In stressful situations, concentrations of various molecules in the human body shift in response to the stressor. These molecules are measurable indicators of stress and are therefore called stress biomarkers. In many stress conditions, such as in overtraining syndrome, early detection of these biomarkers is highly important as the conditions are often not fully reversible. Early detection of the stress symptoms could be achieved with wearable sensors that would continuously monitor health information from different body fluids, such as sweat, urine, saliva, tears and blood. Compared to more conventional electrochemical or optical methods, plasmonic sensing could offer higher sensitivity, better stability and faster data collection while enabling implementation to compact devices. In this work, a sensor chip, based on grating-coupled surface plasmon resonance, is proposed for stress biomarker detection. In this work, we show a highly sensitive grating-based SPR sensor working in concert with a tunable laser within the wavelength range of 1528-1565 nm. The SPR sensor was designed using COMSOL Multiphysics software and was fabricated by means of UV nanoimprinting lithography. The implemented SPR sensor shows sensitivity close to 1200 nm/RIU, with a figure of merit (a ratio between the sensitivity and the full width at half maximum of the SPR dip) exceeding 400. The experimental results are strongly in agreement with COMSOL simulations. Such impressive characteristics of the fabricated sensor are among the best reported in the literature. The sensitivity of the chip was tested with two different stress-related biomarkers: glucose and lactate. With the tested range of 0 to 1.1 M, in the current version of the setup, without a receptor layer, the detection limits of glucose and lactate were 5.9 and 36.9 mM, respectively, which are close to the physiological ranges of these analytes in body fluids. The detection limit can be further improved with the sensor functionalization, thermal stabilization and mechanical isolation. When integrated into a wearable device, this approach has a potential in future healthcare applications, such as in continuous stress monitoring.
Upconversion (UC) luminescence sensing is a technique to improve the detection limit of conventional fluorescence in biosensing that is commonly limited by the autofluorescence-generated background signal. The main limitation of UC materials is their low wavelength conversion efficiency. Many studies have been made to enhance the efficiency of UC materials by optimizing light absorption and energy transfer processes. However, rather low efficiency remains an issue limiting the practical usage of UC materials in biosensors. Plasmon enhancement is a way to improve UC photoluminescence by enhancing the excitation and emission rates. In this study, we modeled and fabricated gold gratings for exciting surface plasmon polaritons (SPPs) at 976-nm wavelength. We aim at increasing the local optical intensity at the locations of UC nanoparticles on a nano-structured plasmonic surface. The UC nanoparticles were adsorbed on the gratings via biomolecule conjugation. UC photoluminescence on the gratings was compared with flat gold surfaces. Experimentally, we achieved UC enhancement up to 70, which is relatively high in comparison with other plasmon-enhanced UC techniques presented in the literature. The results of our work can be applied in various biosensing applications in which low excitation intensity is preferred.
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