Supercritical fluid, deposition methods

A method which uses supercritical fluid/solid phase extraction/supercritical fluid chromatography (SE/SPE/SEC) has been developed for the analysis of trace constituents in complex matrices (67). By using this technique, extraction and clean-up are accomplished in one step using unmodified SC CO2. This step is monitored by a photodiode-array detector which allows fractionation. Eigure 10.14 shows a schematic representation of the SE/SPE/SEC set-up. This system allowed selective retention of the sample matrices while eluting and depositing the analytes of interest in the cryogenic trap. Application to the analysis of pesticides from lipid sample matrices have been reported. In this case, the lipids were completely separated from the pesticides. 

This method has a number of positive features it may be applied to most supercritical fluids with critical temperatures close to ambient deposition of the solid product occurs in a controlled manner, if necessary under an inert atmosphere and the high pressure "stabilizing" conditions are maintained right up to the point of precipitation. The precipitated solid product may then be analysed and characterised by other off-line spectroscopic techniques. In our example, the 13C-NMR spectrum of the solid material, redissolved in d8-toluene, shows the same resonances as those observed with a genuine sample of Cr(CO)4(C2H4)2. 

The first experiments reported here lead us to think that the impregnation of porous supports by drugs can be achieved by means of supercritical fluids. This one-step method yields a final product exempt from any residual trace of toxic solvent. The kinetics of the mass transfer is faster, besides the thermodynamics of the adsorption seems more favourable here. The main problem encountered up to now is the weak solubility of many active molecules in pure C02, which induces a limitation of the percentage of deposited product. However, this difficulty can be overcome by the use of few amount of an entrainer. In particular, ethanol which does not show any toxicity, would greatly extend the range of active substances which could be used. 

Erkey and co-workers [59-61] prepared Pt- and Ru-doped carbon aerogels using a supercritical deposition method. This involved dissolution of an organometallic precursor in a supercritical fluid and the exposure of a solid substrate to this solution. After impregnation of the support with the metal precursor, it was converted to the metal form by different methods. Dimethyl(l,5-cyclooctadiene) platinum(ll) was used as a precursor for Pt [59,60], and two different Ru complexes, trisacetylacetonate Ru(lll) and Ru(cod)(tmhd)2, were used for Ru [61], Monolithic organic and carbon aerogels... 

There are various nanoparticle production methods reported. Most common approaches include solid-state methods (grinding and milling), vapor methods (physical vapor deposition and chemical vapor deposition), chemical synthesis/ solution methods (sol-gel approach and colloidal chemistry), and gas-phase synthesis methods [1]. Chemical approaches are the most popular methods for the production of nanoparticles. Other novel production methods include microwave techniques, a supercritical fluid precipitation process, and biological techniques. 

Specific physicochemical properties of the supercritical fluids offer flexible alternatives to established processes like chemical vapor deposition (CVD), which is used in the preparation of high-quality metal and semiconductor thin films on solid surfaces. Watkins et al. [43] reported a method named chemical fluid deposition (CFD) for the deposition of CVD-quality platinum metal films on silicon wafers and polymer substrates. The process proceeds through hydrogenolysis of dimethyl-(cyclooctadiene)platinum(ll) at 353 K and 155 bar. 

Many methods have been reported for production of nanodiamonds (NDs) such as laser ablation, " plasma-assisted chemical vapor deposition," autoclave synthesis from supercritical fluids, ion irradiation of graphite, chlorination of carbides, electron irradiation of carbon onions, and ultrasound cavitation. Smaller NDs can be prepared by detonation processes that yield aggregates of NDs with sizes of 4-5 nm embedded in a detonation soot composed of other carbon allotropes and impurities. An explosive mixture having an overall negative oxygen balance provides a source of both carbon and energy for the conversion. Because of their small size (2-10 nm) detonation NDs have also been referred to as ultradispersed, nanocrystalline... 

A number of methods have frequently been employed in the production of nanocomposite materials. These include solution intercalation, melt intercalation, polymerization, sol-gel, deposition, magnetron sput-tering, laser, ultrasonication, supercritical fluid, etc. In PHA nanocomposite fabrication, solution intercalation and melt intercalation methods are the most widely explored procedures. However, use of in situ intercalative polymerization, supercritical fluids and electrospinning are shown to be promising and emerging techniques. The performance and quality of a nanocomposite depends on how well the nanofillers disperse or blend into the matrix. Therefore, these methods constitute different strategies to improve the composites thermo-mechanical and physico-chemical properties by enhancing efficient interactions between the nanofiller and the polymer matrices. 

Yen CH, Cui XL, Pan HB, Wang SF, Llin YH, Wan CM. Deposition of platinum nanoparticles on carbon nanotubes by supercritical fluid method. J Nanosci Nanotechno 2005 5 1852-7. 

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