The chemical properties of surfaces are critical to a broad spectrum of products and technologies that include water-repellant fabrics, anti-reflective lenses, packaging, and biomedical implants and prostheses. These properties often define the function, and hence the application of a given material.
As the size of objects is reduced, particularly into the nanometer regime, the effects of surface properties become even more pronounced. These effects are due primarily to the large surface to volume ratio that occurs at small length scales. Surface properties of particular importance in this size regime include surface charge, surface wetting, interfacial energy, and the ability/resistance of materials to adsorb on the surface. The ability to control these properties is essential to a variety of applications of micro- and nanostructures. With the array of tools at our fingertips, Nano-Terra can modify, functionalize, and manipulate surfaces in order to tailor them for specific applications. Below are several examples of our capabilities in this area:
Surface Charge Modification
One example of modifying surfaces is the manipulation of the charge on surfaces. This characteristic is particularly important, for example, in microfluidic applications in which electric fields are used to pump fluids by electroosmotic flow. The charge on the surface of many materials can be altered by plasma-treatment: exposure of the surface to a glow discharge of a gas (e.g., O2). As a result, charged functional groups are produced on the surface. Surface charge can also be modified chemically by coating surfaces with silanizing agent or other charged species.
Modification of the Wetting Properties of Surfaces
The wettability of a surface is characterized by the contact angle that a drop of fluid makes with the surface. A contact angle of 0 degrees represents the complete wetting of a surface, while a contact angle of 180 degrees represents the complete repulsion of the liquid by the surface. Water has a relatively small contact angle on hydrophilic surfaces such as glass (between 5 and 25 degrees), and a larger contact angle on hydrophobic surfaces such as silicon (approximately 70 degrees) and poly(dimethylsiloxane) (PDMS, 109 degrees). It is often desirable or necessary to alter the wettability of materials to make them more/less hydrophilic or more/less hydrophobic. For example, the so-called 'Lotus Effect' in material science is the self-cleaning property observed in the leaves of lotus plants. Their microscopic structure and surface chemistry make it possible for droplets of water to roll off the surface of a leaf, removing particles, mud, and small insects. A droplet of water on a lotus leaf has a contact angle greater than 150 degrees, which is defined as 'superhydrophobic'. There are several applications of the Lotus Effect to industrial products and processes-including paints, windows, roof tiles, fabrics, and other surfaces.
Surface Patterning using Self-Assembled Monolayers (SAMs)
Surface science in the 1970s and early 80s was focused on studying the surface chemistry of metals and metal oxides. This research produced an enormous amount of information about the properties of surfaces, but only under high vacuum. The results of these studies were of limited applicability to metals at atmospheric pressure and were not applicable to organic materials. Many surface scientists were interested in developing a model system for studying the chemistry of organic surfaces. The difficulty with organic surfaces, however, is that they are usually highly disordered, and therefore difficult to study using the techniques of physical chemistry and surface science. Ralph Nuzzo and Dave Allara revolutionized this area by demonstrating that Self-Assembled Monolayers, or SAMs, of alkanethiolates on gold were ordered. SAMs became the model system for studying the properties of organic surfaces because they displayed characteristics that made it possible to study them using physical techniques.
SAMs form spontaneously on surfaces when monomers with the structure, Y(CH2)nX, bind to a surface via the appropriate anchoring group Y. The head group, X, determines the surface properties of the SAM, and the anchoring group, Y, determines the surface required for forming the monolayer. One of the best-studied SAMs is alkanethiolates (where Y=SH) on surface coated with gold or silver. By changing the head group, X, the SAMs exhibit different properties, including wetting, chemical reactivity, the adsorption of molecules, proteins, and cells, and so on. By controlling the head group, surface can be functionalized with specific SAMs and their surface properties can be tailored. This approach to controlling the characteristics of surfaces is particularly powerful because it can be used to create surfaces with regions of different properties by patterning multiple SAMs.
The most versatile approach to patterning SAMs is by microcontact printing (µCP). 'Inking' a topographically patterned elastomeric stamp with alkanethiols, bringing the surface of the stamp into contact with a gold or silver surface, and peeling the stamp away leaves a pattern of SAMs on the surface. The pattern of SAMs can be used in a number of different ways to selectively modify the surface. For example, by patterning a hydrophobic SAM onto a hydrophilic gold surface, hydrophilic liquids applied to the surface can be localized to the bare gold regions, selectively dewetting the SAM-covered regions. This technique can be used to selectively deposit molecules, such as proteins, from the aqueous phase on the bare regions of gold. Similarly, patterns of SAMs can be used as templates for the deposition of a variety of materials, such as polymers, metals, and metal oxides. The patterns of SAMs may also be used as an etch resist for wet etching of the gold or silver surface; this technique is one of the most frequently used applications of µCP and SAMs. This approach can be used to produce electronic circuits and a variety of other structures for different applications. SAMs offer a flexible approach to controlling the chemical and physical properties of surfaces.