Deposition of Avidin Protein on Silicon Oxide Substrate

Chemomechanical functionalization of chemical surfaces is a field of chemistry and physics which has attracted a great deal of attention over the last decade. The atomic force microscope has played an instrumental role in the advancement of nanotechnology using such methods. In my research we investigated the effectiveness of depositing a stable Avidin protein monolayer on a silicon oxide surface. This was accomplished through a multi-step process involving surface chemistry. The atomic force microscope and ellipsometer were used in measuring the thickness and verifying the presence of each layer formed during this multi-step process. Introduction/Background: Nanotechnology, including the development of nanostructures and monolayers on the nanoscale, is at the forefront of science and research. Surface modification and surface chemistry have come to play a major role in this research. One of the first methods developed to manipulate and/or chemomechanically functionalize surfaces was microcontact printing. A well-known group making significant contributions to the field of surface chemistry and modification is the Whitesides Group. They have created selfassembled monolayers on organic surfaces from alkanethiols. The group accomplished this through the implementation of a PDMS stamp with Au or another noble metal. 1 Microcontact printing has proved to serve as a useful tool in surface modification. This method has been used in growing patterned polymers on silicon oxide surfaces. 2 As technology improved nanostructures were able to be developed on surfaces. Among the methods used to accomplish this is dip-pen nanolithography (DPN.) Chad Mirken and his research group 4 are well known for their DPN methods. They have successfully created nanosize structures of specific shapes through “wet-chemical etching.” DPN is used to create layers of Au, Ag, and Pd. Through a “direct-write method” the Mirken Group has been able to use various inks to create single and multicomponent nanostructures. This is a significant improvement from previous surface modification technology. Also found within the same realm of nanotechnology is nanografting. This method of surface modification is able to produce nanostructures of specific size and geometry. This method of nanofabrication demonstrates an improvement in the control of shape, size, and location of these structures on the surface. At the forefront of nanografting is G. Liu of UC Davis and her research group, who have successfully produced nanostructures of single-stranded DNA through the use of nanografting. 3 This method requires the use of the atomic force microscope to scribe or shave off of a layer of surface molecules. This is conducted in the presence of a fluid which allows the chemicals to react and produce a new self-assembled monolayer where the scribing was performed. The nanostructures developed by this technique have been found to be stable and allow researchers to study the conductivity of these structures based on their size. These nanografting experiments consist of imaging under a lower force to obtain an initial image of the surface. Following the initial imaging of the surface the tip is pushed down with a greater force on the surface in the presence of the reacting fluid or molecules, which is known as the scribing step of the process. The molecules in the reaction mixture almost simultaneously attach to the surface where the scanning tip has just scratched, thus creating the self-assembled monolayer. Another effective method to manipulate surfaces on the nanoscale is chemomechanical surface patterning and functionalization, as demonstrated by Brent Wacaser. 5 His techniques use the atomic force microscope to scratch the surface of a hydrogen-terminated silicon wafer, which breaks the Si-H and Si-Si bonds. This technique enables the surface to react with various molecules such as alkenes, alkynes, alkyl halides, alcohols. Wacasser attaches an AFM tip to a fluid cell and immerses it in a liquid containing the reactive molecules. The fluid is placed on the silicon sample wafer and the tip is lowered down into the fluid and prepared to scribe. Smaller, more durable silicon-nitride coated tips are used in these experiments, making it easier to control the size of the modifications on the surface and reduce the damage to the surface. The same tip is used throughout a single experiment to image, scribe, functionalize the surface, and then image again to see the results. Brent Wacasser’s research represents several improvements in the ability to fabricate new surface molecules on the nanoscale with greater accuracy and less damage to surface. Playing an essential and monumental role in the advancement of nanotechnology and development of nanostructures is the atomic force microscope (AFM.) The AFM is capable of producing images on the nanoscale, 10-9, as it is not an optical microscope. It functions by dragging a tip or probe across a surface. The part of the microscope to which the tip is attached is called the cantilever. As the cantilever and tip are being dragged along the surface a laser is deflected off the top of the cantilever into a photodiode detection system (See Figure 1). Through the use of advanced computer software the motion of the tip and cantilever are able to produce a three-dimensional image on the computer. This technology provides an avenue for vast improvements and developments in the nano-world. The two most commonly used modes on the AFM are contact and tapping. Contact mode is when the tip is dragged along the surface while always maintaining contact with the surface, while tapping mode is when the tip “taps” the surface at a certain frequency in order to obtain the desired image. On a normal scan using the atomic force microscope in contact mode, objects can be imaged or measured to widths as small as 5 nm across, or even smaller.