Polymer-based microsensor for soil moisture measurement

A moisture microsensor based on poly(3,4-ethylenedioxythiophene–poly(styrene-sulfonate) (PEDOT–PSS) conductive polymer is developed and presented in this paper. The change in electrical characteristics of the PEDOT–PSS polymer film is used to determine its sensitivity and working mechanism when exposed to different levels of moisture content. The output characteristics, the change in electrical sheet resistance of the PEDOT–PSS film versus the percentage change in relative humidity (%RH), show that the conductivity of the film decreases when it is exposed to increasing levels of moisture content. The moisture sensors thus fabricated based on the PEDOT–PSS thin film were used to detect the gravimetric water content present in highly plastic (CH) soil samples (Buckshot Clay) for geological and geotechnical engineering applications. The Fourier transform infrared spectroscopy (FT-IR) study of the PEDOT–PSS film on a glass substrate showed the incorporation of OH molecules in the film when it was exposed to moisture environment. This incorporation of OH molecules caused the change in resistance of the PEDOT–PSS film when exposed to moisture content. The change in the output resistance of the sensor device was observed to be from 2.5 to 4.0 M ohm when exposed to soil samples with 15–35% change in gravimetric water content.