Exilica’s core products comprise micrometer sized spherical polymer particles or beads, named µ-Sq beads, and sub-micrometer sized hollow silica shells. Unique, core-shell products are also available.
µ-Sq beads
µ-Sq beads are prepared in a one-pot polycondensation reaction and precipitate as the spherical particles shown below in Figure 1.
Figure 1 µ-Sq beads
To look at µ-Sq beads are a blue-black intractable powder, although they can be disintegrated using acid / peroxide mixtures. µ-Sq beads contain C, H, N and O and have no particular magnetic, electronic or optical properties that would otherwise set them apart from other organic polymers, neither do they have any advantageous surface tribological properties (ie. they are not good for use as a dry lubricant). What they do have is an ability to absorb elemental and small molecular species as well as attract a whole range of small, medium and large molecular and biomolecular species to their surface. They have been shown to absorb, in varying amounts, all of the metal elements from lithium to bismuth in the periodic table, including the lanthanides, except for beryllium, tellurium and the radioactives (which we haven’t tried yet). We are currently examining which of these metals the µ-Sq beads retain once absorbed (like selenium) and which can be released upon further washings (like copper). Those that can be released (that we currently know of) do so very readily although we are currently examining the use of µ-Sq beads in chromatography and the separation of elemental species down a column of µ-Sq beads.
Thus far, elemental species have been absorbed into the beads by soaking in aqueous solutions of the elements in any of their water (or acid) soluble forms as well as their ammonium anion species (like ammonium phosphate or ammonium tungstate). The release of such species from µ-Sq beads can be significantly slowed if the µ-Sq beads are spun in commercial polymers, which they do so very readily.
Figure 2 Hollow silica shells
| A few more interesting things about µ-Sq beads |
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μ-Sq beads display a diameter size range of 1.0 - 1.4 μm. |
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μ-Sq beads are thermally stable up to 250°C and from there shrink in size up to full extinction above 600°C. |
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μ-Sq beads are also very robust with a mean nominal rupture stress of 493 ± 113 MPa and a mean rupture deformation of +65% initial diameter. |
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μ-Sq beads can be overcoated with any cationic or zwitterionic polymer (that we know of anyway). |
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μ-Sq beads can absorb volatile formulations, such as fragrances, and can retain such for up to 8 weeks releasing more upon heating after that time. |
Hollow Silica Shells
The hollow silica shells are prepared by a three-stage process and utilise the unique surface properties of µ-Sq beads. Silicon precursors will directly adhere to the surface of µ-Sq beads, thus additional polymerisation of the adsorbed silicon and subsequent removal of the template bead by thermal treatment yields hollow silica shells with shell wall thicknesses of 10% the overall diameter. The resultant size of any individual shell is exactly 50% of that of the template.
Figure 2 shows several electron microscope pictures of the hollow silica shells including a transmission picture and a cross-sectional picture proving that the shells are hollow. To look at, the shells are a nice white powder that feel wonderfully smooth if a portion is run between two fingers, or massaged into the hand. The shells are not impervious; they contain a perforation through which each template bead was removed. This allows the shells to be filled with either liquid and / or gas.We are currently evaluating the storage and release potential of the shells for a whole range of small molecular and medium molecular and biomolecular species, which will aid us in determining just how big of a single (bio)molecular species can we encapsulate.
| A few more interesting things about our hollow silica shells |
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Hollow silica shells display a diameter size range of 500 - 700nm. |
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The hollow silica shells purely consist of amorphous silica. |
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The hollow silica shells are very robust with a mean nominal rupture stress of 438 ± 47 MPa and a mean rupture deformation of +53% initial diameter. |
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The hollow silica shells can be overcoated with any soluble commercial polymer that is known to adhere to glass surfaces, as well as any other charged polymer also know to adhere to glass surfaces. |
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The hollow silica shells are non-toxic to, and adhere nicely to, mammalian cells, especially when overcoated with poly-L-lycine. |
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The hollow silica shells, with an appropriate polymer overcoat, can encapsulate / contain volatile and corrosive formulations. |
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The same production process that is used to prepare the hollow silica shells can also be used to prepare hollow titania shells. These shells, because of the thermal treatment, are produced as an approximately equal mixture of rutile and anatase. |
Core-Shell Products
Our core-shell products are produced by combining the elemental absorption properties of the beads into the production of the hollow silica shells. Thus the result is an outer silica shell that encapsulates inner layers of a chosen metal, usually in the form of a metal oxide (if thermal removal of the template bead is performed in air)
Figure 3 Core-shell products
Figure 3 shows both the elements in the periodic table that have been examined in this manner and also the resultant colours produced. For this, all thermal treatment was performed in air so the encapsulated elemental compounds are primarily oxides, chlorides and oxychlorides.
Figure 4 Encapsulated metals
Figure 4 shows both the outside and a cross-sectional slice of the iron sample (in the form of magnetic iron(III) oxide). The top left picture is a transmission image of encapsulated silver.
Figure 5 Encapsulated phosphors
A few more interesting things about our core-shell products
If the right mixture of elemental compounds are added to the reaction mixture in the production of a core-shell material then multi-component products are possible. For example, Figure 5 shows yttrium vanadate doped with 2% europium, under both normal and ultraviolet light. This mixture was prepared from adding yttrium oxide, ammonium vanadate and europium oxide in appropriate stoichiometric amounts. Encapsulation inside the silica shells can alter not only the appearance but also the physical properties of the entrapped material. For example, copper oxide results as a white substance instead of the expected black, while zinc oxide is produced as a brown substance instead of white.
Figure 6 illustrates what happens (post thermal treatment) to an absorbed metal when the µ-Sq beads are not coated with silica, prior to thermal disintegration. The pictures below are of iron oxide, gold, & silver metal samples following the removal of non-silica coated beads. The top right image is a silver anomoly.
Figure 6 Non-encapsulated metals
