A novel chromone and rhodamine derivative as fluorescent probe for the detection of Zn(II) and Al(III) based on two different mechanisms

In this study, a novel fluorescent probe, 6‑hydroxychromone‑3‑carbaldehyde‑(rhodamine B carbonyl) hydrazine (L), for Zn²⁺ and Al³⁺ was designed and synthesized. Initially, this probe L exhibited inferior fluorescence emission peak centered at 488 nm in EtOH/HEPES solution (3/1, 10.0 μM HEPES, pH 7.4) when excited at 421 nm. After the addition of Zn²⁺, this probe L displayed excellent selectivity towards Zn²⁺ with obvious fluorescence color change from colorless to yellow, which might be attributed to the formation of a 1:1 ligand-metal complex resulting in the inhibition of photo-induced electron transfer phenomenon. Whereas, the prepared Zn²⁺ complex of L could be used as a ratiometric fluorescent probe to detect Al³⁺ on the basis of fluorescence resonance energy transfer mechanism. This ligand-metal complex of Zn²⁺ (LZn) showed high selectivity towards Al³⁺ with obvious enhancement in fluorescence emission intensity at 580 nm and remarkable decrease in fluorescence emission intensity at 488 nm, and the fluorescence color also changed from yellow to pink. Furthermore, the detection limit of the probe L, LZn towards Zn²⁺, Al³⁺ were 1.25 × 10^-7 M and 3.179 × 10^-6 M, respectively. Additionally, the complexation properties of L towards Zn²⁺ and LZn towards Al³⁺ were studied in detail.