Monolayer self-spreading

Figure 13.6 (a) Confocal micrograph of a circularly self-spreading lipid monolayer. A rhodamine-labeled lipid is doped to visualize the spreading behavior, (b) A schematic illustration of the front edge of the self-spreading lipid monolayer [51]. 

In addition to the self-spreading lipid bilayer, it was also found that a lipid mono-layer showed similar spreading behavior on a hydrophobic surface (Figure 13.6) [51]. By fabricating an appropriate hydrophobic surface pattern, the spreading area and direction can be easily controlled. For both the self-spreading bilayer and monolayer, non-biased molecular transportation is an important key concept for the next generation of microfiuidic devices. 

The monolayer resulting when amphiphilic molecules are introduced to the water—air interface was traditionally called a two-dimensional gas owing to what were the expected large distances between the molecules. However, it has become quite clear that amphiphiles self-organize at the air—water interface even at relatively low surface pressures (7—10). For example, x-ray diffraction data from a monolayer of heneicosanoic acid spread on a 0.5-mM CaCl2 solution at zero pressure (11) showed that once the barrier starts moving and compresses the molecules, the surface pressure, 7T, increases and the area per molecule, M, decreases. The surface pressure, ie, the force per unit length of the barrier (in N/m) is the difference between CJq, the surface tension of pure water, and O, that of the water covered with a monolayer. Where the total number of molecules and the total area that the monolayer occupies is known, the area per molecules can be calculated and a 7T-M isotherm constmcted. This isotherm (Fig. 2), which describes surface pressure as a function of the area per molecule (3,4), is rich in information on stabiUty of the monolayer at the water—air interface, the reorientation of molecules in the two-dimensional system, phase transitions, and conformational transformations. 

Protein A is a cell-wall protein of Staphylococcus aureus with a molecular weight of 42,000. Since protein A binds specifically to the Fc part of IgG from various animals, it has been widely used in immunoassay and affinity chromatography. We found that protein A could be spread over the water surface to form a monolayer membrane by the LB method [21]. On the basis of this finding, an antibody array on the solid surface can be obtained by the following two steps. The first step is fabrication of an ordered protein A array on the solid surface by the LB method. The second step is self assembly of antibody molecules on the protein A array by biospecific affinity between protein A and the Fc of IgG as shown in Fig.34. 

The Gibbs adsorption theory (Birdi, 1989,1999, 2002, 2008 Defay et al., 1966 Chattoraj and Birdi, 1984) considers the surface of liquids to be monolayer. The surface tension of water decreases appreciably on the addition of very small quantities of soaps and detergents. The Gibbs adsorption theory relates the change in surface tension to the change in soap concentration. The experiments that analyze the spread monolayers are also based on one molecular layer. The latter data indeed conclusively verifies the Gibbs assumption (as described later). Detergents (soaps, etc.) and other similar kind of molecules are found to exhibit self-assembly characteristics. The subject related to self-assembly monolayer (SAM) structures will be treated extensively (Birdi, 1999). However, no procedure exists that can provide information by direct measurement. The composition of the surface of a solution with two components or more would require additional comments. 

Not all solvents are equally suitable for spreading monolayers. The requirement that the solvent evaporate completely is self-evident. It has been suggested that if the organic solvent dissolves much water, the properties of the monolayer will be different from those in which no water is trapped. Verification that no artifacts are entering the observations from the solvent may be accomplished by conducting duplicate experiments with different solvents. 

To attain minimum permeability to evaporation, a close-packed monolayer under a sufficient state of compression to squeeze out any surface impurities is required. The monolayer must also be self-healing in response to adverse meteorological conditions such as wind, dust and rainfall, so that spreading ability is also needed. To compromise between these requirements, commercial cetyl alcohol (which also contains some steryl, myristyl and oleyl alcohol) has been used successfully. 

Microcontact Printing. An ink of alkanethiols is spread on a patterned PDMS stamp. The stamp is then brought into contact with the substrate, which can range from coinage metals to oxide layers. The thiol ink is transferred to the substrate where it forms a self-assembled monolayer that can act as a resist against etching. 

Fig. 10.27). A fluorinated silane self-assembled monolayer was first applied to the master to aid in the separation of master and stamp. The stamp was formed using one of the following procedures. For thinner stamps, PDMS (Sylgard 184) pre-polymer mixture (1 10) was poured over the PR master and a 250 pm thick, 13-inch x 13-inch Mylar film was placed on top of the liquid PDMS. The prepolymer was evenly spread to less than 50 pm in thickness by rolling a Meyer rod over the surface of the Mylar backing film. 

A guided self-assembly of gold nanoparticles succeeded also on patterned GaAs surfaces. Stmctured Be-doped GaAs surfaces were first decorated with a monolayer of xylyl dithiol molecules. A uniform monolayer of dodecanethiol stabilized 5 nm Au particles, generated by spreading a hexane solntion... 

For DPPC monolayers spread on water there are two species in the system, producing three terms in Eq. 17 (two self-terms, /i o and h and one cross-term, hp ). 

Recently, Brzozowska et al. used NR and ex situ electrochemical techniques to characterize an innovative type of monolayer system intended to serve as a support for a bUayer lipid membrane on a gold electrode surface [51]. Zr ions were used to noncovalendy couple a phosphate-terminated self-assembled monolayer (SAM) formed on a gold surface to the carboxylate groups of negatively charged phos-phatidylserrne (PS). This tethered surface was then used for the formation of a PS hpid bilayer structure formed by vesicle fusion and spreading. NR studies revealed the presence of an aqueous environment associated with the tether layer which arises from nonstoichiometric water associated with the zirconium phosphate moieties [52]. 

Self assembled monolayers (SAMs) have aroused wide spread interest as they provide an opportunity to define the chemical functionality of surfaces with molecular precision, thus creating potential applications related to the control of wettability, biocompatibility and the corrosion resistance of surfaces of a wide range of materials II), Two families of SAMs have received the most attention, SAMs of alkanethiols on gold and alkylsilanes on silicon. 

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