Urea sensor based on tin oxide thin films prepared by modified plasma enhanced CVD

Urea sensing properties of tin oxide thin films are presented here. Tin oxide thin films were deposited by modified plasma enhanced chemical vapor deposition (CVD) technique at the deposition temperatures of 500-800 °C and RF power of 100 W. Film morphology significantly changed with deposition temperature from flakes to tiny crystals having tetragonal rutile structure grown along the [1 1 0] direction. Urease was covalently attached with tin oxide (by soaking in urease solution for 3 h). In general, conductivity of film increases after urease immobilization. The urease immobilized films were found sensitive to urea concentration from 1 to 100 mM. Three different sensitivity regions are observed viz. (i) lower concentrations (below 10 mM); (ii) linear region up to 50 mM and (iii) a saturation region above 50 mM. Sensors are extremely sensitive in region (i). Films deposited at higher temperature resulted in increased urea sensitivity. From the elemental analyses of the films after urease immobilization, urease was found attached with tin oxide, as evident by N 1s peak in the photoelectron spectra. A possible sensing mechanism is presented and discussed.