A parylene-protected nitrate selective microsensor on a carbon fiber cross section

This work describes a novel design for a potentiometric microsensor for nitrate (NO3−) ion based on doped polypyrrole (PPy(NO3−)) films. First, 6–7 μm diameter single carbon fiber filaments are coated with a thin (1–10 μm) insulating layer of parylene C (poly(o-chloro-p-xylylene)). This preparatory step is followed by electrodeposition of a PPy(NO3−) sensing layer on the open carbon fiber cross section. Parylene C provides an excellent protective coating for the fiber while maintaining its flexibility at micro-dimensions. Examination by scanning electron microscopy confirms that the sensing material (PPy(NO3−)) is deposited in the form of micro-beads with diameter of 30–40 μm at the fiber tip. The resulting nitrate sensor exhibits an improvement of approximately half an order of magnitude of NO3− concentration in sensitivity and linear response range relative to a previously reported PPy(NO3−)-based sensor for which the sensing layer was deposited on the carbon fiber walls. The overall sensor geometry, a micro-dimensioned sensing portion accompanied by a flexible, parylene-insulated body, forms a sensor for nitrate with a unique form factor that can be widely used for in situ measurements of variety of environmental samples.