Amperometric carbon fiber nitrite microsensor for in situ biofilm monitoring

A highly selective needle type solid state amperometric nitrite microsensor based on direct nitrite oxidation on carbon fiber was developed using a simplified fabrication method. The microsensorʹs tip diameter was approximately 7 μm (14 μm spatial resolution). At an applied potential of +1.2 V vs. Ag/AgCl, the microsensor exhibited a linear nitrite response from 0 to 25 mg N L−1, a 0.02 mg N L−1 (1.3 μM) limit of detection, and a fast response (<5 s). There was minimal interference with less than 3% of electrode response changes from major chemicals of interest in drinking water and wastewater systems [oxygen, ammonium, monochloramine, nitrate, sodium bicarbonate (alkalinity), chloride, sulfate, and acetate]. Hydrogen ion (pH) affected nitrite measurement by shifting the baseline response, translating into an approximate change of 0.24 mg N L−1 nitrite per 1 pH unit change. Depending on the conditions (e.g., pH, alkalinity, nitrite concentration), pH may need to be taken into account during nitrite measurement. The developed carbon fiber nitrite microsensor successfully measured nitrite in a nitrifying biofilm and is applicable for in situ analysis in other micro-environments (e.g., microbial mats and sediments).