Abstract
Plasmon enhanced upconversion luminescence has drawn extensive attention due to its advances in the fields of optical sensors, bio-detection and medical diagnosis. However, the coupling state between plasmon and upconversion emission is never clearly revealed. Here, we report a novel critical state in plasmon coupled upconversion nanoparticles (UCNPs) that has a long lifetime and is highly stimuli-responsive. The critical state is generated through the precise control over the coupling between plasmonic local field and optical near field. Platinum (Pt) film with thickness of ~7 nm is deposited on photonic crystals to form a periodic plasmonic antenna arrays. UCNPs with inert shells are embedded in plasmonic antenna arrays with a tunable plasmon coupling distance via the thickness control of inert shells. An ultrasensitive critical state is observed for the coupling distance of ~10 nm between Pt film and NaGdF4:Yb/Er luminescent core, accompanied with light confinement by NaYF4/NaGdF4 heterojunction shells. This critical-state can be employed as an ultrasensitive route for fluorescent detection of NH3 and the lowest detection of 0.47 ppm can be achieved that is about 34 times lower than ever reported.
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