Helix Shape Power-Dependent Properties of Single Upconversion Nanoparticles
Jiayan Liao, Dayong Jin, Chaohao Chen, Yiming Li, Jiajia Zhou
Abstract
Nonblinking, nonbleaching, and superbright single upconversion nanoparticles have been recently discovered with nonlinear 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 and 808 nm excitations. We systemically analyze 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 tridoped 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 nonlinear responsive probes for imaging and sensing.