Excitation Laser Power and Temperature-Dependent Luminescence of Optically Trapped Upconversion Particles

Abstract

The routinely adopted ensemble luminescence spectroscopy of rare-earth-ion-doped upconversion micro-/nanoparticles does not provide insights into the role of shell passivation and plasmonic particle incorporation at a single-particle level. Herein, we compare the results of silica shell formation as well as further incorporation of plasmonic nanoparticles onto the green 1 ( 2 H 11/2 to 4 I 15/2 transition, λ�520 nm), green 2 ( 4 S 3/2 to 4 I 15/2 , transition, λ� 540 nm), and red ( 4 F 9/2 to 4 I 15/2 transition) upconversion luminescence of an optically trapped single upconversion particle. Analysis of the results at a single upconversion particle level illustrates that the core−shell particle exhibits a higher intensity for green as well as red emission, and the effect is further pronounced with attachment of plasmonic nanoparticles (NPs) onto the SiO 2 shell. Trapping laser power-dependent studies of an optically trapped particle show a linear increasing behavior for green 1, green 2, and red emissions with a slope value near 1. On the other hand, laser power-dependent studies using the Yb resonant 980 nm laser with a laser power that is 2 orders of magnitude less than the trapping laser show two slope values (changing from less than 1 to more than 1) for the double-logarithmic plot of luminescence of the upconversion microparticle (UCMP) and UCMP@SiO 2 trapped particles, whereas the signal intensity saturates with a single slope value for a UCMP@SiO 2 @Au particle. The temperature-dependent luminescence of the thermally coupled intensity ratio of emission green 1 (520 nm) and green 2 (540 nm) can be applied as a temperature sensor. The thermal sensitivity measurements within the biologically relevant temperature range (298−333 K), using optically trapped particles, reveal distinctions in thermal sensitivity among UCMP, UCMP@SiO 2 , and UCMP@SiO 2 @Au. The results from the present study can find applications in bioimaging, single-cell studies, optical thermometers, photothermal converters, etc.