Rich stochastic dynamics of co-doped Er:Yb fluorescence upconversion nanoparticles in the presence of thermal, nonconservative, harmonic and optical forces

Abstract

The stochastic dynamics of individual co-doped Er:Yb upconversion nanoparticles (UCNP) were investigated from experiments and simulations. TheUCNPwere characterized by high-resolution scanning electron microscopy, dynamic light scattering, and zeta potential measurements. Single UCNPmeasurements were performed by fluorescence upconversion micro-spectroscopy and optical trapping. The mean-square displacement (MSD) from singleUCNPexhibited a time-dependent diffusion coefficient which was compared with Brownian dynamics simulations of a viscoelastic model of harmonically bound spheres. Experimental time-dependent two-dimensional trajectories of individualUCNPrevealed correlated two-dimensional nanoparticle motion. The measurements were compared with stochastic trajectories calculated in the presence of a non-conservative rotational force field. Overall, the complex interplay ofUCNPadhesion, thermal fluctuations and optical forces led to a rich stochastic behavior of these nanoparticles.