Synthesis and characterization of a dansyl-based fluorescent probe for analytical purposes

Among analytical methods that can be useful for detecting nanomaterials in watery samples¹, fluorescence-based sensing remains one of the most promising approaches due to its simple solution assay processing, low cost, rapid response and high sensitivity, as well as its high selectivity through molecular binding design.

With the aim of investigating the development of an easily synthesized dansyl-based fluorescent probe for detecting heavy metal-based nanomaterials in aqueous solution, we have designed a Schiff base ligand H2L. This derives from the reaction of 4-formyl-3- hydroxybenzoic acid with N-(2-aminobenzyl)-5- (dimethylamino)naphthalene-1-sulfonamide. H2L incorporates a suitable O,N-binding do- main that can bind heavy metal ions at the surface of the particles.

H₂L can be obtained in a quick two-step synthesis. In the first step, 2‑(aminomethyl)aniline reacts selectively through the aminomethyl group with dansyl chloride to form N-(2-aminobenzyl)-5-(dimethylamino)naphthalene-1-sulfonamide. Subsequent nucleophilic addition of N-(2-aminobenzyl)-5-(dimethylamino)naphthalene-1-sulfonamide to 4-formyl-3-hydroxybenzoic acid yields the desired Schiff base ligand H₂L.

The obtaining of H₂L has been demonstrated using IR and NMR spectroscopies. The IR spectrum of H₂L shows unequivocally the presence of a new strong sharp band at about 1613 cm^-1, which is attributable to the formation of the imino group. The most remarkable observation in the ¹H NMR spectrum of H₂L is the presence of imine (singlet) proton at about 8.66 ppm and the disappearance at 4.83 ppm of the amino group of N-(2-aminobenzyl)-5-(dimethylamino)naphthalene-1-sulfonamide. This is a clear evidence of the condensation of N-(2-aminobenzyl)-5-(dimethylamino)naphthalene-1-sulfonamide with 4-formyl-3-hydroxybenzoic acid yielding the desired Schiff base ligand H₂L.