A report on the use of principles of physical organic chemistry to create functional surfaces based on self-assembled monolayers (SAMs) [J. Am. Chem. Soc., 111, 321 (1989)] (PDF file) has earned a place among the most highly cited papers in the Journal of the American Chemical Society's 125-year history.
"This paper showed how stable surfaces exposing a single organic functional group could be created by adsorption of thiols on gold--for a dozen different groups, from nonpolar methyl and CF3 groups to halides, nitriles, and carboxylic acids--and described how these surfaces could be used to predict, tailor, and control wettability," says coauthor Colin D. Bain.
Bain was a graduate student with Harvard University chemistry professor George M. Whitesides when the work was done and is now a lecturer at Oxford University's department of chemistry. The other principal investigator on the study was AT&T Bell Laboratories technical staff member Ralph G. Nuzzo, who is currently professor of chemistry and director of the Frederick Seitz Materials Research Laboratory at the University of Illinois, Urbana-Champaign.
The JACS report "was rapidly followed by a series of papers showing how multiple functional groups could be incorporated in a single monolayer, how these groups could be spatially organized on the surface, and how other surface properties, such as surface charge and adhesion, could also be controlled with SAMs," Bain says.
These discoveries have led to a number of important developments, including the following:
- Functional materials and microdevices for a range of applications, from microfluidics to biological sensing.
- "Soft lithography," an alternative to photolithography in which organic chemistry is used to fabricate surface-active microdevices.
- The founding of companies that employ SAM-based technologies, like Surface Logix of Brighton, Mass., and Fluidigm of South San Francisco.
The paper "has been cited so much because of its thorough analysis of the chain-length and end-group dependence of the wetting properties of a range of thiol-derived monolayers," comments associate professor of chemistry Christopher E. D. Chidsey of Stanford University. "Subsequent work very often has referred back to this study as a base to which more complex monolayer systems are compared."
The study is also noteworthy because its principal investigators have remained very influential in designed-interface research, Chidsey adds. "Ralph nurtured work in this field by many others," he says. "And the Whitesides group and many of its former members subsequently made huge contributions to this field and have helped spread the approach embodied in this paper to other monolayer types and to many applications of monolayers."
FUNCTIONALIZED Simple carbon chains with sulfide groups on one end and functional groups of various types (X) on the other self-assemble to form ordered monolayers on substrates. COURTESY OF GEORGE WHITESIDES/MIT
In 1983, a paper by Nuzzo and his colleague David L. Allara first demonstrated the self-assembly of amphiphilic organosulfur compounds on gold surfaces [J. Am. Chem. Soc., 105, 4481 (1983)]. This report and papers that followed "identified alkanethiolate SAMs on gold as highly ordered and easy-to-make stable structures, correctly showed that the organic structure for long-chain n-alkanethiolates was trans-extended, and correctly inferred crystallinity," Whitesides notes. "It was the start of SAMs."
According to Bain, "SAMs on gold are so simple to prepare that chemists who are not surface science experts with a huge range of sophisticated instrumentation can routinely prepare robust, reproducible, well-defined organic surfaces."
Whitesides adds that the ability to tailor SAM composition and properties by organic synthesis "meant it was practical to synthesize interfaces presenting the complex ligands required for studies of biochemistry and biology, and they provided important models with which to study complex interfacial properties."
The JACS paper by Bain, Whitesides, and Nuzzo--along with coworkers E. Barry Troughton, Yu-Tai Tao, and Joseph Evall--added the element of functionality to SAMs and demonstrated some potential applications of functional surfaces.
"These experiments were based on the idea that, since SAMs were ordered, a functional group distal to the thiol would form a close-packed monolayer on the surface," Whitesides says. The organic groups could play useful functional roles such as control of wettability at interfaces between solid surfaces and solution or vapor phases.
"The 1989 paper is one that people cite because it demonstrated the fusion of structure and application and showed how to bridge molecular structure and macroscopic and materials structure in organic surface science," Whitesides says. "It showed that you could use physical organic chemistry to solve problems in materials and surface science."
Papers by several groups had introduced some of these ideas a little earlier. But the classic JACS paper put all the pieces together and "seemed to capture essential elements of those ideas in a particularly deep and compelling way," Nuzzo says.