Quartz crystal microbalance for selenite sensing based on growth of cadmium selenite crystals immobilized on a monolayer of phosphorylated 11-mercapto-1-undecanol

A quartz crystal microbalance (QCM) sensor for selenite ions in aqueous sol ution was constructed based on crystal formation of cadmium selenite, immob ilized with a self-assembly monolayer (SAM) of phosphorylated Il-mercapto-l -undecanol (MUD) on a QCM gold electrode surface. The mass change caused b y the selective adsorption of selenite ions on the cadmium selenite crystal s at the solid/ solution interface was detected by the QCM. The response (- Delta F) of the modified QCM oscillator increased with increasing selenite ion concentrations in sample solutions, ranging from 9.7 x 10(-5) to 9.0 x 10(-4) M at pH 7.4. The synthetic process of anchoring cadmium selenite cry stals on the phosphorylated MUD organic film was also followed by using Xra y photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). The a tomic concentrations measured by XPS confirmed the crystal growth of cadmiu m selenite on the phosphorylated MUD SAM at the QCM gold electrode surface. From the AFM images, changes in surface topographic features were followed : the MUD SAM and phosphorylated MUD on the QCM gold electrode had similar surface roughness; however the difference for the cadmium selenite film on the phosphorylated MUD SAM was clearly seen, The observed QCM frequency cha nge of the modified QCM oscillator per unit lime was found to be proportion al to the square of the supersaturation of cadmium selenite, indicating the crystal growth of cadmium selenite at the solid/solution interface. The mo dified QCM oscillator exhibited selectively strong QCM response to SeO32- i on. In contrast, the responses to tested interfering anions were almost neg ligible. The order of anion selectivities of the present modified QCM senso r was SeO32- much greater than CO32- > SeO42-, SO42-, Br- I, NO3. These sel ectivities were basically attributable to the differences in solubility pro ducts and solubilities for the salts of each anion with cadmium (II) ion.