With the aim of directing lanthanide complex uptake via the mannose receptor, a first generation of luminescent lanthanide complexes has been developed with an α-D-mannose targeting motif. Four complexes were produced to investigate photophysical properties and determine the effect of the coordinated mannose residue on emission intensity. The free hydroxyls of the α-D-mannose residue quenched lanthanide phosphorescence due to their close proximity, though they did not bind the lanthanide centre as observed by q-values ≈1.0 for all complexes between pH 3 and 10. Fluorescent emission was found to vary significantly with pH, though phosphorescent emission was relatively insensitive to pH. This lack of pH sensitivity has the potential to provide stable emission for the visualisation of the endosome-lysosome system where acidic pH is often encountered. 1. Introduction Fluorescent probes and stains that recognize cellular compartments have long been used to visualize cell architecture in fixed and, more recently, live cells. To understand receptor-ligand interaction and internalisation, we need to track each of these components in live cells. Translating this concept into the field of live cell imaging presents a number of complications for probe development. A probe designed for live cell imaging requires long wavelength excitation/emission (to avoid the cell damage that results from low wavelength/high energy light) and a high quantum yield so that small concentrations of probe can be visualised. In addition, high in vivo stability is required, including resistance to any undesirable emission quenching, either by low pH environments or biological media. Lanthanide luminescence is an evolving method for cellular visualisation that has recently been applied to provide feedback on dynamic processes in live cells [1]. Stable lanthanide complexes based on europium (Eu(III)), terbium (Tb(III)), neodymium (Nd(III)) and ytterbium (Yb(III)) are particularly attractive as optical probes for in vivo applications as their excitation and emission wavelengths are in the visible or near infrared (NIR) regions. Trivalent lanthanide ion luminescence resulting from inner-shell 4f-4f transitions is observed as characteristic line-like emission bands (10–20？nm bandwidth), with large Stokes’ shifts and long emission lifetimes (ms range) [2, 3]. These properties support the use of time-resolved detection for cellular imaging, allowing the removal of background autofluorescence in live cells and providing enhanced signal-to-noise ratios when compared to fluorescent analogues [4].
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