Synthesis and Characterization of Carboxylic Acid-Functionalized

This work prompted us to examine whether our polypyrrole−silica ...... Magnetically Responsive Carboxylated Magnetite-Polydipyrrole/Polydicarbazole ...
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Langmuir 1997, 13, 3686-3692

Synthesis and Characterization of Carboxylic Acid-Functionalized Polypyrrole-Silica Microparticles Using a 3-Substituted Pyrrole Comonomer G. P. McCarthy and S. P. Armes* School of Chemistry, Physics and Environmental Science, University of Sussex, Falmer, Brighton BN1 9QJ, U.K.

S. J. Greaves and J. F. Watts Department of Materials Science and Engineering, Surrey University, Guildford, Surrey GU2 5XH, U.K. Received December 19, 1996. In Final Form: April 30, 1997X A series of carboxylated polypyrrole-silica microparticles were synthesized by copolymerizing pyrrole3-acetic acid with pyrrole using a hydrogen peroxide-based oxidant in the presence of an ultrafine silica sol. The effect of varying the proportion of pyrrole-3-acetic acid in the comonomer feed on the physicochemical properties of the resulting carboxylated microparticles has been examined. The particle size distributions and morphologies of the microparticles were assessed using disk centrifuge photosedimentometry and transmission electron microscopy. The compositions of the microparticles were determined using thermogravimetry (silica content) and microanalyses (conducting polymer content), respectively. The surface compositions of the microparticles were examined using X-ray photoelectron spectroscopy and ζ-potential measurements. Increasing the proportion of the pyrrole-3-acetic acid in the comonomer feed led to higher degrees of surface carboxylation, as expected. The reproducibility of these microparticle syntheses was significantly improved compared to that of similar dispersions synthesized using an N-substituted pyrrolic comonomer in an earlier study. Finally, these new microparticles exhibited improved colloid stability after long-term storage under ambient conditions.

Introduction In a series of recent papers, we have reported the preparation and characterization of polyaniline-silica, polypyrrole-silica, and polypyrrole-tin(IV) oxide particles using ultrafine silica or tin(IV) oxide sols as particulate dispersants in aqueous media.1-7 These sols act as high surface area colloidal substrates for the precipitating polyaniline or polypyrrole, yielding stable colloidal dispersions of composite particles which possess a “raspberry” morphology (see Figure 1). These colloidal “raspberries” consist of microaggregates of silica (or tin(IV) oxide) particles held together by the conducting polymer binder component. Both the dimensions of the “raspberries” and the conducting polymer loading can be varied over a wide range by judicious selection of the oxidant type and silica concentration.4,5,7 Small-angle X-ray scattering studies on polyaniline-silica raspberries have confirmed that these materials are true nanocomposites, since the polyaniline chains are confined within nanoscale cavities between the silica particles.8 BET surface area measurements on a series of conducting polymer-inorganic oxides suggested that at least some of these nanocomposites exhibit significant microporosity.9 * Author to whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, July 1, 1997. (1) Gill, M.; Mykytiuk, J.; Armes, S. P.; Edwards, J. L.; Yeates, T.; Moreland, P. J.; Mollett, C. Chem. Commun. 1992, 108. (2) Gill, M.; Armes, S. P.; Fairhurst, D.; Emmett, S. N.; Idzorek, G. C.; Pigott, T. Langmuir 1992, 8, 2178. (3) Stejskal, J.; Kratochvil, P.; Armes, S. P.; Lascelles, S. F.; Riede, A.; Helmstadt, M.; Prokes, J.; Krivka, I. Macromolecules 1996, 29, 6814. (4) Maeda, S.; Armes, S. P. J. Colloid Interface Sci. 1993, 159, 257. (5) Maeda, S.; Armes, S. P. J. Mater. Chem., 1994, 4, 935. (6) Maeda, S.; Armes, S. P. Chem. Mater. 1995, 7, 171. (7) Lascelles, S. F.; Butterworth, M. D.; McCarthy, G. P.; Armes, S. P. to be submitted to Polymer. (8) Terrill, N. J.; Crowley, T.; Gill, M.; Armes, S. P. Langmuir 1993, 9, 2093.

S0743-7463(96)02121-X CCC: $14.00

Figure 1. Schematic representation of the formation of carboxylated polypyrrole-silica microparticles by copolymerization of pyrrole-3-acetic acid with pyrrole using the hydrogen peroxide-based oxidant in the presence of an ultrafine silica sol.

