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Latex and Hollow Particles of Reactive Polypyrrole: Preparation, Properties, and Decoration by Gold Nanospheres Claire Mangeney,* Smaı¨n Bousalem, Carole Connan, Marie-Jose`phe Vaulay, Sophie Bernard, and Mohamed M. Chehimi Interfaces, Traitements, Organisation et Dynamique des Syste` mes (ITODYS), UniVersite´ Paris 7, CNRS (UMR 7086), 1 rue Guy de la Brosse, 75005 Paris, France ReceiVed April 5, 2006. In Final Form: July 19, 2006 Polypyrrole-coated polystyrene latex particles bearing reactive N-amino functional groups (PS-PPyNH2) were prepared by the in-situ copolymerization of pyrrole (Py) and the active amino-functionalized pyrrole (PyNH2) in the presence of 1.33 µm-diameter polystyrene (PS) latex particles. These particles were prepared by dispersion polymerization of styrene using poly(N-vinylpyrrolidone), PNVP, as a steric stabilizer. The functionalized polypyrrole-coated PS particles (PS-PPyNH2) were characterized in terms of their particle size and surface morphology using transmission electron microscopy (TEM). Infrared and X-ray photoelectron spectroscopy (XPS) detected pyrrole-NH2 repeat units at the surface of the latex particles, indicating that this monomer had indeed copolymerized with pyrrole. The coreshell structure of the PS-PPyNH2 particles was confirmed by etching the polystyrene core in THF, leading to the formation of hollow conducting polymer capsules. The PS-PPyNH2 particles were then decorated with citratestabilized gold nanoparticles via electrostatic interactions. Furthermore, etching of the polystyrene core resulted in the formation of gold-decorated PPyNH2 hollow capsules.
1. Introduction The synthesis of colloidal particles with a controlled coreshell morphology has been the subject of numerous studies of the recent years due to the diverse applicability of these particles as building blocks for the creation of photonic crystals, multienzyme biocatalysis, and drug delivery systems.1 The properties of colloidal spheres can be altered by coating the particles with an outer shell that influences the final optical, electrical, thermal, mechanical, magnetic, or catalytic properties. The formation of the corresponding hollow shell architectures from these core-shell systems was also investigated, using various methods.2 For example, polyelectrolytes have been used to create hollow core-shell particles by electrostatically adsorbing successive layers of small nanoparticles onto much larger polystyrene spheres serving as removable templates.3 This layerby-layer technique has been used to create hollow spherical colloids composed of inorganic and organic constituents, including luminescent materials. Over the last 15 years or so there has been increasing interest in the deposition of air-stable organic conducting polymers such as polypyrrole (PPy), polyaniline (PANi), and poly(3,4-ethylenedioxythiophene) (PEDOT) onto colloidal/particulate substrates.4 The advantage of these core-shell colloids is the improvement of polymer processability and unique properties intrinsic in dispersed nanometer- or micrometer-sized materials. Recent works have involved the controlled deposition of conducting polymers onto colloidal sols such as silica5 or hematite6 or various organic * Corresponding author. Phone: 33-1-44 27 68 34. Fax: 33-1-44 27 68 14. E-mail:
[email protected]. (1) Caruso, F. AdV. Mater. 2001, 13, 11. (2) (a) Kim, S. W.; Kim, M.; Lee, W. Y.; Hyeon, T. J. Am. Chem. Soc. 2002, 124, 7642. (b) Wang, D.; Caruso, R. A.; Caruso, F. Chem. Mater. 2001, 13, 364. (3) (a) Caruso, F.; Schuler, C.; Kurth, D. G. Chem. Mater. 1999, 11, 3394. (b) Caruso, F.; Spasova, M.; Susha, A.; Giersig, M.; Caruso, A. R. Chem. Mater. 2001, 13, 109. (c) Kato, N.; Caruso, F. J. Phys. Chem. B 2005, 109, 19604. (4) Armes, S. P. In Handbook of Conducting Polymers, 2nd ed.; Skotheim, T. A., Elsenbaumer, R. L., Reynolds, J. R., Eds.; Marcel Dekker: New York, 1998; Chapter 17, p 423. (5) Armes, S. P.; Gottesfeld, S.; Beery, J. G.; Garzon, F.; Agnew, S. F. Polymer 1991, 32, 2325.
