Novel Lipophilic Fluorophores with Highly Acidity-Dependent Two-Photon Response |
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Authors: | Matt Rammo Dr Aleksander Trummal Dr Merle Uudsemaa Juri Pahapill Katrin Petritsenko Dr Meelis-Mait Sildoja Dr Charles W Stark Dr Sigrid Selberg Prof Ivo Leito Kirsti Palmi Dr Jasper Adamson Prof Aleksander Rebane |
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Institution: | 1. Laboratory of Chemical Physics, National Institute of Chemical Physics and Biophysics, 23, Akadeemia tee, 12618 Tallinn, Estonia;2. Institute of Chemistry, University of Tartu, 14a Ravila st, 50411 Tartu, Estonia |
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Abstract: | Lipophilic fluorophores are widely implemented in nonlinear microscopy; however, few existing membrane-specific probes combine the high brightness of two-photon excited fluorescence (2PEF) with pH sensitivity. Herein we describe four novel two-photon excited fluorophores, based on a coumarin 151 core structure, where lipophilicity is induced by a covalently attached phosphazene moiety. Changing the environmental acidity using trifluoromethanesulfonic (triflic) acid leads to profound changes in the linear fluorescence and 2PEF characteristics, due to chromophores’ switching between neutral- and protonated forms. We characterize this dependence by measuring the two-photon absorption (2PA) spectra over the region λ2PA=550–1000 nm, observing 2PA cross sections of σ2PA=10–20 GM, with an associated 2PEF brightness of 10–13 GM, in neutral solutions of both acetonitrile and n-octanol. Although quantum chemical modelling and NMR measurements show that, at high chromophore concentrations, protonation may be accompanied by a dimerization process, these dimers likely do not form at the lower concentrations used in optical spectroscopy. |
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Keywords: | fluorescent pH indicator lipophilic probe two-photon absorption two-photon cross section two-photon excitation fluorescence |
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