Recent XPS studies10 of a series of conducting polymersilica nanocomposites confirmed that the conducting polymer component was always present at (or very near) the surface of the raspberries, since a N1s signal was observed in each case. On the other hand, the surface Si/N atomic ratios indicated that the surface compositions of these conducting polymer-silica particles were invariably silica-rich, which suggests that a charge stabilization mechanism is responsible for their long-term colloid stability in aqueous media.11 In this regard, it is noteworthy that silica sols, unlike many other oxide sols, are particularly resistant to electrolyte-induced flocculation.12 These XPS observations were supported by aqueous electrophoresis measurements on homopolyaniline- and (9) Maeda, S.; Armes, S. P. Synth. Met. 1995, 73 (2), 151. (10) Maeda, S.; Gill, M.; Armes, S. P.; Fletcher, I. W. Langmuir 1995, 11 (6), 1899. (11) Hunter, R. J. In Foundations of Colloid Science; Clarendon Press: Oxford, 1987; Vol. 1. (12) Healey, T. W. In The Colloid Chemistry of Silica; Bergna, H. E., ed.; A.C.S. Symposium Series No. 234; American Chemical Society: Washington, DC, 1994; p 147.

© 1997 American Chemical Society

Polypyrrole-Silica Microparticles

homopolypyrrole-silica particles;13 ζ-potential vs pH curves for these dispersions were essentially superimposable on that of a silica sol. However, polypyrrolesilica particles containing surface carboxylic acid or amine groups exhibited significant differences in electrophoretic mobility. In 1992, Tarcha and co-workers14 at Abbott Laboratories reported that poly(vinyl alcohol)-stabilized polypyrrole latexes could be used as marker particles in visual agglutination immunodiagnostic assays for the pregnancy hormone hCG, the hepatitis “B” surface antigen, and the AIDS antibody. In this application, the “valued-added” properties of the polypyrrole latex are its strong intrinsic coloration, narrow particle size distribution, and ease of synthesis. In order to minimize the nonspecific binding interactions which are a ubiquitous problem in such assays, the surfaces of the polypyrrole latexes were functionalized with specific binding sites (carboxylic acid or amine groups) for the biological analyte of interest. However, such surface derivatization involved multistep procedures and several transfers between aqueous and nonaqueous solvents. This work prompted us to examine whether our polypyrrole-silica microparticles could be used as marker particles in biomedical diagnostics. In principle, these nanocomposite particles should be more amenable to surface functionalization. For example, there is good literature precedent to suggest that silane chemistry could be used to attach surface amine groups to the raspberry particles via a one-step postpolymerization derivatization in aqueous or alcoholic media.15,16 Alternatively, copolymerization of a suitable carboxylic acidfunctionalized pyrrolic comonomer with pyrrole during the nanocomposite synthesis should lead to carboxylated microparticles directly. Unfortunately, in recent studies by the Sussex group, the amine functionalization protocol usually produced particles with relatively low degrees of amination. Moreover, these dispersions tended to irreversibly flocculate during long-term storage at room temperature.17 However, the latter approach proved more fruitful, and in 1995 we reported the synthesis of carboxylic acid-functionalized polypyrrole-silica microparticles using an N-substituted pyrrolic comonomer.18 These functionalized particles were much more highly absorbing at all wavelengths across the visible spectrum compared to a commercial, extrinsically-dyed polystyrene latex used for diagnostic assays. In a separate collaborative study with the Abbott group,19 it was shown that the assay performance of the carboxylated polypyrrole-silica microparticles was comparable to that of the surfacederivatized polypyrrole latexes described by Tarcha and co-workers. Unfortunately, these initial syntheses of the carboxylated polypyrrole-silica microparticles were much less robust and irreproducible compared to those of the homopolypyrrole-silica dispersions. This was attributed to the well-documented poor copolymerizability of Nsubstituted pyrroles with pyrrole.20 (13) Butterworth, M. D.; Corradi, R.; Johal, J.; Maeda, S.; Lascelles, S. F.; Armes, S. P. J. Colloid Interface Sci. 1995, 174, 510. (14) Tarcha, P. J.; Misun, D.; Finley, D.; Wong, M.; Donovan, J. J. In Polymer Latexes: Preparation, Characterisation and Applications; Daniels, E. S.; Sudol, E. D.; El-Aassar, M. S., Eds.; A.C.S. Symposium Series No. 492; American Chemical Society: Washington, DC, 1992; Vol. 22, p 347. (15) Goodwin, J. W.; Habron, R. S.; Reynolds, P. A. Colloid Polym. Sci. 1990, 268, 766. (16) Philipse, A. P.; Vrij, A. J. Colloid Interface Sci. 1994, 165, 519. (17) McCarthy, G. P.; Wilson, S.; Newby, B.; Johal, J.; Armes, S. P. Unpublished results. (18) Maeda, S.; Corradi, R.; Armes, S. P. Macromolecules 1995, 28 (8), 2905. (19) Pope, M. R.; Armes, S. P.; Tarcha, P. J. Bioconjugate Chem. 1996, 7, 436.

