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Synthesis of polyaniline nanofibers and nanotubes via rhamnolipid biosurfactant templating
Synthetic Metals 161 (2011) 298–306
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Synthesis of polyaniline nanofibers and nanotubes via rhamnolipid biosurfactant templating Panisara Worakitsiri a , Orathai Pornsunthorntawee a , Tuspon Thanpitcha a , Sumaeth Chavadej a,b , Christoph Weder c , Ratana Rujiravanit a,b,∗ a b c
The Petroleum and Petrochemical College, Chulalongkorn University, Bangkok 10330, Thailand Center for Petroleum, Petrochemicals, and Advanced Materials, Chulalongkorn University, Bangkok 10330, Thailand Adolphe Merkle Institute and Fribourg Center for Nanomaterials, University of Fribourg, Route de l’Ancienne Papeterie, CH-1723 Marly 1, Switzerland
a r t i c l e
i n f o
Article history: Received 28 July 2010 Received in revised form 6 November 2010 Accepted 25 November 2010 Available online 18 December 2010 Keywords: Biosurfactants Glycolipids Rhamnolipids Conductive polymers Polyaniline Nanoparticles
a b s t r a c t Polyaniline (PANI) nanorods and nanotubes with average diameters in the range of 121 ± 12 nm to 152 ± 13 nm were synthesized by the oxidative polymerization of aniline using hydrochloric acid as a dopant, ammonium peroxodisulfate as an oxidant, and a rhamnolipid biosurfactant produced by Pseudomonas aeruginosa SP4 as a soft template. The reaction conditions were systematically varied with the objective to maximize the electrical conductivity and aspect ratio of the PANI nanofibers. Optimized conditions, which involved an acid concentration of 0.1 M, an ANI-to-biosurfactant weight ratio of 23:1, and a polymerization time of 6 h, afforded a material with an electrical conductivity of 25 ± 2 S/cm. This material also exhibited the highest degree of crystallinity of all studied samples and matched the electronic properties of a reference material made without the surfactant. © 2010 Elsevier B.V. All rights reserved.
1. Introduction In the last decade, many intrinsically conductive polymers, including polyaniline (PANI), polypyrrole (PPy), polyacetylene (PA), and polythiophene (PTh), have been synthesized by either chemical or electrochemical methods [1]. These polymers contain conjugated sequences of alternating single and multiple bonds, which can be easily reduced or oxidized due to their low ionization potential or high electron affinity, resulting in the alteration of their electrical conductivity from an insulating to a (semi)conducting state [2,3]. Among the electrically conductive polymers, PANI is one of the most attractive candidates, because of its facile synthesis, good environmental stability, ease of electrical conductivity control by changing either the oxidation state or the protonation state, and the low cost of the aniline (ANI) monomer [4–6]. PANI has shown promising commercial viability in a variety of technological applications, including rechargeable batteries [7], conductive coatings or adhesives [8,9], capacitors and other energy storage systems [10], electrical and electrochemical devices [11],
∗ Corresponding author at: The Petroleum and Petrochemical College, Chulalongkorn University, Bangkok 10330, Thailand. Tel.: +66 2 218 4132; fax: +66 2 215 4459. E-mail address: [email protected] (R. Rujiravanit). 0379-6779/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2010.11.039
and sensor applications [12,13]. However, the infusibility of PANI and its limited solubility in common solvents have presented challenges to processing and perhaps hampered a more widespread utilization. Many studies have sought to improve the usefulness of PANI by fabricating composites with polymer matrixes, such as poly(styrene sulfonic acid) (PSSA, which also serves as a dopant) [14] and poly(ethylene-co-vinyl acetate) (PEVA) [15]. In this context, the poor dispersability of PANI in the matrix can cause problems, as this usually limits the electrical conductivity and also negatively impacts the mechanical properties. The use of PANI nanoparticles instead of their macro- or microscopic counterparts has emerged as viable route to address this problem. Particularly attractive in this context are PANI nanofibers or -rods, which due to their high aspect ratio from the basis for low percolation thresholds [16]. PANI nanoparticles can be synthesized by various approaches, which include the use of templates that steer the nanoparticle formation [1]. Template synthesis is considered to be an effective method to produce PANI nanoparticles with controllable size and shape. However, unless the template remains ‘buried’ within the final product such as in materials produced by coating a nanotemplate with the conducting polymer [16], post-synthesis processes are usually required in order to remove the template. In that case, great care has to be taken to avoid adverse effects such as disintegration or aggregation of the nanoparticles and chemical decomposition.
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In the present study, a biosurfactant produced by the microorganism Pseudomonas aeruginosa SP4 was used as a soft template to synthesize PANI nanorods and nanotubes. The approach builds on our recent previous work [17], which has identified l-rhamnosyl3-hydroxydecanoyl-3-hydroxydecanoate, or mono-rhamnolipid (Rha–C10 –C10 ) as the predominant component in the biosurfactant produced by this microorganism. Rhamnolipids possess good physicochemical properties in terms of surface activity, stability, and emulsification activity. Parra et al. [18] reported that these surface-active compounds reduce the surface tension of pure water from 72 mN/m to below 30 mN/m with an interfacial tension in the range of 43 mN/m to below 1 mN/m. Rhamnolipids can self-assemble to form various types of microstructures, including micelles and vesicles, at concentrations greater than the critical micelle concentration (CMC) [19–22]. These rhamnolipid microstructures are able to solubilize, or entrap, both organic and inorganic solutes [19,21], and therefore could serve as a template for the accumulation of ANI monomer and subsequent polymerization. Thus, the aim of this present work was to synthesize PANI nanoparticles by conducting an oxidative polymerization of ANI in the presence of the rhamnolipid biosurfactant as a soft, easily removable template. The PANI product was characterized in terms of morphology, thermal property, crystallinity, and electrical conductivity, and compared to a reference PANI product synthesized in the absence of the template.
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flask and a rotary evaporator was used to remove the solvent at 40 ◦ C under reduced pressure. After solvent evaporation, about 5.2 g of a viscous honey-colored biosurfactant product was obtained per liter of the culture medium. The chemical structure of the most abundant component in the biosurfactant product was identified as Rha–C10 –C10 (73.5%), while the others were characterized as Rha–Rha–C8 –C10 (0.7%), Rha–C8 –C10 (1.5%), Rha–C10 –C12:1 (9.5%), Rha–C10 –C12 (13.3%), and Rha–Rha–C10 –C14:1 (1.4%), with small contributions of their structural isomers [17]. 2.4. Synthesis of polyaniline nanoparticles
P. aeruginosa SP4 was isolated from petroleum-contaminated soil in Thailand [23]. The isolated strain was maintained on nutrient agar slants at 4 ◦ C to minimize the biological activity and was subcultured every month.
The PANI nanoparticles were synthesized in an aqueous solution by the oxidative polymerization of ANI using APS as an oxidant and the rhamnolipid biosurfactant as a template. The synthesis procedure was modified from that of Thanpitcha et al. [26]. Briefly, a desired amount of the ANI monomer (1.0 g, 1.7 g, 2.0 g, or 2.6 g) was added to 50 ml of a 1.8 g/l biosurfactant solution in order to create reaction mixtures with four different ANI-to-biosurfactant weight ratios (11:1, 19:1, 23:1, and 28:1, corresponding to initial ANI concentrations of 20.4 g/l, 34.6 g/l, 40.8 g/l, and 51.0 g/l, respectively) and the solution was vigorously stirred at room temperature for 22 h. Then the mixture was subsequently cooled to 0 ◦ C under mechanical stirring at 300 rpm for 30 min and 100 ml of a 0.1 M hydrochloric acid solution was added drop-wise. The reaction mixture was stirred for another 30 min at 0 ◦ C. After that, 10 ml of a pre-cooled APS solution was added drop-wise into the solution to initiate the polymerization reaction. At this stage, the final ANI concentrations were about 6.4 g/l, 10.8 g/l, 12.8 g/l, and 15.9 g/l and the biosurfactant concentration was approximately 0.6 g/l. The ANI-toAPS molar ratio was kept constant at a molar ratio of 2.6:1. The mixture was further stirred at 0 ◦ C for 6 h to complete polymerization. The resulting suspension was centrifuged at 8500 rpm at 25 ◦ C for 10 min and the supernatant solution was discarded. The isolated product was subjected to dialysis with a large amount of 30%v/v aqueous ethanol for 4 days in order to neutralize the polymer and to remove the biosurfactant template. The dialyzed solution was further centrifuged at 8500 rpm at 25 ◦ C for 10 min and the resulting PANI product in the emeraldine salt (ES) form was subsequently dried in a vacuum oven at ambient temperature for 3 days. The PANI nanoparticles were subsequently kept in desiccators prior to analyses. A sample of PANI that was synthesized in conventional manner in the absence of the biosurfactant template served as a control. The optimum ANI-to-biosurfactant weight ratio for the synthesis of PANI nanoparticles was chosen on the basis of the uniformity of morphology and size, the crystallinity, and the electrical conductivity of the PANI product. The percentage yield was calculated using the following equation:
2.3. Preparation of rhamnolipid biosurfactant
Yield (%) =
An inoculum was first prepared by transferring the bacterial colonies into a nutrient broth and the culture was incubated at 37 ◦ C in a shaking incubator at 200 rpm for 22 h. Then a nutrient broth containing 2% inoculum and 2% palm oil was incubated at 37 ◦ C under aerobic conditions in a shaking incubator at 200 rpm for 48 h to obtain the highest microbial and biosurfactant concentrations [23]. The culture medium was subsequently centrifuged at 8500 rpm at 4 ◦ C for 20 min to remove the bacterial cells. The supernatant was further treated by acidification to pH 2.0 using a 6 M hydrochloric acid solution and the acidified supernatant was left overnight at 4 ◦ C for complete precipitation of the biosurfactant [24]. After centrifugation, the precipitate was removed and dissolved in a 0.1 M sodium bicarbonate solution, followed by extraction with a 2:1 v/v chloroform:ethanol mixture at room temperature [25]. The organic phase was transferred to a round-bottom
where m1 is the weight of the PANI product and m2 is the weight of ANI monomer.
