Coatings, surface modification

Nanoparticle surface modification is of tremendous importance to prevent nanoparticle aggregation prior to injection, decrease the toxicity, and increase the solubility and the biocompatibility in a living system [20]. Imaging studies in mice clearly show that QD surface coatings alter the disposition and pharmacokinetic properties of the nanoparticles. The key factors in surface modifications include the use of proper solvents and chemicals or biomolecules used for the attachment of the drug, targeting ligands, proteins, peptides, nucleic acids etc. for their site-specific biomedical applications. The functionalized or capped nanoparticles should be preferably dispersible in aqueous media. 

In gas-solid chromatography (GSC) the stationary phase is a solid adsorbent, such as silica or alumina. The associated virtues associated therewith, namely, cheapness and longevity, are insufficiently appreciated. The disadvantages, surface heterogeneity and irreproducibility, may be overcome by surface modification or coating with small amounts of liquid to reduce heterogeneity and improve reproducibility 4,15). Porous polymers, for example polystyrene and divinyl benzene, are also available. Molecular sieves, discussed in Chapter 17, are used mainly to separate permanent gases. 

Podgorski, L. and Roux, M. (1999). Wood modification to improve the durability of coatings. Surface Coatings, 82(12), 590-596. 

In the following section, a new technique of surface modification of fillers and curing agents will be discussed plasma polymerization. This technique allows for surface coating of powders, whereby the chemical structure of the coating is determined by the monomer used for the process. The morphology of the substrate is preserved, which is an important precondition for filler treatment. The polarity of the functional groups can be chosen to fit the matrix of the polymer wherein it will be applied. 

Finally, the use of permanent chemical modifiers must be mentioned. Such chemical modification involves coating the graphite tube with a noble metal such as Ir, Pt, W, or Zr. These modifiers behave in much the same way as aqueous Pd in that thermally stable intermetallic compounds are formed on the hot inner surfaces of the coated graphite tube. 

Methyl methacrylate, accounting for 4% of methanol consumption, is produced by the cyanohydrin process utilizing methanol. Methyl methacrylate is used to produce acrylic sheet, surface coating resin, and molding and extrusion powder. Also, there exist minor miscellaneous uses such as modification of acrylic fiber and polyester resin. 

Surface coating using sol-gel technology is mainly used for the preparation of oxide or organofunctional siloxane layers on solid materials. In this case, the material is dipped or spun in the TEOS sol. This procedure leads to the formation of multilayered coatings of irreproducible thickness. Since this type of coating may only be used for axially or radially symmetric materials, alternatives have been developed. Those include spraying, electrophoresis, thermophoresis and ultrasonic pulverization.63 However, these are not of interest in powder modification. 

In the early 90 s, a new technique, Chemical Surface Coating (CSC) was developed by Vansant, Gillis-D Hamers, Van Der Voort and Vrancken, in order to create thin ceramic layers on a substrate by successive chemical modifications. The principles and developments of this technique are the subject of a separate section of this book (cfr. chapter 14). 

In the gas-phase modification of Chemical Surface Coating, the inorganic substrate is subjected to subsequent, single step, one component reactions. This process is repeated in a cyclic way. The reaction temperatures are very low, usually room temperature. In this way, the ceramic precursor is built. Its thickness is a function of the number of CSC cycles involved. A CSC cycle is a subsequent modification of the substrate with 2 gases. Finally, the ceramic precursor is converted towards a ceramic coating by a thermal treatment. 

In the liquid phase modification of Chemical Surface Coating, the treatment of gas 1 and gas 2 are replaced by a modification with e.g. an organosilane. In this case, the thickness of the ceramic precursor can be controlled by varying the amount of water in the reaction phase. When the reaction occurs in an aqueous phase, thick multilayers are created. Reaction circumstances, totally free of water, will yield a monolayer. 

The CSC precursor build-up has been studied after modification of the silica gel surface from the gas phase. This gas phase modification involves the deposition of one molecular layer at the time. For thicker coatings, a cyclic procedure is needed. Liquid phase modification of the silica surface may also yield valuable ceramic precursors. The precursor molecular structure and layer thickness is controlled by other parameters compared to gas phase procedures. Parameters such as reaction solvent, silane concentrations and presence of water are of primal importance. Those have been discussed in detail in chapter 9. In this chapter, the application of silica modified with aminosilanes, will be discussed. The aminopropylsilica is used as a prototype compound for the production of ceramics by liquid phase chemical surface coating. 

Karl C. Vrancken is presently a researcher at the Laboratory of Inorganic Chemistry (University of Antwerp). In this position he substantiated the development of the Chemical Surface Coating method. In his young career as a Doctor in Sciences, he gained full expertise in organosilane chemistry. His current work is concerned with surface modifications and advanced materials preparation. 

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