Multifunctional azo-BODIPY-functionalised upconversion nanoparticles as sensors of hypoxia in biological environments

Abstract

In this work, a hypoxia-sensitive nanoprobe is developed by coating the surface of upconverting core/shell nanoparticles (NaGdF₄: 2%Yb³⁺, 3%Nd³⁺, 0.2%Tm³⁺/NaYF₄) with a non-fluorescent azo-dye based on a boron-dipyrromethene functionalized azo compound. Azo-dyes are able to quench fluorescence emissions due to their NN azo bond, which results in the absorption of most visible emissions of the nanoparticles. However, in a biological environment suffering hypoxia, the azo bond is reduced, which allows the recovery of the nanoparticles’ upconversion emissions. Thereby a near-infrared excitable sensor with an azo-dye is created and for the first time not only enables excitation via NIR at biocompatible 808 nm but also continuous imaging and tracking of the probe in the infrared due to NIR-emissions enabled by the dopant combination since quenching only occurs in the visible. These multifunctional (imaging and sensing) nanoparticles are characterized, their behaviour in reductive and hypoxic environments is determined and the detection of reducing conditions in a hypoxic environment is demonstrated in cells.