Constructions And Applications Of The All-solid-state Ca²⁺-selective Electrodes Based On Amperometric And Colorimetric Signal Readouts

Ion-selective electrodes（ISEs）,as a branch of electrochemical sensors,have been widely used in soil measurement,environmental monitoring,medical treatment and other fields due to their simple detection processes,portable instruments.The traditional ISEs with inner filling solutions need careful storage and frequency maintenance,due to the leakage of the inner filling solutions.In recent years,solid-contact ISEs without inner solutions have been developed,which can effectively avoid the influence of ion flux.Such electrodes show low cost,fast response and portability,and have become a new research hot spot in the field of potentiometric sensors.The principle of solid-contact Ca²⁺-ISEs is:the relationship between the potential response and Ca²⁺activities conforms to the Nernst equation,and the Nernst slope of the electrode is 29.6 m V/decade（25℃）,indicating that a change of 1 m V will cause an activity error of 8%.This means that traditional potential signals are not suitable for detecting small changes in the ion activities.In addition,the readout of the potential response is dependent on A/D converter,which needs complex circuit design.Based on this,this paper proposed new readout strategies for determination of calcium in mineral water,tap water and calcium tablet.The main research contents are as follows:1.A detection method of solid-state calcium ion selective electrode based on current signal readout of redox probe is established.The detection of the all-solid-state Ca²⁺-ISE based on amperometric readout of redox probes have been proposed.The detection principle of this method is as follows:a constant potential was applied between a glassy carbon electrode and the all-solid state Ca²⁺-ISE.The oxidation current of the redox probe on the glassy carbon electrode is modulated by the potential of the ISE through changing the concentrations/activities of Ca²⁺.The concentrations of calcium ion were measured according to the generated oxidation current.The results show that the linear range for detection of Ca²⁺based on the amperometric signal of ide/ferrocyanide redox probes is 1.0×10^-6～1.0×10^-2M,and the slope of response is 22.2μA/dec at the applied potential of-0.2 V.In addition,as compared the potential response with m V lever,the signal resolution of amperometric signal based on redox probes up to thousands of n A when detecting small changes in Ca²⁺activities.Moreover,the amperometric readout method is used to measure the concentrations of calcium ion in mineral water,calcium tablets and seawater,and the results are consistent with those for the traditional potential response,indicating that it is feasible to transform the potential signal of ion-selective electrode into the amperometric signal.2.The detection of the all-solid-state Ca²⁺-ISE based on colorimetric readout of the electrochromic device have been proposed.The detection principle of this method is as follows:The electrically induced discoloration of the preparation of devices and applying constant solid contact between the calcium ion selective electrode potential,change of calcium ion concentration,make solid contact calcium ion selective electrode potentiometric response,and regulation of the electric potential to color device,make its produce change color.To achieve qualitative and semi-quantitative detection of calcium ion concentration.A constant potential was applied between the Prussian blue-based electrochromic device is modulated by the potential of the ISE through changing the concentrations/activities of Ca²⁺.The concentrations of calcium ion were qualitatively and semi-quantitatively measured through the naked eye or the RGB value obtained by the smartphones.The results show that the concentration range of 1.0×10^-6～1.0×10^-2M Ca²⁺can be detected based on the color changes of electrochromic device.The colorimetric readout method is used to measure the concentrations of calcium ion in mineral water and tap water,and the results are consistent with those for the traditional potential response.This work provides a visual and fast method for detection of Ca²⁺through transforming the potential signal of ion-selective electrode into the colorimetric signal.