探討聚氯乙烯/氯化鉀薄膜製程對固態銀/氯化銀參考電極電位穩定性之影響

Abstract

電化學感測器近年廣泛應用於醫療、農業和環境監控等產業，而參考電極為其提供參考電位值，並影響感測的精確度與時間。目前感測器中的參考電極需要較長時間才可達到穩定的電位值，不過使用者希望數分鐘內可完成所有感測程序，使許多系統在參考電極尚未穩定時就讀取數值，使量測結果誤差增加。本論文欲開發一固態式參考電極可於數秒內達到一穩定電位值以減少量測誤差，其層狀結構從聚對苯二甲酸乙二酯（polyethylene terephthalate, PET）基材往上依序為銀/氯化銀網印電極（Ag/AgCl screen-printed electrode）、氯化鉀–聚氯乙烯（KCl-PVC）層和聚氯乙烯（PVC）層 。根據文獻，網印電極表面氯化銀的厚度與緻密程度影響參考電極的電位穩定與使用壽命，以及氯化銀需有飽和的氯離子進行可逆的氧化還原反應。是故，將網印電極浸泡於氯化鐵以提高表面氯化銀比例後，電極表面氯化銀比例提高後較不易產生極化現象，使參考電極的60秒內電位飄移量減少約3～5 mV。接著將不同重量的KCl粉末以PVC固定於已氯化的網印電極上，利用KCl覆蓋率與厚度計算其最佳濃度參數，使氯化銀可與充足的氯離子反應；另外由結果得知，若上層的PVC層未優化，將使過量的KCl在低濃度待測液下不斷溶出且增加電位飄移量。其次，影響參考電極的另一因素為液面接界電位（liquid-junction potential），即離子擴散時在介面造成的電位差。本論文滴覆不同的PVC濃度與厚度在KCl-PVC層上，量測KCl溶出量以分析PVC薄膜的通透性，再以開環電位法量測固態式參考電極的各項能力。從結果說明PVC層內層KCl溶出可能使液面接界現象改變，不僅影響電位飄移量，也降低不同濃度待測液間的電位一致性。最佳參數的固態式參考電極其30秒內電位飄移量約0.5 mV和響應時間為25秒，以及在不同濃度待測液下的電位差異約3 mV。最後藉由與離子選擇電極共同量測鉀離子濃度以及應用於循環伏安法的掃描，皆證實自製固態式參考電極可適用於二極式與三極式系統中。

Electrochemical sensor is widely applied in medical examination, agriculture and environmental monitoring. The major components of the sensor include working electrode and solid-state reference electrode (SSRE). While the reaction of interest is occurring on the working electrode, SSRE provides a basis to measure the potential of working electrode. By providing reference potential, SSRE has to reach its stable state first, and on current SSRE it takes more than 5 minutes, which is much longer than user requirement, i.e. within minutes. Thus, in this study, we develop a layered SSRE that can reach its stable potential in a timely manner, and ultimately enhance the accuracy of the sensor. The layered structures of SSRE from bottom to top include polyethylene terephthalate (PET) substrate, Ag/AgCl screen-printed electrode (SPE) layer, potassium chloride-polyvinyl chloride (KCl-PVC) layer and PVC layer. In previous studies, increasing the thickness and compactness of AgCl facilitates stabling the potential and extends the life of SSRE. Thus, in this study the Ag/AgCl SPEs is chlorized by immersing in FeCl3 solution to increase the deposition of AgCl on the surface of electrode. The results show that the chlorized electrode is less vulnerable to polarization, and the potential drift in 60 seconds is decreased to 1 mV. Furthermore, KCl-PVC layer is covered on the chlorized SPEs. The coverage and thickness of KCl are calculated to ensure that sufficient Cl- maintains reversible redox reaction with AgCl on the surface of SPEs. However, it is found that KCl dissolves into the analyte during measurement, which leads to the potential drifts especially in analyte of low concentration. Hence, an optimized PVC layer is covered on top of KCl to prevent KCl from dissolving. Secondly, different concentrations and thickness of PVC layers are covered on KCl-PVC layer. The results show that the ion diffusion on PVC layer may alter the liquid-junction potential, which is critical on potential drift and reproducible potential in different analytes. By implementing these approaches, the potential drift in 30 seconds of the optimized SSRE is decreased to 0.5 mV, and the response time is shortened to 25 seconds, which is significant improvement compared to the current SSRE. Finally, the optimized SSRE is used in cyclic voltammetry and applied with potassium ion-selective electrodes, and proved to be feasible in electrochemical sensing.