Elimination of concentration dependent luminescence quenching in surface protected upconversion nanoparticles.

Quenching of lanthanide-doped upconversion nanoparticle (UCNP) luminescence at high dopant concentrations is an existing challenge that hampers their desired applications. It has already been reported that concentration quenching is strongly coupled to surface quenching. However, in this study we found that surface quenching is not the only cause for concentration quenching and concentration quenching was still observed in UCNPs with a well-protected surface. By using a mathematical model to better understand the processes, we proposed that this concentration quenching could be minimized by moving the extra sensitizers (Yb3+) to a different layer. To achieve this, a new tri-layer UCNP structure with sensitizers isolated in the middle layer was developed. It was observed that the quenching was reduced, and the luminescence intensity was enhanced up to 11.8 times in comparison with the brightest UCNPs without the sensitizing layer.