Laser-Induced graphene-based Fabry-Pérot cavity label-free immunosensors for the quantification of cortisol

There is a strong and growing need to monitor stress biomarkers in human beings and animals for real-time wellness assessment, which is a key indicator of health. Toward this, we report an optical fiber immunosensor based on laser-induced graphene (LIG) for the potential quantification and monitoring of cortisol hormone. The developed sensors were created using the Fabry-Pérot cavity principle. The cavities were filled with polyimide (PI) and, in some cases, PI mixed with gold nanoparticles. The interferometers were tested for temperature and refractive index, and an increase of two times in the sensitivity to the refractive index was observed due to the addition of the gold nanoparticles. In addition, the PI cavity was partially transformed into LIG using a CO₂ 20 kHz pulsed laser. The resulting LIG morphology was quite porous and presented a leafy texture. The presence of LIG creates a second cavity within the interferometer, and as a consequence, the spectral response of the interferometers resembled the Vernier effect. The refractive index behavior of the LIG-modified interferometers shows improvements in sensitivity up to 15.3 times due to the creation of the LIG cavity. The interferometers were then functionalized with anti-cortisol to promote an affinity for cortisol. Excellent sensitivities of up to -34.7 ± 0.7 nm/log(ng/mL) were achieved within few ng/mL concentration ranges (within the 0.5 to 3 ng/mL). The results obtained show an increase of up to 50 times in sensitivity when compared with other cortisol sensors. The achieved limit of detection was 0.1 ng/mL for the proposed sensors. The sensors’ responses were also shown to be immune to the interference from substances such as glucose, sucrose, fructose, and ascorbic acid. This study lays the foundation for new cortisol biosensors demonstrating a great potential for detection in a simple and highly sensitive way, opening its path for relevant biomedical and environmental monitoring applications in real samples. As a future outline, selectivity studies employing the detection of cortisol in the presence of interfering agents, both using synthetic and real samples, will be accomplished in order to fully investigate the potential of these sensing tools in real-life scenarios.