Self-assembled monolayers surface characterization

The importance of surface characterization in molecular architecture chemistry and engineering is obvious. Solid surfaces are becoming essential building blocks for constructing molecular architectures, as demonstrated in self-assembled monolayer formation [6] and alternate layer-by-layer adsorption [7]. Surface-induced structuring of liqnids is also well-known [8,9], which has implications for micro- and nano-technologies (i.e., liqnid crystal displays and micromachines). The virtue of the force measurement has been demonstrated, for example, in our report on novel molecular architectures (alcohol clusters) at solid-liquid interfaces [10]. 

The parameters K1/ K2/ and K3 are defined by the refractive indices of the crystal and sample and by the incidence angle [32]. If the sample has uniaxial symmetry, only two polarized spectra are necessary to characterize the orientation. If the optical axis is along the plane of the sample, such as for stretched polymer films, only the two s-polarized spectra are needed to determine kz and kx. These are then used to calculate a dichroic ratio or a P2) value with Equation (25) (replacing absorbance with absorption index). In contrast, a uniaxial sample with its optical axis perpendicular to the crystal surface requires the acquisition of spectra with both p- and s-polarizations, but the Z- and X-axes are now equivalent. This approach was used, through dichroic ratio measurements, to monitor the orientation of polymer chains at various depths during the drying of latex [33]. This type of symmetry is often encountered in non-polymeric samples, for instance, in ultrathin films of lipids or self-assembled monolayers. 

Roberts C, Chen CS, Mrksich M, Martichonok V, Ingber DE, Whitesides GE (1998) Using mixed self-assembled monolayers presenting RGD and (EG)3OH groups to characterize long-term attachment of bovine capillary endothelial cells to surfaces. J Am Chem Soc 120 6548-6555... 

A Au-coated substrate is another model surface, to which many surface characterization methods can be applied. To achieve surface-initiated ATRP on Au-coated substrates, some haloester compounds with thiol or disulflde group were developed [80-84]. Self-assembled monolayers (SAM) of these compounds were successfully prepared on a Au-coated substrate and used for ATRP graft polymerization. Because of the limited thermal stability of the S - Au bond, the ATRP was carried out at a relatively low temperature, mostly at room temperature, by using a highly active catalyst system and water as a (co)solvent (water-accelerated ATRP). 

Chemically prepared colloidal gold nanoparticles were immobilized as a submonolayer on Au(lll) surface modified with self-assembled monolayers (SAMs) of 4-aminothiophenol [14]. This submonolayer of Au nanoparticles was subsequently characterized using STM. 

Another well-represented category was that of self-assembled monolayers (SAMS) and other supramolecular systems. The experiments on the SAMS included studies of the surface pKa of one system (110), the kinetics and thermodynamics of the self-assembly process (111), and the characterization of the SAM surface by study of solution contact angles (112). The experiments on supramolecular systems included studies on chemical equilibria in such systems (113, 114, 115), the kinetics of inclusion phenomena (116), and the use of solvatochromic probes in studying inclusion phenomena (117). 

Yamamoto, H., Butera, R. A., Gu, Y. and Waldeck, D. H. Characterization of the surface to thiol bonding in self-assembled monolayer films of C12H25SH on InP(100) by angle-resolved X-ray photoelectron spectroscopy. Langmuir 15, 8640 (1999). 

Formation and characterization of a well-ordered monolayer prepared by adsorbing thiophene onto Au(l 1 1) (96L6167). Exposure of Au(l 1 1) surfaces to an ethanol solution of thiophene produces a stable SAM (self-assembled monolayer) which is strikingly similar to SAMs obtained by reacting Au(l 11) with alkanethiols or dialkylsulfides. 

By their very nature, heterogeneous assemblies are difficult to characterize. Problems include the exact nature of the substrate surface and the structure of the modifying layer. In this chapter, typical examples are given of how surface assemblies can be prepared in a well-defined manner. This discussion includes the descriptions of various substrate treatment methods which lead to clean, reproducible surfaces. Typical methods for the preparation of thin films of self-assembled monolayers and of polymer films are considered. Methods available for the investigation of the three-dimensional structures of polymer films are also discussed. Finally, it will be shown that by a careful control of the synthetic procedures, polymer film structures can be obtained which have a significant amount of order. It will be illustrated that these structural parameters strongly influence the electrochemical and conducting behavior of such interfacial assemblies and that this behavior can be manipulated by control of the measurement conditions. 

The foregoing discussion has focused on self-assembled monolayers formed on essentially flat electrode surfaces whose areas are vastly larger than those occupied by a single adsorbate. This field has now achieved a significant level of sophistication in terms of their structural characterization as well as their rational design for specific functions, e.g. chemically modulated switches. Although somewhat outside the scope of this book, another important area that exploits the unique properties of self-assembled monolayers is monolayer-protected metal clusters or nanoparticles. 

More recent efforts focused on surface modification of conductive polymers by the SECM, fabrication, and characterization of microstructures. Mandler et al. developed an approach for the formation of a 2D conducting polymer on top of an insulating layer. This approach, based on electrostatically binding a monomer (anilinium ions) to a negatively charged self-assembled monolayer of co-mercaptodecanesulfonate [MDS, HS(CH2)ioS03 ] followed by its electrochemical polymerization. The polyanion monolayer exhibited the properties similar to those of a thin polymer film [167]. 

Caldwell, W. B., Campbell, D. j., Chen, K. M., Herr, B. R., Mirkin, C. A., Malik, A., Durbin, M. K., Dutta, P., and Huang, K. G. A highly ordered self-assembled monolayer film of an azobenzenealkanethiol on Au(Ill) - Electrochemical properties and structural characterization by synchrotron in-plane X-ray-diffraction, atomic-force microscopy, and surface-enhanced Raman-spectroscopy. J. Am. Chem. Soc. 1995 117, 6071-6082 ... 

Whitesides, G. M., and Laibinis, P. E. Wet chemical approaches to the characterization of organic surfaces self-assembled monolayers, wetting, and the physical-organic chemistry of the solid-liquid interface. Langmuir, 6, 87 (1990). 

Recently, Brzozowska et al. used NR and ex situ electrochemical techniques to characterize an innovative type of monolayer system intended to serve as a support for a bUayer lipid membrane on a gold electrode surface [51]. Zr ions were used to noncovalendy couple a phosphate-terminated self-assembled monolayer (SAM) formed on a gold surface to the carboxylate groups of negatively charged phos-phatidylserrne (PS). This tethered surface was then used for the formation of a PS hpid bilayer structure formed by vesicle fusion and spreading. NR studies revealed the presence of an aqueous environment associated with the tether layer which arises from nonstoichiometric water associated with the zirconium phosphate moieties [52]. 

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