Highly reliable label-free detection of urea/glucose and sensing mechanism using SiO₂ and CdSe-ZnS nanoparticles in electrolyte-insulator-semiconductor structure

A simple, label-free and cost effective sensor have been studied for reliable urea/glucose sensing, and common serum analyte detection comparable with market available urea "Assay Kit" is also performed by using SiO₂ and CdSe-ZnS nanoparticles in electrolyte-insulator-semiconductor structure for the first time. Thermally grown SiO₂ membrane has shown lower pH detection limit (0.081) and lowest drift rate (2.9 mV/hr) than those of the sputtering and E-beam deposited SiO₂ membranes. The urea detection at physiological buffer pH 7.4 with sensitivity of ∼1.6 mV/mg.dl^-1 at linear range of 6 to 36 mg/dl is shown. The pH detection limit is further reduced (0.074) by using chaperonin protein mediated CdSe-ZnS nanoparticles assembly over SiO₂ surface owing to high pH sensitivity of 55 mV/pH. The sensing mechanism is due to the SiO_x content decreased with increasing pH value. This suggests the lower H⁺ ions absorption on the sensing membrane surface, which is observed by X-ray photo-electron spectroscopy. The glucose concentration is detected by using the core-shell CdSe-ZnS nanoparticles through H₂O₂ sensing because of reduction/oxidation (redox) properties of Zn as well as Zn²⁺ ions generation. Due to the high catalytic activity for H₂O₂ sensitivity, low detection limit of 1 μM is obtained, which will help to detect glucose using this bio-chip in future.