Origin of the Optical Response of a Dye-doped Plasticized Poly(vinyl chloride)-based Photonic Crystal Ion Sensor.

We investigated the origin of the optical responses of a two-dimensional photonic crystal (2D-PhC) optode membrane ion sensor based on plasticized poly(vinyl chloride) (PVC) containing lipophilic dye. We previously developed a highly sensitive potassium ion sensor. However, the contributions of various factors to the sensor's optical response could not be distinguished. Here, we synthesized a lipophilic dye with spectral changes that do not overlap with those of propagated light in the PhC. Using this synthesized dye excluded the contribution of light absorption by the dye to the peak intensity change of the ion sensor. In refractive index standard solutions, the bulk refractive index change upon ion extraction contributed negligibly. The contribution of diffracted light scattering by optical transparency change of the plasticized PVC upon ion extraction accounted for approximately 45% of the ion sensor's total response. These findings are important for the development of highly sensitive plasticized PVC-based 2D-PhC ion sensors.