[9]
The amines were diazotized with sodium nitrite and concentrated HCl in an ice bath.[11] PANI films were suspended in TRISs buffer (pH = 8) and mixed with the diazonium salt solution in an ice bath. The modified films were washed first with 1 L of 1 mol L-1 HCl solution and then with 1 L distilled water. Then the films were dried (dynamic vacuum for 48 h). The nucleophiles used are: thioglycolic acid (HSCH2COOH, TIO), Sodium mercaptopropansulfonate (HSCH2CH2CH2SO3H, MPS), dodecanothiol (HSC12H23, DOT) y Cysteamine (HSCH2CH2NH2, CIA). Measurement set-up. The cloth covered with sensitive material is cut into 1.5 x 1 cm rectangles, which are contacted in the extremes by pressing copper plates. The sensors are placed inside a sample chamber made of a closed 250 ml Erlenmeyer flask, which is thermostatized in a water bath. 25 ml of liquid sample are set inside the flask and left to equilibrate with the head space at the set temperature. In this static set-up the equilibrium conditions can be achieved. The response signal of the sensor was measured with a multimeter connected to a PC via an USB connection and a two probe method was employed for the measurement. Results and Discussion A typical resistance-time response for repetitive exposures to methanol shows a different resistance response to the same compound. The difference could arise from different degree of interaction between methanol and the film and/or different effect on the conductivity. In principle, the conductivity changes could be due to changes in the charge carrier mobility in the polymer chains (an electronic effect) or to changes in the interchain hoping (a swelling effect). Both effects require the interaction of the volatile molecules with the polymer chains. In Fig. 1 it is shown the response of functionalized PANI films to different alcohols. As it can be seen, the materials are sensitive to the presence of the organic molecules and the sensitivity depends on the molecule. Interestingly, increasing the chain length (decreasing polarity) of the alcohol induces an increasing signal (Figure 1.a) for the polymer functionalized with a hydrophilic group (-NH2). Fig. 1. Comparative normalized resistance signal (at saturation) obtained by exposing to different alcohols, sensors made of: (a) hydrophilic polyaniline (PANI-CIA, functionalized by reaction with cysteamine). (b) hydrophobic polyaniline (PANI-DOT, functionalized by reaction with dodecanotiol). Each signal is normalized to the vapor pressure of the volatile at the measurement temperature (25 oC) for comparison. On the other hand, when a hydrophobic polyaniline is produced by incorporation of an alkyl radical (-C12H23), reacting the PANI with dodecanothiol, (PANI-DOT), the trend is different. As it can be seen in Figure 1.b, the signal begins in a negative value (methanol), increases with the chain length of the alcohol till propanol and then decreases. It seems likely that better solvation of the film having alkyl chains are achieved by the alcohols with intermediate alkyl chains. However, it should be expected for alcohols with longer chains to solvate better the PANI-DOT. Fig. 2. Comparative resistance signal (at saturation) of different functionalized polyanilines when exposed to saturated vapor of heptane, at 25 oC. While the effect of alkyl chain length in alcohols is less clear, when the sensors are exposed to an alcane (heptane) lower signal levels are shown overall but a clear signal effect of the sensor material is observed. The sensor based on a hydrophobic polyaniline (PANI-DOT) show a signal nearly 5 times larger than other polymers. It seems that strong van der Waals interactions between the volatile alcane and the long alkyl chains of the attached functional groups induce a strong signal. Conclusions Functionalized polyanilines can be easily prepared by nucleophilic addition of different thiols to polyaniline. The polymers show finite resistance and are sensitive to different volatile molecules. There is an effect of alkyl chain length when the sensors are exposed to alcohols, which is more marked with a polyaniline modified by addition of cysteamine (PANI-CIA). On the other hand, when a hydrophobic polyanililine, modified by addition of dodecanothiol (PANI-DOT), is exposed to different alcohols a non monotonic change of signal is observed. However, PANI-DOT shows a signal nearly five times larger than other polyanilines, when exposed to heptane. It seems that strong interaction between the volatile alcane and the attached functional group induces a large signal. The results suggest that chemical functionalization of polyaniline films allows to tune the molecular sensitivity of resistive sensors.
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