Fluorescence imaging for Fe3+ in Arabidopsis by using simple naphthalene-based ligands

Abstract

A main source of Fe³⁺ exposure for mammals is through plant consumption. Thus, sensitive and selective Fe³⁺ detection in plant tissue is a significant and an urgent need. Although fluorescence probes have been reported for Fe³⁺ in water, the detection of endogenous biological Fe³⁺ in plant tissue remains to be refined due to the high background signal and the thickness of the plant tissue that can hamper the effective application of traditional one-photon excitation. To address these issues, we have synthesized naphthalene-based probes, 1 and 1A. Upon an addition of Fe³⁺ in water–methanol (1 : 1, v/v, pH 7), the fluorescence probes of 1 and 1A were found to dramatically decrease, but no other metal ions had this effect. More interestingly, 1A, which had no diethyl 2,2′-(phenylazanediyl)diacetate moiety, also exhibited high selectivity for Fe³⁺. These results clearly indicate that the Fe³⁺ was bound to the nitrogen and oxygen atoms located near the naphthalene moiety. Furthermore, chemical probes of 1 and 1A were embedded into nanofibrous membrane films NF-1 and NF-1A, respectively, prepared by the electrospinning method for use as a portable chemical probe. Fluorescence changes were examined by immersing the films into solutions of various metal ions. The strong fluorescence intensity of both NF-1 and NF-1A dramatically decreased in accordance with concentration of Fe³⁺ onto the film, which was a “turn-off” system. In contrast, no significant changes of fluorescence intensity were observed compared that of other metal ions, such as Na⁺, K⁺, Zn²⁺, Pb²⁺, Mn²⁺, Cu²⁺, Co²⁺, Ca²⁺, Fe²⁺, and Cd²⁺. The results indicate that both NF-1 and NF-1A could be used to selectively detect Fe³⁺. We also investigated the practicality of both 1 and 1A as imaging probes for Fe³⁺ to operate within living systems like plants. Chemical probes of both 1 and 1A were tested for fluorescence imaging of Fe³⁺ in Arabidopsis. As expected, the fluorescence probes displayed high fluorescence imaging for Fe³⁺ in Arabidopsis.