Spatiotemporal Evolution of Fixed and Mobile Dopant Populations in Silica Thin-Film Gradients as Revealed by Single Molecule Tracking

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• 作者：Chenchen Cui ; Alec Kirkeminde ; Balamurali Kannan ; Maryanne M. Collinson ; Daniel A. Higgins
• 刊名：Journal of Physical Chemistry C
• 出版年：2011
• 出版时间：January 27, 2011
• 年：2011
• 卷：115
• 期：3
• 页码：728-735
• 全文大小：268K
• 年卷期：v.115,no.3(January 27, 2011)
• ISSN：1932-7455

文摘

Investigations of single molecule diffusion and entrapment within sol鈭抔el-derived silica thin film gradients are reported for the first time. Gradient films were prepared on silica-sublayer-coated substrates by infusion-withdrawal dip-coating [Chem. Mater. 2010, 22, 2970]. This method employs a sol of time- varying composition obtained by slowly mixing two different sols in the deposition reservoir. Tetramethoxysilane (TMOS) and methyltrimethoxysilane (MTMOS) were employed as precursor silanes. Films exhibiting macroscopic, unidirectional gradients in methyl content were obtained. Fourier transform infrared (FTIR) microscopy data provide proof of gradient formation, depicting an increase along the gradient in Si-CH₃ absorption at 1275 cm^鈭? relative to Si鈭扥鈭扴i absorption at 1080 cm^鈭?. Widefield fluorescence videos recorded as a function of position for Nile Red-doped films depict spatially varying populations of immobile and mobile molecules. The immobile fraction is believed to be incorporated in the silica sublayer, while the mobile molecules reside in/on the gradient. Diffusion coefficients, D, measured from single molecule tracking data depict, on average, a gradual increase in molecular mobility between the TMOS and MTMOS ends of the gradient. The mobile fraction is observed to split into two separate populations toward the MTMOS end of the gradient. Simultaneously, the width of the D distribution for the most mobile population is found to increase. Comparison with simulated results suggests that the TMOS end of the film is relatively homogeneous, while the MTMOS end exhibits increased heterogeneity. It is concluded the MTMOS end may incorporate TMOS- and MTMOS-rich 鈥渄omains鈥? Plots of mean square displacement in time provide no evidence for confined diffusion, suggesting the molecules move freely between these domains.