Effects of surfactants on the characteristics and ... - Wiley Online Library

Research Article Received: 16 November 2009

Revised: 18 February 2010

Accepted: 28 February 2010

Published online in Wiley Online Library: 20 July 2010

(wileyonlinelibrary.com) DOI 10.1002/pi.2898

Effects of surfactants on the characteristics and biosensing properties of polyaniline Agnieszka Kuczynska,a Aysegul Uygun,b∗ Andrzej Kaim,a Hanna Wilczura-Wachnik,a Ayse Gul Yavuzb and Matt Aldissic Abstract To investigate the effects of surfactants on the properties of polyaniline (PANI), a series of PANIs was synthesized in the presence of surfactants by chemical polymerization of aniline in an acidic medium, using (NH4 )2 S2 O8 as oxidant. Three types of surfactant were used: (i) non-ionic poly(ethylene oxide) (20) sorbitan monolaurate (Tween 20) and poly(ethylene oxide) (20) sorbitan monopalmitate (Tween 40); (ii) cationic (1-tetradecyl)trimethylammonium bromide; and (iii) anionic sodium 1-dodecanesulfonate and sodium 1-pentanesulfonate. The structural, morphological and thermal properties of the various samples were characterized using Fourier transform infrared and UV-visible spectroscopy, scanning electron microscopy and thermogravimetric analysis. Calorimetry was used to compare enthalpy changes during polymerization. The electrochemical and glucose biosensor properties of the PANIs were investigated using cyclic voltammetry and amperometric measurements. PANI-Tween 20 and PANI-Tween 40 were found to be good for immobilization of glucose oxidase enzymes and potential candidates for use in glucose biosensing. c 2010 Society of Chemical Industry Keywords: polyaniline; surfactants; electrochemistry; biosensor; calorimetry

INTRODUCTION

1650

Nanoscale conducting polymers have received considerable attention because of their potential use in applications such as wires,1 chemical sensors,2 biosensors3 and electronic devices.4 Among conducting polymers, polyaniline (PANI) has been studied extensively due to its good conductivity and redox reversibility.5 A variety of methods including seeding polymerization,6 rapidly mixed reaction,7 dilute polymerization,8 self-assembly9,10 and surfactant-directing methods11,12 have been developed to synthesize low-dimensional PANI nanostructures, such as nanofibres, nanotubes and nanosheets.13 The properties of PANI are strongly dependent on the type of dopant, which can significantly affect the chemical and physical properties. Also, the solubility of PANI can be improved using PANI derivatives with functional groups.14 Recently, surfactants have also been employed, in the so-called wet chemical method,15 as soft templates for self-assemblies to control the size and shape of conducting polymers and nanoparticles. A surfactant can affect polymerization kinetics and the final properties of the conjugated polymers. On the basis of the results obtained to date, it is known that the incorporation of a surfactant into a conducting polymer can improve the electrical and morphological properties of the polymer as well as its thermal stability because of the introduction of a bulky hydrophobic component into the polymer structure.15 Additionally, solubility can be improved by the synthesis of nano- to microscale PANI particles, which are easier to disperse in a polymer matrix, or by using an emulsifier, which enhances solubility or film formation. Surfactant additives, whether they are anionic, cationic or non-ionic, can play the role of a dopant. Surfactant molecules can provide a hydrophobic micro-effect with controlled electrochemical catalysis, induce orientation

Polym Int 2010; 59: 1650–1659

or solubilization and greatly improve the quality of polymer ¨ et al. have studied the effect of surfactants on the films. Gok properties of polythiophene.16 Omastová and co-workers have investigated properties of PANI and polypyrrole in the presence of various surfactants.17 However, there are no detailed data on the electrochemistry, thermodynamics and biosensor application of PANI synthesized in the presence of surfactants. Surfactants have been introduced into the PANI backbone during electrochemical18 or chemical polymerization.17 The incorporation of surfactants into conducting polymers can improve their biocompatibility and conformation and give porous surface morphology at the nanoscale to improve enzyme immobilization for enzyme biosensor applications.18,19 In the work reported here, PANI was synthesized chemically in the presence of five different surfactants. Polymerization enthalpy and electrochemical behaviour of PANI samples were investigated using calorimetric and potentiometric methods. The materials were characterized using FTIR and UV-visible spectroscopies, TGA, SEM and conductivity measurements. Biosensor properties were investigated using the amperometric method.

