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Pyrene-Loaded Polypyrrole Microvessels Daria Ke) pinska,† Gary J. Blanchard,‡ Pawez Krysinski,† Jaroszaw Stolarski,§ Krystyna Kijewska,† and Maciej Mazur*,† †
University of Warsaw, Department of Chemistry, Pasteura 1, 02-093 Warsaw, Poland Michigan State University, Department of Chemistry, East Lansing, Michigan 48824-1322, United States § Institute of Paleobiology, Polish Academy of Sciences, Twarda 51/55, 00-818, Warsaw, Poland ‡
bS Supporting Information ABSTRACT: The encapsulation of guest molecules within polymeric hollow nano- or microscale structures is a rapidly developing field of interdisciplinary research due to a variety of applications ranging from drug delivery and sensor fabrication to nanoscale synthesis and bioinspired mineralization. We report on the encapsulation of pyrene within three-dimensional polypyrrole microvessels synthesized by precipitation polymerization of pyrrole onto toluene droplets that contain pyrene. Steady state and time-resolved fluorescence measurements show that the optical response and dynamics of encapsulated pyrene is significantly different from that in the free solution, likely due to interactions with oligomeric species generated during the polymerization process that partition into the organic core of the microvessel. Our results indicate that the encapsulation process can have a significant influence on the local environment of encapsulated species, an issue that is critical from the perspective of potential synthetic or medical applications.
’ INTRODUCTION There has been a great deal of recent effort focused on the synthesis and characterization of hollow polymeric microvessels because of their useful properties and prospects for their application in targeted drug delivery, chemical analysis, molecular electronics, and bioinspired mineralization.15 For many such applications, microvessels formed from conducting polymers are especially attractive since they exhibit controllable permeability through pH or redox potential.6 Hollow three-dimensional polymeric structures can be synthesized by a template method. This method involves precipitation polymerization in the presence of a template which confines the growth of the polymer to the template-solution interface, resulting in the formation of a polymeric replica.7 Subsequent removal of the templating entity produces hollow polymeric structures. Colloidal particles such as solid particles,810 liquid droplets1113 or gaseous bubbles1418 are typically used for this purpose, and removal of the template after polymerization depends on the template used. Typically, etching or dissolution can be used for solid templates. The removal of gaseous or liquid templates, if desired, is more typically based on the permeability of the polymer shell. Hollow structures can be used to encapsulate a variety of chemical species including fluorescent dyes, enzymes, inorganic salts, and metallic nanoparticles, which makes them a promising class of materials for the construction of novel polymeric batteries, sensing devices, solar cells or ion selective electrodes.15 In the work we report here, we demonstrate the encapsulation of pyrene and probe its optical properties and dynamics within the voids of polypyrrole microvessels. Using steady state and r 2011 American Chemical Society
time-resolved fluorescence measurements, we show that the spectroscopic properties and rotational diffusion dynamics of the encapsulated chromophore are different relative to that of the chromophore in free solution. The effects we observe likely arise from the presence of oligomeric pyrrole species confined within the microvessel. This is a significant result because it addresses the uniqueness of chemical environments formed by the microvessels and, in particular, how incorporated species may interact with their surroundings.
