Monolayers self assembled

The self-assembly of an imprinted layer on the surface of a transducer was realized through the adsorption of the template on gold, Si02, or ln02 surfaces followed by treatment with an alkylthiol or organosilane (Hirsch et al. 2003). The first example of this type of sensor was reported in 1987 by Tabushi and coworkers (1987). Octadecylchlorosilane was chemisorbed in the presence of n-hexadecane onto tin dioxide or silicon dioxide for electrochemical detection of phylloquinone, menaqui-none, topopherol, cholesterol, and adamantane. Another MlP-based sensor was 

Self-Assembled Monolayers, Biological Membranes, and Biosensors 

Adsorbed organic layers on surfaces are models for biological bUayers, lipid membranes, and biosensors. EIS is an important tool in the study of these materials. In this chapter, the study of self-assembled monolayers, bUayers, and biosensors will be presented. 

Usually, quite compact layers are obtained. The simplest electrical equivalent model represents the solution resistance in series with the capacitance of a SAM, CsAM (Fig- 12.2a). More detailed analysis reveals that the layers are rarely purely capacitive and their capacitance is in parallel with their resistance, Rsam. leading to a circuit R iCsam Rsam)- Moreover, a diffuse double layer exists at the SAM/solution interface [485,486]. In such a case, the electrical equivalent circuit contains a diffuse-layer capacitance, C, in parallel with the resistance, (Fig. 12.2b). 

However, the SAMs formed are rarely ideal and they contain small defects called pinholes, e.g., bare metal sites or other defects. In such cases, an additional branch must be added in parallel (Fig. 12.2c) consisting of the resistance, R, in series with the parallel connection of the pinhole resistance, Rp, and its capacitance, Cph [488, 489]. The surface coverage of the pinholes is usually very small and does not influence very much the electrode capacitance. The presence of such pinholes can be easily detected using cyclic voltammetry or EIS [483, 490, 491]. The simplified electrical equivalent model of the redox reaction in the presence of pinholes is displayed in Fig. 12.3 [490,492], 

EIS allows for a good characterization of SAMs in the absence and presence of redox species in solution and attached to aUcyl chains. These studies allowed for testing the theory of electron tunneling. SAMs are also used in biosensors (see below). 

In their study, Park et al.100 investigated the frictional properties of fluorine-terminated alkanethiol SAMs grafted to gold surfaces. The frictional properties of the system were investigated by sliding two SAMs past one another at velocities in the stick-slip regime under various external loads. The simulations yield the shear stress as and the kinetic friction coefficient pk can be estimated from the slope of a plot of as versus load, using the relationships contained in Eqs. [4] and [5]. 

An emerging subdiscipline of tribological simulation involves the study of tribochemical reactions—that is, reactions that are activated by pressure and shear. These reactions alter the structure of lubricants and films that are used to protect surfaces from wear. Understanding the effects of these reactions on the intended behavior of these films is of utmost importance. However, simulation studies of tribochemical reactions have been impeded by the difficulty in accurately describing changes in chemical bonding. In a limited number of cases, this can be achieved with the use of reactive FFs, as noted above, whereas in other cases, one must resort to expensive quantum chemical calculations. In this section, we will describe two studies where such methods were used to examine tribochemical reactions. 

The results demonstrated that both compression and shear can induce the formation of C-C bonds between sp-hybridized carbons atoms, which leads to polymerization within the SAM. Interestingly, it was found that the location of these reactive sites within the film could influence the calculated friction. For instance, if the diacetylene components in the chains were close to the tip/film interface, reactions between the film and tip could occur, which led to wear and high friction. On the other hand, if the diacetylene moieties were far from the tip, the reactions did not lead to wear and had little effect on the average calculated friction. These observations demonstrate that a proper treatment of the chemical reactivity of the system may be necessary in some cases to calculate friction accurately. 

