Controllable synthesis of knot-like interpenetrating Co@NC/CMWCNTs composites for simultaneous electrochemical detection of Cd²⁺, Cu²⁺, Hg²⁺, and Pb²⁺

Escalating aquatic contamination by toxic heavy metal ions (HMIs) urgently requires field-deployable electrochemical sensors surpassing traditional methods. This work proposed a controllable synthesis of cobalt-embedded nitrogen-doped carbon/carboxyl-functionalized multi-walled carbon nanotube (Co@NC/CMWCNTs) composites through in situ growth of ZIF-67 on CMWCNTs, followed by optimized carbonization. CMWCNTs-directed morphological evolution was systematically demonstrated, revealing that with increasing CMWCNT ratios, the ZIF-67 crystallite size underwent an initial increase and later decay, and after carbonization, a unique knot-like interpenetrating architecture was formed with CMWCNTs as bridges and Co@NC as nodes. This synergistic structure significantly enhanced electrical conductivity and electrocatalytic activity, as evidenced by electrochemical characterization. Compared to the glassy carbon electrode (GCE), the Co@NC/CMWCNTs/GCE sensor exhibited a 12-fold reduction in charge transfer resistance (28.3 Ω vs. 346.8 Ω), a 21% larger electroactive surface area, and rapid heterogeneous electron transfer kinetics of 3.8 × 10⁻³ cm s -1 . The composite sensor allowed simultaneous detection of Cd²⁺, Pb²⁺, Cu²⁺, and Hg²⁺ in 0.1 M NaAc-HAc buffer (pH 4.5) with high sensitivities (169.7, 199.0, 135.9 and 108.0 μA·μM⁻¹ cm -2 ), low detection limits (40.0, 40.5, 87.5 and 106.2 nM) and observed cross-interference within 10%. This work establishes a materials design paradigm for MOF-derived electrochemical sensors for low-cost on-site environmental monitoring of HMIs.