研究目的
To develop a highly sensitive, fast-responding fiber-optic salinity sensor for ocean salinity measurements by integrating long-period fiber gratings with ionic strength-responsive hydrogels.
研究成果
The hydrogel-coated LPFGs demonstrated high sensitivity (7 nm/M), fast response (<5 s), linear correlation with salinity, good reproducibility, stability, and pH insensitivity in the marine pH range. The sensor provides a robust platform for ocean salinity sensing and has potential for other aqueous salt measurement applications.
研究不足
The sensor uses NaCl solutions instead of real seawater, which may introduce cross-sensitivity errors from other salts (approximately 15%), though these can be compensated mathematically. The study is limited to salt concentrations of 0.4 to 0.8 M and pH around 8.1; performance outside these ranges is not evaluated. The hydrogel coating thickness and fabrication process may affect mechanical robustness in harsh marine environments.
1：Experimental Design and Method Selection:
The study involved designing a salinity sensor using long-period fiber gratings (LPFGs) functionalized with a hydrogel coating. The hydrogel was synthesized via layer-by-layer (LbL) electrostatic assembly of quaternized poly(4-vinylpyridine) (qP4VP) and poly(acrylic acid) (PAA), followed by chemical cross-linking to form a single-component hydrogel. Spectroscopic ellipsometry and optical transmission measurements were used to characterize the hydrogel properties and sensor response.
2：Sample Selection and Data Sources:
Sodium chloride (NaCl) solutions with concentrations from 0.4 to 0.8 M (simulating ocean salinity) at pH 8.1 were used as test samples. Hydrogel coatings were deposited on silicon wafers and LPFGs for characterization.
3：4 to 8 M (simulating ocean salinity) at pH 1 were used as test samples. Hydrogel coatings were deposited on silicon wafers and LPFGs for characterization. List of Experimental Equipment and Materials:
3. List of Experimental Equipment and Materials: Materials included NaCl, NaOH, PAA, branched polyethylenimine (BPEI), poly(methacrylic acid) (PMAA), P4VP, glutaraldehyde, sodium acetate buffer, NaH2PO4, Na2HPO4, HCl, H2O2, and ultrapure water. Equipment included a CO2 laser for LPFG fabrication, optical spectrum analyzer (OSA), spectroscopic ellipsometer (M-2000, J.A. Woollam Co., Inc.), 1H NMR spectrometer (VARIAN Mercury 300), FTIR spectrometer (Bruker Tensor II), and scanning electron microscope (SEM).
4：Experimental Procedures and Operational Workflow:
LPFGs were fabricated using point-by-point CO2 laser irradiation. Hydrogel coatings were deposited via LbL assembly on cleaned LPFGs and silicon wafers, followed by cross-linking with glutaraldehyde and release of PAA. Salinity responses were measured by immersing coated LPFGs in NaCl solutions and monitoring resonance wavelength shifts using OSA. Ellipsometry measured refractive index and thickness changes in hydrogel coatings.
5：Data Analysis Methods:
Data were analyzed using linear fitting for sensitivity calculations, spectroscopic models for ellipsometry data, and software tools for NMR and FTIR spectra processing.
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spectroscopic ellipsometer
M-2000
J.A. Woollam Co., Inc.
Used to measure refractive indices and thicknesses of the hydrogel coatings on silicon wafers under dry and swollen conditions.
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FTIR spectrometer
Bruker Tensor II
Bruker
Used to obtain Fourier-transform infrared spectra of the hydrogel coatings to confirm composition and structural changes.
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optical spectrum analyzer
OSA
Used to measure the transmission spectra of the LPFGs and monitor resonance wavelength shifts during salinity sensing experiments.
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NMR spectrometer
VARIAN Mercury 300
Varian
Used to determine the quaternization degree of qP4VP polymers via 1H NMR spectroscopy.
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CO2 laser
Used for point-by-point irradiation to fabricate the long-period fiber gratings (LPFGs) on optical fibers.
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optical fiber
SMF-28
Corning Optical Communications Inc.
Used as the substrate for fabricating LPFGs and as the sensing element in the salinity sensor.
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motorized translation stage
Used to control the movement during LPFG fabrication with the CO2 laser, ensuring synchronized operations.
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