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Polyaniline nanofiber reinforced nanocomposite coated quartz crystal microbalance based highly sensitive free radical sensors
Sensors and Actuators B 171–172 (2012) 924–931
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Polyaniline nanofiber reinforced nanocomposite coated quartz crystal microbalance based highly sensitive free radical sensors Somik Banerjee, Dimpul Konwar, A. Kumar ∗ Materials Research Laboratory, Department of Physics, Tezpur University, Tezpur 784028, Assam, India
a r t i c l e
i n f o
Article history: Received 8 December 2011 Received in revised form 29 May 2012 Accepted 3 June 2012 Available online 9 June 2012 Keywords: Polyaniline Quartz crystal microbalance Free radical sensors FTIR XRD Dielectric spectroscopy
a b s t r a c t AT-cut quartz crystals with gold electrodes have been coated with PAni nanofiber reinforced PVA nanocomposite thin films for determination of free radicals in solution. The frequency shifts of PAni nanofiber reinforced PVA nanocomposite modified quartz crystal electrode upon exposure to different concentrations of free radicals have been monitored and found to vary linearly with free radical concentration. A sensitivity factor of 133.42, 140.13 and 129.13 Hz ppb−1 for the DPPH, hydroxyl and peroxyl free radicals, respectively within the concentration range of 500–10000 ppb has been observed for direct exposure. The response time has been found to be in the range of 0.5–2.36 s and decreases with the increasing concentration of the analyte. The sensing mechanism of the free radical sensor has been investigated by XRD, FTIR, impedance and ac conductivity spectra of the PAni nanofiber reinforced PVA nanocomposite films before and after exposure to different concentration of the analytes. The results indicate that upon exposure to the analyte, PAni nanofiber reinforced PVA nanocomposites get oxidized by losing hydrogen leading to decrease in mass with a corresponding increase in frequency of the quartz crystal electrode that makes the sensor highly sensitive to free radicals in solution up to ppb level. © 2012 Elsevier B.V. All rights reserved.
1. Introduction: Free radicals can be defined as molecules or molecular fragments with an unpaired electron. The unpaired electron gives certain characteristic properties to the free radical such as paramagnetism and high chemical reactivity [1]. Oxidation by free radicals leads to the degradation of rubber and other polymer products [2]. Deterioration of fats and oils in foodstuffs is also caused by free radical mediated oxidation [3]. In biological systems, free radicals are generated as by-products of complex biochemical reactions such as homolytic bond fission or electron transfer reactions taking place inside the living systems. In general, such reactions proceed either by the absorption of ionizing, ultra-violet, visible, thermal radiation or by redox reactions such as non-enzymic electron transfer reactions, metal catalyzed reactions or enzyme-catalyzed processes. The impact of ionizing radiation on biological material can produce a complex variety of free radical products; ionizing radiation produces mainly H• , OH• and e− aq. when directed onto aqueous solutions [4]. These primary free radicals and the hydrated electrons can interact readily with neighbouring biomolecules. The free radical damage to the living tissue is already well established and recent studies indicate that the progression of various diseases
∗ Corresponding author. Tel.: +91 3712 275553; fax: +91 3712267006. E-mail address: [email protected] (A. Kumar). 0925-4005/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2012.06.005
such as inflammation, infection, cardiac and cerebral ischemia, reperfusion of the ischemic heart leading to potentially lethal arrhythmias, neurodegenerative diseases, cardiovascular diseases, cancer and aging are caused by the uncontrolled oxidation of lipids, proteins and DNA in biological systems [5,6]. Thus detection of free radicals both in vitro and in vivo has become very important in medical science. Techniques employing spin trapping viz., electron spin resonance (ESR) [7] and NMR [8] have been used for detection of free radicals. Scientists have also used chemical labelling by quenching with free radicals, e.g. with nitric oxide (NO) or DPPH (1,1-diphenyl-2-picrylhydrazyl), followed by spectroscopic methods like X-ray photoelectron spectroscopy (XPS) or absorption spectroscopy for the determination of free radicals [9]. But all these techniques are very sophisticated, expensive and time consuming. Thus, the development of cheap and reliable sensors for the detection of very small levels of free radicals is highly desirable and can have immense application in biomedical science, petroleum, polymer and rubber industries. Conducting polymers such as polypyrrole (PPy), polyaniline (PAni), polythiophene (PTh) and their derivatives, have been used as the active layers for different type of sensors since early 1980s [10–13]. In comparison with sensors based on metal oxides that are operated at high temperatures, the sensors made of conducting polymers have many improved characteristics. They have high sensitivities and short response time even at room temperature. Conducting polymers are easy to be synthesized through
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chemical or electrochemical processes, and their molecular chain structure can be modified conveniently by copolymerization or structural derivations. Furthermore, conducting polymers have good mechanical properties, which allow facile fabrication of sensors. As a result, increasing attention has been paid to the sensors fabricated from conducting polymers. The fact that conducting polymers exhibit changes in colour, conductivity, volume, mass, mechanical properties and ion permeability upon doping makes them efficient sensing materials. Being redox active, conducting polymers have the capability of neutralizing free radicals. The emeraldine salt form of polyaniline (PAni) has been found to exhibit excellent free radical scavenging property since they can neutralize the free radical by donating hydrogen atom and thereby changing their oxidation state from emeraldine to pernigraniline [14,15]. It has been observed that as compared to the bulk, PAni nanofibers have even better free radical scavenging property and has been attributed to the availability of more surface reaction sites for scavenging free radicals [15]. The QCM comprises a thin vibrating AT-cut quartz wafer sandwiched between two metal excitation electrodes. When a small amount of mass is adsorbed at the quartz electrode surface, the frequency of the quartz is changed according to the well-known Sauerbrey equation (Eq. (1)) [16]: 2
F =
−2f0 m A(q dq )
1/2
(1)
where F is the measured frequency shift, f0 the original oscillation frequency of the dry crystal, m the mass change, A the piezoelectrically active area of the excitation electrodes, dq the density of quartz and q the shear modulus. Although a surface coating in no way resembles an infinite fluid surrounding the crystal, it is expected that a coating will increase the viscous drag on the face of the crystal and affect its resonant frequency. It has been reported that viscosity plays an important role when the crystal is immersed in a liquid [17]. When the surface of a quartz crystal electrode is coated by a material capable of interaction with the environment of interest, a sensor sensitive to this component can be constructed. The performance characteristics of the QCM sensor (such as selectivity, response time and reversibility) will depend on the chemical nature and physical properties of the coating material. Quartz crystals, coated with various coatings, have been used for adsorption and determination of various compounds [18–23]. In the present study, 5 MHz AT-cut quartz resonators coated with polyaniline nanofiber reinforced PVA nanocomposites have been applied for the continuous monitoring of free radical concentrations in a liquid medium. The effects of increasing free radical concentration on the structure and conformation of the PAni nanofiber reinforced PVA nanocomposite have been investigated using XRD and FTIR, and the charge transport and relaxation mechanisms have been studied using dielectric spectroscopy to have a better insight into the mechanism of free radical sensing by the PAni nanofiber reinforced PVA nanocomposite modified quartz resonators. 2. Experimental details Aniline (p.a Merck) has been distilled under reduced pressure prior to use. All other chemicals are analytical grade reagents. Processible polyaniline (PAni) nanofiber reinforced PVA nanocomposite films are synthesized by in situ rapid mixing polymerization in the medium consisting of the water-soluble non-conducting polymer [polyvinyl alcohol (PVA)] and plasticizer. A plasticizer glycerol has been added at a certain concentration to confer plasticity and flexibility to the films. PVA has been dissolved in milli-Q water after heating at 80 ◦ C with the addition of glycerol. After
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the solution is cooled down to room temperature, the monomer (aniline), oxidant and dopant are added all at the same time and the solution is rapidly mixed (>3000 rpm) for 24 h. Hydrochloric acid (HCl) has been used as the dopant while ammonium peroxydisulfate [APS (NH4 )2 S2 O8 ] as the oxidant during the synthesis. The monomer to the oxidant ratio has been maintained at 2:1. HCl has been added to the solution to maintain a pH < 3. The solution gradually turns green indicating the formation of PAni nanofibers. The solution is then heated to make it viscous and spin coated over the quartz crystals with gold electrodes purchased from Stanford Research System (SRS). The measurements of the frequency shifts as a sensitive response to different concentrations of the free radicals have been acquired using a QCM apparatus (QCM200) also purchased from Stanford Research Systems (SRS). The sensor has not been exposed to a control since the basic idea of fabricating this sensor was the continuous online monitoring of free radicals in solution. Once the sensor response is obtained for a given concentration over a fixed time interval, the sensor output has been brought to zero level using the Lab-view based QCM software (SRS-QCM). The sensor gets ready again for sensing after a typical recovery time on the order of 3–5 min. The response time has been determined from the time required for the sensor to go from the zero level to the next stable state after exposure to the analyte and not from the time required to go to the zero level. The AT cut quartz crystal supplied by Stanford Research Systems (SRS) oscillates at a frequency of 5 MHz, which reduces to about 2.8 MHz after modification with PAni nanofibers reinforced PVA nanocomposites. The response further decreases when medium changes from air to methanol/aqueous. The sensor response has been allowed to become stable and the blank methanol/aqueous background has been subtracted by the software (SRS-QCM). Subsequently, the sensor has been exposed to different concentrations of the free radicals. The morphology of the PAni nanofiber reinforced PVA nanocomposite coated on the quartz crystals with gold electrodes have been studied using a JEOL JSM 6390 LV model scanning electron microscope (SEM), while the size of the PAni nanofibers formed within the PVA matrix have been investigated using a JEOL TEM CX-II transmission electron microscope (TEM). The polymer-coated electrodes have been exposed to different concentrations (500 ppb–10 ppm) of a standard stable free radical, 1,1-diphenyl-2-picrylhydrazil (DPPH) dissolved in HPLC grade methanol (MeOH). The PAni nanofiber reinforced PVA nanocomposite quartz crystal electrodes have also been exposed to different concentrations of peroxyl (ROO• ) free radicals to investigate whether non-oxidizing free radicals can be monitored by the sensor. 2,2-Azobis-isobutyronitrile (AIBN) has been used as the peroxyl radical generator [24]. To evaluate the applicability of the sensor in biological media, sensing of hydroxyl radical (OH• ) radicals in aqueous medium has also been investigated in the present work. The peroxyl radicals are generated in aqueous solutions following the well-known Fenton’s reaction using the pro-oxidant H2 O2 [15]. Since hydrogen peroxide is generated in vivo by several oxidase enzymes in the living body and directly or indirectly leads to the formation of hydroxyl radicals, using H2 O2 as the hydroxyl radical generator resembles to a biological system. In order to have a better understanding of the sensing mechanism, the modifications in the structure and conformation of the active PAni nanofiber reinforced PVA nanocomposite coating of the quartz crystals upon exposure to all the free radicals have been analysed. X-ray diffraction patterns before and after sensing DPPH have been collected using a Rigaku Miniflex diffractometer with Cu K␣ ˚ and FTIR spectroscopy of the nanocomposradiation ( = 1.5406 A) ite films before and after sensing free radicals have been acquired using a Perkin Elmer spectrum 100 FTIR spectrometer. The charge transport mechanism and the conductivity relaxations have been
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Fig. 2. Response curve of the PAni nanofiber reinforced PVA nanocomposite modified QCM sensor after exposure to different concentration of free radicals (DPPH, hydroxyl and peroxyl free radicals) in solution.
