Suitability of emeraldine base polyaniline-PVA composite film for carbon dioxide sensing

Synthetic Metals 156 (2006) 1401–1407

Suitability of emeraldine base polyaniline-PVA composite film for carbon dioxide sensing Mihai Irimia-Vladu ∗ , Jeffrey W. Fergus Auburn University, Materials Research and Education Center, 275 Wilmore Laboratories, Auburn, AL 36849, USA Received 1 April 2006; accepted 12 November 2006 Available online 14 December 2006

Abstract A CO2 sensor based on a composite thin film of emeraldine base polyaniline and poly (vinyl alcohol) cast from N-methyl pyrrolidone on an interdigitated electrode was characterized using impedance spectroscopy. The response of the sensor was slow and smaller in magnitude as compared to reports in the literature for similar sensors. Materials characterization indicated that the desired emeraldine base phase was not present after heat treatment. In addition, the mechanism for carbon dioxide detection previously reported in the literature cannot explain the CO2 sensitivity, because the pH established between CO2 and carbonic acid in the water dissolved in the poly (vinyl alcohol) matrix is not sufficient to induce a significant change in the conductivity of the emeraldine base polyaniline. © 2006 Elsevier B.V. All rights reserved. Keywords: Polyaniline thin film; Materials characterization; Carbon dioxide sensing; Emeraldine base doping characteristic

1. Introduction Respiration or CO2 evolution results from biological activity, so CO2 evolution rate is a useful indicator for many biological activities. The determination of CO2 evolution rate has been used for the evaluation of living microbial biomass in soils [1,2], pesticide damage on soil microorganisms [3], decomposition of leaf litter [4], toxic effects of trace metals on microbial populations [5,6], detection of microorganisms in blood and examination of contaminated bottled juices [7,8]. Precise and rapid determination of microbial contamination is important for monitoring and maintaining the safety of packaged food. The most widely employed technique for the evaluation of microbial contamination of food is the total plate count (TPC), a lengthy method that requires at least 48 h [9,10]. Alternatively, the CO2 evolution rate, which is correlated with the realtime microbial activity of contaminated food, measured with a commercial IR-CO2 analyzer [11,12] or a microrespirometer [13], has been shown to correlate well with the microorganism concentrations determined by the conventional TPC method. Nevertheless, in spite of a very good correlation with published data, both systems are not suitable for miniaturization. ∗

Corresponding author. E-mail address: [email protected] (M. Irimia-Vladu).

0379-6779/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2006.11.005

Polyaniline is an interesting conductive polymer, since it can be cycled between insulating and conductive regimes through two distinctive routes: redox and acid–base chemistry [14,15]. It is also inexpensive and offers a high yield (>90%) of the polymerization reaction [16]. These properties coupled with excellent room temperature stability [17–19] make polyaniline a promising candidate for room temperature sensors and has been used by Ogura and co-workers [20–24] for the detection of carbon dioxide at room temperature. 2. Experimental The preparation method of the polymer composite used in this work follows closely the procedure reported by Ogura et al. [20]. Polyaniline was synthesized by mixing 100 mL of 0.15 M aniline solution and 100 mL of 0.1 M p-toluene sulfonic acid (TSA) solution containing 0.15 M (NH4 )2 S2 O8 , and continuously stirring the mixture for 8 h. The resulting precipitate was separated by filtration through filter paper No. 1 (Watmann Int.), then rinsed several times with methanol and 0.1 M TSA. The precipitate was dried at room temperature under vacuum for a period of 24 h to obtain a dark green powder of TSA-doped polyaniline. The dried cake collected from the filter paper was treated in a 3% NH4 OH aqueous solution for 8 h to convert the conductive emeraldine salt (ES-PAni) to the insulating emeraldine-base

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Table 1 Alternative methods of fabrication employed Heat treatment

EB-NMP concentration (%)

0.06% PVA-NMP

1% PVA-NMP

EB-NMP/PVA-NMP ratio

EB-NMP/PVA-NMP ratio

1/5

1/4

1/2

1/5

1/4

1/2

No HT

0.01 0.1 1

8*, 12, 24* 8 8**, 24**

8, 24 – 8, 24

8, 12, 24 8 8, 24

8 – 8**

8 – –

8 – 8**, 24**

365 ◦ C

0.01 0.1 1

8, 12, 24* – 8, 12, 24*

8 – 8, 24

8, 24 8 8, 24

8 8 8**

– – 8

8 – 8, 24

8, 12, 24 represent the polymerization time (h); the numbers in regular type indicates air and bold type indicates N2 ; *PVA with two different molecular weights (13,000–23,000 or 89,000–98,000) were used; **PVA with three different molecular weights (13,000–23,000, 89,000–98,000 or 124–186,000) were used.

