Polyaniline nanoparticles with controlled sizes using a cross-linked carboxymethyl chitin template
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- Thanpitcha, T., Sirivat, A., Jamieson, A.M. et al. J Nanopart Res (2009) 11: 1167. doi:10.1007/s11051-008-9515-8
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Polyaniline (PANI) nanoparticles were chemically synthesized in the presence of a cross-linked carboxymethyl chitin (CM-chitin) acting as a template. The reaction was performed under acidic conditions and the template was removed after the polymerization of aniline was completed. The morphology of the synthesized PANI was globular with a diameter in the nanometer range. The degree of cross-linking of the CM-chitin played an important role in determining the size of the obtained PANI nanoparticles, which decreased from approximately 392 to 160 nm with increase in concentration of the cross-linking agent, glutaraldehyde, from 0 to 9 μmol, respectively. At a higher glutaraldehyde concentration (18 μmol), an aggregated PANI network was observed due to the incomplete removal of the more highly cross-linked CM-chitin. Molecular characterization (including UV-Visible, FTIR, TGA, and XRD techniques) revealed that the structure of the synthesized PANI nanoparticles is identical to that of conventional PANI. A mechanism is proposed for the formation of PANI nanoparticles in the presence of the cross-linked CM-chitin template.
KeywordsPolyanilineNanoparticlesCross-linked carboxymethyl chitinHydrogelTemplateConductive polymers
Polyaniline, as a conductive polymer, has attracted particular attention because of its unique electronic properties, ease of synthesis, environmental stability, and low cost of the monomer (Bai et al. 2007; Li et al. 2006; Li et al. 2007; Zhang and Wang 2006), as well as its potential applications in many areas, including sensors, actuators, batteries, and anticorrosion coatings (Cho et al. 2004a, b; Thanpitcha et al. 2006). In recent years, much attention has been given to the synthesis of PANI nanostructures, such as nanorods (Stejskal et al. 2006; Wei et al. 2006), nanofibers (Chiou and Epstein 2005; Wang et al. 2007), and nanospheres (Cheng et al. 2005; Zhu and Jiang 2007) with the expectation that such species may show superior properties to conventionally polymerized materials (Cheng et al. 2004; He 2005b; Xing et al. 2006). For example, one might expect enhanced responsiveness for sensor applications (Huang et al. 2004; Virji et al. 2004), improved dispersion (Hopkins et al. 2004), and a lower percolation threshold for electrical conductivity for PANI nanoparticles in composite materials (Banerjee and Mandal 1995).
The synthesis of PANI nanostructures has been achieved by either the chemical or electrochemical polymerization of aniline with the aid of a hard template, such as a zeolite channel (Cao et al. 1992; Wu and Bein 1994), mesoporous silica, MCM-41 (Cho et al. 2004a), track-etched polycarbonate (Liu and Kaner 2004; Martin 1996), anodized alumina (Wang et al. 2002; Xiong et al. 2004a), or a soft template such as a surfactant (Xiong et al. 2004b; Yang et al. 2005), or polyelectrolyte (Lu et al. 2005). Moreover, physical methods such as electrospinning (Li et al. 2006), mechanical stretching (Gu et al. 2005; He et al. 2001), interfacial polymerization (He 2005a; Huang and Kaner 2004), rapid mixing polymerization (Wang and Jing 2008), sonochemical synthesis (Jing et al. 2006, 2007), radiolytic synthesis (Wang and Jing 2005), photolithographic synthesis (Werake et al. 2005), and seeded polymerization (Zhang et al. 2004) have been utilized in the synthesis of PANI nanostructures. Among these methods, the use of hard templates has been shown to offer an effective approach for controlling the morphology, size, and size distribution of the PANI nanostructures (Mazur et al. 2003; Zhang and Wang 2006). Typically, the pores of the hard template can be viewed as nanoreactors, in which nanoparticles of the desired material are synthesized, whose shapes and sizes depend only on the pore topology and pore size (Xiong et al. 2004a). However, the disadvantages of this method are, first, a rather tedious post-synthesis treatment is required to remove the template (Zhang and Wan 2002), and, second, the synthesized PANI nanostructures may be destroyed or form undesirable aggregated structures after release from the template (Zhang and Wang 2006). Therefore, the search for other approaches, which can diminish the drawbacks of the hard template, while retaining the ability to control the morphology, size, and size distribution of the synthesized PANI nanostructures remains a challenge.
Hydrogels are three-dimensional network structures, obtained by physically or chemically cross-linking hydrophilic polymers. Due to the ability of absorbing a large amount of water into the individual pores in their networks, each pore of the hydrogel can function as a single nanoreactor or a template to sequester the monomers for subsequent polymerization. Mohan et al. (2007) used a hydrogel network as a template to prepare well-defined and uniform silver nanoparticles. They found that the size of the silver nanoparticles decreased with increase in the degree of cross-linking of the hydrogel (Mohan et al. 2007). This implies that, by increasing the degree of cross-linking of the hydrogel, smaller pore sizes are obtained, resulting in a more limited space for monomer accumulation (Strachotova et al. 2007). Thus, control of the size of the synthesized nanostructures may be achieved by controlling the degree of cross-linking of the hydrogel template.