∗

Correspondence to: Aysegul Uygun, Suleyman Demirel University, Faculty of Science and Arts, Department of Chemistry, 32260 Isparta, Turkey. E-mail: [email protected]

a University of Warsaw, Faculty of Chemistry, Pasteura 1, 02-093 Warsaw, Poland b Suleyman Demirel University, Faculty of Science and Arts, Department of Chemistry, 32260 Isparta, Turkey c Fractal Systems Inc., Safety Harbor, FL 34695, USA

www.soci.org

c 2010 Society of Chemical Industry

Effects of surfactants on biosensing properties of PANI

EXPERIMENTAL Materials Aniline (Aldrich Chemical Company) was distilled under reduced pressure. Ammonium peroxydisulfate (APS; POCH SA, Poland) and all surfactants (poly(ethylene oxide) (20) sorbitan monolaurate (Tween 20), poly(ethylene oxide) (20) sorbitan monopalmitate (Tween 40), (1-tetradecyl)trimethylammonium bromide (TTAB), sodium 1-dodecanesulfonate (SDS) and sodium 1-pentanesulfonate (SPS); Alfa Aesar, Germany) were used without purification. D-(+)-glucose (anhydrous) (Fluka), phosphate buffer (pH = 7.4, NaH2 PO4 ·2H2 O; Riedel De Haen), glucose oxidase (GOD; EC 1.1.3.4, 179.000 units g−1 , type VII-S from Aspergillus niger; Sigma) and 5% glutaraldehyde (GA) in water solution (Aldrich) were used without purification. N-methylpyrrolidone (NMP), a PANI solvent for UV-visible measurements, was used without further purification. Chemical synthesis of PANI in the presence of surfactants The chemical synthesis of PANI was carried out in a 250 mL three-neck round-bottom flask equipped with magnetic stirrer using APS as the oxidant. First, 0.7 g (0.0031 mol) of oxidant was dissolved in 40 mL of 1.5 mol L−1 HCl solution with the surfactant. Then, 0.38 mL (0.0042 mol) of aniline was rapidly added to the mixture under argon atmosphere. The oxidant/aniline molar ratio used was 0.75 and the aniline/surfactant molar ratio was 7.0.17 Polymerization was carried out at 25 ◦ C for 20 min with controlled stirring under argon atmosphere. Afterwards, the reaction mixture was left standing for one day in the dark at room temperature. The PANI precipitate was then collected using filtration, washed with distilled water and dried at 50 ◦ C under vacuum for 24 h. A reference PANI sample without surfactant was prepared under the same conditions for comparison purposes.

Polym Int 2010; 59: 1650–1659

+0.70 V versus SCE, due to the electro-oxidation of H2 O2 produced enzymatically. Instrumentation FTIR spectra were recorded between 400 and 4000 cm−1 with 4 cm−1 resolution from KBr pellets with a PerkinElmer Spectrum BX FTIR system (Beaconsfield, UK). UV-visible spectral measurements were recorded between 270 and 900 nm using a 1 cm path length quartz cuvette and pure NMP with a PerkinElmer Lambda 20 UV-visible spectrophotometer. The electrical conductivity of PANI was measured with the standard four-probe method using a PCI-DAS6014 current source, a voltmeter and a temperature controller. Dry powder samples were prepared into pellets using a steel die having 13 mm diameter in a hydraulic press under a pressure of 700 MPa. The morphology of PANI was characterized using SEM analysis. Samples were sputter-coated with gold and images were obtained with a scanning electron microscope model Philips XL-30S FEG and VEGA/TESCAN. The thermal stability of PANI was investigated using a PerkinElmer thermal parametric analyser (Beaconsfield, UK) with heating at a rate of 10 ◦ C min−1 under pure nitrogen at a flow rate of 25 mL min−1 . The enthalpy of aniline polymerization in the presence of the various surfactants was determined using calorimetric measurements, with a batch calorimeter type MKR 0.001 (Poland). APS was dissolved in a 40.0 mL solution of HCl and surfactant in a glass beaker. Aniline was dissolved in 2 mL of the same acid solution and kept in a separate vessel with a glass membrane. At the beginning of each measurement, the temperature of both solutions was stabilized at 25 ◦ C. The glass membrane was then broken and aniline solution instantly came into contact with APS solution. At this moment the polymerization reaction in the whole volume was started under continuous stirring. The heat capacity of the calorimeter was calibrated by the response of the reagents to heat at the beginning of each experiment. The following equation was employed in the calibration procedure that preceded each measurement: K=