Fig. 1. The schematic of the interdigitated electrode employed in impedance measurements.

polyaniline (EB-PAni). The base solution was filtered through filter paper No. 1 (Watmann Int.) and the resulting dark blue precipitate dried for 24 h under vacuum at room temperature. The dried emeraldine base powder was collected from the filter paper and stored in a closed vial which was placed in a desiccator. Samples of emeraldine base were extracted from the stock polymer and heat-treated by placing the samples in an oven preheated at 365 ◦ C and holding isothermally for 1 h in a helium gas atmosphere. Stock solutions of emeraldine base polyaniline and poly (vinyl alcohol) were prepared by dissolving 1 mg of heat-treated EB-PAni and 6 mg of PVA of MW = 13000–23000 (Aldrich Inc.) in 10 mL of N-methyl-pyrrolidone (NMP), respectively, to adhere strictly to the procedure reported by Ogura et al. [20,21]. Samples were extracted from the two stock solutions in a ratio of 1:5 (EB-NMP:PVA-NMP), to produce a polymer composite mixture with a 1:30 ratio of EB-PAni:PVA. Alternative methods of fabrication were also investigated. Variations in the procedures include the polymerization time (8, 12 and 24 h), the cover gas during the fabrication (in a fume hood in air or in glove box under nitrogen), the heat treatment of the emeraldine base powders (no heat-treatment or heat-treated for 1 h at 365 ◦ C in helium atmosphere), the EB-NMP solution concentration (0.01, 0.1 and 1%) and the EB-NMP:PVA-NMP ratio (1/5, 1/4, 1/2). The specific conditions used are summarized in Table 1. The sensor performance was essentially the same for all the fabrication conditions investigated. The electrode used for conductivity measurements was fabricated from titanium gold sputtered in a comb-shape configuration on a quartz substrate as shown schematically in Fig. 1. The fingers were 4 mm in length (f) and 200 nm in height (h) with separation distances (d) of 40 ␮m between adjacent fingers. One

microlitre of stock solution was deposited on the interdigitated electrode by dip-coating (a method also employed by Ogura and co-workers [20–24]) and then placed inside a glass chamber and slowly dried under a flowing argon gas atmosphere. The apparatus for evaluation of sensor performance (shown in Fig. 2) consisted of a glass chamber that was partially filled with a supersaturated solution of a salt that established a constant humidity level. The humidity levels fixed by supersaturated salt solutions were ∼60% for NaCl, ∼30% for MgCl2 and ∼4.5% for LiCl. The relative humidity inside the glass chamber was monitored with a humidity sensor (Vaisala HMP35E). The interdigitated electrode covered with the cast polymer composite film was suspended inside the glass chamber about two millimeters above the level of the supersaturated salt solution. The impedance of the composite film was measured with an

Fig. 2. The setup employed in conductivity measurements.

