Long-lived metal complexes open up microsecond lifetime imaging microscopy under multiphoton excitation: from FLIM to PLIM and beyond

• Published in: Chemical science (Cambridge) 2014, Vol.5 (3), p.879-886
• Main Authors: Baggaley, Elizabeth, Botchway, Stanley W., Haycock, John W., Morris, Hayley, Sazanovich, Igor V., Williams, J. A. Gareth, Weinstein, Julia A.
• Format: Article
• Language: English
• Subjects: Coordination compounds; Detection; Excitation; Fluorescence; Imaging; Microscopy; Nanostructure; Time
• ISSN: 2041-6520; 2041-6539
• DOI: 10.1039/C3SC51875B

Lifetime imaging microscopy with sub-micron resolution provides essential understanding of living systems by allowing both the visualisation of their structure, and the sensing of bio-relevant analytes in vivo using external probes. Chemistry is pivotal for the development of the next generation of bio-tools, where contrast, sensitivity, and molecular specificity facilitate observation of processes fundamental to life. A fundamental limitation at present is the nanosecond lifetime of conventional fluorescent probes which typically confines the sensitivity to sub-nanosecond changes, whilst nanosecond background autofluorescence compromises the contrast. High-resolution visualization with complete background rejection and simultaneous mapping of bio-relevant analytes including oxygen - with sensitivity orders of magnitude higher than that currently attainable - can be achieved using time-resolved emission imaging microscopy (TREM) in conjunction with probes with microsecond (or longer) lifetimes. Yet the microsecond timescale has so far been incompatible with available multiphoton excitation/detection technologies. Here we realize for the first time microsecond-imaging with multiphoton excitation whilst maintaining the essential sub-micron spatial resolution. The new method is background-free and expands available imaging and sensing timescales 1000-fold. Exploiting the first engineered water-soluble member of a family of remarkably emissive platinum-based, microsecond-lived probes amongst others, we demonstrate (i) the first instance of background-free multiphoton-excited microsecond depth imaging of live cells and histological tissues, (ii) over an order-of-magnitude variation in the probe lifetime in vivo in response to the local microenvironment. The concept of two-photon TREM can be seen as "FLIM + PLIM" as it can be used on any timescale, from ultrafast fluorescence of organic molecules to slower emission of transition metal complexes or lanthanides/actinides, and combinations thereof. It brings together transition metal complexes as versatile emissive probes with the new multiphoton-excitation/microsecond-detection approach to create a transformative framework for multiphoton imaging and sensing across biological, medicinal and material sciences.