| Summary: | The metal–organic framework (MOF) is a class of materials that exhibits a notable capacity for electron transfer. This unique framework design offers potential applications in various fields, including catalysis, gas storage, and sensing. Herein, we focused on a specific type of MOF called Ti-MOF. To enhance its properties and functionality, the composite material was prepared by incorporating graphitic carbon nitride (g-C<sub>3</sub>N<sub>4</sub>) into the Ti-MOF structure. This composite, known as g-C<sub>3</sub>N<sub>4</sub>@Ti-MOF, was selected as the active material for ion detection, specifically targeting calcium ions (Ca<sup>2+</sup>). To gain a comprehensive understanding of the structural and chemical properties of the g-C<sub>3</sub>N<sub>4</sub>@Ti-MOF composite, several analytical techniques were employed to characterize the prepared g-C<sub>3</sub>N<sub>4</sub>@Ti-MOF composite, including X-ray diffraction (XRD), SEM-EDX, and FT-IR. For comparison, different pastes were prepared by mixing Ti-MOF or g-C<sub>3</sub>N<sub>4</sub>@Ti-MOF, graphite, and <i>o</i>-NPOE as a plasticizer. The divalent Nernstian responses of the two best electrodes, I and II, were 28.15 ± 0.47 and 29.80 ± 0.66 mV decade<sup>−1</sup>, respectively, with concentration ranges of 1 µM–1 mM and 0.1 µM–1 mM with a content 1.0 mg Ti-MOF: 250 mg graphite: 0.1 mL <i>o</i>-NPOE and 0.5 mg g-C<sub>3</sub>N<sub>4</sub>@Ti-MOF: 250 mg graphite: 0.1 mL <i>o</i>-NPOE, respectively. The electrodes showed high sensitivity and selectivity for Ca<sup>2+</sup> ions over different species. The suggested electrodes have been successfully employed for Ca<sup>2+</sup> ion measurement in various real samples with excellent precision (RSD = 0.74–1.30%) and accuracy (recovery = 98.5–100.2%), and they exhibited good agreement with the HPLC.
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