An sp² Patterned Boron Doped Diamond Electrode for the Simultaneous Detection of Dissolved Oxygen and pH

A hybrid sp²-sp³ electrochemical sensor comprising patterned regions of nondiamond-carbon (sp²) in a boron doped diamond (sp³) matrix is described for the simultaneous voltammetric detection of dissolved oxygen (DO) and pH in buffered aqueous solutions. Using a laser micropatterning process it is possible to write mechanically robust regions of sp² carbon into a BDD electrode. These regions both promote the electrocatalytic reduction of oxygen and facilitate the proton coupled electron transfer of quinone groups, integrated into the surface of the sp² carbon. In this way, in one voltammetric sweep (time of measurement ∼4 s) it is possible to determine both the DO concentration and solution pH. By varying the sp² pattern the response can be optimized toward both analytes. Using a closely spaced sp² microspot array, a linear response toward DO, across the range 0.0 to 8.0 mg L^-1 (0.0 to 0.25 mM; sensitivity = -8.77 × 10^-8 A L mg^-1, R² = 0.9991) and pH range 4-10 (sensitivity = 59.7 mV pH^-1, R² = 0.9983) is demonstrated. The electrode is also capable of measuring both DO concentration and pH in the more complex buffered environment of blood. Finally, we show how the peak position for ORR is independent of pH, and thus via measurement of the difference in ORR and pH peak position, internal referencing is possible. Such electrodes show great promise for use in applications ranging from biomedical sensing to water analysis.