Improving an Impedance Biosensor System Design by Studying the Charge Transfer Resistance Mechanism

近幾十年來，電化學阻抗分析頻譜已被廣泛的應用於多種層面，其中生醫感測器為其重要應用之一。於電化學式生醫感測器之量測中，電極表面的修飾與性質為一項影響阻抗非常重要的因素。為了進一步分析量測所得的電化學阻抗，我們使用一等效電路(Randles)將此電化學阻抗分成溶液電阻、電雙層電容、Warburg阻抗與電荷轉移阻抗。在這四項等效電路元件中，電荷轉移阻抗的變化為造成阻抗變化的最主要因素。因此為改善生醫感測器之效能，我們必須了解此電荷轉移阻抗變化的機制。雖然有多篇文獻指出空間障礙與靜電力的影響為改變電荷轉移阻抗的機制的兩項主要因素，但此兩種因素對電荷轉移阻抗影響的程度仍需要更進一步的研究。於此，我們使用導電式原子力學顯微鏡、界達電位（zeta potential）量測儀與電化學阻抗量測法為工具，對此兩種因素進行評估。於本文中，我們使用了七種具導電性的鏈結分子與一個傳統的長碳鏈分子。從我們的實驗結果中可以發現，電荷轉移阻抗與分子阻抗呈一對數關係，而電荷轉移阻抗與電極表面的界達電位呈一指數關係。這個結果說明了空間障礙所造成的阻抗效應遠比靜電力所造成的阻抗效應來的小。 藉由了解此電荷轉移阻抗轉移機制，我們得以設計出一低阻抗之鏈結分子。於本文中，我們利用對此機制的了解，找出一鏈結分子(ATP)不但具有高導電性，並在我們的量測環境中有具有帶正電吸引氧化還原對的效果。藉由使用此導電性鏈結分子，我們得以放大量測到的訊號並增加感測器的靈敏度。也因此，我們能夠利用一個較為簡化的電路量測出感測訊號。此研究有助於電化學式生醫感測器於定點照護（point-of-care）上的發展。Electrochemical impedance spectroscopy (EIS) has been widely used in many applications such as biosensors over these decades. For the development of electrochemical sensor, the condition and property of electrode surface play a crucial role. The factors of how the surface property affects the electrochemical response have been studied for years; however, a more detailed research of the mechanism is still required. In a faradaic EIS, a Randles model is often used to fit the measured impedance data and the circuit element of charge transfer resistance (Rct) dedicates the most of the impedance change. Apart from the energy potential of the redox pair, steric hindrance and electrostatic force are the two well-known factors responsible for the Rct change. To further investigate how these two factors affect the Rct element, we used conductive atomic force microscopy (CAFM), zeta potential measurements and electrochemical method as tools. In this study, 7 kinds of conductive linkers and a conventional alkanethiol linker were used to form the self-assembled monolayers (SAMs) on the gold electrode. From the experimental results, it can be found that the Rct increases logarithmically with monolayer resistance, and decreases exponentially with the surface charge. This result indicates that the steric hindrance plays a minor role in the Rct change when compared to that of the electrostatic force. By this understanding, we can design a low impedance linker to enhance the signal-to-noise ratio. This enhanced signal can also improve the sensor sensitivity and detection limit. Here we found a linker ATP, which possess a good conductive property and ends with a positive charged functional group. By using ATP, we enhanced the measured signal and improved the sensor sensitivity. Therefore, we can use a simplified electronic circuit to make the biomolecule detection. This study is useful for the point-of-care testing implementation of impedance based biosensor