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

Abstract

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.

Architecture of Hydrated Multilayer Poly(methacrylic acid) Hydrogels: The Effect of Solution pH

“…Using a diazide cross-linker, Shi and co-workers fabricated poly­(allylamine hydrochloride) (PAH)/catalase multilayer hydrogels that showed enhanced chemical stability for cell proliferation . In our group, we have shown that multilayer networks of poly­( N -vinylcaprolactam) undergo reversible shrinkage with temperature, while poly­(4-vinylpyridine) (PVP) and PMAA hydrogels reversibly swell up to 20-fold with pH and can be utilized for amino acid sensing. − A recent study by Yang et al showed that PVP-based hydrogels can be used as salinity sensors . Thus, owing to their unique swelling behavior, multilayer hydrogels possess great potential for a wide range of applications, from sensing and controlled drug delivery to antibacterial and anti-icing coatings.…”

“…42−44 A recent study by Yang et al showed that PVP-based hydrogels can be used as salinity sensors. 45 Thus, owing to their unique swelling behavior, multilayer hydrogels possess great potential for a wide range of applications, from sensing and controlled drug delivery to antibacterial and antiicing coatings. Yet, there is a significant lack of knowledge concerning the internal organization of multilayer networks, which limits our understanding of the relationship between the network structure and film properties.…”

Hydrogel Application in Urban Farming: Potentials and Limitations—A Review

“…The first idea of polymer crosslinking was introduced by Charles Goodyear in 1839, aiming for tougher and longerlasting sulfur-crosslinked rubber properties than uncrosslinked rubber [20]. Crosslinking during hydrogel fabrication creates the three-dimensional polymeric network [21] and prevents the dissolution of polymer chains in a solvent [22]. The concentration of the crosslinker significantly affects the swelling capacity of the hydrogel, whereby a higher concentration of crosslinker will result in lower swelling capacity and vice versa [10][11][12].…”

Soft and Stretchable Optical Waveguide: Light Delivery and Manipulation at Complex Biointerfaces Creating Unique Windows for On-Body Sensing

“…Therefore, more and more researchers began to apply stimuli-responsive materials to the field of optical sensing. Stimuli-responsive optical waveguide materials are increasingly used in the field of optical sensings, such as detecting pH, − temperature, − enzyme, small molecules, ,− ion concentration, , etc. Yetisen et al report a phenylboronic acid-functionalized glucose-sensitive hydrogel flexible optical waveguide that can implement continuous glucose monitoring in vivo (Figure D) .…”