Influence of excitation pulse duration on the efficiency of upconversion nanoparticle-based FRET

Accurate and reliable quantification of Förster Resonance Energy Transfer (FRET) is essential for the development of sensitive upconverting nanoparticle (UCNP)-based biosensors. While lifetime-based FRET measurements are generally considered robust, excitation conditions can significantly bias observed efficiencies. Here, we investigate how excitation pulse width and power influence lifetime-derived FRET efficiency in core-shell β-NaYF₄:Yb_0.2@NaYF₄:Yb_0.2,Er_0.02 UCNPs functionalized with Cy3 dyes. Time-resolved upconversion luminescence (UCL) measurements reveal that apparent FRET efficiencies decrease with increasing excitation pulse duration and power. These variations stem from excitation-induced changes in the UCL lifetime, arising from the complex dynamics that accompany the upconversion emission process. A dynamic rate equation model reproduces the experimental trends, confirming that excitation parameters alter emissive state kinetics and thus bias lifetime-based FRET measurements. Our findings identify excitation conditions as a hidden variable in UCNP-FRET experiments and underscore the need for standardized measurement protocols.