Biomaterials Tutorial

Self-Assembled Monolayers (SAMs)

Maxi Boeckl, Dan Graham, M. Jeanette Stein, Benjamin J. Roberts
University of Washington Engineered Biomaterials and National ESCA and Surface Analysis Center for Biomedical Problems (NESAC/BIO)

Self-Assembled Monolayers (SAMs) are ordered molecular assemblies formed by the adsorption of an active surfactant onto a solid surface [1]. These thin crystalline films are an ingeniously simple, yet powerful approach to modifying the surface properties of a material. Since 1983, when SAMs composed of disulfides or alkanethiols on gold (see Figure 1) were discovered by Nuzzo and Allara [2], there has been an exponential growth in SAM research. This high level of interest in SAMs is due to the fact that they are a powerful research tool that is relatively simple to construct and manipulate. SAMs help in the development of new biomaterials. They serve as model surfaces for isolating biological interactions (such as cell signaling, cell adhesion, and protein interactions) on the molecular level.

A primary advantage of SAMs is that changing as little as one atom of the terminal group (see Figure 2) can dramatically alter the macroscopic properties of the surface. Such surface properties include wettability, biocompatibility and protein/cellular adhesion.

SAMs have been studied by a multitude of surface analysis techniques including: ESCA, TOF-SIMS, surface probe microscopies (such as AFM and STM), FTIR, SPR, tunneling electron microscopy (TEM), sum frequency generation (SFG), helium diffraction, electron diffraction, contact angle, ellipsometry, and NEXAFS. This diverse and thorough characterization provides a wealth of background information that scientists can use to design specific experiments.

SAMs have been formed from a variety of compounds and substrates, including (but by no means limited to)fatty acids, trichlorosilanes and trialkoxysilanes on glass and silicon and carboxylic acids and alkyl phosphates on metal oxides [3].

Among these, alkanethiol SAMs adsorbed on gold present several advantages. First, gold is a relatively inert metal that resists oxidation and atmospheric contamination fairly well [4]. Second, gold has a strong specific interaction with sulfur [5]. The predictable binding of sulfur to gold allows the formation of tightly packed monolayers even in the presence of many other functional groups [3]. Third, long-chain alkanethiols form a densely packed, crystalline or liquid-crystalline monolayer due to strong molecular interactions (van der Waals forces) between the long carbon chains [3, 6].

Preparation of alkanethiol SAMs is a simple process. A gold-coated substrate is immersed in a dilute solution of the alkanethiol in ethanol and a monolayer spontaneously assembles on the surface of the substrate over a period of 1-24 hours. A disordered monolayer is formed within a few minutes, during which time the thickness reaches 80 - 90% of its final value. Over the next several hours, van der Waals forces on the carbon chains help pack the long alkanethiol chains into a well-ordered, crystalline layer [7]. In this process contaminants are replaced, solvents are expelled from the monolayer, and defects are reduced while packing is enhanced by lateral diffusion of the alkanethiols [3].

The resulting monolayers assemble with the alkanethiolates in a hexagonal-packing arrangement. The spacing of the sulfur anchors on the gold is 4.97 Å. (Figure 3) [6]. This chain spacing is larger than the ideal distance needed to maximize van der Waals interactions between the chains. Therefore, a natural tilt develops 30° from the normal surface, maximizing molecular interactions between carbon chains as they pack into their final crystalline monolayer (Figure 3) [3, 7, 9, 10]. The importance of van der Waals interactions between the chains is also seen when one considers the chain length. In general, the longer the chain length, the more ordered the monolayer [11, 12].

Figure 3: (a) The gold lattice (white circles) is shown with an alkanethiol overlayer (gray discs) in its hexagonal arrangement. (b) Schematic representation of a tightly packed alkanethiol monolayer. The tilt angle of the individual chains is approximately 30° to the normal degree of the surface. This tilting maximizes the van der Waals interactions between alkane chains.

Mixed Monolayers

The conventional method for designing biologically recognizable surfaces using alkanethiols on gold is to prepare a “mixed monolayer” consisting of two or more alkanethiols with different terminal groups. A mixed monolayer generally consists of a shorter background thiol with a terminal group that provides a neutral surface. The second alkanethiol, typically present in lower concentration, is interdispersed with the background thiol and has the terminal group of interest. While the terminal group of interest provides the desired biological activity of the engineered surface, the monolayer’s stability is often due to the background thiol. It can be advantageous to have only small amounts of a desired functional group present on the surface. For some groups, wider spacing can maximize interaction capabilities. Also, several different functional groups can be introduced at the same time.

There are some concepts to be considered when designing a mixed monolayer. Great differences in chain length or polarity of the terminal group can lead to preferential adsorption of one alkanethiol over another. This can also lead to formation of islands of alkanethiol with one terminal group in the midst of a sea of background thiols, instead of a uniform surface [11, 13].

Applications

At their fundamental level, living cells self-assemble. Thus, an understanding of the mechanisms behind self-assembly will help to understand life. Beyond this though, self-assembly has a myriad of uses.

It is one of the few practical strategies for making nanostructure ensembles. Therefore, the field of nanotechnology and robotics will benefit from self-assembly. It is also common to find self-assembly in more dynamic systems such as smart materials and self-healing structures [14].

Temperature sensitive SAMs that are hydrophobic at one temperature and hydrophilic at another temperature have been utilized to grow cell sheets which can be easily removed by decreasing the temperature.

SAMs that resist the attachment of protein are being studied for a wide range of applications, from the reduction of biofilms on large ships to the prevention of the foreign body response on implants.

In the pharmaceutical field, SAM surfaces are paired with ultrasound technology to create new methods of precision drug delivery. It is safe to say that many fields of study could benefit from the SAM technology.

References:

1. Ulman A. Chem Rev 1996; 96: 1533-1554.

2a. Nuzzo RG, Allara DL. J Am Chem Soc 1983; 105(13): 4481-4483.

2b. Delmarche E, Michel B, Biebuyck HA, Gerber C. Adv Mater 1996; 8(9): 719-4483.

3. Bain CD, Troughton EB, Tao Y-T, Evall J, Whitesides G M, Nuzzo RG. J Am Chem Soc 1989; 111: 321-335.

4. Chesters MA, Somorjai GA. Surf Sci 1975; 52: 21-28.

5. Nuzzo RG. Fusco FA, Allara DL. J Am Chem Soc 1987; 109: 2358-2368.

6. Strong L, Whitesides GM. Langmuir 1988; 4: 546-558.

7. Dubois LH, Nuzzo RG. Annu Rev Phys Chem 1992; 43: 437-463.

8. Biebuyck HA, Bain CD, Whitesides GM. Langmuir 1994; 10: 1825-1831.

9. Bain CD, Whitesides GM. Advanced Materials 1989; 28: 506-512.

10. Porter MD, Bright TB, Allara DL, Chidsey CED. J Am Chem Soc 1987; 109: 3559-3568.

11. Bain CD, Evall J, Whitesides GM. J Am Chem Soc 1989; 111: 7155-7164.

12. Holmes-Farley SR, Bain CD, Whitesides G. Langmuir 1988; 4: 921-937.

13. Bain CD, Whitesides GM. J Am Chem Soc 1989; 111: 7164-7175.

14. Whitesides GM, Grzybowski B. Science 2002; 295: 2418-2421.