Self-assembled monolayers SAMs

SAMs have been of considerable interest for some time [see Chechik, Crooks and Stirhng, Reactions and 

Preparation and purification of polymerised coated substrate linked monolayers by Cu Coupling CuCl (20mg) and degassed Me2CO (3ml) in a reaction vial are treated with TMEDA (61 pi), stirred under N2 at 20° for 30 minutes and the preceding coated substrates are added. The mixture is purged with O2 from a balloon and stirred under O2 for 15 hours. The polymerised coated substrate is collected, rinsed with DMF (lx), with a solution of O.IM sodium diethyldithiocarbamate in DMF (lx), with toluene (lx), with CH2CI2 (2x), sonicated in toluene for 5 minutes, the rinse is repeated, the substrate is finally sonicated for 5 minutes in MeOH, and the polymerised linked substrate blown dry with a stream of N2. 

Preparation and purification of polymerised coated substrate linked monolayers by alkyne metathesis A mixture of trisamidomolybdenum(IV) propyhdyne (5.0mg) and p-nitrophenol (3.3mg) dissolved in trichlorobenzene (3.3ml, 1,2,4-) are places in a vial, the coated substrate is added, the flask is sealed under a vacuum (5 Torr) for 22 hours. The substrate is collected. Rinsed with DMF (lx), O.IM sodimn diethyldithiocarbamate in DMF (lx), toluene (lx), CH2CI2 (2x), then sonicated in toluene for 5 minutes. The 

These monolayers (SAMs) are characterised by UV-Vis, fluorescence and Raman spectroscopy. This is made easy because of the presence of benzene cores within the acetylene and linker chains, these being made of poly(l,4-phenylene-l,3-butadiynylene) polymers. AFM then provides the topology, i.e. shapes (contours), thicknesses and aspect ratios (length and width), of the 2D SAMs. This is done at all stages including those described below. 

Formation and structure of self-assembled monolayers, Chem. 

The most common approach to preparing SAMs is to immerse a clean substrate with a reactive surface in a solution of the coating molecules. Over time, the SAM molecule is chemisorbed to the surface, often initially in a disordered fashion but slowly self-assembling to give a more stable, ordered, close-packed monolayer. Amphiphilic SAMs are also formed at fluid interfaces (e.g. air-water) by a similar self-assembly process. The following sections will introduce examples of SAMs and highlight their use towards nanochemical devices. 

Depending on the conditions, monolayer formation is not the only behaviour observed. For example, if the concentration is raised above a certain critical concentration, the amphilphiles may close in on themselves to form spherical aggregates termed micelles within the bulk water. In a micelle, the hydrophobic tails are packed into the interior of the aggregate, leaving the hydrophilic head groups exposed to the solvent. The point at which these struc- 

Artificial phospholipid vesicles (liposomes) are used to transport vaccines, drugs, enzymes or other substances to target cells or organs. They also make excellent model systems for studying biological ion transport across cell membranes. The vesicles, which are several hundred nanometres in diameter, do not suffer from interference from residual natural ion-channel peptides or ionophores, unlike purified natural cells. For example, the synthetic heptapeptide 5.23 forms pores that promote chloride efflux in vesicle models. Similarly, the ion-pair receptor 2.108 can ferry NaCl from vesicles as an ion-pair ionophore (see Chapter 2, Section 2.6.2), while the hydraphile 5.24 has been shown to transport Na using Na NMR spectroscopy through the bilayer walls of a vesicle model system. 

In 1774, Benjamin Franklin reported the following statement to the British Royal 

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Investigate the differences between LB films and self-assembled monolayer SAMs (Chapter XI). Which are finding more practical use, and what are the potential applications of each ... 

Self-assembled monolayers (SAMs) are molecular layers tliat fonn spontaneously upon adsorjDtion by immersing a substrate into a dilute solution of tire surface-active material in an organic solvent [115]. This is probably tire most comprehensive definition and includes compounds tliat adsorb spontaneously but are neither specifically bonded to tire substrate nor have intennolecular interactions which force tire molecules to organize tliemselves in tire sense tliat a defined orientation is adopted. Some polymers, for example, belong to tliis class. They might be attached to tire substrate via weak van der Waals interactions only. 