Reactive FFs can only be applied to a few specific cases for which they have been developed, such as the hydrocarbon systems discussed in the first part of this section. For other systems, describing tribochemical reactions requires the use of quantum chemical methods. In recent studies, such methods have been applied to investigate the behavior of zinc phosphates (ZPs) in response to high pressures. ZPs form the basis of anti-wear films derived from zinc dialkyldithiophosphates (ZDDPs), which are additives that have 

One key experimental observation regarding the ZP films is that the films found on the tops of asperities are stiffer and exhibit chemical spectra indicative of longer phosphate chain lengths than films found in the valleys between asperities. These observations that differences in the conditions at the two distinct locations alter the elastic and chemical properties of the films. One of the key differences between the tops of asperities and the valleys is the pressure experienced by the zinc phosphates. Since the highest pressures, and greatest potential for wear, are achieved at the tops of the asperities, determining the response of ZPs to these pressures may aid in developing a clear picture of how the anti-wear films work. 

FIGURE 4-13 Structures of common polymeric coatings (a) Nafion, (b) polyvinyllferro-cene (c) polyvinylpyridine id) polypyrrole. 

Cleavage of the S-H bond is central to this monolayer formation  

FIGURE 4-14 Formation of a self-assembled monolayer at a gold sustrate. (Reproduced 

It is not clear yet, how the chemical bond appears either thiols and disulfides bind as thio-lates, whereby the thiol loses its hydrogen atom and the disulfide dissociates, or both bind as 

Silanes react with silanol groups ( SiOH) on silicon surfaces according to the reaction shown in Fig. 10.5 [423] (review Ref. [424]). This reaction, which is usually carried out in organic solvents, is called silanization. Trifunctional silanes bind to the surface silanol groups, 

On the other hand, water molecules in traces, as they appear on the hydrophilic silica surface, activate the surface as well as the silanes. This is due to the two-step process of the silanization. When a silane approaches the surface the groups X will react with the water to form silanol SiOH groups, a reaction called hydrolysis. The hydrolysis determines the overall reaction speed and depends on the groups X [425], It decreases in the series Cl OMe OEt. After hydrolysis the silanol condensates in a second step with the silanol groups from the silica surface. 

Although thiols on gold and silanes on silicon are the most prominent examples, they are not the only self-assembling systems. Various substrates beside gold and silicon oxide are capable of binding SAMt s such as silver, copper, palladium, platinum and metal oxides as zirconium oxides and indium tin oxide (ITO). As organic molecules alcohols ( OH), amines ( NH2), organic acids ( COOH), or isocyanides ( NCO) have been used. 

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 

Cleavage of the S-H bond is eentral to tliis monolayer formation RSH -I- An RS-Au + e- +U+ 

FIGURE 5.7 Schematic representation of electrode preparation and signal generation scheme for the immunoassay based on a film of TTCA. (Reprinted from [35] with permission from Elsevier.) 

S-H bond to the gold surface, followed by the reductive elimination of hydrogen, as shown in Eq. (2) [37]. 

Electrochemical Sensors, Biosensors and Their Biomedical Applications 

FIGURE 5.8 Schematic representation depicting the 20° to 30° tilt angle of alkanethiol chains in a SAM on a gold surface. (Reprinted in part with permission from [39]. Copyright 2005 American Chemical Society.) 

SAM Thin layers controlled by molecule size Steric problems during deposition Immersion 

LB Thin layers of controlled thickness Requires special equipment Langmuir- 

LBL Controlled thickness and wide range in film thickness Limited stability Dip coating 

Immobilization Stable films Requires often high-tempraature Solvent casting 

Plasma and Clean technique No control of polymer structure In-glow discharge 

SAMs represent a powerful approach to covering a surface in a layer of a given functionality. In many cases moderate surface contamination is displaced by the SAM-forming reaction , meaning that surface functionalization with a high degree of perfection can be achieved under ambient conditions. 

of course possible to adsorb more than one single SAM-forming molecule (with different head groups) on a surface, in order to have more control over the precise functionality that is exposed. The density of the functionality can also be tuned by mixing the adsorbate containing the desired head group with an adsorbate terminated in an inert head group that has no function for the particular applica- 

The focus of attention in investigations of SAMs has been on the structure of the adsorbate, and a wide range of spectroscopic and other methods have been applied to elucidate the nature of adsorbed species. In addition, the skills of the organic chemist have been utilized to construct special molecular architectures and incorporate a variety of elements and functional groups for the spectroscopists to target. The kinetics and mechanisms of formation have received much less attention than structural aspects, SAMs usually being considered to form spontaneously.  

