DNA分子於二元自組裝單層膜修飾之金表面的吸附行為

Abstract

為增加藥物輸送時的專一性以及控制其釋放速率，已有不少關於高分子藥物輸送系統的研究，然而此高分子輸送系統大多為共價鍵結，其分子選擇性少且不易釋放。另一方面，由於金具有良好的生物相容性，再加上其表面性質可由自組裝單層膜系統修飾，因此對其表面性質的調控有更大的彈性，且已有不少文獻報導利用奈米金作為載體，將生物分子送入細胞中。由本實驗室之前的研究可知，可由混合不同比例自組裝分子，於金表面沈積具有二元混合官能基的自組裝單層膜(SAM)，並以不同之表面官能基比例，調控其等電位點，希望藉由此一系列比例的微調，使SAM表面於特定pH值變化時，可藉由其表面電性的改變而吸脫附藥物分子，將奈米粒子作為酸鹼感測的藥物載體。本研究以混合不同比例之HS(CH2)8NH2 和 HS(CH2)15COOH 分子的方式，於金基材沈積二元官能基自組裝單層膜，並藉由石英震盪微天平(QCM-D)偵測質體DNA在各pH值環境下，於不同官能基比例的SAM表面吸附時所造成的頻率以及能量消散變化，並以模擬所得的厚度以及密度變化討論DNA分子的吸附行為，以評估此二元官能基表面作為酸鹼感測輸送系統的可能性。實驗結果發現，不論表面官能基的比例，DNA分子於低pH環境下，因表面傾向具有正電，DNA的吸附量均較高pH值為多；此外，吸附量隨著表面易帶正電之NH2官能基的比例增加而增加，顯示表面靜電作用對強負電性的DNA分子之吸附行為有明顯影響。然而其吸附行為並不如等電位點的預期，即表面具有負電位時亦能吸附帶負電之DNA分子，故DNA分子於SAM表面吸附時，須考慮DNA分子尺寸、分子極性以及官能基於環境溶液中實際帶電情形，且於特定pH值環境下，強負電性的DNA分子可產生偶極誘導偶極力吸附於未解離之官能基上。此SAM表面官能基與質體DNA的交互作用力有三：1. NH2官能基可於低pH值質子化形成NH3+並與DNA分子靜電吸附；2.部份COOH官能基於低pH值環境以及NH2官能基於高pH值時，可以偶極誘導偶極力吸附DNA分子3. COOH官能基於高pH值時解離為COO-並呈強負電性表面，並與DNA分子靜電排斥。因此於不同pH值環境下對於DNA分子量吸附的選擇性隨著COOH官能基比例的增加而有明顯差異，且由於其吸附機制的差異以及吸附層密度的影響，由偶極誘導偶極為主的厚度吸附速率，大於以靜電吸附機制為主的厚度吸附速率，但所吸附之DNA具有較鬆散的結構。

Biodegradable polymers have been widely applied in the drug delivery, which has drug-targeting specificity and can control the release rate. Most of these drug molecules are covalently bonded with the polymers and cannot release easily. On the other hand, gold is known to have good biocompatibility and the surface properties can be modified with the use of self-assembled monolayer (SAM) hence it is a promising carrier for drug delivery. In previous work, surface properties of gold surface were tailored with homogeneously-mixed amine and carboxylic acid functional groups and Au nanoparticles with a series of iso-electronic points (IEP) were prepared. In other words, the sign of surface potential flips at a tunable pH. Considering electrostatic interactions, these engineered nanoparticle could switch between adsorbing or desorbing molecules in specific pH range and serve as a pH sensitive drug delivery system. In order to understand how macromolecules interact with these gold substrates modified with different ratio of HS(CH2)8NH2 and HS(CH2)15COOH, Quartz crystal microbalance with dissipation detection (QCM-D) is used. Simultaneously collected frequency and dissipation shift is examined using visco-elastic model to examine the adsorption behavior of plasmid DNA on the binary functional groups modified gold surfaces. The experimental results revealed that the plasmid DNA can adsorbed on the SAM surfaces electrostatically, and the amount of adsorbed plasmid DNA decreased with environmental pH and increased with the ratio of amine functional group on the surfaces. Nevertheless, unlike the case of depositing inorganic materials, it is found that the adsorption behavior of plasma DNA cannot be simply discussed via IEP of the surfaces because dipole-induced dipole interaction between uncharged functional groups on SAM and negatively charged DNA backbone. It is also found that the apparent absorption rate (in terms of thickness) is lower when electrostatic attraction dominated because the deposited DNA film is more dense and rigid.