# Water Proton Spin Relaxivities and Absolute Fluorescent Quantum Yields of Triply and Quadruply Mixed Lanthanide Oxide Nanoparticles

## TL;DR

Researchers developed new lanthanide oxide nanoparticles that can be used for both MRI and fluorescence imaging due to their unique optical and magnetic properties.

## Contribution

The study introduces triply and quadruply mixed lanthanide oxide nanoparticles with enhanced MRI contrast and fluorescence properties.

## Key findings

- PAA-MMLO nanoparticles showed higher MRI relaxivities than commercial agents.
- PAA/PDA-MMLO nanoparticles achieved high quantum yields and long fluorescence lifetimes.
- The nanoparticles are promising for combined T1/T2 MRI and fluorescence imaging.

## Abstract

Multicomponent mixed lanthanide oxide (MMLO) nanoparticles possess considerable potential as multimodal imaging agents because they integrate diverse excellent optical and magnetic properties within a single nanoparticle. Herein, we present triply and quadruply mixed lanthanide oxide nanoparticles, namely, gadolinium (Gd)/dysprosium (Dy)/europium (Eu) oxide (GDEO), Gd/Dy/terbium (Tb) oxide (GDTO), and Gd/Dy/Eu/Tb oxide (GDETO) nanoparticles. Gd3+ can strongly induce positive (T1) contrast in magnetic resonance imaging (MRI), Dy3+ and Tb3+ can generate negative (T2) contrast in MRI, and Eu3+ and Tb3+ emit visible photons that are applicable to fluorescence imaging (FI). All the nanoparticles were grafted with hydrophilic, biocompatible polyacrylic acid (PAA) to enhance colloidal stability and biocompatibility and further grafted with small amounts of an organic photosensitizer, 2,6-pyridinedicarboxylic acid (PDA), to obtain a high absolute fluorescent quantum yield (QY) with an extended fluorescent lifetime (τ). All PAA-MMLO and PAA/PDA-MMLO nanoparticles exhibited nearly monodispersed particle-size distributions with average particle diameters of ~2 nm and displayed considerably higher longitudinal (r1) and transverse (r2) water proton spin relaxivities than commercial molecular MRI contrast agents. The PAA/PDA-GDEO, PAA/PDA-GDTO, and PAA/PDA-GDETO nanoparticles exhibited high absolute QYs of 45, 29, and 61%, respectively, and long τ values of 1–2 ms, making them suitable for time-delayed noise-free fluorescence signal detection. These findings confirm the high potential of PAA-MMLO nanoparticles as T1 and/or T2 MRI contrast agents and PAA/PDA-MMLO nanoparticles as both T1 and/or T2 MRI and FI agents.