Melamine fluorescent sensors from naphthalimide-cyanuric acid conjugates

The development of melamine sensors is an important research theme because contaminating melamine in dairy products or infant powder is toxic to human health, especially for babies. In this research, five novel fluorescent sensors (F1 to F5) containing a 1,8-naphthalimide fluorophore and cyanuric moiety as melamine receptor are designed and successfully synthesized. All target compounds are characterized by 1H and 13C-NMR, High Resolution Mass Spectrometry, and Elemental Analysis. The photophysical properties are investigated by UV-Vis and fluorescence spectroscopy. The selectivity screenings in aqueous DMSO indicate that F3, F4 and F5 give selective fluorogenic changes in the presence of melamine. The sensing mechanism involves the fluorophore aggregation and de-aggregation by multivalent hydrogen-bonding between melamine with cyanuric moiety. The sensor F3 and F4 in 90% water in DMSO show Aggregation-Caused Quenching (ACQ) behavior which results in weak fluorescent emission. After the addition of melamine, the fluorescent signals are selectively enhanced based on the de-aggregation of fluorophore. The sensor F4 provided a detection limit for melamine at 0.8 ppm. On the other hands, the sensor F5 which exhibits an Aggregation-Induced Emission Enhancement (AIEE) in aqueous DMSO shows selective florescence quenching toward melamine upon de-aggregation. The Stern-Volmer constant (Ksv) for signal quenching by melamine is 3 x 103 M-1 with the detection limit of 3.2 ppm.