Ultrasensitive critical-state of upconversion nanoparticles assembled on plasmonic antenna arrays for rapid and accurate determination of NH3

•Nanostructure NaGdF4:Yb3+/Er3+ UCNPs with a shell thickness of 4.95, 10.35 and 20 nm are prepared.•A multilayer functional film by PS microspheres, Pt particles and core/shell UCNPs is designed and presented.•The coupling distance of ultrasensitive critical state is ~10 nm between plasmonic local field and optical near field.•This critical state is employed as an sensitive route for accurate detection of NH3 and the lowest detection of 0.47 ppm.