We prepare optrodes of fiber optic plastic with sol-gel technique. Suitable concentration of carbone nanotubes (CNTs),
phenol red, bromophenol blue and cresol red, design optrodes with fiber optic plastic. The surface charge of silica and
the refractive index, which play an important roll on the fiber, modifies the conditions of light propagation into the
plastic optical fiber. We use the transmittance to measure the pH of a solution or fluid in a range between 3 and 9.