Fig. 1. (a) Transmission electron micrograph and (b) scanning electron micrograph of the PAni nanofiber reinforced PVA nanocomposites.
investigated using the ac conductivity data obtained using a Hioki 3532-50 LCR meter. 3. Results and discussion Quartz crystals with gold electrodes have been coated using an aqueous solution of polyaniline (PAni) nanofiber reinforced PVA nanocomposites employing the spin coating technique. The morphology of the PAni nanofiber reinforced PVA nanocomposite films coated on the quartz crystals have been investigated by electron microscopy. Fig. 1a and b shows the transmission electron micrograph (TEM) and the scanning electron micrograph of the PAni nanofiber reinforced PVA nanocomposite films coated on the quartz crystals using spin-coating technique. The diameter of the PAni nanofibers formed within the PVA matrix can, however, be determined using the transmission electron micrograph (Fig. 1a). It is observed that PAni nanofibers having average diameters in the range of 30 nm are formed after the rapid mixing polymerization reaction. The scanning electron micrograph (SEM) of the sample clearly shows randomly oriented fiber like structures of PAni. Though it is not possible to exactly determine the size of the nanofibers, it gives an idea of the surface morphology of the PAni nanofiber reinforced PVA nanocomposites coated on the quartz crystals. The response curves of quartz crystals coated with PAni nanofiber reinforced PVA nanocomposite for different concentration of 1,1-diphenyl-2-picrylhydrazil (DPPH), hydroxyl (OH• ) and
peroxyl (ROO• ) free radicals in solution are shown in Fig. 2. It is observed that the frequency of the crystal increases sharply with increase in concentration of free radicals irrespective of the oxidizing and non-oxidizing free radicals. However, it has been observed that the modified quartz crystals are more sensitive to the DPPH free radicals as compared to the hydroxyl and peroxyl free radicals. This response shows that the PAni nanofiber reinforced PVA nanocomposite coated quartz electrode is sensitive to very low concentrations of free radicals. According to the Sauerbrey equation (Eq. (1)), the increase in the frequency indicates that the mass of the quartz crystal decreases after reacting with the free radicals in the solution. We have not considered the viscosity effect primarily because the polymer coatings used in the present work are ultrathin (∼70 nm) and of the same composition prepared under similar conditions [25]. Moreover, for complex fluids, the viscosity is shear-rate dependent, thus, the relevant frequency for QCM measurements is on the order of MHz, whereas most conventional rheometers are capable of measuring responses up to several hundred Hz only. Thus, it is possible that the measured viscosity from a rheometer at low frequencies (<102 Hz) may be irrelevant for the QCM measurements (>106 Hz). It has also been observed that viscosity effects are more pronounced when the entire quartz crystals are coated [17], however in the present case only one surface of the crystal has been coated. Thus, in the present work, the Sauerbrey’s equation (Eq. (1)) has been considered to be valid and the mass change as a function of time upon exposure to different concentrations of the free radicals determined from Eq. (1) is displayed in Fig. 3. It is observed that the mass of the quartz crystal decreases with the increase in the analyte concentration. However, in order to construct a sensor a number of factors such as the linearity, sensitivity, the limit of detection and the response time have also been investigated and optimized. It is highly desirable that the response obtained by a sensor should be linear against different concentration of the analytes. In order to investigate this, the PAni nanofiber reinforced PVA nanocomposite coated quartz crystal electrodes have been directly exposed to various concentrations of free radicals from 500 ppb to 10 ppm. The frequency shifts as a function of concentration initially show a linear behaviour from 500 ppb to 1000 ppb but beyond that the frequency shifts gradually decrease and in spite of being linear in the concentration range above 1000 ppb (i.e. up to 10 ppm), the slope of the linear curve decreases. Fig. 4 shows the calibration curves constructed by plotting the frequency shifts against the concentration of the
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Fig. 3. Variation of mass of the PAni nanofiber reinforced PVA nanocomposite coated quartz crystal upon exposure to different concentrations of free radicals (DPPH, hydroxyl and peroxyl free radicals) in solution.
three types of free radicals in solution investigated in the present work within the concentration range of 500–1000 ppb. The inset of Fig. 4 shows the frequency shift in the concentration range of 1000 ppb–10 ppm. Two different slopes have been observed in the plots. Thus the linearity of the sensor decreases with increase in concentration of the analyte. The sensitivity is defined as the slope of the calibration graph [26]. Since two slopes have been observed from the calibration curve in different concentration regimes and there is a decrease in the slope with increasing concentration, there is a decrease in the sensitivity with increasing concentration. However, it has been observed that the sensor is linear in two different concentration regimes viz., 500–1000 ppb and 1000 ppb–10 ppm. The sensitivity of the PAni nanofibers reinforced PVA nanocomposites modified quartz crystals have been determined separately in these two regimes and are tabulated in Table 1. The observed decrease in linearity and sensitivity can be attributed to the fact that after reacting with a free radical viz., DPPH, benzenoid ring in the PAni chains get oxidized to quinoid ring with the elimination of one hydrogen atom [15]. PAni nanofibers used as coatings for the quartz crystals are in the emeraldine form and upon exposure to highly reactive
Fig. 4. Calibration curve for determination of free radicals in solution within the concentration range of 500–1000 ppb. Inset shows the calibration curve for the concentration range of 1000 ppb–10 ppm. The frequency shifts of a PAni nanofiber reinforced PVA nanocomposite modified quartz crystal electrode in direct contact to the free radical solutions have been recorded after 2.5 min of exposure.
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Fig. 5. Representative plots for calculating the response time of the sensor for different concentrations of free radicals (DPPH) in solution.