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impedance gain phase analyzer (Solartron Model SI 1260) using a frequency range of 3.2 × 107 –1 Hz. The CO2 response was determined by alternating between pure argon (99.9% purity, Air Gas Inc.) and Ar + 5% CO2 (99.8% purity, Air Gas Inc.). Each gas was flowed inside the glass chamber until the resistance of the thin film stabilized to within ±0.5% and then the gas was changed. Fourier transformed infrared spectroscopy (Perkin-Elmer Spectrum GX FTIR Spectrometer) was performed on powder samples of emeraldine base polyaniline in the KBr pellet mode. Equal portions of emeraldine base from the stock powder were each heat treated for 1 h in a helium atmosphere at a temperature of 100, 150, 200, 250, 300 or 365 ◦ C. UV–vis spectrophotometry analysis (CARY 3E UV-visible spectrophotometer) was performed by adding two drops of 3% emeraldine base polymer solution in NMP to a 10 mL cuvete of pure NMP. Each emeraldine powder had been previously treated for 1 h in a helium atmosphere at a temperature of 100, 150, 200, 250, 300 or 365 ◦ C before being dissolved in NMP. Differential scanning calorimetry analysis (TA Instruments 2910 Differential Scanning Calorimeter) was performed on emeraldine base powder samples. Under nitrogen gas flow, the analyzed sample was held for 30 min at 100 ◦ C and then ramped at 20 ◦ C/min to 400 ◦ C. After cooling down to the room temperature, a second run of the experiment was restarted using the same conditions. X-ray diffraction (Rigaku DMaxB Diffractometer) was performed on emeraldine base sample powders heat-treated for 1 h in a helium atmosphere at temperatures of 100, 150, 175, 205, 225, 245, 275, 300 or 365 ◦ C. The scan range (2θ) was 5–45◦ at a scan rate of 5◦ min−1 . 3. Results and discussion 3.1. Sensor performance The mechanism of CO2 detection proposed by Ogura and co-workers [20,23] is displayed in Fig. 3. The CO2 reacts with water from PVA to create a bicarbonate ion (HCO3 – ) and a proton, which protonates the polyaniline. Ogura observed that as the CO2 partial pressure increases, the conductivity increases, presumably due to an increase in the amount of protonation. Typical impedance spectra, as shown in Fig. 4, consisted of a depressed semicircle in the Z’–Z” plot. The intercept of each semicircle with the horizontal axis represents the real part of

Fig. 3. The CO2 detection mechanism-adapted from refs. [20,23].

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Fig. 4. Impedance spectra for a sample exposed to pure argon and Ar + 5% CO2 in 30% relative humidity.

the impedance [25]. The magnitude of response R/R of the sensor varied between 2 and 15% for all the humidity levels employed. This response was very small compared to Ogura and co-workers who reported conductivity increases of up to 2 orders of magnitude [20–24]. After three or four cycles between argon and argon + 5% CO2 , the drift of the sensor in this work exceeded half the dynamic range. The sensors reported by Ogura and coworkers [20–24] were stable with minimal drift throughout the testing period of up to 35 days. The 90% time response (t90 ) of the sensor in this work was in the range of few hours to as high as 24 h, which is much slower than the response reported by Ogura and co-workers who reported values t90 in the range of a second to few minutes [20–24]. The response of the sensor when the gas was changed from pure argon to Ar + 5% CO2 was faster than when the gas was changed back to argon. This trend is consistent with Ogura et al. although, as mentioned above, their response was much faster. Ogura ascribed this discrepancy in response times to the slow removal of the weakly bound water in the case of inert gas flow. 3.2. Materials characterization Materials characterization was performed to investigate the effects of the heat treatments used by Ogura at either 280 ◦ C to achieve the full conversion of the emeraldine salt to emeraldine base [21–24] or at 380 ◦ C to eliminate the TSA residue from the polymer chains [20]. 3.2.1. FTIR spectroscopy A comparison of the FTIR spectra between emeraldine base powders not heat treated or heat-treated for 1 h at various temperatures in helium gas atmosphere is shown in Fig. 5. The characteristic peaks marked on the graph are summarized in Table 2 along with the respective vibrational mode assigned in the literature [20,26–33]. The FTIR spectra indicate that emeraldine base undergoes undesirable transformations when heat-treated at 365 ◦ C. In fact, the evolution of FTIR peaks indicates that the emeraldine base powder becomes a mixture of leucoemeraldine and cross-linked emeraldine base at heattreatment temperatures higher than 250 ◦ C. Scherr et al. [34] reported that emeraldine base cross-links if treated for 4 h at 300 ◦ C and proposed that the cross-linking process is chemical in nature and occurs through a linkage between imine nitrogens that generate a phenazine-type structure. Mil-

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Fig. 5. Comparison among FTIR spectra of emeraldine base without heat treatment or with heat treatment for 1 h at various temperatures in helium atmosphere.

