  Tunable and ultrasensitive surface plasmon resonance (SPR) sensors are highly
desirable for monitoring stress hormones such as cortisol, a steroid hormone
formed in the adrenal glands in the human body. This paper describes the
detection of cortisol using a bimetallic SPR sensor based on highly anisotropic
two-dimensional material, i.e., phosphorene. Thicknesses of bi-metal layers,
such as copper (Cu) and nickel (Ni), is optimized to achieve strong SPR
excitation. The proposed sensor is rotated in-plane with a rotation angle
around the z-axis to obtain phosphorene anisotropic behavior. The performance
parameters of the sensor are demonstrated in terms of higher sensitivity
(347.78 degree/RIU), maximum angular figure of merit (1780.3), and finer limit
of detection of 0.026 ng/ml. Furthermore, a significant penetration depth (203
nm) is achieved for the proposed sensor. The obtained results of the above
parameters indicate that the proposed sensor outperforms the previously
reported papers in the literature on cortisol detection using the SPR
technique.