free radicals, get reduced to the pernigraniline form. When the concentration of the free radicals is increased the potency of the PAni nanofibers to donate hydrogen atom decreases as they continuously undergo a transition from emeraldine to pernigraniline form. This is the reason why the sensitivity of the sensor and also the linearity decreases after the sensor is exposed to concentrations (>1000 ppb). Since the sensor fabricated in the present work was ideally designed for continuous on-line monitoring of free radicals in solution, we have not used a control solution to restore the PAni nanofibers reinforced PVA nanocomposite coating to their pristine state. However, for discrete monitoring of free radicals the coating can be easily restored to its original condition for sensing purpose by immersing in a 1 M HCl solution for about 24 h followed by washing it thoroughly. The response time can be defined as the time required for a sensor output to change from its previous state to a final settled value within a tolerance band of the correct new value. Fig. 5 shows a diagram to illustrate the methodology adopted for the calculation of response times of the sensor to different concentrations of free radicals in solution with response to DPPH as a standard. Fig. 5 shows the time required by the PAni nanofiber reinforced PVA nanocomposite coated QCM sensors to reach a stable final output value from the initial value after reacting with different concentrations of DPPH. The response times of the sensor for sensing different concentration of the three types of free radicals investigated in the present work has been tabulated in Table 1. It has been observed that the response time of the sensor decreases as the concentration of the free radical in solution increases from 500 ppb to 10 ppm. However, in order to support the above hypothesis and understand the possible mechanism for which PAni nanofiber reinforced PVA nanocomposites act as an active layer for the quartz crystal while sensing free radicals, some more experiments have been conducted. The mechanism of free radical sensing by PAni nanofiber reinforced PVA nanocomposites have been investigated by studying the variations in the X-ray diffraction patterns, FTIR spectra and the impedance and ac conductivity spectra of the nanocomposites before and after reaction with different concentrations of free radicals in the range of 500 ppb–10 ppm. Results for the standard free radical DPPH have been reported in the present paper to visualize the variations in the active layer upon interaction with the analyte that can provide vital information about the sensing mechanism. Fig. 6a shows the X-ray diffraction patterns of the PAni nanofibers reinforced PVA nanocomposites before and after exposure to different concentration of analytes in the 2 range of 10–40◦ . The (1 0 0) reflection peak of PAni centered at 2 = 20◦ can be
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Table 1 Response time and sensitivity of the PAni nanofibers reinforced PVA nanocomposites coated QCM sensor for different concentrations of free radicals. Concentration (ppb)
500 600 700 800 900 1000 5000 10000
Sensitivity (Hz ppb−1 )
Response time (s) DPPH
Hydroxyl
Peroxyl
DPPH
Hydroxyl
Peroxyl
1.231 1.092 0.959 0.909 0.779 0.731 0.653 0.567
1.724 1.617 1.582 1.501 1.326 1.213 1.095 0.951
2.362 2.147 1.983 1.764 1.538 1.472 1.315 1.205
133.42 (500–1000 ppb)
140.13 (500–1000 ppb)
129.13 (500–1000 ppb)
28.41 (1000 ppb–10 ppm)
19.01 (1000 ppb–10 ppm)
15.87 (1000 ppb–10 ppm)
observed in all the patterns. In order to have a deeper understanding of the X-ray diffraction patterns, a single line approximation method [27] has been used for calculating the domain length (L) and the strain (ε) in the PAni nanofiber reinforced PVA nanocomposite samples before and after reaction with different concentrations of the analyte DPPH. Fig. 6b displays the X-ray diffraction patterns in the range of 2 = 18–24◦ . The inset of Fig. 6b shows the variation of the normalized integral intensity (I/I0 ) of the PAni nanofibers reinforced PVA nanocomposites as a function of analyte concentration. The variation in the domain length and strain in the PAni nanofiber reinforced PVA nanocomposites as a result of increasing the concentration of the analyte has been presented in Table 2. It has been observed that the domain length (L) i.e. the local range of order within the PAni chains in the PAni nanofiber reinforced PVA nanocomposite samples decreases as the concentration of the analyte increases, while the strain is found to increase. Table 2 also depicts the variation in the d-spacing of the (1 0 0) reflection corresponding to the parallel periodicity of PAni chains in the nanofibers before and after exposure to increasing analyte concentrations. The thickness of the films is also given in Table 2. It is observed that the d-spacing increases as the analyte concentration increases. This indicates that the -stacking between the PAni chains decreases [28]. Decrease in the normalized integral intensity with increasing analyte concentration confirms the decreasing degree of crystallinity of the PAni nanofibers reinforced PVA nanocomposites upon exposure to DPPH. The FTIR spectrum of the PAni nanofiber reinforced PVA nanocomposites before and after reaction with the analyte is shown in Fig. 7. The band around 3300 cm−1 is attributed to the N H stretching vibrations while that at 1650 cm−1 is a signature of the N H bending vibration of polyaniline (PAni). The peak observed at 815 cm−1 is ascribed to the N H out of plane bending vibration. The strong band observed at 1140 cm−1 and the band at 1200 cm−1 are due to the C C stretching and C C twisting of the alkyl chain. The C N stretching peak of the polymer is observed at 1336 cm−1 . The C Cl stretching vibration observed around 600 cm−1 confirms the fact that the PAni nanofibers are in doped states. The vibrational bands around 1460 and 1400 cm−1 are assigned to C C stretching vibration of the quinoid and benzenoid ring of PAni, respectively.
The major variation in the FTIR spectra has been observed for the peaks corresponding to the C C stretching vibrations of the para di-substituted benzene (benzenoid) and the quinone diimine (quinoid). It has been observed that the C C stretching peak due to the benzenoid ring has decreased significantly whereas a corresponding increase in the C C stretching peak due to the quinoid ring has increased but the increase is not that significant as can be observed from Fig. 8. However, the decrease in the intensity of the C C stretching vibration for the PAni nanofiber reinforced PVA nanocomposites after reaction with free radicals indicates that the conjugation length in the nanocomposites decrease with the increase in the concentration of the analyte. A decrease in the conjugation length is an indication of the fact that the conductivity of the material decreases. In order to prove this fact the variations in the impedance spectra and the ac conductivity spectra of the PAni nanofiber reinforced PVA nanocomposites before and after exposure to the free radicals have been studied. In order to analyze the conductivity relaxation mechanisms in the PAni nanofibers reinforced PVA nanocomposites before and after exposure to the DPPH analyte, the imaginary part (Z ) of the complex impedance (Z∗ ) as a function of frequency have been taken into consideration. Fig. 9 shows the variation of Z as a function of frequency for the PAni nanofiber reinforced PVA nanocomposites after exposure to different concentrations of DPPH in solution. The inset of Fig. 9 shows the variation of the imaginary part (Z ) of the complex impedance (Z∗ ) for the pristine sample. It is known that PAni exhibits two conductivity relaxation peaks in the Z spectra depending upon the amount of benzenoid and quinoid structures present in the PAni chain [29]. The lower frequency peak can be attributed to the phase of oxidized repeat units (quinoid units) and the higher frequency peak to the phase of reduced repeat units (benzenoid units) of PAni [29]. Z vs. log ω spectra has been fitted with the following equation in order to determine the conductivity relaxation time:
Z =
n i=1
Zi [(ω 0i )1−˛i cos(˛i /2)]
(2)
1 + 2(ω 0i )1−˛i sin(˛i /2) + (ω 0i )2(1−˛i )
Table 2 Comparison of the domain length (L) and the strain (ε), d-spacings in the PAni nanofiber reinforced PVA nanocomposites before and after exposure to different concentrations of DPPH free radicals. Thickness of the films has also been tabulated. Concentration of free radical (DPPH) in ppb
Domain length (L) (Å)
Strain (ε)
d-Spacings (Å)
Thickness (nm)
0 500 600 700 800 900 1000 5000 10000
22.66 20.18 18.21 17.13 13.45 10.11 9.82 9.23 8.91
1.16 1.45 1.91 2.43 2.61 3.01 3.14 3.54 3.71
4.25 4.26 4.28 4.30 4.32 4.35 4.36 4.48 4.49
65.9 63.5 70.4 62.8 66.4 71.3 64.4 68.6 72.5
± ± ± ± ± ± ± ± ±
2.2 5.1 3.7 0.9 0.7 2.6 2.1 1.6 1.5
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Fig. 8. Comparison of the intensity of the C C stretching vibration due to the benzenoid and quinoid structures of the PAni nanofiber reinforced PVA nanocomposites before and after exposure to DPPH in solution.
Fig. 6. (a) X-ray diffraction pattern of the PAni nanofiber reinforced PVA nanocomposite over the 2 range from 10◦ to 40◦ and (b) Variation of the (1 0 0) reflection peak of PAni before and after exposure to different concentration of DPPH free radical in solution.
Fig. 7. FTIR spectra of PAni nanofiber reinforced PVA nanocomposites before and after exposure to different concentrations of DPPH in solution.
Z is the contribution of each conductivity relaxation mechanism to the real part of the complex impedance, Z = Ri . ω is the angular frequency, ˛i is a parameter which is correlated with the mean width of the curve of each mechanism and obtains values of 0 ≤ ˛i < 1. The parameter 0i = 1/2f0i describes the mean relaxation time of each conductivity relaxation mechanism and corresponds to the frequency of maximum Z value. It is evident from Fig. 9 that as the concentration of the analyte increases the relaxation peak observed in the Z spectra shifts from higher frequency to lower frequency region and the value of the imaginary part Z of complex impedance (Z∗ ) increases by almost 106 times, which is an indication of the fact that there is indeed a transformation from the benzenoid to quinoid structure in the PAni chains upon exposure to the analyte and the transformation varies directly with concentration of the analyte. In fact the peak due to benzenoid units is no longer observed in the impedance spectra of the PAni nanofibers reinforced PVA nanocomposites after exposure to the analyte. Table 3 shows the conductivity relaxation times obtained from the best fits of the experimental data in Fig. 9 to Eq. (2). We find that there is an increase in the relaxation time with increasing analyte concentration. Since relaxation
Fig. 9. Z spectra of the PAni nanofiber reinforced PVA nanocomposites before and after exposure to different concentrations of DPPH free radical in solution. The solid lines are the best fits of the experimental data according to Eq. (2).
