Orthophosphate ion-sensors based on a quartz-crystal microbalance coated with insoluble orthophosphate salts

Abstract

A nanogram gravimetric sensor for orthophosphate ions in aqueous solution was constructed based on surface adsorption of orthophosphate ions on insoluble orthophosphate salts, such as CePO 4, CrPO 4 and BiPO 4, immobilized on the electrodes of a quartz-crystal microbalance (QCM). The mass change caused by the selective adsorption of orthophosphate ion on solid/ aqueous interfaces of the immobilized insoluble orthophosphate salts was detected by the QCM. The microcrystal particles of each orthophosphate salt was embedded within the adhesion layer of the silicone rubber. The observed frequencies of the QCM ion-sensors, thus made, were found to decrease with increase in the orthophosphate ion concentrations in adjacent sample solutions ranging 10 −6–10 −2 M at pH 7.0. These responses were mainly due to the single-ion adsorption of orthophosphate ions. The order of ion selectivities for the frequency response of the QCM sensors modified using CePO 4 and CrPO 4 was: orthophosphate ionarsenate ioncitrate ion⪢F −1, SO 4 2−, ClO 4 −, OAc −, NO 3 −, Br − and SCN −, and using BiPO 4 immobilized QCM was: orthophosphate ionarsenate ion⪢F −, SO 4 2−, ClO 4 −, OAc −, NO 3 −, Br − and SCN −. These selectivities were attributed to the differences in solubility products or solubilities for the salts of each anion with Ce 3+, Bi 3+ or Cr 3+; the molecular weight of each anion was also taken into account.