Metal surface effects on single upconverting nanoparticle luminescence and thermometry signals†

Metal surfaces can alter the luminescence emitted by nanoparticles through a variety of effects including quenching, plasmonic enhancement, and optical interference-, reflection-, and absorption-related phenomena. While many of these effects are well-established, multiple such effects typically occur in parallel in realistic measurement scenarios, making the relative importance of each effect difficult to discern. As imaging and sensing applications in which luminescent nanoparticles are placed on metal surfaces continue to grow, a detailed understanding of how metal surfaces modify nanoparticle luminescence is increasingly important for optimizing and ensuring correct interpretation of the measurement results. Here, we systematically investigate how metal surfaces affect the luminescence emitted by individual NaYF₄:Yb³⁺,Er³⁺ upconverting nanoparticles (UCNPs) ∼27 nm in diameter using a judiciously selected set of five different metal coatings with varying optical and thermal properties. We find that the average single-UCNP emission intensity is determined by an interplay between quenching and reflection effects. Consequently, the average single-UCNP emission intensity is correlated with the reflectance of the underlying metal coating, but non-radiative decay rate changes also play an important role, leading to different average single-UCNP emission intensities for metal coatings with near-identical reflectances. We also evaluate metal surface effects on the common ratiometric thermometry signal of NaYF₄:Yb³⁺,Er³⁺ UCNPs and find that the intrinsic temperature dependence of the luminescence intensity ratio is unaffected by the underlying material. The only differences observed are the result of laser-induced heating for sufficiently absorbing metal coatings on low thermal conductivity substrates, in accordance with the predictions of an analytical heat transfer model.