The water soluble fluorescein-based ligand 1 forms a non-fluorescent complex with Cu(2+). This complex serves as a fluorescent sensor for amino acids in the 10(-3) M concentration range. Since the signal response is very fast, the sensor can be used to detect the hydrolytic activity of various proteases (trypsin, chymotrypsin, subtilisin) on bovine serum albumin as a whole protein substrate, and more generally to follow reactions releasing or removing free amino acids, in real time.