I2 Rt Tk

(1)

where K is the electric calibration constant, R the resistance of the calibration heater (= 48.5 m), Tk the temperature change during calibration, I the current used in calibration (= 150 mA) and t the heating time during calibration (= 250 s). The numerical values of the calibration constant K were used in the enthalpy polymerization calculation according to H =

KTr nPANI

(2)

where Tr is the temperature change during polymerization and nPANI the number of moles of aniline. Electrochemical experiments were carried out using a Gamry 300 potentiostat/galvanostat (Gamry Instruments, USA). Cyclic voltammetry measurements were performed using the three-electrode cell described above.

RESULTS AND DISCUSSION Characterization of PANI Figure 1 shows FTIR spectra of PANI samples formulated in the presence and absence of surfactants. All characteristic peaks


wileyonlinelibrary.com/journal/pi

1651

Preparation of enzyme electrodes PANI (0.002 g) was dispersed in 20 µL of acetonitrile, and 10 µL of this solution was dropped onto a platinum electrode and dried at room temperature. An enzyme solution was prepared by dissolving 5 mg of GOD in 1 mL of phosphate buffer solution (0.1 mol L−1 , pH = 7.4). A 1 µL aliquot of 5% GA solution and 5 µL of the enzyme solution were mixed thoroughly. Subsequently, 5 µL of the mixture was dropped onto the surface of the electrode modified with PANI prepared with the various surfactants. The enzyme electrode was then characterized after a 30 min resting period at room temperature to allow for crosslinking and stabilization. The sensor response was investigated using chronoamperometry with a conventional three-electrode cell, using a modified platinum working electrode, a saturated calomel reference electrode (SCE) and a platinum wire as an auxiliary electrode. The working platinum electrode had a 1 mm2 surface area. Prior to electropolymerization, it was polished to a mirror finish using alumina slurries. Electrochemical measurements were carried out in the threeelectrode cell described above. Oxygen was introduced into this cell at a constant flow rate to obtain an oxygen-saturated solution. Oxygen flow above the solution was continued to keep it saturated during the measurements. In order to determine the steady-state background current of the enzyme (working) electrode, a potential of +0.70 V versus SCE was applied to the working electrode. Once the current value had been determined, known amounts of glucose were added to the cell from a stock glucose solution, followed by stirring for 5 s. The enzyme electrode response to glucose was measured amperometrically at a constant potential of

www.soci.org

www.soci.org

A Kuczynska et al.

Figure 1. FTIR spectra: (a) PANI; (b) PANI-Tween 20; (c) PANI-Tween 40; (d) PANI-TTAB; (e) PANI-SDS; (f) PANI-SPS.

Figure 2. Comparison of UV-visible spectra of PANI and PANI prepared in the presence of surfactants.

1652

are observed in the spectra, indicating that polymerization of aniline with the various surfactants had indeed taken place. The 3440 cm−1 peak is attributed to the N–H stretching vibration. The 1473–1485 cm−1 bands are assigned to the B–NH–B vibration (were B denotes benzenoid ring). The 1558–1560 cm−1 bands are assigned to –N Q N–structure (Q were denotes quinone). The 1108–1142 cm−1 bands are assigned to Q N+ H–Q or B–NH+ –B stretching vibrations. The 1237–1302 cm−1 bands are assigned to C–N+ and C–N+• stretching vibrations and correspond to π -electron delocalization induced in the polymer by protonation. Finally, the 793–815 cm−1 bands are due to outof-plane deformations of C–H on rings.20 – 23 The additional bands between 2500 and 3200 cm−1 may confirm incorporation of the


surfactant into the PANI structure. For example, the 2900 cm−1 peaks in the spectra of PANI-SDS and PANI-SPS are due to the –CH2 stretching vibrations of the surfactants. Moreover, shifts observed for peaks assigned to B–NH–B and –N Q N–vibrations of PANI are most likely due to PANI–surfactant interactions. Figure 2 shows the UV-visible absorption spectra of PANI synthesized in the presence of the various surfactants and dissolved in NMP. Two characteristic absorption bands at 333 and 626–640 nm are observed. The first absorption is due to the excitation of the nitrogen in the benzenoid segments (π → π ∗ transition), while the second (n → π ∗ ) is ascribed to the molecular exciton associated with the protonated quinine diimine structure of PANI.15,22,24,25 NMP, exerting weak base properties, is a good