The literature on SAMs will be considered in this chapter only when there is information that elucidates the adsorption of thiols relevant to flotation, and the results of these publications will be cited in the following 

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A logical division is made for the adsorption of nonelectrolytes according to whether they are in dilute or concentrated solution. In dilute solutions, the treatment is very similar to that for gas adsorption, whereas in concentrated binary mixtures the role of the solvent becomes more explicit. An important class of adsorbed materials, self-assembling monolayers, are briefly reviewed along with an overview of the essential features of polymer adsorption. The adsorption of electrolytes is treated briefly, mainly in terms of the exchange of components in an electrical double layer. 

As discussed in Chapter III, the progression in adsoiptivities along a homologous series can be understood in terms of a constant increment of work of adsorption with each additional CH2 group. This is seen in self-assembling monolayers discussed in Section XI-IB. The film pressure r may be calculated from the adsorption isotherm by means of Eq. XI-7 as modified for adsorption from dilute solution ... 

Deposited Langmuir-Blodgett films take on many of the same stmctures as the Langmuir monolayers discussed in Section IV-4C, and they are often compared to the self-assembling monolayers described in Section XI-IB. The area... 

Investigate the differences between LB films and self-assembled monolayer SAMs (Chapter XI). Which are finding more practical use, and what are the potential applications of each ... 

Freunscht P, Van Duyne R P and Schneider S 1997 Surface-enhanced Raman spectroscopy of trans-stilbene adsorbed on platinum- or self-assembled monolayer-modified silver film over nanosphere surfaces Chem. Phys. Lett. 281 372-8... 

Straniok S J, Parikh A N, Tao Y-T, Allara D L and Weiss P S 1994 Phase separation of mixed-oomposition self-assembled monolayers into nanometer soale moleoular domains J. Phys. Chem. 98 7636... 

Salmeron M, Neubauer G, Folch A, Tomitori M, Ogletree D F and Sautet P 1993 Viscoelastic and electrical properties of self-assembled monolayers on Au(111) films Langmuirs 3600... 

Salmeron M, Liu G-Y and Ogletree D F 1995 Molecular arrangement and mechanical stability of self-assembled monolayers on Au(111) under applied load Force in Scanning Probe Methods ed H-J Guntherodt et al (Amsterdam Kluwer)... 

Figure Bl.22.3. RAIRS data in the C-H stretching region from two different self-assembled monolayers, namely, from a monolayer of dioctadecyldisulfide (ODS) on gold (bottom), and from a monolayer of octadecyltrichlorosilane (OTS) on silicon (top). Although the RAIRS surface selection rules for non-metallic substrates are more complex than those which apply to metals, they can still be used to detemiine adsorption geometries. The spectra shown here were, in fact, analysed to yield the tilt (a) and twist (p) angles of the molecular chains in each case with respect to the surface plane (the resulting values are also given in the figure) [40].

C2.4 Organics films (Langmuir-Blodgett films and self-assembled monolayers)... 

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. 

Gao W, Dickinson L, Grozinger C, Morin F G and Reven L 1996 Self-assembled monolayers of alkylphosphonic acids on metal oxides Langmuir M 6429-35... 

Sheen C W, Martensson J, Shi J, Parikh A N and Allara D L A 1992 New class of organized self-assembled monolayers—alkane thiols on GaAs(IOO) J. Am. Chem. Soc. 114 1514-5... 

Aronoff Y G, Chen B, Lu G, Seto C, Schwartz J and Bernasek S L 1997 Stabilization of self-assembled monolayers of carboxylic acids on native oxides of metals J. Am. Chem. Soc. 119 259-62... 

