Characterization of rhodamine B hydroxylamide as a highly selective and sensitive fluorescence probe for copper(II)

Rhodamine B hydroxylamide (1) is characterized as a highly selective and sensitive fluorescence probe for Cu(2+). Under the optimized conditions, the probe exhibits specific absorbance-on and fluorescence-on responses to Cu(2+) only. This remarkable property may allow Cu(2+) to be detected directly in the presence of the other transition metal ions, and such an application has been demonstrated to human serum. The reaction mechanism is also investigated and proposed as that the hydroxylamide group of 1 binds Cu(2+), and the subsequent complexation of Cu(2+) displays a high catalytic activity for the hydrolytic cleavage of the amide bond, causing the release of fluorophore (rhodamine B) and thereby the retrievement of absorbance and fluorescence. The recovered fluorescence intensity is proportional to the concentration of Cu(2+) in the range 1-20 microM. The detection limit for Cu(2+) is 33 nM (k=3). The reaction mechanism described here may be useful for developing excellent spectroscopic probes with cleavable active bonds for other analytes.