Monolayer sensing

Figure Bl.21.1 shows a number of other clean umeconstnicted low-Miller-index surfaces. Most surfaces studied in surface science have low Miller indices, like (111), (110) and (100). These planes correspond to relatively close-packed surfaces that are atomically rather smooth. With fee materials, the (111) surface is the densest and smoothest, followed by the (100) surface the (110) surface is somewhat more open , in the sense that an additional atom with the same or smaller diameter can bond directly to an atom in the second substrate layer. For the hexagonal close-packed (licp) materials, the (0001) surface is very similar to the fee (111) surface the difference only occurs deeper into the surface, namely in the fashion of stacking of the hexagonal close-packed monolayers onto each other (ABABAB.. . versus ABCABC.. ., in the convenient layerstacking notation). The hep (1010) surface resembles the fee (110) surface to some extent, in that it also...

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). 

In some of these models (see Sec. Ill) the surfactants are still treated as flexible chains [24]. This allows one to study the role of the chain length and chain conformations. For example, the chain degrees of freedom are responsible for the internal phase transitions in monolayers and bilayers, in particular the hquid/gel transition. The chain length and chain architecture determine the efficiency of an amphiphile and thus influence the phase behavior. Moreover, they affect the shapes and size distributions of micelles. Chain models are usually fairly universal, in the sense that they can be used to study many different phenomena. 

The first step in the preparation of an LB film is the successful spreading of a monolayer of the material of interest, which may be molecular, polymeric, or particulate and need not be amphiphilic in the traditional sense. This is accomplished by depositing drops of a dilute solution of the material in an appropriate spreading solvent onto the water surface. The concentration is generally millimolar or less, and the solvent selected should be one that will spread across the surface rapidly and evaporate without remaining at the surface or dissolving into the subphase. Common spreading solvents include chloroform, benzene,... 

Luderer F, Walschus U (2005) Immobilization of Oligonucleotides for Biochemical Sensing by Self-Assembled Monolayers Thiol-Organic Bonding on Gold and Silanization on Silica Surfaces. 260 37-56... 

Yoon, H.C., Lee, D., and Kim, H.-S. (2002) Reversible affinity interactions of antibody molecules at functionalized dendrimer monolayer affinity-sensing surface with reusability. Anal. Chim. Acta 456, 209-218. 

Xu, T., Zach, M.P., Xiao, Z.L., Rosenmann, D., Welp, U., Kwok, W.K. and Crabtree, G.W., Self-assembled monolayer-enhanced hydrogen sensing with ultra thin palladium films, Applied Physics Letters, 86, 203104, 2005. 

Tao, A. Kim, E Hess, C. Goldberger, J. He, R. Sun, Y. Xia, Y. Yang, P. 2003. Langmuir-Blodgett silver nanowire monolayers for molecular sensing with high sensitivity and specificity. Nano. Lett. 3 1229-1233. 

The two examples discussed here arc typical in the sense that metal adsorbates with atoms that are smaller than those of the substrate tend to form commensurate layers, while adsorbates with bigger atoms tend to form incommensurate monolayers [7]. 

A. Tao, F. Kim, C. Hess, J. Goldberger, R. He, Y. Sun, Y. Xia and P. Yang, Langmuir-Blodgett silver nanowire monolayers for molecular sensing using surface-enhanced Raman spectroscopy, Nano Lett., 3(9) (2003) 1229-1233. 

Still with an enzyme monolayer, the synthesis and current responses of a system that involves simultaneous attachment of the cosubstrate to the electrode coating are then described. The next step consists in constructing a multilayered coating constituted by successive layers of enzyme built thanks to antigen-antibody interactions. Sensing the diffusion of the cosubstrate through the film thus constructed provides evidence for spatial order and an estimate of the distances between layers. 

We have little information on the way low molecular weight molecules and oligomers adsorb (19). Apparently below DP s of about 100 they lie flat on the surface for concentrations up to a monolayer of segments, then seem to form thicker islands of smectic or nematic structure. Ordered condensed mono, -di, -or multi-layers are primarily the arrangements of smaller, especially amphipa-tic molecules on liquid-liquid interfaces. Polymers are too large to adsorb, in the ordinary sense, on micelles but segments of linear polymers may act as nucleation centers for micelles of small molecules which probably is one of the mechanisms for the lipid-, or detergent-, polymer interaction. 

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