Document Type

Dissertation

Date of Degree

Spring 2016

Degree Name

PhD (Doctor of Philosophy)

Degree In

Chemistry

First Advisor

M. Lei Geng

Abstract

Nanoporous materials have been widely used in the fields of biological and chemical sensing, chemical separation, heterogeneous catalysis and biomedicine due to their merits of high surface area-to-volume ratio, chemical and thermal stabilities, and flexible surface modification. However, as the nature of nanoporous materials, they are inherently heterogeneous in the micro- and nanoenvironments. The environmental heterogeneity plays a decisive role in determining the performance of various applications of nanoporous materials. In order to provide an in-depth understanding of the nanoporous materials, it is of great interest to investigate the environmental heterogeneity in them. Single molecule spectroscopy, combined the quantitative confocal fluorescence imaging which possesses the capability of optical sectioning, has demonstrated to be a powerful tool to approach the environmental heterogeneity inside nanoporous materials. Single molecule spectroscopy is an ultrasensitive technique for probing molecular transport and properties of individual molecules. This technique has been extensively used in the research of environmental heterogeneity in nanoporous materials since it removes the issues of ensemble averaging and directly approaches detailed information that is obscured in ensemble measurements. In order to proficiently interpret single molecule data, we developed a comprehensive methodology – single molecule counting – for characterizing molecular transport in nanoporous silica. With this methodology as a tool, the nanoenvironmental heterogeneity inside the nanopores of C18-derivatized silica particles was explored by probing single molecular diffusion inside the pores. By employing single molecule ratiometric spectroscopy and a solvatochromic fluorophore as viii reporter of local environment, the gradient in nanopolarity as well as the nanoviscosity along the C18 layer after the inclusion of solvent was uncovered. The chemical properties of solute molecules at the nanopore surface are ultimately controlled by the energetics of the solute-interface interactions. The imaging of distribution of energies would be a decisive approach to assess the fundamental heterogeneity of the interface. To this end, we investigated the ΔG distribution of C18-derivatized nanoporous silica particles with quantitative confocal imaging. The pixel-to-pixel and particle-to-particle analysis showed the existence of ΔG heterogeneity between particles as well as within individual particles. The heterogeneity in ΔG could be partially responsible for band broadening in chemical separations and significantly affect overall reaction yield when using nanoporous materials as solid support for heterogeneous catalysis.

Public Abstract

Nanoporous material is extensively used in fields of scientific research including biological sensing, chemical separation and catalytic science. It possesses several unique properties such as large surface area, flexible surface modification and the most importantly for this thesis, the inherent heterogeneity in the microenvironments. It is of great interest to explore the environmental heterogeneity since it could significantly affect the performance of various applications of nanoporous material. Single molecule spectroscopy, combined with confocal fluorescence microscopy was demonstrated to be a powerful tool to explore the heterogeneity inside nanopores of material.

Single molecule spectroscopy is an ultrasensitive technique that allows the detection of single molecules. Unlike bulk measurement of providing an averaged single number that represents the measurement results of thousands molecules, single molecule measurement reveals a certain level of fluctuation in the measured parameter since only one molecule is measured at a time and each molecule reports unique information of its local microenvironment. Therefore, tremendous information of the heterogeneity in material that is hidden in bulk measurement can be resolved. Single molecule counting was established as a comprehensive methodology for characterizing single molecular transports. A research to apply this methodology to probe heterogeneities in nanopolarity and nanoviscosity of C18-modified nanoporous silica particles was accomplished with single molecule confocal microscopy.

The ΔG heterogeneity between particles as well as within individual particles was also confirmed by quantitative confocal imaging. This heterogeneity could significantly contribute to several issues in certain applications, for example, the band broadening in chromatographic separation.

Keywords

publicabstract, Analytical chemistry, Confocal microscopy, Fluorescence spectroscopy, Single molecule spectroscopy

Pages

xviii, 238

Bibliography

226-238

Copyright

Copyright 2016 Yan Hu

Included in

Chemistry Commons

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