Broad range and highly sensitive optical pH sensor based on Hierarchical ZnO microflowers over tapered silica fiber

In present work we report the fabrication and characterization of optical fiber pH sensor using smart hydrogel coating over Zinc Oxide (ZnO) microflowers. ZnO microflowers were hydrothermally grown over chemically etched tapered multi-mode fiber (MMF). Mainly, the sensing head contains two layers in which one is ZnO microflower and another is pH sensitive hydrogel layer. The variation in pH of liquid around the sensing probe causes the swelling and shrinkage of hydrogel which leads the change in its refractive index (RI). The work of ZnO microflower is to enhance the evanescent waves that interact with the change in RI of hydrogel which causes the variations of wavelength dips with respect to each pH solutions. The sensitivity was estimated by using wavelength interrogation technique. The ZnO microflower based sensor is highly sensitive for wide range pH values (both high and low range of pH varied from pH = 3 to pH = 7 and pH = 7 to pH = 11, respectively). The sensitivity of ZnO microflower based MMF sensor was found to be 2.59 nm/pH and 0.70 nm/pH for both acidic and basic media, respectively. In addition to high sensitivity, the proposed sensor shows fast response time and good stability and reproducibility.