Impedance variation with different relative humidities of PAni/Mn nanofibres
DIVYANSHI SRIVASTAVA and R K SHUKLA∗
Department of Physics, University of Lucknow, Lucknow 226007, India
∗Author for correspondence (rajeshkumarshukla_100@yahoo.co.in)
MS received 19 March 2018; accepted 19 October 2019
Abstract. This paper presents the humidity sensing properties of surface-modified polyaniline (PAni). In this study, the impedance response and dielectric properties of pure- and doped-PAni have been investigated as a function of relative humidity (RH%) and frequency. PAni and PAni/Mn composite samples are synthesized by one-step interfacial polymerization process. The structural properties and surface morphologies of the prepared materials have been characterized by X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM), respectively. XRD confirms the formation of PAni and it shows semi-crystalline behaviour. FESEM shows granular, porous and well-distributed structure. It has been observed that the porosity and nanogranular structure increased with increasing doping percentage. Here, we observe that porous and granular structure of Mn-doped PAni shows better response and recovery time (∼28 s) and decreases in electrical impedance. Dielectric constants, dielectric loss and AC conductivity have also been discussed with variations in frequency and relative humidity.
Keywords. Polyaniline; FESEM; XRD; humidity sensing.
1. Introduction
Organic polymer has achieved significant research interest due to its easy synthesis, low weight and low preparation cost. Polymers consisting of organic and inorganic con- stituents have been received considerable attention in several applications [1–4] as they can combine the mechanical, elec- trical and optical properties of both the constituents. Among the varieties of organic–inorganic composites, polyaniline (PAni)-based nanocomposites are widely being used in vari- ous applications, such as protective coating, supercapacitors and sensors. One of the promising applications of polymer is towards humidity sensing in the present era, because of high absorption capability due to its porous structure.
A good humidity sensor should have the properties like high response values, quick response and recovery time, low-cost and stability at room temperature. Usually, they are oper- ated at room temperature, so their response and recovery time are comparatively long. Humidity sensors with response and recovery time in the range of seconds is found to be good in numerous applications like agriculture, weather control, food processing also in medical conditions like anesthesia, pul- monary function diagnostics [5–9], etc. Conducting polymers with porous nature shows high sensitivity towards electri- cal and optical properties. It has been reported [10–14] that composite forms of conducting polymers have different and enhanced properties in comparison to the individual one. In case of humidity sensing of conducting polymers, absorbed water molecules lead to an increase in the conductivity by
charge transfer [15,16]. Increase in conductivity is due to the formation of H-bonding between water molecules and nitro- gen of polymer chain. It facilitates the proton exchanges and therefore, increases in interchain charge transfer [17–20].
Aniline has been polymerized in combining with other met- als, such as PAni/graphene, PAni/CNT, PAni/Ag, PAni/Au, etc. [21–26]. Several studies have been reported recently on composites of PAni-doped with metal oxides. Jain et al[27] synthesized PAni doped with weak acid by chem- ical method and discuss the humidity sensing properties.
They observed the response time for an increase in rel- ative humidity values by 10%, is found to be 4–5 s and recovery time is found to be around 10 s. Parvatikar et al [28] synthesized PAni/tungsten oxide composite and investi- gated their electrical and humidity sensing properties. Pandey et al [29] prepared PAni nanocomposites and investigated their gas-sensing and chemiresistive response. Also, McGov- ern [30] prepared PAni blend and used it as sensing medium for construction of resistance-based humidity sensor. Simi- larly, Aussawasathienet al[31] reported that HCSA-doped PAni/polystyrene electrospun nanofibres are used for sensing hydrogen peroxide and glucose prepared by electrospinning technique.
Transition metal oxides, such as Mn has been widely stud- ied due to its low-cost and environmental stability. PAni/Mn composites can be prepared by either chemical or electro- chemical methods. PAni/Mn composite combine the prop- erties of Mn, such as environmental stability and low cost with the properties of PAni, such as electrical conductivity, 0123456789().: V,-vol
low-cost, stability and processability. This paper reports the investigation of humidity sensing measurements of PAni and PAni/Mn composite and dielectric properties at frequencies 100 Hz–5 MHz.
