Noncovalent adsorption

On the other hand, noncovalent adsorption leaves CNTs electronic and optical properties almost intact, as the type of modification is through van der Waals, 7T-7T stacking, H-bonding and electrostatic interactions, that do not determine any pyramidalization process. Such benefit is traded off by a decreased stability of CNT-functionalized systems, as a consequence of the weaker interactions. 

The noncovalent adsorption of proteins by p.CP is experimentally simple, but suffers from the disadvantage that the attachment can be reversible by rinsing the pattern with certain buffers and detergents or replacement by other proteins in solution. Moreover, the orientation of the deposited protein is not controlled. Delamarche et al. proposed the use of stamps modified with poly(ethylene oxide) silanes.100 The modification was conducted by oxidation of the PDMS stamp and reaction with APTES to yield an amino-functionalized surface. The next step was the reaction with homobifunctional cross-linker BS3 to bind surface amino groups with poly(ethylene glycol) (PEG) chains (Fig. 14.10). 

Figure 2 Immobilization strategies, (a) A schematic representation of noncovalent adsorption. The lipid tail of a neo-glycolipid adheres to a hydrophobic surface via noncovalent interactions, (b) Strategies used for covalent immobilization of glycans.

There are distinct methods to improve the enzyme stability either by simple coating of enzymes by ILs or by noncovalent adsorption or by covalent linking of the enzymes with the solid supports such as polymers, nanoparticles, carbon nanotubes, and encapsulation in hydrogels. One of the interesting and new features of ILs as an immobilization support for enzymes evolved out of its incompetence for retaining... 

Noncovalent adsorption of native carbohydrate probes on a substrate surface is the simplest way to prepare carbohydrate microarrays. This method relies on the formation of a variety of noncovalent interactions between the surface and the arrayed carbohydrates. In addition to its simplicity and high-throughput characteristics in array construction, these approaches may be favorable in supporting the preservation of the native structure of spotted carbohydrate antigens since there is no need to modify the carbohydrates before microarray application. However, given that the saccharides are noncovalently immobilized on an array substrate, the efficiency of immobilization must be verified for each spotted carbohydrate. 

Electrochemical polymerization of pyrrole on an SWNT electrode using an aqueous HCl 0.5 M solution as electrolyte, resulted in deposition of a PPy film onto the SWNT layer leading to a composite with a bilayer structure, as demonstrated by Raman spectroscopy [112]. Anew method was developed by S.Cosner eta/, in 2008 [111] SWNTs were functionalized by electropolymerizable pyrrole groups following covalent and noncova-lent strategies. The covalent pyrrole grafting was carried out by ester formation between pyrrole alcohol and chemically oxidized SWNTs. The strong Ti-interactions between pyrene and SWNTs were exploited for the noncovalent adsorption of a new pyrene-pyrrole derivative on the pristine CNT surface. The pyrrole-ester-SWNTs were solubilized in THE and electropolymerized by controlled potential electrolysis at 0.95 V. The PPy/SWNT... 

X-ray reflectivity measurements confirmed that after self-assembly and reaction of the alkoxysilyl group with the silicon oxide a 1-3 nm thick polymer layer had attached to the surface. One important issue was to clarify, whether the junction-point functional polymer was attached to the silicon surface by covalent bonds formed by the TEOS functionality, or by mere noncovalent adsorption of the PS chains at the surface. To shed light on this issue, we carried out additional deposition experiments using the allyl-functional block copolymer PS-(AGE)-PEO instead of the reactive PS-(TEOS)-PEO. X-ray reflectivity results for PS-(TEOS)-... 

The interactions between solutes and solvents are noncovalent in nature (barring the occurrence of chemical reactions), and therefore fall into the same category as those that govern molecular recognition processes, the formation and properties of liquids and solids, physical adsorption, etc. Hydrogen bonding, in its many manifestations, is a particularly prominent and important example. 

The vast majority of functionalization methods of carbon nanotubes belong to two broad categories (a) covalent and (b) noncovalent functionalization of the external CNT surface. The former is achieved by covalent attachment of functional groups to the C-C double bond of the n-conjugated framework. The latter is based on the adsorption through van der Waals type bonds of various functional entities. 

Bendahl, L., Hansen, S. H., and Gammelgaard, B. (2001). Capillary modified by noncovalent anionic polymer adsorption for the capillary zone electrophoresis, micellar electrokinetic capillary chromatography and capillary electrophoresis mass spectrometry. Electrophoresis 22, 2565-2573. 

CNTs can be easily doped by noncovalent means via molecular adsorption, an aspect that has been considerably exploited to develop ultrasensitive field effect transistor sensors [88-91]. However, substitutional doping with B and N to confer p and n character to the CNTs has also been carried out [92]. Such doped systems can be more susceptible to react with donors or acceptors molecules (depending on the doping) allowing the chemically reactivity to increase. 

Immobilisation methods are treated in detail in chapter 6. Most enzyme immobilisation methods used in coimection with non-conventional media rely on noncovalent interactions between the support and the enzyme. The reason why this works well in many cases is that enzymes normally have a low tendency to dissolve in the reaction media used. Adsorption or deposition on porous supports are often used methods. It is important to remember that other substances (for example salts and other polar substances) are often immobilised on the support because they are present during the immobilisation procedure and not soluble in the reaction medium. Those substances influence the microenviromnent of the enzyme and thereby its catalytic activity. 

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