Expanding the scope of biomolecule monitoring with ratiometric signaling from rare-earth upconverting phosphors

Upconversion (UC) is a powerful mulitphoton mechanism that converts low energy photons into higher energy emission. One of the most investigated UC systems is upconverting phosphors (UCPs). Here, a new, one-pot synthetic procedure was used to prepare water dispersible, visibly emissive, rare-earth doped UCPs that were capped with the functional groups oleic acid (OA), 6-aminohexanoic acid (AHA), and 6-maleimidohexanoic acid (MHA). These synthesised UCPs were characterised by UC luminescence, dynamic light scattering (DLS), transmission electron microscopy (TEM), and powder X-ray diffraction (pXRD). This study expands upon our previous proof-of-principle work in demonstrating the use of UCPs (both synthesised and commercial) to detect on the level and function of biological analytes, from enzymes to key disease biomarkers (PETNR, glucose oxidase, vitamin B12, and cytochrome c). By tailoring the absorption profile of the biomolecule cofactors to the UCP emission, a wide-range of analytes can be utilised. We also demonstrate the ability of our system to reversibly monitor the addition of enzyme substrates via repeat oxidation and reduction of pentaerythritol tetranitrate reductase (PETNR).