Spider dragline silk for PH sensing

We develop a fiber pH sensor using a spider dragline silk (SDS). SDS obtained from an Araneus Ventricosus is a pH-sensitive natural protein. We wrap the SDS on a tapered single-mode optical fiber (TSOF) to fabricate the sensing structure. The fiber taper is too thin to confine the laser propagating in the fiber, leading to the leakage of the strong evanescent field. Therefore, the higher-order modes will couple to the SDS, and we may obtain a multimode interference (MMI) spectrum. The pH of the solution will change the protein structure of the SDS. Thus, the effective refractive index (RI) of the SDS will vary with pH. Therefore, the change of pH causes the shift of the MMI spectrum. The pH detection range is 6.5 to 7.9. Compared with other PH sensors, the materials used in this sensor are biocompatible and the sensitivity of this sensor is competitive. The maximum pH sensitivity is as high as 7.7 nm of wavelength shift per pH level. In our experiment, the resolution of optical spectrum analyze is 0.2 nm. Therefore, the pH change that corresponds to the minimum detectable wavelength shift is 0.03. The sensor is low in cost, environmental-friendly, and has good repeatability.