Specific pH Sensor Based on Nitrogen/Carbon Nanotube-Modified Commercial Field-Effect Transistor for Detection of Rumen pH in Ruminants In Situ

A small and stroke resistant pH sensor was fabricated using a commercial metal-oxide field-effect transistor and was modified with single-walled carbon nanotubes (SWCNTs) for direct evaluation of the rumen pH in cows via a stomach tube. Integrated circuit (IC: BS250) was selected as the selective probe for pH detection. For this purpose, the plastic cover on the surface of the drain was drilled to bare the drain surface, followed by modification with nitrogen-doped SWCNTs by the hot-wall chemical vapor deposition process. According to the figures of merit, fabricated sensor was linear from 4.2 to 8.1 pH unit. Detection limit was 1.23 × 10
^-9
M. Relative standard deviation for five replicate analyses was 3.47%. Based on 90% of maximum response, the response times were estimated to be ~60 s for in vivo and ~12.4 s for in vitro measurements. Calibration sensitivity was measured to -62.1 pH/decade. No interference was observed after setting at least 200-fold of interfering agents, such as PO
₄
^3-
, SO
₄
^2-
, Ca
²⁺
, globulin, and acetic acid in a rumen fluid with 6.61 × 10
^-7
M of H
⁺
. The ease of conducting of the sensor to the rumen through a stomach tube and in situ measurement of the rumen pH was evaluated in four dairy cows. The results were reliable compared with those obtained by a glass membrane electrode on samples drained by rumenocentesis. Slaughterhouse testing of the sensor on rumen contents of sheep (n=8) and cows (n=2) revealed that its relative error percentage could approach to maximum 10% compared with glass membrane sensors.