Fluorescent nanodiamonds for luminescent thermometry in the biological transparency window

Fluorescent nanodiamonds (FNDs) have attracted recent interest for biological applications owing to their biocompatibility and photostability (absence of photoblinking and bleaching). For optical thermometry, nitrogen-vacancy (NV) color centers and silicon-vacancy color centers in diamonds have demonstrated potential, where the NV has the highest sensitivity. However, NV is often excited with green light, which can cause heating and photodamage to tissues, as well as autofluorescence that decreases sensitivity. To overcome these limitations, we report temperature sensing using NV centers excited by deep red light (660 nm), plus another color center that can be excited with NIR light; the nickel (Ni) complex. The NV center measures temperature using diamond lattice expansion while the Ni complex measures temperature using phonon sideband strength.