自組裝結構增強茶葉黑色素半導體特性

Abstract

黑色素是一種存在於植物、動物和微生物的獨特色素。黑色素主要功能在於保護生物免於太陽輻射的直接照射；黑色素不僅是個「顏色」，同時也是具有清除自由機活性功能的抗氧化劑，亦是重金屬的螯合劑，也對有毒有機分子具有強吸收性。黑色素本身無毒且具有生物親和力；是對抗有毒化學物和重金屬的天然保護劑。

我們成功的在茶葉中萃取出在生理酸鹼值下可溶解的黑色素。茶葉黑色素是個具有單一分子量14 kD的均勻高分子。我們使用掃瞄式電子顯微鏡第一次發現到茶葉黑色素自組裝的奈米結構，同時也發現了茶葉黑色素的奈米晶體。對於黑色素的奈米粒子的物性、化性和生物特性方面的研究是非常重要的。

本實驗利用茶葉黑色素製作非晶態薄膜於矽晶圓基材，並且成功發現茶葉黑色素薄膜具有半導體導性質。經由對此非晶態薄膜的直接量測，發現其與金屬之間有蕭基能障存在；利用奈米結構觀察其在微觀尺度下的導電特性，發現蕭基能障轉趨不明顯。茶葉黑色素非晶態薄膜的導電性可以經由摻雜提升，經由摻雜處理過的薄膜，其導電性明顯提高，同時伴隨著對金屬能障的縮小。茶葉黑色素的導電性較動物性黑色素略高，更是合成黑色素的一百倍之多。

在適當條件下，茶葉黑色素可形成三腳狀的自組裝構造，此構造同樣具有半導體的特性。茶葉黑色素自組裝構造的導電性較非晶態薄膜為高，大約有三十倍之譜，且與非晶態薄膜相較，蕭基能障幾乎完全消失。同樣地，此自組裝結構的導電性也會依摻雜程度的不同而產生變化；除此之外，經由掃描式電子顯微鏡的觀察，我們更發現，自組裝結構的親疏水性及形成狀況與摻雜程度有密切關係。

我們相信茶葉黑色素的半導體特性，加以天然無毒的生物相容性，不論在學術或是應用方面絕對具有無窮潛力，若能更進一步地了解其特性、結構、與生物體的交互作用，未來必定能在奈米生物電子的領域中開花結果。

Melanin is a ubiquitous black pigments from plant, animal, to microbes. The primary function of melanin is to protect life from direct sun radiation. Melanin is not just a color. It is a strong antioxidant with free radical scavenging activity, a strong heavy metal chelator, a strong toxic organics absorber, and is non-toxic and bio-absorbable itself; thus it serves as a natural guardian against hostile chemical and heavy-metal environments.

We have extracted melanin from tea which is soluble at physiological pH. Tea melanin is non-toxic and exhibits all biological activities as melanins extracted from other resources. Tea melanin is homo-polymer with single molecular weight of 14 kD. We disclose, for the first time, the nano-structure of self assembled tea melanin using scanning electron microscopy. We also obtained nano-crystal of tea melanin. Thus is plausible to explore the chemical, physical, and biological properties of melanin nano-particle.

Melanins are also thought to be solid state biomacromolecular semiconductors. In 1974, McGinness et al. showed that eumelanin could behave as an amorphous electrical switch. They postulated that these materials may be disordered organic semiconductors. Semiconductor properties were observed in melanin derived from tea. The formation of tea melanin fiber enhanced several orders of this conductivity. Schottky barrier with threshold voltage of ca. 5 volts was observed for amorphous thin film; however, such phenomenon was negligible for melanin fibers. In this thesis, it is described that our attempts to unravel the structure-conductivity relationships of these mysterious biological conductors.