外电场操控单分子的偶极取向极化特性研究

Dipole orientation polarization property of single-molecule manipulated by external electric field

单分子偶极取向的有效操控对于提高单分子荧光收集效率及荧光共振能量转移等相关研究具有重要的意义.本文通过测量单分子荧光偏振特性的变化,研究了外部电场作用下极性单分子的偶极取向极化特性,实现了外电场对单分子偶极取向的有效操控.研究发现电场方向与单分子样品表面平行时,掺杂在聚甲基丙烯酸甲酯聚合物薄膜中的取向随机分布的极性单分子荧光偏振方向呈现出双峰统计分布规律,表明在溶剂挥发过程中外电场诱导极性单分子偶极取向进行了重新分布.

The dipole orientation of single-molecule plays an important role in improving the fluorescence collection efficiency and promises to have applications in super-resolution imaging, protein folding, and Forster resonance energy transfer between fluorophores. However, these applications are realized usually by precisely manipulating the orientation of the dipole moment of single molecules. Here, the dipole orientation of 1,1’-dioctadecyl-3,3,3’,3’,-tetramethylindodicarbocyanine(DiD) single molecules with the permanent dipole moment of 14.9 D is manipulated by using an external electric field of 3500 V/mm. Single DiD molecules are prepared by using mixed solvent of chloroform and dimethyl sulfoxide. The dipole orientation of single molecules is manipulated by an external electric field during the evaporation of solvent. The fluorescence of single molecules is measured by splitting the fluorescence collected by an objective into the S-polarized and P-polarized beams, and the fluorescence polarization of single molecules can be calculated by measuring the intensities of two orthogonal channels(Is and IP). The distribution of dipole orientation angle(α) for single DiD molecules in poly-(methyl methacrylate)(PMMA) film is analyzed statistically, and its changes are compared under different electric fields. It is found that the dipole orientation angle a of single DiD molecules in the PMMA film without applying electric field obeys a single-peak Gaussian distribution with the most probable value of 41°, which results from the fluorescence dichroism signal of the high numerical aperture objective. Applying a perpendicular electric field to the surface of single-molecule sample, the distribution of dipole orientation angle a of single DiD molecules can be still fitted by a single-peak Gaussian function with the most probable value of44.2°. The dipole orientation of single DiD molecules under the perpendicular electric field changes little.However, by applying a parallel electric field to the surface of single-molecule sample, the dipole orientation angle a of single DiD molecules changes prominently. It obeys a two-peak Gaussian distribution with the most probable values of ~32° and 55.5°,indicating that the orientation polarization of the dipole moment occurs to the single DiD molecules in PMMA film. The dipole orientation of single polar molecules tends to the parallel electric field in this case.