A Co(II)-Organic Coordination Polymer and Its Hydroxyl Multiwalled Carbon Nanotube Composite Incorporating 2-Iodo-4-sulfobenzoate Exhibiting Dual Electrochemical Sensing Performance for NO2– and Fe3+

The novel Co(II) coordination polymer incorporating 2-iodo-4-sulfobenzoic acid (H₂isba) and a pliable 1,4-bis(benzimidazole-1-methyl) benzene (bdbmb) ligand, {[Co(bdbmb)(H₂O)₄]·isba·2H₂O·2DMA}_n (Co(II)-CP), was synthesized. In addition, the composite of Co(II)-CP with the short hydroxyl multiwalled carbon nanotubes (Co(II)-CP@HCNTs) was prepared via an in situ preparation strategy. Amperometric current response reveals that the glass carbon electrodes (GCEs) of Co(II)-CP and Co(II)-CP@HCNTs exhibit highly sensitive electrochemical sensing toward NO₂^- and Fe³⁺ in the corresponding electrolytes. Co(II)-CP@HCNTs demonstrate superior electrocatalytic performance toward NO₂^- oxidation and Fe³⁺reduction to Co(II)-CP. The sensing mechanisms for NO₂^- and Fe³⁺ were illustrated with Hirshfeld surface analysis and density of states calculations. Furthermore, this methodology was successfully implemented on a miniaturized screen-printed electrode (SPE) platform. Among the three tested electrodes, the Co(II)-CP@HCNT-modified SPEs exhibited superior sensing capabilities, showing response ranges of 0.002-20 mM for nitrite and 0.002-38 mM for ferric ions, respectively. The calculated detection limits reached 0.12 μM for NO₂^- (δ = 0.00396 μA, N = 10) and 0.30 μM for Fe³⁺ (δ = 0.00201 μA, N = 10). Moreover, Co(II)-CP@HCNTs/SPE was further validated through practical applications for the detection of both analytes in real-world samples in the respective electrolytes.