2. Experimental 2.1. Materials Nutrient broth (Difco, USA) was prepared at a fixed concentration of 8 g/l, as recommended by the supplier. Agar powder was supplied by Himedia Co., Ltd. (India). Palm oil was purchased from Morakot Industry, Co., Ltd. (Thailand). Hydrochloric acid and ethanol were provided by J.T. Baker Co., Ltd. (Malaysia). Chloroform was purchased from Labscan Asia Co., Ltd. (Thailand). Sodium bicarbonate and sodium hydroxide pellets were supplied by Ajax Finechem (Australia). ANI monomer, purchased from Merck (Germany), was distilled under reduced pressure prior to use. Ammonium peroxodisulfate (APS) and N-methyl-2-pyrrolidinone (NMP) were supplied by Sigma–Aldrich (Germany). 2.2. Bacterial strain and culture growth conditions
m1 × 100 m2
2.5. Doping of polyaniline nanoparticles The PANI nanoparticles were originally isolated in the ES form. For certain experiments, the material was dedoped by dispersion in a 0.4 M sodium hydroxide solution for 3.5 h. Then the dedoped PANI was filtered and excessively washed with distilled water until neutral. The dedoped material was re-doped by dispersion in a 1.5 M hydrochloric acid solution for 24 h before the polymer was isolated by filtration and subsequently dried in a vacuum oven. The doping process of the PANI samples was done prior to being analyzed with wide-angle X-ray diffraction (WAXD) and electrical conductivity measurements.
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2.6. Analytical methods and measurements 2.6.1. Scanning electron microscopy (SEM) The morphology of the PANI nanoparticles was investigated by scanning electron microscopy (JOEL, model JSM-5410LV) at 20 kV. The PANI nanoparticles were dispersed in distilled water before being dropped on to a brass stub. Samples were coated with a thin layer of gold by using an ion sputtering device (JOEL, model JFC-1100E). The diameter of the synthesized PANI nanoparticles was determined from SEM micrographs using SemAfore software, version 4.00 (JEOL (Skandinaviska) AB, Sweden). 2.6.2. Transmission electron microscopy (TEM) The internal feature of the obtained PANI products and the morphology of the biosurfactant microstructures were observed under a transmission electron microscope (JEOL, model JEM-2100). A drop of either a PANI suspension in distilled water or the biosurfactant solution was placed onto a copper grid. Only the biosurfactant solution sample was stained with 1% uranyl acetate aqueous solution. The excess of the sample solution was then removed by absorbing the drop with a piece of filter paper. The grid was dried in a vacuum desiccator for at least 6 h before being analyzed. 2.6.3. Dynamic light scattering (DLS) measurements DLS experiments were conducted to determine the sizes of the biosurfactant microstructures, expressed as the hydrodynamic diameter. A Malvern Zetasizer Nanoseries, model S 4700, dynamic light scattering instrument was used at 25 ± 1 ◦ C in the hydrodynamic diameter range of 0.6–6000 nm. The sizes of the biosurfactant microstructures were calculated from the diffusion coefficients obtained from computer analysis using the software provided with the instrument. 2.6.4. Fourier-transformed infrared (FT-IR) spectroscopy A model 670 Thermo Nicolet Nexus FT-IR spectrometer was used to acquire FT-IR spectra of the rhamnolipid biosurfactant and the various PANI samples. The FT-IR spectra were collected using 16 scans in a range of 4000–40 cm−1 at a resolution of 4 cm−1 . All spectra were corrected for atmospheric carbon dioxide. 2.6.5. Ultraviolet–visible (UV–vis) spectroscopy The UV–vis spectra of the synthesized PANI nanoparticles were measured using a Shimadzu, model 2550, UV–vis spectrophotometer in the wavelength range of 200–800 nm. NMP was used as the solvent to prepare solutions of the PANI in the emeraldine base (EB) form. 2.6.6. Thermogravimetric analyses (TGA) TGA was used to assess the thermal stability of the rhamnolipid biosurfactant and the various PANI samples using a Dupont, model 2950, thermogravimetric analyzer in the temperature range of 50–800 ◦ C with a heating rate of 10 ◦ C/min under a nitrogen atmosphere. 2.6.7. Wide angle X-ray diffraction (WAXD) experiments An Rigaku, model D/MAX-2000, X-ray diffractometer was used to characterize the crystalline structure of the PANI samples that had been doped as indicated in Section 2.5. The WAXD analysis was carried out in a continuous mode with a scan speed of 5◦ /min covering the angles 2 from 5◦ to 50◦ . Cu K␣1 was used as the X-ray source.
Fig. 1. Yield of PANI synthesized at an acid concentration of 0.1 M and a polymerization time of 6 h in the presence of biosurfactant template at four different ANI-to-biosurfactant weight ratios compared to the conventional PANI synthesized at the corresponding ANI concentrations (6.4 g/l, 10.8 g/l, 12.8 g/l, and 15.9 g/l).
using a custom-made two-point probe connected with an electrometer/high resistance meter (Keithley, model 7517A) at room temperature and a relative humidity of 50%. 2.7. Statistical analysis The experimental data are presented in terms of arithmetic averages of at least three replicates and the standard deviations are indicated by error bars. The analyses were done using SigmaPlot software, version 8.02 (SPSS Inc., UK). 3. Results and discussion 3.1. Template synthesis of PANI PANI nanoparticles were synthesized in the presence of the rhamnolipid biosurfactant produced by P. aeruginosa strain SP4 by the oxidative polymerization of aniline using hydrochloric acid as a dopant and APS as an oxidant. Keeping the biosurfactant concentration in the polymerization mixture constant at about 0.6 g/l, the final ANI concentration was systematically varied from 6.4 g/l to 15.9 g/l at which four different ANI-to-biosurfactant weight ratios (11:1, 19:1, 23:1, and 28:1) were studied. The ANI-to-APS molar ratio was kept constant at 2.6:1. Fig. 1 shows the yield of the synthesized PANI (in percentage of the used monomer) at four different ANI-to-biosurfactant weight ratios (11:1, 19:1, 23:1, and 28:1, corresponding to ANI concentrations of 6.4 g/l, 10.8 g/l, 12.8 g/l, and 15.9 g/l, respectively) compared to the conventional PANI synthesized at the corresponding ANI concentrations. Increasing the ANI content in the polymerization mixture from 6.4 g/l to 15.9 g/l (concomitant with an increase in the ANI-to-biosurfactant weight ratio from 11:1 to 28:1) gradually decreased the yield of the synthesized PANI from 34.6 ± 1.5% to 25.5 ± 0.4%. The same trend was also observed for the reference experiments conducted in the absence of the biosurfactant (referred to as conventional PANI); therefore, the presence of the biosurfactant template did not affect the reaction yield in a noticeable manner. 3.2. Morphology and possible formation mechanism
2.6.8. Electrical conductivity measurements The electrical conductivity of the synthesized PANI samples that had been doped as indicated in Section 2.5 was measured by
Fig. 2 shows representative SEM micrographs of the PANI nanoparticles synthesized at the four different ANI-to-
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Fig. 2. SEM images of (a) conventional PANI and PANI nanoparticles synthesized at an acid concentration of 0.1 M and a polymerization time of 6 h in the presence of biosurfactant template at ANI-to-biosurfactant weight ratios of (b) 11:1, (c) 19:1, (d) 23:1, and (e) 28:1.
biosurfactant weight ratios along with conventional PANI prepared in the absence of the biosurfactant. From the SEM images, it is evident that the conventional PANI and the PANI synthesized at the lowest ANI-to-biosurfactant weight ratio (11:1) exhibited similar irregular shaped structures due to the aggregation of primary PANI particles of roughly spherical shape. By contrast, the PANI synthesized in the presence of the biosurfactant template at ANI-to-biosurfactant weight ratios of 19:1 or higher was of fibrillar structure. It was also observed that the average diameter of the PANI nanofibers increased from 121 ± 12 nm to 152 ± 13 nm upon increasing the ANI-to-biosurfactant weight ratio from 19:1 to 28:1. The results indicate that, at an appropriate ANI-to-biosurfactant weight ratio, the biosurfactant can template the synthesis of PANI nanostructures with high aspect ratio – length-to-diameter ratio. This parameter expresses the average geometry of the synthesized particles in a quantitative manner and is important, as it governs the threshold concentration for the formation of percolating network structures if the nanoparticles are dispersed in a polymer matrix.
