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

## TL;DR

Researchers improved upconverting nanoparticle-based biosensing by optimizing nanoparticle design for better energy transfer efficiency and multicolor detection.

## Contribution

A novel core@shell UCNP design with optimized Tm3+ concentration and multicolor FRET capabilities is introduced.

## Key findings

- A 4% Tm3+ shell concentration maximized FRET efficiency with surface-bound dyes.
- Four ATTO dyes were successfully differentiated using a single UCNP donor via spectral and time-domain analysis.
- The study provides a framework for designing efficient, multicolor, and wash-free UC biosensing platforms.

## Abstract

Upconverting nanoparticles (UCNPs) have emerged as promising
alternative
donors for resonance energy transfer (FRET)-based biosensing. However,
employing UCNPs in FRET assays remains challenging because they display
relatively small absorption cross sections and are relatively large
as compared to the Förster distance. Thousands of individual
donor ions in each UCNP are located within various distances from
surface-bound acceptors, complicating the data analysis. While previous
studies have explored how the composition and architecture of UCNPs
influence FRET, many reports remain qualitative, and multicolor UC-FRET
systems involving a single donor and multiple acceptors are less commonly
studied than single-donor-single-acceptor systems. To address these
challenges, we synthesized UCNPs with an absorbing core (Yb3+-doped)/active shell (Yb3+, Tm3+-doped) nanoparticles
systematically varying Tm3+ concentrations to optimize
the FRET efficiency to surface-bound organic acceptors. A shell composition
containing 4% Tm3+ yielded the highest FRET efficiency.
Moreover, four distinct ATTO dyes showing spectral overlap with the
blue emission of Tm3+ were used as acceptor dyes on the
surface of UCNPs to evaluate FRET efficiencies in spectral and time
domains. The differentiation of the four ATTO dyes on one type of
upconverting donor nanoparticles using a simple ratiometric approach
lays the foundation for the design of multiplexed bioassays. Our results
offer a strategy for improving UC-FRET sensitivity through smart core–shell
UCNPs designs, donor concentration tuning, and provide important insights
into the rational design of more efficient, multicolor, and wash-free
UC biosensing platforms.