ton and Monkman [35] confirmed that chemical cross-linking process rendered by treating the sample at temperatures greater than 180 ◦ C is responsible for the sample hardening and insolubility in NMP. In addition, the latter group suggested that physical cross-links (chain entanglements) might also accompany the mainly chemical crosslinking process. Chen and Lee [31] and Ding et al. [30] confirmed the chemical nature of the cross-linking process occurring at temperatures of 150–300 ◦ C, showing that the process is responsible for the diminishing in the intensity of the peak centered at ∼1160 cm−1 and attributed to N Q N stretching type vibration. Different from Chen and Lee, Ding et al. observed multiple peaks centered at ∼1160 cm−1 and attributed their presence to a more ordered, partially crystalline structure of their emeraldine base. Subsequent to heat treatment, these multiple peaks transformed into a single peak of much lower intensity, representative to an amorphous structure. The chemical nature of cross-linking process, occurring through bonding between imine nitrogens, is consistent with the FTIR spectra in Fig. 5. The multiple peaks having an infrared absorption in the range of ∼1145–1164 cm−1 gradually diminish in intensity with increasing temperature and finally vanish for an emeraldine powder treated at a temperature of 365 ◦ C. The spectrum of emeraldine base for which no heat treatment was performed (i.e. the bottom curve in Fig. 5) displays two bands associated with N H stretching vibrations: a broad band centered at ∼3284 cm−1 associated with hydrogen bonded N H stretching vibrations and a minor band centered at ∼3365 cm−1 associated with non-hydrogen bonded, free N H stretching Table 2 Characteristic peaks and vibrational modes assigned to emeraldine base Peak (cm−1 )

Vibrational mode

Reference

3365 3284 1596 1497 1299 1145–1164 1040

Free N H stretching Interchain H bonded N H stretching C C stretching-quinoid rings C C stretching-benzenoid rings C N stretching-aromatic amine N Q N stretching Symmetric SO2 stretching in TSA residues

[26–29] [26–29] [20,21,26–32] [20,21,26–32] [20,21,26–29,32] [20,26–32] [20]

vibrations. These two values are in good agreement with the published values of ∼3290 and ∼3383 cm−1 , respectively [26–29]. In contrast, the spectrum of leucoemeraldine base contains only the band at ∼3383 cm−1 , which is characteristic of free, non-hydrogen bonded N H stretching vibrations [26–29]. As the heat-treatment temperature increases above 250 ◦ C the hydrogen bonded N H stretching vibration band centered at ∼3284 cm−1 disappears, but the peak at ∼3383 cm−1 shifts to ∼3404 cm−1 for a sample heat-treated at 365 ◦ C. In addition, the quinoid and benzenoid bands centered, respectively, at ∼1596 and ∼1497 cm−1 are severely reduced intensity, and broadened, which, together with the disappearance of the hydrogen bonded N H stretching vibration, indicate that at temperatures exceeding 250 ◦ C the emeraldine base undergoes a chemical transformation (reduction) to leucoemeraldine base. This is in agreement with reports of Mathew et al. [32,33] who showed that chemical cross-linking is accompanied by reduction of the emeraldine polymer to leucoemeraldine base and suggested that the reductant is in fact the polymer itself. Thus, the cross-linking reaction between imine nitrogen and quinoid ring producing the phenazine structure also generates a hydrogen atom (H+ + e− ). The generated proton reacts with a neighboring quinoid group resulting in a simple reduction of the polymer. Ding et al. [30], Chen and Lee [31], Mathew et al. [32], Scherr et al. [34] and Milton and Monkman [35] have reported that once the emeraldine powder became cross-linked at high temperature, the powder could not be processed in appropriate solvents (i.e. DMSO or NMP) or strong acids (i.e. H2 SO4 ). In this work, the powder heat treated at temperatures exceeding 250 ◦ C is not completely soluble in NMP. The higher the heat treatment temperature, the lower the solubility of the powder and the less intense the blue color of the resulting solution. After heat-treatments above 300 ◦ C, the color of the solution obtained after the dissolution of the respective powder is no longer blue (the color which appears after the dissolution of emeraldine base in NMP) but yellowish brown, which is characteristic to the leucoemeraldine-NMP solution. After about 24 h, a residue of undissolved polymer sediments at the bottom of the flask. The dissolved polymer is actually the leucoemeraldine base (as determined by UV–vis spectrophotometry), whereas the undissolved one is presumably the cross-linked emeraldine base. Fig. 5 also shows that the peak centered at 1040 cm−1 due to the SO2 symmetrical stretching vibration [20] (i.e. the peak that indicates the presence of TSA residue in the emeraldine chains) is severely reduced during the heat treatment until it finally disappears after heat-treatments at temperatures exceeding 300 ◦ C. The removal of the TSA residue from the polymer chains by heating the polymer at 380 ◦ C represents the purpose of heat treatment performed by Ogura et al. [20] and is probably the only desirable effect of the high temperature treatment of the polymer. 3.2.2. UV–vis spectrophotometry UV–vis spectrophotometry confirms the transformation of emeraldine base in leucoemeraldine base during the high temperature treatment. The collected spectra taken for emeraldine

