Low-cost optical fiber sensors for the portable detection of cortisol and <i>E. coli</i>

Detection of biomarkers, as well as bacterial infection levels, plays an important role in clinical analysis. Development in research for fast, high-sensitivity, and selective sensors has a key role in decision-making during a health issue. Aligning to these features, low-cost biomarkers and bacteria population detection methods should be developed in order to make clinical trials more accessible to the whole of society. In this context, optical sensors based on surface plasmon resonance (SPR) phenomena have been intensively studied due to their easy functionalization and fast and amplified response. This work aimed at the optical detection of some analytes in aqueous solutions. SPR-based D-shape sensors employing gold-deposited polymer optical fibers (POF) were used in a portable platform to detect concentrations of cortisol, the stress hormone, and <i>E. coli</i>, a bacteria strain. The sensors showed good linearity in response to increasing cortisol concentrations, but were less consistent in detecting the <i>E. coli</i> bacteria. However, <i>E. coli</i> sensors possess a sensitivity of -0.38 ± 0.06 nm/log(CFU/mL), bearing a resemblance to the sensitivity obtained for cortisol sensors with -0.41 ± 0.04 nm/log(ng/mL). The biosensors had limits of detection (LODs) lower than those of existing literature, with <i>E. coli</i> detection of 3.72~ × ~10^-7 CFU/mL, and cortisol detection of 1.73~ × ~10^-4 pg/mL. Small topography profile differences of about 2.0-3.4 nm were found through atomic force microscopy (AFM), equivalent to the thickness of the used antibodies, suggesting they were deposited parallel to the gold surface. In the future, methods with the potential to assure antibody verticality should be explored.