Gold-based SAMs

The modification of glass surfaces can be readily achieved through silanisation chemistry (Figure 3.2). In contrast to gold-based SAMs, the silanisation reactions typically require slightly harsher conditions. They are therefore generally not carried out in the presence of the peptides but are used to modify the glass surface with other functionalities before attachment of the peptide material. 

The different stages of the preparation of peptide surfaces can be confirmed with surface-sensitive physical and chemical analysis techniqnes. For gold-based SAMs, Mrksich and co-workers have introduced a matrix-assisted laser desorption ionisation time-of-ftight (MALDI-TOF) mass spectrometry-based analysis procedure with which they are able to identify the presence of various surface functional groups via their mass (Yeo Mrksich, 2006 Yeo et al., 2003). Although this method is applicable to SAMs, it is not strictly a surface sensitive technique, as the desorption process in MALDI is not confined to the uppermost layer of a material. 

Fig. 14 Schemes of the Hg-drop LAJ incorporating AZO-based SAM (a) in the trans form and (b) in the cis form, (c) I-V curves for junctions assembled by incorporating either the trans or the cis isomers, (d) I-V curves measured under in-situ photoisomerization by alternating irradiation at X = 370 nm and X = 450 nm through the semitransparent gold electrode...

While electrochemical experiments provide useful information regarding electron transport through these molecular monolayers, construction of real devices requires formation of a top contact so that solid-state transport measurements can be made. The fabrication of contacts to molecular layers has been the major obstacle to the development of molecular electronic devices, whether based on thiol-based SAMs on gold or covalently attached molecules on silicon. The most popular approach to making contacts involves evaporation of metals onto the molecular layer, which is likely to result in at least partial penetration of the monolayer, and may possibly damage the molecules in the layer. 

Thiols and related molecules have the following attractive features. The mono-layers can be easily formed on gold and other metals by exposing them to the thiol dissolved in solution. A particularly important feature of thiol-based SAMs is the strength of the thiol-metal bond. This high bond strength, typically of the order of... 

Alkanethiol-based or alkylsiloxane-based SAMs have been profitably employed in all these instances to probe the distance effect in electron-transfer dynamics. The thiol-based SAMs have the virtue that the spacer length can be predictably altered simply by varying the number of methylene units in the chain. The distance dependence of is embodied in the parameter f, the decay coefficient (for a critical discussion of the subtleties involved in the extraction and interpretation of this parameter, see Ref. [399]). A value of 0.49 0.07 has been reported for f for n-InP-alkanethiol-ferrocyanide interfaces [403]. This value is smaller than its counterpart for corresponding films on gold surfaces which range from 0.6 to 1.1 per methylene unit. The reason for this difference is not entirely clear, although several hypotheses were advanced by the authors [403]. 

Representative examples of such types of measurement involve thiol-based SAMs on gold, with ferrocene as the electroactive group, and hydrocarbons of varying degrees... 

We can synthesize various component molecules in which predesigned structures are specifically coded with the rules necessary for the molecular assembly. This looks as if hiomolecules in nature, such as DNAs or proteins, are coded by the structural specificity to express their functions. In the case of an alkanethiol-hased SAM, the code is set in two structural parts, that is, a thiol substituent and an alkane chain. The thiol substituent is a soft binder effective to specific surfaces, such as gold, to which the component molecules can be chemically adsorbed from the solution or gas phase [5]. The alkane chain is a building block to form a monolayer and can serve as the structural unit that enables the spontaneous organization of the tightly packed mono-layer through van der Waals interaction (or hydrophobic interaction in water) between the alkane chains. A schematic of alkanethiol-based SAM structure is shown in Fig. 1 [1]. 

Fig. 1 A schematic of alkanethiol-based SAM structure depicted using a structural model of an n-dodecanethiolate SAM (C12-SAM) on a gold (111) substrate [1], The definition of film thickness and tilt angle ((/>).

STM All thiol-based SAMs assembled spontaneously on gold have never exhibited perfect crystalline domains with single molecular lattice. STM is capable of observing the appearance of local disorder or molecular pinholes as a real-space imaging with atomic resolution. This method makes it possible to characterize such local... 

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