Fluorescent and colorimetric detection of Fe(III) and Cu(II) by a difunctional rhodamine-based probe

A new rhodamine B hydrazone derivative (probe L) was synthesized and characterized. The probe L had sufficiently satisfactory selective response to Fe³⁺ and Cu²⁺ ions among various interferential metal ions, and high sensitivity with the detection limit of 4.63×10^-9M and 5.264×10^-7M for Fe³⁺ and Cu²⁺ ions, respectively. In the presence of Fe³⁺, the probe L exhibited turn-on orange fluorescence accompanied by color change from colorless to pink. Toward Cu²⁺, the probe L showed significant color change from colorless to red purple. These remarkable orange fluorescence and color change made probe L suitable naked-eye identify for Fe³⁺ and Cu²⁺ ions. By means of Job's plot, Benesi-Hildebrand studies and FTIR spectra, both 1:1 binding modes (L-Fe³⁺ and L-Cu²⁺) were confirmed. The coordination mechanism and turn on/off fluorescence for L-Fe³⁺ and L-Cu²⁺ complexes were well explained by theoretical calculations. Moreover, probe L could be used as a quick, simple, visual test strip for Fe³⁺ and Cu²⁺ detection.