In order to examine the internal features of the fibrillar PANI structures, the samples were further subjected to TEM analysis. Fig. 3 shows representative TEM micrographs of the PANI nanoparticles synthesized via the biosurfactant templating. The image of materials synthesized at an ANI-to-biosurfactant weight ratio of 23:1 (Fig. 3a) suggests that these samples comprised both PANI nanotubes and nanorods with a rough surface. However, if the ANIto-biosurfactant weight ratio increased to 28:1, nanorods become the more dominant species (Fig. 3b). The formation of nanostructures can be explained based on nucleation and growth theories. Li et al. [27] reported that the formation of PANI was initiated by some nuclei either formed homogeneously in the parent phase or heterogeneously grown on other species, such as preformed particles or the reactor surface. In order to trace the formation mechanism of the PANI nanofibers synthesized via the biosurfactant templating, DLS and TEM techniques were used to study the aggregation behavior of the rhamnolipid biosurfactant in the distilled water, in the presence of
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Fig. 3. TEM images of PANI nanotubes and PANI nanorods synthesized via biosurfactant templating at ANI-to-biosurfactant weight ratios of (a) 23:1 and (b) 28:1.
the ANI monomer, and in acidic solution. The ANI-to-biosurfactant weight ratio was kept constant at 23:1. Fig. 4 shows representative TEM micrographs of the biosurfactant microstructures while Fig. 5 shows the results from DLS measurements expressed as the hydrodynamic diameter. At an initial biosurfactant concentration of 1.8 g/l, the rhamnolipid biosurfactant self-assembled in distilled water to form vesicular structures with an average hydrodynamic diameter of 126 ± 2 nm. After adding the ANI monomer to the biosurfactant solution at the ANI concentration of 40.8 g/l, rhamnolipid vesicles with an average hydrodynamic diameter of 211 ± 3 nm were observed. The observed increase in the vesicle size is indicative of the solubilization of the ANI monomer in the biosurfactant vesicles. After adding the HCl solution to the mixture, the biosurfactant and the ANI concentrations decreased to 0.6 g/l and 12.8 g/l, respectively. In acidic solution, the rhamnolipid biosurfactant formed larger vesicle structures with an average hydrodynamic diameter of 242 ± 1 nm. Although the biosurfactant and impurity concentrations were able to significantly affect the size of rhamnolipid microstructures [19–21], it seemed that the influence of solution pH should be more dominant in the current work. Champion et al. [19] reported that a pH decrease of the medium from 8.0 to 6.0 led to an increase of the size of rhamnolipid microstructures from micelles (5–10 nm in diameter) to
large vesicles (larger than 250 nm in diameter). The rhamnolipid compounds behave as anionic surfactants at pH values of above 4 due to the dissociation of the carboxylic acid group comprised in their hydrophilic moiety [28]. However, in acidic solution, the rhamnolipid biosurfactant are protonated; therefore, the charge repulsion between the adjacent polar head groups of the biosurfactant molecules is reduced, promoting the self-assembly process and the formation of larger microstructures. Fig. 6 shows the proposed formation mechanism of the PANI nanotubes and nanorods synthesized via the biosurfactant templating technique. In distilled water, the rhamnolipid biosurfactant spontaneously self-assembles to form vesicular structures. Upon adding the ANI monomer into the biosurfactant solution, the monomer can be solubilized in the rhamnolipid vesicles in the outer bilayer region (vesicle “A” in Fig. 6). The ANI monomer is an amphiphilic molecule consisting of a hydrophilic amino group and a hydrophobic benzenoid ring. Hence, it possibly interacts with the biosurfactant molecule in the vesicle membrane via hydrophobic interactions, perhaps between the benzenoid ring of the ANI monomer and the hydrocarbon chains in the hydrophobic moiety of the rhamnolipid compounds, as well as hydrophilic interactions via hydrogen bonding between the polar amino group of ANI and the carboxylate group of the rhamnolipid biosurfactant [26]. The addi-
Fig. 4. TEM images of rhamnolipid vesicles at the biosurfactant concentration of 1.8 g/l dissolved (a) in distilled water and (b) in the presence of 40.8 g/l ANI monomer (ANI-to-biosurfactant weight ratio of 23:1).
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yields PANI nanotubes whereas the polymerization of that solubilized in the inner aqueous core (vesicle B) leads to the formation of the dense PANI nanorods. Nevertheless, it should be noted that, upon increasing the ANIto-biosurfactant weight ratio, the migration of the anilinium ion into the inner aqueous core of the rhamnolipid vesicles becomes more favorable due to a higher amount of ANI. Therefore, the dense PANI nanorods were dominantly produced, as shown in Fig. 3. 3.3. Chemical characterization of synthesized PANI
Fig. 5. Hydrodynamic diameters of rhamnolipid vesicles at the biosurfactant concentration of 1.8 g/l (a) in distilled water, (b) in the presence of 40.8 g/l ANI monomer, and (c) at the biosurfactant concentration of 0.6 g/l in acidic solution with 12.8 g/l ANI monomer. The ANI-to-biosurfactant weight ratio is kept constant at 23:1 and the inset figures are the visible characteristics of the obtained solutions.
tion of hydrochloric acid into the biosurfactant solution containing the ANI monomer converts the amino groups (–NH2 ) of the ANI into the protonated anilinium ion (–NH3 + ), which is more hydrophilic and drives the monomer into the inner aqueous core of the rhamnolipid vesicles (vesicle “B” in Fig. 6). As the APS oxidant is added into the reaction mixture, the polymerization takes place, and PANI nanostructures are produced via the aggregation and elongation of the rhamnolipid vesicles during the polymerization. The polymerization of the ANI solubilized in the vesicle membrane (vesicle A)
Fig. 7 shows the FT-IR spectra of the rhamnolipid biosurfactant produced by P. aeruginosa SP4, the conventional PANI, and the PANI nanoparticles synthesized at four different ANI-to-biosurfactant weight ratios. As shown in Fig. 7a, the FT-IR spectrum of the biosurfactant exhibits the important absorption bands at 3364 cm−1 , 2925 cm−1 , 2854 cm−1 , 1745 cm−1 , and 1300–1100 cm−1 , indicating the chemical structure of rhamnolipid compounds. According to the work of Pornsunthorntawee et al. [17], the weak broad band located at 3364 cm−1 can be attributed to the O–H stretching vibrations of hydroxyl groups, while the strong absorption peaks at 2925 cm−1 and 2854 cm−1 are assigned to the C–H stretching vibrations of the hydrocarbon chain positions. The characteristic peak observed at 1745 cm−1 corresponds to the C O stretching vibrations of the carbonyl group, and the C–O stretching bands in the range of 1300–1100 cm−1 related to the bonds formed between carbon atoms and hydroxyl groups in the chemical structures of the rhamnose rings. As shown in Fig. 7b, the FT-IR spectra of the conventional PANI exhibits the characteristic peaks at 1568 cm−1 , 1484 cm−1 , 1291 cm−1 , 1103 cm−1 , and 792 cm−1 , which were assigned to the C C stretching of the quinoid ring, the C C stretching of the benzenoid ring, the C–N stretching, the N Q N stretching (Q representing the quinoid ring), and the out-ofplane deformation of C–H in the 1,4-disubstituted benzene ring, respectively [29,30]. These absorption peaks are characteristics of PANI in the ES form – the doped conducting state of PANI (PANI ES). This form was obtained because the synthesis was conducted under acidic conditions. The FT-IR spectra of the PANI nanoparticles synthesized at four different ANI-tobiosurfactant weight ratios (Fig. 7c–f) were identical to that of the conventional PANI, indicating that the ANI-to-biosurfactant weight ratio did not affect the electronic state of the PANI products. The FT-IR spectra also indicate the complete biosurfactant template removal from the synthesized PANI because there was no characteristic absorption peaks of the rhamnolipid compounds appearing in the FT-IR spectra of the PANI nanoparticles. UV–vis spectroscopy was used to examine the electronic states of the synthesized PANI, both in the ES and EB forms (data not shown). To study the as-prepared PANI ES, the samples were dispersed in distilled water. The UV–vis spectrum of the PANI samples synthesized in the presence of the biosurfactant template at four different ANI-to-biosurfactant weight ratios showed a characteristic absorption peak centered around 410 nm and a broad band at 810 nm. These two characteristic absorption bands are diagnostic for the protonated ES form – the conducting state of the PANI [26]. The PANI nanoparticles synthesized with the biosurfactant route were dissolved in NMP; under these conditions, the polymer can be de-doped through deprotonation by the slightly basic solvent, to afford PANI EB [31]. Indeed, the UV–vis spectra showed two absorption peaks at 330 nm and 630 nm, which are assigned to the –* transition of the benzenoid ring and the exciton absorption of the quinoid ring, respectively, and are characteristics of the EB form of PANI [32]. The UV–vis spectra of the PANI nanoparticles synthesized at the four different ANI-to-biosurfactant ratios – in
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Fig. 6. Proposed mechanism for the formation of PANI nanotubes and PANI nanorods synthesized via biosurfactant templating.
