Self-assembled monolayer

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A self assembled monolayer (SAM) is an organized layer of amphiphilic molecules in which one end of the molecule, the “head group” shows a special affinity for a substrate. SAMs also consist of a tail with a functional group at the terminal end as seen in Figure 1.

SAMs are created by the chemisorption of hydrophilic “head groups” onto a substrate from either the vapor or liquid phase^[1] followed by a slow two-dimensional organization of hydrophobic “tail groups”^[2]. Initially, adsorbate molecules form either a disordered mass of molecules or form a “lying down phase” ^[1], and over a period of hours, begin to form crystalline or semicrystalline structures on the substrate surface^[3][4]. The hydrophilic “head groups” assemble together on the substrate, while the hydrophobic tail groups assemble far from the substrate. Areas of close-packed molecules nucleate and grow until the surface of the substrate is covered in a single monolayer.

Adsorbate molecules adsorb readily because they lower the surface energy of the substrate^[3] and are stable due to the strong chemisorption of the “head groups.” These bonds create monolayers that are more stable than the physisorbed bonds of Langmuir-Blodgett films^[5][6]. Thiol-metal bonds, for example, are on the order of 100 kJ/mol, making the bond stable in a wide variety of temperature, solvents, and potentials^[4]. The monolayer packs tightly due to van der Waals interactions^[3][6], thereby reducing its own free energy^[3]. The adsorption can be described by the Langmuir adsorption isotherm if lateral interactions are neglected. If they cannot be neglected, the adsorption is better described by the Frumkin isotherm^[4].

Contents 1 Types of SAMs 2 Preparation of SAMs 3 Characterization of SAMs 4 Applications of SAMs 5 References 6 Further reading

[edit] Types of SAMs

Selecting the type of head group depends on the application of the SAM^[3]. Typically, head groups are connected to an alkyl chain in which the terminal end can be functionalized (i.e. adding –OH, -NH3, or –COOH groups) to vary the wetting and interfacial properties^[5][7]. An appropriate substrate is chosen to react with the head group. Substrates can be planar surfaces, such as silicon and metals, or curved surfaces, such as nanoparticles. Thiols and disulfides are the most commonly used on noble metal substrates. Currently, gold is the standard for these head groups. Gold is an inert and biocompatible material that is easy to acquire. It is also easy to pattern via lithography, a useful feature for applications in nanoelectromechanical systems (NEMS)^[3]. Additionally, it can withstand harsh chemical cleaning treatments^[4]. Silanes are generally used on nonmetallic oxide surfaces^[3].

[edit] Preparation of SAMs

Metal substrates for use in SAMs can be produced through physical vapor deposition techniques, electrodeposition or electroless deposition^[3]. Alkanethiol SAMs produced by adsorption from solution are made by immersing a substrate into a dilute solution of alkane thiol in ethanol for 12 to 72 hours at room temperature and dried with nitrogen^[3][4][8]. SAMs can also be adsorbed from the vapor phase. For example, chlorosilane SAMs (which can also be adsorbed from the liquid phase), are often created in a reaction chamber by silanization in which silane vapor flows over the substrate to form the monolayer^[9].

[edit] Characterization of SAMs

The structures of SAMs are most commonly determined using scanning probe microscopy techniques such as atomic force microscopy (AFM) and scanning tunneling microscopy (STM). More recently, however, diffractive methods have also been used^[3]. The structure can be used to characterize the kinetics and defects found on the monolayer surface. These techniques have also shown physical differences between SAMs with planar substrates and nanoparticle substrates.

Kinetics

There is evidence that SAM formation occurs in two steps, an initial fast step of adsorption and a second slower step of monolayer organization. Many of the SAM properties, such as thickness, are determined in the first few minutes. However, it may take hours for defects to be eliminated via annealing and for final SAM properties to be determined^[1][4]. The exact kinetics of SAM formation depends on the adsorbate, solvent and substrate properties. In general, however, the kinetics are dependent on both preparations conditions and material properties of the solvent, adsorbate and substrate^[1]. Specifically, kinetics for adsorption from a liquid solution are dependent on^[3]:

• Temperature – room temperature preparation improves kinetics and reduces defects.
• Concentration of adsorbate in the solution – low concentrations require longer immersion times^[3][4] and often create highly crystalline domains^[4].
• Purity of the adsorbate – impurities can affect the final physical properties of the SAM
• Dirt or contamination on the substrate – imperfections can cause defects in the SAM

The final structure of the SAM is also dependent on the chain length and the structure of both the adsorbate and the substrate. Steric hindrance and metal substrate properties, for example, can affect the packing density of the film^[3][4], while chain length affects SAM thickness^[6].

