Novel fluorescent sensors based on 8-aminoquinoline receptor

Abstract

Two series of amidoquinoline-based fluorescent sensors are designed and synthesized for the selective detection of glucosamine and zinc ion in aqueous media. For the glucosamine sensors, three amidoquinoline-naphthalimide dyads are designed and synthesized in 67–73% overall yields in 3 steps from commercially available starting materials. Compounds with unsubstituted and nitro-naphthalimide show excellent selective fluorescent responses towards glucosamine with the enhancement of fluorescence quantum yields by 14 folds. The determination of HOMO-LUMO levels by linear sweep voltammetry suggests that the sensing mechanism likely involves the inhibition of photo-induced electron transfer (PET) between the aminoquinoline and naphthalimide moieties by glucosamine. The association constants of 1.55x104 and 1.45x104 M-1, along with the glucosamine detection limits of 1.06 and 0.29 µM are determined for unsubstituted and nitro-naphthalimide, respectively. The application of the nitro-naphthalimide derivative as a fluorescent probe for real-time detection of cellular glucosamine at micromolar level in living Caco-2 cells is also demonstrated.

For the zinc ion sensors, four derivatives of 8-amidoquinolines have been successfully synthesized by extension of the p-conjugated system on the quinoline ring and incorporation of either the salicylaldimine or its reduced amino form. All four compounds show selective fluorescence enhancement by zinc (II) ion, attributing to chelation-enhanced fluorescence (CHEF) effects, in which the deprotonation of the amido –NH and phenolic -OH causes the internal charge transfer (ICT) process and results in the bathochromic shift of the emission spectra. The fluorescent signals of the four compound are observed at different wavelengths in the range of 320 to 390 nm depending on the p-conjugated systems. The sensing mechanism is verified by 1H-NMR titration, Mass Spectrometry, and the X-ray crystal structure of the sensor with salicylaldimine and unsubstituted aminoquinoline, which suggests a 1:1 binding stoichiometry between this fluorophore and zinc ion. The detection limits of 0.024 to 0.431 µM and the association constants ranging from 7.0x 103 to 1.2x 104 M-1 are estimated for the four sensors.