Monitoring nitrate ions is essential in agriculture, food industry, health sector and aquatic ecosystem. We show that a conducting polymer, poly(3,4-ethylenedioxythiophene) (PEDOT), can be used for nitrate sensing through a process in which nitrate ion uptake leads to oxidation of PEDOT and change of its optical properties. In this study, a new platform is developed in which a single-mode fibre coated at the tip with PEDOT is used for nitrate sensing. A crucial step towards this goal is introduction of carbonate exposure to chemically reduced PEDOT to a baseline value. The proposed platform exhibits the change in optical behaviour of the PEDOT layer at the tip of the fibre as it undergoes chemical oxidation and reduction (redox). The change in optical properties due to redox switching varies with the intensity of light back reflected by the fibre coated with PEDOT. The proposed platform during oxidation demonstrates linear response for the uptake of nitrate ions in concentrations ranging between 0.2 and 40 parts per million (ppm), with a regression coefficient R2=0.97 and a detection limit of 6.7 ppm. The procedure for redox switching is repeatable as the back reflection light intensity reaches ±1.5% of the initial value after reduction.
Simple optical force sensors have many uses but suffer from relatively low sensitivity or low fabrication-tolerance. We have demonstrated that pure skew rays can enhance the sensitivity of a bend-loss-based force sensor over the mixture of rays created from a normal incidence by a factor of 3.8 to enable a sensitivity of 0.126 dB/N. The dynamic range was measured from 222.2 mN to >14.1 N. The response/recovery times were found to be 500 and 600 ms, respectively. We also showed a compression-loss-based force sensor exhibiting a small deviation of 6.7% in sensitivity of 0.015 dB/N against the changes in the launch angle of light. The dynamic range was tested from 875.0 mN to >24.2 N. The response/recovery times were observed as 350 and 300 ms, respectively. The sensitivity of these force sensors can be further enhanced with geometry and fiber-material changes, and the enhancement technique could be extended to other designs.
Existing sensing technologies lack the ability to spatially resolve multiple sources of water or humidity without relying on the deployment of numerous inline sensors. A fully distributed approach has the potential to unlock a diverse range of applications, such as humidity mapping and liquid-depth measurements. We have explored a new direction toward what is, to the best of our knowledge, the first non-bending fully distributed water/humidity sensors. This new class of sensors was made possible from the first combination of small-core exposed-core fiber, a hydrophilic polyelectrolyte multilayer coating, and coherent optical frequency-domain reflectometry. Their non-bending nature enables deployment in a wider range of environments compared to the bending type based on water-induced fiber bending. The sensing mechanism involves monitoring back-reflected optical signals Manuscript
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