Introduction to ratiometric lanthanide sensors
Luminescence occurs when an atom or molecule in an excited state emits photons and returns to the ground state, with the excess energy being released in the form of a photon (radiative decay process). Lanthanide metal ions have found an ever increasing use in many areas of research such as synthesis, coordination chemistry, material science and photonic devices. Over the last three decades considerable interest in the chemistry of lanthanide complexes has focused on their use as biological imaging agents and luminescent probes in the detection of chemical species. In light of this the most interesting lanthanides are that emit in the visible region (Sm3+, Eu3+, Tb3+ and Dy3+) and in the near infrared spectral range (Nd3+, Ho3+, Er3+ and Yb3+), in which biological tissue is relatively transparent (Fig.1.).
Figure 1: (left) Typical low-resolution emission spectra of luminescent lanthanide complexes. (right) The colour observed of selected lanthanide(III) complexes of a common macrocyclic ligand incorporating an antenna chromophore, excited at 355 nm. (Bunzli et al. 2003, images used with consent)
The lanthanide ions exhibit strong, relatively long-lived luminescence consist of several well defined, sharp and narrow bands providing a fingerprint like emission profile (lightprint). Emissive lanthanide (f-block) complexes are already used in various sensors and assays because of their resistance to photo-bleaching and photo-fading, high optical brightness and longer lifetimes (allowing time-gated detection of undesired autofluorescence). This trend is set to develop further for molecular imaging applications, exploring emissive probes that can signal information on the local environment - ranging from bio-fluid samples in vitro, to targeted probes for use in cellulo and in vivo.
In recent work, the Parker group and FScan Ltd. have sought to develop responsive optical probes (Fig.2), based on the understanding and appreciation of the chemistry of emissive lanthanide complexes.
Figure 2: Schematic diagram showing the general concept of the lanthanide(III)based sensor compounds detailing how an organic antenna can be used to sensitise lanthanide(III) ion luminescence.
Arising from the photophysical properties of the lanthanide ions, effective lanthanide emission can be achieved by using an antenna group absorbing at a suitable wavelength with maximised light output upon excitation.
We are resolved to explore systems in which the emitted light encodes information about the chemical composition of the local environment or the sole presence of a target marker / analyte, through modulation of the emitted lightprint. Thus, systems have been developed that signal changes in pH, hydrogencarbonate, citrate, lactate and urate concentration. Each system operates under the conditions found within the complex nature of various biological fluids or cells (variable concentrations of protein, oxygen, competing ions and quenching species), and reports on the target analyte via a change in the intensity of at least two emission bands, i.e. it provides a ratiometric analysis that is independent of probe concentration. This is a particular feature of europium(III) complexes, as the relative simplicity of the spectral emission profile has allowed reliable correlations to be made regarding the immediate envionment of the metal centre based on the relative intensities of selected emission bands. The precision of the spectral measurement, which is an inherent feature of such ratiometric assays, allows less than 1% variance in the measured intensity ratio for a given analyte concentration.
Alternate strategies based on the use of two different lanthanide(III)complexes of a common macrocyclic ligand have also been explored. Lanthanide sensors have been developed based on the finding that certain low molecular weight reductants, such as ascorbate and urate, are able to quench the lanthanide excited state, leading to a reduction in both the emission lifetime and intensity of selected bands in the observed lightprint. The method is based on an examination of Eu/Tb (red lght vs. green light) emission intensity ratios, with the europium complex showing an enhanced susceptibility to the quenching effect of the target analyte. As this assay involves two complexes of the same ligand, other non-specific effects, that may affect the observed emission intensity for a given sample, occur to the same extent for the Eu and Tb complexes.
In summary, lanthanide(III) complexes and more specifically Eu(III) complexes are ideal candidates for clinical applications, forming the basis of ratiometric sensors for various analytes, such as lactate, citrate and urate in a variety of biological fluids. These lanthanide-based assays not only benefit from fast response, high resolution and straightforward ‘user friendly’ physical and chemical properties, but also significantly reduce the material usage and acquisition time required by currently used enzyme-based methods. Moreover, these measurements require the same spectrophotmetric equipment commonly found in analytical laboratories.
For more detailed information on lanthanide technology please visit our ' Literature' section.
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