2. Experimental
2.1 Synthesis
Pure aniline, sulphuric acid and MnSO4 powder were pur- chased from Sigma-Aldrich. Ethanol, acetone and deionized water (DI) have been used for washing. All the chemicals were used as received without further purifications. The solution was prepared with DI. Before the experiments, the solution was deaerated by passing Ar gas through the solution for at least 10 min.
Thin films used for sensing were fabricated by electrochem- ical polymerization. Electrochemical experiment was carried out in a three-electrode connected cell using salt bridge. The working and counter electrodes were of indium-tin-oxide (ITO) (40 ohm cm−2) and platinum mesh, respectively, and the Ag/AgCl was used as reference electrode. Film deposited on anode for 20 min at 1 mA current and 0.8 V potentialvs.
Ag/AgCl by chronoamperometry. Acetone and ethanol are used to wash the electrodes. To deposit films on anode, an electrolyte solution of 0.15 M of aniline and 0.15 M sulphuric acid to make 100 ml solution with different (1, 2 and 5) wt%
of MnSO4. Proper solubility of MnSO4 in DI was made by
Figure 1. Film structure used for sensing.
stirring it for 10 min and then, added dropwise into the solution at room temperature. The obtained green colour films were washed with distilled water and dried in the gentle stream of nitrogen at room temperature.
2.2 Preparation
The prepared film on ITO, as shown in figure1, was used to measure the impedance variation and dielectric properties of the film with respect to the variation of frequency in the range between 100 Hz–5 MHz and relative humidity from 20 to 90 (RH%) in a closed humidity chamber, as shown in figure2.
The variations in relative humidity is controlled by saturated salts CH3COONa, K2CO3, KI, NaCl and K2SO4, which cor- respond to RH% of 20, 40, 60, 80 and 90, respectively. The measurement of dielectric and impedance properties with controlled application of frequency and RH% is done by LCR meter.
3. Results
Surface morphologies of the samples were studied with field emission scanning electron microscope (FESEM, JEOL Japan JSM-7610 F). X-ray diffraction (XRD) patterns of the samples were recorded with Rigaku Ultima IV equipped with CuKα radiation (λ = 1.5403 Å) at the scan rate of 2◦(2θ)min−1ranging from 10 to 70◦. To check the electrical properties like dielectric constants (real and imaginary) were measured at frequency range of 100 Hz–5 MHz at humidity ranges between 10 and 90 RH% using high frequency LCR meter (6500 P Wayne Kerr).
3.1 XRD analysis
The crystallographic structures of materials were determined by powder XRD system. XRD pattern (figure 3) of pure PAni obtained two broad peaks centred at 2θ = 19.47 and 25.19◦ [32–34] shows their semi-crystalline behaviour and corresponds the crystal planes (100) and (110), respectively [35–37]. Here, emeraldine salt (ES)-1 form was obtained,
Figure 2. Set up for sensing and dielectric properties measurement.
Figure 3. XRD of PAni and PAni/Mn samples.
which has pseudo-orthorhombic crystal structure. The value of 2θfor the peak along the plane (100) i.e. 2θ=19.47, 19.46, 19.70 and 20.30◦. Peaks are shifted towards higher angles as the dopant concentration is increased in the samples.
The crystallite size of the samples was calculated using Debye Scherrer method given below:
tds=kλ/βcosθ, (1)
whereλ =1.5403 Å is the wavelength of X-ray for CuKα, β (FWHM) of the diffraction peaks is taken in radian,θ is
the diffraction angle,tds is crystallite size andk is Scherrer constant (k=0.99).
Crystallite size, FWHM andd-spacing of pure- and Mn- doped PAni samples are shown in the table1. It can be seen that the particle size decreases with the Mn concentration in the composite increases.
Broadening of peak along plane (100) is observed. This broadening occurs either due to the crystallite size or strain in the films. To calculate particle size and strain WH plot has been employed in figure4. The line equation as calculated from WH-plot arey= −0.1881x+0.0586,y= −0.1596x+ 0.05516, y= −0.3184x+0.09005 and y= −0.44108x+ 0.11632 for samples 0, 1, 2 and 5 wt%, respectively. The strain and particle size are calculated by comparing the line equation with
βcosθ= Cλ tWH
+2εsinθ (2)
The correction factorC is taken as 1. Determined strain and particle size are shown in table2. The strains can be tensile or compressive and are indicated by+ve and−ve signs, respec- tively.
