to study the rate of polymer growth at the droplet surface. A laser beam (λ ) 442 nm) was focused onto the interface between a toluene droplet and the aqueous polymerization solution, and the intensity of the reflected light was recorded as a function of time. We show in Figure 3b the polypyrrole deposition curve. The intensity of the reflected light gradually decreases with pyrrole polymerization due to the formation of an absorptive polymer shell on the droplet surface. The morphology of the polymeric deposits was studied using scanning electron microscopy. We show in Figure 4a a SEM image of polypyrrole capsules supported on a glass surface.

Toluene-Filled Polypyrrole Microvessels

Figure 3. (a) Optical microscopy image of toluene droplets adsorbed on glass and (b) deposition of polypyrrole onto the surface of an individual droplet; variation of intensity of light reflected from the droplet surface as a function of polymerization time.

There are relatively large round-shaped structures resembling spherical caps that have collapsed as a result of toluene evaporation from the microvessels. As estimated from the image, the wall thickness of the structures is ca. 0.1-0.2 µm. A number of much smaller spherical capsules scattered on the surface are also visible. The histogram at Figure 5b shows the microvessel size distribution which is a bimodal distribution with maxima at 4.4 ( 2.1 µm (1σ) and 14.5 ( 3.0 µm (1σ). The surface microvessel loading density is estimated to be ca. 1.6 × 104 vessels/cm2. The cap-like structures can be attributed to deposition of polypyrrole onto surface-adsorbed droplets (Figure 3a). The diameter of the smaller vessels produced on the surface is close to the size of nonsupported vessels from the bulk emulsion (Figure 1). It is possible that following toluene droplet adsorption and introduction of the polymerization solution, some residual nonadsorbed toluene droplets remained in the proximity of the surface. As a result, after polymer deposition these small droplets were produced in bulk solution and ultimately accumulated on the substrate surface. The chemical identity of the solid-supported microvessels was confirmed using Raman spectroscopy. Due to submicrometer spatial resolution of confocal Raman microscopy, we were able to record the spectra of individual supported microvessels. The spectrum shown in Figure 5 is similar to these reported in the literature for polypyrrole.49,50 An intense band at 1594 cm-1 is associated with the CdC stretching mode of polypyrrole. The 1375 cm-1 band is assigned to asymmetric C-N stretch, and the peak 1236 cm1 is associated with asymmetric C-H in-plane

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Figure 4. (a) SEM image of surface-supported polypyrrole capsules; inset: magnification of the image and (b) histogram presenting size distribution of the supported capsules.

bending mode. Several bands attributable to polarons and bipolarons are also seen in this spectrum. The 938 and 1087 cm-1 bands are associated with ring deformation and the symmetric C-H in-plane bending of the bipolaron structure, respectively. Much weaker, but still observable bands for these modes for the polaron structures are seen at 966 and 1055 cm-1, respectively. In these measurements, no signals associated with encapsulated toluene could be detected, likely due to evaporation of the toluene as a result of irradiation by the laser beam. Encapsulation of Fluorescent Dye. It is possible using microvessel structures to encapsulate predetermined chemical species by incorporation of the chosen target molecule into the microdroplets prior to polymer deposition. We have chosen perylene to illustrate this capability. Perylene is a convenient hydrophobic fluorescent molecule with well established spectroscopic properties. In the encapsulation procedure, the perylenecontaining droplets were prepared through sonication, adsorbed onto the glass surface and then coated with polypyrrole. The size distribution of the perylene containing droplets is comparable to those of neat toluene droplets. The corresponding microscopy image is included as Supporting Information (Figure III). The encapsulated species were detected using epifluorescence microscopy. We show in Figure 6a the optical microscopic image of emission from the microvessels excited with blue light. One can see several bright spots which are due to perylene fluorescence from within the microvessels (Figure 6b). The bands at 444, 471, and 504 nm are all characteristic of perylene emission,51 confirming the encapsulation of the chromophore and the absence of reaction between the chromophore

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Figure 5. Raman spectrum of an individual polypyrrole capsule supported on glass.

in a position to determine whether or not the confinement we have imposed affects the properties of the toluene in any way. To address this question we have used time-resolved fluorescence spectroscopy to measure the rotational diffusion dynamics of perylene within the microvessels. To make this measurement, the solid-supported microvessels containing perylene were excited with polarized picosecond laser pulses and polarized perylene emission transients, I|(t) and I⊥(t), from the vessels were collected (Figure 7a). These data were permuted according to eq 1 to create the induced orientational anisotropy decay function, R(t).

R(t) )

I|(t) - I⊥(t) I|(t) + 2I⊥(t)

(1)

The decay functionality of R(t) provides information on the rotational diffusion dynamics of perylene entrapped in the microvessels (Figure 7b). These data decay with a singleexponential functionality with a time constant of τOR ) 64 ( 5 ps. This value is somewhat slower than that for perylene in bulk (bulk) toluene, τ OR ) 16 ( 1 ps.52 We will return to a discussion of these comparative values after we consider the significance of a single exponential decay functionality. Chuang and Eisenthal53–55 have developed a model for the interpretation of rotational diffusion data that allows for consideration of the shape of the volume swept out by the reorienting chromophore. For perylene, the chromophore π-system is in the xy plane, and the z axis is perpendicular to the perylene plane, and a single exponential decay functionality is consistent with the chromophore reorienting as a prolate ellipsoid, where Dx > Dy ) Dz. In Chuang and Eisenthal’s formulation Figure 6. (a) Fluorescence microscopy image of solid-supported polypyrrole capsules filled with perylene-containing toluene and (b) emission spectrum of perylene entrapped in the polypyrrole capsules; excitation: 420 nm.

and the pyrrole monomers during the polymerization process. With the chromophore encapsulated in the microvessels, we are

R(t) ) R(0) exp(-6Dzt)

(2)

So there is limited information on the relative values of the Cartesian components of the rotational diffusion constant under these conditions. For a spherical rotor, Dz would be equal to D.