’ MATERIALS AND METHODS Chemicals. All chemicals were of the highest quality commercially available: Toluene (Aldrich, 99,9%), pyrrole (Aldrich, 98%), pyrene (Aldrich, 99%), iron(III) chloride (Aldrich, 97%). All aqueous solutions were prepared from high purity water (Milli Q Plus). Instrumentation. The scanning electron microscopy images of the samples were recorded using LEO 435 VP or LEO 1530 microscopes. The transmission electron microscopy images were acquired with a JEM 1400 TEM (JEOL Co., Japan). UVvisible absorbance measurements were performed with a Lambda 25 (PerkinElmer) spectrometer. Optical emission and excitation spectra were collected with a Fluorolog FL32-IHR320 spectrometer (Horiba Jobin Yvon). A Nikon Eclipse LV 100 optical microscope operating in fluorescence mode with UV-2A filter was used for fluorescence imaging. Raman spectra were recorded with a LabRAM HR spectrometer (Horiba Jobin Yvon) attached to an Received: July 29, 2011 Revised: September 5, 2011 Published: September 07, 2011 12720
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Figure 1. (a) SEM and (b) TEM image of free-standing polypyrrole microvessels. Inset: Histogram showing the distribution of microvessel diameters. Olympus BX41 confocal microscope. The excitation source was an Excelsior-CDRH diode-pumped laser (Spectra Physics) operating at 532.3 nm. Fourier transform infrared spectra in transmission mode in KBr pellets were acquired with a Nicolet 8700 spectrometer and the reflection mode spectra of the microvessels cast onto a gold slide were recorded with a Nicolet 6700 Continuμm FTIR microscope (Thermo Electron Corporation). Fluorescence anisotropy decay measurements of pyrene were performed using a time-correlated single photon counting (TCSPC) instrument that has been detailed elsewhere.19 The pump 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 second harmonic (532 nm, 2.5 W average power) of this laser is used to pump synchronously a Coherent 7022 cavity dumped dye laser. The dye laser produces 660 nm 5 ps pulses at a 4 MHz repetition rate using Kiton Red 620 laser dye (Exciton). The output of the dye laser was frequency-doubled using a Type I LiIO3 SHG crystal, and a color glass filter was used to separate the fundamental from the second harmonic output. Fluorescence transients were detected using Hamamatsu R3809U microchannel plate photomultiplier tubes, with wavelength selection provided by Spectral Products CM112 subtractive double monochromators. The detection electronics is 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 unit. The instrument response function for this system is typically 40 ps fwhm. Microvessel Construction for ex Situ Measurements. Toluene/water emulsions were prepared by sonicating a mixture of toluene (100 μL) and water (3 mL) for 30 s at 400 W using an ultrasonic processor (UP400S, Hielscher Ultrasound Technology). Nonsupported polypyrrole microvessels were formed from a polymerization bath that 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 microvessels were separated by centrifugation and dried (this procedure yields ca. 3 mg of microvessels). For the formation of solid-supported polypyrrole microvessels, the quartz substrate on which they were supported was placed in the emulsion (3 mL) for ca. 5 min to allow adsorption of toluene droplets. The slide was oriented horizontally and the droplets were accumulated predominantly on the bottom side of the substrate. The emulsion was then exchanged for polymerization solution (prepared by mixing 2 mL of aqueous 0.1 M pyrrole with 2 mL of aqueous 0.05 M FeCl3 solution) and left for 30 min. Following the completion of the reaction, the substrate was removed, rinsed with deionized water, and dried. 12721
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Figure 2. Consecutive Raman spectra of an individual toluene droplet placed in polymerization solution. Polymerization time: (a) 2 min, (b) 7 min, (c) 12 min, and (d) 17 min. The encapsulation of pyrene in the polymeric microvessels was accomplished using a 25 mM solution of pyrene in toluene in place of neat toluene to prepare the polypyrrole microvessels as described above.
Preparation of the Samples for in Situ Measurements of the Microvessels. To avoid evaporation of toluene from the microvessels (or evaporation of toluene droplets), all in situ experiments were performed in such a way that the microvessels/droplets were in contact with an aqueous solution. For in situ Raman measurements a drop of emulsion mixed with polymerization solution (prepared as described above) was placed between two silica slides and the laser beam was focused with the microscope objective onto a selected toluene microdroplet. Raman spectra were acquired every 1 min. For reference Raman measurements, the experimental setup was the same, but instead an emulsion with or without monomer/oxidant was used, and the concentrations of the other components were kept the same as for the preparation of free-standing microvessels. For fluorescence microscopy imaging after completion of the polymerization reaction, free-standing microvessels loaded with pyrene were separated by centrifugation and a drop of the suspension was placed between two silica slides and microscopic images were acquired. The steady state and time-resolved fluorescence measurements were performed on solid-supported microvessels loaded with pyrene. The toluene/ water emulsion was poured into a 1 mm path length quartz cuvette for 5 min and then exchanged for polymerization solution as described
above. After completion of the reaction, the polymerization solution was exchanged for distilled water and the fluorescence spectra were acquired.
