2, University of Regensburg, Regensburg, , Germany
Upconversion nanoparticles (UCNPs) are a unique class of inorganic phosphors capable of absorbing near-infrared light (NIR) and converting it, through the sequential absorption of photons, to ultraviolet (UV) and visible emissions. UCNPs are especially promising for biomedical applications due to their high photostability, and high signal to noise ratio due to the weak autofluorescent response and deep tissue penetration of NIR; however, UCNPs suffer from low upconversion efficiencies leading to the need for high excitation power densities, severly heating the UCNPs' environment, causing a high degree of thermal toxicity. Therefore, developing upconversion nanoparticles (UCNPs) capable of emitting intense visible light emissions in response to low power density NIR excitations is essential for their functional utility in applications including sub-cellular labeling, bioimaging, and biosensing. To this end, we have rationally designed a single-crystal core-shell-shell “sandwich” structured UCNP capable of minimizing deleterious energy back transfer, yielding intense visible emissions in response to low power density 980 nm NIR excitations. This core-shell-shell “sandwich” structured UCNP shows a remarkable enhancement of 63 times for total visible luminescence region, as well as an 80 times enhancement in the green emission range (510nm - 570nm) relative to typical β-NaYF4:Yb 20 mol%, Er 2 mol% co-doped UCNPs. As a proof of concept, we have constructed a highly sensitive biosensor for the detection of dopamine at pico-molar level concentrations. The exceptional upconversion luminescence endows “sandwich” structured UCNPs with great potential for biosensing, as well as other material and biological applications.