利用奈米間隙之共平面電極探討表面電位與電雙層電容之關聯並作為生物感測器之應用

An Incremental Double-Layer Capacitance of A Planar Nano Gap and Its Application in Biosensors

薛孝亭(Hsiao-Ting Hsueh)

指導教授 ： 林致廷

國立臺灣大學/電機資訊學院/生醫電子與資訊學研究所/博士(2016年)

https://doi.org/10.6342/NTU201600421

摘要

表面電位為固液介面間最重要的性質之一，可藉由外加電壓或者表面改質來作調控。因此，表面電位可作為一良好之指示器來觀察表面性質的變化，例如生物分子接合的現象。因此在本論文中，我們提出了利用共平面電極且電極間隙為奈米尺寸的結構，可用來量測間隙表面電位之變化，作為表面電位變化之指示器，更將此指示器作為生物分子感測元件使用，期能透過此簡單架構的元件以達成重點醫療檢驗之目標。
由於間隙表面電位會影響溶液表面離子的分布，而表面離子的分布亦會決定了電極的電雙層電容組成，因此，我們所提出的架構，即是利用這樣的原理來進行量測。我們利用的電化學阻抗分析以及循環伏安法量測電雙層電容因間隙表面不同化學性修飾所致之變化，並由實驗結果驗證了表面電位以及電雙層電容之間的關係。更由調整間隙寬度的實驗中提出了兩條路徑的等效電路模型來解釋現象。其中一條路徑與傳統路徑一致，此條路徑遠離間隙表面，因此其電路特性並不會受到間隙表面電位之影響。而另一條路徑則貼近表面，因此，此條路徑會受到表面電位所調控。此外，由調整溶液離子強度量測阻抗變化，證實了表面電位會藉由縮短電雙層之厚度而影響電雙層電容。
其次，在驗證了基本機制後，我們將此指示器應用於生物分子量測上。我們在量測環境中含有高濃度(10µg/ml)牛血清蛋白為干擾物的條件下，成功量測到電容隨著心肌鈣蛋白T的濃度的變化，證實了此晶片可做為心肌鈣蛋白T的感測元件用。心肌鈣蛋白T為最重要用來檢測心肌損傷之生物標記物。可檢驗的範圍由10 pg/ml 到 1 µg/ml，最低極限為10 pg/ml，此數值可滿足目前醫學上判斷心肌損傷之標準(35 pg/ml) ，因此其結果提供了此晶片可應用醫學檢驗上之可行性。更由於此晶片之結構簡單，反應迅速，具有可最為重點醫療檢驗晶片之潛力。
最後，我們討論了三個會影響此感測器表現與性質的三個大方向。分別是反應時間、元件結構以及抗原性質三個面向。在反應時間部分，我們藉由提高抗體與抗原之反應時間，證實了可提高反應之感測極限。而在元件結構部分，我們提高的間隙的高度以增加表面電流與電場的分佈比例，證實了3D間隙結構可提高元件之敏感度。在抗原性質部分，我們探討了抗原不同電性、結構、聚集以及大小等特性，皆會影響感測器之表現。儘管結果除了證實蛋白質的複雜性外，我們更從這些複雜性中，找到數個可解釋結果之脈絡，而這些脈絡可在未來做感測器特性之預測提供重要的參考。

關鍵字

電雙層電容 ； 循環伏安法 ； 生物感測器 ； 奈米間隙 ； 共平面電極 ； 德拜長度

並列摘要

Surface potential is one of the most important properties at solid-liquid interfaces. It can be modulated by the voltage applied on the electrode or by the surface properties. Hence, surface potential is a good indicator for surface modifications, such as biomolecular bindings. In this dessertation, we proposed to use a planar nano-gap structure to monitor surface-potential difference and biomedical application.
Based on the proposed architecture, the variance of surface-potential difference can be determined by electrical double layer capacitance (EDLC) between the nano-gap electrodes. In this work, we used electrochemical impedance spectroscopy to demonstrate the relationship between surface potential and EDLC by chemically modifying surface properties and by physically tuning the gap width. Then, we proposed two pathway equivalent circuit model to explain the relationship. One pathway is away from the surface which is independent of gap surface potential and the other one is near the surface which is dependent of gap surface potential. Further, buffer concentration experiment proved gap surface potential modulated near surface pathway by contracting electrodes debye length.
Next, we showed the proposed planar nano-gap device provide the capability for cardiac-troponin T (cTnT) measurements with co-existed 10 µg/ml BSA interference by using cyclic voltammetry. cTnT is one of the most important biomarker of myocardial necrosis. This detection is by monitoring of surface-potential variation and differs from traditional capacitive biosensors. The detection dynamic range of our device is from 10 pg/ml to 1 µg/ml and the detection limit is less than 10 pg/ml in diluted PBS buffer (0.01X PBS). These results demonstrated the planar nano-gap architecture having ability on clinical examination. Moreover, because of the simplicity and fast response of the proposed mechanism, this device has high potential on point-of-care test application.
Final part is optimization of the proposed mechanism on bio-detection. We discuss three categories of variables that modulate sensing characteristics. They are response time, electrode structure and antigen features. In response time, we modulated the incubation time of antigen and antibody and showed improvement detection limit. In electrode structure part, we raised the height of gap and the experiment results demonstrated the improvement of sensitivity. In antigen feature part, we compare different charges, sizes and structure of antigen such as catenin, CRP and Hb. Though, the result demonstrates the complexity of protein, there are still some tendency in these features.

並列關鍵字

Electric double layer capacitance ； Cyclic voltammetry ； Biosensor ； Nano-gap ； Coplanar electrode ； debye length

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