Endoplasmic reticulum-targeted fluorescent probes for metal-free tracking of carbon monoxide in living cells

Endoplasmic Reticulum (ER) targetable fluorescent probes, ER-NapNO2 and ER-NapAllyl, have been designed and synthesized based on 1,8-naphthalimide fluorophore, for the selective and targeted detection of carbon monoxide (CO) in aqueous buffer medium of pH 7.4 at 37 degrees C. ER-targetable moiety, methyl sulfonamide unit, was attached with the one part with the fluorophore and another part was connected with CO reactive zone i.e. 4nitrobenzyle and allyl group. Very weak fluorescence signals were observed with both the probes due to the possible PET process operating from naphthalimide moiety to electron withdrawing units i.e.-NO2 and allyl group. Interestingly, both of the probes exhibited their selective detection behavior towards CO in aqueous reaction buffer in absence of any metal ions e.g., palladium, rhodium, iron etc. The possible expulsion of highly fluorescent naphthalimide-derivative was identified from the reaction mixture during the reaction of both the probes and CO. The detection technique is found to highly selective towards CO over various reactive oxygen, nitrogen, sulfur, and other biological species with low detection limit, 0.080 & mu;M and 0.161 & mu;M of ER-NapNO2 and ER-NapAllyl respectively. Theoretical study using time dependent density functional theory (TDDFT) optimized the end-products formed after the reaction with CO and probes which excellently corroborate with the experimental observation. Also, the sensing reactions are exothermic with computed free energy changes for ERNapNO2 and ER-NapAllyl are -158.74 kcal/mol and -28.51 kcal/mol respectively. Cellular confocal fluorescence microscopic experiments showed that these probes are also efficient to detect CO in ER specific regions in living cells.