Wobbling Molecules Probe Wiggle Room in Nanochannels

Single-molecule wobbling measurements help to elucidate molecular confinement within cylindrical nanopores.

Wide-field fluorescence images (upper) acquired simultaneously in two orthogonal polarizations (double-ended arrows) depicting the 1D diffusional motions of rod-shaped single molecules confined within surfactant-templated mesoporous silica (bottom scheme).
Image courtesy of Higgins et al. © 2013 American Chemical Society.
Wide-field fluorescence images (upper) acquired simultaneously in two orthogonal polarizations (double-ended arrows) depicting the 1D diffusional motions of rod-shaped single molecules confined within surfactant-templated mesoporous silica (bottom scheme). The images depict the maximum signal level at each pixel in the video data.

The Science

Researchers have developed a new method to interrogate the space accessible to molecules in one-dimensional (1D) nanopores. The method uses video microscopy to quantify the wobbling motions of single dye molecules diffusing through the pores.

The Impact

Measurements of single molecule wobbling will afford a better understanding of the molecular-level origins behind the unique selectivities of nanopore-based materials for chemical separations and catalysis. The results will aid in the engineering of nanoporous materials optimized for use in applications relevant to energy science, including membrane separators for fuel cells and batteries and nanoporous supports for catalysis.

Summary

Molecular confinement within nanoporous media plays an essential role in the stereoselectivity of certain catalytic reactions and in the size-based selectivity of chemical separations using zeolites and molecularly-imprinted polymers. BES-funded researchers have established a new approach to investigate the confinement and motion of single molecules diffusing within cylindrical silica nanopores. The simultaneous single-molecule tracking and emission polarization measurements reveal strongly polarized fluorescence from rod-shaped dye molecules exhibiting 1D diffusion within solvent- and surfactant-filled silica nanopores. Analysis indicates that the molecules are, on average, oriented with their long axes parallel to the long axis of the nanopores. Variations in the fluorescence emission polarization are attributed to orientational motions of the molecules within the nanopore and permit the wobbling angle to be calculated. The wobbling angle provides a measure of the ‘wiggle room’ accessible to each molecule. Initial results indicate that the wiggle room is much smaller (ca. 1 nm) than the silica pore diameter (ca. 4 nm).  This difference is attributed to confinement of the molecules by interactions with the liquid in the pore. These investigations will help clarify the molecular-level origins of the unique selectivity afforded by nanoporous materials, and thus will aid in the design of better materials for separations and catalysis.

Contact

Daniel A. Higgins
Kansas State University
[email protected]

Takashi Ito
Kansas State University
[email protected]

Funding

DOE Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division

Publications

Rajib Pramanik, Takashi Ito, and Daniel A. Higgins “Single Molecule Wobbling in Cylindrical Mesopores”, J. Phys. Chem. C 2013, 117 (7), 3668-3673. [DOI: 10.1021/jp400479w]

Rajib Pramanik, Takashi Ito, and Daniel A. Higgins “Molecular Length Dependence of Single Molecule Wobbling within Surfactant and Solvent Filled Silica Mesopores”, J. Phys. Chem. C 2013, 117 (29), 15438–15446. [DOI: 10.1021/jp404991m]

Daniel A. Higgins, Khanh-Hoa Tran-Ba, and Takashi Ito “Following Single Molecules to a Better Understanding of Self-Assembled One-Dimensional Nanostructures” (Perspective), J. Phys. Chem. Lett. 2013, 4 (18), 3095-3103. [DOI: 10.1021/jz401215r]

Highlight Categories

Program: BES , CSGB

Performer: University