the ES as well as the EB form – were identical to that of the conventional PANI, suggesting that the use of the biosurfactant did not affect the electronic state of the PANI product. 3.4. Thermal properties of synthesized PANI Fig. 8 shows the TGA thermograms of the rhamnolipid biosurfactant produced by P. aeruginosa SP4, the conventional PANI, and the PANI nanoparticles synthesized in the presence of the biosurfactant. As shown in Fig. 8a, the TGA thermogram of the rhamnolipid
Fig. 7. FT-IR spectra of (a) rhamnolipid biosurfactant, (b) conventional PANI, and PANI nanoparticles synthesized at an acid concentration of 0.1 M and a polymerization time of 6 h in the presence of biosurfactant template at ANI-to-biosurfactant weight ratios of (c) 11:1, (d) 19:1, (e) 23:1, and (f) 28:1.
biosurfactant exhibits only one weight loss step at 400 ◦ C, which is associated with the decomposition of the biosurfactant. By contrast, three discrete weight losses at approximately 80 ◦ C, 280 ◦ C, and 500 ◦ C were observed in the TGA thermograms of all PANI samples (Figs. 8b–f). These events are related to the loss of water, the elimination of the dopant, and the degradation of the PANI, respectively [3]. The TGA results show that the use of the surfactant did not affect the thermal property of the PANI product. In addition, the absence of the characteristic weight loss step associated with the rhamnolipid biosurfactant in the TGA thermograms of the PANI
Fig. 8. TGA thermograms of (a) rhamnolipid biosurfactant, (b) conventional PANI, and PANI nanoparticles synthesized at an acid concentration of 0.1 M and a polymerization time of 6 h in the presence of biosurfactant template at ANI-to-biosurfactant weight ratios of (c) 11:1, (d) 19:1, (e) 23:1, and (f) 28:1.
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Table 1 Crystallinities (I6.6 /I25.9 ) and electrical conductivities of conventional PANI and PANI nanoparticles synthesized at four different ANI-to-biosurfactant weight ratios in the doped state. ANI-tobiosurfactant weight ratio
Crystallinity (I6.6 /I25.9 )
Electrical conductivity (S/cm)
Conventional PANI 11:1 19:1 23:1 28:1
0.5 – 0.4 0.6 0.5
5.2 0.1 0.1 24.8 6.7
± ± ± ± ±
0.6 0.0 0.0 1.7 3.1
investigated here, including the conventionally synthesized PANI, suggesting that the biosurfactant template affected both morphology and degree of crystallinity of the produced PANI. Fig. 9. XRD patterns of (a) conventional PANI and PANI nanoparticles synthesized at an acid concentration of 0.1 M and a polymerization time of 6 h in the presence of biosurfactant template at ANI-to-biosurfactant weight ratios of (b) 11:1, (c) 19:1, (d) 23:1, and (e) 28:1.
nanoparticles confirms that the biosurfactant template was completely removed from the synthesized PANI product. 3.5. Crystallinity of synthesized PANI WAXD experiments were carried out to characterize the crystalline structure of the PANI products after they had been doped by immersion in a 1.5 M hydrochloric acid solution. Fig. 9 shows the WAXD patterns of the conventional PANI compared to those of the PANI nanoparticles synthesized at the four different ANIto-biosurfactant weight ratios. The X-ray diffraction pattern of the PANI synthesized at the ANI-to-biosurfactant weight ratio of 11:1 displays two broad peaks at 2 equal to 20.0◦ and 25.6◦ , indicating that the PANI was largely amorphous [29]. Furthermore, the WAXD patterns of the conventional PANI and the PANI synthesized at the ANI-to-biosurfactant weight ratios of 19:1 or higher exhibit distinct peaks at 2 equal to 6.6◦ , 20.8◦ , and 25.9◦ , corresponding ˚ 4.3 A, ˚ and 3.5 A, ˚ respectively. Chen et al. to d-spacings of 13.6 A, [33] reported that a reflection at 2 equal to 6.6◦ is due to either the interplanar distance between PANI molecules or the reflection to periodic distance between dopant and N atom on the adjacent main chain, while Han et al. [3] and Zhang et al. [29] reported that the distinct peaks at 2 equal to 20.8◦ and 25.9◦ can be ascribed to the periodicity parallel and perpendicular to the polymer chains, respectively. According to the work of Anikumar and Jayakannan [34], the two peaks located at 2 equal to 20.1◦ and 25.5◦ were generally observed in doped PANI while the peak at 2 equal to 6.4◦ was only observed for PANI with a highly ordered orientation. Thus, from the WAXD patterns, it appears that the conventional PANI and the PANI synthesized at the ANI-to-biosurfactant weight ratio of 19:1 or higher are partially crystalline and are of a structure that is characteristic of PANI ES. Anikumar and Jayakannan [35] reported that the ratio of the peak intensity at 2 equal to 6.4◦ and 26.0◦ can be used to express the degree of crystallinity of PANI samples. The materials prepared in this study were analyzed in this manner and the ratio of the peak intensities at 2 equal to 6.6◦ and 25.9◦ (I6.6 /I25.9 ) are shown in Table 1. The data show that the value of I6.6 /I25.9 (and therewith the crystallinity of the polymer) increases with increasing ANIto-biosurfactant ratio to the maximum at an ANI-to-biosurfactant weight ratio of 23:1; the trend is broken for the highest ANI-tobiosurfactant ratio of 28:1. Thus, the WAXD results indicate that the PANI synthesized at an ANI-to-biosurfactant weight ratio of 23:1 possessed the highest crystallinity among the PANI samples
3.6. Electrical conductivities of synthesized PANI Table 1 shows the electrical conductivities of the conventional PANI and the PANI nanoparticles synthesized in the presence of the biosurfactant. All PANI samples were doped with a 1.5 M hydrochloric acid solution. It was found that an increase in the ANI-to-biosurfactant weight ratio from 11:1 to 23:1 strongly increased the electrical conductivity of the PANI nanoparticles from 0.1 ± 0 S/cm to 25 ± 2 S/cm. However, when the ANI-to-biosurfactant weight ratio increased to 28:1, the electrical conductivity of the PANI dropped to 7 ± 3 S/cm. It is known that the crystal structure and the degree of crystallinity are important factors controlling the electrical conductivity of the PANI in the aspect that the electrical conductivity increases with the level of crystallinity [2]. As shown in Table 1, the PANI nanoparticles synthesized at the ANI-to-biosurfactant weight ratio of 23:1 shows the highest electrical conductivity among the studied samples, including the conventional PANI, perhaps because of their highest crystallinity.
4. Conclusions The present work used a rhamnolipid biosurfactant produced by P. aeruginosa SP4 as a soft template for the synthesis of PANI nanoparticles. In comparison to the conventional PANI, the PANI synthesized in the presence of the biosurfactant template showed a uniform morphology and size with higher crystallinity and electrical conductivity. It might be concluded that the rhamnolipid compounds should be a good candidate for the template synthesis of conductive polymeric nanoparticles with controllable morphology and size with high conductivity. The biosurfactant can be readily removed under mild conditions and may thus represent certain advantages over a number of synthetic surfactants – sodium dodecylsulphate (SDS) [36] and nonylphenol ethoxylate(9) (NP-9) [37] – that were previously used to create PANI nanostructures.
Acknowledgements This work was financially supported by the Master Research Grant (MAG) and the BRG 5080030 Grant under the Thailand Research Fund (TRF), and the 90th Anniversary of Chulalongkorn University Fund under the Ratchadaphiseksomphot Endowment Fund. The authors would like to acknowledge Assoc. Prof. Anuvat Sirivat, The Petroleum and Petrochemical College, Chulalongkorn University, for his assistance in electrical conductivity measurements.
