We propose a new pH-sensing scheme using a methylene blue adsorption on an optical fiber cladding surface. Interactions between the silica and hydroxyl ions of a base solution induce the surface of the silica negatively charged. The charged surface attracts the positively charged chromophores of methylene blue. As the pH of the solution is reduced, the electrostatic attraction will also be reduced. This electrostatic attraction can change the transmitted light intensity of the cladding mode, since the boundary condition changes. We also carried out a simulation to verify the effect from external refractive index change around a long-period fiber grating. Our results confirm that the wavelength shift by external refractive index change is negligible compared to the transmitted light intensity variation of the cladding mode. By using a long-period grating pair, we can detect the cladding mode transmittance variations. Experimentally, we showed the possibility of pH sensing in the $1.5{mu}m$ infrared region.