以離子場效電晶體為基礎之微型酵素型生物感測器及其參考系統開發

Abstract

本論文旨在開發一種以電化學感測原理結合半導體製程製作的場效電晶體，成為具有對生物訊號有感測能力的微型生物感測器。係利用酸鹼值離子感測場效電晶體(pH-Ion Selective Field Effect Transistor, pH-ISFET)為基礎，在其感測層表面固定化生物材料，例如酵素，來達到生物感測器的功能。此外，為達到微小化之目的，其參考電位系統也一併加入設計考量，以期與後段訊號讀出電路設計能有良好匹配。 pH-ISFET係以金氧半場效電晶體(MOSFET)為基礎發展出之感測器，其金屬閘以離子感應層(Ion Sensing Layer)、酸鹼緩衝溶液(pH Buffer Solution) 和一參考電極取代。場效電晶體的臨界電壓(Threshold voltage)隨著感應層和待測溶液的表面接觸電位變化而改變，而感應層和待測溶液的表面接觸電位變化和溶液中的氫離子濃度有直接的關係。可藉此將電化學反應中離子濃度變化轉換成電子訊號，達到感測的目的。利用此元件特性，改變其感測層特性，將可開發出特定目的之感測器，例如可感測生物訊號的生物感測器。 本論文發現某些質子交換膜，例如NafionTM，結合一些高分子材料可以應用在微型固態電極(Solid-state Reference Electorde, SRE)以及場效電晶體型參考電極(Reference Electrode Field Effect Transistor，REFET)上，此類的固態電極有降低干擾達到穩定電位的能力，同時使得整個感測元件結構簡單。另外，利用NafionTM及包埋法（entrapment）可製作生物分子的包埋層，例如酵素場效電晶體(Enzyme Field-Effect Transistor, EnFET)。 為實現微型生物感測器的開發，此研究為分為三個進程。首先，我們研究並尋找出最佳化製程條件之ZrO2作為離子感應層的ISFET。這樣的元件擁有約58mV/pH高靈敏以及小於1mV/hr的低電壓飄移值，極具作為生物型感測器基礎的優勢，同時也針對其對其它離子(例如鉀離子和鈉離子)的干擾特性作一分析。其次，利用NafionTM等質子交換膜結合高分子材料，例如：polyimide、PR、P3HT等，在不同的材料比例、不同的加熱溫度和不同的結構下來製作微型化固態參考電極(SRE)以及場效電晶體型參考電極(REFET)，這樣的參考電極擁有極低的氫離子感測靈敏度，可與高氫離子感測靈敏度的ISFET結合成差動對來完成一個完整的微型離子感測場效電晶體。如此完成的元件具備轉換成生物感測器的基礎。第三部份根據已完成的微型離子感測場效電晶體為平台，再次利用NafionTM來包埋或混合生物感測材料以形成感測層，探討其生物訊號感測能力、飽和點、重現性以及操作範圍。 在此論文中，我們將詳述酸鹼離子感測器、微小化參考電極以及尿素生物感測器的製作流程及量測條件，並且分析各種感應特性。並針對後段補償電路和差動電路對元件之需求條件作電性匹配調整。

In this thesis, a miniaturized urease biosensor constructed with an enzymy field-effect transistor (EnFET) and solid state reference systems were designed and developed. The EnFETs and the reference systems were developed based on the ion-sensitive FET (ISFET) technology. The ISFETs based biosensors have the advantages of rapid response, small size, high input-impedance and low output-impedance as well as the applicability of semiconductor and integrated-circuit technologies. The ISFET, which is a MOSFET with the gate connection separated in the form of a reference gate immersed in aqueous solution which is contact with the sensing layer above gate oxide. It is working based on the approach of electrochemical measurement of pH. With incorporating the entrapment immobilization technology, the biological or ion-insensitive materials can be attached on the ISFETs and transform the functionalities to be EnFETs or reference FETs (REFETs). Entrapment was a popular method to immobilize the various biological materials, such as urease, glucose and protein, with ISFETs. This method needs supporting matrixes provided by polymers with resistant to chemical attack and low interference to the materials entrapped. In our study, the NafionTM was found to be suitable candidate as supporting material. To develop a miniaturized ISFET based biosensor, three programs were executed. Firstly, the ZrO2-gated ISFET was fabricated and characterized. The optimal post annealing process was determined to be 600 , 30 mins. with N2 gas. The fabricated ISFET demonstrated good performances of high sensitivity, linearity, low drift and hysteresis. Secondly, the immobilization method was developed. The Nafion was utilized as supporting matrix, which entrapped ion-insensitive polymers, to produce REFET. Meanawhile, the EnFET was fabricated with similar approach of immobilizing urease in Nafion. Finally, for the miniaturization purpose, the reference systems, such as solid-state reference electrode (SRE) and reference FET (REFET) with quasi-reference electrode (QRE), were integrated with the sensors in the on-chip level. With differential measurements, the biosensors demonstrated comparable detecting performances.