Sulfate ion sensing based on a quartz-crystal microbalance immobilized with barium sulfate crystals grown on a self-assembled monolayer of phosphorylated 11-mercapto-1-undecanol Original Research Article

A self-assembly monolayer (SAM) of phosphorylated 11-mercapto-1-undecanol (MUD) on a quartz-crystal microbalance (QCM) gold electrode was served as an interfacial layer for subsequent adhesion of barium sulfate crystals. The subsequent process of anchoring barium sulfate crystals on the phosphorylated MUD organic film was followed using X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). The atomic concentration measured by XPS supported the growth of barium sulfate crystal on the phosphorylated MUD SAM on the QCM gold electrode surface. The AFM image of the barium sulfate crystals grown on the phosphorylated MUD SAM showed a smaller surface roughness than those of the MUD and of the phosphorylated MUD SAM. The observed QCM frequency change per unit time of the modified QCM oscillator was found to be proportional to the square of the relative supersaturation of barium sulfate. This also indicated the crystal growth of barium sulfate at the solid/solution interface. The response (−ΔF) of the modified QCM oscillator increased with increasing sulfate ion concentrations in adjacent sample solutions ranging 6.8×10−5–4.5×10−3 M at pH 7.2. These responses were due to the selective adsorption of sulfate ions on the barium sulfate crystals at the solid/solution interface. The relative standard deviation was found to be less than 3%. The order of relative anion selectivities for the QCM frequency change of the modified QCM oscillator was: sulfate ion>carbonate ion≫NO−3, I−, Br−, SCN−. These relative selectivities were attributed to the differences in solubility product and solubilities for the salts of each anion with barium(II) ion.