A highly selective liquid membrane nitrite microsensor based on the hy
drophobic ion-carrier aquocyano-cobalt(III) -hepta(2-phenylethyl)-cobr
ynate is described, The sensor has a tip diameter of 10 to 15 mu m. Th
e response is log-linear in freshwater down to 1 mu M NO2- and in seaw
ater to 10 mu M NO2-. A method is described for preparation of relativ
ely large polyvinyl chloride (PVC)-gelled liquid membrane microsensors
with a tip diameter of 5 to 15 mu m, having a hydrophilic coating on
the tip, The coating and increased tip diameter resulted in more sturd
y sensors, with a lower detection limit and a more stable signal than
uncoated nitrite sensors with a tip diameter of 1 to 3 mu m. The coati
ng protects the sensor membrane from detrimental direct contact with b
iomass and can be used for all PVC-gelled liquid membrane sensors mean
t for profiling microbial mats, biofilms, and sediments. Thanks to the
se improvements, liquid membrane sensors can now be used in complex en
vironmental samples and in situ, e.g., in operating bioreactors. Examp
les of measurements in denitrifying, nitrifying, and nitrifying/denitr
ifying biofilms from wastewater treatment plants are shown. In all of
these biofilms high nitrite concentrations were found in narrow zones
of less than 1 mm.