矽單晶上金屬超微電極之製備與電化學應用

Abstract

超微電極(ultramicroelectrode)在電化學偵測上扮演著非常重要的角色。微小電極面積、高質傳速度(mass transfer rate)和低介面電容(surface capacitance)等特性使其可以應用在生物細胞的性質偵測、微量分析和化學反應機制的研究中。然而超微電極因電極面積小，所偵測到的反應電流也較低，因此訊雜比(signal-to-noise ratio)較差。電極設計上可以用微電極矩陣來解決此一問題，也就是許多微電極同時進行偵測，如此一來可以大幅提升反應電流，同時又維持擁有超微電極的性質。傳統上，超微電極矩陣是利用半導體製程技術，在矽晶片表面設計金屬超微電極，儀器使用成本較為高昂。在本篇論文研究中，我們使用簡易的高溫鍛燒和電化學沉積技術，成功的在矽晶片表面上製備出白金超微電極矩陣。製程上，我們利用銅和氧化矽薄膜在高溫中特殊的作用機制，於晶片表面上產生金屬氧化物的矩陣；接著使用電化學沉積方式在金屬氧化物上成長白金電極。利用掃描式電子顯微鏡SEM測得白金超微電極矩陣之各島狀電極結構的平均寬為500nm，長為數微米；掃描式歐傑影像儀則用以分析表面元素的分布狀態，以確定其電化學活性區域和電極成長的過程。超微電極的電化學特性則分別使用Ru(NH3)6II/III和Ferrocene做為標準品進行分析，在低循環式伏安法掃描速率下，我們合成出的電極顯示出steady-state 的I-V圖，證明了該電極具有超微電極的特性。應用上，我們使用該電極來對微量鎘離子進行偵測。搭配上差式脈波伏安法(differential pulse voltammetry)和陰極脫附伏安法(anodic stripping voltammetry)兩項技術，其偵測極限達數百ppb。本論文研究所開發之超微電極製備技術，相較於利用半導體蝕刻技術而言，其成本低廉許多，且製備方式較為容易，可應用在生醫偵測或環境樣品採樣快速分析上。

Ultramicroelectrodes (UMEs) are wildly used in trace sample analysis and some kinetic reaction studies because of their fast mass transfer and low charging current properties. However, the small effective size of UMEs also leads to a decrease in Faradaic signal and results in a low signal-to-noise ratio. Ultramicroelectrode arrays (UMEAs) were thus developed to solve the problem. In general, the silicon technology used to fabricate UMEAs involves photolithography and chemical etching for properly controlling their structure. However, the technology uses sophisticated instruments, which makes the fabrication process costly. Its solvent waste also raises environmental concerns. In this study, we introduced a novel and facile method to fabricate UMEAs on silicon wafers. It has the advantages of low fabrication cost and of posing no harm to the environment. The UMEs were electrochemically developed from a copper-based line structure which was produced by the annealing in oxygen environment of a copper thin films deposited on silica-covered silicon chip. The resulting UMEA structure (termed silica-isolated UMEA, SUMEA) was studied using scanning electron microscopy (SEM) and scanning Auger electron microscopy (SAM); the SUMEA electrochemical properties were determined by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) in both ferrocene (organic) or ruthenium hexamine (aqueous). The results showed that there were multiple UMEs on a single chip. The Faradaic signal was enhanced while the unique UME properties were preserved. The kinetic calculations from the CVs were also agreed with the UME properties in SUMEA. Further electrical properties of SUMEA were analyzed using EIS, in which an equivalent circuit was assumed to describe the electrical conduction in SUMEA. The relative electrical parameters of the SUMEA were obtained using the impedance fitting in Nyquist plot, and confirmed by the kinetic calculation of the steady-state cyclic voltammogram, which shows the assumed equivalent circuit is suitable to describe the electrical behavior of the SUMEA.