Monolayer Langmuir-Schaefer

Honciuc A, Jaiswal A, Gong A, Ashworth K, Spangler CW, Peterson IR, Dalton LR, Metzger RM (2005) Current rectification in a Langmuir-Schaefer monolayer of fullerene-bis-[4-diphenylamino-4 -(N-ethyl-N-2 -ethyl)amino-l,4-diphenyl-l,3-butadiene] malonate between Au electrodes. J Phys Chem B109 857-871... 

The other method of monolayer transfer from the air/water interface onto solid substrates is illustrated in Figure 2. This method is called the Langmuir-Schaefer technique, or horizontal lift. It was developed in 1938 by I. Langmuir and V. Schaefer for deposition of protein layers. Prepared substrate horizontally touches the monolayer, and the layer transfers itself onto the substrate surface. The method is often used for the deposition of rigid monolayers and for protein monolayers, hi both cases the apphcation of the Lang-muir-Blodgett method produces defective films. 

FIG. 26 Cyclic voltammograms of 40 monolayers of Langmuir-Schaefer films of cytochrome P450SCC on indium-tin oxide glass plate (ITO) in 10 mM phosphate buffer at a scan rate of 20 mV/s between 0.4 and —0.4 V vs. Ag/AgCl. LS films on ITO worked as the working electrode, platinum as the counter, and Ag/AgCl as the reference electrode. Cholesterol dissolved in X-triton 100 was added 50 p.1 at a time (1) with cholesterol, (2) 50 p.1 of cholesterol, (3) 100 p.1 cholesterol, and (4) 150 p.1 of cholesterol. 

Fig. 8 shows an AFM image of a DPPC bilayer formed using a combination of the LB technique and the Langmuir-Schaefer (LS) method [18]. In this approach, the first monolayer is deposited onto the substrate using the LB dipping technique. The second monolayer is deposited on the first using the LS approach where the substrate is positioned parallel with the air-water interface and transferred through the interface. This results in a Y-type lipid bilayer supported on a substrate. 

Methodologies for the fabrication of biomimetic membranes vary somewhat from one biomimetic membrane to another. However, a number of experimental procedures for the formation of lipid monolayers and bilayers on solid supports are common to several biomimetic membranes. The most popular procedures are vesicle fusion, Langmuir-Blodgett and Langmuir-Schaefer transfers, and rapid solvent exchange. The formation of lipid monolayers and bilayers on gold and... 

Fig. 2 The construction of a polymer-cushioned lipid bilayer membrane. (A) Architecture constructed in a sequential way first, onto the functionalized substrate a polymer layer (cushion) is deposited by adsorption from solution and covalent binding, followed by the (partial) covalent attachment of a lipid monolayer containing some anchor lipids as reactive elements (B) able to couple the whole monolayer to the polymer cushion. (C) Alternatively, a lipopolymer monolayer, organized, e.g., at the water-air interface can be co-spread with regular low-mass amphiphiles and then transferred as a mixed monolayer onto a solid support, prefunctionalized with reactive groups, able to bind covalently to the polymer chains of the lipopolymer molecules, (B). (D) By a fusion step (or a Langmuir Schaefer transfer) the distal lipid monolayer completes the polymer-tethered membrane architecture...

This chapter deals primarily with monolayers of surfactants at fluid interfaces, but some attention is also given to (nano) coatings such as Langmuir-Blodgett (LB) and Langmuir-Schaefer (LS) films, self-assembled monolayers (SAMs), and layers obtained by alternating polyelectrolyte deposition. Such coatings may be applied for the functionalization of surfaces, for instance, to achieve biocompatibility of biomaterials, improve specificity and selectivity of biosensors and membranes, and control immobilization of enzymes or cells in bioreactors. 

Another procedure for transferring monolayers is the Langmuir-Schaefer (LS) method, in which the solid support is moved downward until it horizontally touches the monolayer at the liquid interface. After contacting for a few or a few tens of seconds the solid substrate is withdrawn or pushed down into the liquid subphase. The LS method is far less popular than the LB method but it may be preferred for use with monolayers that are rather rigid. 

In the Langmuir-Schaefer method, a flat substrate is placed horizontally onto a compressed monolayer on the liquid-air interface. When the substrate is lifted horizontally and separated from the water subphase, the monolayer is transferred onto the substrate (Figure 4). The method is useful to transfer viscous films as well as monolayers of lipids and proteins. ... 

The grafting-to method is based on the synthesis of polymer chains in solution, which are then physically or covalently bound to a solid surface. Various grafting-to methods have been used (i) spreading and fusion of micelles or vesicles on solid supports, (ii) transfer of monolayers (Langmuir-Blodgett (LB) and Langmuir-Schaefer (LS)), and (iii) LbL methods. ... 

The second way of preparing L-B monolayer structures, the horizontal lifting method, was introduced by Langmuir and Schaefer. In this method, a compressed monolayer first is formed at the water-air interface, and a flat substrate is then placed horizontally on the monolayer film. When the substrate is lifted and separated from the water surface, the monolayer is transferred onto the substrate, as depicted in Fig. 15(d). 

Viscosity, defined as the resistance of a liquid to flow under an applied stress, is not only a property of bulk liquids but of interfacial systems as well. The viscosity of an insoluble monolayer in a fluid-like state may be measured quantitatively by the viscous traction method (Manheimer and Schechter, 1970), wave-damping (Langmuir and Schaefer, 1937), dynamic light scattering (Sauer et al, 1988) or surface canal viscometry (Harkins and Kirkwood, 1938 Washburn and Wakeham, 1938). Of these, the last is the most sensitive and experimentally feasible, and allows for the determination of Newtonian versus non-Newtonian shear flow. 

Langmuir I, Schaefer VJ (1938) Activities of urease and pepsin monolayers. J Am Chem Soc 60 1351-1360... 

Figure 7.38. XPS spectrum of an ionic liquid, [EMIM][Tf2N], detailing the C(ls) and N(ls) regions. Since there are no peaks from the Au substrate, the film thickness is hkely >10nm. Also shown (right) is the comparison between XPS, ultraviolet photoelectron spectroscopy (UPS, Hel = 21.2eV, Hell = 40.8 eV radiation), and metastable impact electron spectroscopy (MIES). Whereas XPS and UPS provide information from the first few monolayers of a sample, MIES is used for zero-depth (surface only) analysis, since the probe atoms are excited He atoms that interact with only the topmost layer of sample. Full interpretations for these spectra may be found in the original work Hofft, O. Bahr, S. Himmer-lich, M. Krischok, S. Schaefer, J. A. Kempter, V. Langmuir 2006, 22, 7120. Copyright 2006 American Chemical Society.

Another way of transferring monolayers to solid substrates is the horizontal dipping method, first applied by Langmuir and Schaefer to deposit proteins onto... 

Proteins are found either not to desorb or to desorb only with great difficulty from quiescent interfaces. Langmuir and Schaefer (1939) calculated, on the basis of the Gibbs adsorption equation, that compression of a monolayer of protein of molecular weight 35,000 by 15 mN m-1 should increase its solubility by a factor of 1095. This results from the large area occupied by the molecule at the interface and the resultant large pressure increment of solubility. The failure of protein monolayers to desorb readily on compression was thus taken as an indication of irreversibility. 

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