3.2 FESEM analysis
Figure 5 signifies the FESEM images of pure (figure 5a) and doped PAni (figure5b, c and d). Figure5 also reveals that pure PAni product consists of a large amount of highly branched interconnected nanofibres rather than isolated or single fibres. Most of the nanofibres grow from each other.
Interconnected nanofibre improves the movement of charge carriers to improve the conductivity. Nanostructure provides large surface area for absorption. Porous surface of the film contains large amount of void space, which facilitates the adsorption phenomena. In this case, doped PAni and pres- ence of Mn shows the formation of granules, porous and well-distributed structure of doped PAni. Change in mor- phology of composite with improved characteristics has been observed. FESEM images shows compact granular morphology of Mn-doped PAni at lower magnification. At higher magnification, granular agglomeration clumps of small Table 1. XRD parameters were calculated using Debye–Scherrer method.
Sample FWHM (Radian) d-spacing (nm) Crystallite size (nm) Average (nm)
PAni 0.0272 0.455 5.672 7.151
0.0181 0.353 8.631
PAni/Mn 1% 0.0286 0.455 5.393 6.403
0.0211 0.354 7.414
PAni/Mn 2% 0.0362 0.4502 4.269 5.825
0.0211 0.3530 7.382
PAni/Mn 5% 0.0392 0.4370 3.933 4.827
0.0272 0.3546 5.722
Figure 4. Strain graphs of PAni/Mn samples with Mn concentrations of (a) 0, (b) 1, (c) 2 and (d) 5 wt%.
structure are found on the surface of the film. Granular structures provide a large surface area, which plays effective role in humidity sensing. Morphology of the structure remains same at higher magnification. It has also been observed that this porous and granular structure has been increased with increasing Mn%. Reportedly, oxidative polymerization in acidic medium is responsible for the formation of granular structure of the conducting emeraldine form of PAni, pro- viding the high conductivity of several orders [38]. Jiaxing et al [39] has reported that with different mineral acids it yields uniform nanofibrous structure with average diame- ter of 30–50 nm. High concentration of mineral acid yields high quality of nanofibres even at room temperatures. It is also reported that granular morphology of PAni was pre- pared in strong acidic medium (0.1 M sulphuric acid) [40].
Oxidation of aniline under mild acidic medium produces nan- otubes, whereas microspheres can be obtained by oxidation in alkaline medium. Among these structures, granular struc- ture shows conducting form of PAni and higher conductivity of the order of∼4 and 1.1×10−9S cm−1 in its base form [41].
Table 2. Strain and particle size calculated from WH-plot.
Sample Twh Strain
PAni 2.622 −0.094
PAni/Mn 1% 2.792 −0.079
PAni/Mn 2% 1.710 −0.159
PAni/Mn 5% 1.323 −0.220
3.3 Humidity sensing and dielectric properties
The characteristics curves of impedance against RH of the samples are given in figure 6at different frequencies. Val- ues were measured at 100 Hz (i), 1 kHz (ii), 10 kHz (iii), 100 kHz (iv), 1 MHz (v) and 5 MHz (vi). We can see that the change in impedance is of the order of 102, which shows the high sensitivity of the sensor. An inversely proportional rela- tionship was observed between RH% and impedances. This impedance variation can be observed in three ways: variations with frequencies, with the variation of RH% and with the vari- ation of doping of Mn. Dependence of impedance on RH%
Figure 5. FESEM images of PAni/Mn samples prepared with Mn concentrations of (a) 0, (b) 1, (c) 2 and (d) 5 wt%.