Preparation of Solutions for Reference Steady State and Time-Resolved Fluorescence Measurements. The reference fluorescence measurements of pyrene were performed in neat toluene, toluene with pyrrole (2 M) and toluene with pyrrole oligomers (ca. 103 M of bipyrrole, as determined from UVvis data), respectively. In all solutions, the concentration of pyrene was kept at 105 M.
’ RESULTS AND DISCUSSION Polymer microvessels were fabricated by oxidative polymerization of pyrrole onto the surface of toluene droplets dispersed in an aqueous medium. The droplets act as soft templates that limit the polymer growth exclusively to their surface. This method results in the formation of hollow vessels filled with an organic solvent. We show in Figure 1a an SEM image of microvessels prepared by the polymerization of pyrrole on the surfaces of toluene droplets. The microvessels are formed in a range of diameters, with the average being ca. 2.6 μm (a histogram revealing size distribution of the microvessels is shown in the inset to Figure 1a). The corresponding TEM image (Figure 1b) reveals that the structures contain an empty hollow region surrounded by a polymer wall that is ca. 70 nm thick. 12722
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Figure 3. Free standing pyrene-loaded polypyrrole microvessels: (a) SEM image, (b) TEM image. Inset: Histogram showing the distribution of microvessel diameters.
The polymerization of pyrrole on the surface of toluene droplets was monitored with Raman confocal microscopy. The laser beam was focused onto an individual microdroplet placed in polymerization solution sandwiched between two silica slides, and consecutive Raman spectra were collected (Figure 2). The spectrum recorded 2 min after initiation of the reaction shows several vibrational modes attributable to toluene.20 These are the bands at 521, 787, 1004, 1031, 1211, and 1381 cm1. The signals gradually decrease with increasing polymerization time and become nonresolvable after ca. 10 min. Several modes characteristic of poly- or oligopyrrole21 are seen in the spectrum after 2 min of polymerization and do not change significantly in subsequent scans. The most prominent band in the spectrum at ca. 1604 cm1 is due to CdC stretching in the pyrrole ring. The mode at 1337 cm1 is associated with the asymmetric CN stretch. The band at 1261 cm1 is attributable to asymmetric CH in plane bending. Several modes in the 9001150 cm1 range are due to polarons and bipolarons in the polymer backbone. The Raman
data raise two important questions: why the toluene signals disappear when the polymer is deposited onto the droplet and why the modes attributable to polypyrrole do not change with reaction time. The decrease of the toluene bands can be explained by the forming polypyrrole coating absorbing the Raman scattered light from the toluene. The excitation beam interacts with toluene producing a weak Raman signal that is absorbed by the polypyrrole vessel wall. The vibrational modes attributable to polypyrrole are seen in the Raman spectrum after 2 min of polymerization and do not change considerably with the progress of the reaction. To clarify this observation, we recorded the Raman spectrum of the pyrrole containing toluene droplet suspended in aqueous solution, but without addition of the oxidant. The spectrum shown in Figure S1 (Supporting Information, spectrum c) exhibits only toluene bands and no signal is assigned to monomeric pyrrole. For comparison, the spectra of neat toluene droplets in the aqueous 12723
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Figure 4. FTIR spectrum in transmission mode of (a) pyrene and (b) pyrene-loaded polypyrrole microvessels.
