Flow immunosensor system with an electrode replacement unit for continuous cortisol monitoring for fish

- A flow immunosensor system was designed for continuous cortisol monitoring for fish.
- The sensor can be replaced in seconds by using proposed replacement unit.
- Compare with conventional ELISA method, the measurement time was greatly reduced.

Blood cortisol concentration change in fish is widely used as an indicator of stress. In this study, we developed a flow system which equipped by an electrode replacement unit which can replace sensor conveniently. Multiple sensors sequentially disposed vertically in the holder, and the conductive portion of the sensor to be used was bonded with which set in the bottom of the holder. After measurement, next sensor was inserted into the holder and the replacement would be completed by sequentially moving. The flow system was constructed by replaceable working electrodes which sequence placed in the electrode replacement unit and count electrode into a reaction loop which operated by micro-pump. During the measurement, the potassium ferricyanide was transported into the flow loop and applied a voltage (+ 250 mV). Output current values were measured both before and after the immune-reaction and the change between them was calculated to determine the cortisol levels. As a result, replacement of the immunosensor performed in only several seconds and a good linear relationship was observed between the sensor response and cortisol in the range of 0-40 ng ml− 1. We also performed a measurement of cortisol levels in plasma of tilapia (Oreochromis niloticus) both by proposed biosensor system and ELISA which got a good correlation. We believe that the proposed system can provide a simple and rapid detection of cortisol levels for fish and its principle could be a foundation of a continuous in vivo monitoring of fish cortisol change in fish in the future.