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Contents lists available at ScienceDirect
Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Synthesis of polyaniline nanofibers and nanotubes via rhamnolipid biosurfactant templating Panisara Worakitsiri a , Orathai Pornsunthorntawee a , Tuspon Thanpitcha a , Sumaeth Chavadej a,b , Christoph Weder c , Ratana Rujiravanit a,b,∗ a b c
The Petroleum and Petrochemical College, Chulalongkorn University, Bangkok 10330, Thailand Center for Petroleum, Petrochemicals, and Advanced Materials, Chulalongkorn University, Bangkok 10330, Thailand Adolphe Merkle Institute and Fribourg Center for Nanomaterials, University of Fribourg, Route de l’Ancienne Papeterie, CH-1723 Marly 1, Switzerland
a r t i c l e
i n f o
Article history: Received 28 July 2010 Received in revised form 6 November 2010 Accepted 25 November 2010 Available online 18 December 2010 Keywords: Biosurfactants Glycolipids Rhamnolipids Conductive polymers Polyaniline Nanoparticles
a b s t r a c t Polyaniline (PANI) nanorods and nanotubes with average diameters in the range of 121 ± 12 nm to 152 ± 13 nm were synthesized by the oxidative polymerization of aniline using hydrochloric acid as a dopant, ammonium peroxodisulfate as an oxidant, and a rhamnolipid biosurfactant produced by Pseudomonas aeruginosa SP4 as a soft template. The reaction conditions were systematically varied with the objective to maximize the electrical conductivity and aspect ratio of the PANI nanofibers. Optimized conditions, which involved an acid concentration of 0.1 M, an ANI-to-biosurfactant weight ratio of 23:1, and a polymerization time of 6 h, afforded a material with an electrical conductivity of 25 ± 2 S/cm. This material also exhibited the highest degree of crystallinity of all studied samples and matched the electronic properties of a reference material made without the surfactant. © 2010 Elsevier B.V. All rights reserved.
1. Introduction In the last decade, many intrinsically conductive polymers, including polyaniline (PANI), polypyrrole (PPy), polyacetylene (PA), and polythiophene (PTh), have been synthesized by either chemical or electrochemical methods [1]. These polymers contain conjugated sequences of alternating single and multiple bonds, which can be easily reduced or oxidized due to their low ionization potential or high electron affinity, resulting in the alteration of their electrical conductivity from an insulating to a (semi)conducting state [2,3]. Among the electrically conductive polymers, PANI is one of the most attractive candidates, because of its facile synthesis, good environmental stability, ease of electrical conductivity control by changing either the oxidation state or the protonation state, and the low cost of the aniline (ANI) monomer [4–6]. PANI has shown promising commercial viability in a variety of technological applications, including rechargeable batteries [7], conductive coatings or adhesives [8,9], capacitors and other energy storage systems [10], electrical and electrochemical devices [11],
∗ Corresponding author at: The Petroleum and Petrochemical College, Chulalongkorn University, Bangkok 10330, Thailand. Tel.: +66 2 218 4132; fax: +66 2 215 4459. E-mail address: [email protected] (R. Rujiravanit). 0379-6779/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2010.11.039
and sensor applications [12,13]. However, the infusibility of PANI and its limited solubility in common solvents have presented challenges to processing and perhaps hampered a more widespread utilization. Many studies have sought to improve the usefulness of PANI by fabricating composites with polymer matrixes, such as poly(styrene sulfonic acid) (PSSA, which also serves as a dopant) [14] and poly(ethylene-co-vinyl acetate) (PEVA) [15]. In this context, the poor dispersability of PANI in the matrix can cause problems, as this usually limits the electrical conductivity and also negatively impacts the mechanical properties. The use of PANI nanoparticles instead of their macro- or microscopic counterparts has emerged as viable route to address this problem. Particularly attractive in this context are PANI nanofibers or -rods, which due to their high aspect ratio from the basis for low percolation thresholds [16]. PANI nanoparticles can be synthesized by various approaches, which include the use of templates that steer the nanoparticle formation [1]. Template synthesis is considered to be an effective method to produce PANI nanoparticles with controllable size and shape. However, unless the template remains ‘buried’ within the final product such as in materials produced by coating a nanotemplate with the conducting polymer [16], post-synthesis processes are usually required in order to remove the template. In that case, great care has to be taken to avoid adverse effects such as disintegration or aggregation of the nanoparticles and chemical decomposition.
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In the present study, a biosurfactant produced by the microorganism Pseudomonas aeruginosa SP4 was used as a soft template to synthesize PANI nanorods and nanotubes. The approach builds on our recent previous work [17], which has identified l-rhamnosyl3-hydroxydecanoyl-3-hydroxydecanoate, or mono-rhamnolipid (Rha–C10 –C10 ) as the predominant component in the biosurfactant produced by this microorganism. Rhamnolipids possess good physicochemical properties in terms of surface activity, stability, and emulsification activity. Parra et al. [18] reported that these surface-active compounds reduce the surface tension of pure water from 72 mN/m to below 30 mN/m with an interfacial tension in the range of 43 mN/m to below 1 mN/m. Rhamnolipids can self-assemble to form various types of microstructures, including micelles and vesicles, at concentrations greater than the critical micelle concentration (CMC) [19–22]. These rhamnolipid microstructures are able to solubilize, or entrap, both organic and inorganic solutes [19,21], and therefore could serve as a template for the accumulation of ANI monomer and subsequent polymerization. Thus, the aim of this present work was to synthesize PANI nanoparticles by conducting an oxidative polymerization of ANI in the presence of the rhamnolipid biosurfactant as a soft, easily removable template. The PANI product was characterized in terms of morphology, thermal property, crystallinity, and electrical conductivity, and compared to a reference PANI product synthesized in the absence of the template.
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flask and a rotary evaporator was used to remove the solvent at 40 ◦ C under reduced pressure. After solvent evaporation, about 5.2 g of a viscous honey-colored biosurfactant product was obtained per liter of the culture medium. The chemical structure of the most abundant component in the biosurfactant product was identified as Rha–C10 –C10 (73.5%), while the others were characterized as Rha–Rha–C8 –C10 (0.7%), Rha–C8 –C10 (1.5%), Rha–C10 –C12:1 (9.5%), Rha–C10 –C12 (13.3%), and Rha–Rha–C10 –C14:1 (1.4%), with small contributions of their structural isomers [17]. 2.4. Synthesis of polyaniline nanoparticles
P. aeruginosa SP4 was isolated from petroleum-contaminated soil in Thailand [23]. The isolated strain was maintained on nutrient agar slants at 4 ◦ C to minimize the biological activity and was subcultured every month.
The PANI nanoparticles were synthesized in an aqueous solution by the oxidative polymerization of ANI using APS as an oxidant and the rhamnolipid biosurfactant as a template. The synthesis procedure was modified from that of Thanpitcha et al. [26]. Briefly, a desired amount of the ANI monomer (1.0 g, 1.7 g, 2.0 g, or 2.6 g) was added to 50 ml of a 1.8 g/l biosurfactant solution in order to create reaction mixtures with four different ANI-to-biosurfactant weight ratios (11:1, 19:1, 23:1, and 28:1, corresponding to initial ANI concentrations of 20.4 g/l, 34.6 g/l, 40.8 g/l, and 51.0 g/l, respectively) and the solution was vigorously stirred at room temperature for 22 h. Then the mixture was subsequently cooled to 0 ◦ C under mechanical stirring at 300 rpm for 30 min and 100 ml of a 0.1 M hydrochloric acid solution was added drop-wise. The reaction mixture was stirred for another 30 min at 0 ◦ C. After that, 10 ml of a pre-cooled APS solution was added drop-wise into the solution to initiate the polymerization reaction. At this stage, the final ANI concentrations were about 6.4 g/l, 10.8 g/l, 12.8 g/l, and 15.9 g/l and the biosurfactant concentration was approximately 0.6 g/l. The ANI-toAPS molar ratio was kept constant at a molar ratio of 2.6:1. The mixture was further stirred at 0 ◦ C for 6 h to complete polymerization. The resulting suspension was centrifuged at 8500 rpm at 25 ◦ C for 10 min and the supernatant solution was discarded. The isolated product was subjected to dialysis with a large amount of 30%v/v aqueous ethanol for 4 days in order to neutralize the polymer and to remove the biosurfactant template. The dialyzed solution was further centrifuged at 8500 rpm at 25 ◦ C for 10 min and the resulting PANI product in the emeraldine salt (ES) form was subsequently dried in a vacuum oven at ambient temperature for 3 days. The PANI nanoparticles were subsequently kept in desiccators prior to analyses. A sample of PANI that was synthesized in conventional manner in the absence of the biosurfactant template served as a control. The optimum ANI-to-biosurfactant weight ratio for the synthesis of PANI nanoparticles was chosen on the basis of the uniformity of morphology and size, the crystallinity, and the electrical conductivity of the PANI product. The percentage yield was calculated using the following equation:
2.3. Preparation of rhamnolipid biosurfactant
Yield (%) =
An inoculum was first prepared by transferring the bacterial colonies into a nutrient broth and the culture was incubated at 37 ◦ C in a shaking incubator at 200 rpm for 22 h. Then a nutrient broth containing 2% inoculum and 2% palm oil was incubated at 37 ◦ C under aerobic conditions in a shaking incubator at 200 rpm for 48 h to obtain the highest microbial and biosurfactant concentrations [23]. The culture medium was subsequently centrifuged at 8500 rpm at 4 ◦ C for 20 min to remove the bacterial cells. The supernatant was further treated by acidification to pH 2.0 using a 6 M hydrochloric acid solution and the acidified supernatant was left overnight at 4 ◦ C for complete precipitation of the biosurfactant [24]. After centrifugation, the precipitate was removed and dissolved in a 0.1 M sodium bicarbonate solution, followed by extraction with a 2:1 v/v chloroform:ethanol mixture at room temperature [25]. The organic phase was transferred to a round-bottom
where m1 is the weight of the PANI product and m2 is the weight of ANI monomer.