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gravimetric analysis (TGA) of the emeraldine polymer shows no major weight loss between 200 and 300 ◦ C, which is consistent with many reports [20,30,31,36,40]. Therefore, the thermal process centered at ∼225 ◦ C can be attributed to the crosslinking of the polymer chains as suggested by Wei et al. [39] and others [30–35]. The emeraldine base should display a small endothermic peak in the range 200–250 ◦ C due to the glass transition [30,31,34,39], but this endothermic peak is apparently offset by the large exothermic peak due to crosslinking of the polymer chain, so DSC is not a suitable technique to visualize the glass transition in emeraldine base [30,39].

Fig. 6. UV–vis spectra of emeraldine base samples heat-treated for 1 h at various temperatures in helium atmosphere.

base samples treated at various temperatures are shown in Fig. 6. The lower curves in the figure contain two peaks of interest: a peak centered at ∼328 nm due to a π–π* type absorption of benzenoid rings and a peak centered at ∼630 nm due to exciton type absorption of quinoid rings, both of which are in good agreement with values reported in the literature [20,29,33,36–38]. The gradual disappearance of the peak centered at ∼628 nm and the persistence instead of the peak centered at ∼328 nm indicates the emeraldine base to leucoemeraldine base transformation after treatment at temperatures exceeding 250 ◦ C, since the leucoemeraldine base does not contain quinoid rings. 3.2.3. Differential scanning calorimetry/thermogravimetric analysis The cross-linking of emeraldine base has been observed using DSC [39]. The DSC thermogram in Fig. 7 shows that the emeraldine powder heated at 400 ◦ C contains an exothermic peak centered at about 225 ◦ C (∼250 ◦ C reported by Wei et al.). This peak is not present in a DSC thermogram taken immediately after cooling the sample to room temperature, indicating that the peak corresponds to an irreversible process. Thermo-

Fig. 7. DSC thermograms of emeraldine base sample held isothermal for 30 min at 100 ◦ C and ramped up 20 ◦ C/min to 400 ◦ C.

3.2.4. X-ray diffraction The cross-linking phenomenon occurring at temperatures exceeding 250 ◦ C is also shown in the X-ray diffraction spectra (Fig. 8) of powder samples treated at various temperatures for 1 h in helium atmosphere. The peaks are indexed for the emeraldine base class I (EB-I) crystal structure, according to Jozefowicz et al. [41]. The main crystalline peak at 2θ ∼ = 19.5 in Fig. 8 is consistent with other reports [14,28,29,39,42–44] in which polyanilines in the emeraldine oxidation state are divided into two classes, which differ substantially in their crystallinity and compactness. Class I emeraldine is generated when the polymer is fabricated in the conductive emeraldine salt form (ES-I) and then converted to the corresponding insulating base form (EB-I) through treatment with a strong base. Class II emeraldine materials are generated directly in the nonprotonated, insulating base form (EB-II) and then converted to the respective salt type (ES-II) through a treatment with a protonating acid. In addition, the EB-II crystal structure can be generated by dissolving the EB-I type in an appropriate solvent (e.g. tetrahydrofuran (THF), dymethyl sulfoxyde (DMSO) or N-methyl-2-pyrrolidone (NMP) [41–43]). Pouget et al. [42,44] and Jozefowicz et al. [41,43] showed that very little crystallinity is expected for the emeraldine base in the EB-I form, which is the class produced by the procedure used in this work and by Ogura et al. [20]. Studying the structure of polypyrrole and the effect of dopant anion on crystallinity of the polymer, Warren et al. [45] found

Fig. 8. X-ray diffraction of emeraldine base without heat treatment or heattreated at various temperatures for 1 h in helium atmosphere. The diffraction peaks are indexed according to similar reports in literature.

