Surface chemical gradients are materials that exhibit continuous, gradually varying chemical or physical properties along and across the length of a substrate. As a result, each point on the gradient surface can represent an individual sample. They are broadly classified as chemical and physical gradients depending upon the properties that the gradient exhibits. A physical gradient involves a continuous variation of physical properties such as surface roughness and film porosity on the micrometer scale. Chemical gradients involve a gradual variation of chemical properties such as polarity, acidity and basicity, etc. Such gradients have found various applications in cell adhesion, nanoparticle absorption, etc. Because of the multitude of potential applications of acid-base gradient materials in separation science and biological applications, the main work of this dissertation work is focused on the preparation and fundamental, molecular level investigation of acid-base gradients on siloxane surfaces. In this work, we focused on the preparation and characterization of surface charge gradients. Charged gradients are gradients that contain charged functional groups that are spatially distributed along the length of the substrate. They can interact with each other or with other species in solution by electrostatic interactions. They can also play a key role in governing the interaction of macromolecules and bacteria on surfaces, the wetting of surfaces, the layer-by-layer (LBL) assembly of thin films, reactions in catalysis, and the separation of charged species in chromatography. Therefore, understanding localized interactions between surface functional groups and charged species in solution are particularly relevant to the development of surfaces resistant to biofouling, antimicrobial surfaces, catalytic surfaces, multi-layered composite thin films, and imprinted surfaces for chemical sensing and separations. Thus, it is of great of interest to develop methodologies to create and study heterogeneous and homogeneous charged surfaces with well-defined properties. There have been several different methods developed for the preparation of charged gradients. First a chemical gradient is prepared and then the chemical gradient is converted to charged gradient by a chemical approach. Silane-based methods for the preparation of chemical gradients are among those that are widely used because of the straightforwardness of the chemistry involved and also the availability of silanes with various chemical functionalities. A few of these silane based approaches such as the vapor-diffusion method and liquid diffusion method have been used for various applications so far. Most of these methods are only able to prepare surface chemical gradients for a specific application mainly because of their limitations in terms of gradient-length scale and chemistry involved. In this work, we used a procedure already developed in our lab to prepare chemical gradients from different functionalized alkoxysilanes; we call this procedure the ‘controlled-rate infusion method (CRI)’. This method can be adapted to different substrates and can form gradients at various length-scales, such as few hundred microns to tens of centimeters. The CRI method involves the infusion of an organoalkoxysilane solution into a container with a substrate mounted vertically so that time-dependent exposure along the substrate forms a gradient in chemical functionality from bottom to the top. The most important attribute of this method is that the local steepness of the gradient can be systematically controlled by simply changing the rate of infusion. The steepness of the gradient can also be changed at predefined positions along its length by programming the rate of infusion. CRI can also be used to prepare gradients containing multiple functionalities, termed multicomponent chemical gradients. The different chemical functionalities can be oriented in different directions to produce gradients where functionalities can be oriented along the same or opposed directions producing aligned and opposed multicomponent chemical gradients, respectively. In this work, the multicomponent gradients were converted to charge gradients via chemical reaction with 30% H2O2. Using controlled rate infusion and this technique, aligned or opposed multicomponent charge gradients containing NH3+, SO3- and SiO- groups were prepared. By infusing 3-aminopropyltriethoxysilane (APTEOS) and 3-mercaptopropyltriethoxysilane (MPTMOS) in the same or opposed direction, gradients containing charged species in different locations relative to each other along the length of the substrate were made. The gradient properties in each case were different and correlated to the way they were prepared i.e., where the gradients were oriented in an aligned or opposed fashion. Surface wettability and local surface charge, etc were found to be entirely different depending on the type of charge gradients (aligned and opposed). In another example, SiO- and NH3+ opposed gradients were prepared by infusing APTEOS on different base layers prepared from tetramethoxysilane (TMOS), phenyltrimethoxysilane (PTMOS), dimethyldimethoxysilane (DMDMOS) or octyltrimethoxysilane (OTMOS) followed by protonation of the surface amines. The gradient profiles and surface wettability were found to be independent of each other and dependent of the type of the base layer. In summary, this dissertation work focuses mainly on the preparation of multicomponent charge gradients and their molecular level characterization by a multitude of different analytical methods including XPS spectroscopy, tapping mode atomic force microscopy (TM-AFM), zeta potential measurement, and SCA and DCA measurements. CRI has incredible flexibility and adaptability, which was confirmed by extending it to different siloxane base films and creating gradients with different functionalities. Multicomponent charge gradients containing acid and base functionalities can be prepared and optimized for and acid base catalysis reactions such Michael addition as well as aldol, Henry, and Knoevenagel condensations.
| Identifer | oai:union.ndltd.org:vcu.edu/oai:scholarscompass.vcu.edu:etd-5948 |
| Date | 01 January 2017 |
| Creators | Ashraf, Kayesh |
| Publisher | VCU Scholars Compass |
| Source Sets | Virginia Commonwealth University |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | Theses and Dissertations |
| Rights | © The Author |
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