We present the optical coupling of the silver nanoparticles (AgNPs)-conjugated dye molecule into fiber optical modes for detecting fluorescence with the enhanced signal-to-noise (S/N) ratio. This near field coupling of the excited state of organic dye (FAM) molecules into the fiber multimodes occurs by immobilizing them on the exposed surface of fiber core, permitting the coupled light to be guided along the fiber for detection. This fiber based scheme is the first attempt to single out the fluorescence using fiber modes not for carrying excitation light but only for collecting emission light via the dye-fiber coupling. The emission-selective coupling into fiber modes turns out to be effective in reducing the unwanted background noise arising from both the false detection of excitation light and bulk autofluorescence. This scheme differs from the previously reported fluorescence sensors based on waveguides where guided modes at λex excite dye molecules via their evanescent fields. In addition, the local fields enhanced by AgNPs in close proximity to FAM molecules on the fiber core surface increase the rates of dye excitation and radiative decay/AgNP supported surface plasmon coupled emission. While focusing on demonstrating the proof-of-concept of the scheme presented, we obtain the maximum of 4.2-fold enhancement of the signal-to-noise (S/N) ratio in detecting fluorescence as compared to a conventional fluorescence detection scheme. The results presented in the fiber-based scheme may find an application where high S/N ratio fluorescence based biochemical assay is required in a small-sized device with remote sensing capability.
Keywords: Fiber coupled fluorescence; Fluorescence based assay; Localized surface plasmon resonance (LSPR); Near-field coupling; Silver nanoparticles.
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