Upconversion Particle as a Local Luminescent Brownian Probe: A Photonic Force Microscopy Study

Near-infrared (NIR) light sensitive
lanthanide-doped NaYF4 upconversion particles (UCPs) are
gaining increasing attention as
local probes in biomedical applications. Here, we implemented a photonic
force microscope (PFM) to manipulate and study the optical properties
of trapped single UCPs, β-NaYF4:Yb,Er. In particular,
we focused on the mechanisms of the optical trapping of nonspherical
UCPs of different sizes, in the range 0.5–2 μm, as well
as on their upconversion photoluminescence (UCL) properties under
excitation with the strongly focused laser beam (λ = 1064 nm)
of the PFM, operating at power densities up to 14.7 MW cm–2. A careful analysis of UCL under such conditions points to three
emission peaks at 469, 503.6, and 616.1 nm, which were enhanced by
the high laser power density. The analysis of Brownian motion was
used to quantify the thermal fluctuations of the particle inside the
optical trap as well as the particle sizes and optical forces acting
in the two dimensions perpendicular to the optical axis. A steep dependence
of UCL as a function of the particle diameter was found for UCPs having
sizes smaller than the focal spot (∼900 nm) of the NIR laser.