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In the present work we describe the synthesis of carboxylated polypyrrole-silica microparticles using a 3-substituted pyrrole-based comonomer, pyrrole-3-acetic acid, in combination with the hydrogen peroxide-based oxidant system described by Yamamoto and co-workers21 (see Figure 1). These colloid syntheses proved much more reliable, and the resulting microparticles have been extensively characterized in terms of their particle morphology and size distribution, conducting polymer content, and surface composition using transmission electron microscopy, disk centrifuge photosedimentometry, thermogravimetric analysis, X-ray photoelectron spectroscopy, and ζ-potential measurements, respectively. Experimental Section Monomer Synthesis. Pyrrole-3-acetic acid was synthesized according to a four-step literature procedure.22 The first step involved protection of the pyrrole N-H group via tosylation. The resulting N-tosyl pyrrole was acetylated using acetic anhydride in CH2Cl2 under Friedel-Crafts conditions and then 1-tosylpyrrole-3-methacrylate was obtained via thallation using K-10 clay as a solid support. Finally, quantitative hydrolysis of both the N-tosyl and methyl ester protecting groups was achieved using 5 M NaOH in methanol at reflux for 12 h. The structure and purity of the resulting pyrrole-3-acetic acid monomer were verified by 1H NMR spectroscopy. Synthesis of Polypyrrole-Silica Microparticles. The synthesis of homopolypyrrole-silica microparticles has been described previously in some detail.4,5,7 The protocol employed in the present study was based on these earlier procedures. Typically, 0.144 g of pyrrole was stirred with 1.04 g (dry weight) of a commercial ultrafine silica sol (kindly donated by Nyacol Products; nominal average particle diameter, 20 nm; provided as a 34 w/w % aqueous dispersion) and hydrogen peroxide (0.319 g of a 27.5 w/w % solution in water). This mixture was diluted to 14 mL with deionized water, and the solution pH was adjusted to pH 2.0 by the addition of 1.2 M HCl. Then 0.15 mL of a 0.01 M FeCl3‚6H2O aqueous solution was added and the total volume was made up to 15 mL with deionized water. The polymerization was allowed to proceed for 24 h with constant stirring. The reaction mixture was then centrifuged at 5000 rpm for 40 min using a Beckman J2-21 centrifuge, and the resulting black sediment was redispersed in deionized water with the aid of an ultrasonics bath. This centrifugation-redispersion cycle was repeated three times in order to remove the excess, nonaggregated silica particles and any (in)organic byproducts of the reaction. Essentially, the same procedure was utilized for the synthesis of carboxylated polypyrrole-silica microparticles, except that the pyrrole monomer was replaced with pyrrole/pyrrole-3-acetic acid mixtures at various comonomer feed ratios (see Table 1). In addition, two initial silica concentrations were investigated (3.4 and 6.8 w/w %). The polymerizations were carried out at both 4 and 25 °C, and three different oxidants were examined, namely FeCl3‚6H2O, (NH4)2S2O8, and H2O2/Fe3+/HCl.22 A carboxylated polypyrrole-silica dispersion was also synthesized using the 1-(2carboxyethyl)pyrrole comonomer (at 75% of the comonomer feed) reported by Maeda and co-workers18 and the hydrogen peroxide oxidant. Finally, polypyrrole and poly(pyrrole-3-acetic acid) homopolymers were synthesized as bulk powder reference materials in the absence of any silica sol using the hydrogen peroxide-based oxidant. Chemical Composition. The microanalytical carbon contents of the carboxylated polypyrrole-silica microparticles were quantitatively determined by CHN elemental microanalyses using a Perkin-Elmer 2400 instrument. Since the carbon contents of the polypyrrole homopolymer and poly(pyrrole-3acetic acid) bulk powders were essentially identical (54.3% and 54.1%, respectively), the conducting polymer contents of the microparticles were readily determined by comparing their carbon contents to those of either of these reference materials. The (20) (a) Neoh, K. G.; Kang, E. T.; Tan, T. C.; Tan, K. L. J. Appl. Polym. Sci. 1989, 38, 2009. (b) Kiani, M. S.; Mitchell, G. R. Synth. Met. 1992, 46, 293. (21) Liu, C. F.; Maruyama, T.; Yamamoto, T. Polymer 1993, 25, 363. (22) Ho-Hoang, A.; Fache, F.; Boiteux, G.; Lemaire, M. Synth. Met. 1994, 62, 277.

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Table 1. Effect of Varying the Pyrrole-3-acetic Acid/ Pyrrole Comonomer Feed Ratio, Oxidant Type, Initial Silica Concentration, and Polymerization Temperature on Colloid Formation comonomer sample feed ratio no. (COOH-Py/Py) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

0:100 25:75 50:50 75:25 75:25 75:25 75:25 100:0 100:0 100:0 75:25 75:25 75:25 75:25 75:25 75:25

oxidant type H2O2/Fe3+/HCl H2O2/Fe3+/HCl H2O2/Fe3+/HCl H2O2/Fe3+/HCl H2O2/Fe3+/HCl H2O2/Fe3+/HCl H2O2/Fe3+/HCl H2O2/Fe3+/HCl FeCl3‚6H2O (NH4)2S2O8 FeCl3‚6H2O FeCl3‚6H2O (NH4)2S2O8 (NH4)2S2O8 (NH4)2S2O8 (NH4)2S2O8

initial polym colloid silica conc temp formed? (w/w %) (°C) (yes/no) 6.8 6.8 6.8 6.8 6.8 3.4 3.4 6.8 6.8 6.8 3.4 6.8 3.4 6.8 3.4 6.8

