Manganese oxide nanoparticles electrodeposited on graphenized pencil lead electrode as a sensitive miniaturized pH sensor

pH monitoring in micro volume samples is required for environmental and clinical analysis. Low cost, miniaturized and stable metal oxide based pH sensors could be a suitable alternative to glass electrodes. In this study, a sensitive potentiometric solid state pH sensor based on manganese oxide nanoparticles electrodeposited on graphenized pencil lead electrode (MnO₂/GPLE) was reported. The prepared MnO₂/GPLE was carefully characterized by SEM, XRD and electrochemical techniques. To miniaturize the prepared pH sensor, a stainless steel 304 needle was used as a reference electrode. Selectivity, response time, stability and reproducibility of the miniaturized pH sensor were studied and compared with conventional glass pH electrode. According to experimental results, a near-Nernstian slope of − 57.051 mV/pH and linearity over the pH range of 1.5–12.5 were obtained for the developed MnO₂/GPLE pH sensor. The prepared sensor represented high ion selectivity to mono-valence and multi-valence ions with \(- \log K_{{{\text{A}},{\text{B}}}}^{{{\text{Pot}}}}\) values around 6.49. A fast response time of 20 s in acidic medium and 60 s in alkaline medium, long-term stability and reproducibility in 2 months, the simplicity of fabrication, low cost and accuracy makes this sensor as a suitable choice for rapid pH recording in micro volume samples. The MnO₂/GPLE pH sensor was successfully used for the pH monitoring of human tear, human blood, saliva, apple juice, lemon juice, milk, and vinegar samples with satisfactory results.