latexes.7-10 The corresponding hollow shell architectures were also obtained, after core extraction, and evaluated for various applications such as encapsulation materials.11 An interesting characteristic of the conducting polymer capsule is that transport rates of small molecules into the capsule core are affected by the oxidation state of the conductive polymer,12 a feature of potential utility in many molecular uptake and release scenarios. Despite a massive amount of work on the synthesis and characterization of colloidal particles coated with different conductive polymer shells and their corresponding hollow capsules, depositing noble metal nanocrystals on conducting polymer-coated polystyrene spheres has proven to be challenging. The main benefit from coating nanoparticles on flat or spherical surfaces is to avoid the spread of the nanoparticles to the environment, due to the bonding between the substrate and the coated nanoparticles. Furthermore, gold nanoparticles immobilized onto a particle template can, after selective chemical attack of the template, result in the formation of submicrometric gold hollow spheres.13 It is worth noting that gold nanoparticle clusters retain their localized surface plasmon resonance (LSPR) absorption band when immobilized on functionalized particles.14 The very high sensitivity of gold (and silver) nanocolloid plasmon bands to changes in the refractive index at particle surfaces has been advantageously applied to the real-time monitoring of ligand-receptor interactions in both liquid and solid phases.15 (6) Partch, R.; Gangolli, S. G.; Matijevic, E.; Cai, W.; Arajs, S. J. Colloid Interface Sci. 1991, 144, 27. (7) Beadle, P.; Armes, S. P.; Gottesfeld, S.; Mambourquette, C.; Houlton, R.; Andrews, W. D.; Agnew, S. F. Macromolecules 1992, 25, 2526. (8) Liu, C.; Maruyama, T.; Yamamoto, T. Polym. J. 1993, 25, 363. (9) (a) Omastova, M.; Pavlinec, J.; Pionteck, J.; Simon, F.; Kosina, S. Polymer 1998, 39, 6559. (b) Omastova, M.; Simon, F. J. Mater. Sci. 2000, 35, 1743. (10) Okubo, M.; Fujii, S.; Minami, H. Colloid Polym. Sci. 2001, 279, 139. (11) (a) Yang, Y.; Chu, Y.; Yang, F.; Zhang, Y. Mater. Chem. Phys. 2005, 92, 164. (b) Cho, S.-H.; Kim, W.-Y.; Jeong, G.-K.; Lee, Y.-S. Colloids Surf. A 2005, 255, 79. (12) Marinakos, S. M.; Novak, J. P.; Brousseau, L. C., III; House, A. B.; Edeki, E. M.; Feldhaus, J. C.; Feldheim, D. L. J. Am. Chem. Soc. 1999, 121, 8518. (13) Chah, S.; Fendler, J. H.; Yi, J. J. Colloid Interface Sci. 2002, 250, 142. (14) (a) Oldenburg, S. J.; Averitt, R. D.; Westcott, S. L.; Halas, N. J. Chem. Phys. Lett. 1998, 288, 243. (b) Westcott, S. L.; Oldenburg, S. J.; Lee, T. R.; Halas, N. J. Langmuir 1998, 14, 5396.
10.1021/la060910f CCC: $33.50 © 2006 American Chemical Society Published on Web 10/25/2006
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Figure 1. Schematic representation of the assembly of negatively charged gold nanoparticles on the surface of positively charged core-shell polypyrrole-polystyrene latex particles bearing surface-protonated N-propylamino groups.
