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High-throughput microfluidics allows precision color-tuning of upconverting nanoparticles

Screening of a novel synthetic space achieves white-light emission

Rare-earth doped upconversion nanoparticles (UCNPs) have been of great interest for biological imaging and security applications. For white light emission, UCNPs are a promising alternative to commonly used heterogeneous materials (e.g. polymers, organic compounds and quantum dots). Such heterogeneous systems bring multiple problems, such as phase separation, energy transfer, low stability, and the need for multiple excitation sources. Most of these can be overcome by using a homogeneous system, such as UCNPs. Additionally, UCNPs are nontoxic and thus a good alternative to quantum dots, for example. However, precision tuning of the properties of UCNPs is a non-trivial task due to their sensitivity and complexity.

Shangkun Li and coworkers from the deMello Group have developed a novel synthetic route to the fabrication of UCNPs with precisely-tuned emission characteristics. The synthesis was performed in a segmented flow microfluidic reactor, which eliminated cross contamination. In this reactor, the elemental components of the synthesis were separated into individual precursors, allowing precise tuning of doping concentrations and precursor ratios. Equipped with inline photoluminescence spectroscopy, the reactor allowed for real-time parameter screening and synthesis optimization.

In NaYF4:Yb,Er,Tm NCs, a host lattice of NaYF4 integrates the rare-earth elements Yb3+, Er3+ and Tm3+. This complex system of multiple elements makes reaction optimization exceedingly difficult. In this work, the separation of all elemental components into individual precursors allowed for precise tuning of doping concentrations and ratios. After screening experiments were performed, doping concentrations of 60% Yb, 0.45% Er and 1.05% Tm achieved pure white light emitting NCs. Finally, the applicability of the color-tunable UCNPs in the security sector was demonstrated in an anti-counterfeit device. In this a security check with two separate indicators is necessary, which was provided through a homogenous color of the NCs without excitation and three emission profiles upon IR-excitation.

Written by Julia Nette

Read the full paper here

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