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that the ratio of the peak half width to the peak height (HW/H) of the X-rays diffraction peak reflects order in the polymer backbone. The smaller the value of the HW/H ratio, the higher the order. Wei et al. [39] used the ratio of the peak half width to the peak height (HW/H) as a measure of order in the polymer backbone in polyaniline and found that the HW/H decreased with increasing heat-treatment temperature, reached a minimum at a temperature immediately above the glass transition temperature and then increased again as the heat-treatment temperature further increased. The overlapping peaks lead to uncertainty in the determination of the width of the peak, so full width at half maxima (FWHM) has been used in this work. The calculated FWHM in Table 3 shows a minimum at ∼225 ◦ C, which is consistent with the results reported by Wei et al. [39], who indicated that this corresponds to a maximum degree of order near the glass transition temperature. 3.3. Possible reasons for the sensor poor performance The heat-treatment process (introduced by Ogura and coworkers [20–24] in their development of the emeraldine base CO2 sensor) leads to undesirable transformations that occur concomitantly: the crosslinking of the polymer and the reduction of emeraldine base to leucoemeraldine base. The leucoemeraldine base can be processed in NMP [14] but cannot be protonated by a protonating agent with a pH greater than −0.2 (e.g. 1N HCl used by Huang et al. [15,46]). Increases in the conductivity of leucoemeraldine occur through oxidative doping when it transforms to the emeraldine salt, but never through protonation [15,46]. On the other hand, cross-linked emeraldine base, which can be protonated, is virtually insoluble in NMP [30–35], so the proportion of emeraldine base chains in the deposited polymer film is expected to be very low compared to the leucoemeraldine base chains. Nevertheless, even without the heat treatment step that transformed the emeraldine base to leucoemeraldine, the sensor performance did not improve indicating there is another reason for the poor performance. The weak response through the carbonic acid protonation route of polyaniline is consistent with results of MacDiarmid and co-workers [15,47–49] and Jozefowicz et al. [50], who showed that very little or virtually no protonation of emeraldine base occurs if the pH of the protonating bath is greater than 4. Moreover, the emeraldine sample is not significantly protonated even Table 3 Full width at half maxima for heat-treated emeraldine base samples Heat temperature (◦ C)

FWHM

No heat treatment 100 145 175 205 225 245 275 300 365

2.3 2.2 2.1 1.9 1.7 1.6 1.8 1.9 2.5 14.1

Fig. 9. Comparison of the doping percentage as a function of pH of protonating bath for the two classes of emeraldine base; the figure also displays the pH interval of carbonic acid generated in water by carbon dioxide gas detected by Ogura.

at a pH of 2, whereas a maximum protonation level is reached when the pH of the bath approaches zero. The pKa of the imine sites in emeraldine chain (i.e. the sites that were shown to protonate first [14,15,47]) is 2.55 [50,51]. Although, as shown in Fig. 9, there is some difference in the pH for protonation between Class I [48] and Class II materials [50], in both cases pH levels less than 4 are needed for protonation. The pH established by bubbling CO2 through water according to CO2 + H2 O = H+ + HCO3 −

(1)

is 3.95 for a CO2 partial pressure of 105 Pa using the reasonable assumption that at CO2 partial pressures greater than 36 Pa the proton and the bicarbonate concentrations are equal [52]. Therefore, 100% carbon dioxide at atmospheric pressure generates an infinitesimal protonation level for a “very insulating” emeraldine base sample (the initial conductivity of about 10−10 S/cm). The emeraldine base reported by Ogura et al. [20] had an initial conductivity of 10−5 S/cm, which, according to the results of MacDiarmid and co-workers [15,45–47], would require a pH of 3 or lower to generate any measurable change in conductivity. The carbon dioxide detection interval reported by Ogura and coworkers [20–24] (5–500 Pa) would correspond to a pH in water in the limits between 6.0 and 4.5, which would not protonate the emeraldine base, indicating that their proposed model does not explain the observed response. 4. Conclusions The sensor developed following the procedure reported by Ogura and co-workers [20–24] offered poor sensing characteristics: severe drift, limited dynamic range and an unacceptably long response time. Materials characterization showed that the heat treatment of emeraldine base at temperatures exceeding 250 ◦ C leads to both cross-linking and reduction of emeraldine to leucoemeraldine base. However, elimination of this heat treatment to maintain the desired emeraldine phase did not improve

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