In view of biomedical sensing and therapeutic applications, gold nanoshells offer two appealing properties: on one hand, they scatter light in the near-infrared, enabling optical molecular cancer imaging, and on the other hand, they absorb light, therefore allowing selective destruction of targeted carcinoma cells through photothermal therapy.16 It is expected that the decoration of conducting polymer capsules by gold nanoparticles provides novel bifunctional materials with (i) permeability that depends on the oxidation state of the conducting polymer layer and (ii) optical properties of the gold nanoparticles coating. Such a gold-decorated conducting polymer capsule could be a promising candidate for drug delivery system with photothermal therapeutic capabilities. Although no reports have been published yet on the synthesis of gold-nanoparticle-coated conducting polymer capsules, a few methods have already been applied to coating nanoparticles of noble metals on conducting polymer-coated polystyrene (PS) latexes. For example, Armes and co-workers17 synthesized metalclad latex particles by electroless deposition of gold via redox interaction between Au(III) ions and conducting polymers such as polypyrrole, polyaniline, and poly(3,4-ethylenedioxythiophene). However, this method did not allow homogeneous gold coatings to be obtained. In the present work, an alternative route is proposed for the surface modification of polypyrrole latexes by gold in the form of nanoparticles and for the formation of gold-decorated conducting polymer capsules. The method developed herein consists of the electrostatic assembly of negatively charged gold nanoparticles onto positively charged polystyrene-polypyrrole core-shell particles bearing surfaceprotonated N-propylamino groups. This method presents the advantage to perfectly control the size and dispersity of the gold nanocoating. Figure 1 presents a schematic of the procedure employed to prepare the gold-decorated polypyrrole latexes and the corresponding hollow capsules. The dispersion polymerization of styrene was chosen for the preparation of the polystyrene core, the choice being dictated by the better colloidal stability obtained by this method compared to emulsion polymerization. Poly(N-vinylpyrrolidone) was used as a steric stabilizer. The reactive coatings consist of copolymers of pyrrole and N-propylamino-substituted pyrrole (pyrroleNH2) in the initial 50:50 (%) ratio. PS-PPyNH2 latex particles were characterized by FTIR, X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). Selected batches of particles were incubated with gold nanoparticles in order to monitor the formation of assemblies between the carrier (polypyrrole particle) and the supported nanoparticles. Then the polystyrene core was selectively removed (15) (a) Englebienne, P. Analyst 1998, 123, 1599. (b) Haes, A. J.; Van duyne, R. P. J. Am. Chem. Soc. 2002, 124, 10, 596. (16) Loo, C.; Lowery, A.; Halas, N.; West, J.; Drezek, R. Nano Lett. 2005, 5, 4, 709. (17) Akif Khan, M.; Perruchot, C.; Armes, S. P.; Randall, D. P. J. Mater. Chem. 2001, 11, 2363.
by etching the uncoated and gold-decorated latexes in THF, leading to the formation of the corresponding hollow capsules. 2. Experimental Section 2.1. Materials. Styrene (Aldrich) was purified by passing through a column of activated neutral alumina. Poly(N-vinylpyrrolidone), with a nominal molecular weight of 360 000, was purchased from Aldrich and used without further purification. Pyrrole (Fluka) was purified by passing through a column of activated basic alumina (Acros) prior to use. The 1-(3-aminopropyl)pyrrole comonomer was synthesized as previously described in ref 18. 1-(2-Cyanoethyl)pyrrole (Acros), FeCl3‚6H2O (Aldrich), R-azoisobutyronitrile (AIBN) (Fluka), hydrogen tetrachloroaurate (Aldrich), and sodium citrate (Aldrich) were used without further purification. All aqueous solutions were prepared with deionized water. 2.2. Synthesis of Uncoated Polystyrene Latex Particles. The sterically stabilized polystyrene latex particles were prepared using the procedure described by Lascelles and Armes.19 Briefly, PNVP stabilizer (7.5 g) was dissolved in 400 mL of 2-propanol in a threenecked round-bottomed flask. The mixture was heated to 75 °C under a nitrogen purge for 24 h. A solution of styrene (83.3 mL) containing AIBN (0.75 g) was added dropwise to the vigorously stirred PNVP/2-propanol solution. The reaction was left to proceed at 70 °C for 24 h under continuous mechanical stirring at 400 rpm. The polymerization was allowed to proceed for 24 h before cooling to room temperature. The resulting milky-white mixture was centrifuged, the supernatant was decanted and replaced with deionized water, and the white sediment was redispersed. This centrifugationredispersion cycle was repeated several times in order to remove nonadsorbed PNVP stabilizer. 2.3. Synthesis of Surface-Functionalized Polypyrrole-Coated Polystyrene Latex Particles. The coating procedure consists of the in-situ copolymerization of the monomers in the presence of polystyrene latex. Pyrrole and PyNH2 were premixed in 50:50 (23 10-3: 23 10-3 mol) molar ratios. This comonomer mixture was added to a vigorously stirred solution containing 1 g of dry weight PS and 1.8 g of FeCl3‚6H2O. The solution was stirred at 75 °C for 24 h. The resulting colloidal particles were isolated by five centrifugation-redispersion cycles and redispersed in deionized water. The composite poly(pyrrole/pyrroleNH2)-coated PS particles are abbreviated PS-PPyNH2. The polystyrene core was selectively extracted from PS-PPyNH2 latex particles using THF. The solution was stirred at room temperature for 24 h, and the resulting hollow capsules were isolated by five centrifugation-redispersion cycles. 2.4. Coating of PS-PPyNH2 with Gold Nanoparticles. Gold particles (12 nm-sized) were prepared according to literature procedures.20 Before use, all glassware was washed with freshly prepared aqua regia (3:1 HCl:HNO3) followed by extensive rinsing with doubly distilled water. Sodium citrate (39 mM, 50 mL) was poured into boiling 1.0 mM HAuCl4 (500 mL) under vigorous stirring. After the onset of a deep red color, boiling and stirring were continued for 15 min. The solution was cooled to room temperature under continuous stirring to yield a mother solution evaluated as 16 nM (18) Goller, M. I.; Barthet, C.; McCarthy, G. P.; Corradi, R.; Newby, B. P.; Wilson, S. A.; Armes, S. P.; Luk, S. Y. Colloid Polym. Sci. 1998, 276, 1010. (19) Lascelles, S. F.; Armes, S. P. J. Mater. Chem. 1997, 7, 1339. (20) Freeman, R. G.; Hommer, M. B.; Grabar, K. C.; Jackson, M. A.; Natan, M. J. J. Phys. Chem. 1996, 100, 718.