phase, with and without oxidant, are shown in Figure S1, curves a and b, in the Supporting Information. The absence of pyrrole signals in the spectrum confirms that the monomer cannot be detected in the polymerization mixture under our experimental conditions. The lack of pyrrole Raman signal is due to the absence of the resonance enhancement effect associated with polypyrrole. In contrast, only a minute amount of polypyrrole produced on the surface of toluene droplet is detectable in the Raman spectrum due to resonance enhancement since the polymer exhibits an absorption band at ca. 450 nm (vide supra), close to the excitation wavelength. When polymeric species are precipitated on the droplet’s surface, their Raman signal is relatively strong, and it is the same absorbance band that likely attenuates toluene Raman scattering from within the droplet. Another important question is why the polymer grows directly on the toluene droplet surface and only a small amount of featureless polymer is formed in bulk solution. If the reaction proceeds in homogeneous aqueous solution, in the absence of toluene droplets, then a precipitate forms. When the reaction proceeds in the emulsion, the polymer forms exclusively on the toluene droplets. The primary reason for the deposition of the polymer at the toluene/water interface is the fact that the monomer immediately transfers from the aqueous to the organic phase before the polymerization is started. The initial concentration of pyrrole in the aqueous phase after mixing the polymerization solution with the emulsion is 28 mM. The partition coefficient of pyrrole between water and toluene is ca. 7 106, determined spectroscopically; thus, the equilibrium concentration of the monomer in the aqueous phase diminishes to ca. 105 M, while that in
toluene droplets reaches the value of ca. 1.4 M (the toluene to water volume ratio in the mixture is 0.02). We believe the increased concentration of monomer in the toluene droplets is responsible for the formation of polypyrrole exclusively at the droplet surface. In this situation, the polymerization proceeds heterogeneously, at the interface between the aqueous phase, where the oxidant is dissolved, and the organic phase, where monomer is present, yielding a polymeric coating around the toluene droplet. The monomer present in the toluene core is consumed gradually by the polymerization reaction, and possibly also some intermediate species (e.g., oligomers) may accumulate in the organic phase. One of the main advantages of forming microvessels with a hydrophobic core is the ability to confine guest species within their volume. In this work, we demonstrate the encapsulation of pyrene, a hydrophobic polycyclic aromatic hydrocarbon (PAH) characterized by a moderately high fluorescence quantum yield and an emission spectrum with vibronic structure that is dependent on the polarity of its environment.22 The encapsulation of pyrene was accomplished by using a pyrene-containing toluene solution instead of neat toluene to prepare the droplets. Shown in Figure 3a is SEM image of pyrene-loaded microvessels. The diameter of the structures is slightly smaller (ca. 2.0 μm) than that of nonloaded microvessels (Figure 1). The corresponding TEM image (Figure 3b) does not show any features in the microvessel hollow core region. A simple calculation confirms that when toluene is evaporated, for a typical 2 μm diameter microvessel, the dry pyrene content should be ca. 0.013 μm3. If the pyrene deposit were to form a hollow sphere adjacent to the 12724
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Figure 5. Fluorescence microscopy image of pyrene-loaded free-standing microvessels suspended in water.