Figure 6. Impedance variations with relative humidity (RH%) at frequencies (i) 100 Hz, (ii) 1 kHz, (iii) 10 kHz, (iv) 100 kHz, (v) 1 MHz, and (vi) 5 MHz of PAni/Mn sample with Mn concentrations of (a) 0, (b) 1, (c) 2 and (d) 5 wt%.
can be observed with frequency as impedance shows inverse relationship with frequency in figure7. Change in the electri- cal properties of PAni/Mn is due to the adsorption, absorption of water molecules on the surface of samples and formation of electrical charge conduction complexes [42]. Water molecules increase the charge concentration and charge mobility on the surface of samples. Variations in impedance characteristic curve can also be observed with respect to the Mn doping
%. It shows that with increment in Mn doping %, impedance reduces for all the frequencies. It can be due to the reduction in particle size as observed in XRD. Nanoscale grain size leads to more grain boundaries and more active sites are available
for water condensation, which produces more charge carriers for electrical conduction. At low RH%, water molecules are less and high at higher RH%. Therefore, impedance reduces at higher RH%. However, at lower frequencies, the reduc- tion rate of impedance is∼75%. As the frequency increases, reduction rate increases to about 85%. We can also understand it as sensitivity. The percentage of sensitivity was calculated as [43]:
Sensitivity= |I2−I1|
|I1| ×100, (3)
Figure 7. Impedance variations with frequencies at RH%: (i) 20, (ii) 40, (iii) 60, (iv) 80, (v) 90 of PAni/Mn sample with Mn concentrations of (a) 0, (b) 1, (c) 2 and (d) 5 wt%.
whereI1andI2are the impedances of the sample at minimum and maximum measured humidity levels, respectively.
The sensitivity graph in figure8clearly defines the varia- tion of sensitivity with different frequency values. At lower frequencies like 100 Hz or 1 kHz, sensitivity percentage was found to be decreasing continuously and after it reaches a certain frequency point, sensitivity % started increasing and reaches maximum at 5 Hz. It can also be correlated with adsorption and absorption of water molecules on the sur- face of the sensors. At lower frequency, the adsorbed water
molecules may not respond to the applied electromagnetic frequency. Sensitivity obtained its minimum value for 10 kHz.
After 10 kHz, adsorbed water molecules started contributing their compatibility with applied frequency and therefore, it started gradually increasing and obtained maximum value at 5 MHz. As we increase the doping percentages, sensitivity of the samples was observed to be improving. It can also be attributed with the particle size decreasing with increase in doping percentage, which enhances the charge mobility and surface area to charge conduction. Incorporation of Mn
Figure 8. Variations of sensitivity with frequency variations of PAni/Mn sample with Mn concentrations of (a) 0, (b) 1, (c) 2 and (d) 5 wt%.
as metallic filler in PAni leads to the formation of organic–
inorganic (polymer–metal oxide) composite. Variations have been observed due to the presence of Mn particles. XRD revealed the decrement in particle size, which may be linearly related to the conductivity. Conductivity is found to be pro- portional to doping % of Mn and inversely proportional to the particle size. The presence of Mn particles in the electrolyte enhances the current conduction and charge transfer dur- ing electropolymerization process at the same potential with increasing the weight percentage (wt%). This enhancement seems to be due to the interaction between hybrid structure and agglomeration of Mn. It has also been reported that in the samples, as the particle size decreases, number of voids increases, which provide higher sensitivity to the samples for humidity [44,45]. The time required by the sensor to measure the change in impedance by changing the relative humidity from 20 to 95% and from 95 to 20% are the response and recovery time, respectively. The response and recovery time
Figure 9. Response and recovery time of pure and doped with Mn concentrations of 1, 2 and 5 wt%.
Figure 10. Real and imaginary dielectric constants with frequency at different RH% for (a) pure PAni, (b) PAni/Mn 1 wt%, (c) PAni/Mn 2 wt% and (d) PAni/Mn 5 wt%.
Figure 11. Graphs show dielectric constants (i) and (ii) and dielectric loss (iii) with RH% at 10 kHz of PAni/Mn sample with Mn concentration of (a) 0, (b) 1, (c) 2 and (d) 5 wt%.
graphs are shown in figure9for all the samples. Here, response time (Tres) and recovery time (Trec) vary in a range between 19<Tres<28 and 25<Trec<28 s, respectively, for all the samples.
To analyse the dielectric properties, plots have been drawn as a function of frequency as shown in figure 10.
