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In fluorescence imaging studies of biological mechanisms, cyanine dyes have been employed as fluorescent labels. In
particular, tricarbocyanines have the advantage that light at their emission and absorption maxima in the near-infrared
(NIR) region around 650-900 nm can penetrate deeply into tissues. We successfully developed two types of cyanine dyes
whose fluorescence properties change upon specific reaction with nitric oxide (NO) or zinc ion. The mechanism of
fluorescence modulation of the NO probes involves photoinduced electron transfer, and the fluorescent intensity can
change at the same wavelengths. We synthesized a series of amine-substituted tricarbocyanines in order to examine the
correlation between the electron-donating ability of the amine and the fluorescence peak wavelength. We found that
changing the electron-donating ability of the amine substituent altered the absorption and emission wavelengths. Then,
we synthesized dipicolylcyanine (DIPCY), consisting of tricarbocyanine as a fluorophore and dipicolylethylenediamine
as a heavy metal chelator, and investigated its response to various heavy metal ions. DIPCY can work as a ratiometric
fluorescent sensor for zinc ion. This fluorescence modulation of amine-substituted tricarbocyanines should be applicable
to dual-wavelength measurement of various biomolecules or enzyme activities. Thus, we have established two
mechanisms for modulating the fluorescence properties of cyanines.
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