This account describes recent developments of ion sensors, mainly from the author's laboratory and their response mechanisms based on inorganic metal salt and oxide materials, which includes ion-selective electrodes (ISEs) and quartz crystal microbalance (QCM) sensors. The processes of the selective adsorption and desorption of insoluble inorganic salts at their solid/solution interfaces are fundamentally related to highly component-ion selective crystallization reactions at the solid/aqueous

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... terfaces. This process is analytically relevant and still attractive as a nearly ideal ion/molecular recognition site, which is followed in situ by a membrane potential charge (ISE) and QCM. First, we relate the charge-separation process caused by primary ion adsorption with the potentiometric response of ion-selective electrodes based on insoluble inorganic salts, such as silver halides and metal sulfides. It is shown that QCM sensors modified with various insoluble inorganic salts can detect selective primary ion adsorption and/or multilayer salt deposition at solid/solution interfaces. This new approach allows most insoluble inorganic salts to be used to sense materials for this sensing system. Also discussed are the potentiometric response mechanisms of ion-selective electrodes based on inorganic materials having a three-dimensional network structure with interstitial ions.