Cortisol (MW = 362.5) is a steroid hormone produced in the human body by the adrenal gland in response to elevated levels of stress. Normal levels of cortisol in the human body range from 6-23 microgram per deciliter. Cortisol has also been widely studied in order to better understand fish physiology, with applications to fish ecology and fish farming, since cortisol levels can also be employed as a biomarker for reproductive activity. 

Following immobilization of the polyclonal antibody to cortisol atop an Au electrode, we have detected cortisol in both PBS buffer solution and fish blood plasma, with a detection range up to 0.5 microgram/ml.  Binding of cortisol approximately follows the Langmuir adsorption isotherm, and the detection range is limited at high concentrations by saturation of the antibody film.  This allows the use of sample dilution to reduce the physiological concentration of cortisol down to the range in which it can be detected by impedance biosensing.  The advantage of this approach is that non-specific adsorption is also dramatically reduced by sample dilution.  Non-specific adsorption in this system is unmeasurable, due to both the use of sample dilution and the use of BSA for surface blocking.