Effect of Electrostatic Fields on Self-Assembly at Surfaces

Stanford University, Stanford CA

Investigators

Abstract

CBET-0827822 Melosh Self-assembly at interfaces involves a delicate balance between charge, bonding strength, transport, and reversibility. External fields or non-uniform charge distributions can alter assembly behavior in surprising ways by accelerating, inhibiting or changing the assembly process. In particular, many molecular and biological species are themselves highly-charged systems which may also undergo significant conformational and electrostatic reconfiguration during reaction/assembly. This general problem of self-assembly in external fields and charge-redistribution during the assembly process has received little attention, yet is vital for the goals of top-down/bottom-up patterning or more traditional applications such as DNA microarrays and biosensors. This project will systematically measure how electric fields modulate self-assembly at interfaces, and develop a fully-vetted theoretical model. A new optical technique will be applied to monitor the spatial and temporal build-up of assembling species as well as the ionic double layer with single nanometer accuracy, providing new, quantitative information on the dynamics of highly charged species. In these experiments DNA serves as an ideal model system, as preliminary studies have shown that ionic strength and electric fields have some effect on assembly, but a complete theoretical model has not been established. DNA transport to the interface, hybridization, and melting kinetics will be studied as a function of length, potential field, mismatch locations, and charge density. The intellectual merit of this project is to elucidate how electric fields and ion distributions affect self-assembly mechanisms. Fundamentally, this work will fill a void in our understanding of how highly-charged species interact and react at surfaces under an external bias. While small ion and colloidal behavior in external fields are well-established areas, self-assembly under electric fields, the tendency of monomer reconfiguration during assembly, and the effect of force distribution on DNA within field gradients have not been explored. Mean-field and Brownian dynamics theoretical models will be developed that can replicate the results of these experiments, providing deeper insight into their mechanism. The broader impacts of this project include developing models to predict the hybridization and melting dynamics of DNA on surfaces, providing straight-forward error correction methods for nanomaterials assembly, educate graduate and undergraduate students, and to disseminate these findings. Development of new methods to control interfacial activity is important for separations, bio-fouling, DNA-array technology and coatings. A full understanding of the interplay between assembly and fields will allow design of pulse algorithms and conditions through which to greatly accelerate DNA hybridization while reducing mis-matches and cross-contamination, a critical problem for commonly used DNA microarrays, and can be implemented in a number of applications. These results will be widely shared with other researchers, foremost through development of a website with free public access to the codes and protocols used. In addition to scientific and technological impact, this proposal incorporates an educational outreach component as well. Undergraduate research, curriculum development, scientific professional development, and tutoring elementary school students are an integral part of the research program.