inner side of the microvessel wall, then the thickness of the pyrene sphere would be ca. 1 nm. It is thus not surprising that pyrene cannot be resolved in the TEM image under our imaging conditions. The incorporation of pyrene was demonstrated by FTIR spectroscopy. Shown in Figure 4 (curve b) is the infrared spectrum of pyrene-loaded microvessels. The spectrum reveals vibrational bands of both polypyrrole23,24 and pyrene25,26 (for comparison, the spectrum of pyrene is shown, curve a). The 1549 cm1 mode is attributable to CdC/CC stretching vibration of the pyrrole ring. The 1464 cm1 band is assigned to ring breathing with contributions from CdC/CC and CN. The 1291 cm1 and 1039 cm1 bands are due to CH in-plane bending modes while 912, 681, and 655 cm1 signals are assigned to CH out-of-plane deformation, CC out-of plane ring deformation and NH out of plane vibrations, respectively. Several bands attributable to pyrene are also seen in the spectrum. The most intense modes are seen at 668, 709, 749, and 840 cm1. The localization of the chromophore within the microvessels was probed by fluorescence microscopy. In a configuration similar to that used for Raman measurements, the microvessels were placed in an aqueous solution and sandwiched between two quartz slides. The microscopic image (Figure 5) shows a group of microvessels filled with pyrene-containing toluene solution. The vessels appear as bright round spots against a dark background, confirming that the dye is uniformly distributed within the microvessel voids. Interestingly, no fluorescence signal is seen when the sample is allowed to dry (Figure S2, Supporting Information). This can be understood as the pyrene partitioning into the polypyrrole walls upon evaporation of the solvent, with the direct pyrenepolypyrrole interaction quenching the pyrene emission. The encapsulation of pyrene was studied further with steady state fluorescence spectroscopy. For these measurements, we used microvessels supported on a quartz substrate since the interpretation of the experimental data from this structural motif was more straightforward due to limited scattering and absorption effects by the polymer wall of the microvessel. The excitation beam was directed onto a microvessel interior through a cuvette’s quartz window at ca. 45 to the normal and the fluorescence
signal was collected in front face geometry. The preparation of solid supported microvessels required the adsorption of toluene droplets onto a quartz slide followed by deposition of polypyrrole onto the droplet-containing surface. We show in Figure 6 an SEM image of polypyrrole microvessels supported on a quartz substrate. The corresponding histogram (inset to Figure 6) reveals a bimodal distribution of microvessel diameters with maxima at ca. 3 and 8.5 μm. This finding suggests that there exist two types of the structures on the surface; microvessels that were formed in the bulk of the reaction mixture prior to accumulation on the surface (diameter ca. 3 μm) and microcapsules that were produced through deposition of the polymer onto already adsorbed toluene droplets (ca. 8.5 μm). The larger microvessels are likely formed because the buoyant force drives the larger droplets preferentially toward the supporting surface, resulting in larger diameter solid-supported microvessels.12 We show in Figure 7 the excitation and emission spectra of pyrene contained in the microvessels (for comparison, the excitation and emission spectra of neat polypyrrole, which reveals no fluorescence signal, and the emission spectrum of solid pyrene are shown in Supporting Information as Figures S3, S4, and S5, respectively). The emission bands centered at ca. 384 nm exhibit vibronic structure characteristic of pyrene.27 The emission spectrum of pyrene depends sensitively on the polarity of its microenvironment. This sensitivity is manifested through the vibronic band ratio of the I band (374 nm) and III band (384 nm). For pyrene encapsulated in the microvessels this ratio is ca. 1.04, higher than the ratio for pyrene contained in toluene/water emulsion (ca. 0.77, Figure S6, Supporting Information). The fluorescence spectrum of pyrene also reveals a broad band centered near 471 nm. This band is assigned to excimer emission. Since the corresponding excitation spectrum does not change regardless whether the emission is collected at 384 or 471 nm (Figure S7, Supporting Information), we can conclude that the excimers are formed dynamically.28 For encapsulated pyrene the intensity of the excimer band at 471 nm relative to the monomer band at 384 nm is ca. 11.3 which is more than four times higher than that observed in bulk toluene (I471 nm/384 nm = 4.6 at 25 mM). This increased band ratio for 12725
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Figure 6. SEM image of solid-supported polypyrrole microvessels. Inset: Histogram showing the distribution of microvessel diameters.