Carboxymethyl chitin (CM-chitin) is an anionic water-soluble derivative of chitin obtained by the carboxymethylation reaction of chitin powder with monochloroacetic acid under basic condition. Additionally, solutions of CM-chitin can be cross-linked with glutaraldehyde to form hydrogels. This suggests the possibility that such hydrogels can serve as the reaction template for the synthesis of PANI nanostructures.
In the present work, we report a simple process to synthesize PANI nanoparticles with controllable sizes, and with narrow size distribution, by using cross-linked CM-chitin as the hydrogel template. A mechanism for the formation of PANI nanoparticles in cross-linked CM-chitin hydrogels is proposed, and the effect of glutaraldehyde concentration, used as the cross-linking agent, on the size of the PANI nanoparticles is explored.
The aniline monomer, purchased from Merck, was distilled under reduced pressure prior to use. AR grade ammoniumperoxodisulfate (APS) was also purchased from Merck.
Chitin, with a degree of deacetylation (%DD) equal to 20%, measured by the method of Baxter et al. (Baxter et al. 1992), was prepared from shrimp shell (Penaeus merguiensis), kindly supplied by Surapon Food Co. Ltd, Thailand. AR grade hydrochloric acid, sodium hydroxide, monochloroacetic acid, glutaraldehyde, ethanol, and acetone were purchased from Labscan and used as received.
Preparation of carboxymethyl chitin (CM-chitin)
CM-chitin, with a degree of substitution (DS) equal to 0.43, was prepared by the reaction of chitin powder with monochloroacetic acid under basic conditions, according to the method described by Wongpanit et al. (Wongpanit et al. 2005). In a typical procedure, CM-chitin was prepared by suspending 5 g of chitin powder in 100 g of 42% w/w NaOH. The suspension was stored under reduced pressure for 30 min. Then, 160 g of crushed ice was added to the suspension and the mixture was stirred at below 5 °C for 30 min. A pre-cooled solution at a temperature below 5 °C, containing 27 g monochloroacetic acid in 70 mL of 14% w/w NaOH, was slowly added to the mixture with vigorous stirring. The reaction was maintained at 0–5 °C for 30 min. After settling at room temperature overnight, the mixture was neutralized with glacial acetic acid, and subsequently dialyzed in running water, followed by dialysis with distilled water for 1 day. The dialysate was centrifuged at 10,000 rpm for 10 min to remove insoluble material. A white solid was recovered from the supernatant by adding it drop-wise into acetone. The product was washed with ethanol, filtered, and dried in a vacuum at room temperature.
Synthesis of polyaniline nanoparticles
PANI nanoparticles were synthesized in aqueous solution by the oxidative polymerization of aniline using APS as an oxidizing agent, in the presence of the cross-linked CM-chitin acting as a hydrogel template. The synthesis procedure is described as follows: CM-chitin powder (0.5 g) was added to each of the four solutions of glutaraldehyde, prepared by dissolving various amounts of glutaraldehyde (0, 3, 9, and 18 μmol) in 49.5 g of distilled water, to achieve 1 wt% of distinct glutaraldehyde-added CM-chitin solution. The mixtures were magnetically stirred overnight to complete the dissolution and cross-linking reaction of CM-chitin. A total of 8 g of aniline (0.086 mol) was then poured into the CM-chitin solution and the mixture was cooled to 0 °C with mechanical stirring at 300 rpm for 1 h. Next, 100 mL of 1.5 M HCl was added drop-wise into the suspension in a period of 30 min and the suspension was maintained with mechanical stirring for 30 min. A pre-cooled solution, at a temperature below 5 °C, containing 10 g NH4S2O8 (0.048 mol) in 100 mL of 1.5 M HCl, was added drop-wise within 30 min and the suspension was stirred at 0 °C for 4 h to complete the polymerization. The resulting suspension was centrifuged at 11,000 rpm for 20 min and the precipitate was subjected to dialysis with an excess amount of distilled water until becoming neutral. The precipitated product was filtered, washed with distilled water to remove the cross-linked CM-chitin template, dried under reduced pressure for 2 days, and kept in desiccators prior to use.
UV-Visible spectra were obtained from a Shimadzu UV-VIS spectrometer (model 2550) in the wavelength range 250–900 nm. Aqueous 1.5 M HCl and N-methyl-2-pyrrolidone (NMP) were used as solvents to prepare, respectively, the emeraldine salt form (doped state) of PANI (PANI ES) and the emeraldine base form (undoped state) (PANI EB) at a concentration of 0.3 g L−1.
