Optical humidity sensor using porphyrin immobilized Nafion composite films

A Nafion–TMPyP composite film was used in an optical humidity sensor. The composites were prepared with various molar ratios between the sulfonic groups and TMPyP molecules, R = [–SO3H]/[TMPyP]. The UV–vis measurement of the composite films suggested that the TMPyP molecules were well dispersed at in the range of R = 60–100, but formed aggregates at R ≤ 20. Furthermore, the FTIR indicated that sulfonic group interacted with TMPyP. The reflectance change with relative humidity (%RH) occurred at 426.4 and 465.4 nm with the isosbestic point at 437.5 nm, and also in the Q-band region. The sensors of R = 30 and 40 sensitively responded to humidity in the lower humidity region (10–20%RH), but saturated or even decreased with further increase in humidity. At R = 60, the sensitivity slightly decreased but a measurable range was extended to around 70%RH. Remarkable deterioration in sensitivity occurred at R = 100. The TMPyP molecules were stably immobilized on a Nafion matrix, even in liquid water, without solving out.