Evaluation of Nafion-Crystal Violet films for the construction of an optical relative humidity sensor

The composition of Nafion-Crystal Violet films and casting processes were studied in order to optimise the performance of an optical relative humidity sensor. The films were immobilised on a glass disc support, by placing a 5.0 mu L drop of a methanolic solution of a mixture of various compositions (molar ratio) of Nafion and Crystal Violet. The homogeneity of the films, and also their reversibility to humidity, increased when the molar ratio of Nafion to Crystal Violet was increased. The homogeneity was even further improved when these films were treated with a 5.0 mu L drop of methanol, leading to films of higher durability. The durability of films decreased with decrease in the film thickness. A methanol-treated, 5 mu m thickness film with a Nafion:Crystal Violet molar ratio of 10.1 showed a linear response range from 30 to 70% relative humidity (r(2) > 0.99), with good stability and reversibility.