Toluene-Filled Polypyrrole Microvessels

J. Phys. Chem. B, Vol. 114, No. 46, 2010 14895 we recover a R(0) value of ca. 0.24, somewhat lower than what is typically seen for perylene in bulk solvents. Such a diminution in the value of R(0) could be accounted for by the interaction of the chromophore π-system with a medium other than bulk solvent, such as a polymer, oligomer or monomer. Again, this finding is not conclusive evidence for the existence of perylene-(poly)pyrrole interactions, but it is consistent with such an interaction.57 Conclusion We have demonstrated a means to prepare micrometer-sized polypyrrole microvessels through the deposition of polymer onto the surface of toluene droplets, either dispersed in an aqueous medium or deposited onto a solid support. The polypyrrole microvessels can be used to encapsulate chromophores or other species, and we have demonstrated this effect using perylene. Fluorescence miscroscopy shows that the chromophore resides within the microvessels and time-resolved emission data indicate that perylene molecules reorient more slowly in the microvessels than in the corresponding bulk liquid. While the details of this difference in behavior remain under examination, this finding is consistent with the chromophore interacting to a measurable extent with the poly- oligo- or monomeric pyrrole. Our findings highlight the significance of poly/oligo/monomer partitioning into the encapsulated liquid during polymerization procedure. We are currently working on the development of synthetic procedures that allow for lowering concentrations of monomer/ oligomers in the droplets. Our efforts are focused on the application of functionalized monomers containing polar groups, which will decrease the solubility of the monomer in the droplets. The effect of other solvents for droplet preparation or decrease of polymerization temperature is also under investigation.

Figure 7. Fluorescence time transients of perylene encapsulated in polypyrrole containers: (a) emission decays at parallel (i) and perpendicular (ii) position of the polarizer and (b) decay of anisotropy function.

The observation of a 64 ps reorientation time in the encapsulated system and 16 ps in bulk toluene suggests that the local environment presented to the chromophore is somewhat different in the two cases. We make this observation with some caution because the reorientation time constants for perylene under these two conditions were acquired by different means and we have not deconvoluted the instrument response function from the TCSPC data we report here. Despite these caveats, the reorientation time constants for perylene in toluene are not the same for encapsulated and bulk solvent conditions. While it is unlikely that the viscosity or the density of the solvent is significantly different for these two experimental conditions, the confinement of the perylene within a polypyrrole vessel of several micrometer dimensions would allow for significant interactions between the chromophore and the polymer. If there were significant chromophore-polymer interactions, it could account for a slowing of the chromophore rotational motion.54,56,57 We note here that, in addition to the presence of polypyrrole in the microvessel wall, it is also possible for there to be oligomeric and monomeric pyrrole present in the toluene microdroplet, and interactions between such species and the chromophore could also account for our findings. Further experiments with higher time resolution will be required to resolve the molecular-scale details of such an interaction. One piece of information that is consistent with the possibility of perylene-polymer/oligomer/monomer interactions is the value of the zero time anisotropy we recover. For our microvessels,

Acknowledgment. Special thanks are due to Dr. Damian Pociecha for his valuable comments on the manuscript and help in the experimental work. The funding for this work was provided by the Ministry of Science and Higher Education within project N204 117 32/3116, years 2007-2010. Infrared and fluorescence spectroscopy measurements were conducted in the Structural Research Laboratory (SRL) at the Department of Chemistry, University of Warsaw, Poland. S.R.L. was established with the 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. G.J.B. and the TCSPC measurements were supported by the National Science Foundation under Grant CHE 0808677. Supporting Information Available: SEM images illustrating the effect of oxidant/monomer ratio on the morphology of polymeric deposits are shown and discussed in section 1. Fluorescence microscopy image of perylene-containing toluene droplets is shown in section 2. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Caruso, R. A.; Antonietti, M. Chem. Mater. 2001, 13, 3272. (2) Wang, Y.; Angelatos, A. S.; Caruso, F. Chem. Mater. 2008, 20, 848. (3) Lou, X. W.; Archer, L. A.; Yang, Z. C. AdV. Mater. 2008, 20, 3987. (4) Skrabalak, S. E.; Chen, J. Y.; Sun, Y. G.; Lu, X. M.; Au, L.; Cobley, C. M.; Xia, Y. N. Acc. Chem. Res. 2008, 41, 1587. (5) Skotheim, T. A.; Reynolds, J. R. Handbook of conducting polymers, 3rd ed.; Terje, T. A., Reynolds, J. R., Ed.; CRC Press: Boca Raton, FL, 2007.

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