Figure10shows real and imaginary permittivities,εandε with frequency at RH% for pure and Mn-doped PAni at room temperature. Capacitances (Cp) was used to calculate real and imaginary permittivities,εandεand loss tangent using the equations given below [46]:
ε= Cpd
ε0A, (4)
ε= G
ωC0, whereC0= ε0A
d . (5)
ε=Dε, (6)
tanδ= ε
ε, (7)
wheredis the thickness of the film,Athe electrode area,ε0
the dielectric permittivity in vacuum (8.85×10−12f m−1),Cp the parallel capacitance,Gis the conductance,ωthe angular
frequency, D the dissipation factor and the parallel capacitanceCpis obtained directly from the measurements.
Both the real ε and imaginary ε dielectric constants exhibit typical behaviour. Figure 10 shows the frequency- dependent dielectric constant, as it is clear from the figure that the dielectric constant decreases with increase in fre- quency. For lower value of frequency, the dielectric constant attains higher values, whereas at higher frequency, the dielec- tric constant decreases. The decrease in dielectric constant with frequency can be explained as dielectric relaxation phe- nomena [47]. Large values observed forεandεare related to the charge carriers accumulated at the interfaces. The decrease ofε andε with increase in frequency indicates that the charge carrier localization [48] is unstable and easily affected by the external frequency. The decrease of dielec- tric constants with frequency can be explained as follows: at low frequencies, dielectric constants for polar materials are due to the contribution of multi-component of polarizabil- ity (electronic, ionic, orientation and interfacial). When the frequency is increased, the dipole will no longer be able to rotate sufficiently rapidly and follow the applied field. So, their oscillations begin to lag behind the field. As the fre- quency is further increased, the dipole will be completely unable to follow the field and the orientation stopped, so the dielectric constant decreases at a higher frequency approach- ing a constant value due to the interfacial polarization [49,50].
Figure 12. Graphs show dielectric constants (i) and (ii) and dielectric loss (iii) with RH% at 5 MHz of PAni/Mn sample with Mn concentration of (a) 0 , (b) 1, (c) 2 and (d) 5 wt%.
Figures11and12represent dielectric constantvs. RH% at 10 kHz and 5 MHz frequency for all the samples. At 5 MHz sensitivity of the sample is high and low at 10 kHz, therefore, variation of dielectric constants and dielctric loss tangent with the variation of RH% at 10 kHz and 5 MHz is shown in figures11and12. From the figures, as we increase the RH%
value of dielectric constants (εandε) decreases. It can be due to the increase in percentage of water molecules on the surface of pure and doped PAni as the water molecules may not respond to the applied electromagnetic field. Investigation also reveals that in the presence of Mn particles, dielectric loss and dielectric constant start decreasing, which is due to the increase in conductivity and decrease in particle size. The dielectric constants tend to decrease with doping in the rela- tively high frequency region. This can be due to the formation of bonds between Mn ions with electron pair of NH group of polymer chain. Dipolar properties of NH group become weaker and therefore, dipole moment decreases. The varia- tion of the dielectric loss (tan δ) recorded as a function of RH% for pure PAni and PAni/Mn composites. It is observed that the dielectric loss decreases with RH%. For all wt%, the loss tangent is independent of RH in the low RH% region, but suddenly increases at higher RH% region. This can be due to the presence of large number of space charge carri- ers and defects in sample [51]. Consequently, the dielectric
loss (tanδ) value decreases with respect to increase in the wt% of Mn-doped PAni composite, which is the expected one.
4. Conclusion
The film was made on ITO substrate by the electrochem- ical method to fabricate a resistive type humidity sensor.
The sensitivity of the PAni/Mn humidity sensor is high. The response and recovery times of the sensor lie in the range between 19 < Tres < 28 and 25 < Trec < 28 s, respec- tively, with different doping percentages and with different frequencies. The relation of impedance and RH is obvi- ously affected by the measured frequency, especially in the low frequency range. The dielectric property of the sensor depends on measured frequency and RH. In the low RH range, no space–charge polarization takes place, and the dielectric constant is high with the frequency. In the high RH range, the polarization catches up with the change in the elec- trical field direction, and the dielectric constant decreases.
The graph of the dielectric loss decreases with higher doping.
Acknowledgements
We are grateful to UP state government through the Centre of Excellence Scheme for providing XRD facility at the Depart- ment of Physics, University of Lucknow.
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