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Table 3 Values of relaxation times ( Q and B ), dc conductivity, s parameter and obtained from the best fits of the experimental data of imaginary impedance and ac conductivity with Eqs. (2) and (3). Table 2 also presents the value of dielectric constants (ε ) at 50 Hz for the nanocomposites films before and after exposure to different concentrations of the free radical. Concentration (ppb)
Relaxation time (s)
a b
“s” parameter
ε (at 50 Hz and 303 K)
5.13 × 10−3 5.02 × 10−6 4.19 × 10−6 5.39 × 10−7 4.24 × 10−8 – – – –
0.81 0.92 0.91 0.88 0.97 – – – –
82 61 45 41 35 26 16 8 4
B b
Q a 0 500 600 700 800 900 1000 5000 10000
d.c. (S/cm)
−4
4.63 × 10 5.24 × 10−6 6.21 × 10−6 9.87 × 10−6 2.38 × 10−4 2.58 × 10−4 1.30 × 10−3 6.41 × 10−2 3.82 × 10−2
1.66 × 10−7 – – – – – – – –
Q stands for conductivity relaxation time corresponding to the quinoid unit.
B stands for conductivity relaxation time corresponding to the benzenoid unit.
is related to charge transport by hopping mechanism, an increase in relaxation time indicates the increase in hopping length with consequent reduction in the conductivity [30]. The plot of ac conductivity of the PAni nanofiber reinforced PVA nanocomposites before and after exposure to the analyte is shown in Fig. 10. The ac conductivity of disordered materials follows the Joncher’s law [31] given by Eq. (3) = d.c. + Aωs
(3)
where is the total conductivity is the sum of the frequency independent dc conductivity given by d.c. and the frequency dependent conductivity a.c. = Aωs , where s is the frequency exponent which generally lies between 0 and 1, A is proportionality constant. However, in some cases it has been observed that “s” can perhaps have values more than 1 [32]. The experimentally acquired data have been fitted to the Eq. (3) and it has been observed that the experimental data show a reasonably good fit to Eq. (3), however the ac conductivity spectra of the sample exposed to 900 ppb of DPPH and beyond show deviations from the ideal curve at lower frequencies (<10 kHz); which may be ascribed to the electrode polarization effects. The origin of electrode polarization in this case is not pretty clear but it may be assumed that with increasing free radical concentration in solution there are morphological changes in the PAni nanofiber reinforced PVA nanocomposites. The ac conductivity in general obeys the universal power law with the exponent “s” varying from 0 to 1 as can be seen from Table 3; however, as obtained
Fig. 10. Ac conductivity spectra of the PAni nanofiber reinforced PVA nanocomposite before and after exposure to different concentration of DPPH in solution. The solid lines are the best fits of the experimental data according to Eq. (3).
from the best fits of the experimental data there is a decrease in the value of dc conductivity (Table 3) as the concentration of free radicals are increased, which may be ascribed to the decrease in conjugation length in the PAni nanofiber reinforced PVA nanocomposites upon exposure to different concentrations of free radicals due to the partial benzenoid to quinoid transformation. This result is corroborated by the decrease in the degree of crystallinity and stacking obtained from the X-ray diffraction analysis. The increase in the conductivity relaxation time also supports the fact that the conductivity of the PAni nanofibers reinforced PVA nanocomposites decreases with increasing analyte concentration.
4. Conclusions PAni nanofiber reinforced PVA nanocomposites synthesized using rapid mixing polymerization has been used to modify 5 MHz AT cut quartz crystals to act as sensors. It has been observed that these modified QCM sensors can be used to detect ppb level of free radicals in solution. The linearity of the sensor has been found to be excellent with a linear regression coefficient of 0.9685 within the concentration range of 500–1000 ppb. Beyond 1000 ppb, although the response of the sensor is linear but there is a decrease in the slope. The sensitivity of the sensor has been found to be 133.42, 140.13 and129.13 Hz ppb−1 for the DPPH, hydroxyl and peroxyl free radicals, respectively within the concentration range of 500–1000 ppb. However, the linearity and sensitivity of the sensor decreases beyond that concentration since due to continuous monitoring the active sensing layer loses its potential to donate hydrogen atom to neutralize the free radicals. The sensitivity and linearity can be restored by using a control of 1 M HCl solution if continuous detection of free radicals is not desired. The response time of the sensor has been determined and it is observed that the response time decreases with the increase in the analyte concentration. It has been observed that the local range of order in the PAni nanofiber reinforced PVA nanocomposite decreases upon exposure to different concentration of the analyte which indicates that the material amorphizes after reacting with free radicals. FTIR spectra analysis reveals a partial transformation from benzenoid to quinoid structures in the PAni chains that can significantly decrease the conjugation length and effect charge conductivity through the material. The impedance plots also indicate that with the increase in the concentration of the analyte, the benzenoid structures get oxidized to the quinoid structure and as such a shift in the relaxation peak towards lower frequency region is observed. The relaxation time has been found to increase indicating an enhancement in the carrier hopping length. The ac conductivity plots show reasonably good fit with the universal power law and it has been observed that the dc conductivity of the PAni nanofiber
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reinforced PVA nanocomposites obtained from the best fits of the experimental data decreases with the increase in the concentration of the analyte. QCM based sensors modified with PAni nanofiber reinforced PVA nanocomposites can be a very cost effective and sustainable method for the detection of harmful free radicals in food and polymer products and also for in vitro sensing of free radicals from biological systems. However, the applicability of these sensors in real biological media needs to be investigated before realizing the potential of these sensors in biomedical engineering. Work is underway for using and optimizing these sensors for detection of free radicals in food and biological systems. Acknowledgements The authors acknowledge the financial assistance provided by DST (vide Project No. SR/S2/CMP-28/2008). Help extended by the staff of the Electron microscopy division, SAIF, Shillong for TEM is also acknowledged. References [1] A. Boveris, Biochemistry of free radicals: from electron to tissue, Medicine (Buenos Aires) 58 (1998) 350–356. [2] A.V. Tobolsky, A. Mercurio, On the catalyzed oxidative degradation of natural rubber networks, Journal of the American Chemical Society 81 (1959) 5539–5540. [3] G. Scott, Atmospheric Oxidation and Antioxidants, vol. I–III, Elsevier, Amsterdam, 1993. [4] P.A. Riley, Free radicals in biology: oxidative stress and the effects of ionizing radiation, International Journal of Radiation Biology 65 (1994) 27–33. [5] B. Halliwell, Antioxidants in human health and disease, Annual Review of Nutrition 16 (1996) 33–50. [6] O.I. Aruoma, B. Halliwell, E. Gajewski, M. Dizdaroglu, Damage to the bases in DNA induced by hydrogen peroxide and ferric ion chelates, Journal of Biological Chemistry 264 (1989) 20509. [7] J.L. Zweier, P. Kuppusamy, Electron paramagnetic resonance measurements of free radicals in the intact beating heart: a technique for detection and characterization of free radicals in whole biological tissues, Proceedings of the National Academy of Sciences of the United States of America 85 (1988) 5703–5707. [8] V. Khramtsov, L.J. Berliner, T.L. Clanton, NMR spin trapping: detection of free radical reactions using a phosphorus-containing nitrone spin trap, Magnetic Resonance in Medicine 42 (1999) 228–234. [9] P. Molyneux, The use of the stable free radical diphenylpicrylhydrazyl (DPPH) for estimating antioxidant activity, Songklanakarin Journal of Science and Technology 26 (2004) 211–219. [10] H. Bai, G. Shi, Gas sensors based on conjugated polymers, Sensors 7 (2007) 267–307. [11] D. Nicolas-Debarnot, F. Poncin-Epaillard, Polyaniline as a new sensitive layer for gas sensors, Analytica Chimica Acta 475 (2003) 1–15. [12] Q. Ameer, S.B. Adeloju, Polypyrrole-based electronic noses for environmental and industrial analysis, Sensors and Actuators, B 106 (2005) 541–552. [13] S.T. McGovern, G.M. Spinks, G.G. Wallace, Micro-humidity sensors based on a processable polyaniline blend, Sensors and Actuators, B 107 (2005) 657–665. [14] M. Gizdavic-Nikolaidis, J. Travas-Sejdic, P.A. Kilmartin, G.A. Bowmaker, R.P. Cooney, Evaluation of antioxidant activity of aniline and polyaniline, Current Applied Physics 4 (2004) 343–346. [15] S. Banerjee, J.P. Saikia, A. Kumar, B.K. Konwar, Antioxidant activity and haemolysis prevention efficiency of polyaniline nanofibers, Nanotechnology 21 (2010) 045101 (8pp). [16] G. Sauerbrey, The use of quartz oscillators for weighing thin layers and for microweighing, Zeitschrift fur Physik 155 (1959) 206–222. [17] D. James, D.V. Thiel, G.R. Bushell, W.K. Busfield, A. Mackay-Sim, Phase change and viscosity effects on a quartz crystal microbalance, Analyst 119 (1994) 2005–2007. [18] X. Wang, B. Ding, M. Sun, J. Yu, G. Sun, Nanofibrous polyethyleneimine membranes as sensitive coatings for quartz crystal microbalance-based formaldehyde sensors, Sensors and Actuators, B 144 (2010) 11–17. [19] M. Matsuguchi, Y. Kadowaki, Poly(acrylamide) derivatives for QCM-based HCl gas sensor applications, Sensors and Actuators, B 130 (2008) 842–847. [20] P.-G. Su, Y.-P. Chang, Low-humidity sensor based on a quartz-crystal microbalance coated with polypyrrole/Ag/TiO2 nanoparticles composite thin films, Sensors and Actuators, B 129 (2008) 915–920.
