Optical fiber immunosensors based on SPR for cortisol detection

• Autor(a) principal: Soares, Maria Simone Fernandes
• Data de Publicação: 2021
• Tipo de documento: Dissertação
• Idioma: eng
• Título da fonte: Repositório Científico de Acesso Aberto de Portugal (Repositórios Cientìficos)
• Texto Completo: http://hdl.handle.net/10773/33682
• topic: Biosensor; Cortisol; D-shape; Polymeric optical fiber (POF); Silica optical fiber (SOF); Surface plasmon resonance (SPR)

Stress is a normal physiological and behavioral response to a stimulus that somehow disturbs the maintenance of homeostasis, leading to changes in cortisol (stress hormone) levels. When stress is persistent and uncontrolled, it can severely affect several areas, such as human health and some applications in marine biology, including aquaculture production. Currently, the detection of cortisol is performed in laboratories using conventional techniques which have several disadvantages, one of them being the long waiting time for a response. Therefore, it is essential to develop biosensors that detect cortisol directly in point of care (POC). Biosensors are miniaturized analytical devices capable of detecting and quantifying specific biomarkers. In addition, they allow real-time monitoring, overcoming the disadvantages of conventional techniques. Silica optical fibers (SOF) and polymeric optical fibers (POF) coated with gold (Au) were used in this work to develop immunosensors for cortisol detection. Throughout the entire experimental process, simulation of the response to the refractive index (RI) of the optical fibers coated with Au was also carried out to compare the experimental results with those obtained from the simulation. In the laboratory, Au-coated fibers were initially characterized at RI with eight glucose concentrations ranging from 1.333 to 1.386 RI units (RIU). The obtained sensitivity with Au-SOF was 1646.67 ± 91.66 nm/RIU, being lower than the simulated (2138.95 ± 142.65 nm/RIU), and for Au-POF the sensitivity presented a value of 1566.81 ± 96.87 nm/RIU, was not possible to obtain results by the simulation. Subsequently, the fibers were functionalized with anti-cortisol antibody (AB) and passivated with bovine serum albumin (BSA), using cysteamine which has a thiol group and an amine group to allow binding to Au and AB, respectively. After this procedure, both immunosensors were tested for a range of cortisol concentrations from 0.01 to 100 ng/ml, in which the SOF immunosensors had a mean total resonance wavelength shift of 3.70 ± 0.39 nm, and a limit of detection (LOD) of 1.46 ng/mL, whereas for POF immunosensors it was not possible to trace a calibration line. Control tests on sensors functionalized with anti-cortisol AB were also performed using glucose (50 mg/dL and 500 mg/dL) and cholesterol (170 mg/dL and 240 mg/dL). The functionalization and detection test of immunosensors were accompanied by morphological and chemical characterization. Finally, a preliminary test of a SOF immunosensor regeneration showed promising results. In the future, it will be necessary to perform regeneration and reuse tests to validate the applied procedure.