An Extended-Gate FET Biosensor Designed for Real-Time and Continuous Detection of Urea Concentration, a Biomarker of Renal Function

This study aimed to develop a field-effect transistor (FET)-type biosensor with an enzyme-modified extended-gate (EG) electrode for the continuous monitoring of urea concentration as an indicator of renal function. Urease, which hydrolyzes urea, was employed as the recognition element, and a silk fibroin matrix was used for enzyme immobilization. The sensor exhibited a reversible voltage response to increases and decreases in urea concentration in phosphate-buffered saline (PBS), and showed sufficient sensitivity within the clinically relevant blood urea range (17-43 mg/dL). The optimal pH for sensor operation was 6.0-7.0, and external light illumination of the EG surface had a negligible effect when a differential circuit was employed. When the molar concentration of PBS was varied from 1 to 100 mmol/L, the lower limit of urea detection increased from 0.1 to 10 mg/dL, accompanied by a decrease in the voltage response. These observations are attributed to the reduction in Debye length at the EG surface and the increase in the buffer capacity of PBS.