利用微悬臂梁表面应力研究聚N-异丙基丙烯酰胺分子的构象转变

Investigation of conformation transition of poly(N-isopropylacrylamide) by surface stress detection using micro-cantilever

提出了一种基于微悬臂梁传感技术研究大分子折叠构象转变的新方法.通过分子自组装的方法将热敏性的聚N异丙基丙烯酰胺(PNIPAM)分子链修饰到微悬臂梁的单侧表面,用光杠杆技术检测温度在20—40℃之间变化时由于微悬臂梁上的PNIPAM分子在水中的构象转变所引起的微悬臂梁变形.实验结果显示:在升温过程中,微悬臂梁的表面应力发生了变化并且导致微悬臂梁产生了弯曲变形,这个过程对应着微悬臂梁上的PNIPAM分子从无规线团构象到塌缩小球构象的构象转变.在降温过程中,微悬臂梁发生了反方向的弯曲变形,这对应着PNIPAM分子从塌缩小球构象向无规线团构象的构象转变.整个温度变化过程中构象转变是连续进行的,而在低临界溶解温度(约32℃)附近转变幅度较大,这与自由水溶液中PNIPAM分子的无规线团塌缩小球构象转变相对应.实验结果还显示:由于PNIPAM分子在塌缩过程中氢键的形成和链段间可能的缠结效应,整个温度循环过程中微悬臂梁的变形是不可逆的且有明显的迟滞效应.

A new method based on micro-cantilever sensors was presented and used to investigate conformation transition of macromolecules. Poly（N-isopropylacrylamide） （PNIPAM）were grafted onto one surface of a micro-cantilever by self assemble monolayer method. Then the micro-cantilever was immersed into distilled water in which the temperature can be adjusted in the range of 20--40 ~C. The deflection of the micro-cantilever induced by conformation transition of PNIPAM chains was measured using optical lever technique. The results show that the micro-cantilever deflects upon heating and opposite deflection occurs upon cooling, which indicates that the surface stress of the micro-cantilever changes when the conformation.of PNIPAM changes upon heating and cooling. The surface stress changes continuously over the range of 20-40℃. However, a sharp change appears around the low critical solution temperature （ - 32℃ ）, at which a coil-globule transition occurs in free aqueous solution of PNIPAM. The whole process is irreversible and shows a clear hysteresis, which can be attributed to hydrogen bonding and the nossible chain entanalement formed during the collanse nrocess.