Effect of nanostructure on the urea sensing properties of sol-gel synthesized ZnO

Urea sensing properties of zinc oxide in thick film form are presented here. Zinc oxide thick films were prepared on Al-sheet by conventional doctor blade method using organic additives. Flower-like structure and nanobelts of ZnO was synthesized by solution method using zinc acetate dihydrate and sodium hydroxide. Structural morphology significantly changed with precursor concentration (0.3-0.5 M) from a belt to flower-like structure. Urease was covalently attached with zinc oxide (by soaking in urease solution containing 100 units for 3 h). In general, conductivity of film increases after urease immobilization. The urease immobilized films were found sensitive to urea concentration from 1 mM 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). Nanobelt structure of ZnO resulted in highest urea sensitivity. Sensors exhibited linear response when tested for different concentration of human urine in buffer solution (1:9 to 4:6). The sensor responses are reproducible, reliable, reversible and selective, with a response time of 6 s.