Comparison of cobalt phthalocyanine (CoPc) and reduced graphene oxide functionalized CoPc hybrid as sensing element for heavy metal ions in water

In this study, heavy metal ion (HMI) sensing properties of 4-pyridinyl-oxadiazole tetrasubstituted cobalt phthalocyanine (CoPc) and reduced graphene oxide (rGO) modified CoPc hybrid in water samples were comparatively investigated by quartz crystal microbalance (QCM) technique. The adsorption equilibrium isotherms of four main HMIs (As^{3 +}, Hg²⁺, Pb²⁺, and Cd²⁺) from aqueous solutions onto CoPc and rGO/CoPc were also studied and modelled. Results showed that the presence of carboxyl groups and strong Lewis base nature of rGO improves the HMI sensitivity of rGO/CoPc nanocomposite. A sensitivity value of 3.9 × 10⁴ Hz/mg l ⁻¹ was obtained for As³⁺ ions with rGO/CoPc while it was 6.7 × 10² Hz/mg l⁻¹ with CoPc. A set of long-term stability and reusability tests was performed on both CoPc and CoPc/rGO hybrid based sensors for Pb²⁺ ions. CoPc/rGO hybrid exhibits excellent long-term stability after ∼ 500 h, maintaining about 95 % of its initial sensitivity, compared to 80 % for the CoPc-based sensor. The adsorption equilibrium isotherms of the four HMIs from aqueous solutions onto CoPc and rGO/CoPc were modelled using five different (Langmuir, Freundlich, Elovich, Temkin, and Jovanovic) two parameter adsorption isotherm models. Regression analysis showed that the functionalization of the CoPc with rGO has a strong effect on the HMIs adsorption isotherm.