以新穎性電化學安培法發展不具電化學活性分子之量測技術以及微晶片電泳電化學偵測平台的開發

Novel avenues in determination of electrochemically inactive species and construction of a new amperometric platform in microchip electrophoresis

陳繼浩(Chi-Hao Chen)

指導教授 ： 林孟山

淡江大學/理學院/化學學系博士班/博士(2013年)

https://doi.org/10.6846/TKU.2013.00307

摘要

本研究第一部分基於銅金屬的配位能力，發展在環境以及生醫檢測上具有重要指標性但不具電化學活性偵測物質的檢測方法。實驗上基於銅金屬在氧化時，由於價態轉變需要額外配位基螯合的特性，將電極表面難溶的氧化層轉變為可溶性的銅錯合物，並藉由流注系統將之帶離電極表面，使得內層金屬銅層可繼續氧化產生氧化電流訊號。藉由量測此氧化訊號的大小可對不具電化學活性的分子如多胺、肌酸酐以及易毒化分子如鈉乃浦、嘌呤進行電化學分析。
實驗首先先以一系列多胺分子對整個量測機制進行研究，實驗結果發現多牙配位基且能與銅離子螯合成五圓環或六圓環的錯合物才能達到較高的靈敏度，因此推論本方法具有結構的選擇性，此外此選擇性可藉由緩衝溶液的酸鹼值改變配位基的帶電性質進行調整。實驗條件經最佳化後，多胺分子可在0.1 M磷酸緩衝液，pH 10下，對於精胺、亞精胺、腐胺以及屍胺四種多胺分子的偵測極限分別可達到50, 60,110以及270 nM。
鈉乃浦為雙硫醯胺化合物，為環境常見之殺菌農藥，根據國際殺蟲劑殘留物聯席會議裁定，鈉乃浦的殘留量需小於7 ppm，而此分析物不易以電化學進行分析。然而以結構上，鈉乃浦適合本研究所開發的機制。經最佳化探討後，最佳化條件為:50m M 磷酸緩衝液pH 5.75，偵測電位為- 0.125V，線性範圍為0.2 μM to 10 μM (or 0.051 to 2.56 ppm)。此方法不受環境分子如SDS，腐植酸以及樟腦的影響。然而，由於鈉乃浦的螯合處容易與其他金屬結合，造成有效濃度的下降，因此利用陽離子交換樹脂將樣品中的重金屬先行交換出來，可防止重金屬的干擾。
在醫學研究上，肌酸酐是評估腎功能的重要指標，正常人血清中的肌酸酐濃度約0.6~1.2 mg/dL，每天排放於尿中的總量為1~2.5g。由於肌酸酐完全不具有電化學活性，傳統上偵測此一分子須依序經由肌酸酐酶、肌酸酶以及肌氨酸氧化酶三個酵素轉換出過氧化氫之後進行量測，或是利用肌酸酐去亞胺酶轉換出銨根離子再進行偵測，因此在偵測線性、選擇性以及實用性上都尚待改善。而本研究發現肌酸酐符合本偵測機制，可在無酵素輔助下直接對肌酸酐進行分析。最佳化的分析條件為100 mM磷酸緩衝液(pH 7.0)，偵測電位為-100 mV，其線性範圍為25 μg/dL to 1.5 mg/dL。本方法除了尿酸外不受其他傳統易氧化物質的干擾，而在包覆上一層Nafion 薄膜後，可排除尿酸的干擾。
接者基於本機制可對尿酸進行偵測，因此對於尿酸的嘌呤結構類似物如黃嘌呤、次黃嘌呤、腺嘌呤、鳥嘌呤以及咖啡因進行研究，這些嘌呤類分子雖然是具有電化學活性之分子，然而偵測上需要相當高的氧化電位且會造成嚴重的店及毒化現象。而本方法是利用配位的方式進行偵測，分析物本身不會進行氧化，因此可大幅提升分析的再現性。而研究結果顯示除了咖啡因外，其餘的嘌呤類似物的偵測極限都可低於μM，而無法偵測咖啡因的原因推測是來自於螯合官能基產生的立體障礙所致。
第二部份則是開發微晶片電泳系統搭配整合式電化學偵測平台，希冀能以簡單的電泳分離增加電化學系統的選擇性。然而為了降低製作成本以及增加後續電極材料的可變性，實驗上選擇利用翻模法製作PDMS微流道，並以網版印刷以及濕式蝕刻的方式先行在玻璃基材上製作出電極溝槽，之後填入導電碳膠而完成整合式電化學偵測平台的製作。在不需要顯微鏡的輔助下，偵測系統與微流道貼合的變異性僅有2.2%。本系統雖然是利用導電碳膠作為去偶合電極，但在電場強度為200V/cm下，此電化學偵測系統背景電流值的變異仍僅有1.4 pA，且不會產生短路的現象。而利用多巴胺以及鄰苯二酚對此系統的分析特性的進行評估。其線性分別為0.1 到 100 μM (R = 0.998) 以及 0.2 到 200 μM (R=0.996)。理論板數分別達15700以及34600(plate/m)。此外由於導電碳膠可以輕易用丙酮清除，因此電極的部份可輕易進行去除並重新製作。
最後一部份則是發展簡易的電滲流強度評估方法。在電泳分析中，電滲流佔有舉足輕重的腳色，傳統上需要加入中性分子或是利用各種物理或化學的方式改變局部緩衝液的組成才可對電滲流進行評估。而本研究中所開發的偵測平台上，發現去偶合電極除了可消除高壓電場的干擾外，還會將溶液中的氧氣轉變為過氧化氫，因此藉由改變去偶合電極與工作電極之間的距離便可在不添加任何物質或改變溶液組成的情況下以對電滲流的強度進行評估。最後也應用於評估分析物的電泳性質上。

