Jan. 17, 2003 Santa Barbara, Calif. -- A team of researchers, including University of California at Santa Barbara (UCSB) chemical engineer Samir Mitragotri and colleagues from the Massachusetts Institute of Technology (MIT) and UC Berkeley, have designed and demonstrated a reversible surface switch. Their findings, published in the Jan. 17 issue of Science, represent a new paradigm for surface design that incorporates for the first time a temporal control without altering surface chemistry. The new approach to surface design makes possible the dictation of surface properties as a function of time.
This broad, enabling technology will likely spawn a variety of new applications. The researchers write, "these findings may, with further study, have implications in dynamic regulation of macroscopic properties, such as wettability, adhesion, friction or biocompatibility. Potential applications might include microfluidics, microengineering of smart templates for bioseparation or data storage, or microfabrication of controlled-release devices."
"Ideally," Mitragotri explained, "a monolayer, such as a self-assembled monolayer [SAM] of alkanethiolates deposited on gold, would be used to establish nanometer-thin interfaces whose properties can be dictated as a function of space and time."
An alkanethiol molecule is a linear chain of several carbon atoms. Imagine the linear chains protruding like porcupine quills from a gold wafer.
In the reported experiments, most of the chain is hydrophobic (adverse to water), but the topmost part (a carboxyl group [CO2H]) is hydrophilic (attracted to water). Applying an electrical field to the gold wafer causes the negatively charged carboxyl heads to be attracted to the positively charged surface, so that the chains bend over towards the gold wafer base, thereby exposing the hydrophobic middle of the chain. The surface has switched from hydrophilic to hydrophobic and can readily be switched back with the removal of the electric field.
The reported experiments demonstrate the principle of reversible switching between hydrophilic and hydrophobic properties, but other properties can be switched depending on the desired application.
The key trick is to arrange the alkanethiol molecules at just the right distance from each other so that they have room to bend over, but not so far apart that the chains have too much flexibility for control.
"We did a lot of work in the beginning," said Mitragotri, "to find out what the ideal spacing should be between the chains. Based on our simulations of molecular dynamics, we found that an area per molecule of 0.65 nanometer square is optimal. Once we identified how far apart the molecules should be, we synthesized a different intermediate molecule with a temporary top to the chain. That temporary molecule's spatial extent enables the requisite spacing of the chains." [The intermediate is a derivative of mercaptohexadecanoic acid with a globular end group, technically ((16-mercapto) hexadecanoic acid (2-chlorophenyl) diphenylmethyl ester, MHAE).]
It is as if the alkanethiolates are holding open umbrellas and can get only as close to one another as the expanse of the open umbrellas allows. This phase of the research involved a two-step process of determining what the head group should be and then actually making that head.
Finally, the researchers remove the umbrella or bulky head group using hydrolysis; in its wake is the hydrophilic carboxyl group ready to bend towards the gold wafer with the application of an electrical field.
Mitragotri, assisted by his UCSB chemical engineering graduate student, Jagannathan Sundarum, led the efforts in computational design of the alkanethiol monolayer--i.e., figuring out the optimum separation between the chains necessary to achieve reversible control.
"To collect accurate information about the structure of the alkanethiol monolayer," said Mitragotri, "we had to describe how each atom interacts with other atoms. The optimum distance provided by our simulations (0.65 nanometer square) is a strong function of the interactions between the atoms. If we chose incorrect interactions, we would have ended up with an incorrect separation." The team thereby determined the necessary size of the bulky head group and then predicted the electric fields necessary to bend the chains.
The first author of the "Science" article is Joerg Lahann, a postdoctoral fellow in the laboratory of MIT chemical engineer Robert Langer, the article's senior author. Other MIT-based authors are graduate student Thanh-Nga Tran and undergraduate research assistant Hiroki Kaido. The remaining two authors are Berkeley chemist Gabor A. Somorjai and his postdoctoral fellow Saskia Hoffer.
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