Multi-parametric sensing by multi-channel molecular fluorescent probes based on excited state intramolecular proton transfer and charge transfer processes

Considering the applications of fluorescent probes and the information they provide, their brightness of fluorescence and photostability are of paramount importance. However, in the case of steady-state fluorescence spectroscopy and fluorescence microscopy, the amount of information can be increased by the application of multi-channel probes, via a multi-band fluorophore introduced in the probe molecule. In most cases, the use of such a multi-band (or multi-channel) fluorophore can also be combined with the concomitant introduction of one or several analyte receptors. Most often, the design of ratiometric probes with multi-band fluorescence emission are based on phenomena such as photoinduced intramolecular charge transfer (ICT) or excited state intramolecular proton transfer (ESIPT). Although ICT probes were up to recently the most popular, ESIPT probes and among them 3-hydroxyflavone derivatives, were shown to be the most productive. Several general problems were resolved by this family of probes, as for example the measurement of local dielectric constant, local H-bond accepting ability, water local concentration and ATP concentration in small volumes. Incorporation of such multi-channel probes into lipid membranes allowed to measure the different membrane potentials and to detect cell apoptosis. Also, it enabled to recognize and characterize the rafts formation in different lipid bilayers and peculiar features of the charged membrane interface. Such probes are also able to provide a concentration-dependent fluorescence signals upon binding of H(+), Mg(2+)and Ba(2+)ions, and thus to recognize these different cations. The multi-channel probes are effective tools in the study of interactions of macromolecules such as peptides, proteins and nucleic acids. The most useful feature is that they inform simultaneously about several physical parameters, in this way giving a better insight in the investigated system. Thus, by comparing the reviewed probes with other modern fluorescent approaches, it can be concluded they are more informative and accurate tools.