Bipyridine-linked three-dimensional covalent organic frameworks for fluorescence sensing of cobalt(II) at nanomole level

A novel fluorometric method based on bipyridine-linked three-dimensional covalent organic frameworks (COFs) was developed for the determination of Co2+. The COFs were synthesized by the polyreaction of tetrakis(4-aminophenyl)methane (TAPM), 2,2′-bipyridine-5,5′-diamine (Bpy), and 4,4′-biphenyldicarboxaldehyde (BPDA) under solvothermal conditions. The fluorescence of the COFs, with excitation/emission peaks at 324/406 nm, is quenched by Co2+. Under the optimal conditions, the fluorescence quenching degrees (F0–F) of the resulted COFs linearly enhance as the concentrations of Co2+ increase in the range 0.01 to 0.25 μM, and a limit of detection of 2.63 nM is achieved. The fluorescence response mechanism was discussed in detail. This proposed approach has also been successfully employed to determine Co2+ in complex samples (shrimp and tap water), and satisfactory recoveries (88.1 ~ 109.7%) was obtained. The relative standard deviations are below 4.9%.