Silica or silicon dioxide (SiO2) is the most important constituent in the making of many commercial glasses. It is the structural basis of all traditional glass and is continually proving to be invaluable in the development of high-tech glasses. Single-component silica glass is one of the simplest glass systems in terms of composition, but some of its properties are far from simple. Vitreous silica behaves anomalously in many areas such as its volume-temperature relationship and its negative thermal expansion below room temperature. These properties lead one to consider the structure of the silica network.
The structure of vitreous silica may be one of the most highly debated issues in glass science of the past century. Scientists have gone from believing that silica was made of tiny crystallites, to proving that it definitely has no crystallites in a matter of a few years. The current knowledge is the generally accepted theory that glass consists of slightly distorted SiO4
tetrahedra joined at the corners. Practically all of the oxygen atoms in silica are bridging to two silicon atoms. The angle between two neighboring tetrahedra (Si-O-Si) is the bond angle. This bond angle along with the torsion or rotation angles allow the tetrahedra to significantly alter their arrangement from a crystal such that no long-range order exists. Given the silica structure of the bulk, the surface structure is very different owing to the fact that the bonds are altered by the discontinuity. It is generally accepted that silica has various hydroxyl molecules on the surface. There are basically three forms of hydroxyls or silanols; geminal, vicinal and single. A single silanol is a silicon atom on the surface bound to a single OH molecule, a geminal silanol consists of two OH molecules bound to a single silicon atom on the surface, and a vicinal silanol has two OH molecules each bound to a
silicon atom on the surface with hydrogen bonding between the OH molecules.
The properties of many glasses depend on the condition of the surface.
Clearly mechanical properties will depend on surface defects such as scratches or pits with fewer defects leading to stronger glasses. Several optical properties of glasses are highly dependent on the roughness of the surface, a smoother surface will reflect light more specularly. These properties are not intuitively affected by surface chemistry though. However, scratches on a glass surface will have unsatisfied bonds and these bonds will tend to react with water. This is why a scored glass piece breaks easier after having been wiped with water. On the chemical scale, glass properties are still affected by the surface. Liquid chromatography is a method by which organic molecules are separated in a column that usually contains silica gel.
The chemical nature of the surface of the silica greatly affects the success of the separation in terms of time and resolution.
It is applications such as these that require scientists to understand the surface chemistry of silica. There are many instruments that can provide surface sensitive data, but these instruments often require silica forms that are not always utilized in application. Infrared spectroscopy often requires the powder form in order for the signal to be appreciable. The same can be said for BET nitrogen adsorption analysis or X-ray photoelectron spectroscopy. Bulk samples are usually required for glancing incidence X-ray techniques and microscopy techniques such as scanning electron microscopy and atomic force microscopy. Herein lies the problem though, how does one characterize the surface of a fiber or the inside of a capillary tube without altering the fundamental form of the glass and therefore the surface chemical structure?
The current work attempts to deal with this dilemma by utilizing small biological molecules to characterize surfaces that are otherwise difficult or impossible to characterize by conventional means. In order to accomplish
this goal, the work must begin with common surfaces and common characterization techniques to create a correlation between surface chemistries and observed reactions. The previous work that led to this study, suggested that Chinese hamster ovary cells can distinguish between glass surfaces where other characterization techniques cannot. It is known that proteins form the interface between glass and proliferating cells and so proteins were chosen as the surface probe in the current study. The hypothesis of this work is that proteins can be used to characterize glass surface. More specifically, that certain groups on a protein interact with the surface silanols of the silica. In this work, the following terms and their definitions will be employed. Silica is any material comprised essentially of SiO2. Vitreous silica is amorphous silica or silica glass. Crystalline silica refers to cristobalite, tridymite or quartz. Quartz is the low temperature crystalline form of silica. Quartz glass or fused quartz indicates vitreous silica formed by the melting of quartz.
The main proteins investigated are streptavidin and mouse immunoglobulin G (IgG). These proteins were selected because of their asymmetry and binding orientation sensitivity. Upon binding to a glass surface, these proteins may or may not present a binding site for a secondary protein, biotin and anti-mouse IgG, respectively. Unfortunately, this work did not achieve the stage whereupon the secondary protein binding capabilities were examined.
Silicon dioxide (SiO2) was chosen as the composition to be studied and it was obtained in various forms. These forms include quartz single crystals, slides, cane, fiber, fumed Cab-o-Sil® and micron-sized spheres made via the Stöber process. The quartz single crystals were investigated because of their regular atomic arrangement. It was hypothesized that by using the known crystal structure of a given plane, one could correlate any repeated interaction of proteins to atomic groupings on the quartz surface. These specific interactions could then be correlated to the non-periodic structure of
amorphous silica and in so doing, determine the high affinity sites of proteins for silica.
The amorphous silica forms were exposed to various surface treatments, including, ethanol cleaning, HF acid etching, water plasma treatments, and 1000°C heat treatments. These surface treatments were intended to alter the surface chemistry of the silica by changing the type and population of surface silanols. The surfaces were characterized using a number of techniques, which included atomic force microscopy and chemical force microscopy. The proteins were then introduced to the various samples via incubation adsorption for 30 minutes. The adsorbed protein was analyzed either directly or indirectly using fluorescence spectroscopy, bicinchoninic acid protein assay, sodium dodecylsulfide polyacrylamide gel electrophoresis, atomic force microscopy and chemical force microscopy. The results were collected, analyzed, and conclusions were drawn as to the efficacy of utilizing proteins to characterize the surface of silica. This thesis deals with the interactions of proteins with the silica surface as a function of form and surface treatments.