Latex and Hollow Particles of ReactiVe Polypyrrole in colloid from UV/vis absorption, by taking into account the average particle diameter of 12 nm ( 1 nm as determined by direct observation in transmission electronic microscopy. All measurements were performed at room temperature and pressure. The coating was investigated by the reaction of PS-PPyNH2 with gold nanoparticles. Gold nanoparticles, 16 nM in aqueous suspension, were added to the PS-PPyNH2 latex particles (0.15 g dry weight, total volume 2 mL), and the mixture was left to react for 16 h. After incubation, the samples were centrifuged and washed thoroughly with distilled water in order to remove the free and/or loosely bound nanoparticles. The centrifugation-redispersion cycles were repeated until the washings were clear and colorless. The products were characterized by TEM. Etching of the polystyrene core was done in the same way as previously described for the precursors PS-PPyNH2 latex particles. 2.5. Analytical Techniques. Transmission electron microscopy (TEM) micrographs were obtained using a JEOL JEM 100CXII UHR operating at 100 kV. Solutions containing the latex particles were cast onto Formvar-coated copper grids, and the solvent was allowed to evaporate. FT-IR spectra were recorded using a Nicolet Magna 550 Series II instrument. Spectra were typically averaged over 20 scans at 4 cm-1 resolution. X-ray photoelectron spectroscopy (XPS) measurements were performed using a Thermo VG ESCALAB 250 instrument equipped with a monochromatic Al KR X-ray source (1486.6 eV). The X-ray spot size was 650 µm. The pass energy was set at 150 and 40 eV for the survey and the narrow scans, respectively. Charge compensation was achieved with a combination of electron and argon ion flood guns. The energy and emission current of the electrons were 4 eV and 0.35 mA, respectively. For the argon gun, the energy and the emission current were 0 eV and 0.1 mA, respectively. The partial pressure for the argon flood gun was 2 × 10-8 mbar. These standard conditions of charge compensation resulted in a negative but perfectly uniform static charge. Data acquisition and processing were achieved with the Avantage software, version 1.85. Spectral calibration was determined by setting the main C1s component at 285 eV. The surface composition was determined using the manufacturer’s sensitivity factors. The fractional concentration of a particular element A (%A) was computed using %A )
(IA/sA)
∑(I /s )
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Figure 2. SEM micrographs of (a) uncoated PS latex and (b) coreshell PS-PPyNH2 latex particles.
× 100
n n
where In and sn are the integrated peak areas and the sensitivity factors, respectively.
3. Results and Discussion 3.1. Chemical Composition and Morphology of PSPPyNH2 Particles. Figure 2 displays SEM micrographs of PS and PS-PPyNH2 particles. Particles are spherical and have quasimonodisperse size distribution. The number-average diameters (Dn) of particles were measured directly from the SEM images. In the common procedure, the sizes of 500 particles are measured and the values are averaged. The calculated diameter values of the actual latex particles are collected in Table 1. One should keep in mind that the size data determined by SEM, which is a local technique, have a limited statistical relevance compared to the values that could be obtained by scattering experiments, which probe larger areas. However, from the diameters estimated by SEM, it is worth noting that the number-average particle diameter increases from around 1.33 µm for PS particles to around 1.51 µm for PS-PPyNH2 particles. The difference between these two diameters (180 nm) gives an estimation of the conductive overlayer thickness, i.e., around 90 nm. However, this evaluation should be considered as semiquantitative because the change in diameter is only 12% of the total diameter of particles.
