Sampling Sub-Diffraction Temperature Gradients with Spectrally Orthogonal Nanoparticle Luminescence

Recording the temperature-dependent luminescence emitted by an isolated single nanoparticle offers one strategy for performing far-field optical thermometry with spatial resolution below the diffraction limit. However, such measurements are inherently restricted to probing the temperature at a single spatial point. Here, we demonstrate an approach to sampling temperature gradients at multiple points within a subdiffraction region by simultaneously collecting the emission from different nanoparticle species with spectrally orthogonal temperature-dependent luminescence. Taking advantage of the narrow spectral bands and wavelength tunability of lanthanide-doped upconverting nanoparticle (UCNP) emission, we use a single laser to excite both NaYF₄:Yb³⁺,Er³⁺ and NaYF₄:Yb³⁺,Tm³⁺ UCNPs and concurrently acquire their spectrally distinct temperature-dependent luminescence. The emission spectra and temperature response obtained from tandem UCNP pairs consisting of one NaYF₄:Yb³⁺,Er³⁺ and one NaYF₄:Yb³⁺,Tm³⁺ UCNP are in excellent agreement with corresponding measurements using isolated individual UCNPs of each composition. To demonstrate the utility of this approach, we use a tandem pair of UCNPs located ∼108 nm from each other to probe the sharp temperature gradient resulting from laser heating of an isolated silver nanodisk. While the diffraction-limited emission spots of the UCNPs overlap nearly completely, we can distinguish a temperature difference of ∼19 K between their two locations. This capability is particularly applicable to scenarios that would benefit from multiple temperature data points, but where the majority of the sample surface must remain accessible for other purposes, such as in the case of plasmonic and photothermal catalysis.