Helix Shape Power-Dependent Properties of Single Upconversion Nanoparticles

Non-blinking, non-bleaching and super-bright single upconversion nanoparticles have been recently discovered with non-linear power-dependent properties and can be switchable under dual beam excitations, which are ideal for super-resolution microscopy, single-molecule tracking, and digital assays. Here, we report that the brightness of Nd3+-Yb3+-Er3+ doped nanoparticles displays a pair of unusual double helix shapes as the function of power densities of 976 nm and 808 nm excitations. We systemically analyse the power-dependent emission spectra, lifetimes, and power-intensity double-log slopes of single upconversion nanoparticles, which reveal that the dynamic roles of Nd3+ ions in the tri-doped nanosystem with underlining electron population pathways are power dependent. That is, at high power 808 nm excitation, Nd3+ ions can directly emit upconverted luminescence, with their conventional role of sensitization saturated in the Nd3Yb3+Er3+ energy transfer systems. Moreover, we confirm that the universal helix shape phenomena commonly exist in a set of eight batches of core-shell nanoparticles regardless of the doping concentrations of Nd3+, Yb3+ and Er3+ ions in the sensitization shell, migration shell, and active core, though the crossing nodes occur at different excitation power ranges. This study emphasizes the important role of power-dependent properties in both improving the upconversion emission efficiency and the design of non-linear responsive probes for imaging and sensing.

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