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[21] P. Si, J. Mortensen, A. Komolov, J. Denborg, P.J. Møller, Polymer coated quartz crystal microbalance sensors for detection of volatile organic compounds in gas mixtures, Analytica Chimica Acta 597 (2007) 223–230. [22] X. Wang, B. Ding, J. Yu, M. Wang, F. Pan, A highly sensitive humidity sensor based on a nanofibrous membrane coated quartz crystal microbalance, Nanotechnology 21 (2010) 055502 (6pp). [23] V.V.R. Sai, S. Mahajan, A.Q. Contractor, S. Mukherji, Immobilization of antibodies on polyaniline films and its application in a piezoelectric immunosensor, Analytical Chemistry 78 (2006) 8368–8373. [24] V. Krishnamachari, L.H. Levine, P.W. Pare, Flavonoid oxidation by the radical generator AIBN: a unified mechanism for quercetin radical scavenging, Journal of Agricultural and Food Chemistry 50 (2002) 4357–4363. [25] B.D. Vogt, E.K. Lin, W.-L. Wu, C.C. White, Effect of film thickness on the validity of the Sauerbrey equation for hydrated polyelectrolyte films, Journal of Physical Chemistry B 108 (2004) 12685–12690. [26] J.C. Miller, J.N. Miller, Statistics for Analytical Chemistry, second ed., Ellis Horwood, UK, 1988, p. 117. [27] Th. H. de Keijer, J.I. Langford, E.J. Mittemeijer, A.B.P. Vogels, Use of the voigt function in a single-line method for the analysis of X-ray diffraction line broadening, Journal of Applied Crystallography 15 (1982) 308–314. [28] A. Kumar, S. Banerjee, J.P. Saikia, B.K. Konwar, Swift heavy ion irradiation induced enhancement in the antioxidant activity and biocompatibility of polyaniline nanofibers, Nanotechnology 21 (2010) 8, 175102. [29] A.B. Afzal, M.J. Akhtar, M. Nadeem, M.M. Hassan, Investigation of structural and electrical properties of polyaniline/gold nanocomposites, Journal of Physical Chemistry C 113 (2009) 17560–17565. [30] Y.T. Ravikiran, M.T. Lagare, M. Sairam, N.N. Mallikarjuna, B. Sreedhar, S. Manohar, A.G. MacDiarmid, T.M. Aminabhavi, Synthesis, characterization and low frequency AC conduction of polyaniline/niobium pentoxide composites, Synthetic Metals 156 (2006) 1139–1147. [31] A.K. Jonscher, Dielectric Relaxation in Solids, Dielectric Press, London, 1983. [32] A.N. Papathanassiou, I. Sakellis, J. Grammatikakis, Universal frequencydependent ac conductivity of conducting polymer networks, Applied Physics Letters 91 (2007) 122911.
Biographies
Mr. Somik Banerjee did his Master degree (M.Sc.) in Physics from Tezpur University. He is presently doing research in the field of conducting polymer based nanostructured materials under the supervision of Prof. A. Kumar in the Materials Research Laboratory, Department of Physics, Tezpur University, Assam, India.
Mr. Dimpul Konwar is a M.Sc. student in Nanoscience and Technology in the Department of Physics, Tezpur University. He is presently pursuing his M.Sc. project work under the supervision of Prof. A. Kumar.
Prof. Ashok Kumar did his M.Tech. and Ph.D. from IIT Kanpur in Materials Science. He has published more than seventy papers in national and international journals. He has published eight book chapters and edited one book on nanofibers. Presently, he is a Professor in the Department of Physics, Tezpur University.
Contents lists available at SciVerse ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Polyaniline nanofiber reinforced nanocomposite coated quartz crystal microbalance based highly sensitive free radical sensors Somik Banerjee, Dimpul Konwar, A. Kumar ∗ Materials Research Laboratory, Department of Physics, Tezpur University, Tezpur 784028, Assam, India
a r t i c l e
i n f o
Article history: Received 8 December 2011 Received in revised form 29 May 2012 Accepted 3 June 2012 Available online 9 June 2012 Keywords: Polyaniline Quartz crystal microbalance Free radical sensors FTIR XRD Dielectric spectroscopy
a b s t r a c t AT-cut quartz crystals with gold electrodes have been coated with PAni nanofiber reinforced PVA nanocomposite thin films for determination of free radicals in solution. The frequency shifts of PAni nanofiber reinforced PVA nanocomposite modified quartz crystal electrode upon exposure to different concentrations of free radicals have been monitored and found to vary linearly with free radical concentration. A sensitivity factor of 133.42, 140.13 and 129.13 Hz ppb−1 for the DPPH, hydroxyl and peroxyl free radicals, respectively within the concentration range of 500–10000 ppb has been observed for direct exposure. The response time has been found to be in the range of 0.5–2.36 s and decreases with the increasing concentration of the analyte. The sensing mechanism of the free radical sensor has been investigated by XRD, FTIR, impedance and ac conductivity spectra of the PAni nanofiber reinforced PVA nanocomposite films before and after exposure to different concentration of the analytes. The results indicate that upon exposure to the analyte, PAni nanofiber reinforced PVA nanocomposites get oxidized by losing hydrogen leading to decrease in mass with a corresponding increase in frequency of the quartz crystal electrode that makes the sensor highly sensitive to free radicals in solution up to ppb level. © 2012 Elsevier B.V. All rights reserved.
1. Introduction: Free radicals can be defined as molecules or molecular fragments with an unpaired electron. The unpaired electron gives certain characteristic properties to the free radical such as paramagnetism and high chemical reactivity [1]. Oxidation by free radicals leads to the degradation of rubber and other polymer products [2]. Deterioration of fats and oils in foodstuffs is also caused by free radical mediated oxidation [3]. In biological systems, free radicals are generated as by-products of complex biochemical reactions such as homolytic bond fission or electron transfer reactions taking place inside the living systems. In general, such reactions proceed either by the absorption of ionizing, ultra-violet, visible, thermal radiation or by redox reactions such as non-enzymic electron transfer reactions, metal catalyzed reactions or enzyme-catalyzed processes. The impact of ionizing radiation on biological material can produce a complex variety of free radical products; ionizing radiation produces mainly H• , OH• and e− aq. when directed onto aqueous solutions [4]. These primary free radicals and the hydrated electrons can interact readily with neighbouring biomolecules. The free radical damage to the living tissue is already well established and recent studies indicate that the progression of various diseases
∗ Corresponding author. Tel.: +91 3712 275553; fax: +91 3712267006. E-mail address: [email protected] (A. Kumar). 0925-4005/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2012.06.005
such as inflammation, infection, cardiac and cerebral ischemia, reperfusion of the ischemic heart leading to potentially lethal arrhythmias, neurodegenerative diseases, cardiovascular diseases, cancer and aging are caused by the uncontrolled oxidation of lipids, proteins and DNA in biological systems [5,6]. Thus detection of free radicals both in vitro and in vivo has become very important in medical science. Techniques employing spin trapping viz., electron spin resonance (ESR) [7] and NMR [8] have been used for detection of free radicals. Scientists have also used chemical labelling by quenching with free radicals, e.g. with nitric oxide (NO) or DPPH (1,1-diphenyl-2-picrylhydrazyl), followed by spectroscopic methods like X-ray photoelectron spectroscopy (XPS) or absorption spectroscopy for the determination of free radicals [9]. But all these techniques are very sophisticated, expensive and time consuming. Thus, the development of cheap and reliable sensors for the detection of very small levels of free radicals is highly desirable and can have immense application in biomedical science, petroleum, polymer and rubber industries. Conducting polymers such as polypyrrole (PPy), polyaniline (PAni), polythiophene (PTh) and their derivatives, have been used as the active layers for different type of sensors since early 1980s [10–13]. In comparison with sensors based on metal oxides that are operated at high temperatures, the sensors made of conducting polymers have many improved characteristics. They have high sensitivities and short response time even at room temperature. Conducting polymers are easy to be synthesized through
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chemical or electrochemical processes, and their molecular chain structure can be modified conveniently by copolymerization or structural derivations. Furthermore, conducting polymers have good mechanical properties, which allow facile fabrication of sensors. As a result, increasing attention has been paid to the sensors fabricated from conducting polymers. The fact that conducting polymers exhibit changes in colour, conductivity, volume, mass, mechanical properties and ion permeability upon doping makes them efficient sensing materials. Being redox active, conducting polymers have the capability of neutralizing free radicals. The emeraldine salt form of polyaniline (PAni) has been found to exhibit excellent free radical scavenging property since they can neutralize the free radical by donating hydrogen atom and thereby changing their oxidation state from emeraldine to pernigraniline [14,15]. It has been observed that as compared to the bulk, PAni nanofibers have even better free radical scavenging property and has been attributed to the availability of more surface reaction sites for scavenging free radicals [15]. The QCM comprises a thin vibrating AT-cut quartz wafer sandwiched between two metal excitation electrodes. When a small amount of mass is adsorbed at the quartz electrode surface, the frequency of the quartz is changed according to the well-known Sauerbrey equation (Eq. (1)) [16]: 2
F =
−2f0 m A(q dq )
1/2
(1)