4 4 4 4 21 4 21 4 4 4 4 4 21 21 4 4

yes yes yes yes yes yes yes no no no no no no no no no

silica contents of the carboxylated polypyrrole-silica microparticles were determined directly by thermogravimetric analysis (Perkin-Elmer TGA-7 instrument; scan rate, 20 °C per min in air). The observed weight losses were attributed to quantitative pyrolysis of the conducting polymer, with the resulting white residues assumed to be pure silica. In calculating the silica content of the microparticles a small correction was made to allow for the surface dehydration of the silica particles. Their water content was determined by thermogravimetric analysis of the original dried silica sol. Fourier transform infra-red (FTIR) spectroscopy studies were performed on the dried microparticles dispersed in KBr disks using a Nicolet Magna-IR 550 spectrometer (average of 64 scans per spectrum at 4 cm-1 resolution). Particle Size Analysis. The particle size distributions of the carboxylated polypyrrole-silica microparticles were determined using a Brookhaven Instruments disk centrifuge photosedimentometer (DCP) at temperatures ranging from 23 to 27 °C, operating in the external line-start mode.23 The following protocol was employed: The dispersions were diluted to 0.1-0.5 w/v % using deionized water, and 33 v/v % methanol was added. After sonication in an ultrasonics bath to ensure complete dispersion of the particles, a 0.20 mL aliquot was injected into the disk cavity, which contained the spin fluid (comprised of 15 mL of deionized water and 1 mL of methanol). Depending on the size of the microparticles, centrifugation rates of 5000-8000 rpm were selected and it was assumed that the highly absorbing particles had the same optical properties (extinction coefficient) as carbon black. Standard deviations were calculated assuming normal statistics for the size distributions. The particle densities required for DCP analyses were determined using a Micromeritics Accu-Pyc 1330 helium pycnometer; each density value was the average of five measurements. The values obtained from this method were in good agreement with densities calculated from the elemental microanalyses and thermogravimetry data assuming additivity. This instrument was also used to determine the densities of the polypyrrole and poly(pyrrole-3-acetic acid) homopolymers, which were found to be 1.53 and 1.45 g cm-3, respectively. Particle Morphology. Transmission electron microscopy (TEM) studies were made on diluted dispersions of the microparticles dried onto carbon-coated copper grids (3.0 mm diameter, e.g. Bio-Rad) using a Hitachi 7100 instrument. Surface Composition Studies. The surface compositions of the three carboxylated polypyrrole-silica microparticles were examined by X-ray photoelectron spectroscopy (XPS) using a VG Scientific ESCALAB Mk. II spectrometer interfaced to a VG 5000S data system based on a DEC PDP 11/73 computer. Samples were mounted on double-sided adhesive tape: excess loose powder was shaken off, leaving enough sample to cover the analysis area (ca. 10 mm2) in the center of the specimen stub. (23) Holsworth, R. M.; Provder, T.; Stansbrey, J. J. In Particle size distribution: assessment and characterisation; Provder, T., Ed.; A.C.S. Symposium Series No. 332; American Chemical Society: Washington, DC, 1987; p 191.

Operating conditions were as follows: the Al KR X-ray source was used at a power of 450 W (i.e. 13 kV potential and 34 mA emission current). The spectrometer was operated in the fixed analyzer transmission mode at a pass energy of 50 eV (survey spectra) or 20 eV (high-resolution spectra). The base pressure in the sample chamber during analysis was approximately 3 × 10-8 mbar. Good-quality survey spectra were obtained with a single scan; core-line high-resolution spectra were integrated over 5-10 scans, depending on the intensity of the spectral region of interest. Total spectral acquisition times per sample were typically 10-20 min. Spectral analysis was carried out using the standard VGS 5000S software for quantification and peak fitting; quantification was based on peak areas calculated from the high-resolution spectra. Minor sample charging effects of the order of 2-3 eV were observed for some of the samples. The errors incurred by ignoring the low hydrogen contents of the samples were negligible. ζ-Potential vs pH measurements were carried out using a Malvern ZetaMaster S instrument. Three carboxylated polypyrrole-silica dispersions in deionized water were examined, and the solution pH was adjusted using either HCl or KOH. ζ-Potentials were calculated from mobilities using the Henry equation. A homopolypyrrole-silica dispersion synthesized using the hydrogen peroxide oxidant was also examined as a reference material. Color Intensity Studies. The absorbance (color intensity) of one of the carboxylated polypyrrole-silica microparticles synthesized in the present study (sample 4 in Table 1) was compared to that of “SuperCarboxyl Hi-Dye” (an extrinsicallydyed, carboxylated-polystyrene latex of 125 nm diameter kindly donated by Polymer Laboratories) and also that of carboxylated polypyrrole-silica microparticles synthesized using the 1-(2carboxyethyl)pyrrole comonomer18 at the same comonomer feed ratio in conjunction with the hydrogen peroxide oxidant. Each dispersion was diluted to 30 mg dm-3, and the absorption spectra were recorded over the 400-900 nm range using a Perkin-Elmer Lambda 2S UV/vis spectrophotometer.

