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

Area of Science:

• Materials Science
• Optical Engineering
• Environmental Monitoring

Background:

• Accurate, real-time ocean salinity data is vital for scientific research, commercial activities, and defense.
• Existing salinity measurement methods may lack the speed, sensitivity, or real-time capabilities required for certain applications.

Purpose of the Study:

• To develop and demonstrate a highly sensitive, fast-responding fiber-optic salinity sensor.
• To integrate long-period fiber gratings (LPFGs) with an ionic strength-responsive hydrogel for enhanced salinity detection.

Main Methods:

• Synthesized a submicron-thick hydrogel using layer-by-layer assembly of partially quaternized poly(4-vinylpyridine) (qP4VP) and poly(acrylic acid), followed by cross-linking.
• Utilized spectroscopic ellipsometry to analyze hydrogel swelling/deswelling in response to varying salt concentrations (0.4-0.8 M).
• Coated LPFGs with the synthesized hydrogel and measured the resultant shift in resonance wavelength (RW) with changes in salinity.

Main Results:

• The hydrogel exhibited robust and reversible swelling/deswelling in solutions with salt concentrations from 0.4 to 0.8 M at pH 8.1.
• The hydrogel's refractive index changes, induced by swelling/deswelling, caused significant shifts in the LPFG's RW.
• LPFGs coated with the hydrogel showed a sensitivity of 7 nm RW shift/M (125.5 pm/‰) with measurement times under 5 seconds.

Conclusions:

• The developed fiber-optic salinity sensor offers rapid, accurate, and real-time ocean salinity measurements.
• The linear correlation between RW shift and salt concentration simplifies salinity quantification.
• This technology holds significant potential for scientific, commercial, and defense applications requiring precise salinity monitoring.