Supersensitive sensing based on upconversion nanoparticles through cascade photon amplification at single-particle level

Single-particle sensing strategy is expected to achieve super-sensitivity with extremely low detection limit by improving the contact surface with targeted object and excluding the statistical effect of nanoparticle ensembles. However, single-particle sensing based on upconversion nanoparticles (UCNPs) is rarely reported, limited by the detection difficulty of single particles. Herein, we designed a cascade photon amplification structure of surface plasmonic photonic crystals consisting of polymethyl methacrylate (PMMA) opal photonic crystals (OPCs) with plasmonic CsxWO3 NPs onto its surface voids. Finite-difference time-domain simulations and atomic force microscopy (AFM) characterization evidence that UCNPs at single-particle level (abbreviated as UCNP) are located at the maximum amplification of optical field. Combined with the advanced fluorescence microscope equipped with AFM, the upconversion emissions of single NaYF4:Yb3+, Er3+ NP (~45 nm) are greatly boosted ~1600 folds along with lower pumping thresholds by cascade photon amplification effect for the first time. As a proof of concept, such enhanced UCNP is employed as a fluorescent probe to develop a novel UCNP senor for supersensitive dithiothreitol detection with a detection limit of 0.25 nM, two orders of magnitude lower than the previous reports. This work provides a new attempt to develop high performance of UCNP for constructing supersensitive sensors.