Figure 3. TEM micrographs of (a) uncoated PS latex and (b) coreshell PS-PPyNH2 latex particles. Table 1. Properties of Uncoated and PPyNH2-Coated Polystyrene Latex Particlesa particles
Dn (µm)
standard deviation
shell thickness (nm)
PS PS-PPyNH2 hollow PPyNH2 gold-decorated PS-PPyNH2 gold-decorated PPyNH2 capsules
1.33 1.51 1.53 1.53 1.51
0.11 0.16 0.17 0.16 0.12
90 40 95 b
a Error bars are 0.01 µm. b The shell thickness of gold-decorated PPyNH2 hollow capsules could not be determined using TEM images.
TEM microscopy allowed comparison of the surface morphology of PS and PS-PPyNH2 particles taken from the same batches as the particles imaged by SEM. As shown in Figure 3, the conducting polymer coating produces slight modifications of the surface morphology. The uncoated PS particles (Figure 3a) have a smooth, featureless surface morphology. In contrast, for PS-PPyNH2 particles (Figure 3b), the conducting polymer overlayer induces a weak roughening of the surface. These modifications in the surface morphology and roughness of the PS-PPyNH2 latex particles are in line with previously published data.19
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Figure 4. FTIR spectra of (a) pure PPyNH2 film, (b) PNVP-stabilized PS particles, and (c) PS-PPyNH2 latex particles, in the 650-2000 cm-1 region.
FTIR spectroscopy proved to be useful to characterize the presence of the PPyNH2 coating on the surface of the polystyrene particles. Since an attempt to prepare a homopolymer of PPyNH2 was unsuccessful, the reactive coatings consist of copolymers of pyrrole and N-propylamino-substituted pyrrole (pyrroleNH2) in the initial 50:50 (%) ratio. Figure 4 displays the FTIR spectra of pure PPyNH2 film, PNVP-stabilized PS particles, and PSPPyNH2 latex particles, in the 650-2000 cm-1 region. In the spectrum of pure PPyNH2 films (see Figure 4a), prepared electrochemically, the vibrational bands associated with PPyNH2 are routinely seen at frequencies of 1560 cm-1 and lower. The antisymmetric ring stretching is observed at 1552 cm-1 as well as the C-H in-plane deformation vibrations at 1312, 1195, and 1048 cm-1. Figure 4b depicts the spectrum of uncoated polystyrene particles, which shows the characteristic absorbances of a monosubstituted arene, with two bands at 1493-1454 cm-1 for the CdC stretch and two others at 756 and 700 cm-1 for the CdC-H out-of-plane bending. An additional weak feature is observed at ca. 1603 cm-1, assigned to the pyrrolidone carbonyl of the PNVP stabilizer. In Figure 4c, the spectrum of PPyNH2-coated particles appeared to be the combination of the pure PPyNH2 spectrum (cf. Figure 4a) and the spectrum of pure polystyrene (cf.Figure 4b), suggesting the effective coating of PPyNH2 on the polystyrene particles. Figure 5 displays XPS survey spectra of PS and PS-PPyNH2 particles. The main peaks are C1s, N1s, and O1s centered at 285, 400, and 532 eV, respectively. In the PS-PPyNH2 spectrum, N1s and O1s relative peak intensities are fairly high by comparison to the same peaks from the underlying PNVP-stabilized PS. Particularly, one can note the increase in the N1s/C1s peak intensity ratio as the PS particles are coated with the PPyNH2 conducting polymer. This can easily be explained by the presence of around 27% of nitrogen relative to carbon in PPyNH2 while PS particles only contain a small contribution of nitrogen brought by the PNVP stabilizer. The relative increase of oxygen is due to overoxidation of the conducting polymer. Furthermore, new minor peaks appear in the PS-PPyNH2 spectrum, Cl2p (at 198 eV) and Fe2p3/2 (at ∼710 eV), which are assigned respectively to the chloride dopants and to the insertion of iron in the form of FeCl2 resulting from the reduction of the oxidizing agent FeIII.
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Figure 5. XPS survey scans of PNVP-stabilized PS and PS-PPyNH2 particles. Table 2. Apparent Surface Chemical Composition As Determined by XPSa samples
C
N
O
Cl
PS-PNVP PS-PPy PS-PPyNH2 PPyNH2 capsules
92.0 72.1 68.2 70.0
3.3 10.8 11.4 12.4
4.7 15.2 19.3 17.2
1.9 1.1 0.4
a
The standard deviation is about 10% for all elements.