Polym Int 2010; 59: 1650–1659


www.soci.org

Table 1. UV-visible, yield and conductivity results Wavelength (nm) Polymer

λ1 (π → π ∗ )

λ2 (n → π ∗ )

AQ /AB

Yield (g)

Conductivity at 25 ◦ C (S cm−1 )

334 334 332 333 333 333

630 635 626 634 640 633

0.62 0.64 0.54 0.68 0.68 0.65

0.24 0.29 0.31 0.24 0.35 0.22

1.98 2.10 2.23 3.65 0.80 0.22

PANI PANI-Tween 20 PANI-Tween 40 PANI-TTAB PANI-SDS PANI-SPS

Figure 3. Cyclic voltammograms of 0.1 mol L−1 aniline in the presence of surfactants.

Polym Int 2010; 59: 1650–1659

electrochemistry findings. TTAB not only acts as surfactant but also as a doping agent to PANI, which increases the charge carriers in the polymer and has an impact on the conductivity and dielectric properties of the polymer. This may also be a factor in the increase in conductivity and crystallinity of the polymer. This means that a surfactant such as TTAB has a more important role in modifying the properties of PANI than the other the surfactants. The electrochemical behaviour of the PANI samples was investigated during oxidation of aniline in the presence and absence of the surfactants. The cyclic voltammograms are compared in Fig. 3. The oxidation potential of the parent PANI is 0.99 V and undergoes a very small shift in the presence of the surfactants. However, the oxidation currents of PANI are very different from each other. Based on the oxidation currents, the electroactivity decreases in the following order: PANI-TTAB >> PANI-Tween 40 > PANI-Tween 20 > PANI > PANI-SPS > PANI-SDS. This is exactly the same order as that for conductivity. The calorimetric signals registered during aniline polymerization are shown in Fig. 4. In all cases, a small endothermic effect (characteristic threshold) is observed before the main signal. We believe that this effect is due to a combination of the positive mixing enthalpy of the solutions and the exothermic polymerization reaction. As a result, a characteristic threshold on the temperature versus time curve is observed. Our results indicate



1653

solvent for PANI, where the polymer chains are unfolded. The interaction of NMP and surfactants is so strong that the resulting product is susceptible to dedoping.26 Thus, PANI in the presence of surfactants might exhibit emeraldine base behaviour in the UV-visible spectra. As can be seen in Fig. 2, the weakest second band absorption is that for PANI-Tween 40. This means that interactions between PANI chains and Tween 40 molecules are the strongest compared to the other surfactants. Such an explanation is confirmed by the TGA data (discussed below). Taking into account that the first weight loss (ca 100 ◦ C) is due to loss of water, the thermal stability of PANI-Tween 40 is the highest (discussed in more detail below). The wavelengths, intensity ratios of quinone absorption (AQ ) to benzenoid absorption (AB ), yields and conductivity results for the PANI samples are listed in Table 1. The AQ /AB intensity ratio varies from 0.54 to 0.68, and the yield value is practically unchanged. The presence of surfactants does not significantly change the oxidation form of PANI. But, the conductivities are quite different in the presence of the various surfactants. The PANI conductivity decreases in the following order: PANITTAB > PANI-Tween 40 > PANI-Tween 20 > PANI > PANI-SDS > PANI-SPS. PANI-TTAB has the highest conductivity value among all samples whereas the anionic surfactants are responsible for low conductivities. These results are also confirmed by

www.soci.org

A Kuczynska et al.

