Hydrophobic surfaces, enhancement

Figure 15. The electrowetting effect. (According to Mugele et al. [260].) (a) If a voltage V is applied between a liquid and an electrode separated by an insulating layer, the contact angle of the liquid-solid interface is decreased and the droplet flattens , (b) Hydrophobic surfaces enhance the effect of electro wetting. For electrowetting on dielectrics (EWOD) several individual addressable control electrodes (here on the bottom) and a large counter-electrode are used. The droplet is pulled to the charged electrodes.

Density functional theory study of aqueous-phase rate acceleration and endo/exo selectivity of the butadiene and acrolein Diels-Alder reaction72 shows that approximately 50% of the rate acceleration and endo/exo selectivity is attributed to hydrogen bonding and the remainder to bulk-phase effects, including enforced hydrophobic interactions and cosolvent effects. This appears to be supported by the experimental results of Engberts where a pseudothermodynamic analysis of the rate acceleration in water relative to 1-propanol and 1-propanol-water mixtures indicates that hydrogen-bond stabilization of the polarized activated complex and the decrease of the hydrophobic surface area of the reactants during the activation process are the two main causes of the rate enhancement in water.13... 

The ProteinChip System from Ciphergen Biosystems uses patented SELDI (Surface-Enhanced Laser Desorption/Ionization) ProteinChip technology to rapidly perform the separation, detection, and analysis of proteins at the femtomole level directly from biological samples. ProteinChip Systems use ProteinChip Arrays which contain chemically (cationic, anionic, hydrophobic, hydrophilic, etc.) or biochemically (antibody, receptor, DNA, etc.) treated surfaces for specific interaction with proteins of interest. Selected washes create on-chip, high-resolution protein maps. This protein mass profile, or reten-tate map of the proteins bound to each of the ProteinChip Array surfaces, is quantitatively detected in minutes by the ProteinChip Reader. 

Also for MALDI, there is a special case worth mentioning. Surface-enhanced laser desorption/ionization (SELDI) is a technique that utilizes special sample plates [196, 197]. These have different modified surfaces, for example, hydrophobic, anionic, or antibody treated. Which type of surface to select depends on the application. After application of analyte the surface is washed according to a protocol leaving only the desired components on the target. Finally, a MALDI matrix is applied before analysis in the spectrometer. See Chapter 12 for an application example of SELDI. 

Fullerene showed antibacterial activity, which can be attributed to different interactions of C60 with biomolecules (Da Ros et al., 1996). In fact, there is a possibility to induce cell membrane disruption. The fullerene sphere seems not really adaptable to planar cellular surface, but for sure the hydrophobic surface can easily interact with membrane lipids and intercalate into them. However, it has been demonstrated that fullerene derivatives can inhibit bacterial growth by unpairing the respiratory chain. There is, first, a decrease of oxygen uptake at low fullerene derivative concentration, and then an increase of oxygen uptake, which is followed by an enhancement of hydrogen peroxide production. The higher concentration of C60 seems to produce an electron leak from the bacterial respiratory chain (Mashino et al., 2003). 

Avseenko et al. (2001) immobilized antigens onto aluminum-coated Mylar films by electrospray (ES) deposition. Various surface modifications of the metallized films were studied to determine their abilities to enhance sensitivity. The plastic surfaces were firsf cleaned by plasma discharge treatment, followed by coating with proteins (BSA and casein) or polymers such as poly (methyl methacrylate) or oxidized dextran, or they were exposed to dichlorodimethyl silane to create hydrophobic surfaces. Protein antigen was prepared in 10-fold excess sucrose and sprayed onto the surfaces to form arrays with spot diameters between 7 and 15 pm containing 1 to 4 pg protein. 

In a related approach, arrays with different types of surface chemistries such as hydrophobic, hydrophilic, anionic, and affinity are used to absorb certain protein groups from biological or patient samples. The chip-absorbed proteins are then directly detected by surface-enhanced laser desorption/ionization time-of-flight MS (SELDl-TOF MS) (Issaq et al. 2002). The resulting protein masses can be used in pattern analysis and thereby provide a useful diagnostic tool. 

Moreover, fluorine substitution reduces polarisability, increases the hydrophobic surface area and provides an enhanced driving force for desolvatation (estimated driving force 0.2-0.5 kcal/mol) [13]. 

Most work which has been done has used metal oxides as the adsorbent. There is a need for more work on more hydrophobic surfaces (such as that in Chapter 17). Most systematic studies on metal oxides have used fairly homogeneous surfaces. For practical application to situations where adsorption on more heterogeneous surfaces is of interest (e.g., detergency, flotation, enhanced oil recovery), more study must be done on these... 

The stronger excitonic interaction in EB assemblies than that of ACR or AMAC is apparently due to a greater hydrophobic surface area of the former, as estimated from computer modeling studies (MacSpartan). Such increased hydrophobic surface is not expected from their structures (three six-membered ring systems) it also results in an enhanced entropic contribution to the binding energy when the probe is transferred from the aqueous phase to the interior of BAZrP, where there is little or no water. Therefore, the formation of these supramolecular assemblies may indeed involve a large entropic component, but this needs to be demonstrated experimentally. 

The hydrophobias are a case where protein nanofibers can play a dual role in creating a biosensor. They can aid in the immobilization of bioactive components within a biosensor and also add further functionality to the transducing element of a biosensor device. Hydrophobins are self-assembling [3-sheet structures observed on the hyphae of filamentous fungi. They are surface active and aid the adhesion of hyphae to hydrophobic surfaces (Corvis et al., 2005). These properties can be used to create hydrophobia layers on glass electrodes. These layers can then facilitate the adsorption of two model enzymes glucose oxidase (GOX) and hydrogen peroxidase (HRP) to the electrode surface. The hydrophobin layer also enhances the electrochemical properties of the electrodes. 

PMBQ represents a special class of polyelectrolytes with hydrophobic functional groups, which tends to form aggregates in solution and exhibits a high affinity to hydrophobic surfaces. The presence of hydrophobic parts in PMBQ enhanced its flocculation performance in different dispersions. We have shown that formation of hydrophobic aggregates of PMBQ was responsible for the considerable broadening of the flocculation window that is extremely important and desirable for industrial application. 

Using SFS, Davies and co-workers [77-79] reported enhanced adsorption and competitive adsorption at the hydrophobic surface, reminiscent of that seen at the air-solution interface. For the SDS/PEO mixture [79], competitive adsorption was observed at low concentrations, whereas at higher SDS concentrations, PEO was depleted from the surface. Similar observations were made from IR-ATR measurements by Poirier et al. [80] on CieTAB/PSS mixtures at the silica-solution interface. However, the technique could not distinguish between depletion or surface complex formation. Similar trends were also reported by Fielden et al. [76] for SDS/AM-MAPTC mixtures on mica. For the PEI/SDS mixture at the hydrophobic interface [76], the SFS measurements indicated a higher degree of order and hence adsorption due to complexation at the interface. This was also shown to be strongly pH dependent [81],... 

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