Practical and Responsive NIR to NIR Time-Resolved Imaging Probes for Biological Applications

Many fluorescent dyes, such as fluorescein and cyanine dyes, are well known markers for locating various organelles, thus enabling many in-vitro/in-vivo molecular processes to be monitored. However, bioimaging using these organic fluorescent dyes often suffers from the problem of having significant background noise, which are caused by autofluorescence of the biological samples, scattered light from solid substrates, and the luminescence from the optical components, even via two-photon excitation.

To solve this problem, increasing attention has been focused on time-resolved fluorescence technique using lanthanide complexes as fluorescence labels. Time-resolved spectroscopy can determine the excited lifetimes which provide insight of the excited dynamics and the decay pathways of the excited chromophore/materials. With this technique, it is possible, for example, to extract information based on differences in their fluorescence lifetime. The nano-second autofluorescecne can be eliminated via time gated system. Therefore, clean and clear in-vitro images can be obtained by the milli/micro-second imaging agents.

The main objective of the research proposed herein is to develop new organic- lanthanide complexes as novel time-resolved imaging luminescent bioprobes which can be excited and emit in the “tissue-transparent” near-infrared region. These new complexes can be used in in-vitro organelles/signaling pathway imaging and can be further developed into in-situ sensing method for targeting disease, i.e. gastric adenocarcinoma. Recently, we have demonstrated that ytterbium(III) porphyinato complex, through Yb(III) 5F5/2 5F7/2 transitions, can serve as near-infrared (NIR) emissive probe in solution/in-vitro. To support this proposed work, we have performed the following proof-of-concept experiments: (1) we have successfully synthesized water-soluble porphyrin-based ytterbium(III)/neodymium(III) complexes that could be excited by one- (linear) and two-photon to give responsive emission with high quantum efficiency; and (2) we have observed an enhancement of neodymium(III) emission via linear/two-photon excitation upon binding to phosphatidylserine derivatives. With these preliminary results and the development of NIR to NIR confocal microscope in our sister institutions, we are poised to develop a new generation of time-resolved in-vitro imaging probes that are highly sensitive, selective and could give a remarkable emission enhancement to reveal concentrations of phosphatidylserine derivatives for early diagnosis of cancer and also further extend our finding as bio-conjugation to label particular antibodies for biomedical applications. This proposal could be a big step forward in developing much higher resolution of in-vitro/in-vivo imaging probes for practical biological applications.