where F is the measured frequency shift, f0 the original oscillation frequency of the dry crystal, m the mass change, A the piezoelectrically active area of the excitation electrodes, dq the density of quartz and q the shear modulus. Although a surface coating in no way resembles an infinite fluid surrounding the crystal, it is expected that a coating will increase the viscous drag on the face of the crystal and affect its resonant frequency. It has been reported that viscosity plays an important role when the crystal is immersed in a liquid [17]. When the surface of a quartz crystal electrode is coated by a material capable of interaction with the environment of interest, a sensor sensitive to this component can be constructed. The performance characteristics of the QCM sensor (such as selectivity, response time and reversibility) will depend on the chemical nature and physical properties of the coating material. Quartz crystals, coated with various coatings, have been used for adsorption and determination of various compounds [18–23]. In the present study, 5 MHz AT-cut quartz resonators coated with polyaniline nanofiber reinforced PVA nanocomposites have been applied for the continuous monitoring of free radical concentrations in a liquid medium. The effects of increasing free radical concentration on the structure and conformation of the PAni nanofiber reinforced PVA nanocomposite have been investigated using XRD and FTIR, and the charge transport and relaxation mechanisms have been studied using dielectric spectroscopy to have a better insight into the mechanism of free radical sensing by the PAni nanofiber reinforced PVA nanocomposite modified quartz resonators. 2. Experimental details Aniline (p.a Merck) has been distilled under reduced pressure prior to use. All other chemicals are analytical grade reagents. Processible polyaniline (PAni) nanofiber reinforced PVA nanocomposite films are synthesized by in situ rapid mixing polymerization in the medium consisting of the water-soluble non-conducting polymer [polyvinyl alcohol (PVA)] and plasticizer. A plasticizer glycerol has been added at a certain concentration to confer plasticity and flexibility to the films. PVA has been dissolved in milli-Q water after heating at 80 ◦ C with the addition of glycerol. After
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the solution is cooled down to room temperature, the monomer (aniline), oxidant and dopant are added all at the same time and the solution is rapidly mixed (>3000 rpm) for 24 h. Hydrochloric acid (HCl) has been used as the dopant while ammonium peroxydisulfate [APS (NH4 )2 S2 O8 ] as the oxidant during the synthesis. The monomer to the oxidant ratio has been maintained at 2:1. HCl has been added to the solution to maintain a pH < 3. The solution gradually turns green indicating the formation of PAni nanofibers. The solution is then heated to make it viscous and spin coated over the quartz crystals with gold electrodes purchased from Stanford Research System (SRS). The measurements of the frequency shifts as a sensitive response to different concentrations of the free radicals have been acquired using a QCM apparatus (QCM200) also purchased from Stanford Research Systems (SRS). The sensor has not been exposed to a control since the basic idea of fabricating this sensor was the continuous online monitoring of free radicals in solution. Once the sensor response is obtained for a given concentration over a fixed time interval, the sensor output has been brought to zero level using the Lab-view based QCM software (SRS-QCM). The sensor gets ready again for sensing after a typical recovery time on the order of 3–5 min. The response time has been determined from the time required for the sensor to go from the zero level to the next stable state after exposure to the analyte and not from the time required to go to the zero level. The AT cut quartz crystal supplied by Stanford Research Systems (SRS) oscillates at a frequency of 5 MHz, which reduces to about 2.8 MHz after modification with PAni nanofibers reinforced PVA nanocomposites. The response further decreases when medium changes from air to methanol/aqueous. The sensor response has been allowed to become stable and the blank methanol/aqueous background has been subtracted by the software (SRS-QCM). Subsequently, the sensor has been exposed to different concentrations of the free radicals. The morphology of the PAni nanofiber reinforced PVA nanocomposite coated on the quartz crystals with gold electrodes have been studied using a JEOL JSM 6390 LV model scanning electron microscope (SEM), while the size of the PAni nanofibers formed within the PVA matrix have been investigated using a JEOL TEM CX-II transmission electron microscope (TEM). The polymer-coated electrodes have been exposed to different concentrations (500 ppb–10 ppm) of a standard stable free radical, 1,1-diphenyl-2-picrylhydrazil (DPPH) dissolved in HPLC grade methanol (MeOH). The PAni nanofiber reinforced PVA nanocomposite quartz crystal electrodes have also been exposed to different concentrations of peroxyl (ROO• ) free radicals to investigate whether non-oxidizing free radicals can be monitored by the sensor. 2,2-Azobis-isobutyronitrile (AIBN) has been used as the peroxyl radical generator [24]. To evaluate the applicability of the sensor in biological media, sensing of hydroxyl radical (OH• ) radicals in aqueous medium has also been investigated in the present work. The peroxyl radicals are generated in aqueous solutions following the well-known Fenton’s reaction using the pro-oxidant H2 O2 [15]. Since hydrogen peroxide is generated in vivo by several oxidase enzymes in the living body and directly or indirectly leads to the formation of hydroxyl radicals, using H2 O2 as the hydroxyl radical generator resembles to a biological system. In order to have a better understanding of the sensing mechanism, the modifications in the structure and conformation of the active PAni nanofiber reinforced PVA nanocomposite coating of the quartz crystals upon exposure to all the free radicals have been analysed. X-ray diffraction patterns before and after sensing DPPH have been collected using a Rigaku Miniflex diffractometer with Cu K␣ ˚ and FTIR spectroscopy of the nanocomposradiation ( = 1.5406 A) ite films before and after sensing free radicals have been acquired using a Perkin Elmer spectrum 100 FTIR spectrometer. The charge transport mechanism and the conductivity relaxations have been
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Fig. 2. Response curve of the PAni nanofiber reinforced PVA nanocomposite modified QCM sensor after exposure to different concentration of free radicals (DPPH, hydroxyl and peroxyl free radicals) in solution.
Fig. 1. (a) Transmission electron micrograph and (b) scanning electron micrograph of the PAni nanofiber reinforced PVA nanocomposites.
investigated using the ac conductivity data obtained using a Hioki 3532-50 LCR meter. 3. Results and discussion Quartz crystals with gold electrodes have been coated using an aqueous solution of polyaniline (PAni) nanofiber reinforced PVA nanocomposites employing the spin coating technique. The morphology of the PAni nanofiber reinforced PVA nanocomposite films coated on the quartz crystals have been investigated by electron microscopy. Fig. 1a and b shows the transmission electron micrograph (TEM) and the scanning electron micrograph of the PAni nanofiber reinforced PVA nanocomposite films coated on the quartz crystals using spin-coating technique. The diameter of the PAni nanofibers formed within the PVA matrix can, however, be determined using the transmission electron micrograph (Fig. 1a). It is observed that PAni nanofibers having average diameters in the range of 30 nm are formed after the rapid mixing polymerization reaction. The scanning electron micrograph (SEM) of the sample clearly shows randomly oriented fiber like structures of PAni. Though it is not possible to exactly determine the size of the nanofibers, it gives an idea of the surface morphology of the PAni nanofiber reinforced PVA nanocomposites coated on the quartz crystals. The response curves of quartz crystals coated with PAni nanofiber reinforced PVA nanocomposite for different concentration of 1,1-diphenyl-2-picrylhydrazil (DPPH), hydroxyl (OH• ) and
peroxyl (ROO• ) free radicals in solution are shown in Fig. 2. It is observed that the frequency of the crystal increases sharply with increase in concentration of free radicals irrespective of the oxidizing and non-oxidizing free radicals. However, it has been observed that the modified quartz crystals are more sensitive to the DPPH free radicals as compared to the hydroxyl and peroxyl free radicals. This response shows that the PAni nanofiber reinforced PVA nanocomposite coated quartz electrode is sensitive to very low concentrations of free radicals. According to the Sauerbrey equation (Eq. (1)), the increase in the frequency indicates that the mass of the quartz crystal decreases after reacting with the free radicals in the solution. We have not considered the viscosity effect primarily because the polymer coatings used in the present work are ultrathin (∼70 nm) and of the same composition prepared under similar conditions [25]. Moreover, for complex fluids, the viscosity is shear-rate dependent, thus, the relevant frequency for QCM measurements is on the order of MHz, whereas most conventional rheometers are capable of measuring responses up to several hundred Hz only. Thus, it is possible that the measured viscosity from a rheometer at low frequencies (<102 Hz) may be irrelevant for the QCM measurements (>106 Hz). It has also been observed that viscosity effects are more pronounced when the entire quartz crystals are coated [17], however in the present case only one surface of the crystal has been coated. Thus, in the present work, the Sauerbrey’s equation (Eq. (1)) has been considered to be valid and the mass change as a function of time upon exposure to different concentrations of the free radicals determined from Eq. (1) is displayed in Fig. 3. It is observed that the mass of the quartz crystal decreases with the increase in the analyte concentration. However, in order to construct a sensor a number of factors such as the linearity, sensitivity, the limit of detection and the response time have also been investigated and optimized. It is highly desirable that the response obtained by a sensor should be linear against different concentration of the analytes. In order to investigate this, the PAni nanofiber reinforced PVA nanocomposite coated quartz crystal electrodes have been directly exposed to various concentrations of free radicals from 500 ppb to 10 ppm. The frequency shifts as a function of concentration initially show a linear behaviour from 500 ppb to 1000 ppb but beyond that the frequency shifts gradually decrease and in spite of being linear in the concentration range above 1000 ppb (i.e. up to 10 ppm), the slope of the linear curve decreases. Fig. 4 shows the calibration curves constructed by plotting the frequency shifts against the concentration of the
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Fig. 3. Variation of mass of the PAni nanofiber reinforced PVA nanocomposite coated quartz crystal upon exposure to different concentrations of free radicals (DPPH, hydroxyl and peroxyl free radicals) in solution.