Figure 7. Excitation and emission spectra of pyrene contained in solid-supported microvessels.
encapsulated pyrene is likely associated with an interaction between chromophores that is mediated in some manner by the microvessels. To understand the possible causes for this finding, we first need to determine whether the pyrene concentration in the microvessels changed due to encapsulation. The initial concentration of the pyrene in toluene phase is 25 mM, but it is possible that this concentration may be decreased for several reasons, as a consequence of encapsulation (e.g., leakage to the surrounding aqueous phase, degradation, loss of toluene through the vessel walls). To address this problem we examined the dependence of the excimer-to-monomer band ratio on solution concentration. This ratio varies monotonically with concentration in toluene as shown in Figure S8 (curve a, Supporting Information). However,
as discussed above, for 25 mM concentration the I471 nm/384 nm value is ca. 4 times lower than that in the microvessels. Since it is unreasonable to assume that the concentration increases several times in the microvessels, we are left to conclude that the different excimer-to-monomer ratio is associated in some manner with an interaction between chromophore(s) and some other constituent present in the microvessel. To test this hypothesis, we performed the polymerization reaction in a bulk water/toluene biphasic system where the toluene phase was in contact with aqueous polymerization solution for 30 min, replicating the reaction conditions used for preparation of microvessels. Pyrene was dissolved in this solution to yield 25 mM concentration and an emission spectrum was recorded. The excimer-to-monomer band ratio is ca. 12.3, which is only slightly higher than that 12726
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determined for pyrene-loaded microvessels (the corresponding calibration curve is shown in Figure S8, curve b, Supporting Information). This finding demonstrates that the pyrene concentration does not change significantly with encapsulation. The differences between the steady state emission spectra of encapsulated and nonencapsulated pyrene could be explained by the composition of the fluid in the microvessel being different than toluene and pyrene alone. The FTIR spectrum of toluenefilled microvessels reveals bands attributable to both polypyrrole and toluene (Figure S9, Supporting Information), however, a broad vibrational mode observable at ca. 3500 cm1 cannot be attributed to either the polymer or the chromophore. This band characteristic of the NH stretching vibration29 can be assigned to monomeric or oligomeric pyrrole that may be contained in the microvessels. To evaluate this possibility in more detail, we recorded the UVvis absorption spectrum of the toluene phase (without pyrene) that was in contact with aqueous polymerization solution (Figure 8a). The spectrum reveals a band with a maximum at ca. 281 nm, consistent with the formation of pyrrole oligomers, generated as side products of polymerization reaction. Since the electronic spectra of pyrrole oligomers depend sensitively on the chain length, the position of the absorption band can be used to identify their structure. The 281 nm band is associated with dimeric pyrrole.3032 Its spectrum is different from that of pyrrole30 and polypyrrole, which exhibits bands at 450 and 870 nm, Figure 8b. Assuming the molar absorption coefficient at 2 103 L 3 mol1 3 cm1,33 we estimate the concentration of the dimer to be ca. 103 M. Some small amount of higher oligomers can be also detected after preconcentration of the solution, with
)
two bands with maxima at 323 and 337 nm, attributable to trimers30 and tetramers,33 respectively (inset to Figure 8a). Interestingly, the presence of oligomers in the toluene phase not only affects the excimer to monomer band ratio in the fluorescence spectrum but also influences the overall observable emission intensity. With increase of oligomer concentration the intensity of fluorescence is increased several orders of magnitude (Figure S10, Supporting Information). This result points out to considerable interaction of pyrene with oligomer resulting in modifications in its spectral characteristics. The presence of dissolved species in the encapsulated toluene gives rise to the changes in the spectral properties of the dye, and one means of evaluating its interaction is through the motional dynamics of the chromophore. We have measured the rotational diffusion dynamics of the chromophore contained in the microvessels. The central issue is the extent to which encapsulated species interact with the oligopyrrole species present in the solution. Before we discuss these results, it is useful to review the spectroscopic properties of pyrene.22,34 The dominant absorption band in pyrene is the S2 r S0 transition at ca. 330 nm which is polarized along the short axis of the molecule. Emission is from the S1 state and the polarization of the S0 r S1 transition is along the long molecular axis. For this reason, the induced orientational anisotropy function (eq 1) for pyrene generated from the polarized fluorescence transients exhibits a negative zero-time anisotropy in solution. The solid supported microvessels containing pyrene were excited at 330 nm with polarized picosecond pulses and the polarized emission transients I (t) and I^(t) collected at 400 nm were combined to produce the experimental R(t) function. I ðtÞ I^ ðtÞ I ðtÞ þ 2I^ ðtÞ )
RðtÞ ¼
)
Figure 8. Electronic spectra of the following: (a) pyrrole oligomers partitioned into toluene phase from the polymerization solution, (inset: the spectrum of preconcentrated solution of oligomers); (b) polypyrrole microvessels.
