Quantitative measurement of chloride ion concentration has an important role in various fields of electrochemistry, medical science, biology, metallurgy, architecture, etc. Among them, its importance of architecture is ever-growing due to unexpected degradations of building structure. These situations are caused by corrosion of reinforced concrete (RC) structure of buildings. And chloride ions are the most powerful factors of RC structure corrosion. Therefore, precise inspection of chloride ion concentration must be required to increase the accuracy of durability monitoring.

Well-known chloride ion sensors are using silver / silver chloride electrode, copper / copper sulfate electrode, and several types of organic based optical chloride ion sensors. But, these methods had each constraint, such as high cost, low stability, durability issue at high pH condition, quick signal converting and precise observation. These problems made it hard to analyze the concentration of chloride ions in specific areas.

Multi-walled Carbon nanotubes (MWCNTs) have high chemical resistivity, large surface area and superior electrical property.¹ Thus, it is suitable for the channels of electrical signals made by the sensor. Unfortunately, MWCNTs do not have the chloride ion sensing characteristics. Therefore, silver nanoparticles were added to give the sensing property. CuBTC, one of the metal organic frameworks (MOFs), was employed as a material to improve the sensing property because of its hydrophilicity and high surface area to volume ratio.²

In this study, sensing elements was synthesized by chemical reaction process. At first, MWCNTs were functionalized with a mixture of sulfuric acid and nitric acid because of enhancement of solubility in solution and surface activation. And functionalized MWCNTs, silver nanoparticles, and CuBTC were synthesized on PTFE membrane, one by one. Electroless deposition process was performed to deposit the silver nanoparticles.³ CuBTC was produced by room temperature synthesis.⁴ Surface morphology and composition analysis were characterized by scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDS), respectively. X-ray photoelectron spectroscopy (XPS) was also performed to confirm the existence of sensing materials. The electrical properties of sensors were measured by semiconductor analyzer. The chloride ion sensing characteristics was confirmed with the variation of the resistance at 1 V.

References

1. Saito, R., Dresselhaus, G., & Dresselhaus, M. S. Physical properties of carbon nanotubes (Vol. 35). London: Imperial college press.
2. Chui, S. S. Y., Lo, S. M. F., Charmant, J. P. H., Orpen, a G. & Williams, I. D. A Chemically Functionalizable Nanoporous Material [Cu₃(TMA)₂(H₂O)₃]_n. Science (80-. ). 283, 1148–1150 (1999).
3. Houk, R. J. T. et al. Silver Cluster Formation, Dynamics, and Chemistry in Metal - Organic Frameworks. Nano Lett., 9, 3413-3418 (2009).
4. Tranchemontagne, D. J., Hunt, J. R. & Yaghi, O. M. Room temperature synthesis of metal-organic frameworks: MOF-5, MOF-74, MOF-177, MOF-199, and IRMOF-0. Tetrahedron 64, 8553–8557 (2008).