Non-toxic fluorescent alanine–fluorescein probe with green emission for dual colorimetric/fluorimetric sensing

Two new fluorescent bio-inspired dyes 1 and 2 containing a fluorescein unit linked to a protected with a Boc group (N-tert-butyloxycarbonyl), and unprotected alanine residue were synthesized and characterized. The acid–base behavior was studied in water/ethanol (60/40) and at pH = 8 a strong emission fluorescence, followed by a color change from colorless to yellow was observed. Photophysical studies in water HEPES buffered solution toward alkaline, alkaline earth, transition, and post- transition metal ions were performed. Compounds 1 and 2 revealed to be very sensitive to Hg2+ with colorimetric changes from yellow to pink in the presence of HEPES buffer. Moreover, compound 1 show a color change from yellow to orange in the presence of the neurotransmitter molecule dopamine. Compound 2 also reveals to be sensitive to Cu2+ and Ni2+, being able to detected and quantified in HEPES buffer ca. 0.37 ppm and ca. 1.50 ppm of Cu2+, respectively. Very small silver nanoparticles coated with compound 2 of ca. 3.68 ± 0.93 nm were successfully synthesized and characterized. Toxicological studies with both compounds were also carried out.