Defects

Though the slow step in SAM formation often removes defects from the film, defects are included in the final SAM structure. Defects can be caused by both external and intrinsic factors. External factors include the cleanliness of the substrate, method of preparation, and purity of the adorbates^[3][4]. SAMs intrinsically form defects due to the thermodynamics of formation. “The high coverage of the adsorbate present in the SAM is, in fact, thermodynamically unstable”^[10].

Nanoparticle Properties

The structure of SAMs is also dependent on the curvature of the substrate. SAMs on nanoparticles including colloids and nanocrystals, “stabilize the reactive surface of the particle and present organic functional groups at the particle-solvent interface”^[11]. These organic functional groups are useful for applications, such as immunoassays, that are dependent on chemical composition of the surface^[3].

[edit] Applications of SAMs

Areas of application for SAMs include biology, electrochemistry and electronics, nanoelectromechanical systems (NEMS) and microelectromechanical systems (MEMS), and everyday household goods. SAMs can serve as models for studying membrane properties of cells and organelles and cell attachment on surfaces^[3]. SAMs can also be used to modify the surface properties of electrodes for electrochemistry, general electronics, and various NEMS and MEMS^[3]. For example, the properties of SAMs can be used to control electron transfer in electrochemistry. They can serve to protect metals from harsh chemicals and etchants. SAMs can also reduce sticking of NEMS and MEMS components in humid environments. In the same way, SAMs can alter the properties of glass. A common household product, Rain-X, utilizes SAMs to create a hydrophobic monolayer on car windshields to keep them clear of rain.

[edit] References

1. ^ ^{a b c d} Schwartz, D.K., Mechanisms and Kinetics of Self-Assembled Monolayer Formation. Annu. Rev. Phys. Chem. 2001, 52, 107.
2. ^ Wnek, Gary, Gary L. Bowlin. Encyclopedia of Biomaterials and Biomedical Engineering. Informa Healthcare, 2004, 1331-1333.
3. ^ ^{a b c d e f g h i j k l m n o p q} Love et al. Self-Assembled Monolayers of Thiolates on Metals as a Form of Nanotechnology. Chem. Rev. 2005, 105, 1103-1169.
4. ^ ^{a b c d e f g h i j} Vos, Johannes G., Robert J. Forster, Tia E. Keyes. Interfacial Supramolecular Assemblies. Wiley, John & Sons, Incorporated, 2003, 88-94.
5. ^ ^{a b} Madou, Marc. Fundamentals of Microfabrication: The Science of Miniaturization. Boca Raton: CRC, 2002, 62-63.
6. ^ ^{a b c} Kaifer, Angel. Supramolecular Electrochemistry. Coral Gables: Wiley VCH, 2001, 191-193.
7. ^ Saliterman, Steven. Self-assembled monolayers (SAMs). Fundamentals of BioMEMS and Medical Microdevices. Bellingham: SPIE Press, 2006, 94-96.
8. ^ Wysocki, . “Self-Assembled Monolayers (SAMs) as Collision Surfaces for Ion Activation.”
9. ^ Knieling, T.; Lang, W.; Benecke, W. Gas phase hydrophobisation of MEMS silicon structures with self-assembling monolayers for avoiding in-use sticking. Sensors and Actuators, B: Chemical (2007), B126(1), 13-17.
10. ^ Love et al. Self-Assembled Monolayers of Thiolates on Metals as a Form of Nanotechnology. Chem. Rev. 2005, 105, pg 1122.
11. ^ Love et al. Self-Assembled Monolayers of Thiolates on Metals as a Form of Nanotechnology. Chem. Rev. 2005, 105, pg 1128.

[edit] Further reading

• I. Rubinstein, E. Sabatani, R. Maoz and J. Sagiv, Organized Monolayers on Gold Electrodes, in Electrochemical Sensors for Biomedical Applications, C.K.N. Li (Ed.), The Electrochemical Society 1986: 175.
• N. Faucheux, R. Schweiss, K. Lützow, C. Werner, T. Groth. Self-assembled monolayers with different terminating groups as model substrates for cell adhesion studies. Biomaterials 2004;25:2721-2730.
• S. R. Wasserman, Y. T. Tao, G. M. Whitesides. Structure and Reactivity of Alkylsiloxane Monolayers Formed by Reaction of Alkyltrichlorosilanes on Silicon Substrates. Langmuir 1989;5:1074-1087
• H.E. Hoster, M. Roos, A. Breitruck, C. Meier, K. Tonigold, T. Waldmann, U. Ziener, K. Landfester, R.J. Behm, Structure Formation in Bis(terpyridine)Derivative Adlayers – Molecule-Substrate vs. Molecule-Molecule Interactions, Langmuir 23 (2007) 11570
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