Results and Discussion Maeda et al. have previously reported18 the synthesis of carboxylated polypyrrole-silica particles prepared using 1-(2-carboxyethyl)pyrrole as a comonomer with pyrrole. XPS18 and ζ-potential13 studies confirmed the presence of carboxylic acid groups at the surface of these microparticles. Although this ‘proof of concept’ study demonstrated the general validity of the approach, the colloid syntheses proved rather irreproducible and were surprisingly sensitive to parameters such as the order of addition of the reagents and the duration of the reaction. Another problem encountered was the relatively poor long-term colloid stability exhibited by these dispersions. Although relatively small weight-average particle diameters (∼110 nm) were obtained in some cases, size distributions were generally somewhat broader than those normally considered desirable for immunodiagnostic strip assays. On the other hand, Pope and co-workers19 have recently shown that neither a low degree of dispersion nor broad size distributions seem to be detrimental to the assay performance of carboxylated polypyrrole-based microparticles. Replacing the 1-(2-carboxyethyl)pyrrole comonomer with pyrrole-3-acetic acid was considered to be potentially advantageous for the synthesis of carboxylated microparticles, since the latter monomer should copolymerize more readily with pyrrole. No colloids were obtained in the homopolymerization of pyrrole-3-acetic acid using any of the oxidants usually employed for polypyrrole-silica colloid syntheses, i.e., (NH4)2S2O8, FeCl3, or the H2O2/FeCl3/HCl catalytic system described by Yamamoto’s group.4,5,21 Even when the rate of polymerization was reduced by lowering the polymerization temperature to 4 °C, no stable dispersions were formed (see samples 8-10 in Table 1). This was considered unfortunate, since homopolymerization of pyrrole-3-acetic

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acid in the presence of a silica sol had been expected to lead to the highest possible concentration of surface carboxylic acid groups. Similar negative results were reported by Maeda et al. for the homopolymerization of 1-(2-carboxyethyl)pyrrole.18 In contrast, copolymerization of pyrrole-3-acetic acid with pyrrole led to stable dispersions of microparticles, but only for the H2O2/FeCl3/HCl system (see samples 2-7 in Table 1). With this catalytic oxidant the rate of polymerization is relatively slow; using more powerful oxidants such as (NH4)2S2O8 or FeCl3 gave faster rates of polymerization but always resulted in macroscopic precipitation (see samples 11-16 in Table 1). With the hydrogen peroxide-based oxidant, microparticle formation appeared to be relatively insensitive to silica sol concentration, polymerization temperature, and comonomer feed ratio (in contrast, a similar initial silica concentration of 3.4 w/w % proved rather marginal for successful microparticle syntheses based on the 1-(2-carboxyethyl)pyrrole comonomer18). Colloid yields were in the 1-2 w/w % range depending on the silica content of the microparticles, with lower yields being obtained at lower silica loadings. In the present study the effect of varying the proportion of pyrrole-3-acetic acid in the comonomer feed from 25% to 75% (samples 2-4 in Table 1) on the properties of the resulting carboxylated microparticles was examined in some detail. It is noteworthy that our microparticle syntheses based on the copolymerization of 1-(2-carboxyethyl)pyrrole with pyrrole using the hydrogen peroxide-based oxidant (not shown in Table 1) gave stable colloidal dispersions but somewhat irreproducible particle sizes (diameters ranged from 137 to 399 nm) and compositions (conducting polymer contents ranged from 19 to 76%). Thus, the significant improvement in reproducibility achieved using the the pyrrole-3-acetic acid comonomer cannot be attributed merely to changing the oxidant from FeCl3 to hydrogen peroxide. The long-term colloid stabilities of the three carboxylated dispersions prepared using the pyrrole-3acetic acid comonomer (samples 2-4 in Table 1) were also much improved compared to those of microparticles synthesized using the 1-(2-carboxyethyl)pyrrole comonomer. The former dispersions showed only minimal signs of incipient flocculation after standing at room temperature for 6 months at a solution pH of 6.4 (see the two DCP curves depicted in Figure 2). Figure 3 shows the weight-average particle size distributions obtained by DCP for the three successful carboxylated polypyrrole-silica dispersions described in Table 1 (samples 2-4). Increasing the proportion of pyrrole-3-acetic acid in the comonomer feed ratio clearly results in larger weight-average particle diameters and broader particle size distributions. Thus, high degrees of carboxylation can only be obtained at the expense of reduced control over the size distributions of the microparticles. The relatively small particle size of samples 1 and 2 (see Tables 1 and 2) meant that their cleanup was somewhat problematic, and several additional centrifugation-redispersion cycles were necessary to remove the excess, nonaggregated ultrafine silica particles from sample 2. However, even this proved inadequate for cleaning up sample 1, which remained contaminated with residual silica sol. Excellent agreement was obtained between the conducting polymer contents calculated from microanalytical data and the silica contents determined from thermogravimetric analyses. A good correlation was also obtained between these silica contents and the particle densities determined using helium pycnometry. It is noteworthy that only a relatively low silica content was required for the formation of stable dispersions of mi-

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Figure 2. Weight-average particle size distribution curves obtained by disk centrifuge photosedimentometry analysis of carboxylated polypyrrole-silica microparticles. This dispersion was prepared under the same conditions as sample 2 in Table 1, except that the polymerization was carried out at 21 °C rather than at 4 °C. Curve a was obtained for the freshly-prepared dispersion, and curve b, for the same dispersion stored for 6 months at room temperature at pH 6.4.