2. Experimental 2.1. Materials Nutrient broth (Difco, USA) was prepared at a fixed concentration of 8 g/l, as recommended by the supplier. Agar powder was supplied by Himedia Co., Ltd. (India). Palm oil was purchased from Morakot Industry, Co., Ltd. (Thailand). Hydrochloric acid and ethanol were provided by J.T. Baker Co., Ltd. (Malaysia). Chloroform was purchased from Labscan Asia Co., Ltd. (Thailand). Sodium bicarbonate and sodium hydroxide pellets were supplied by Ajax Finechem (Australia). ANI monomer, purchased from Merck (Germany), was distilled under reduced pressure prior to use. Ammonium peroxodisulfate (APS) and N-methyl-2-pyrrolidinone (NMP) were supplied by Sigma–Aldrich (Germany). 2.2. Bacterial strain and culture growth conditions
m1 × 100 m2
2.5. Doping of polyaniline nanoparticles The PANI nanoparticles were originally isolated in the ES form. For certain experiments, the material was dedoped by dispersion in a 0.4 M sodium hydroxide solution for 3.5 h. Then the dedoped PANI was filtered and excessively washed with distilled water until neutral. The dedoped material was re-doped by dispersion in a 1.5 M hydrochloric acid solution for 24 h before the polymer was isolated by filtration and subsequently dried in a vacuum oven. The doping process of the PANI samples was done prior to being analyzed with wide-angle X-ray diffraction (WAXD) and electrical conductivity measurements.
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2.6. Analytical methods and measurements 2.6.1. Scanning electron microscopy (SEM) The morphology of the PANI nanoparticles was investigated by scanning electron microscopy (JOEL, model JSM-5410LV) at 20 kV. The PANI nanoparticles were dispersed in distilled water before being dropped on to a brass stub. Samples were coated with a thin layer of gold by using an ion sputtering device (JOEL, model JFC-1100E). The diameter of the synthesized PANI nanoparticles was determined from SEM micrographs using SemAfore software, version 4.00 (JEOL (Skandinaviska) AB, Sweden). 2.6.2. Transmission electron microscopy (TEM) The internal feature of the obtained PANI products and the morphology of the biosurfactant microstructures were observed under a transmission electron microscope (JEOL, model JEM-2100). A drop of either a PANI suspension in distilled water or the biosurfactant solution was placed onto a copper grid. Only the biosurfactant solution sample was stained with 1% uranyl acetate aqueous solution. The excess of the sample solution was then removed by absorbing the drop with a piece of filter paper. The grid was dried in a vacuum desiccator for at least 6 h before being analyzed. 2.6.3. Dynamic light scattering (DLS) measurements DLS experiments were conducted to determine the sizes of the biosurfactant microstructures, expressed as the hydrodynamic diameter. A Malvern Zetasizer Nanoseries, model S 4700, dynamic light scattering instrument was used at 25 ± 1 ◦ C in the hydrodynamic diameter range of 0.6–6000 nm. The sizes of the biosurfactant microstructures were calculated from the diffusion coefficients obtained from computer analysis using the software provided with the instrument. 2.6.4. Fourier-transformed infrared (FT-IR) spectroscopy A model 670 Thermo Nicolet Nexus FT-IR spectrometer was used to acquire FT-IR spectra of the rhamnolipid biosurfactant and the various PANI samples. The FT-IR spectra were collected using 16 scans in a range of 4000–40 cm−1 at a resolution of 4 cm−1 . All spectra were corrected for atmospheric carbon dioxide. 2.6.5. Ultraviolet–visible (UV–vis) spectroscopy The UV–vis spectra of the synthesized PANI nanoparticles were measured using a Shimadzu, model 2550, UV–vis spectrophotometer in the wavelength range of 200–800 nm. NMP was used as the solvent to prepare solutions of the PANI in the emeraldine base (EB) form. 2.6.6. Thermogravimetric analyses (TGA) TGA was used to assess the thermal stability of the rhamnolipid biosurfactant and the various PANI samples using a Dupont, model 2950, thermogravimetric analyzer in the temperature range of 50–800 ◦ C with a heating rate of 10 ◦ C/min under a nitrogen atmosphere. 2.6.7. Wide angle X-ray diffraction (WAXD) experiments An Rigaku, model D/MAX-2000, X-ray diffractometer was used to characterize the crystalline structure of the PANI samples that had been doped as indicated in Section 2.5. The WAXD analysis was carried out in a continuous mode with a scan speed of 5◦ /min covering the angles 2 from 5◦ to 50◦ . Cu K␣1 was used as the X-ray source.
Fig. 1. Yield of PANI synthesized at an acid concentration of 0.1 M and a polymerization time of 6 h in the presence of biosurfactant template at four different ANI-to-biosurfactant weight ratios compared to the conventional PANI synthesized at the corresponding ANI concentrations (6.4 g/l, 10.8 g/l, 12.8 g/l, and 15.9 g/l).
using a custom-made two-point probe connected with an electrometer/high resistance meter (Keithley, model 7517A) at room temperature and a relative humidity of 50%. 2.7. Statistical analysis The experimental data are presented in terms of arithmetic averages of at least three replicates and the standard deviations are indicated by error bars. The analyses were done using SigmaPlot software, version 8.02 (SPSS Inc., UK). 3. Results and discussion 3.1. Template synthesis of PANI PANI nanoparticles were synthesized in the presence of the rhamnolipid biosurfactant produced by P. aeruginosa strain SP4 by the oxidative polymerization of aniline using hydrochloric acid as a dopant and APS as an oxidant. Keeping the biosurfactant concentration in the polymerization mixture constant at about 0.6 g/l, the final ANI concentration was systematically varied from 6.4 g/l to 15.9 g/l at which four different ANI-to-biosurfactant weight ratios (11:1, 19:1, 23:1, and 28:1) were studied. The ANI-to-APS molar ratio was kept constant at 2.6:1. Fig. 1 shows the yield of the synthesized PANI (in percentage of the used monomer) at four different ANI-to-biosurfactant weight ratios (11:1, 19:1, 23:1, and 28:1, corresponding to ANI concentrations of 6.4 g/l, 10.8 g/l, 12.8 g/l, and 15.9 g/l, respectively) compared to the conventional PANI synthesized at the corresponding ANI concentrations. Increasing the ANI content in the polymerization mixture from 6.4 g/l to 15.9 g/l (concomitant with an increase in the ANI-to-biosurfactant weight ratio from 11:1 to 28:1) gradually decreased the yield of the synthesized PANI from 34.6 ± 1.5% to 25.5 ± 0.4%. The same trend was also observed for the reference experiments conducted in the absence of the biosurfactant (referred to as conventional PANI); therefore, the presence of the biosurfactant template did not affect the reaction yield in a noticeable manner. 3.2. Morphology and possible formation mechanism
2.6.8. Electrical conductivity measurements The electrical conductivity of the synthesized PANI samples that had been doped as indicated in Section 2.5 was measured by
Fig. 2 shows representative SEM micrographs of the PANI nanoparticles synthesized at the four different ANI-to-
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Fig. 2. SEM images of (a) conventional PANI and PANI nanoparticles synthesized at an acid concentration of 0.1 M and a polymerization time of 6 h in the presence of biosurfactant template at ANI-to-biosurfactant weight ratios of (b) 11:1, (c) 19:1, (d) 23:1, and (e) 28:1.
biosurfactant weight ratios along with conventional PANI prepared in the absence of the biosurfactant. From the SEM images, it is evident that the conventional PANI and the PANI synthesized at the lowest ANI-to-biosurfactant weight ratio (11:1) exhibited similar irregular shaped structures due to the aggregation of primary PANI particles of roughly spherical shape. By contrast, the PANI synthesized in the presence of the biosurfactant template at ANI-to-biosurfactant weight ratios of 19:1 or higher was of fibrillar structure. It was also observed that the average diameter of the PANI nanofibers increased from 121 ± 12 nm to 152 ± 13 nm upon increasing the ANI-to-biosurfactant weight ratio from 19:1 to 28:1. The results indicate that, at an appropriate ANI-to-biosurfactant weight ratio, the biosurfactant can template the synthesis of PANI nanostructures with high aspect ratio – length-to-diameter ratio. This parameter expresses the average geometry of the synthesized particles in a quantitative manner and is important, as it governs the threshold concentration for the formation of percolating network structures if the nanoparticles are dispersed in a polymer matrix.
