Optical Super-Resolution Nanothermometry via Stimulated Emission Depletion Imaging

Modern technologies ranging from next-generation electronics to energy conversion and storage 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 minimally 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, highly (~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 intensities. 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.

Here, we demonstrate an optical super-resolution nanothermometry technique based on temperature-dependent STED imaging and spectroscopy of highly Tm-doped UCNPs [1]. First, we show that individual highly Tm-doped UCNPs enable both ratiometric thermometry and STED imaging. We then measure temperature-dependent emission spectra of individual UCNPs and identify a peak intensity ratio that shows sensitive, repeatable temperature dependence with good particle-to-particle uniformity. Using a custom-built STED imaging and spectroscopy system, we also demonstrate single-UCNP spectroscopic depletion. We further 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 create uniform UCNP monolayers and multilayers that can subsequently be placed on a sample surface. By scanning the surface of a UCNP-coated substrate at different uniform temperatures, we can record temperature-dependent ratios at each pixel and generate temperature maps in both diffraction limited and STED modes. We also perform measurements on a Joule-heated microstructure and show that STED nanothermometry can resolve a temperature gradient that is undetectable with diffraction limited thermometry. These results indicate the potential of STED nanothermometry to uncover local temperature heterogeneities across a broad range of applications.

[1] Z. Ye, B. Harrington, and A.D. Pickel, “Optical Super-Resolution Nanothermometry via
Stimulated Emission Depletion Imaging of Upconverting Nanoparticles,” Science Advances 10,
eado6268 (2024).