Piezoelectric Crystal Liquid Flow Detection Systems Based on Alkyl (C18)-Benzo-15-Crown 5 for Organic Compounds and Metal Ions

Abstract

A water in soluble long-chain crown ether alkyl (C18)-benzo-15-crown-5 was synthesized and applied as a coating material on quartz crystal membranes of a liquid flow piezo electric crystal sensor. The oscillating crown ether-coated piezo electric (PZ) crystal with a home-made computer inter face was prepared as a liquid chromato graphic (LC) detector for organic species and metal ions in aqueous solutions. The oscillating frequency of the quartz crystal decreased due to the adsorption of organic molecules or metal ions on crown ether molecules. Effects of functional group, molar mass, steric hindrance, and polarity of organic molecules on frequency responses of the crown ether coated PZ crystal detector were investigated. The frequency responses of the crown ether coated PZ crystal detector for various molecules were in the order: amines > carboxylic acids > alcohols > ketones. The crown ether PZ detector also exhibited good sensitivity for some heavy metal ions and the frequency shifts were in the order: Cr3+ » Pb2+ > Co2+ > Cd2+ > Ni2+ > Cu2+. The crown ether coated piezo electric crystal LC detector demonstrated low detection limits for various polar organic molecules, e.g., 6.0 × 10−5 M for propylamine, and metal ions, e.g., 2.9 × 10−5 M (1.8 ppm) for Cu2+; the crown ether PZ detector also gave good reproducibility when re used. A quite sensitive electrochemical quartz crystal microbalance (EQCM) detection system was also set-up for detecting trace heavy metal ions in solutions. The variation in frequency of the PZ crystal and the diffusion current were observed simultaneously after the reduction in heavy metal ions such as Cu2+ and Ni2+. The EQCM detection system exhibited fairly good sensitivity, e.g., 112 Hz/ppm for Cu2+ and a good detection limit, e.g., 0.13 ppm for Cu2+ ions. Comparison between EQCM and PZ detection systems was made and discussed.