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Salicylic acid (SA) is a vital regulator of induced defense mechanisms in plants. Progressive variations in the SA levels have been reported in many droughts, salt, and cold/heat-stressed plants, as well as after pathogen/herbivore attacks. Hence, real-time and in situ monitoring of this phytohormone will facilitate the early identification of crop stresses so that immediate interventions can be implemented to mitigate productivity losses and maintain the quality of the product (e.g., quality of produce and cotton fibers). However, the lack of in situ and localized analysis capabilities in current technologies limits their use in crop fields. Toward this endeavor, this work reports an LSPR (localized surface plasmon resonance)- based fiber-optic sensor functionalized with a conjugate of gold nanoparticles and copper-based metal-organic framework (CuMOF) for selectively measuring SA levels in sap. For this purpose, the distal end of an optical fiber was coated with gold nanoparticles that exhibited LSPR upon excitation by light’s electromagnetic field. The gold nanoparticles-coated fiber was covalently functionalized with CuMOF, which selectively oxidized SA. This oxidation reaction resulted in a variation in the local refractive index, thereby leading to a change in the LSPR intensity level. The fiber-tip sensor exhibited a sensitivity of up to 0.0117 % light reflection variations per μM concentration of SA and a limit of detection of 37 μM. Plant sap was used to calibrate the sensor for precision measurements. The fiber-tip sensor was also demonstrated to measure the SA levels in the stem of a live plant. Considering excellent dynamic detection range (100-1000 μM) and sensitive thiol chemistry, the developed sensor could hold promises in future in situ probe development for real-time measurements of phytohormones in sap.
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