An On–Off\nElectroanalysis Strategy for Sensitive\nRatiometric Detection of Multiple Metal Ions Based on Dopamine-Bridged\nMIL-101(Fe)

The susceptibility of electroanalysis to interference\nfrom various\nfactors has garnered increasing attention on designing a reliable\nelectrochemical platform for multiple metal ions. In this work, an\non–off ratiometric electrochemical strategy was proposed using\nDA-MIL-101(Fe), where dopamine (DA) served as an internal reference.\nThe interaction between DA molecules and metal ions realized ratiometric\ndual-signaling, enabling the high recognition of metal ions. The large\nspecific surface area and catalytic efficiency of MIL-101(Fe) could\nfacilitate the binding with DA and the electron transfer of metal\nions. The binding of metal ions to DA decreased the oxidation signal\nof DA, while the current responses of metal ions increased. Electrochemical\nanalysis of metal ions with different valence states (Ag<sup>+</sup>, Cd<sup>2+</sup>, Cu<sup>2+</sup>, Pb<sup>2+</sup>, Bi<sup>3+</sup>, In<sup>3+</sup>) at the DA-MIL-101(Fe) was investigated, and the\ncalculated binding energies revealed the specific recognition mechanism\nbetween phenolic hydroxyl group of DA and different metal ions. Focusing\non Pb<sup>2+</sup> as a representative target, the proposed DA-MIL-101(Fe)\ndemonstrated excellent electrochemical performance by employing the\ncurrent ratio, <i>I</i><sub>Pb</sub><sup>2+</sup>/<i>I</i><sub>DA</sub>, for the qualitative and quantitative analysis\nof Pb<sup>2+</sup> in concentration range 0.20–10.0 μM.\nIt exhibited anti-interference capability and high stability, which\nwas successfully applied to detect Pb<sup>2+</sup> in different real\nsamples with satisfactory related standard deviations (RSDs) (1.84%–5.42%).\nThe results verified the possibility of DA-MIL-101(Fe) as a ratiometric\nplatform on detection of metal ions in practical applications, offering\na promising avenue for the development of a ratiometric sensor.