本論文我們提出一個將單分子去氧核醣核酸(DNA)端點釘附於表面之新策略。此策略是透過先散佈外層修飾streptavidin並帶正電的量子點至一帶負電的玻璃基板上，並利用它們去鉤附端點有修飾biotin的DNA分子。此外，我們進一步研究在直流和交流電場的影響下，端點釘附之DNA的拉伸與彈性鬆弛行為。
本論文第三章，我們演示如何實現釘附作用並尋求最佳實驗條件來促進單分子的研究，此實驗條件包括添加非離子型界面活性劑以消除電滲流、增加溶液黏度去延長鬆弛時間以及控制添加量子點的數量使得在DNA分子無相互糾纏的前提下獲得足夠的DNA分子釘附的量。
本論文第四章，我們施加不同大小的DC電場拉伸端點釘附的DNA並以獲得拉伸力-長度特徵曲線。我們亦藉由不可延展和可延展的wormlike chain models迴歸實驗數據去測得DNA的persistence length(l_p)。我們出乎意料地發現l_p不超過10nm並且比一般文獻上常見的50nm短，這說明在我們的系統裡DNA分子較難拉伸，原因可能為DNA鏈滴(blobs)與表面界面活性劑交聯。這個交聯效應可以透過存在著兩個鬆弛時間的結果來支持，而我們觀察到第二鬆弛時間較長，且效應也隨拉伸程度的減少而更明顯。
本論文第五章，我們檢視在高頻交流電場下DNA的鬆弛行為。在一交流電場頻率下，我們發現第一鬆弛時間會隨著交流電場增加而有先減後増的趨勢，且不論DNA拉伸長或短，其變化趨勢均類似。這個非單調變化可歸因於與長度相關的極化機制，此機制涉及於在長拉伸時的介電極化與短拉伸時的導電極化之間的相互競爭。
本論文的最後部份，我們施加一具偏壓不為零的交流電場拉伸DNA。相較於第四章的結果，在平均電場強度方面，於低電場時拉伸會較長，而在高電場時會較短。針對低電場的情形，我們亦透過關閉偏壓，在一平均電場為零的交流電場下測量DNA(第一)鬆弛時間，並發現其確實比第四章結果來的長。這裡我們所量測的DNA拉伸及鬆弛時間行為意味著交流電場可能施加額外的拉伸力而使DNA更加延展。

關鍵字：DNA端點釘附、拉伸、鬆弛、交流極化、具偏壓之交流電場

In this thesis we propose a simple strategy for tethering single DNA molecules onto a surface. This is realized by first spreading streptavidin-coated, positively charged quantum dots on a negatively charged glass and then using them to anchor the biotin-attached end of a DNA molecule. In addition, we further study the stretching and elastic relaxation of end-tethered DNA under the influence of DC or/and AC electric fields.
In Chapter 3, we show how to realize the tethering and seek optimal conditions for expediting single molecule studies, including the addition of non-ionic surfactants for eliminating electroosmotic flow, the increase in the solution viscosity for prolonging the relaxation time, and the control of the amount of added quantum dots for obtaining a sufficient amount of tethered DNA molecules without being entangled to each other.
In Chapter 4, we stretch end-tethered DNA under varying strengths of DC electric fields to determine the characteristic force-extension curve. We also measure the persistence length l_p of DNA by fitting the data using both inextendable and extendable wormlike chain models. We find, surprisingly, that l_p is no more than 10nm and much shorter than 50nm commonly reported in literatures, suggesting that DNA molecules in our setup are hard to extend perhaps due to the entanglement of DNA blobs (but not the backbone of DNA) to the surface surfactants. This entanglement effect is supported by the existence of two relaxation times and the observation that the second one is longer especially when the extent of stretch is smaller.
In Chapter 5, we examine the relaxation behavior of DNA under high frequency AC electric fields. At a given AC frequency, we observe that the first relaxation time first decreases and then increases as the field strength increases. This trend is found to be similar regardless of the extent of stretch. This non-monotonic change in the relaxation time could be attributed to the length-dependent polarization mechanism, involving the competition between dielectric polarization for large stretch and conductive polarization for small stretch.
In the last part of this thesis, we apply an AC biased electric field having a non-zero mean to stretch DNA. Compared to the results under DC electric fields in Chapter 4, in terms of the averaged field strength, the chain extension can be greater at low fields, whereas it becomes shorter at high fields. For the low-field case, we also measure the (first) relaxation time of DNA in a zero-mean AC electric field by turning off the bias, and find that it is indeed longer than that in Chapter 4. Both stretching and relaxation results suggest that AC electric fields might impose additional stretching forces to make DNA more extendable.

Keywords: end-tethered DNA, stretching, relaxation, AC polarization, AC bias

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