Figure 4. Temperature profiles of the exothermic oxidation of aniline in the presence of various surfactants.

that the enthalpy of aniline polymerization strongly depends on the type of surfactant used, with the numerical values being in the following order: PANI-SDS < PANI-Tween 20 < PANI-Tween 40 < PANI-TTAB < PANI < PANI-SPS. According to the literature, the enthalpy of polymerization is strongly dependent on the reaction conditions27,28 such as APS/aniline ratio and acid concentration.29 The oxidant to aniline molar ratio of 0.75 used in our experiments was chosen based on the literature.30 With this ratio, no other reactions in the backbone ring take place. All calorimetric data are given in Table 2. The enthalpy of polymerization without a surfactant is approximately −124 kcal mol−1 , and is similar to that of PANI-TTAB and PANI-SPS. The enthalpy of PANI-Tween 20 and PANI-Tween 40 has nearly the same exother-

mic value (−108 to −109 kcal mol−1 ). The lowest exothermic enthalpy value (−89 kcal mol−1 ) is obtained for PANI-SDS. Comparing these data, we conclude that intermolecular interactions between PANI and the surfactant are in agreement with the enthalpy of polymerization order given above, with the strongest among them being between PANI and SDS. Surprisingly, we notice an opposite influence on the aniline polymerization thermal effect for both anionic surfactants (SPS and SDS). The highest and lowest enthalpy of polymerization values are found for SPS and SDS, respectively. We believe that such a result correlates with the size of the non-polar part of the anionic surfactant. Based on our results, we can conclude that, in general, the presence of surfactants reduces the thermal effect of

Table 2. Enthalpy of aniline polymerization in the absence and presence of various surfactants Surfactant

Chemical structure and type

H (kcal mol−1 )

Tween 20

−108.1

Tween 40

−109.4

TTAB

−122.5

SDS

−89.1

SPS

−127.5

1654

None


–


−124.5

Polym Int 2010; 59: 1650–1659


www.soci.org

Figure 5. TGA curves of the various PANI samples.

aniline polymerization in the order cationic > non-ionic > anionic surfactant except SPS. Unfortunately, no detailed explanation can be given for SPS and SDS behaviour based on the data presented so far. Additionally, we checked the influence of increasing the oxidant to aniline ratio on the enthalpy of aniline polymerization. Under these conditions, the polymerization was performed in the presence of Tween 20 for an oxidant to aniline ratio of 2. The obtained thermal effect is −217.8 kcal mol−1 , which is higher compared to results obtained under the conditions used in all our experiments. The thermal stability of the PANI samples was investigated using TGA. The results are shown in Fig. 5. For the protonated conducting form, a three-step decomposition process has been proposed in the literature.15,27 – 30 The first weight loss (50–100 ◦ C) indicates the loss of water molecules from the polymer matrix. The second weight loss occurs between 160 and 285 ◦ C, and this may be attributed to the loss of dopant anions bound to the PANI chain. The third decomposition step occurs between 249 and 470 ◦ C (Table 3). In the final step, the PANI backbone decomposes after the elimination of dopant from the polymer structure. Generally, it is noticed that thermal stability of PANI is clearly affected by the presence of the surfactants. Tween 40 has the highest initial thermal degradation temperature (ca 180 ◦ C). The profile of the TGA curves provides a good indication of the significant weight loss for PANI-SDS and PANI-Tween 40. The other PANI samples have similar weight loss characteristics.

Polym Int 2010; 59: 1650–1659

Sample PANI

PANI-Tween 20

PANI-Tween 40

PANI-TTAB

PANI-SDS PANI-SPS

Ti (◦ C)

Tm (◦ C)

Tf (◦ C)

Weight loss (%)

175 255 483 172 249 475 700 189 344 462 162 452 686 141 272 164 327 462

221 358 526 235 338 551 732 254 365 508 207 519 734 179 344 221 371 526

255 465 720 250 360 581 772 285 401 564 252 569 791 243 473 264 423 603

9.0 24.0 7.5 7.4 4.3 3.5 54.9 19.6 9.0 6.0 7.1 8.6 3.6 4.1 52.9 8.3 3.4 5.4

a Ti , initial degradation temperature; Tm , maximum degradation temperature; Tf , final degradation temperature.

method at the oxidation site on the biosensor. The reactions are as follows: GOD Glucose + O2 −−−−−−−−→gluconic acid + H2 O2

(3)

H2 O2 −−−→ O2 + 2H+ + 2e−

(4)

Conducting polymers and their composites have been used as matrices for the entrapment of GOD and serve as the working electrode for sensing glucose.31 – 33 Usually, the monomer



1655

Glucose biosensing and SEM results The catalytic reaction of a glucose biosensor is as follows. GOD (b-Dglucose: oxygen 1-oxidoreductase) catalyses the oxidation of b-Dglucose to D-glucono-1,5-lactone and hydrogen peroxide, using molecular oxygen as the electron acceptor. D-Glucono-1,5-lactone then hydrolyses to gluconic acid and simultaneously hydrogen peroxide is released. During the enzyme-catalysed reaction, the hydrogen peroxide is detected using the amperometric current

Table 3. TGA resultsa

www.soci.org

A Kuczynska et al.