Table 2 summarizes the atomic percent determined for PS, PS-PPyNH2, and homopolypyrrole-coated polystyrene particles (PS-PPy). By comparison to PS coated with a homopolypyrrole layer, the N/C atomic ratio is even higher, in agreement with the slight increase of nitrogen content in the pyrroleNH2 repeat unit (theoretical ratio ) 2/7) compare to pyrrole (theoretical ratio ) 1/4). This provides direct evidence for the incorporation of pyrroleNH2 repeat units in the conducting copolymer overlayers. The high-resolution C1s spectra are shown in Figure 6a for the uncoated and polypyrrole NH2-coated PS particles. For PS, the C1s region exhibits a sharp main component centered at 285 eV and minor components centered at 286.3 (C-N), 288 (NCdO), and 291.5 eV (shake-up satellite). The latter component is characteristic of the aromatic pendent phenyl group from styrene repeat units. For PS-PPyNH2 particles, the C1s region experiences a dramatic change, resulting in the widening of the main component centered at 285 eV, due to the introduction of C-C/ C-H, C-N, and CdN bonds particularly. It is noteworthy that the CdO carbon from the polypyrrole oxidation induces a distinct component at 288 eV that is not observed at the surface of the underlying PS latex particles. Moreover, the shake-up satellite, the fingerprint of polystyrene, decreases in intensity following coating by the conducting shell. This actually occurs only when polypyrrole coating is uniform and quite thick.21 For patchy polypyrrole coatings, the polystyrene shake-up satellite remains very well detected.22 This modification of the C1s signal is a clear indication of a thick and continuous coating of the PS particles by the PPyNH2 conducting polymer shell. The peak-fitted N1s core-line spectrum of PS-PPyNH2 particles, displayed in Figure 6b, shows that three nitrogen environments are present, centered at 398, 400, and 401.5 eV; (21) Perruchot, C.; Chehimi, M. M.; Delamar, M.; Lascelles, S. F.; Armes, S. P. Langmuir 1996, 12, 3245. (22) Chehimi, M. M.; Azioune, A.; Bousalem, S.; Ben Slimane, A.; Yassar, A. In Colloid Polymers. Synthesis and Characterization; Elaissari, H., Ed.; Surfactant Science Series; Marcel Dekker: New York, 2003; Vol. 115, Chapter 10.
Latex and Hollow Particles of ReactiVe Polypyrrole
Figure 6. (a) High-resolution C1s XP spectra of PNVP-stabilized PS and PS-PPyNH2 particles and (b) peak-fitted N1s core-line spectrum of PS-PPyNH2 particles.
these peaks are assigned to dN-, -NH-, and -N+-, respectively. The existence of protonated amine is supposed to lead to the attachment of the gold nanoparticles to the PSPPyNH2 particles surface. 3.2. Characterization of Hollow PPyNH2 Particles. PSPPyNH2 particles were converted to hollow microspheres by immersing the particles in THF. Presumably, the dissolution of PS occurs via transport of the solvent through the conducting polymer shell to the core. Figure 7a shows the SEM micrograph of the hollow PPyNH2 microspheres after etching the PS core. Most of the hollow PPyNH2 capsules appear to be intact, but some microspheres display holes, indicating that they are hollow. The PPyNH2 residues duplicated the original template structure very well and were not out of shape, which suggests a high strength for the PPyNH2 shells. The formation of hollow PPyNH2 capsules resembles that of the hollow conductive polymer microspheres synthesized with sulfonated polystyrene template.11a However, this contrasts with the case of homopolypyrrole-coated PS latex particles, which led to the formation of broken egg shells after selective extraction of the core in THF.19 It follows that functionalized polypyrrole (here PPyNH2) has mechanical and/or microporous properties that permit it to withstand the selective extraction of the core and hence the formation of hollow capsules. The size of the hollow capsules (∼1.53 µm) is greater than that of the core-shell particles (see Table 1). Considering that, in the early stages of the THF exposure, the core is swollen with the solvent molecules, this should lead to an increase in the particle size, resulting in the expansion of the PPyNH2 shell as well. Then, after having passed a state of gelation, the mobility of the PS chains is high enough to enable diffusion through the expanded PPyNH2 shell.
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Figure 7. Micrographs of PPyNH2 hollow capsules after etching of the polystyrene core in THF, obtained by (a) SEM and (b) TEM.