In order to examine the internal features of the fibrillar PANI structures, the samples were further subjected to TEM analysis. Fig. 3 shows representative TEM micrographs of the PANI nanoparticles synthesized via the biosurfactant templating. The image of materials synthesized at an ANI-to-biosurfactant weight ratio of 23:1 (Fig. 3a) suggests that these samples comprised both PANI nanotubes and nanorods with a rough surface. However, if the ANIto-biosurfactant weight ratio increased to 28:1, nanorods become the more dominant species (Fig. 3b). The formation of nanostructures can be explained based on nucleation and growth theories. Li et al. [27] reported that the formation of PANI was initiated by some nuclei either formed homogeneously in the parent phase or heterogeneously grown on other species, such as preformed particles or the reactor surface. In order to trace the formation mechanism of the PANI nanofibers synthesized via the biosurfactant templating, DLS and TEM techniques were used to study the aggregation behavior of the rhamnolipid biosurfactant in the distilled water, in the presence of
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Fig. 3. TEM images of PANI nanotubes and PANI nanorods synthesized via biosurfactant templating at ANI-to-biosurfactant weight ratios of (a) 23:1 and (b) 28:1.
the ANI monomer, and in acidic solution. The ANI-to-biosurfactant weight ratio was kept constant at 23:1. Fig. 4 shows representative TEM micrographs of the biosurfactant microstructures while Fig. 5 shows the results from DLS measurements expressed as the hydrodynamic diameter. At an initial biosurfactant concentration of 1.8 g/l, the rhamnolipid biosurfactant self-assembled in distilled water to form vesicular structures with an average hydrodynamic diameter of 126 ± 2 nm. After adding the ANI monomer to the biosurfactant solution at the ANI concentration of 40.8 g/l, rhamnolipid vesicles with an average hydrodynamic diameter of 211 ± 3 nm were observed. The observed increase in the vesicle size is indicative of the solubilization of the ANI monomer in the biosurfactant vesicles. After adding the HCl solution to the mixture, the biosurfactant and the ANI concentrations decreased to 0.6 g/l and 12.8 g/l, respectively. In acidic solution, the rhamnolipid biosurfactant formed larger vesicle structures with an average hydrodynamic diameter of 242 ± 1 nm. Although the biosurfactant and impurity concentrations were able to significantly affect the size of rhamnolipid microstructures [19–21], it seemed that the influence of solution pH should be more dominant in the current work. Champion et al. [19] reported that a pH decrease of the medium from 8.0 to 6.0 led to an increase of the size of rhamnolipid microstructures from micelles (5–10 nm in diameter) to
large vesicles (larger than 250 nm in diameter). The rhamnolipid compounds behave as anionic surfactants at pH values of above 4 due to the dissociation of the carboxylic acid group comprised in their hydrophilic moiety [28]. However, in acidic solution, the rhamnolipid biosurfactant are protonated; therefore, the charge repulsion between the adjacent polar head groups of the biosurfactant molecules is reduced, promoting the self-assembly process and the formation of larger microstructures. Fig. 6 shows the proposed formation mechanism of the PANI nanotubes and nanorods synthesized via the biosurfactant templating technique. In distilled water, the rhamnolipid biosurfactant spontaneously self-assembles to form vesicular structures. Upon adding the ANI monomer into the biosurfactant solution, the monomer can be solubilized in the rhamnolipid vesicles in the outer bilayer region (vesicle “A” in Fig. 6). The ANI monomer is an amphiphilic molecule consisting of a hydrophilic amino group and a hydrophobic benzenoid ring. Hence, it possibly interacts with the biosurfactant molecule in the vesicle membrane via hydrophobic interactions, perhaps between the benzenoid ring of the ANI monomer and the hydrocarbon chains in the hydrophobic moiety of the rhamnolipid compounds, as well as hydrophilic interactions via hydrogen bonding between the polar amino group of ANI and the carboxylate group of the rhamnolipid biosurfactant [26]. The addi-
Fig. 4. TEM images of rhamnolipid vesicles at the biosurfactant concentration of 1.8 g/l dissolved (a) in distilled water and (b) in the presence of 40.8 g/l ANI monomer (ANI-to-biosurfactant weight ratio of 23:1).
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yields PANI nanotubes whereas the polymerization of that solubilized in the inner aqueous core (vesicle B) leads to the formation of the dense PANI nanorods. Nevertheless, it should be noted that, upon increasing the ANIto-biosurfactant weight ratio, the migration of the anilinium ion into the inner aqueous core of the rhamnolipid vesicles becomes more favorable due to a higher amount of ANI. Therefore, the dense PANI nanorods were dominantly produced, as shown in Fig. 3. 3.3. Chemical characterization of synthesized PANI
Fig. 5. Hydrodynamic diameters of rhamnolipid vesicles at the biosurfactant concentration of 1.8 g/l (a) in distilled water, (b) in the presence of 40.8 g/l ANI monomer, and (c) at the biosurfactant concentration of 0.6 g/l in acidic solution with 12.8 g/l ANI monomer. The ANI-to-biosurfactant weight ratio is kept constant at 23:1 and the inset figures are the visible characteristics of the obtained solutions.
tion of hydrochloric acid into the biosurfactant solution containing the ANI monomer converts the amino groups (–NH2 ) of the ANI into the protonated anilinium ion (–NH3 + ), which is more hydrophilic and drives the monomer into the inner aqueous core of the rhamnolipid vesicles (vesicle “B” in Fig. 6). As the APS oxidant is added into the reaction mixture, the polymerization takes place, and PANI nanostructures are produced via the aggregation and elongation of the rhamnolipid vesicles during the polymerization. The polymerization of the ANI solubilized in the vesicle membrane (vesicle A)
Fig. 7 shows the FT-IR spectra of the rhamnolipid biosurfactant produced by P. aeruginosa SP4, the conventional PANI, and the PANI nanoparticles synthesized at four different ANI-to-biosurfactant weight ratios. As shown in Fig. 7a, the FT-IR spectrum of the biosurfactant exhibits the important absorption bands at 3364 cm−1 , 2925 cm−1 , 2854 cm−1 , 1745 cm−1 , and 1300–1100 cm−1 , indicating the chemical structure of rhamnolipid compounds. According to the work of Pornsunthorntawee et al. [17], the weak broad band located at 3364 cm−1 can be attributed to the O–H stretching vibrations of hydroxyl groups, while the strong absorption peaks at 2925 cm−1 and 2854 cm−1 are assigned to the C–H stretching vibrations of the hydrocarbon chain positions. The characteristic peak observed at 1745 cm−1 corresponds to the C O stretching vibrations of the carbonyl group, and the C–O stretching bands in the range of 1300–1100 cm−1 related to the bonds formed between carbon atoms and hydroxyl groups in the chemical structures of the rhamnose rings. As shown in Fig. 7b, the FT-IR spectra of the conventional PANI exhibits the characteristic peaks at 1568 cm−1 , 1484 cm−1 , 1291 cm−1 , 1103 cm−1 , and 792 cm−1 , which were assigned to the C C stretching of the quinoid ring, the C C stretching of the benzenoid ring, the C–N stretching, the N Q N stretching (Q representing the quinoid ring), and the out-ofplane deformation of C–H in the 1,4-disubstituted benzene ring, respectively [29,30]. These absorption peaks are characteristics of PANI in the ES form – the doped conducting state of PANI (PANI ES). This form was obtained because the synthesis was conducted under acidic conditions. The FT-IR spectra of the PANI nanoparticles synthesized at four different ANI-tobiosurfactant weight ratios (Fig. 7c–f) were identical to that of the conventional PANI, indicating that the ANI-to-biosurfactant weight ratio did not affect the electronic state of the PANI products. The FT-IR spectra also indicate the complete biosurfactant template removal from the synthesized PANI because there was no characteristic absorption peaks of the rhamnolipid compounds appearing in the FT-IR spectra of the PANI nanoparticles. UV–vis spectroscopy was used to examine the electronic states of the synthesized PANI, both in the ES and EB forms (data not shown). To study the as-prepared PANI ES, the samples were dispersed in distilled water. The UV–vis spectrum of the PANI samples synthesized in the presence of the biosurfactant template at four different ANI-to-biosurfactant weight ratios showed a characteristic absorption peak centered around 410 nm and a broad band at 810 nm. These two characteristic absorption bands are diagnostic for the protonated ES form – the conducting state of the PANI [26]. The PANI nanoparticles synthesized with the biosurfactant route were dissolved in NMP; under these conditions, the polymer can be de-doped through deprotonation by the slightly basic solvent, to afford PANI EB [31]. Indeed, the UV–vis spectra showed two absorption peaks at 330 nm and 630 nm, which are assigned to the –* transition of the benzenoid ring and the exciton absorption of the quinoid ring, respectively, and are characteristics of the EB form of PANI [32]. The UV–vis spectra of the PANI nanoparticles synthesized at the four different ANI-to-biosurfactant ratios – in
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Fig. 6. Proposed mechanism for the formation of PANI nanotubes and PANI nanorods synthesized via biosurfactant templating.
