Planar substrates, adsorption

Nanoparticles have been used extensively for the immobilization of biomolecules [3]. In addition to their biocompatibility they can produce a unique microenvironment that provides improvement in the freedom of orientation for affinity binding with advantages over planar substrates, an increase in surface area for higher probe loading capacities, and enhanced diffusion of amplification agents. Modification of electrode surfaces with nanoparticles can be carried out by simple electrostatic adsorption or covalent attachments such... 

Adsorption on Planar Substrates. The simplest type of adhesive bond occurs when a liquid is contacted with a planar solid with which it is totally immiscible and into which it cannot diffuse. Bonding is limited to physical and/or chemical adsorption at specific sites on the substrate surface. A sharp and planar interface is formed. This is the usual situation when an organic adhesive adheres to a very smooth inorganic substrate. The time-dependent process during which interfacial bonds form is called wetting. In general, it involves an increase in the... 

In recent years self-assembly processes involving electrostatic interactions have been used in order to build up multilayered materials. Polyelectrolyte multilayers can be formed by alternating exposure of a charged substrate to solutions of positive or negative polyelectrolytes [80]. This principle of layer formation has not only been achieved by adsorption to planar substrates, but even to colloidal particles [81], which offers the possibility for investigations by NMR methods. 

Immiscible Planar Substrates. Consider the simplest case in which a liquid adhesive is placed on a molecularly smooth solid substrate with which it is totally immiscible. The time-dependent process whereby the adhesive and substrate come into intimate contact is called wetting. The interface is a plane across which molecular forces of attraction, also denoted intrinsic adhesion, exist between the liquid and solid. These forces range in magnitude from strong covalent or ionic chemical bonds to weaker physical adsorption, e.g., H-bonding, dipole-dipole, and van der Waals interactions. 

Hereafter, we assume the polymers to form an adsorbed layer around the colloidal particles, with a typical thickness much smaller than the particle radius, such that curvature effects can be neglected. In that case, the effective interaction between the eolloidal particles with adsorbed polymer layers can be traced back to the interaction energy between two planar substrates covered with polymer adsorption layers. In the case when the approaeh of the two particles is slow and the absorbed polymers are in full equilibrium with the polymers in solution, the interaction between two opposing adsorbed layers is predominantly attractive [45, 46], mainly beeause polymers form bridges between the two siufaees. Recently, it has been shown that there is a small repulsive eomponent to the interaetion at large separations [47]. 

Measurements on Flat Surfaces. Using planar substrates, polymer adsorption can be studied under well-defined conditions of surface eneigies and known surface area. These measurements provide important insists into the structure and dynamics of adsorbed pol3rmers, which are often not experimentally achievable using dispersed particles. 

The artificial lipid bilayer is often prepared via the vesicle-fusion method [8]. In the vesicle fusion process, immersing a solid substrate in a vesicle dispersion solution induces adsorption and rupture of the vesicles on the substrate, which yields a planar and continuous lipid bilayer structure (Figure 13.1) [9]. The Langmuir-Blodgett transfer process is also a useful method [10]. These artificial lipid bilayers can support various biomolecules [11-16]. However, we have to take care because some transmembrane proteins incorporated in these artificial lipid bilayers interact directly with the substrate surface due to a lack of sufficient space between the bilayer and the substrate. This alters the native properties of the proteins and prohibits free diffusion in the lipid bilayer [17[. To avoid this undesirable situation, polymer-supported bilayers [7, 18, 19] or tethered bilayers [20, 21] are used. 

STM has also been used to examine the dynamics of potential-dependent ordering of adsorbed molecules [475-478]. For example, the reversible, charge-induced order-disorder transition of a 2-2 bipyridine mono-layer on Au(lll) has been studied [477]. At positive charges, the planar molecule stands vertically on the surface forming polymeric chains. The chains are randomly oriented at low surface charge but at higher potentials organize into a parallel array of chains, which follow the threefold symmetry of the Au(l 11) substrate as shown in Fig. 34. Similar results were found for uracil adsorption on Au(lll) and Au(lOO) [475,476]. 

If adsorption is a necessary prerequisite for electron transfer, one would intuitively expect that this would lead to a substrate-electrode complex of defined structure (e.g., an aromatic ring system would be held with its planar surface facing the electrode surface). The exact chemical consequences of this arrangement are hard to predict, but at least one implication would be one of stereochemistry. If the configuration ad sorbant-surface is held for a definite period of time after electron transfer has taken place, any following fast product-forming steps would occur with the intermediate shielded from chemical attack on one side by the electrode, and therefore the stereochemistry of the reaction should be affected. 

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