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ð1Þ
The induced orientational anisotropy decay contains information on the rotational diffusion dynamics of pyrene as well as any interactions between the chromophore and its local environment. Because we have spectroscopically selected monomer emission for these measurements, our experimental data are not consistent with excimer reorientation. Moreover, the excimers form as a complex between a ground state molecule and an excited molecule and the time constant for the onset of excimer formation is in excess of the reorientation time constant we measure. We show in Figure 9 the anisotropy decay of pyrene confined within supported microvessels. These data reveal a single-exponential anisotropy decay with a time constant of 247 ps, and zero-time anisotropy of 0.06. The positive zero-time anisotropy value for pyrene indicates an interaction between the chromophore and some species within the microvessel that is sufficient to perturb the vibronic coupling between the chromophore S1 and S2 states. The single-exponential anisotropy decay can be interpreted in the context of pyrene sweeping out a prolate volume as it rotates.35 This finding is not surprising but comparison of the anisotropy decay time constant for confined pyrene to that of pyrene in bulk toluene solution reinforces the notion that there are significant interactions between pyrene and the microvessel contents. The reorientation time is considerably longer for the encapsulated pyrene than it is for the same chromophore in bulk toluene solution. The reorientation time of pyrene in toluene is too fast to be measured accurately using our system and, indeed, this is the result we recover (Figure S11, Supporting Information). 12727
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Figure 9. Fluorescence anisotropy decay curve for pyrene encapsulated in solid-supported microvessels.
We estimate the reorientation time constant for pyrene in toluene to be 26 ps using the DebyeStokesEinstein equation in the stick-limit. This estimate is likely high because slip-limit behavior is expected for nonpolar solventsolute systems. The slower rotational diffusion time constant seen for pyrene in the microvessels is likely associated with interactions between the chromophore and pyrrole oligomers contained within the microvessel. We compared the anisotropy decay data for encapsulated pyrene with these for pyrrole-containing bulk toluene. For this system we observe single exponential anisotropy decay with R(0) = 0.09 and τOR = 62 ps (Figure S12, Supporting Information). It is important to note that we observe a positive R(0) value for both the pyrene/pyrrole bulk system and for pyrene in microvessel, consistent with there being a significant interaction between the chromophore and pyrrole resulting in a perturbation of the spectroscopic response of the chromophore. However, the orientational relaxation time for the pyrene/pyrrole system is longer than for pyrene in bulk toluene but is a factor of 4 slower than that seen for the encapsulated chromophore. Our results suggest that pyrene interacts with oligopyrrole species that are contained in the microvessel. Pyrene was dissolved in the solution containing oligomers and time-resolved data were acquired. For this system, R(0) = 0.08 and the reorientation time is 222 ps (Figure S13, Supporting Information), essentially the same as the reorientation time constant for the encapsulated chromophore. We believe that interaction of the pyrene aromatic system with oligopyrrole(s) mediates the pyrene S1S2 vibronic coupling, resulting in the R(0) data we have recorded. The formation of a pyrene/oligomer conjugate (possibly through ππ stacking interactions) may lead to observed slower rotational diffusion time due to the larger hydrodynamic volume of the reorienting species. In this context, the steady state data become more understandable. After the generation of pyrrole oligomers in the organic phase, the chromophore-solvent interactions are replaced by the chromophore-oligomer interactions, modifying the spectral response of the fluorophore. Such a phenomenon is not surprising since pyrene is known to be sensitive to its local microenvironment. There is one more piece of information that can be extracted from the time-resolved fluorescence anisotropy data of the
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Figure 10. Release curve of pyrene from solid supported microvessels to the aqueous phase. The data shown are the dependence of the fluorescence intensity at 384 nm vs time.
