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.
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.
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