Marine salinity sensing using long-period fiber gratings enabled by stimuli-responsive polyelectrolyte multilayers

A highly sensitive fiber-optic salinity sensor synergistically combining long-period gratings (LPG) and stimuli-responsive polyelectrolyte multilayers is demonstrated. The LPG coupled with LP_0,10 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 0.5 M over the 0.1–0.8 M range at pH 7.5. A significant blue shift with a sensing response of 36 nm/M was observed from 0.5 to 0.8 M, 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.