Abstract
A highly sensitive fiber-optic salinity sensor synergistically combining long-period gratings (LPG) and stimuli-responsive polyelectrolyte multilayers is demonstrated. The LPG coupled with LP0,10 cladding mode was coated with ionic-strength-responsive chitosan (CHI)/poly (acrylic acid) (PAA) polyelectrolyte multilayers via the layer-by-layer (LbL) assembly technique. This LbL-coated LPG was exposed to NaCl solutions with varying concentrations for salinity measurement. The LPG resonance wavelength underwent a change from red shift to blue shift at the salt concentration of 0.5M over the 0.1–0.8M range at pH 7.5. A significant blue shift with a sensing response of 36nm/M was observed from 0.5 to 0.8M, relevant to that of seawater. This sensitivity is one order of magnitude higher than that obtained using as fabricated LPG without the stimuli-responsive LbL multilayers as well as documented studies. The mechanism associated with the salinity response of the LbL multilayers is discussed.
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