Figure 3. Weight-average particle size distribution curves obtained by disk centrifuge photosedimentometry for three carboxylated polypyrrole-silica dispersions synthesized at various comonomer feed ratios (samples 2-4 in Table 1).

croparticles; for example, the silica content of sample 4 is only approximately 7.7% by mass. The chemical compositions of the same three carboxylated polypyrrole-silica microparticles are summarized in Table 3. Copolymer compositions were calculated by comparing the C/N ratios of the three copolymers to those found for the respective polypyrrole and poly(pyrrole-3acetic acid) homopolymers synthesized as ‘bulk powders’. These calculations assume the same doping levels for the two homopolymers and the three copolymers and that the copolymerization chemistry is not affected by the presence of the silica sol. Reasonable agreement was found between

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Figure 4. Transmission electron micrographs of three carboxylated polypyrrole-silica microparticles (samples 2-4 in Table 1). Note the change in morphology from “raspberry”-like to near-spherical as the proportion of silica in the microparticles is decreased. Table 2. Weight-Average Particle Size, Density, and Chemical Composition for Three of the Carboxylated Polypyrrole-Silica Dispersions (Samples 2-4 in Table 1) weight-average conducting particle silica polymer a b c sample no. diameter density content contentd (COOH-Py/Py) (nm) (g/cm3) (wt %) (wt %) 1 (0:100) 2 (25:75) 3 (50:50) 4 (75:25)

58 ( 10 67 ( 11 94 ( 13 196 ( 33

2.10 1.85 1.55 1.50

93.9 70.4 13.9 7.7

5.7 29.3 85.2 92.8

a Determined by disk centrifuge photosedimentometry. b Determined by helium pycnometry. c Determined by thermogravimetric analysis. d Determined by elemental microanalysis.

Table 3. Effect of Varying the Comonomer Feed Ratio on the Final Copolymer Composition of the Carboxylated Polypyrrole-Silica Microparticles

sample no. 2 3 4 COOH-PPy bulk PPy bulk

elemental comonomer microanalyses copolymer feed ratio C N C/N composition (COOH-Py/Py) (%) (%) ratio (COOH-Py/Py) 25:75 50:50 75:25 100:0a 0:100a

15.9 46.2 50.3 54.1 54.3

4.2 11.4 11.1 11.1 15.8

4.41 4.73 5.29 5.68 4.01

30:70 48:52 81:19 100:0 0:100

a These homopolymers were synthesized as bulk powders in the absence of the silica sol.

the calculated copolymer compositions and the corresponding initial comonomer feed ratios, which suggests that good copolymerizability was achieved, as expected. Transmission electron microscopy studies (see Figure 4) of these three carboxylated polypyrrole-silica dispersions confirmed the effect of increasing particle size with increasing pyrrole-3-carboxylic acid content suggested by the DCP analyses. With regard to particle morphology, a marked decrease in surface roughness with increasing pyrrole-3-acetic acid content is evident. This is consistent with the decrease in silica content of the microparticles indicated by both thermogravimetric analyses and elemental microanalyses (see Table 2). Fourier transform infrared spectra for the three carboxylated polypyrrole-silica dispersions are displayed in

Figure 5. Fourier transform infrared spectra of three carboxylated polypyrrole-silica microparticles (samples 2-4 (ac, respectively) in Table 1). The 1706 cm-1 band corresponds to the characteristic CdO stretch of the carboxylic acid group. Note that its intensity increases as the proportion of pyrrole3-acetic acid comonomer in the microparticles is increased.

Figure 5. The broad peak at 1117 cm-1 is characteristic of silica and is the superimposition of three Si-O stretching vibrations.24 The peaks relating to the doped polypyrrole chains occur at 1632, 1570, 1386, and 893 cm-1.25 With regard to the present study, the most important peak is the feature at 1706 cm-1; which is (24) Armes, S. P.; Gottesfeld, S.; Beery, J. G.; Garzon, F.; Agnew, S. F. Polymer 1991, 32, 2325. (25) Bjorklund, R. B.; Liedberg, B. Chem. Commun. 1986, 1293.

