Combining Ratiometric Thermometry and Super-Resolution Imaging of Upconverting Nanoparticles

Modern electronic, data storage, and energy conversion devices increasingly combine nanoscale dimensions with challenging operating conditions, including extreme temperatures, high pressures, large electromagnetic fields, and harsh chemical environments. In tandem, thermal properties play an outsize role in determining the overall performance of these technologies. Engineering improved performance thus requires the ability to visualize heat flow using non- invasive thermometry techniques with nanoscale spatial resolution. Conventional far-field optical techniques enable non-contact measurements, but such approaches fundamentally lack the spatial resolution required to resolve nanoscale temperature heterogeneities. Upconverting nanoparticles (UCNPs) are popular luminescent thermometers with Boltzmann-distributed emission intensity that facilitates thermometry via temperature-dependent spectral peak intensity ratios, an approach known as “ratiometric” thermometry. UCNP coatings have been applied for temperature mapping with diffraction limited spatial resolution. Additionally, single-UCNP measurements have been used to circumvent the diffraction limit, but this approach only allows for single-point nanothermometry. Recently, heavily (~10%) Tm-doped UCNPs were shown to enable a super-resolution imaging method called stimulated emission depletion (STED) that has been widely used in biological imaging to circumvent the optical diffraction limit. Compared with other common STED imaging probes, UCNPs require much lower STED laser powers. Separately, this same UCNP composition, but with a much lower Tm concentration of ~1%, has been used for diffraction-limited ratiometric thermometry, suggesting the possibility of adapting STED imaging for far-field optical temperature mapping with sub-diffraction limited spatial resolution.<br/> <br/>Here, we demonstrate that individual heavily Tm-doped UCNPs can be applied both for ratiometric thermometry and STED imaging. We measure temperature-dependent emission spectra of individual UCNPs as function of temperature, and we identify several temperature-dependent peak intensity ratios. While the temperature dependence deviates from the Boltzmann-distributed emission intensity that has been observed for lightly Tm-doped UCNPs, these ratios show sensitive, repeatable temperature dependence with good particle-to-particle uniformity. Using a custom-built STED imaging and spectroscopy system, we successfully demonstrate single-UCNP spectroscopic depletion for the first time, a key requirement for ratiometric STED nanothermometry. We also show that the sub-diffraction limited imaging resolution of our system is maintained from room temperature up to 400 K. Using an interfacial self-assembly method, we demonstrate the ability to create uniform UCNP monolayers spanning regions many microns wide, which can subsequently be placed on a sample surface. We then scan the surface of a monolayer-coated substrate that is heated to different uniform temperatures and successfully generate uniform temperature maps by recording temperature-dependent ratios at each pixel. These results indicate that temperature-dependent STED imaging of heavily Tm-doped UCNP monolayers has excellent potential to facilitate optical super-resolution thermometry of structures with nanoscale temperature heterogeneities in the near future.