Abstract
This paper reports on high-accuracy simulation of a grating structure based fiber optic plasmonic sensor for salivary cortisol sensing. Gratings of SiO2 and SiC (one at a time) in combination with a thin Ag layer are considered to be in direct contact with analyte medium (solutions containing different concentrations of cortisol) considering that the groove regions are also filled with analyte. The optimization of Ag layer thickness is carried out to achieve maximum power loss (PL) corresponding to cortisol concentration variation. The variation of PL (in dB) spectra with the angle of incidence (α) is the sensing mechanism of the proposed scheme. Sensing performance is extensively analyzed in terms of sensitivity, limit-of-detection (LOD) and figure-of-merit (FOM) that incorporates both the sensitivity and the width of the corresponding PL curves. While the sensitivity and FOM values are significantly large, the results also reveal that in angular interrogation mode (AIM), an average LOD of 9.9 pg/mL and 9.8 pg/mL is obtained for SiO2 and SiC-based sensor designs, respectively. When the intensity interrogation method (IIM) in place of AIM is considered, an average LOD of 22.6 fg/mL and 68.17 fg/mL is obtained for SiO2 and SiC-based sensor designs, respectively. LOD (with IIM, in particular) is considerably better than the present-state-of-art related to cortisol monitoring. Pragmatic model for possible practical implementation of sensor scheme is also discussed. The involvement of optical fiber in the proposed sensor design makes it possible to implement it as a flexible sensor or for wearable solution for cortisol detection via sweat monitoring as well as for measuring cortisol level in aquaculture tanks where concentration levels are much lower than 10 ng/mL.
Highlights
The human immune system is prone to be altered by allostatic stress
The involvement of optical fiber in the proposed sensor design makes it possible to implement it as a flexible sensor or for wearable solution for cortisol detection via sweat monitoring as well as for measuring cortisol level in aquaculture tanks where concentration levels are much lower than 10 ng/mL
When the LOD was represented under an intensity interrogation mode, the average LOD
Summary
The human immune system is prone to be altered by allostatic stress. This stress level can be identified by the important biomarkers such as, cortisol, norepinephrine (NE), serotonin, neuropeptide. The quantification of cortisol is based on arduous separation methods such as, electrochemical immunosensing, or immunoassay in case of conventional laboratory detection techniques [8] These techniques provide sensitive detection of biosamples, several drawbacks are associated with them such as, large sample volume (0.2–2 mL), extensive analyze time, the requirement of skilled personnel and specialized instruments. In context of SPR based cortisol sensors, Stevens et al reported cortisol detection in human saliva with a detection limit of 0.36 ng/mL [11]. The present work is focused on the detection of salivary cortisol using a plasmonic grating based fiber optic SPR (FOSPR) sensor using the angular interrogation mode. The analysis of sensor’s performance in terms of the limit-of-detection (LOD) is presented based on angular interrogation and intensity interrogation methods
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