A Method for Improving the Optical Properties of a Fluoregenic Di-metal Chelator as a Zn2+Ion Fluorescent Probe by Using a Bridging Substrate

Abstract

Fluorescent probe based detection methods for metal ions are indispensable tools in many fields, including medical diagnostics, environmental monitoring, living cell studies, and electronics. These methods have multiple advantages over other methods, such as high sensitivity, low cost, ease of application, and versatility. Numerous fluorescent metal ion probes have been designed using many strategies, including fluorogenic metal chelators, fluorescent dye tagged oligonucleotides, catalytic signal amplification, and chemodosimetors. Fluorogenic metal chelators, which consist of a fluorogenic unit (signaling site) covalently linked to chelating moieties (receptor units) with an appropriate spacer, are a general type metal ion probe. The recognition of chelating moieties with metal ions induces a change in the photophysical properties of the fluorescent probe. This is converted into an optical signal expressed as an enhancement or quenching of the fluorophore emission. The recognition can be enhanced to utilize additive reagents that provide additional binding sites for metal ions. For example, 8-aminoquinolino-β-cyclodextrin, developed by Liu et al., exhibited cooperative binding to Zn ion with 1-adamatanoic acid. It also detected Zn ions more efficiently than without 1adamatanoic acid because 1-adamatanoic acid bound to βcyclodextrin to provide additional binding site for Zn ions. This strategy can easily expand to improve the optical properties of metal ion sensors with two metal binding sites. Di-metal complexes may include bridging substrates to complete the metal coordination sphere, and bridging substrates can modulate the properties of the resulting cascade complexes. In particular, bridging substrates provided additional metal ion binding sites and enhanced the binding properties of metal ions to metal ion ligands. Various fluorescent molecules with two metal binding sites have been synthesized as efficient probes for detecting metal ions, such as Zn and Ag. Scaffolds of di-metalic fluoregenic probes have also been devised to detect anions. Most of these molecules can be used as metal ion probes. The optical properties of these molecules can be improved by bridging substrates, which provide additional metal ion binding sites. To validate this concept, (9,10-bis[(2,2-dipicolylamino)methyl]anthracene) (1) was adapted as a Zn ion probe because [Zn2(1)] 4+ efficiently detects phosphate derivatives as potential bridging substrates to provide additional metal binding sites. In various phosphate derivatives, ATP was adapted as a bridging substrate to provide additional metal binding sites because [Zn2(1)] 4+ fluorescence is most enhanced by binding with ATP. Therefore, a combination of 1 and ATP (1/ATP) was prepared as a Zn fluorescent probe. The optical properties of 1/ATP, compared to those of 1, can be improved by providing additional binding sites for metal ions using ATP, as depicted in Scheme 1. To determine the optimal ratio of 1 and ATP, fluorescence titration of ATP was conducted using a 5 μM solution of 1 with 2 equivalents of Zn ions. The fluorescence emission spectra of [Zn2(1)] 4+ in the presence of varied ATP concentrations were evaluated. [Zn2(1)] 4+ fluorescence increased nearly proportionally to ATP concentration and was maximized at one equivalent of ATP (see Supporting information). A sensing system (1/ATP) consiting of 1 (5 μM) and ATP (5 μM) was prepared to evaluate the cooperative binding effect of the bridging substrate by providing additional binding sites for Zn ions. Fluorescence changes of 1/ATP in the presence of various concentrations of Zn were examined. The addition of Zn ions induced greatly enhanced fluorescence. The binding zinc ions within the bis-(2-picolyl)amine in 1 prevents the photo induced electron transfer process