Polypyrrole-Silica Microparticles

attributable to the CdO stretch for carboxylic acid. No such feature was observed in the IR spectrum of the homopolypyrrole-silica microparticles (not shown). The intensity of the peak at 1706 cm-1 increases as the proportion of pyrrole-3-acetic acid in the comonomer feed ratio is increased, as expected. Thus, these FTIR spectra provide good qualitative evidence for the incorporation of the pyrrole-3-carboxylic acid comonomer into the microparticles. However, this observation does not confirm that the carboxylic acid groups are at the surface of the microparticles; such verification requires a surface-specific technique such as XPS, which has a sampling depth of 2-5 nm (see below). XPS survey spectra (not shown) were recorded for the three carboxylated polypyrrole-silica dispersions. The surface Si/N atomic ratios of these dispersions decreased as the proportion of silica was reduced. This trend is consistent with the changes in particle morphology observed by transmission electron microscopy. More importantly, the XPS studies also confirmed the presence of carboxylic acid groups at, or very near, the surface of the particles. Figure 6 shows the core-line C1s spectra for the four carboxylated polypyrrole-silica microparticles prepared at various comonomer feed ratios. A distinct shoulder is evident at higher binding energy (∼289.5 eV), which is characteristic of the carboxylic acid carbon atom; this feature is noticeably absent in the corresponding C1s spectrum of the homopolypyrrole-silica microparticles. Table 4 summarizes the semiquantitative data obtained from peak-fitting, which allows an estimate of the surface concentration of carboxylic acid groups as a proportion of the total surface carbon content. Clearly, the increasing surface concentration of carboxylic acid groups indicated by XPS correlates with an increasing proportion of the pyrrole-3-acetic acid in the comonomer feed, as expected. ζ-Potential vs pH curves for the three carboxylated polypyrrole-silica dispersions are shown in Figure 7. A distinct ‘knee’ at approximately pH 4 is observed in each case. This is in good agreement with the pKa of an aliphatic carboxylic acid. Very similar results were reported by Butterworth et al. for a carboxylated polypyrrole-silica dispersion prepared using the 1-(2-carboxyethyl)pyrrole comonomer.13 In the present work, the more negative ζ-potentials at pH 4 are in accordance with the increasing degree of (surface) carboxylation suggested by the IR and XPS data. Thus, these ζ-potential measurements confirm the presence of surface carboxylic acid groups. It is interesting to note that the curve obtained for sample 2, which was synthesized with the lowest proportion of pyrrole-3-acetic acid, is almost superimposable onto that of the homopolypyrrole-silica sample. This indicates a relatively low degree of carboxylation for this dispersion. Current commercial ‘marker’ particles include dyed polystyrene latexes, which are often surface-functionalized with either carboxylic acid or amine groups to facilitate the covalent attachment of a biological analyte of interest in the immunodiagnostic assay. Figure 8 compares the color intensity of one such commercial marker (SuperCarboxyl Hi-Dye; a blue, extrinsically-dyed, carboxylated polystyrene latex from Polymer Laboratories) with that of the carboxylated polypyrrole-silica microparticles prepared at a 75:25 comonomer feed ratio (sample 4 in Table 1). The conducting polymer-based microparticles absorb visible light much more strongly than the commercial polystyrene latex at all wavelengths in the 400900 nm range. This effect is most prominent at shorter wavelengths: at 400 nm the microparticles have an optical absorbance approximately three times that of the commercial dyed latex. It is noteworthy that there is a qualitative difference between the absorption spectra of the two carboxylated polypyrrole-silica dispersions syn-

Langmuir, Vol. 13, No. 14, 1997 3691

Figure 6. Peak fitting of the C1s core-line XPS spectra of (1) unfunctionalized homopolypyrrole-silica microparticles and (2-4) three carboxylated polypyrrole-silica dispersions synthesized with an increasing proportion of pyrrole-3-acetic acid in the comonomer feed (the numbers correspond to samples 2-4 in Table 1). Note the increasing shoulder at approximately 289.5 eV due to the carbonyl carbon of the surface carboxylic acid groups. Table 4. Surface Composition Data for the Three Carboxylated Polypyrrole-Silica Dispersions (Samples 2-4 in Table 1) As Determined by X-ray Photoelectron Spectroscopy sample no.

copolymer composition (COOH-Py/Py)

surface Si/N atomic ratioa

surface carboxylic acid contentb (%)

1 2 3 4

0:100 30:70 48:52 81:19

18.8 11.2 0.8 0.2

0 2 4 7

a As determined from the Si b 2p and N1s core-line spectra. As determined from the C1s core-line spectra. These values reflect the carboxylic carbon content as a percentage of the total surface carbon content.

thesized using the pyrrole-3-acetic acid and the 1-(2carboxyethyl)pyrrole comonomers at the same 75:25 comonomer feed ratio. In principle, the reduced absorbance at longer wavelengths (700-900 nm) observed for the microparticles synthesized using the N-substituted

3692 Langmuir, Vol. 13, No. 14, 1997

McCarthy et al.