(a)

(b)

(c)

Figure 6. (a) Changes in the response of PANI-Tween 20/GOD enzyme electrode with glucose concentration. (b) Amperometric response of the PANITween 40/GOD, PANI-Tween 20/GOD and parent PANI/GOD enzyme electrodes for varying concentration of glucose. (c) Bioelectrode responses to glucose addition for PANI-Tween 20/GOD, PANI-Tween 40/GOD and parent PANI/GOD electrodes. Fitting of the sensor response in terms of current density for glucose concentration.

1656 wileyonlinelibrary.com/journal/pi


Polym Int 2010; 59: 1650–1659


www.soci.org

Table 4. Comparison of response time, R2 and Km for various electrodes Sample

Km

R2

Linear range (mmol L−1 )

PANI PANI-Tween 20 PANI-Tween 40

15.2 26.7 5.2

0.9244 0.9954 0.9951

0.7–10.3 0.7–12.0 0.7–10.3

is electropolymerized onto a platinum electrode by cyclic voltammetry in a buffer solution containing GOD, which is entrapped in the polymeric film.34 In our case, the electrode surface was surfactant-modified PANI formulated by drop-coating and drying, followed by drop-coating the enzyme solution on the surface of the surfactant-modified PANI electrode. Parent PANI/GOD, PANI-Tween 20/GOD and PANI-Tween 40/GOD electrodes exhibit a response to varying glucose concentrations whereas PANI with the other surfactants does not respond to glucose. This result can be explained in two ways. First, Tween 20 and Tween 40 compounds can link covalently to the amine groups of GOD because of their carbonyl group (C O). This type of bonding can create a structure more stable than that attained by physical aggregation. Both glutaraldehyde crosslinker and Tween 20 and Tween 40 surfactants having C O groups in the PANI-modified enzyme electrode have reinforced compact tertiary structures resulting in enzyme stabilization for immobilizing. Second, PANI has a high electroactivity in the presence of Tween 20 and Tween 40 relative to SDS and SPS, as seen from Fig. 3. This electroactivity and current ranges could be useful for enzyme sensor behaviours. However, the maximum current of PANI-TTAB is 0.3 µA which is ca 104 times higher than those of PANI-Tween 20 and PANI-Tween 40. After enzyme immobilization, PANI-TTAB becomes less electroactive because TTAB does not have functional groups to increase the interaction between PANI and GOD enzyme. It has been reported that the polythiophene/SiO2 system has high GOD enzyme activity in the presence of non-ionic surfactant Tween 20,19 and that PANI has good cholesterol biosensor properties in the presence of non-ionic Triton X100.18 Figure 6(a) shows the current response versus time in the presence of various glucose concentrations for PANI-Tween 20/GOD. The linear dynamic ranges and equations of the response (a)

Equation

Response time (s)

y = 478.67x + 6.0217 y = 1655.8x + 2.4932 y = 492x + 2.2937

20 30 40

time and regression coefficient values of the electrodes are given in Table 4 and Fig. 6(b). It is evident from our data that higher response times are observed for electrode materials prepared in the presence of surfactants. The Michaelis–Menten constant (Km ) for the enzyme immobilized on PANI-Tween 20 is higher than that for the parent PANI or PANI-Tween 40, possibly due to the diffusional limitation imposed on the flow of substrate and product molecules by the fibrous layers of the grafted PANIs.33 SEM results indicate that PANI-Tween 20 has the most fibrillar structure, which affects diffusion, and this could be the reason for the observed results. According to the Lineweaver–Burk form of the Michaelis–Menten equation, the relation between the reciprocal of response current and the reciprocal of glucose concentration is linear. Generally, Km is used to evaluate enzyme activity. As enzyme loading increases, a larger fraction of the GOD locates near to the surface of the electrode, so that the average enzyme–substrate binding increases, with Km decreasing accordingly.33,34 Km values were obtained from Lineweaver–Burk plots.35 The Km values of GOD entrapped on the electrodes modified with parent PANI, PANITween 20 or PANI-Tween 40 indicate a high affinity for glucose which is comparable to that of modified electrodes based on Prussian blue and PANI/GOD31 or gold nanoparticle/conductive PANI nanocomposite36 and others.24,26,36 – 38 The sensor response y (in terms of current density) to concentration x (mmol L−1 ) for the experimental data is well described by a linear function. As an example, for PANI-Tween 20, y = 1655.8x + 2.4932 (R2 = 0.9954). All parameters are listed in Table 4, and Fig. 6(b) shows that the sensor between 0.7 and 10–12.0 mmol L−1 glucose (R2 = 0.9244, 0.9954, 0.9951) gives a linear response with respect to analyte concentration, and a short response time (20–40 s),39 – 44 which suggests that our sensing (b)

