Scientists in Durham, Hong Kong and Spain have shown that two-photon microscopy can be used to reveal the cell localisation of a range of terbium peptide conjugates. 

David Parker and colleagues from the University of Durham and collaborators from City University of Hong Kong and Institute of Chemical Research of Catalonia, Spain, have synthesised a terbium complex conjugated to a range of peptide conjugates. These peptide conjugates include a C₁₂ chain, Lys-Arg₇, Arg₇, a tetraguanidium cation and human serum albumin. 

Two-photon microscopy is a fluorescence imaging technique that is used to image living tissue. It was used here to excite the sample at 720 nm, sensitising the chromophore incorporated into the terbium complex and subsequently generating lanthanide luminescence. This is the first time that two-photon microscopy has been used for terbium-based systems.

Luminescent probes are of interest as they offer high resolution, they can be selectively excited and also designed to emit light with frequencies that encode information about the local environment. However, the challenge is to develop optical imaging probes that have the ability to be transported into a cell and be directed towards a specific cellular compartment. 

Parker has shown that the peptide conjugates localise in lysosomes and the guanidinium analogues are directed to the mitochondria. However, these peptide conjugates are rather toxic and can give rise to apoptosis (programmed cell death).

The future plan for this work is 'to develop non-toxic and targeted optical probes that signal key information in their spectral emission profile about local concentration changes of key bioactive ions and molecules,' said Parker. These challenges 'are based on europium complexes, as they can be used as responsive probes to monitor within living cells in real time, local pH changes or monitor variations in the concentration of anions such as bicarbonate, lactate or citrate. These ideas, in principle, can then be applied to monitor the concentration of key bioactive species in tissues or selected bio-fluids. This could then be used in diagnostic tests to profile diseased states,' explained Parker.

Emma Shiells