Sensing of biologically relevant d-metal ions using a Eu(III)-cyclen based luminescent displacement assay in aqueous pH 7.4 buffered solutionw

Over the past decades the importance of metals in biology has been increasingly recognised. Approximately one third of the human proteome contains metal cations, either in the form of cofactors, with catalytic function, or as structural support elements. Moreover, a growing number of diseases have been identified that are characterised by metal imbalance in cells and tissues. Hence, the identification and quantification of these in the physiological environment is of great current importance. Recently, the use of lanthanide luminescent systems as sensors for metal ions has been explored by us and others. Due to their long wavelength emission and long excited state lifetimes these are ideal candidates for use in biological environments. Recently we developed the heptadentate cyclen Eu(III) complex 1 Eu, Scheme 1, possessing an alkyl thiol group, which facilitates its adsorption onto gold nanoparticles, as well as onto flat gold surfaces, at the same time as removing an effective N–H oscillator from the cyclen structure. Herein we demonstrate the use of this structure for application in lanthanide luminescent sensing for metal cations using displacement assays. The formation of a ternary complex between 1 Eu and the water soluble antenna 4,7-diphenyl-1,10-phenanthroline-disulfonate (BPS) gives rise to the formation of 1 Eu BPS, a red emitting Eu(III)-luminescent species, Scheme 1, upon excitation of the BPS antenna. BPS has previously been shown to effectively sensitise Eu(III), possessing good water solubility, at the same time as being a known ligand for the selective colorimetric sensing of Fe(II). We foresaw that the Eu(III) emission of 1 Eu BPS could be modulated, or ‘‘switched off’’ through the displacement of the BPS antenna in the presence of Fe(II); for which it has high affinity. The Fe(II) induced displacement would be reflected in the emission of the hypersensitive D0F2 transition of Eu(III) in particular, upon formation of Fe BPS3 and 1 Eu, the latter being poorly luminescent in the absence of the antenna, possibly enabling us also to achieve enhancement in the detection limit of BPS for Fe(II), over that currently seen for colorimetric sensing of Fe(II). The synthesis and characterisation of 1 and its Eu(III) complex, 1 Eu, have been previously described by us. By measuring the excited state lifetimes of 1 Eu in H2O and D2O, the hydration state (q), or the number of metal bound water molecules was confirmed as two in aqueous solution. The formation of a ternary complex between BPS and 1 Eu to give 1 Eu BPS, Scheme 1, was next monitored in pH 7.4 buffered solution by observing the evolution of the sensitised Eu(III) emission at long wavelengths. The results (see Fig. S1–S4, ESIw) clearly demonstrated that BPS functions as an efficient sensitizer for Eu(III), occurring through an energy transfer process from S1 via the T1 of the antenna to the Eu(III) D0 excited state, with concomitant deactivation to the FJ ground states (J = 0–4), upon excitation at BPS lmax at 278 nm. As expected, no significant emission occurred from 1 Eu in the absence of this antenna, but upon formation of 1 Eu BPS, a ca. 2.5-fold emission enhancement was observed for DJ = 2 (Fig. S4, ESIw), along with a minor change in the F2/ F1 emission ratio from 6.0 0.2 to 6.5 0.1. This indicates direct binding of BPS to the Eu(III). Concomitantly, the fluorescence emission was also monitored (Fig. S3, ESIw). The observed changes in the BPS fluorescence and the Eu(III) emission (e.g. Fig. S3 and S4, ESIw) were analysed, and both displayed Scheme 1 Schematic representation of the displacement assay developed for the selective sensing of Fe(II) using 1 Eu BPS in aqueous solution.