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Title: Multicolor Upconversion Förster Resonant Energy Transfer Using Optimized Yb@YbTm Core@Shell Nanoparticles

Authors:G. Bękarski, K. Prorok, F. Štětina, M. Misiak, H. H. Gorris, A. Bednarkiewicz

Journal: ACS Nano

DOI: 10.1021/acsnano.5c13869

Förster resonance energy transfer (FRET) between lanthanide-doped upconverting nanoparticles (UCNPs) and organic dyes has been gaining increasing attention as a tool for next-generation biosensing platforms. The interest arises from the advantages of upconversion, including strongly reduced background signals and minimized spectral excitation/ emission cross-talk, which together enhance detection sensitivity. An additional benefit of using Tm³⁺ ions – compared with the commonly studied Er³⁺ ions – is their significantly wider spectral gap between emission bands in the visible range (approximately 150 nm vs. 100 nm). This broader spectral gap is particularly advantageous for multiplexed assays involving multiple analytes.

In this work, the group of prof. Bednarkiewicz together with the group of prof. Gorris from Masaryk University in Brno, Czech Republic, optimized the concentration of Tm³⁺ ions in core@shell nanocrystals of the composition NaYF₄:50%Yb³⁺@NaYF₄:20%Yb³⁺, x%Tm³⁺ for efficient FRET and compared two classical approaches for evaluating FRET efficiency: donor emission quenching and donor luminescence lifetimes shortening in the presence of an acceptor. By directly attaching the dyes to the nanoparticle surface, they achieved high energy transfer efficiencies of approximately 90% and 40%, respectively, for ATTO 488. However, these approaches yield different numerical values due to the complex nature of upconversion processes, energy migration within the Yb³⁺/Tm³⁺ network and the repopulation of excited Tm³⁺ states, therefore, they cannot be interpreted independently.   

The most sensitive measure of FRET proved to be ratiometric detection, based on the ratio of acceptor to donor emission intensities in well-defined spectral windows (500 – 614 nm and 435 – 485 nm). Although this method does not provide mechanistic insight into the FRET process, it enabled the lowest limits of detection reaching 3 . The wide spectral separation between the blue and red Tm³⁺ emission bands was further exploited to develop a simple multiplexing system capable of distinguishing four ATTO dyes using only a pair of optical filters. This approach demonstrated the potential of Tm³⁺ - based UCNPs for constructing multicolor, highly sensitive FRET biosensing platforms.