In this work, we report a simple electrochemical sensor that detects proteinuria upon immersion into a sample. We immobilized peptidases in conjunction with glutamate oxidase (GluOx) on a Prussian blue-modified glassy carbon (PB-GC) electrode. In the sensor, albumin, which is a major protein in urine, was degraded by peptidases to produce glutamate that was detected by GluOx on PB-GC. Initially, we validated the strategy for the detection of proteinuria using glutamate sensing. We quantified the amount of glutamate produced by degrading human serum albumin with HCl and carboxypeptidase A (CPA). The results indicated that an increase in the enzyme degradation rate was necessary for daily proteinuria sensing that needs to be completed within 1 h. Therefore, we investigated the use of endopeptidase in conjunction with CPA. The combination of proteinase K and CPA liberated glutamate from albumin at 17.5 times higher comparing to solely CPA. The subsequent glutamate assay using peptidase-modified GluOx-PB-GC electrode did not exhibit changes in the electrochemical signal with increases in the glutamate concentration, because peptidase degraded the GluOx on the modified electrode. To prevent this, GluOx was encapsulated in ZIF-8. Glutamate was successfully detected using peptidase and GluOx encapsulated in the ZIF-8-modified PB-GC (peptidase-ZIF-8/GluOx-PB-GC) electrode. Finally, an albumin assay was performed using the peptidase-ZIF-8/GluOx-PB-GC electrode, wherein albumin was detected with a sensitivity of 0.1 mg/mL within 30 min. Our simple and easy-to-use method for albumin detection provides a viable sensor for daily urinalysis.
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