three types of free radicals in solution investigated in the present work within the concentration range of 500–1000 ppb. The inset of Fig. 4 shows the frequency shift in the concentration range of 1000 ppb–10 ppm. Two different slopes have been observed in the plots. Thus the linearity of the sensor decreases with increase in concentration of the analyte. The sensitivity is defined as the slope of the calibration graph [26]. Since two slopes have been observed from the calibration curve in different concentration regimes and there is a decrease in the slope with increasing concentration, there is a decrease in the sensitivity with increasing concentration. However, it has been observed that the sensor is linear in two different concentration regimes viz., 500–1000 ppb and 1000 ppb–10 ppm. The sensitivity of the PAni nanofibers reinforced PVA nanocomposites modified quartz crystals have been determined separately in these two regimes and are tabulated in Table 1. The observed decrease in linearity and sensitivity can be attributed to the fact that after reacting with a free radical viz., DPPH, benzenoid ring in the PAni chains get oxidized to quinoid ring with the elimination of one hydrogen atom [15]. PAni nanofibers used as coatings for the quartz crystals are in the emeraldine form and upon exposure to highly reactive
Fig. 4. Calibration curve for determination of free radicals in solution within the concentration range of 500–1000 ppb. Inset shows the calibration curve for the concentration range of 1000 ppb–10 ppm. The frequency shifts of a PAni nanofiber reinforced PVA nanocomposite modified quartz crystal electrode in direct contact to the free radical solutions have been recorded after 2.5 min of exposure.
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Fig. 5. Representative plots for calculating the response time of the sensor for different concentrations of free radicals (DPPH) in solution.
free radicals, get reduced to the pernigraniline form. When the concentration of the free radicals is increased the potency of the PAni nanofibers to donate hydrogen atom decreases as they continuously undergo a transition from emeraldine to pernigraniline form. This is the reason why the sensitivity of the sensor and also the linearity decreases after the sensor is exposed to concentrations (>1000 ppb). Since the sensor fabricated in the present work was ideally designed for continuous on-line monitoring of free radicals in solution, we have not used a control solution to restore the PAni nanofibers reinforced PVA nanocomposite coating to their pristine state. However, for discrete monitoring of free radicals the coating can be easily restored to its original condition for sensing purpose by immersing in a 1 M HCl solution for about 24 h followed by washing it thoroughly. The response time can be defined as the time required for a sensor output to change from its previous state to a final settled value within a tolerance band of the correct new value. Fig. 5 shows a diagram to illustrate the methodology adopted for the calculation of response times of the sensor to different concentrations of free radicals in solution with response to DPPH as a standard. Fig. 5 shows the time required by the PAni nanofiber reinforced PVA nanocomposite coated QCM sensors to reach a stable final output value from the initial value after reacting with different concentrations of DPPH. The response times of the sensor for sensing different concentration of the three types of free radicals investigated in the present work has been tabulated in Table 1. It has been observed that the response time of the sensor decreases as the concentration of the free radical in solution increases from 500 ppb to 10 ppm. However, in order to support the above hypothesis and understand the possible mechanism for which PAni nanofiber reinforced PVA nanocomposites act as an active layer for the quartz crystal while sensing free radicals, some more experiments have been conducted. The mechanism of free radical sensing by PAni nanofiber reinforced PVA nanocomposites have been investigated by studying the variations in the X-ray diffraction patterns, FTIR spectra and the impedance and ac conductivity spectra of the nanocomposites before and after reaction with different concentrations of free radicals in the range of 500 ppb–10 ppm. Results for the standard free radical DPPH have been reported in the present paper to visualize the variations in the active layer upon interaction with the analyte that can provide vital information about the sensing mechanism. Fig. 6a shows the X-ray diffraction patterns of the PAni nanofibers reinforced PVA nanocomposites before and after exposure to different concentration of analytes in the 2 range of 10–40◦ . The (1 0 0) reflection peak of PAni centered at 2 = 20◦ can be
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Table 1 Response time and sensitivity of the PAni nanofibers reinforced PVA nanocomposites coated QCM sensor for different concentrations of free radicals. Concentration (ppb)
500 600 700 800 900 1000 5000 10000
Sensitivity (Hz ppb−1 )
Response time (s) DPPH
Hydroxyl
Peroxyl
DPPH
Hydroxyl
Peroxyl
1.231 1.092 0.959 0.909 0.779 0.731 0.653 0.567
1.724 1.617 1.582 1.501 1.326 1.213 1.095 0.951
2.362 2.147 1.983 1.764 1.538 1.472 1.315 1.205
133.42 (500–1000 ppb)
140.13 (500–1000 ppb)
129.13 (500–1000 ppb)
28.41 (1000 ppb–10 ppm)
19.01 (1000 ppb–10 ppm)
15.87 (1000 ppb–10 ppm)
observed in all the patterns. In order to have a deeper understanding of the X-ray diffraction patterns, a single line approximation method [27] has been used for calculating the domain length (L) and the strain (ε) in the PAni nanofiber reinforced PVA nanocomposite samples before and after reaction with different concentrations of the analyte DPPH. Fig. 6b displays the X-ray diffraction patterns in the range of 2 = 18–24◦ . The inset of Fig. 6b shows the variation of the normalized integral intensity (I/I0 ) of the PAni nanofibers reinforced PVA nanocomposites as a function of analyte concentration. The variation in the domain length and strain in the PAni nanofiber reinforced PVA nanocomposites as a result of increasing the concentration of the analyte has been presented in Table 2. It has been observed that the domain length (L) i.e. the local range of order within the PAni chains in the PAni nanofiber reinforced PVA nanocomposite samples decreases as the concentration of the analyte increases, while the strain is found to increase. Table 2 also depicts the variation in the d-spacing of the (1 0 0) reflection corresponding to the parallel periodicity of PAni chains in the nanofibers before and after exposure to increasing analyte concentrations. The thickness of the films is also given in Table 2. It is observed that the d-spacing increases as the analyte concentration increases. This indicates that the -stacking between the PAni chains decreases [28]. Decrease in the normalized integral intensity with increasing analyte concentration confirms the decreasing degree of crystallinity of the PAni nanofibers reinforced PVA nanocomposites upon exposure to DPPH. The FTIR spectrum of the PAni nanofiber reinforced PVA nanocomposites before and after reaction with the analyte is shown in Fig. 7. The band around 3300 cm−1 is attributed to the N H stretching vibrations while that at 1650 cm−1 is a signature of the N H bending vibration of polyaniline (PAni). The peak observed at 815 cm−1 is ascribed to the N H out of plane bending vibration. The strong band observed at 1140 cm−1 and the band at 1200 cm−1 are due to the C C stretching and C C twisting of the alkyl chain. The C N stretching peak of the polymer is observed at 1336 cm−1 . The C Cl stretching vibration observed around 600 cm−1 confirms the fact that the PAni nanofibers are in doped states. The vibrational bands around 1460 and 1400 cm−1 are assigned to C C stretching vibration of the quinoid and benzenoid ring of PAni, respectively.