Laibinis P E, Flickman J J, Wrighton M S and Whitesides G M 1989 Orthogonal self-assembled monolayers— alkanethiols on gold and alkane carboxylic-acids on alumina Science 245 845-7... 

Folkers J P, Gorman C B, Laibinis P E, Buchholz S and Whitesides G M 1995 Self-assembled monolayers of long-chain hydroxamic acids on the native oxides of metals Langmuir 813-24... 

Woodward J T, Ulman A and Schwartz D K Self-assembled monolayer growth of octadecylphosphonic acid on mica Langmuir 12 3626-9... 

Deiamarche E, Michei B, Biebuyck FI A and Gerber C 1996 Golden interfaces the surface of self-assembled monolayers Adv. Mater. 8 719-29... 

Kumar A, Biebuyok FI A and Whitesides G M 1994 Patterning self-assembled monolayers—applioations in materials soienoe Langmuir 10 1498-511... 

Wilbur J L, Kumar A, Kim E and Whitesides G M 1994 Miorofabrioation by mioroeontaot printing of self-assembled monolayers Adv. Mater. 6 600-4... 

Pale-Grosdemange C, Simon E S, Prime K L and Whitesides G M 1991 Formation of self-assembled monolayers by ohemisorption of derivatives of oligo(ethylene glyool) of struoture FIS(CFl2),, (OCFl2CFl2)meta-OFI on gold J. Am. Chem. Soc. 113 12-20... 

Flaussling L, Ringsdorf FI, Sohmitt F J and Knoll W Biotin-funotionalized self-assembled monolayers on gold— surfaoe-plasmon optioal studies of speoifio reoognition reaotions Langmuir 1837-40... 

Spinke J, Liley M, Sohmitt F J, Guder FI J, Angermaier L and Knoll W 1993 Moieouiar reoognition at self-assembled monolayers—optimization of surfaoe funotionalization J. Chem. Phys. 99 7012-9... 

Sohierbaum K D, Weiss T, Thoden van Velzen E U, Engbersen J F J, Reinhoudt D N and Gdpel W 1994 Moieouiar reoognition by self-assembled monolayers of oavitand reoeptors Sc/e/ ce 265 1413-5... 

Harder P, Grunze M, Dahint R, Whitesides G M and Laibinis P E 1998 Molecular conformation in oligo(ethylene glycol)-terminated self-assembled monolayers on gold and silver surfaces determines their ability to resist protein adsorption J. Rhys. Chem. B 102 426-36... 

Feldman K, Hahner G, Spencer N D, Harder P, and Grunze M 1999 Probing resistance to protein adsorption of oligo(ethylene glycol)-terminated self-assembled monolayers by scanning force microscopy J. Am. Chem. Soc. at press... 

DiMilla P A, Folkers J P, Biebuyck H A, Harter R, Lopez G P and Whitesides G M 1994 Wetting and protein adsorption of the self-assembled monolayers of alkanethiolates which are supported on transparent films of gold J. Am. Chem. Soc. 116 2225-6... 

Batchelder D N, Evans S D, Freeman T L, Haussling L, Ringsdorf H and Wolf H 1994 Self-assembled monolayers containing polydiacetylenes J. Am. Chem. See. 116 1050-3... 

Koutsos V, van der Vegte E W, Grim PCM and Hadziioannou G Isolated polymer chains via mixed self-assembled monolayers morphology and friction studied by scanning force microscopy Macromolecules 116-23... 

Hostetler M J, Stokes J J and Murray R W 1996 Infrared speotrosoopy of three-dimensional self-assembled monolayers n-alkanethiolate monolayers on gold oluster oompounds Langmuir M 3604... 

Colvin V L, Goldstein A N and Alivisatos A P 1992 Semioonduotor nanoorystals oovalently bound to metal surfaoes using self-assembled monolayers J. Am. Chem. Soc. 114 5221... 

Siepmann JI and IR McDonald 1993a. Domain Formation and System-size Dependence in Simulatio of Self-assembled Monolayers. Langmuir 9 2351-2355. 

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