Development of a micro-planar amperometric bile acid biosensor for urinalysis

The determination of bile acid concentration in urine is useful for the screening and diagnosis of various hepatobiliary diseases. Currently, there is no concise method to determine bile acid concentration in urine. This study describes a bile acid biosensor fabricated by electrochemical technique for urinalysis. The micro-planar electrodes employed for the study consisted of a working electrode (platinum), a counter electrode (platinum) and a reference electrode (silver/silver chloride (Ag/AgCl)). The sensor chip was coated with Nafion® using a spin-coater in order to both eliminate many interference species in urine and achieve long-term stability of the reference electrode. Nafion® coating allowed the sensor chip to prevent the electrode reaction from interference species in urine, because it is charged negative strongly (Nafion® contains sulfonic acid group). Three enzymes (bile acid sulfate sulfatase: BSS, β-hydroxysteroid dehydrogenase: β-HSD, and NADH oxidase: NHO) were immobilized by glutaraldehyde (GA: cross-linker) onto the sensor chip, because the immobilization of enzymes by GA is simple and commonly carried out. The sensor chip was able to detect bile acid in buffer solution. The optimum enzyme ratio immobilized onto the sensor chip was BSS:β-HSD:NHO = 4:4:20 U/1 chip. There was a relationship between the concentration of bile acid and the response current value. The dynamic range of the sensor chip was 2–100 μM for bile acid. Additionally, bile acid in the urine specimen could be detected using this bile acid biosensor. We present a simple and rapid bile acid biosensor with high sensitivity and high reproducibility.