TEM micrograph in Figure 7b further evidences the hollow nature of the PPyNH2 residues with a sharp contrast between the dark edge and the pale center. The sphere shell thickness dTEM, determined from the TEM images, is approximately 40 nm. This value is far below that previously obtained by calculating the difference between PSPPyNH2 and PS particles diameters (i.e. d ∼ 90 nm). As the overall diameter of the hollow capsule is higher than that of the plain original particle, it could partly explain the decrease in polymer thickness as it has expanded. However, it can be calculated that the mass of the PPyNH2 shell decreases by about 50% from PS-PPyNH2 core-shell particles to hollow PPyNH2 capsules. It thus seems that, although the diffusion of polystyrene chains through the PPyNH2 layer does not disrupt the conducting polymer capsules, it however leads to a partial leaching of some PPyNH2 chains into the solution. It is also possible that THF removes oligomers of aminated polypyrrole. FTIR spectra, shown in Figure 8, of the THF-treated PSPPyNH2 particles were recorded in order to confirm the extraction of the PS core. After treatment with THF, one notices a clear decrease of the PS characteristic bands (at 1493 and 1454 cm-1) relative to the PPy peak (at 1552 cm-1): the PS bands lose approximately 75% of their intensity. This last feature shows that the treatment of the PS-PPyNH2 particles with THF effectively extracts, at least partially, the PS core from the coreshell particles. The fact that the PS bands do not disappear completely may be explained by the presence of still a small amount of PS inside the PPyNH2 shell or by some extracted PS forming aggregates suspended in the latex suspension. One should note that the
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Figure 8. FTIR spectra of PS-PPyNH2 particles (solid line) and THF-treated PS-PPyNH2 particles (dotted line) in the 1300-1900 cm-1 region.
PNVP characteristic band is still visible after extraction of the PS core, showing that the PNVP stabilizer remains entrapped inside the PPyNH2 shell. XPS spectra recorded for the THF-treated PS-PPyNH2 particles indicated a very slight increase of the N/C atomic ratio from 0.17 to 0.18 following PS extraction (see Table 2). This is in line with a polymer shell that resembles more and more that of pure PPyNH2 (N/C ∼ 0.27). 3.3. Decoration with Gold Nanoparticles. Aqueous suspensions of 12 nm citrate-stabilized gold nanoparticles (polydispersity index 1.08) were prepared by the method developed by Natan et al.20 They were then combined with PS-PPyNH2 particles, with the amount of gold particles in slight excess of that estimated for full monolayer coverage of the latex particles surface. Nonadsorbed gold particles were always observed after nanoparticle self-assembly, indicating that the PS-PPyNH2 particles were the limiting reagent. The resulting core-shell particle ensembles were then precipitated, rinsed, and redispersed several times to remove loosely bound nanoparticles. Scanning electron microscopy (Figure 9a) allowed determination of the particle diameters. The mean latex particle diameter increases from 1.51 µm for PS-PPyNH2 particles to 1.53 µm for gold-decorated PS-PPyNH2 particles. The difference between these two diameters (around 20 nm) allows one to get an estimation of the gold deposit thickness, ca. 10 nm. This thickness matches to the gold nanoparticles diameter. However, the TEM image of the spherical microparticle ensembles, displayed in Figure 9b, shows that the deposition of the gold nanoparticles forms a submonolayer of single nanoparticles coexisting with domains of nanoparticle aggregates. It is likely that the electrostatic attraction between positively charged PS-PPyNH2 and negatively charged gold nanoparticles is the driving force for the assembly of gold nanoparticles at the surface of the PS-PPyNH2 particles. This is in agreement with a reported study23 of the assembly of gold nanoparticles on aminosilanized glass plates, which demonstrated the role of electrostatic interactions for the attachment of gold nanoparticles. The latex surface positive charges of PS-PPyNH2 particles come from (i) the protonated amino groups and (ii) the polypyrrole backbone, which is positively charged in its oxidized form. (23) Seitz, O.; Chehimi, M. M.; Cabet-Deliry, E.; Truong, S.; Felidj, N.; Perruchot, C.; Greaves, S. J.; Watts, J. F. Colloids Surf. A 2003, 225.
Figure 9. Micrographs of gold nanoparticle-coated PS-PPyNH2 latex obtained by (a) SEM and (b) TEM.
Figure 10. XPS survey spectrum of gold-decorated PS-PPyNH2 latex particles. The inset shows the high-resolution Au4f XP spectrum.
