Functional SAMs

During organ formation, cells are continuously lost from the SAM, and these cells have to be replaced through cell divisions in the stem cell zone in order to maintain a functional SAM. The rate of cell divisions and the size of the stem cell population has to be coordinated with the rate of organ formation, which may vary during different developmental phases. How is this coordination achieved, and how can cells in the SAM communicate their position and their fate to each other An... 

A different approach was followed by Blackledge et al. [124]. They used the catalytic activity of a palladium coated SPM-tip to selectively generate a pattern into different co-functionalized SAMs. 

These monolayers form themselves spontaneously by adsorption of suitable components from a diluted solution directly onto a surface. The formation of ordered and orientated monomolecular layers by spontaneous adsorption from a diluted solution is called self-assembling, the respective layers are called self-assembled monolayers (SAM) or self-organized monolayers. In the first part of this review, the coating of surfaces with functionalized SAMs will be examined. After that, some aspects of the attachment of the biomolecules onto these SAM-modified surfaces will be discussed. 

Towards fabrication of SWNT-based molecular electronic devices, two methods have been used to assemble the 03-SWNTs on functionalized SAMs of OPEs, as shown in Figure 5.10. The first, termed chemical assembly , is based on a condensation reaction between the carboxylic acid functionalities of O3-SWNTs and the amine functionalities of SAMs to form amides. The results show that O3 -SWNTs coat the amino-terminated SAM with a high degree of surface coverage. The second method is based on physical adsorption via layer-by-layer (LBL) deposition with bridging of metal cations, i.e., Fe3+ on carboxylate terminated SAMs or Cu2+ on thiol-terminated SAMs. The oxidatively shortened 03 -SWNTs are shown to be perpendicular to the surface with random adsorption of longer tubes. The patterned nanotube assemblies may be useful in hybridized electronic devices, where device functions can be modified by the orientation and stacking of SWNTs, and the properties of the SAM. 

Clearly, these considerations imply that the preparation of functionalized SAMs via postsynthetic procedures requires reactions proceeding under mild conditions compatible with the structure and the integrity of the self-assembled monolayer. In addition, these reactions should, ideally, proceed quantitatively these considerations actually limit the reactions commonly used for SAM functionalization to a handful. In fact, the number of reactions used for postsynthetic modifications of SAMs is relatively small, and is essentially limited to acylation reactions (either amidations... 

Of course, a poly(ethylene oxide)-functionalized SAM may be further functionalized with suitable groups to ensure protein grafting on the SAM surface. The best choice for these purposes are active esters functionalization of oligo(ethylene oxide) SAMs with biotin85 has also been reported. The adsorption of poly(ethylene oxide)-functionalized disulfides on gold surfaces has been proven to give SAMs with protein-resistant surfaces comparable to those obtained from oligo(ethylene oxide)-functionalized thiols.86... 

The stability of poly(ethylene oxide)-functionalized SAMs under different storage conditions such as in air, under nitrogen, water, phosphate-buffered saline, and... 

Zhou et al. studied the selective LbL assembly of NCs on microcontact printed carboxylate-functionalized SAMs.91 The LbL assembly was achieved by alternating adsorption of NCs functionalized with hydrophobic trioctylphosphine oxide and 2-mercaptoethanesulfonic acid, and positively charged linear poly(ethyleneimine) (LPEI). By adsorption of 11-mercaptoundecylhexa(ethylene glycol) in the noncon-tacted areas, nonspecific interactions were minimized. A uniform and linear growth of the polymer-NC layers was observed. 

Over the last decade, a considerable number of reactions has been studied (11,35) (i) olefins oxidation (38,39), hydroboration, and halogenation (40) (ii) amines silylation (41,42), amidation (43), and imine formation (44) (iii) hydroxyl groups reaction with anhydrides (45), isocyanates (46), epichloro-hydrin and chlorosilanes (47) (iv) carboxylic acids formation of acid chlorides (48), mixed anhydrides (49) and activated esters (50) (v) carboxylic esters reduction and hydrolysis (51) (vi) aldehydes imine formation (52) (vii) epoxides reactions with amines (55), glycols (54) and carboxyl-terminated polymers (55). A list of all the major classes of reactions on SAMs plus relevant examples are discussed comprehensively elsewhere (//). The following sections will provide a more detailed look at reactions with some of the common functional SAMs, i.e hydroxyl and carboxyl terminated SAMs. 

Crafting of Functionalized SAMs onto Silicon Wafer... 

Three different techniques are frequently used to obtain SAMs Langmuir-Blod-gett techniques, involving an air-water interface to transfer the assembled film to a solid substrate solution adsorption of film molecules onto the substrate and a vapor-phase molecular self-assembling technique [2], which uses vapor deposition of the film onto the substrate. Our functionalized SAMs were prepared by the last of these techniques, which had been slightly improved in the laboratory... 

With chemically modified AFM tips, adhesion forces between the tip and a two-component self-assembled monolayer on a gold electrode have been studied [87]. Utilizing the different strengths of interaction between the modified tip (methyl and carboxyl terminating group functionalized), SAM areas with methyl and carboxyl end groups could be distinguished. 

Fig. 4 Structure of NHS ester-functionalized SAM on gold n = 2, 10, 15) and hydrolysis reaction in aqueous NaOH...

It has been described the formation of porphyrin- and sapphyrin-containing self-assembled monolayers on electrochemically prepared Au surfaces by a multistep approach After electrochemical preparation the Au substrate was characterized by SERS and cyclic voltammetry. SERS characterization was also used to study the basic architecture and properties of the porphyrin- and sapphyrin modified SAMs and their interactions with fluoranthene, 1,10-phenantroline and adenine. In general, it can be proposed that macrocycle oligopyrrole functionalized SAMs could be useful in the analysis and detection of polyaromatic and heterocyclic compounds. 

Binding of Metal Ions to Functionalized SAMs General Phenomena and Case Studies... 

An abundant scheme for the preparation of surface metal-organic complexes is ligand exchange, where one of the ligands of a metal complex in solution is substituted with a surface-confined ligand, leading to immobilization of the metal ion in the functional SAM. Examples of this process include Co and Os terpyri-dine complexes [22] Ru tetraamine [23] and dinuclear Ru [24] complexes salen complexes with Fe, Co, and Mn [25] and Zr acetylacetonate [26]. 

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