Supercapacitive Carbon-Coated δ-MnO₂ Nanosheets as a Solid-Contact Material for Self-Testing Potassium-Ion Biosensors

Carbon-coated δ-MnO₂ nanosheets (δ-MnO₂@C) are developed as a solid-contact material for all-solid-state potassium-ion biosensors. The synthesis is designed to be facile and scalable: manganese(II) acetate and oxamide are coprecipitated and subsequently oxidized by persulfate treatment, achieving simultaneous crystallization of δ-MnO₂ and carbon coating in a one-pot process at room temperature. Benefiting from their large specific surface area, efficient redox activity, and good electrical conductivity, δ-MnO₂@C exhibits a high specific capacitance of 165.9 F g^{- 1} at 1.0 A g^{- 1} when used as an electrode material for supercapacitors. Practical biosensing applicability is demonstrated by employing δ-MnO₂@C as a solid-contact material into a potassium-ion sensing strip, enabling accurate detection of K⁺ concentrations across a wide range (1.0 × 10^{- 4 -} 1.0 × 10^{- 1} M). Notably, δ-MnO₂@C-employed potentiometric potassium-ion sensor achieved 1.4-fold faster response time compared with conventional activated carbon contacts. Comparative studies reveal that sensor response time is governed primarily by the areal capacitance of the solid-contact material rather than its surface area or porosity. Owing to the simplicity of the synthesis and the superior performance, δ-MnO₂@C provides a promising pathway toward portable self-testing potassium-ion biosensors for daily monitoring of blood potassium levels in patients with renal failure.