It can be particularly difficult to measure pH in vivo using current electrochemical sensors due to sensor drift and fouling of the sensor surface. Sensors suitable for measuring pH, particularly in vivo, may comprise: a sensor tail comprising a first working electrode, a second working electrode, and at least one other electrode； a first active portion located upon the first working electrode, the first active portion comprising a substance having pH-dependent oxidation-reduction chemistry； and a second active portion located upon the second working electrode, the second active portion comprising a substance having oxidation-reduction chemistry that is substantially invariant with pH. A difference between a first signal from the first active portion and a second signal from the second active portion may be correlated to a pH value for a fluid.
