Fabrication of surface amine gradients by controlled-rate infusion for chromatographic applications

Surface gradients are materials that exhibit a variation in properties such as chemical composition, topography etc., in a continuous and/or discrete fashion. They are broadly classified as chemical and physical gradients depending upon the properties that gradient exhibits. Among those, chemical gradients on various surfaces have been a great interest in materials research for the last two decades. This is due to the applications of gradients in various fields such as biological sciences, separation science, etc. There have been several different approaches developed for the preparation of chemical gradients. Silane-based approaches 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 were 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. Hence, there is a need to develop additional procedures for the preparation of chemical gradients that can be adaptable to different substrates and use them to form gradients at various length-scales such as few hundred microns to tens of centimeters. Using 3-aminopropyltriethoxysilane (APTEOS) as the precursor, a simple method was developed to prepare surface amine gradients termed ‘controlled-rate infusion (CRI). The CRI method involves the infusion of an organoalkoxysilane solution into a container with a substrate mounted vertically so that time-based exposure along the substrate forms a xxiv gradient in chemical functionality. 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. The ability to manipulate the gradient profile is particularly important for applications that rely on mass transport and/or those that require spatial control of gradient properties. CRI can also be used to study the reactivity of aminoalkoxysilanes that contain mono, di and tri amino groups and also amines with various substitutions (secondary and tertiary amines). The gradient profiles in each case were different and correlated to their reactivity with surface silanols. Among those aminoalkoxysilanes, ethylenediamine and diethlenetriamine were found to be very good chelating agents to bind with metal ions. Thus, gradients prepared with diamine and triamine were used as ligand density gradients for the investigation of metal-amine complex formation using two different metal ions (Cu and Zn). The above mentioned work formed the first part of this dissertation (up to Chapter 5). The latter part concentrates on fabrication and application of surface gradients for stationary phases for chromatographic separation. For the demonstration of proof-of-principle of the application of gradient stationary phases, the CRI approach was employed to prepare continuous stationary phase gradients on HPTLC plates for planar chromatography. The SiOH groups on the activated HP-TLC plates were reacted with 3-aminopropyltriethoxysilane (APTEOS) in a time dependent fashion by using a programmable syringe pump to control the rate of APTEOS infusion into the deposition reservoir. The shape (profile) of the gradient was controlled by the rate of infusion and the gradients were visualized by utilizing a concentration-dependent color formation reaction between amine groups and ninhydrin. The advantages of such gradients in optimizing the xxv retention and separation of various components in different mixtures was illustrated using mixtures of (i) four weak acids and bases and (ii) three widely-used over-the-counter drugs. Thus, the gradient stationary phase was successfully fabricated and applied as a planar chromatographic support. The next step of this research was to prepare continuous amine gradients on silica capillaries as these would be useful in open-tubular capillary electrochromatography for separation of complex mixtures and also to study the retention behavior of various components. This was achieved by carefully infusing micro-volumes of APTEOS solution into the activated silica capillaries. To characterize these gradients, a unique set-up was developed to measure streaming potential and zeta potential along the gradient capillary. Since surface amine groups exist as positively charged groups at acidic and neutral pHs, the measured zeta potential increased from the low amine end to the high amine end of the