It is generally said that an appeal to the eye is more effective than an appeal to the ear, which may be the basis of the proverb “Seeing is believing”. Methods to “see into the body” or “see into cells” are essential for the diagnosis and treatment of disease, as well as for research into the basic processes of life. It is desirable that the methods used should not be invasive, i.e., should not involve cutting into the body or isolating cellular constituents. Therefore, techniques to visualize physiological or pathophysiological changes in the body and cells have become increasingly important in biomedical sciences.
Compared to other technologies such as radioisotope labeling, magnetic resonance imaging (MRI), electron spin resonance (ESR) spectroscopy, and electrochemical detection, fluorescence imaging has many advantages for this purpose, because it enables highly sensitive, less-invasive and safe detection using readily available instruments. Another advantage of fluorescence imaging we should emphasize here is that the fluorescence signal of a molecule can be drastically modulated, so that probes relying on activation, not just accumulation, can be utilized. Today, fluorescent probes based on small organic molecules have become indispensable tools in modern biology because they provide dynamic information concerning the localization and quantity of the molecules of interest, without the need for genetic engineering of the sample. It is also expected that technology using fluorescent probes will play a pivotal role in the field of drug discovery, with applications in both academia and industry. For in vivo molecular imaging, fluorescent probes are administered to the subject and emit a signal within the body. It should be clear that, in order to achieve successful imaging, the role of appropriate chemical design for activation of the probes, is extremely important.
At present, activatable probes for cell or in vivo
imaging, which emit an increased fluorescence signal after reaction with the target biomolecules, are not very common.1–3)
Over the past decade, however, we have developed a variety of such probes, based on several design strategies, including the mechanisms listed below.
(1) Photoinduced electron Transfer (PeT) mechanism2,4–9)
i) acceptor-excited PeT (a-PeT) mechanism10)
ii) donor-excited PeT (d-PeT) mechanism11)
(2) Förster Resonance Energy Transfer (FRET) mechanism12–14)
(3) Intramolecular Charge Transfer (ICT) mechanism15)
(4) Spirocyclization mechanism16)
This review focuses on three mechanisms (a-PeT, d-PeT, and spirocyclization) for control of fluorescence characteristics, which have been established by our group and others, together with some bioimaging applications of probes utilizing these mechanisms.