A second class of monolayers based on van der Waal s interactions within the monolayer and chemisorption (in contrast with physisorption in the case of LB films) on a soHd substrate are self-assembled monolayers (SAMs). SAMs are well-ordered layers, one molecule thick, that form spontaneously by the reaction of molecules, typically substituted-alkyl chains, with the surface of soHd materials (193—195). A wide variety of SAM-based supramolecular stmctures have been generated and used as functional components of materials systems in a wide range of technological appHcations ranging from nanoHthography (196,197) to chemical sensing (198—201). 

Surface properties are generally considered to be controlled by the outermost 0.5—1.0 nm at a polymer film (344). A logical solution, therefore, is to use self-assembled monolayers (SAMs) as model polymer surfaces. To understand fully the breadth of surface interactions, a portfoHo of chemical functionahties is needed. SAMs are especially suited for the studies of interfacial phenomena owing to the fine control of surface functional group concentration. 

Langmuir-Blodgett films (LB) and self assembled monolayers (SAM) deposited on metal surfaces have been studied by SERS spectroscopy in several investigations. For example, mono- and bilayers of phospholipids and cholesterol deposited on a rutile prism with a silver coating have been analyzed in contact with water. The study showed that in these models of biological membranes the second layer modified the fluidity of the first monolayer, and revealed the conformation of the polar head close to the silver [4.300]. 

The expectation of the structural dependence of lubrication motivated great numbers of investigations that intended to prepare and use highly ordered organic films, such as the Langmuir-Blodgett (L-B) hlms and Self-Assembled Monolayers (SAMs), as solid lubricant, which will be discussed more specihcally in Section 4. 

This chapter introduces three kinds of surface organic modihcation hlms on a magnetic head that we have studied. These are polyfluoroalkylmethacrylate films, X-1P films, and self-assembled monolayers (SAMs). It also reviews the works of surface lube on a hard disk surface. In the last, the challenges on the development of a magnetic recording system are discussed. 

Self-assembled monolayers (SAMs) [8] The layers are formed by heterologous interaction between reactive groups, such as thiols, and noble metals, such as gold or silver. Since the molecules are selectively adsorbed on these metals, film growth stops after the first monolayer is completed. The molecular aggregation is enthalpy driven, and the final structure is in thermodynamic equilibrium. 

When appropriate molecular-molecular and molecular-surface interactions are present, an ordered monolayer is formed spontaneously on surfaces (Figure 16.1). This process is called self-assembly (SA) and monolayers formed in this manner are called self-assembled monolayers (SAMs). 

For transition and precious metals, thiols have been successfully employed as the stabilizing reagent (capping reagent) of metal nanoparticles [6]. In such cases, various functionalities can be added to the particles and the obtained nanoparticles may be very unique. It is well known that thiols provide good self-assembled monolayers (SAM) on various metal surfaces. When this SAM technique is applied to the nanoparticle preparation, nanoparticles can be covered constantly by functionalized moieties, which are connected to the terminal of thiol compounds. 

In order to prevent the irrevisible adhesion of MEMS microstructures, several studies have been performed to alter the surface of MEMS, either chemically or physically. Chemical alterations have focused on the use of organosilane self-assembled monolayers (SAMs), which prevent the adsorption of ambient moisture and also reduce the inherent attractive forces between the microstructures. Although SAMs are very effective at reducing irreversible adhesion in MEMs, drawbacks include irreproducibility, excess solvent use, and thermal stability. More recent efforts have shifted towards physical alterations in order to increase the surface roughness of MEMS devices. 

Fig. 2.4 Self-assembled monolayer (SAM) structures from organosilane compounds.

Self-assembled monolayers (SAMs) of alkanethiols, HS(CH2)nX, where X denotes various functional groups, are frequently used to prepare model surfaces [3-6]. Alkanethiols or alkanedisulfides chemisorb from a solution onto a surface coated... 

Coupling of affinity molecules to surfaces also can be enhanced by the use of discrete PEG linkers. Nishimura et al. (2005) modified an amino surface with a NHS-PEG -maleimide crosslinker to create a hydrophilic self-assembled monolayer (SAM) surface that was thiol reactive for the conjugation of sulfhydryl-modified RNAs. This array then was used to investigate the binding specificity of synthetic kanamycins with selected RNA sequences to prove the specific interaction of ribosomal RNA with this molecule. The PEG linkers on surfaces provide lower nonspecific binding character than alkyl linkers, when preparing SAM surfaces for affinity interactions. 

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