Antifouling Copper Surfaces Interfere with Wet Chemical Nitrate Sensors: Characterization and Mechanistic Investigation

Wet chemical sensors autonomously sample and analyze water using chemical assays. Their internal fluidics are not susceptible to biofouling (the undesirable accumulation of microorganisms, algae, and animals in natural waters) due to the harsh chemical environment and dark conditions; however, the sample intake and filter are potentially susceptible. This paper describes the use of copper intake filters, incorporated to prevent fouling, on two different wet chemical nitrate sensors that each use different variants of the Griess assay (in particular, different nitrate reduction steps) to quantify nitrate concentrations. When the copper filters were used, measurements were perturbed in both sensors. Here we describe how the interference was first encountered in field testing and how it was subsequently replicated in laboratory testing. We show how the interference is due to the presence of copper ions from the filters and propose a mechanism for how it interferes with the assay, accounting for differences between the different versions of the Griess assay used in each sensor, and discuss strategies for its management. The findings are not just limited to wet chemical sensors but also more broadly applicable to any laboratory nitrate or nitrite analysis based on the Griess assay.