XPS analysis were carried out on the gold-decorated PSPPyNH2 particles. The survey spectrum, shown in Figure 10, displays the typical elements of the PS-PPyNH2 particles, with carbon (at 285 eV), nitrogen (at 400 eV), oxygen (at 532 eV), and chloride (at 200 eV). In addition to these peaks, a strong pair of doublets can be detected at ca. 84 and 340 eV, which are assigned to photoelectrons originating from Au 4f and Au 4d energy levels, respectively. The Au/N atomic ratio is calculated to be ∼0.19. Close inspection of the peak-fitted Au 4f core-line spectrum, shown in the inset of Figure 10, reveals two components in the Au 4f5/2 and 4f7/2 energy levels. The first component of the 4f7/2 peak, centered at ca. 84 eV, is characteristic of zerovalent gold. In addition, one observes a small component at ca. 86-87 eV, assigned to the
Latex and Hollow Particles of ReactiVe Polypyrrole
Figure 11. Micrographs of gold-decorated PPyNH2 hollow capsules after etching the polystyrene core in THF, obtained by (a) SEM and (b) TEM.
presence of Au(III) species. These species are possibly due to incomplete reduction of [AuCl4]- during gold nanoparticle synthesis, resulting in the adsorption of citrate anions to neutralize these positive charges, forming a negatively charged shell around the gold nanoparticles. The colloidal stability of the gold-decorated latexes was assessed qualitatively. The high degree of dispersion of the PSPPyNH2 particles is possible because the conducting polymer is located within the solvated PNVP steric stabilizer layer. The diameter of the gold deposits (around 10-12 nm) is lower than the estimated 20-30 nm thickness of the PNVP steric stabilizer layer. However, once the PPyNH2-coated polystyrene latex is decorated with gold, the degree of aggregation increases significantly. This can be explained by the high Hamaker constant of gold (like all metals): the surface-confined gold nanoparticles thus acts as heteroflocculants for the latex particles. 3.4. Characterization of Hollow Au-Decorated PPyNH2 Particles. We further explored the formation of hollow Au-
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decorated PPyNH2 capsules: first, gold-decorated PS-PPyNH2 particles were synthesized and then the polystyrene core was extracted using a THF solution. Figure 11a shows a SEM image of Au-decorated PPyNH2 hollow capsules. The capsules appear to be intact, with a spherical shape, and narrowly dispersed. It is noteworthy that the mean diameter of the gold-decorated hollow capsules (around 1.51 µm) is slightly lower than that of the corresponding gold-decorated core-shell structured particles. This contrasts with the case of gold-free particles, the diameter of which increased after the core extraction. Such a difference in behavior between uncoated PS-PPyNH2 particles and golddecorated particles can be attributed to the presence of the gold nanocoating that strengthens and hardens the PPyNH2 shell and prevents it from expanding while the solubilized polystyrene diffuses through the conducting polymer layer. A TEM micrograph of Au-decorated PPyNH2 capsules is shown in Figure 11b. It was not possible, from the TEM images, to determine the sphere shell thickness, due to the presence of the gold nanoparticles adsorbed on the PPyNH2 hollow capsules. One observes that the gold deposits keep the same spatial arrangement as prior to the core extraction. This indicates that the gold nanoparticles are strongly adhering at the surface of the PPyNH2 layer. The absence of single particles observed on the TEM images suggests a strong tendency of the Au-PPyNH2 capsules to form clusters, in agreement with the decreased colloidal stability. XPS allowed the study of the effect of the THF-induced core extraction on the chemical composition of the particle surface. The gold signal, at 84 eV (Au4f7/2), remains unchanged after THF treatment of the gold-decorated PS-PPyNH2 particles with a Au/N atomic ratio of 0.20, indicating that the gold nanocoating is not affected by the THF treatment.
4. Conclusions Polypyrrole-polystyrene latex particles bearing surface Npropylamino functional groups were prepared in aqueous solution by copolymerization of pyrrole and N-aminated pyrrole (pyrroleNH2) using FeCl3 in the presence of PS latexes. The PS-PPyNH2 particles were characterized in terms of chemical composition, size, morphology, and core-shell structure. TEM indicated a narrow size distribution of the particles with a diameter around 1.51 µm. The core-shell structure of the PS-PPyNH2 particles was evidenced by etching the polystyrene core in THF leading to the formation of hollow conducting polymer capsules. The aminated particles proved to be effective for the electrostatic attachment of gold nanoparticles. Furthermore, etching of the polystyrene core resulted in the formation of gold-decorated PPyNH2 hollow capsules. These new materials open up opportunities for the development of new drug delivery systems with photothermal therapeutic capabilities. LA060910F
