Ocean Salinity Sensing Using Long-Period Fiber Gratings Functionalized with Layer-by-Layer Hydrogels

• Published in: ACS omega Vol. 4; no. 1; pp. 2134 - 2141
• Main Authors: Yang, Fan, Hlushko, Raman, Wu, Di, Sukhishvili, Svetlana A, Du, Henry, Tian, Fei
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 31.01.2019
• ISSN: 2470-1343; 2470-1343
• DOI: 10.1021/acsomega.8b02823

Rapid, accurate, and real-time measurements of ocean salinity are of great importance for a host of scientific, commercial, and defense applications. We demonstrate a highly sensitive, fast-responding fiber-optic salinity sensor that integrates long-period fiber gratings (LPFGs) with ionic strength-responsive hydrogel. The submicron-thick hydrogel was synthesized via layer-by-layer electrostatic assembly of partially quaternized poly­(4-vinylpyridine) (qP4VP) and poly­(acrylic acid), followed by chemical cross-linking. Spectroscopic ellipsometry measurement of a hydrogel made of 37% quaternized qP4VP showed robust and reversible swelling/deswelling in solutions with salt concentrations ranging from 0.4 to 0.8 M (22.8–44.7 g/kg) around pH 8.1. The swelling/deswelling process induced large changes in the refractive index of the hydrogel, leading to resultant shift in the resonance wavelength (RW) of LPFGs. The salinity-dependent optical response of the hydrogel-coated LPFGs is in good agreement with ellipsometry measurement. LPFGs coated with the hydrogel exhibited a sensitivity of 7 nm RW shift/M (125.5 pm/‰) with a measurement time less than 5 s. The shift in the resonance wavelength correlated linearly with salt concentration, making quantification of measured salinity straightforward.