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

Abstract

Investigations of single molecule diffusion and entrapment within sol-gel-derived silica thin film gradients are reported for the first time. Gradient films were prepared on silica-sublayer-coated substrates by infusionwithdrawal 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-CH 3 absorption at 1275 cm -1 relative to Si-O-Si absorption at 1080 cm -1 . 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 MTMOSrich "domains". Plots of mean square displacement in time provide no evidence for confined diffusion, suggesting the molecules move freely between these domains.

Controlled structure and hydrophilic property of polymethylhydrosiloxane thin films attached on silicon support and modified with phosphorylcholine group

“…3b and was about 3 times higher than in the bulk (z) as measured by IR absorption (Table 2), showing the preferential reaction of PL at the uppermost layer by the casting method. This method may be useful to those developing thin film gradients for application in material science [79] and surface of biocompatible polymers [80].…”

Amine-phenyl multi-component gradient stationary phases

“…We have also fabricated two-component gradients using controlled rate infusion (CRI) [23][24][25] and infusion-withdrawal dip coating (IWDC) [26][27][28]. These two methods involve a variation of the sol-gel process [29][30][31].…”

“…Gradient formation is governed by the rate of infusion, the rate of reaction, and the concentration of the silane. In IWDC, the siloxane-coated substrate is suspended in a sol prepared from tetramethoxysilane and synchronized syringe pumps are used to infuse an organoalkoxysilane into the deposition reservoir, while the mixed sol (and/or the slide) is withdrawn to produce a film [26][27][28]. In both of these approaches, one component is unreacted Si OH and/or Si(OR) x groups due to incomplete condensation and the other is the pre-determined organic modifier (e.g., CH 3 , NH 2 , C 6 H 5 , etc.).…”

Vapor-Phase Plotting of Organosilane Chemical Gradients

“…They have been used previously in combinatorial materials chemistry, high-throughput investigations of molecule–surface interactions, catalysis, chemical separations, , molecular combing of DNA, and for driving the motions of liquid droplets, vesicles, nanoparticles, macromolecules, and cells . Gradient fabrication has been accomplished by a variety of methods, including vapor diffusion, , solution diffusion, ,, microfluidic mixing, inkjet printing, photolithography, contact printing, electrochemical methods, dip coating, − and controlled-rate infusion methods. , Deposition methods that rely upon precursor diffusion alone do not afford any control over the gradient profile or steepness, which is determined by Fick’s laws. Furthermore, few such methods allow for patterning of the film.…”

“…Although photolithography and contact printing afford control over the gradient profile and can be used to make patterned films, changing the gradient characteristics or film pattern requires the fabrication of a new mask or stamp. Although both dip-coating − and controlled-rate infusion , methods allow the gradient profile to be manipulated, neither allows for the deposition of patterned films. Inkjet printing is a direct-write method that provides the flexibility required to produce gradients of arbitrary shape, location, and profile, but it requires careful optimization of the ink properties (e.g., surface tension, viscosity, and solvent volatility) to achieve proper jetting. , Furthermore, the printing of reactive materials by inkjet methods frequently leads to clogging of the print nozzles.…”