Toward a Label-Free Electrochemical Impedance Immunosensor Design for Quantifying Cortisol in Tears

Cortisol is a viable biomarker for monitoring physiological, occupational, and emotional stress and is normally present in tear fluid at approximately 40 nM, or higher as a result of stress. We present characterization and quantification of cortisol via several electrochemical methods versus the standard enzyme-linked immunosorbent assay, commonly known as ELISA. We also present a prototyped design of a disposable test strip and handheld sensor based on label-free electrochemical impedance spectroscopy to quantify cortisol levels in tear fluid within approximately 90 seconds. Electrochemical characterization of the cortisol molecule was conducted using cyclic voltammetry, amperometric i-t, and square wave voltammetry. Lower limits of detection for these techniques were not sufficient to quantify cortisol and phycological tear ranges: 0.1 M, 0.23 M, and 193 M for cyclic voltammetry, amperometric i-t, and square wave voltammetry, respectively. However, electrochemical impedance spectroscopy (EIS) was to be the best mode of cortisol quantification and comparison to ELISA technique (detection range of ~ 138 pM - 552 nM). The initial EIS biosensor obtained a lower limit of detection of 59.76 nM with an approximate 10% relative standard deviation. The cortisol assay and tear collection prototype presented here offer a highly reproducible and ultra-low level of detection with a label-free and rapid response.