Self-assembled monolayers pinholes

Studies have shown that defects, such as pinholes or grain boundaries, exist in the self-assembled monolayers.11,25 In addition to the irregularities introduced during the self-assembly process, another cause of such defects is the roughness of the substrate surface. For... 

On the other hand, it is possible to study electron transfer to an electroactive species held at a fixed distance (10-30 A) from the electrode surface by a suitable spacer, such as an adsorbed monolayer (Section 14.5.2) (75, 76). One approach is based on the use of a blocking monolayer, such as a self-assembled monolayer of an alkane thiol or an insulating oxide film, to define the distance of closest approach of a dissolved reactant to the electrode. This strategy requires knowledge of the precise thickness of the blocking layer and assurance that the layer is free of pinholes and defects, through which solution species might penetrate (Section 14.5). Alternatively, the adsorbed monolayer may itself... 

FIGURE 5.7 Schematic view of the processes involved in the SECM measurements of ET across an electroactive SAM. (A) Mediated ET, (B) direct electron tunneling through monolayer, and (C) ET through pinholes. (Adapted from Liu, B., Bard, A.J., Mirkin, M.V., and Creager, S.E., Electron transfer at self-assembled monolayers measured by scanning electrochemical microscopy, J. Am. Chem. Soc., 126, 1485-1492, 2004. Copyright 2004 American Chemical Society.)... 

Van der Waals forces between the methylene groups orient the monolayer. Such a self-assembly process thus results in well-organized and stable mono-layers, with the hydrocarbon tails packed parallel to each other, tilted at -30° relative to the surface normal (Fig. 4.14). The closely-packed pinhole-free films (surface coverage of 9 x 10 10mol/cm2) block transport of species to the... 

Time dependencies measured in Ref. 26 showed that the adsorption of lipid at the ITIES is fully reversible. The lipid adsorption on and desorption from the ITIES occur at about the same time scale. The monolayer self-assembled at the ITIES is in dynamic equilibrium with dissolved lipid molecules that allows any pinhole defects to be healed. In contrast, self-assembly on the solid/liquid interface often involves irreversible adsorption and requires high purity of chemicals, complex pretreatment, and preparation protocols to obtain a low-defect-density molecular monolayer (32). 

A conventional BLM can also be formed by a modified L B technique. It uses essentially a lipid monolayer spread from lipid in a volatile solvent (e.g., -hexane) on each aqueous chamber with the aqueous solution below the hole. Raising the aqueous solution level on both sides above the hole results in the two monolayers combining into a BLM in the aperture, thereby forming a so-called solvent-free BLM.It should be mentioned, however, that there is a major difference between the BLM and multilayers formed by the L B technique. A BLM, formed either by the conventional painting method or self-assembling on substrates (e.g., freshly cleaved metallic wire and agar gel—see later sections) is a dynamic liquid-like structure which is capable of accommodating a host of modifiers such as polypeptides, proteins, oil-soluble compounds, etc. In contrast, a L B multilayer of bimolecular thickness, albeit more stable than a BLM, usually contains pinholes and is in a solid state [16,17]. 

Lu X, Yuan H, Zuo G, Yang J (2008) Study of the size and separation of pinholes in the self-assembled thiol-porphyrin monolayers on gold electrodes. Thin Solid Films 516 (18) 6476-6482... 

---