Conjugate and non-conjugate controls of a sensitizer to enhance dye-sensitized upconversion luminescence

Abstract

Although dye sensitization is an effective strategy to improve the upconversion luminescence of lanthanide-doped upconversion nanoparticles (UCNPs), the sensitization effect is limited due to the high probability of dye aggregation. To address the problem, the tetraphenylethene group is introduced into cyanine dye, which not only inhibits the tight packing of cyanine dyes on the UCNP surface but also increases the fluorescence quantum yield. As a result, a 135-fold enhancement of upconversion luminescence in dye-sensitized UCNPs is achieved compared to that in non-sensitized UCNPs under excitation at 785 nm with the power density of 5 W cm⁻². Moreover, a facile method is found that triethylamine (TEA) can further increase the upconversion luminescence by 7 times. Spectral properties of cyanine dyes on the UCNP surface and in the aggregated state suggest that TEA can also suppress the close packing of cyanine dyes. We speculate that the inhibitory effect of TEA originates from the electrostatic control effect. Furthermore, the highly efficient upconversion nanoparticles has been successfully used in security printing.