關鍵字

安培法 ； 多胺 ； 肌酸肝 ； 鈉乃浦 ； 嘌呤 ； 微晶片 ； 電滲流

並列摘要

The goal of this thesis is to develop a new amperometric scheme to deal with those electrochemically inactive species by using a copper based or copper plated electrode. Since the target analytes such as polyamines, purines, nabam and creatinine can be used as a ligand for cupric ions, the passive oxide layer on the copper plating electrode would be converted into a soluble cupric or cuprous complex after reacting with these analytes. This behavior exposed the inner copper layer to the solution and an oxidation reaction would be induced to further regenerate the passive layer and the intensity of the oxidation current can be used to reflect the concentration of these analytes.
Firstly, the basic behaviors of this scheme were investigated respect to the polyamines. The results of this approach show that this new amperometric scheme possesses better sensitivity in those analytes that could form a stable five or six member-ring complex with cupric ion, and the selectivity of this scheme would be
further regulated by the charge polarity of the chelating site. Moreover, in the optimal experimental, the detection limit of spermine, spermidine, putrescine and cadaverine is 0.05, 0.06, 0.11 and 0.27 μM, respectively (S/N = 3).
Nabam is a dithiocarbamate based fungicide which is extensively used today. According to the requirement of the Joint FAO /WHO Meeting on Pesticide Residues (JMPRs), the tolerance limit of this pesticide is 7 ppm. Electrochemical determination of the nabam has rarely been successful due to its poor redox property and surface contamination. However, nabam is not only a fungicide but also a famous chelating molecule; thus, it would be a suitable candidate for this scheme. Under a set of optimal condition with operating potential at -0.125 V in a 50 mM phosphate buffer, pH 5.75, the dynamic range of nabam is 0.2 μM to 10 μM (or 0.051 to 2.56 ppm), which meets the requirement of JMPR. This method is free from the common environmental interferences. However, a simple cation exchange column was used to release the chelated nabam molecule from other metal-nabam complexes.
Creatinine is an important index of renal function in the clinical diagnosis, which maintains a range between 0.6 to 1.2 mg/dL in the serum for a health subject that is independently of dietetic habit. Traditionally, because the creatinine is an electrochemically inactive molecule, most approaches require a series of enzymatic reactions to facilitate their electrochemical applications. However, a serious interference from each enzymatic intermediates and poor linearity are commonly encountered problems. Here, creatinine was identified as a target ligand to be determined via this special scheme. After a careful optimization, this scheme can be conducted at potential of -0.1 V in a phosphate buffer (pH 7). The linearity of creatinine is started from 0.025 to 1.5 mg/dL. This method does not suffer from most biological interferences except uric acid. However, a Nafion® coated copper plated electrode successfully overcame the interference of the uric acid with a slightly decreased sensitivity of creatinine. The feasibility of further clinical application is demonstrated by evaluating the creatinine concentration in a urine sample.
Finally, because this scheme shows a highly sensitive in uric acid, therefore, the feasibility of its structural analog such as xanthine, hypoxanthine, guanine, and adenine were investigated. Although these purine derivatives can be directly determined with a carbon based electrode at relative high oxidation potential, it would encounter several problems including biological interferences and surface contamination. However, because the operating potential of this proposed scheme can be held below 0 V and the analyte would not be really oxidized in the proposed scheme. Moreover, because one of the structural analog, caffeine, could not be performed in this scheme, which implies this scheme has not only a structural selectivity but also a stereo selectivity.
The following project described a novel method to fabricate an integrated amperometric platform for an off-channel based microchip electrophoresis. A simple screen printed protocol combining a wet etching procedure was used to define the three electrode system, decoupler electrode, and their conductive circuit on a glass substrate. Subsequently, a carbon based amperometric platform was obtained by filling the conducted carbon ink into these cavities. The variation of reassembled of this device is only 2.2 % without the assistance of the microscope (N = 6). This device was characterized by dopamine (DA) and catechol (CA). Under the optimum conditions, this device possesses a low background with a very low noise level of 1.4 pA (peak to peak). The linear range for DA and CA are 0.1 to 100 μM (R = 0.998) and 0.2 to 200 μM (R = 0.996) with sensitivity of 82.17 and 37.51 pA/μM, respectively, and a theoretical plate number is 15700 and 34600 (plate/m) for DA and CA, respectively. Finally, the carbon ink electrode can be easily removed by acetone solution, which indicates this platform could be easily renewed when necessary.
The next work focused on the development of a simple method to estimate the electroosmotic flow (EOF) without using any complicated electric profile or physical alteration. In our off channel microfludic device, it is found the carbon ink based decoupler electrode maintains its reductive capacity after the electric field is turned
off. This residual reduction capacity would continuously convert the dissolved oxygen to hydrogen peroxide and accumulated on the decoupler electrode. Subsequently this sample plug was used as a marker driven by EOF and would be delivered to reach the working electrode during the next electrophoretic run. In this scheme, the EOF has been proven to be determined by using a single microchannel only. Subsequently, this scheme was transferred into a cross type microchannel, and the electrophoretic property of the target analyte can be estimated after a simple calculation with the retention time of the self-generate marker and the target analyte.

並列關鍵字

Amperometry ； Polyamine ； Creatinine ； Nabam ； Purine ； Microchip ； Electroosmotic flow

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