Investigation of sp 2 -Carbon Pattern Geometry in Boron-Doped Diamond Electrodes for the Electrochemical Quantification of Hypochlorite at High Concentrations

An electrochemical sensor that contains patterned regions of sp -carbon in a boron-doped diamond (BDD) matrix is presented for the quantitative detection of hypochlorite (OCl ) at high concentrations in the alkaline, chemically oxidizing environment associated with bleach. As BDD itself is unresponsive to OCl reduction within the solvent window, by using a laser micromachining process, it is possible to write robust electrochemically active regions of sp -carbon into the electrochemically inert sp -BDD electrode. In this work, four different laser patterned BDD electrodes are examined, and their response compared across a range of OCl concentrations (0.02-1.50 M). A single macrospot (0.37 mm diameter disk) electrode and a closely spaced microspot (46 μm diameter disk) hexagonal array electrode, containing the same surface area of sp -carbon, are shown to provide the most linear response toward OCl reduction. Finite element modeling (FEM) is employed to better understand the electrochemical system, due to the complexity of the electrode geometry, as well as the need to include contributions from migration and Ohmic drop at these high concentrations. FEM data suggest that only a small percentage (∼1 × 10 %) of the total laser-machined sp area is active toward the OCl reduction process and that this process is kinetically very sluggish (∼ = 1 × 10 cm s ). The sensitivity at the array electrode (-0.127 ± 0.004 mA M ; = 0.992) is higher than that at the single-spot electrode (-0.075 ± 0.002 mA M ; = 0.996) due to the enhanced effect of transport to the edges of the microspots, shown via simulation. The electrodes returned a relatively stable response over a greater than 3 month period of use in the OCl solutions, demonstrating these hybrid sp -BDD electrodes show promise for long-term monitoring applications in the harsh environments associated with bleaching applications.