Optimal Sensitizer Concentration in Single Upconversion Nanocrystals

Abstract

Each single upconversion nanocrystal (UCNC) usually contains thousands of photon sensitizers and hundreds of photon activators to up-convert near-infrared photons into visible and ultraviolet emissions. Though in principle further increasing the sensitizers' concentration will enhance the absorption efficiency to produce brighter nanocrystals, typically 20% of Yb ions has been used to avoid the so-called "concentration quenching" effect. Here we report that the concentration quenching effect does not limit the sensitizer concentration and NaYbF is the most bright host matrix. Surface quenching and the large size of NaYbF nanocrystals are the only factors limiting this optimal concentration. Therefore, we further designed sandwich nanostructures of NaYbF between a small template core to allow an epitaxial growth of the size-tunable NaYbF shell enclosed by an inert shell to minimize surface quenching. As a result, the suspension containing 25.2 nm sandwich structure UCNCs is 1.85 times brighter than the homogeneously doped ones, and the brightness of each single 25.2 nm heterogeneous UCNC is enhanced by nearly 3 times compared to the NaYF: 20% Yb, 4% Tm UCNCs in similar sizes. Particularly, the blue emission intensities of the UCNCs with the sandwich structure in the size of 13.6 and 25.2 nm are 1.36 times and 3.78 times higher than that of the monolithic UCNCs in the similar sizes. Maximizing the sensitizer concentration will accelerate the development of brighter and smaller UCNCs as more efficient biomolecule probes or photon energy converters.

Electrochromic Switch Devices Mixing Small‐ and Large‐Sized Upconverting Nanocrystals

“…When illuminated with NIR radiation, incident low‐energy photons are primarily absorbed by the Yb 3+ sensitizers and then the energy is nonradiatively transferred to adjacent activators generating visible or ultraviolet upconversion emissions. Recent advances have enabled a precise control over the particles' crystallographic structure, morphology, size, composition, core@shell architecture, and specific surface functionalization . Hence, UCNPs are promising candidates for a wide range of applications, spanning bioimaging and therapeutics, thermal sensing, photovoltaics, anticounterfeiting, full‐color volumetric 3D display technology, and single‐particle tracking …”

A New Generation of NIR‐II Probes: Lanthanide‐Based Nanocrystals for Bioimaging and Biosensing

“…Due to the concentration quenching effects and the contradictory impact of efficient resonant energy transfer from Yb 3+ sensitizer to the Ln 3+ activator, the optimal doping ratio of Yb 3+ /Ln 3+ is needed to optimize to obtain the highest emission efficiencies in these Yb 3+ –Ln 3+ pairs. However, increasing Yb 3+ concentration (by substitution for Gd 3+ or Y 3+ in the host matrix) would inevitably lead to the particle size being uncontrollable . This is because the heavy Yb 3+ with smaller ionic radius ( r = 1.125 Å) than Gd 3+ ( r = 1.193 Å) or Y 3+ ( r = 1.159 Å) in NaGdF 4 or NaYF 4 host, respectively, tends to form larger nanocrystals owing to the higher crystal growth rate .…”

“…Therefore, the optical properties for different nanocrystal batches are essentially not comparable because of the size effect. To reduce this unfavorable influence, an optically inert Lu 3+ , which can closely mimic the effect of Yb 3+ , can be codoped to maintain the total concentration of Yb 3+ and Lu 3+ in the host . The similar ionic radius and comparable chemical properties between Yb 3+ and Lu 3+ ( r = 1.117 Å) ensure a negligible effect of local crystallization distortion on the emission intensity, which helps to preserve the particle size under identical synthesis conditions.…”

Production of a fluorescence resonance energy transfer (FRET) biosensor membrane for microRNA detection

“…42 Yb 3+ ions act as sensitizers in the system, which absorb infrared radiation (980 nm) and nonradiatively transfer their excitation to Ho 3+ ions, thus populating not only the emitting levels but also the intermediate levels. 43,44 …”