生物粒子在熵障礙管柱中的泳動現象

Electrophoretic Separation of Bioparticles through Entropic Barriers

鄭光鈴(Kuang-Ling Cheng)

指導教授 ： 諶玉真

國立臺灣大學/工學院/化學工程學研究所/博士(2009年)

https://doi.org/10.6342/NTU.2009.01949

摘要

摘要

本文利用布朗動力學模擬法來模擬球型生物粒子在熵障礙中的泳動機制。文中探討了尺寸不同但zeta電位相同的球型粒子，在以深區域與窄通道所形成的熵障礙分離管柱中電泳的現象。我們發現粒子的泳動率和電場強度及粒子尺寸有關。在弱電場區域中，粒子以布朗運動為主，出口效應決定了粒子從熵障礙中逃脫的速度，因此，泳動率隨著粒子直徑增加而降低。在強電場區域中，粒子傾向沿著電流線移動，出口效應影響小，但小粒子因為布朗運動顯著而易偏移流線，降低了平均速度，因此，泳動率隨著粒子直徑的增加而增大。在介於其中之中間電場區域中，粒子泳動率趨於一致，使得電泳分離解析度變差。
文中亦討論在布朗運動逃脫和作用力驅使之情形下，球型粒子從圓球體相連而成的熵障礙中逃脫的機制。在採用布朗運動模擬法，並以因次分析法分析結果，我們發現，在沒有外力作用下，第一逃脫時間 是粒子直徑 、球體半徑 、孔洞半徑 的函數，粒子逃脫速度正比於出口效應。在作用力驅使下，泳動率 是粒子直徑 、球體半徑 、孔洞半徑 及電場強度 的函數。在弱電場區域中，布朗運動佔優勢，出口效應明顯，因此，泳動率隨著粒子直徑減少而增加。泳動率隨電場強度而非線性的增加，會造成粒子解析度下降。
文中亦利用球形粒子的分離機制來解釋高分子在熵障礙中泳動之結果，其中泳動率和電場強度與高分子長度有關。當高分子之特徵尺寸小於孔洞的半徑，其電泳機制與球形粒子一致。由於高分子的擴散係數反比於其長度，即 ，在弱電場區域中，高分子受布朗運動影響大，出口效應顯著，因此，泳動率隨長度增加而減小；在強電場區域中，由於出口效應影響小，因此，隨泳動率隨長度增加而變大。然而，當高分子之特徵尺寸大於孔洞的半徑，高分子在熵障礙中的受困時間，與高分子的停留於出口處的時間與變形時間有關，其中 是決定高分子逃脫快慢的主要因素，因為長度越長的高分子擴散係數小，容易停留在出口處而形成灘頭堡，逃離熵障礙，造成泳動率隨長度增加而增大。我們模擬的結果，印證了實驗的觀察，也對熵障礙管柱的設計提供了參考方向。

關鍵字

電泳 ； 熵障礙 ； 布朗動力學模擬法 ； 帶電粒子 ； 布朗運動

並列摘要

Abstract

The size separation of Brownian particles with the same -potential in an electrophoretic microchannel with alternating thick regions and narrow constrictions is studied theoretically. The electrophoretic mobility is field-dependent and generally increases with field strength. In weak fields, Brownian diffusion dominates and the migration is controlled by the entrance effect. Therefore, smaller particles migrate faster than larger ones. The separation resolution worsens as the field is increased. The results of simulation indicates that the greater field and smaller thick region depth raise separation speed while the smaller period length of the nanofilter enhances the separation resolution. In strong fields, however, the particle tends to follow electric field lines. Smaller particles are susceptible to Brownian motion and thus influenced by the nonuniform electric field in the well significantly. As a result, larger particles possess higher mobility.
Brownian escape from a spherical cavity through small holes and force-driven transport through periodic spherical cavity have also been investigated by Brownian dynamic simulations and scaling analysis. The mean first passage time and force-driven mobility are obtained as a function of particle diameter , hole radius ， cavity radius , and external field strength. In the absence of external field, the escape rate is proportional to the exit effect. In weak fields, Brownian diffusion is still dominant and the migration is controlled by the exit effect. Therefore, smaller particles migrate faster than larger ones. In this limit the relation between Brownian escape and force-driven transport can be established by the generalized Einstein-Smoluchowski relation. As the field strength is strong enough, the mobility becomes field-dependent and grows with increasing field strength. As a result, the size selectivity diminishes.
The separation of bead-spring polymers with different lengths but in an electrophoretic microchannel with alternating thick regions and narrow constrictions is studied by Brownian dynamics simulations. The result is similar to spherical particles. For Rouse polymers, Brownian diffusion is dominant and the migration is controlled by the exit effect under weak fields. As a result, shorter polymers migrate faster than longer ones. However, the situation is reversed under strong fields. The polymers are likely to follow electric field lines. Shorter polymers are influenced by Brownian motion. Therefore longer polymers migrate faster than shorter ones. We show that the coupling mechanism is Brownian diffusion and force-driven migration. For , polymers are entropically trapped in the thick regions and able to escape driven by an electric field. The trapping lifetime is length-dependent. In agreement with reported experimental results, longer DNA molecules have higher electrophoretic mobility. We show that the coupling mechanism between transverse diffusion and kinetic escape by deformation is responsible for this counterintuitive behavior. For each escape attempt, the probability for longer polymers to be successfully dragged through narrow constriction before the diffuse away form the entrance increases as chain length increases.
Our simulation results agree with the experimental observations in periodically constricted microchannels. Our theoretical results provide useful guidance for design of a microchannel device based on periodic entropic barriers for efficiently separation a mixture of bioparticles based on size differences.

並列關鍵字

electrophotetic ； entropic barriers ； Brownian dynamics simulation ； bioparticles ； Brownian motion

參考文獻

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