Measurement of chloride-ion concentration with long-period grating technology

• Published in: Smart materials and structures 2007-06, Vol.16 (3), p.665-672
• Main Authors: Tang, Jaw-Luen, Wang, Jian-Neng
• Format: Article
• Language: English
• Subjects: Condensed matter: electronic structure, electrical, magnetic, and optical properties; Exact sciences and technology; Fundamental areas of phenomenology (including applications); General equipment and techniques; Instruments, apparatus, components and techniques common to several branches of physics and astronomy; Measurement and testing methods; Nanocrystals and nanoparticles; Optical properties and condensed-matter spectroscopy and other interactions of matter with particles and radiation; Optical properties of low-dimensional, mesoscopic, and nanoscale materials and structures; Physics; Servo and control equipment; robots; Solid mechanics; Structural and continuum mechanics
• ISSN: 0964-1726; 1361-665X
• DOI: 10.1088/0964-1726/16/3/013

A simple and low-cost long-period fiber grating (LPG) sensor suited for chloride-ion concentration measurement is presented. The LPG sensor is found to be sensitive to the refractive index of the medium around the cladding surface of the sensing grating, thus offering the prospect of development of practical sensors such as an ambient index sensor or a chemical concentration indicator with high stability and reliability. We measured chloride ions in a typical concrete sample immersed in salt water solutions with different weight concentrations ranging from 0% to 25%. Results show that the LPG sensor exhibited a linear decrease in the transmission loss and resonance wavelength shift when the concentration increased. The measurement accuracy for the concentration of salt in water solution is estimated to be 0.6% and the limit of detection for chloride ions is about 0.04%. To further enhance its sensitivity for chloride concentrations, we coated a monolayer of colloidal gold nanoparticles as the active material on the grating surface of the LPG sensor. The operating principle of sensing is based on the sensitivity of localized surface plasmon resonance of self-assembled gold colloids on the grating section of the LPG. With this method, a factor of two increase in the sensitivity of detecting chemical solution concentrations was obtained. The advantages of this type of fiber-optic sensor are that it is compact, relatively simple to construct and easy to use. Moreover, the sensor has the potential capability for on-site, in vivo and remote sensing, and it has potential use as a disposable sensor.