Polym Int 2010; 59: 1650–1659



1657

Figure 7. (A) SEM images of (a) parent PANI and (b) PANI-TTAB. (B) SEM images: (a) parent PANI before GOD immobilization; (b) parent PANI after GOD immobilization; (c) PANI-Tween 20 before GOD immobilization; (d) PANI-Tween 20 after GOD immobilization.

www.soci.org (a)

(b)

(c)

(d)

A Kuczynska et al.

Figure 7. (Continued).

1658

materials could be viable candidates for use in glucose-sensing electrodes. The morphology of the parent PANI and surfactant-modified PANI powder samples was investigated using SEM. Surfactantfree PANI (Fig. 7A(a)) has a more aggregated structure than the surfactant-modified PANI. PANI-Tween 40 and PANI-TTAB (Fig. 7A(b)) have a more globular nanostructure. The difference in morphology of the PANI samples is caused by the different molecular structure of the surfactants used in polymerization. Various reasons are given in the literature for these types of morphology.10 Additionally, SEM micrographs of dispersed samples were compared for glucose biosensor applications (Fig. 7B). For the parent PANI (Figs 7B(a) and (b)) and PANI-Tween 20 (Figs 7B(c) and (d)), the analysis of SEM images after GOD


immobilization confirms that the surface of the PANI nanofibres is covered with GOD and also reveals aggregated shapes.

CONCLUSIONS The use of surfactants in conjunction with aniline polymerization has a clear influence on the thermal effect of the polymerization reaction with the heat of polymerization increasing in the following order: anionic SDS < non-ionic < cationic < without surfactant < anionic SPS. In our opinion the difference between the two anionic surfactants (SPS and SDS) is caused by the size of the hydrophobic part of the surfactant molecules. The analysis of the products, particularly spectral and TGA investigations, suggests incorporation of the surfactant into the


Polym Int 2010; 59: 1650–1659

Effects of surfactants on biosensing properties of PANI PANI structure. The electrical conductivity of the doped PANI is very much surfactant-dependent, with conductivity increase in the case of PANI-Tween 20 and PANI-TTAB, and decrease in the case of PANI-Tween 40, PANI-SDS and PANI-SPS relative to the surfactant-free PANI material. It was found that the PANITTAB (cationic surfactant) has the highest conductivity among all samples. The PANI samples synthesized using surfactants can be used for the fabrication of novel glucose biosensors with short response times. The amperometric response of enzyme-coated electrodes to glucose was enhanced when PANI was used with the non-ionic surfactant. The highest linear range from 0.7 to 12.0 mmol L−1 of glucose was obtained for the PANI-Tween 20 electrode.

ACKNOWLEDGEMENTS The Scientific and Technological Research Council of Turkey is acknowledged for funding this work (project no. 107T880). The work at the Faculty of Chemistry, University of Warsaw, was supported by grants 501/68-BW-179220 and 501/64-BST-143121.

