Poly(alkoxy-bithiophenes) sensors for organic vapours

This work reports electrical resistance measurements and on line UV–Vis–NIR spectra for sensors, fabricated from poly(3,30-dipentoxy- 2,20-bithiophene) (I) or poly[3,30-2-(2-ethoxy)ethoxy-2,20-bithiophene] (II) doped with FeCl3, Fe(ClO4)3, or CuCl2, and tested against vapours of o-xylene, n-butanol, benzene, methanol, n-heptane, and 3-pentanone over a 50–66,444 ppm (v/v) concentration range in argon. Each sensor was exposed repeatedly and randomly to the test vapours in a wide concentration range over a maximum of 41 times and 25 days. Neat argon sweeping was performed between test vapour injections to recover base line conditions and test sensor reversibility. The percentage changes of electrical resistance, relative to the value measured before exposure, and of the sensor absorbance at 880–1400 nm, both varied linearly according to the concentration used. The sensitivities a were calculated from the linear regression analysis of 80 sets of DR, C values for a total of about 400 measurements, in order to assess the influence of several parameters, i.e. the chemical nature of the sensor, the fabrication, the exposure time to the test vapour, and the ageing in air. The data analysis demonstrated that sensitivity depends on the chemical nature of both the sensor and the test vapour and on the length of exposure. Data collected with a 90 s exposure gave the narrower range of standard deviation values, amounting to 14% of the calculated a value in the worst case, whereas a values over the range of sensor and test vapour combinations varied from 3:4 �� 10��5 for I doped FeCl3 versus MeOH to 231 �� 10��5 for I doped with Fe(ClO4)3 versus o-xylene. A significant base line drift was observed at the longest working time, but it was not proven to affect the sensitivity of the sensor. The concomitant changes of the sensors and of the electronic spectra offer intriguing scope to further investigate the nature of the sensor–test vapour interaction.