Formaldehyde fluorescent sensors from 1,8-Naphthalimide derivatives

Abstract

Three derivatives of 1,8-naphthalimide were systematically designed and synthesized to investigate the effect of substitution pattern on their photophysical properties and sensing behaviors towards formaldehyde in aqueous media. The key substituents, the hydrazino (-NHNH2) and the 2-methoxyethylamino groups (-NHCH2CH2OCH3), were installed at the 4-position of 1,8-naphthalimide by a nucleophilic replacement, and at the imide position by a condensation reaction with 1,8-naphthalic anhydride precursor. All target compounds were obtained in good overall yields of 60-80%. Compounds with two hydrazine moieties (R3) showed excellent selective fluorescent responses towards formaldehyde with the 3.5-fold fluorescence enhancement in 5% aqueous acetic acid solution at 520 nm. The sensing mechanism likely involves the inhibition of photo-induced electron transfer (PET) between the hydrazine and naphthalimide moieties by formaldehyde. The detection limit of the R3 probe was calculated to be 0.1 mM. Other aldehydes and related analysts did not show significant interference in the fluorescence spectrum. Moreover, the sensitivity of the R3 probe toward formaldehyde was successfully enhanced by protonation of the hydrazine moiety with HCl gas to give R3.HCl. The fabrication of R3.HCl in sol-gel was successfully demonstrated. The doping of 10 µM of R3.HCl in silica sol-gel led to a sensor which was capable of formaldehyde detection at 2 to 0.02 µM by naked eyes.