REFERENCES 1 Kong J, Franklin NR, Zhou Ch, Chapline MG, Peng S, Cho K, et al, Science 287:622–625 (2000). 2 Bobacka J, Ivaska A and Lewenstam A, Electroanalysis 15:366–374 (2003). 3 McQuade DT, Pullen AE and Swager TM, Chem Rev 100:2537–2574 (2000). 4 Malhotra BD and Singhal R, Pramana J Phys 61:331–343 (2003). 5 Zhao XY, Zang JB, Wang YH, Bian LY and Yu JK, Electrochem Commun 11:1297–1300 (2009). 6 Xing S, Zhao C, Jing S and Wang Z, Polymer 47:2305–2313 (2006). 7 Li D and Kaner R, J Am Chem Soc 128:968–975 (2006). 8 Chiou NR and Epstein AJ, Adv Mater 17:1679–1683 (2005). 9 Zhang Z, Wei Z and Wan M, Macromolecules 35:5937–5942 (2002). 10 Wei Z, Zhang Z and Wan M, Langmuir 18:917–921 (2002). 11 Li G and Zhang Z, Macromolecules 37:2683–2685 (2004). 12 Li G, Peng H, Wang Y, Qin Y, Cui Z and Zhang Z, Mater Lett 63:592–594 (2009). 13 Zhang D and Wang Y, Mater Sci Eng. 134:9–19 (2006). 14 Mav I and Zigon M, Synth Met 119:145–146 (2001).

www.soci.org ¨ A, Synth Met 157:235–242 (2007). 15 Yavuz AG and Gok ¨ A, Omastová M and Yavuz AG, Synth Met 157:23–29 (2007). 16 Gok 17 Stejskal J, Omastová M, Fedorova S, Prokeˇs J and Trchová M, Polymer 44:1353–1358 (2003). 18 Khan R, Solanki PR, Kaushik A, Singh SP, Ahmad S and Malhotra BD, J Polym Res 16:363–373 (2009). 19 Uygun A, Yavuz AG, Sen S and Omastova M, Synth Met 159:19–20 (2009). 20 Bhadra S, Singha NK and Rubber DK, Pol J Vet Sci 104:1900–1904 (2007). 21 Hino T, Namiki T and Kuramoto N, Synth Met 56:1327–1332 (2006). 22 Guimard NK, Gomez N and Schmidt CE, Prog Polym Sci 32:876–921 (2007). 23 Ayad MM, Amer WA and Stejskal J, Thin Solid Films 517:5969–5973 (2009). 24 Chunming Y, Zheng F and Pingmin Z, J Central South Univ Technol 6:127–129 (1999). ´ ´ 25 Rubio Retama J, Lopez Cabarcos E, Mecerreyes D and Lopez-Ruiz B, Biosens Bioelectron 20:1111–1117 (2004). 26 Rannou P and Nechtschein M, Synth Met 84:755–756 (1997). ˇ enková I, Trchová M, Blinova NV and Stejskal J, Thin Solid Films 27 Sedˇ 515:1640–1646 (2006). 28 Fu Y and Elsenbaumer RL, Chem Mater 6:671–677 (1994). 29 Ansari R and Keivani MB, E J Chem 3:202–217 (2006). 30 Ding L, Wang X and Gregory RV, Synth Met 104:73–78 (1999). 31 Garjonyte R and Malinauskas A, BiosensBioelectron 15:445–451 (2000). 32 Gerard M, Chaubey A and Malhotra BD, Biosens Bioelectron 17:345–359 (2002). ˇ G, Karakıs¸la M, Altıntas¸ B, Metin AU, Saçak M and 33 Bayramoglu Arıca MY, Process Biochem 44:880–885 (2009). 34 Borole DD, Kapadi UR, Mahulikar PP and Hundiwale DG, Polym Adv Technol 15:306–312 (2004). 35 Lineweaver H and Burk D, J Am Chem Soc 56:658–666 (1934). 36 Xian Y, Hu Y, Liu F, Xian Y, Wang H and Jin L, Biosens Bioelectron 21:1996–2000 (2006). 37 Wei D and Ivaska A, Chem Anal (Warsaw) 51:839–852 (2006). 38 Pumera M, Sanchez S, Ichinose I and Tang J, Sens Actuators B 123:1195–1205 (2007). 39 Ahuja T, Ahmad Mir I, Kumar D and Rajesh K, Biomaterials 28:791–805 (2007). 40 MacDiarmid AG, Synth Met 125:11–22 (2002). 41 Liu G and Freund MS, Macromolecules 30:5660–5665 (1997). 42 Xue H, Shen Z and Li Y, Synth Met 124:345–349 (2001). 43 Blinova NV, Stejskal J, Trchova M and Prokes J, Polymer 47:42–48 (2006). 44 Ozden M, Turk J Chem 23:89–98 (1999).

1659

Polym Int 2010; 59: 1650–1659

