Coordination mechanism of cyanine dyes on the surface of core@active shell β-NaGdF4:Yb3+,Er3+ nanocrystals and its role in enhancing upconversion luminescence

Sensitization of lanthanide-doped upconversion nanocrystals (UCNCs) using organic dyes with broad and intense optical absorption is an interesting approach for efficient excitation energy harvesting and enhancement of upconversion luminescence of such UCNCs. In this work, ultrasmall (∼6.5 nm in diameter) β-NaGdF4:Yb3+,Er3+ core and related core@shell UCNCs were sensitized by six NIR-excitable cyanine dyes with a wide range of functional groups and optical properties. The greatest UC enhancement of 680-times was observed for the conjugate between Cy 754 dye and NaGdF4:Yb3+,Er3+@NaGdF4:10%Yb3+,30%Nd3+ core@shell UCNCs excited using 754 nm laser. The enhancement was estimated relatively NaGdF4:Yb3+,Er3+@NaGdF4:10%Yb3+,30%Nd3+ core@shell UCNCs capped with oleic acid and excited using similar intensity (75 W/cm2) of 980 nm laser. UC intensity measurements for identical dye-sensitized UCNCs carried out in methanol and in deuterated methanol under argon, as well as in air, allowed us to reveal the connection of the dye triplet states with UCNC sensitization as well as of the hydroxyl groups with quenching of excited states of lanthanide ions. For UCNCs dispersed in methanol, the strong quenching UC luminescence was always observed, including core@shell UCNCs (with ∼ 2 nm shell). A strong influence of the triplet states of the dyes was observed for the two dyes Cy 754 and Cy 792, which bind firmly to UCNCs and allow the distances between the dye and UCNC to be reduced, whereas the contribution of this sensitization pathway is very insignificant for Cy 740 and Cy 784 dyes, which bind weaker to UCNCs.