Abstract
Upconverting phosphors (UCPs) convert multiple low energy photons into higher energy emission via the process of photon upconversion and offer an attractive alternative to organic fluorophores for use as luminescent probes. Here, UCPs were capped with functionalized silica in order to provide a surface to covalently conjugate proteins with surface‐accessible cysteines. Variants of green fluorescent protein (GFP) and the flavoenzyme pentaerythritol tetranitrate reductase (PETNR) were then attached via maleimide‐thiol coupling in order to allow energy transfer from the UCP to the GFP or flavin cofactor of PETNR, respectively. PETNR retains its activity when coupled to the UCPs, which allows reversible detection of enzyme substrates via ratiometric sensing of the enzyme redox state.
Highlights
Upconverting phosphors (UCPs) convert multiple low energy photons into higher energy emission via the process of photon upconversion and offer an attractive alternative to organic fluorophores for use as luminescent probes
While UC has recently been shown in small molecule complexes,[3] the most common systems are based on YbIII!ErIII or YbIII!TmIII rare-earth ion pairs doped into an inert matrix (e.g., NaYF4, Gd2O2S, etc.),[2] and more recently NdIII ions have been used in place of YbIII to enable excitation at 808 nm, where water and biological tissue absorb less strongly.[1f]
We have previously demonstrated significant diffusion-controlled quenching of UCP upconversion emission by the oxidized flavin cofactor of the enzymes pentaerythritol tetranitrate reductase (PETNR)[8] and glucose oxidase,[9] as well as to vitamin B12, and the heme cofactor of cytochrome c.[9]
Summary
Upconverting phosphors (UCPs) convert multiple low energy photons into higher energy emission via the process of photon upconversion and offer an attractive alternative to organic fluorophores for use as luminescent probes. Inset shows “on–off” apparent energy transfer (AET) concept with PETNR on the surface of UCPs. and environmental sensing applications than by monitoring enzymes or substrates directly.
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