Silica bead

Soak silica beads in concentrated nitric acid at ca. 90°C overnight to remove organic impurities. Rinse the beads thoroughly with deionized water until neutral. If the silica beads are colored, repeat this step until they become colorless. 

If a bulky silanation agent such as octyidimethylchlorosilane is used, there will be unreacted silanols left. These silanols can be end capped by reacting the silica beads with trimethylchlorosilane by repeating steps 4-7. 

Proteins. A chiral stationary phase with immobilized a -acid glycoprotein on silica beads was introduced by Hermansson in 1983 [18, 19]. Several other proteins such as chicken egg albumin (ovalbumin), human serum albumin, and cellohy-drolase were also used later for the preparation of commercial CSPs. Their selectivity is believed to occur as a result of excess of dispersive forces acting on the more retained enantiomer [17]. These separation media often exhibit only modest loading capacity. 

Small chiral molecules. These CSPs were introduced by Pirkle about two decades ago [31, 32]. The original brush -phases included selectors that contained a chiral amino acid moiety carrying aromatic 7t-electron acceptor or tt-electron donor functionality attached to porous silica beads. In addition to the amino acids, a large variety of other chiral scaffolds such as 1,2-disubstituted cyclohexanes [33] and cinchona alkaloids [34] have also been used for the preparation of various brush CSPs. 

The two best selectors resulting from Li s screening, DNB-L-ala and DNB-L-leu, were then prepared on a larger scale, attached to silica beads modified with 3-amino-propyl-triethoxysilane, and the CSPs were packed into columns. Respective separation factors of 4.7 and 12 were found for the separation of racemic naphthyl leucine ester 17 using these CSPs. 

It has been shown that for naphthalene contained in coal tar globules, the area-dependent mass transfer coefficient for globules was 10 greater than when the substrate was coated on microporous silica beads, and that this was an important factor in determining the rate of mineralization (Ghoshal and Luthy 1996). 

In order to probe the influence of Au and KOAc on the vinyl acetate synthesis chemistry, four different catalysts were synthesized. All of these catalysts were prepared in a manner exemplified in prior patent technology [Bissot, 1977], and each contained the same palladium loading in an egg-shell layer on the surface of a spherical silica support. The palladium content in the catalyst was easily controlled by adjusting the solution strength of palladium chloride (PdClj) added to the porous silica beads prior to its precipitation onto the support by reaction with sodium metasilicate (Na SiOj). The other two catalyst components (Au and KOAc) were either present or absent in order to complete the independent evaluation of their effect on the process chemistry, e.g., (1) Pd-i-Au-hKOAc, (2) Pd-i-KOAc, (3) Pd-hAu, and (4) Pd only. 

Spherical microparticles are more difficult to manufacture and can be prepared by several methods. One method prepares silica hydrogel beads by emulsification of a silica sol in an immiscible organic liquid [20,21,24,25]. To promote gelling a silica hydrosol, prepared as before, is dispersed into small droplets in a iater immiscible liquid and the temperature, pH, and/or electrolyte concentration adjusted to promote solidification. Over time the liquid droplets become increasingly viscous and solidify as a coherent assembly of particles in bead form. The hydrogel beads are then dehydrated to porous, spherical, silica beads. An alternative approach is based on the agglutination of a silica sol by coacervation [25-27], Urea and formaldehyde are polymerized at low pH in the presence of colloidal silica. Coacervatec liquid... 

J. Wang, G. Liu, and Y. Lin, Bioassay label based on electroactive silica beads. Small 2, 1134-1138... 

Other composite photocatalysts were prepared by mounting immobilized anatase particles on mesoporous silica and silica beads [189-191], The behavior of anatase-mounted activated carbons was also studied in detail [192-194], It was even suggested that carbon-coated anatase exhibits better performance in photocatalysis than anatase itself, demonstrating high adsorptivity, inhibition of interaction with organic binders, etc. [195,196],... 

Cross-reactive sensing arrays were developed to detect odors and vapors in an artificial nose manner. Solvatochromic dyes such as Nile Red are adsorbed on the surface or embedded into various polymeric or porous silica beads. The beads respond to analyte vapor by a change in fluorescence maxima or/and intensity due to changes of polarity inside the bead. A portable instrument and preliminary field test for the detection of petroleum products was recently described [106]. 

As was demonstrated, a variety of polymeric materials are used for preparation of dye-doped beads. Dye-doped silica beads are also extremely popular due to their chemical robustness, biocompatibility and simplicity in preparation and further functionalization of the surface [55]. Thus, polymeric, silica and Ormosil beads (which occupy intermediate position) are widely used as nanosensors and labels. On the other hand, quantum dots possess much higher cytotoxicity which often limits their application in biological systems. 

Although capillary columns packed with typical modified silica beads have been known for more then 20 years [30,31], it is only now that both the chro-... 

The plug flow profile would only be distorted in very narrow bore capillaries with a diameter smaller than the thickness of two double-layers that then overlap. To achieve an undisturbed flow, Knox suggested that the diameter should be 10-40 times larger than 6 [15]. This can easily be achieved in open capillaries. However, once the capillary is packed with a stationary phase, typically small modified silica beads that carry on their own charged functionalities, the distance between adjacent double-layers is only a fraction of the capillary diameter. However, several studies demonstrated that beads with a submicrometer size can be used safely as packings for CEC columns run in dilute buffer solutions [15,35]. 

Preparing the outlet end-frit at the desired distance from the column end by sintering the silica beads using heating to a temperature of over 550 °C. 

Dittmann at al. later developed a very simple method for preparing such stationary phases [41 ]. They packed a capillary with 3-pm ODS beads and then drew a heated wire along the capillary to achieve sintering of the beads. Since changes in the drawing speed directly affected both EOF and retention, they inferred that the heat treatment led to detachment of a part of the C18 ligands from the silica beads. 

Chirica and Remcho first created the outlet frit, packed the column with ODS beads, and then fabricated the inlet frit. The column was filled with aqueous solution of a silicate (Kasil) and the entrapment achieved by heating the column to 160 °C [105,106]. The monolithic column afforded considerably reduced retention times compared to the packed-only counterpart most likely due to a partial blocking of the pores with the silicate solution. This approach was recently extended to the immobilization of silica beads in a porous organic polymer matrix [107]. 

Fig. 30. Effect of field strength and percentage of acetonitrile in the mobile phase on electroosmotic flow in a packed capillary column. (Reprinted with permission from [35]. Copyright 2000 Elsevier). Conditions capillary column 100 pm i.d., total length 38 cm, active length 8.5 cm packed with 0.5-pm C8 silica beads, mobile phase acetonitrile/25 mmol/1 TRIS-HCl buffer pH = 8, temperature 20 °C, marker thiourea...

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