Specific Recognition of Cysteine in a Mixed‐Valence Bimetallic Organic Framework with Unique Channel Properties

C NMR, UV-Vis absorption spectra, electrochemical analysis, and density functional theory calculations complementarily validate the valence state switching of cobalt centers in ZnCoBTCHx, the host-guest interactions and significant electron transfer between Cys and the framework. Additionally, isothermal titration calorimetry data suggest potential adsorption occurring within the channels of ZnCoBTCHx. In contrast, the mixed-valence CdCoBTCHx possesses a monoporous architecture wherein all potential N/O donor atoms of the ligands are engage in metal coordination. CdCoBTCHx displays a colorimetric response not only to Cys but also to other sulfur-containing amino acids. Notably, ZnCoBTCHx represents the first reported MOF that exhibits the ability to selectively recognize Cys residues in polypeptides and enzymes. This distinctive property enables ZnCoBTCHx to serve as a promising sensor for thiolase activity, underscoring its significant potential for biomedical applications.