Surface functionalization the ability to alter the chemical or biological properties of a solid substrate is now an important component in a number of technical fields, including chemical separations, sensing, catalysis, and bioengineering. The simplest means of adding functionality to a surface is the physical adsorption of functional molecules with little or no control over the thickness of the resulting film or the final orientation and position of the functional groups with respect to the surface. In contrast, Langmuir-Blodgett (LB) film deposition allows precise control over film thickness and also provides for consistent orientation and positioning of the functional groups. Self-assembled monolayers (SAMs) also result in the creation of a densely packed monolayer film with the added benefit of stronger interactions between the monolayer and the solid substrate. The disadvantage to both of these techniques is that they require the synthesis of molecules with the desired functional group at one end and either a hydrophilic headgroup (LB film) or reactive group (SAM) at the other end. This á−ù synthesis is time consuming and limits the number of functional surfaces that can be created. These methods can be simplified and improved by borrowing concepts from 3-D crystal engineering, specifically those of host guest inclusion compounds. The PI has demonstrated the creation of ternary host-guest monolayers at the air water interface based on hydrogen bonding between alkanesulfonate amphiphiles and guanidinium ?spacer? counter-ions which form cavities into which a variety of alkyl substituted functional guest molecules can be inserted. These host-guest monolayers can be transferred to solid substrates via LB deposition in order to form functionalized surfaces.
The proposed work has the capacity to transform current methods of surface functionalization by opening up a large library of functionalized surfaces that can be formed using a common host monolayer system into which can be inserted a multitude of alkyl substituted guest molecules.
The nano-related project is broken down into three sections: 1) characterization of host-guest LB films based on guanidinium sulfonate host monolayers, including film stability, and the kinetics of insertion and exchange of commercially available homologous guest molecules; 2) synthesis and characterization of host-guest LB films based on sterically hindered amphiphiles amphiphiles possessing an integral ?spacer? moiety; and 3) synthesis and characterization of analogous sterically hindered host-guest SAM films that are strongly bound to the solid substrate, providing greater stability and allowing the functionalization of non-planar substrates such as 3D scaffolds and nanoparticles.
Broader Impacts:
The nano-related project has the potential to benefit society by enabling the production of materials and devices with tailored surface functionality, with applications in sensing, biocompatibility, separations, and catalysis. This research will also advance discovery, while promoting teaching and learning at the high school, undergraduate, and graduate levels by providing: (1) research opportunities for undergraduate students, including underrepresented minorities and women during both the regular academic year and in a summer research program and their participation in outreach programs to encourage high school students, particularly women, to pursue careers in engineering; (2) coordinated recruitment of graduate students from under-represented groups with Virginia Tech?s Office of Graduate Student Recruiting into an interdisciplinary research program and their participation in outreach programs such as C-Tech2 and opportunity to mentor undergraduate researchers, and (3) the inclusion of knowledge and examples drawn from this work in my elective Self-Assembly course and our undergraduate Unit Operations laboratory course. The results of this research will be widely disseminated to the scientific and lay communities in peer-reviewed journals, in presentations at multi-disciplinary conferences, in undergraduate symposia, and in course web pages.
Results of the work: This research is aimed at developing a new method for changing the properties of material surfaces at the molecular level. Materials interact with their environment primarily through their surfaces, so changing the surface properties has profound effects on material and device performance. The Virginia Tech team led by Dr. Martin created a universal "host" surface that can accept any number of interchangeable "guest" molecules to create a whole library of surface properties. The research has resulted in several new methods for the creation of nanometer scale host layers containing gaps on interfaces of materials. These gaps are designed to capture "guest" molecules with different properties, such as biocompatibility, chemical reactivity, or sensing behavior. The research has also confirmed the significant finding that surface properties are changed on the introduction or removal of guest molecules into the host surface structure. The end result of this work is a completely new approach for changing surface chemistry where different functional groups can easily be added, removed, and exchanged in the universal host to adjust material properties for specific applications. For instance, the interaction of cells with a surface could be controlled via the addition of biological receptors in order to improve the compatibility of an implanted biomedical device. These "host-guest" thin films could have applications in a variety of fields, including chemical sensing, catalysis, electronics, and biomedical device engineering. Intellectual Merit of the research: The research addresses the need for new methods of creating functional surfaces. The work aims to do this by relying on the self-assembly of molecules, due to reversible intermolecular interactions, into a host-guest thin film on a solid surface. The work has resulted in several new routes for the formation of low-density host self-assembled monolayers containing cavities into which guest molecules can insert. The work has also demonstrated the feasibility of guest insertion into and removal from the host monolayers. Finally, the work is also providing insight into the structure and physical properties of host-guest thin films, particularly with regards to the phenomena of guest molecule addition and removal. Broader impacts of this activity: The ongoing research activities will benefits society by allowing the easy creation of functional material surfaces with applications in a number of areas, including sensing, electronics, and biotechnology. The Martin group is working to broaden the participation of underrepresented groups through interactions with C-Tech2, a Virginia Tech program that introduces high school aged girls to careers in science and technology, and the Center for the Enhancement of Engineering (CEED) Discovery Camp and the NASA-Inspires camps, which are targeted at elementary and middle school students. These students were introduced to topics in chemical engineering with hands-on demonstrations, including discussions of the importance of interfacial properties and surface chemistry for many applications. Dr. Martin has also emphasized his research in recruitment activities aimed at freshmen and high school students, and in his involvement as a faculty resource for freshmen students in the Hypatia (women) and Galileo (men) engineering-themed dormitories at Virginia Tech. The work advances discovery and understanding while promoting teaching, training, and learning through the training of graduate students and undergraduate researchers in an active research setting. The results of the research are being disseminated broadly to enhance scientific and technological understanding through presentations at national scientific meetings and through the preparation of several scientific articles .