FTIR spectra were recorded using a Thermo Nicolet Nexus 670 FTIR spectrometer in the absorbance mode at 32 scans with a resolution of 4 cm−1. The spectra in the frequency range 4,000–400 cm−1 were measured using a deuterated triglycerine sulfate detector (DTGS) with a specific detectivity of 1 × 109 cm Hz1/2 w−1.
The morphologies of the polyaniline nanoparticles were investigated using a scanning electron microscope (JOEL, model JSM-5800LV) at 15 kV. The polyaniline sample was prepared by the dispersion of the synthesized polyaniline powder in distilled water, pipetting on to a brass stub, and drying before gold sputtering.
Thermogravimetric analysis was performed using a DuPont Instrument TGA 5.1 (model 2950) in the temperature range of 30 to 800 °C at a heating rate of 10 °C min−1 under a nitrogen atmosphere.
X-ray diffraction (using a Rigaku, model D/MAX-2000) was carried out in the continuous mode with a scan speed of 5° min−1, covering angles 2θ between 5° and 50°. Cu Kα1 was used as the X-ray source.
The electrical conductivity was measured at 25 °C using a custom-made two-point probe with an electrometer/high resistance meter (Keithley, model 7517A).
Results and discussion
Morphology and possible mechanism
In addition, it was found that the size of the PANI nanoparticles can be controlled by varying the degree of cross-linking of the CM-chitin. From Fig. 2, it is seen that the glutaraldehyde concentration used to prepare the cross-linked CM-chitin template determines the size of the resulting PANI nanoparticles (see Fig. 2b–d). Specifically, as the glutaraldehyde concentration increased from 3 μmol (1CMCT-3Glu) to 9 μmol (1CMCT-9Glu), the size of PANI nanoparticles produced in the cross-linked CM-chitin decreased from 246 ± 38 to 160 ± 19 nm, as shown in Fig. 2c and d, respectively. Thus, a higher degree of cross-linking of CM-chitin (i.e., a higher glutaraldehyde concentration) results in the formation of smaller-sized PANI nanoparticles, presumable because of a reduction in the size of the pore spaces within the network structure (Mohan et al. 2007). However, as seen in Fig. 2e, at 18 μmol of glutaraldehyde concentration (1CMCT-18Glu), an aggregated PANI network was obtained. This result may be due, in part, to more limited penetration of the aniline monomer inside the pores of the cross-linked CM-chitin so that polymerization occurs both in the inner pores and the outer surfaces of the cross-linked CM-chitin. Moreover, the higher degree of cross-linking (CM-chitin solution containing 18 μmol glutaraldehyde) creates difficulty in template removal due to the stronger intermolecular interaction between the CM-chitin chains. This results in the presence of residual cross-linked CM-chitin in the resulting PANI product, as confirmed by FTIR and TGA results.
Thermogravimetric analysis (TGA)
Wide angle X-ray diffraction (WAXD)
Electrical conductivity of the PANI nanoparticles in the pellet form
Specific conductivity (S cm−1)
3.72 × 10−11 ± 9.86 × 10−13
15.6 ± 0.32
3.99 × 10−11 ± 1.52 × 10−12
24.4 ± 1.58
3.52 × 10−11 ± 2.36 × 10−12
26.8 ± 2.06
3.07 × 10−11 ± 7.97 × 10−13
39.8 ± 2.13
2.01 × 10−11 ± 9.80 × 10−13
31.5 ± 0.98
Uniformly globular PANI nanoparticles with a diameter in the nanometer range were successfully synthesized by the oxidative polymerization approach in the presence of a cross-linked CM-chitin template. The molecular structures of the synthesized PANI nanoparticles were identical to those of the conventional PANI. The amount of glutaraldehyde-added CM-chitin was found to play an important role in determining the size of the obtained PANI nanoparticles. By increasing the concentration of the crosslinker, glutaraldehydes, PANI nanoparticles of smaller sizes were obtained. The formation mechanism of the PANI nanoparticles in the cross-linked CM-chitin template is proposed according to the accumulation and polymerization of the aniline monomer within the pores of the cross-linked CM-chitin network. The electrical conductivities of the compressed PANI nanoparticles was significantly higher than that of the conventional PANI, attributed to a higher orientation of PANI molecules induced by interaction with the cross-linked CM-chitin template. Therefore, the use of cross-linked CM-chitin, with a specific degree of cross-linking, as a template for the synthesis of PANI nanoparticles may be a viable approach to obtain bulk quantities of PANI nanoparticles with controlled sizes and narrow size distribution.
The authors gratefully acknowledge Thailand Research Fund (Royal Golden Jubilee Ph.D Scholarship), National Nanotechnology Center (NANOTEC), NSTDA, and The Conductive and Electroactive Polymers Research Unit, Chulalongkorn University, Thailand, for their financial support of this work. AMJ acknowledges The National Science Foundation for financial support through award DMR0513010, Polymers Program. We also acknowledge Surapon Food Public Co. Ltd. for supplying the material for this work.