Light-emitting diode excitation for upconversion microscopy: a quantitative assessment

TL;DR

This study demonstrates that incoherent LED excitation combined with time-gated detection enables sensitive upconversion microscopy at the single-nanoparticle level, offering a low-power alternative to laser-based methods for biological imaging.

Contribution

It introduces a novel approach using LED excitation and time-gated detection for upconversion microscopy, reducing reliance on high-power lasers.

Findings

LED excitation is about 3 times less effective than laser excitation for UCNPs.

Time-gated detection effectively eliminates LED background noise.

Hydrophilic UCNPs enable cellular imaging with LED excitation.

Abstract

Lanthanide-based upconversion nanoparticles (UCNPs) generally require high power laser excitation. Here we report wide-field upconversion microscopy at single-nanoparticle sensitivity using incoherent excitation of a 970-nm light-emitting diode (LED). We show that due to its broad emission spectrum, LED excitation is about 3 times less effective for UCNPs and generates high background compared to laser illumination. To counter this, we use time-gated luminescence detection to eliminate the residual background from the LED source, so that individual UCNPs with high sensitizer (Yb3+) doping and inert shell protection become clearly identified under LED excitation at 1.18 W cm-2, as confirmed by correlated electron microscopy images. Hydrophilic UCNPs are obtained by polysaccharide coating via a facile ligand exchange protocol to demonstrate imaging of cellular uptake using LED excitation. These results suggest a viable approach to bypassing the limitations associated with high-power lasers when applying UCNPs and upconversion microscopy to life science research.