Blue emission-dominated NaYbF 4 @NaYF 4 :2%Ho@NaYF 4 upconversion nanoparticles for detecting ascorbic acid.

The blue emission-dominated NaYbF 4 @NaYF 4 :2%Ho@NaYF 4 (CSS) upconversion nanoparticles (UCNPs) were designed to detect ascorbic acid (AA). The blue emission of CSS was increased, ∼34 and ∼8 times higher than that of NaYF 4 :2%Ho@NaYbF 4 @NaYF 4 (CSS (I)) and NaYbF 4 @NaYF 4 :20%Yb,2%Ho@ NaYF 4 (CSS (II)), respectively. The dominant blue emission of CSS might be attributed to three aspects: (1) the protected NaYbF 4 core for high near-infrared absorption, (2) the spatial separation of Yb 3+ and Ho 3+ preventing back energy transfer, and (3) their inert shell inhibiting surface quenching. It is worth noting that the fluorescence resonance energy transfer (FRET) of the CSS (donor) to MnO 2 nanosheets (acceptor) was developed to trace AA. The detection limit is 0.75 μM and the detection range is 0.5-100 μM. In summary, our work not only explored the blue emission mechanism of Ho 3+ , but also constructed a CSS/MnO 2 -based nanosensing platform for the rapid and sensitive detection of AA. The blue emission of Ho 3+ has promising applications in the field of sensing and detection.