the ES as well as the EB form – were identical to that of the conventional PANI, suggesting that the use of the biosurfactant did not affect the electronic state of the PANI product. 3.4. Thermal properties of synthesized PANI Fig. 8 shows the TGA thermograms of the rhamnolipid biosurfactant produced by P. aeruginosa SP4, the conventional PANI, and the PANI nanoparticles synthesized in the presence of the biosurfactant. As shown in Fig. 8a, the TGA thermogram of the rhamnolipid
Fig. 7. FT-IR spectra of (a) rhamnolipid biosurfactant, (b) conventional PANI, and PANI nanoparticles synthesized at an acid concentration of 0.1 M and a polymerization time of 6 h in the presence of biosurfactant template at ANI-to-biosurfactant weight ratios of (c) 11:1, (d) 19:1, (e) 23:1, and (f) 28:1.
biosurfactant exhibits only one weight loss step at 400 ◦ C, which is associated with the decomposition of the biosurfactant. By contrast, three discrete weight losses at approximately 80 ◦ C, 280 ◦ C, and 500 ◦ C were observed in the TGA thermograms of all PANI samples (Figs. 8b–f). These events are related to the loss of water, the elimination of the dopant, and the degradation of the PANI, respectively [3]. The TGA results show that the use of the surfactant did not affect the thermal property of the PANI product. In addition, the absence of the characteristic weight loss step associated with the rhamnolipid biosurfactant in the TGA thermograms of the PANI
Fig. 8. TGA thermograms of (a) rhamnolipid biosurfactant, (b) conventional PANI, and PANI nanoparticles synthesized at an acid concentration of 0.1 M and a polymerization time of 6 h in the presence of biosurfactant template at ANI-to-biosurfactant weight ratios of (c) 11:1, (d) 19:1, (e) 23:1, and (f) 28:1.
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Table 1 Crystallinities (I6.6 /I25.9 ) and electrical conductivities of conventional PANI and PANI nanoparticles synthesized at four different ANI-to-biosurfactant weight ratios in the doped state. ANI-tobiosurfactant weight ratio
Crystallinity (I6.6 /I25.9 )
Electrical conductivity (S/cm)
Conventional PANI 11:1 19:1 23:1 28:1
0.5 – 0.4 0.6 0.5
5.2 0.1 0.1 24.8 6.7
± ± ± ± ±
0.6 0.0 0.0 1.7 3.1
investigated here, including the conventionally synthesized PANI, suggesting that the biosurfactant template affected both morphology and degree of crystallinity of the produced PANI. Fig. 9. XRD patterns of (a) conventional PANI and PANI nanoparticles synthesized at an acid concentration of 0.1 M and a polymerization time of 6 h in the presence of biosurfactant template at ANI-to-biosurfactant weight ratios of (b) 11:1, (c) 19:1, (d) 23:1, and (e) 28:1.
nanoparticles confirms that the biosurfactant template was completely removed from the synthesized PANI product. 3.5. Crystallinity of synthesized PANI WAXD experiments were carried out to characterize the crystalline structure of the PANI products after they had been doped by immersion in a 1.5 M hydrochloric acid solution. Fig. 9 shows the WAXD patterns of the conventional PANI compared to those of the PANI nanoparticles synthesized at the four different ANIto-biosurfactant weight ratios. The X-ray diffraction pattern of the PANI synthesized at the ANI-to-biosurfactant weight ratio of 11:1 displays two broad peaks at 2 equal to 20.0◦ and 25.6◦ , indicating that the PANI was largely amorphous [29]. Furthermore, the WAXD patterns of the conventional PANI and the PANI synthesized at the ANI-to-biosurfactant weight ratios of 19:1 or higher exhibit distinct peaks at 2 equal to 6.6◦ , 20.8◦ , and 25.9◦ , corresponding ˚ 4.3 A, ˚ and 3.5 A, ˚ respectively. Chen et al. to d-spacings of 13.6 A, [33] reported that a reflection at 2 equal to 6.6◦ is due to either the interplanar distance between PANI molecules or the reflection to periodic distance between dopant and N atom on the adjacent main chain, while Han et al. [3] and Zhang et al. [29] reported that the distinct peaks at 2 equal to 20.8◦ and 25.9◦ can be ascribed to the periodicity parallel and perpendicular to the polymer chains, respectively. According to the work of Anikumar and Jayakannan [34], the two peaks located at 2 equal to 20.1◦ and 25.5◦ were generally observed in doped PANI while the peak at 2 equal to 6.4◦ was only observed for PANI with a highly ordered orientation. Thus, from the WAXD patterns, it appears that the conventional PANI and the PANI synthesized at the ANI-to-biosurfactant weight ratio of 19:1 or higher are partially crystalline and are of a structure that is characteristic of PANI ES. Anikumar and Jayakannan [35] reported that the ratio of the peak intensity at 2 equal to 6.4◦ and 26.0◦ can be used to express the degree of crystallinity of PANI samples. The materials prepared in this study were analyzed in this manner and the ratio of the peak intensities at 2 equal to 6.6◦ and 25.9◦ (I6.6 /I25.9 ) are shown in Table 1. The data show that the value of I6.6 /I25.9 (and therewith the crystallinity of the polymer) increases with increasing ANIto-biosurfactant ratio to the maximum at an ANI-to-biosurfactant weight ratio of 23:1; the trend is broken for the highest ANI-tobiosurfactant ratio of 28:1. Thus, the WAXD results indicate that the PANI synthesized at an ANI-to-biosurfactant weight ratio of 23:1 possessed the highest crystallinity among the PANI samples
3.6. Electrical conductivities of synthesized PANI Table 1 shows the electrical conductivities of the conventional PANI and the PANI nanoparticles synthesized in the presence of the biosurfactant. All PANI samples were doped with a 1.5 M hydrochloric acid solution. It was found that an increase in the ANI-to-biosurfactant weight ratio from 11:1 to 23:1 strongly increased the electrical conductivity of the PANI nanoparticles from 0.1 ± 0 S/cm to 25 ± 2 S/cm. However, when the ANI-to-biosurfactant weight ratio increased to 28:1, the electrical conductivity of the PANI dropped to 7 ± 3 S/cm. It is known that the crystal structure and the degree of crystallinity are important factors controlling the electrical conductivity of the PANI in the aspect that the electrical conductivity increases with the level of crystallinity [2]. As shown in Table 1, the PANI nanoparticles synthesized at the ANI-to-biosurfactant weight ratio of 23:1 shows the highest electrical conductivity among the studied samples, including the conventional PANI, perhaps because of their highest crystallinity.
4. Conclusions The present work used a rhamnolipid biosurfactant produced by P. aeruginosa SP4 as a soft template for the synthesis of PANI nanoparticles. In comparison to the conventional PANI, the PANI synthesized in the presence of the biosurfactant template showed a uniform morphology and size with higher crystallinity and electrical conductivity. It might be concluded that the rhamnolipid compounds should be a good candidate for the template synthesis of conductive polymeric nanoparticles with controllable morphology and size with high conductivity. The biosurfactant can be readily removed under mild conditions and may thus represent certain advantages over a number of synthetic surfactants – sodium dodecylsulphate (SDS) [36] and nonylphenol ethoxylate(9) (NP-9) [37] – that were previously used to create PANI nanostructures.
Acknowledgements This work was financially supported by the Master Research Grant (MAG) and the BRG 5080030 Grant under the Thailand Research Fund (TRF), and the 90th Anniversary of Chulalongkorn University Fund under the Ratchadaphiseksomphot Endowment Fund. The authors would like to acknowledge Assoc. Prof. Anuvat Sirivat, The Petroleum and Petrochemical College, Chulalongkorn University, for his assistance in electrical conductivity measurements.
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