A dual-function probe based on naphthalene for fluorescent turn-on recognition of Cu2+ and colorimetric detection of Fe3+ in neat H2O.

A simple naphthalene derivative, 6-hydroxy-2-naphthohydrazide (NAH), was designed and synthesized through two facile steps reactions with the 6-hydroxy-2-naphthoic acid (NCA) as the starting material. In neat H2O (10% 0.01 M HEPES buffer, v/v, pH = 7.4), probe NAH showed a highly selective and sensitive response towards Fe3+ via perceptible color change and displayed "turn-on" dual-emission fluorescence response for Cu2+. The binding stoichiometry ratio of NAH/Cu2+ and NAH/Fe3+ were all confirmed as 1:1 by the method of fluorescence job's plot and UV-Vis job's plot, respectively. Probe NAH can be used over a wide pH range for the determination of Fe3+ (2.0-10.0) and Cu2+ (6.0-10.0) without interference from other co-existing metal ions. A possible detection mechanism was the hydrolysis of NAH upon the addition of Fe3+ or Cu2+, thereby leading to the formation of 6-hydroxy-naphthalene-2-carboxylic acid (NCA) which was further confirmed by the various spectroscopic techniques including FT-IR, 1H NMR titration and HRMS. Moreover, NAH was successfully applied to the detection of Cu2+ and Fe3+ in tap water, ultrapure water and BSA.