The major variation in the FTIR spectra has been observed for the peaks corresponding to the C C stretching vibrations of the para di-substituted benzene (benzenoid) and the quinone diimine (quinoid). It has been observed that the C C stretching peak due to the benzenoid ring has decreased significantly whereas a corresponding increase in the C C stretching peak due to the quinoid ring has increased but the increase is not that significant as can be observed from Fig. 8. However, the decrease in the intensity of the C C stretching vibration for the PAni nanofiber reinforced PVA nanocomposites after reaction with free radicals indicates that the conjugation length in the nanocomposites decrease with the increase in the concentration of the analyte. A decrease in the conjugation length is an indication of the fact that the conductivity of the material decreases. In order to prove this fact the variations in the impedance spectra and the ac conductivity spectra of the PAni nanofiber reinforced PVA nanocomposites before and after exposure to the free radicals have been studied. In order to analyze the conductivity relaxation mechanisms in the PAni nanofibers reinforced PVA nanocomposites before and after exposure to the DPPH analyte, the imaginary part (Z ) of the complex impedance (Z∗ ) as a function of frequency have been taken into consideration. Fig. 9 shows the variation of Z as a function of frequency for the PAni nanofiber reinforced PVA nanocomposites after exposure to different concentrations of DPPH in solution. The inset of Fig. 9 shows the variation of the imaginary part (Z ) of the complex impedance (Z∗ ) for the pristine sample. It is known that PAni exhibits two conductivity relaxation peaks in the Z spectra depending upon the amount of benzenoid and quinoid structures present in the PAni chain [29]. The lower frequency peak can be attributed to the phase of oxidized repeat units (quinoid units) and the higher frequency peak to the phase of reduced repeat units (benzenoid units) of PAni [29]. Z vs. log ω spectra has been fitted with the following equation in order to determine the conductivity relaxation time:
Z =
n i=1
Zi [(ω 0i )1−˛i cos(˛i /2)]
(2)
1 + 2(ω 0i )1−˛i sin(˛i /2) + (ω 0i )2(1−˛i )
Table 2 Comparison of the domain length (L) and the strain (ε), d-spacings in the PAni nanofiber reinforced PVA nanocomposites before and after exposure to different concentrations of DPPH free radicals. Thickness of the films has also been tabulated. Concentration of free radical (DPPH) in ppb
Domain length (L) (Å)
Strain (ε)
d-Spacings (Å)
Thickness (nm)
0 500 600 700 800 900 1000 5000 10000
22.66 20.18 18.21 17.13 13.45 10.11 9.82 9.23 8.91
1.16 1.45 1.91 2.43 2.61 3.01 3.14 3.54 3.71
4.25 4.26 4.28 4.30 4.32 4.35 4.36 4.48 4.49
65.9 63.5 70.4 62.8 66.4 71.3 64.4 68.6 72.5
± ± ± ± ± ± ± ± ±
2.2 5.1 3.7 0.9 0.7 2.6 2.1 1.6 1.5
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Fig. 8. Comparison of the intensity of the C C stretching vibration due to the benzenoid and quinoid structures of the PAni nanofiber reinforced PVA nanocomposites before and after exposure to DPPH in solution.
Fig. 6. (a) X-ray diffraction pattern of the PAni nanofiber reinforced PVA nanocomposite over the 2 range from 10◦ to 40◦ and (b) Variation of the (1 0 0) reflection peak of PAni before and after exposure to different concentration of DPPH free radical in solution.
Fig. 7. FTIR spectra of PAni nanofiber reinforced PVA nanocomposites before and after exposure to different concentrations of DPPH in solution.
Z is the contribution of each conductivity relaxation mechanism to the real part of the complex impedance, Z = Ri . ω is the angular frequency, ˛i is a parameter which is correlated with the mean width of the curve of each mechanism and obtains values of 0 ≤ ˛i < 1. The parameter 0i = 1/2f0i describes the mean relaxation time of each conductivity relaxation mechanism and corresponds to the frequency of maximum Z value. It is evident from Fig. 9 that as the concentration of the analyte increases the relaxation peak observed in the Z spectra shifts from higher frequency to lower frequency region and the value of the imaginary part Z of complex impedance (Z∗ ) increases by almost 106 times, which is an indication of the fact that there is indeed a transformation from the benzenoid to quinoid structure in the PAni chains upon exposure to the analyte and the transformation varies directly with concentration of the analyte. In fact the peak due to benzenoid units is no longer observed in the impedance spectra of the PAni nanofibers reinforced PVA nanocomposites after exposure to the analyte. Table 3 shows the conductivity relaxation times obtained from the best fits of the experimental data in Fig. 9 to Eq. (2). We find that there is an increase in the relaxation time with increasing analyte concentration. Since relaxation
Fig. 9. Z spectra of the PAni nanofiber reinforced PVA nanocomposites before and after exposure to different concentrations of DPPH free radical in solution. The solid lines are the best fits of the experimental data according to Eq. (2).
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Table 3 Values of relaxation times ( Q and B ), dc conductivity, s parameter and obtained from the best fits of the experimental data of imaginary impedance and ac conductivity with Eqs. (2) and (3). Table 2 also presents the value of dielectric constants (ε ) at 50 Hz for the nanocomposites films before and after exposure to different concentrations of the free radical. Concentration (ppb)
Relaxation time (s)
a b
“s” parameter
ε (at 50 Hz and 303 K)
5.13 × 10−3 5.02 × 10−6 4.19 × 10−6 5.39 × 10−7 4.24 × 10−8 – – – –
0.81 0.92 0.91 0.88 0.97 – – – –
82 61 45 41 35 26 16 8 4
B b
Q a 0 500 600 700 800 900 1000 5000 10000
d.c. (S/cm)
−4
4.63 × 10 5.24 × 10−6 6.21 × 10−6 9.87 × 10−6 2.38 × 10−4 2.58 × 10−4 1.30 × 10−3 6.41 × 10−2 3.82 × 10−2
1.66 × 10−7 – – – – – – – –
Q stands for conductivity relaxation time corresponding to the quinoid unit.
B stands for conductivity relaxation time corresponding to the benzenoid unit.
is related to charge transport by hopping mechanism, an increase in relaxation time indicates the increase in hopping length with consequent reduction in the conductivity [30]. The plot of ac conductivity of the PAni nanofiber reinforced PVA nanocomposites before and after exposure to the analyte is shown in Fig. 10. The ac conductivity of disordered materials follows the Joncher’s law [31] given by Eq. (3) = d.c. + Aωs
(3)
where is the total conductivity is the sum of the frequency independent dc conductivity given by d.c. and the frequency dependent conductivity a.c. = Aωs , where s is the frequency exponent which generally lies between 0 and 1, A is proportionality constant. However, in some cases it has been observed that “s” can perhaps have values more than 1 [32]. The experimentally acquired data have been fitted to the Eq. (3) and it has been observed that the experimental data show a reasonably good fit to Eq. (3), however the ac conductivity spectra of the sample exposed to 900 ppb of DPPH and beyond show deviations from the ideal curve at lower frequencies (<10 kHz); which may be ascribed to the electrode polarization effects. The origin of electrode polarization in this case is not pretty clear but it may be assumed that with increasing free radical concentration in solution there are morphological changes in the PAni nanofiber reinforced PVA nanocomposites. The ac conductivity in general obeys the universal power law with the exponent “s” varying from 0 to 1 as can be seen from Table 3; however, as obtained
Fig. 10. Ac conductivity spectra of the PAni nanofiber reinforced PVA nanocomposite before and after exposure to different concentration of DPPH in solution. The solid lines are the best fits of the experimental data according to Eq. (3).
from the best fits of the experimental data there is a decrease in the value of dc conductivity (Table 3) as the concentration of free radicals are increased, which may be ascribed to the decrease in conjugation length in the PAni nanofiber reinforced PVA nanocomposites upon exposure to different concentrations of free radicals due to the partial benzenoid to quinoid transformation. This result is corroborated by the decrease in the degree of crystallinity and stacking obtained from the X-ray diffraction analysis. The increase in the conductivity relaxation time also supports the fact that the conductivity of the PAni nanofibers reinforced PVA nanocomposites decreases with increasing analyte concentration.
4. Conclusions PAni nanofiber reinforced PVA nanocomposites synthesized using rapid mixing polymerization has been used to modify 5 MHz AT cut quartz crystals to act as sensors. It has been observed that these modified QCM sensors can be used to detect ppb level of free radicals in solution. The linearity of the sensor has been found to be excellent with a linear regression coefficient of 0.9685 within the concentration range of 500–1000 ppb. Beyond 1000 ppb, although the response of the sensor is linear but there is a decrease in the slope. The sensitivity of the sensor has been found to be 133.42, 140.13 and129.13 Hz ppb−1 for the DPPH, hydroxyl and peroxyl free radicals, respectively within the concentration range of 500–1000 ppb. However, the linearity and sensitivity of the sensor decreases beyond that concentration since due to continuous monitoring the active sensing layer loses its potential to donate hydrogen atom to neutralize the free radicals. The sensitivity and linearity can be restored by using a control of 1 M HCl solution if continuous detection of free radicals is not desired. The response time of the sensor has been determined and it is observed that the response time decreases with the increase in the analyte concentration. It has been observed that the local range of order in the PAni nanofiber reinforced PVA nanocomposite decreases upon exposure to different concentration of the analyte which indicates that the material amorphizes after reacting with free radicals. FTIR spectra analysis reveals a partial transformation from benzenoid to quinoid structures in the PAni chains that can significantly decrease the conjugation length and effect charge conductivity through the material. The impedance plots also indicate that with the increase in the concentration of the analyte, the benzenoid structures get oxidized to the quinoid structure and as such a shift in the relaxation peak towards lower frequency region is observed. The relaxation time has been found to increase indicating an enhancement in the carrier hopping length. The ac conductivity plots show reasonably good fit with the universal power law and it has been observed that the dc conductivity of the PAni nanofiber
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Biographies
Mr. Somik Banerjee did his Master degree (M.Sc.) in Physics from Tezpur University. He is presently doing research in the field of conducting polymer based nanostructured materials under the supervision of Prof. A. Kumar in the Materials Research Laboratory, Department of Physics, Tezpur University, Assam, India.
Mr. Dimpul Konwar is a M.Sc. student in Nanoscience and Technology in the Department of Physics, Tezpur University. He is presently pursuing his M.Sc. project work under the supervision of Prof. A. Kumar.
Prof. Ashok Kumar did his M.Tech. and Ph.D. from IIT Kanpur in Materials Science. He has published more than seventy papers in national and international journals. He has published eight book chapters and edited one book on nanofibers. Presently, he is a Professor in the Department of Physics, Tezpur University.