Engineering water-tolerant core/shell upconversion nanoparticles for optical temperature sensing

Luminescence thermometry is a promising approach using upconversion nanoparticles (UCNPs) with a nanoscale regime in biological tissues. UCNPs are superior to conventional fluorescent markers, benefiting from their autofluorescence suppression and deep imaging in tissues. However, they are still limited by poor water solubility and weak upconversion luminescence intensity, especially at a small particle size. Recently, YVO₄:Er⁺³,Yb⁺³ nanoparticles have shown high efficiency upconversion (UC) luminescence in water at single-particle level and high contrast imaging in biological models. Typically, a 980-nm laser triggers the UC process in the UCNPs, which overlaps with maximum absorption of water molecules that are dominant in biological samples, resulting in biological tissues overheating and possible damaging. Interestingly, neodymium (Nd⁺³) possesses a large absorption cross section at the water low absorption band (808 nm), which can overcome overheating issues. In this Letter, we introduce Nd⁺³ as a new near-infrared absorber and UC sensitizer into YVO₄:Er⁺³,Yb⁺³ nanoparticles in a core/shell structure to ensure successive energy transfer between the new UC sensitizer (Nd⁺³) to the upconverting activator (Er⁺³). Finally, we synthesized water-tolerant YVO₄:Er⁺³,Yb⁺³@Nd⁺³ core/shell nanoparticles (average size 20 nm) with strong UC luminescence at a biocompatible excitation wavelength for optical temperature sensing where overheating in water is minimized.