Present status of, and future prospects for, upconverting phosphors in proximity-based bioassay

Upconverting labels based on luminescent lanthanide ions (e.g. Er3+, Tm3+) have a number of potentially attractive features for ligand-binding bioassays. The labels can be excited with near-infrared light that does not excite background luminescence and emit highly structured luminescence that can be detected selectively. Recent work has shown the potential of such labels as energy donors in assays based on energy transfer to absorbing acceptor species. Although published work to date claims that the energy transfer is a radiationless resonance ('FRET') process this has not in fact been demonstrated conclusively. In addition, upconverting labels so far studied are not always well characterised and some potential difficulties have not been anticipated. Commercial phosphors similar to those used in one published paper can be heterogeneous in composition and hence in spectroscopic properties. A marked variation in spectral properties with power density of excitation is also found, which has implications for assay design. Surprisingly, in some samples a 'photobleaching' effect is seen which might impact on quantification of luminescence measurements. FRET-based assays are sensitive only to short range interaction. Assay formats other than FRET are possible and in particular near-field coupling of upconverted emission to conventionally labeled microspheres or energy transfer from 'whispering gallery' modes at the surface of upconverting microspheres are suggested as possible alternatives. (C) 2007 Elsevier B.V. All rights reserved.