A highly selective naphthalimide-based ratiometric fluorescent probe for the recognition of tyrosinase and cellular imaging.

Tyrosinase is polyphenolic oxidase enzyme associated with the progression of various diseases. Therefore, for the recognition of tyrosinase, naphthalimide-based ratiometric fluorescent sensor probe was designed and synthesized. 3-Hydroxyphenyl, as the substrate unit for the enzyme, is an important feature of this design, which avoids the interference of other bio-analytes for the recognition of tyrosinase. When the sensor probe was excited at 425 nm, an intense blue emission band emerged at 467 nm. However, upon the addition of tyrosinase to the probe solution, the monophenolic unit oxidized to o-dihydroxy and consequently released the 4-aminonaphthalimide unit. As the oxidation reaction proceeded, the fluorescence emission at 535 nm started to increase gradually with an increase in the concentration of enzyme. Therefore, the sensor probe gives the ratiometric changes via fluorescence spectroscopy. The probe affords high selectivity and sensitivity to tyrosinase with a detection limit of 0.2 U mL. Furthermore, live cell images were recorded to assay the endogenous enzyme in A375 cells, which also show a dual color change in the presence of the L3 probe.