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 (LPFG) with ionic strength-responsive hydrogel. Submicron-thick hydrogels were synthesized via layer-by-layer (LbL) assembly of partially quaternized poly(4-vinyl pyridine) (qP4VP) and poly(acrylic acid) (PAA), followed by chemical crosslinking of qP4VP and removal of PAA. Spectroscopic ellipsometry studies of hydrogels with 37% quaternized qP4VP revealed robust and reversible swelling/deswelling behavior of the coatings in solutions with different salt concentrations at pH 7.5. The performance of hydrogel-coated LPFG for the monitoring of sodium chloride solution in the salinity relevant range of 0.4 to 0.8 M was investigated. The swelling/deswelling process induced remarkable changes in the refractive index of the coating, resulting in robust shift in the resonance wavelength of LPFG. The hydrogel-coated LPFG exhibited a sensitivity of 7 nm/M with a response time less than 1 second. There is a linear correlation between the resonance wavelength shift and the salt concentration, making quantification of measured salinity straightforward.
Manganese is an important heavy metal element that influences nervous system. Detection of manganese in various mediums has thus attracted lots of attentions. Here we report a study on silver nanoparticles functionalized long-period fiber grating (LPFG) for manganese sensing. Silver nanoparticles (AgNPs) with a size in the range of 70nm10nm were synthesized with polyvinyl pyrrolidone (PVP)-glycol. The interplay between arginine, an agent that can cause aggregation of AgNPs, and Mn2+ leads to refractive index change in the AgNPs colloidal solution, thus a shift in the resonance wavelength of LPFG that is surrounded by the colloidal solution. A sensitivity of 0.2nm shift/10-6M was achieved using such strategy. We believe the integration of nanoparticles with LPFG represents a promising sensing strategy for more advanced applications important for not only environmental but also health science.