Upconversion particle-based optical tweezers for sensing applications

Optical tweezers use focused laser beams to manipulate small particles, primarily for force sensing. Recent advances in nanoscale-trapping approaches have enabled the development of multiplexed sensing applications, such as temperature and viscosity detection. Upconversion particles (UCPs) and, in particular, lanthanide-doped nano-/micro-crystals (~6 nm to 6 μm) exhibit particular anti-Stokes emission properties, which facilitate their visualization when trapped and the detection of changes to their properties based on temperature and orientation. Their ion resonance enhances the trapping force, enabling the manipulation of smaller particles and their use for force sensing. Here we provide step-by-step instructions to build UCP-based holographic optical tweezers systems, including super-resolved photonic force microscopy and fluorescence optical tweezers. We detail the characterization of the setup for subfemtonewton-scale force sensing and include nanoprobe functionalization, force sensitivity validation and comparison with known forces. We further include the procedures for temperature and viscosity sensing, such as calibrating polarized spectra, initiating UCP rotation and analyzing viscosity via spectral fluctuations. Applications, including nanoparticle-DNA-coated gold film interactions and temperature distribution near single cells, are shown as well. The procedure typically requires 6 days to complete and is suitable for users with expertise in photonics.