encapsulated pyrene. The single exponential anisotropy decay (Figure 9) indicates that all the emitting species reside in a single microenvironment. If the pyrene emission were seen from both the toluene solution core and the polymeric shell, then we would expect at least two anisotropy decay components. The other interesting issue is the permeability of the microvessel’s polymer shell. The question is whether the encapsulated species can be released from the hollow structure. To test this possibility we prepared pyrene-loaded microvessels supported on the inner side of a cuvette wall, filled the cuvette with distilled water and recorded the emission spectra of pyrene being released to the aqueous phase. Shown in Figure 10 is release curve of the encapsulated pyrene (emission collected at 384 nm). The data show slow, gradual release of the chromophore until a plateau is achieved after ca. 10 min. The final concentration, as determined from absorbance measurements is close to the solubility of pyrene in water (0.50.7 μM36). This finding clearly demonstrates that polypyrrole is permeable to pyrene, and the release phenomena are governed mainly by the solubility of the chromophore in the external aqueous phase.
’ CONCLUSIONS We have successfully encapsulated pyrene in polypyrrole microvessels formed by oxidative polymerization of pyrrole onto micrometer-sized droplets of pyrene-containing toluene. The encapsulated fluorophore exhibits relatively slow rotational dynamics and a positive zero-time anisotropy, likely due to interactions with oligopyrrole which partitions into the organic core during polymer deposition. The chromophore-oligopyrrole interactions appear to be sufficiently strong to perturb the polarization of the absorbing band relative to the emitting band. It is also possible that the pyrene is interacting with the walls of the microvessels, rendering the chromophore motionless. Whether these intermolecular interactions are predominantly between pyrene and solution phase oligopyrrole, the walls of the microvessels, or both, cannot be resolved at the present time. We are investigating this issue and will report our results in the forthcoming paper. 12728
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’ ASSOCIATED CONTENT
bS
Supporting Information. Raman spectra of toluene droplets; emission and excitation spectra of reference materials (pyrene, polypyrrole, pyrene-doped polypyrrole); excitation and emission spectra of pyrene in the presence of pyrrole oligomers; FTIR spectrum of toluene-filled free-standing microvessels; anisotropy decays for pyrene in bulk toluene, in the presence of pyrrole and pyrrole oligomers. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT Special thanks are due to Mr. Piotr Olejnik for his assistance with FTIR measurements. This work was supported by the Ministry of Science and Higher Education through National Centre for Research and Development, Project No. PBR 0923/ R/T02/2010/10. G.J.B. gratefully acknowledges support from the U.S. National Science Foundatiozn under Grant 0808677. The FTIR measurements were carried out with the support of the project financing agreements POIG.02.02.00-14-024/08-00. The TEM measurements were performed in the Laboratory of Electron Microscopy, Nencki Institute of Experimental Biology, Warsaw, Poland. The TEM equipment was installed under the project sponsored by the EU Structural Funds: Centre of Advanced Technology BIMEquipment purchase for the Laboratory of Biological and Medical Imaging. Steady state fluorescence measurements were conducted in the Structural Research Laboratory (SRL) at the Department of Chemistry, University of Warsaw, Poland. SRL was established with the financial support of the European Regional Development Fund in the Sectoral Operational Programme “Improvement of the Competitiveness of Enterprises, years 2004200600 , Project No: WKP_1/1.4.3./1/2004/72/72/165/2005/U.
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dx.doi.org/10.1021/la202966k |Langmuir 2011, 27, 12720–12729