In collaboration with Chehimi and co-workers,26 the extent of DNA adsorption onto carboxylated polypyrrolesilica microparticles (sample 3 in Table 1) has been recently investigated and compared to that obtained for unfunctionalized homopolypyrrole-silica particles, polypyrrole bulk powder, and colloidal silica sol. It was found that no DNA adsorption was detected for the silica sol, but the extent of adsorption onto the carboxylated polypyrrolesilica microparticles, the unfunctionalized polypyrrolesilica microparticles, and the polypyrrole bulk powder was found to be 6.4, 1.3, and 8.0 mg/g, respectively. Hence, the presence of the carboxylic groups at the surface of the microparticles promoted DNA adsorption. Similar studies are currently in progress utilizing proteins such as human chorionic gonadotrophin (hCG). Conclusions

Figure 7. ζ-Potential vs pH curves for three carboxylated polypyrrole-silica dispersions (samples 2-4 in Table 1). The “knee” at around pH 4 corresponds to the ionization of the surface carboxylic acid groups. Note the increasing negative ζ-potential with increasing pyrrole-3-acetic acid content.

Figure 8. Visible absorption spectra of (a) carboxylated polypyrrole-silica microparticles synthesized using the pyrrole3-acetic acid comonomer (sample 4 in Table 1, comonomer feed contained 75% pyrrole-3-acetic acid, hydrogen peroxide-based oxidant); (b) carboxylated polypyrrole-silica microparticles synthesized using the 1-(2-carboxyethyl)pyrrole comonomer (comonomer feed contained 75% 1-(2-carboxyethyl)pyrrole, hydrogen peroxide-based oxidant); and (c) an extrinsically-dyed, surface-carboxylated commercial polystyrene latex designed for use in diagnostic assays (Supercarboxyl Hi-Dye, e.g. Polymer Laboratories). The solids concentration of all three dispersions was 30 mg dm-3.

comonomer could be due to overoxidation of the conjugated polymer backbone. However, since both these dispersions were synthesized using the same hydrogen peroxide-based oxidant, this difference is more likely to be due to increased torsional twisting of the conducting polymer chains caused by the sterically bulkier N-substituted comonomer. On the other hand, the visible absorption spectrum of the carboxylated polypyrrole-silica particles prepared using the 1-(2-carboxyethyl)pyrrole comonomer in the present study is also qualitatively different from that reported by Maeda and co-workers for similar microparticles synthesized using the FeCl3 oxidant at the same 75:25 comonomer feed ratio.18 Thus the choice of oxidant can apparently influence the optical properties of these dispersions. In conclusion, carboxylated polypyrrole-silica microparticles synthesized using the hydrogen peroxide oxidant at a given comonomer feed ratio exhibit a higher optical absorbance if pyrrole-3-acetic acid is selected as the functional comonomer in preference to 1-(2-carboxethyl)pyrrole.

Although the pyrrole-3-acetic acid comonomer is rather more difficult to synthesize compared to 1-(2-carboxyethyl)pyrrole, its use in combination with the Yamamoto oxidant system is definitely advantageous for the synthesis of carboxylated polypyrrole-silica microparticles. Such microparticle syntheses are much more robust and reproducible, and the resulting dispersions exhibit much improved long-term colloid stability. Particles as small as 60 nm can be obtained, although complete removal of the excess silica sol from such dispersions may be problematic. Size distributions can be relatively narrow at low degrees of carboxylation; increasing the proportion of pyrrole-3-acetic acid comonomer in the feed results in larger particle diameters and broader size distributions. Both XPS and ζ-potential measurements indicate that the pyrrole-3-acetic acid comonomer is present at, or very near, the surface of the microparticles. Thus, covalent attachment of biological analytes should be feasible. In view of the above observations, the optimum pyrrole-3acetic acid/pyrrole comonomer feed ratio for microparticle syntheses is probably approximately 50:50. This feed ratio gives microparticles which possess a narrow size distribution, an average weight-average particle diameter of 94 nm (which is small enough for rapid strip migration but large enough to allow efficient cleanup via centrifugation-redispersion cycles), and a reasonable degree of surface carboxylation. Finally, comparison with a commercial dyed polystyrene latex indicates that the conducting polymer-based microparticles are much more strongly absorbing across the entire visible spectrum. Therefore, it is suggested that carboxylated polypyrrole-silica particles synthesized using pyrrole-3-acetic acid are excellent candidates for new marker particles in immunodiagnostic strip assays. Preliminary studies indicate that significantly higher levels of DNA can be adsorbed onto the carboxylated polypyrrole-silica microparticles compared to noncarboxylated microparticles. Acknowledgment. G.P.McC. and S.P.A. thank the EPSRC (GR/K01841) for financial support in the form of a postdoctoral fellowship to G.P.McC. Dr. M. R. Simmons is thanked for his assistance with the synthesis of the pyrrole-3-acetic acid monomer. Dr. J. Thorpe of the University of Sussex is thanked for his assistance with the TEM studies. LA962121B (26) Saoudi, B.; Jammul, N.; Chehimi, M. M.; McCarthy, G. P.; Armes, S. P. J. Colloid Interface Sci., accepted for publication.