All-Solid-State Separated Potassium Electrode Based on SnO[sub 2]/ITO Glass

Abstract

An all-solid-state potassium electrode based on tin oxide ͑SnO 2 ͒/indium tin oxide ͑ITO͒ glass is developed in this paper. The sensing membrane of the electrode is prepared by mixing poly͑vinyl chloride͒ ͑PVC͒, bis͑2-ethylhexyl͒ sebacate ͑DOS͒, valinomycin ͑VAL͒, and potassium tetrakis͑p-chlorophenyl͒ borate ͑KTpClPB͒. The mixed solution is dissolved in tetrahydrofuran ͑THF͒. Then, it is dropped on the sensing window of SnO 2 /ITO glass. In addition, the Ag/AgCl wire, which is coated PVC membrane containing 3.5 M KCl powder, is used as a miniaturized reference electrode in this study. The optimum experiment of the potassium sensor is conducted by observing the characteristics of sensitivity and selectivity in different weight ratios. After the optimum experimental trials, the best weight ratio of the potassium sensor is PVC:DOS:VAL:KTpClPB ϭ 33:66:4:2. In this optimum condition, the sensitivity is 56.4 mV/decade with linear concentration range between 1 ϫ 10 −5 M and 1 M. The low detection limit of the sensor reaches to 8 ϫ 10 −6 M, and response time is Ͻ1 s. Moreover, the sensor is slight interfered by sodium, calcium, and magnesium ions. The pH variation does not affect its electromotive force in the range between pH 6 and pH 12. The sensor possesses reusable properties showing that it can be repeated, measuring 30 times.The chemical sensors have been developed for many years because those need a simple analytical technique for real application. The growing trend of chemical sensors is miniaturization and all solid state. 1-5 Among the chemical sensors, the potassium sensor is very important for the clinical and medical fields to detect the potassium ion concentration in serum, urine, and food. 2 The normal range of potassium ion concentration in the human serum is between 3.5 mM and 4.7 mM; hence, hypopotassemia, alkalosis, cirrhosis of liver, and diuretic drugs can be detected by measuring the potassium ion in blood. 6 The potassium sensors have been developed for many years; there are many types, such as ion selective electrode ͑ISE͒, ion sensitive field effect transistor ͑ISFET͒, and others types. Umezawas et al. summarized the papers on potassium ion sensors with different ionophores and membrane composition in the years between 1989 and 1997. 7 The performances of sensitivity and selectivity coefficients were summarized in Ref. 7. Afterward, many papers on potassium ion sensors were focused on ionophores and the membrane matrix. 8-10 In addition, the application of the all-solidstate potassium sensor is starting a trend because the device possesses advantages of a long lifetime, low cost, and short response time. Moreover, a miniaturized reference electrode, such as Ag/AgCl wire coated with a PVC membrane containing KCl powder, was used because the size of the miniaturized reference electrode is smaller than reference electrode with a liquid inner salt bridge, such as REF 251 ͑Radiometer, France͒, used in commercial applications. 5 Indium tin oxide ͑ITO͒ glass is a material of solar cell and thinfilm tra...

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