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Silica is a versatile coating that has wide applicability for many applications. nanComposix’s ability to manipulate the properties of the silica shells enables the engineering of very sophitcated structured and is considered one of our core competencies. Silica coatings are an amorphous network of silicon and oxygen that encapsulate the particle with a coating that can be many nanometers thick. The silica is added to the particle surface via the base catalyzed condensation of various silanes, usually alkoxyl silanes such as tetraethylethoxysilane (TEOS). This is known as Stober growth. Through the selection of various silanes, different chemical functionality can be integrated within and on the surface of the silica coating.

In addition, the porosity of the silica surface can be modified by the addition of various templating molecules which can subsequently be removed, giving rise to pores. For instance mesoporous shells can be formed by templating off surfactants such as cetyltrimethylammonium bromide (CTAB).

Silica coated particles have good salt stability and are one of the best coatings to preserve the optical properties of metal nanoparticles when integrated into composites and coatings, acting as an insulating steric layer that prevents the surface plasmons of particles from interacting with other particles even when the particles themselves may be in physical contact.

Standard silica coatings have a hydroxyl surface giving the particles a large negative zeta potential at neutral and basic pH. Zeta potential measurements of 80 nm-diameter silica colloids as a function of pH show that the isoelectric point of silica nanoparticles is close to pH 2.

Amine-functionalized silica is useful for binding studies, conjugation with carboxyl-containing molecules through EDC/NHS coupling, or binding to dyes and molecules with isothiocyanate (ITC) or amine-reactive esters. The amines at the colloid surface can be protonated at acidic pH to yield particles with a large positive zeta potential. Measurement of zeta potential versus pH for 120 nm amine-terminated silica particles indicates an isoelectric point near pH 7.5.

Through the incorporation of other silanes, the surface can be rendered hydrophobic. It is important to note that the silica on the surface of the particles grown by Stober growth may be quite labile and can dissolve away to silicic acid species. The incomplete hydrolysis of the silane precursors may encourage this dissolution. While this may only cause the loss of some surface layers, if these layers contain the functional groups of interest the particles may lose their properties. For applications where stability of the silica shell in aqueous solutions is important, we strongly recommend using aluminosilicate shells instead of silica. Any silica shell can be transformed into an aluminosilicate shell via a proprietary solution based method (nanoComposix patent US 9,675,953).

Advantages

  • Dispersion in a wide range of solvents with a high degree of stability.
  • Retains optical properties of the metal nanoparticles when integrated into composite and coatings
  • Can be dried down and redispersed without agglomeration.
  • Allow the engineering of the particle surface with a wide range of functionality
  • Allows engineering of controlled aggregates
  • Allows exquisite control of the surface functionality

Representative Source: tetraethyl orthosilicate (Sigma Aldrich, 333859)

Comparison to Alternatives

  • Displaceable: Not displaceable – strongly bound to the particle surface
  • Soluble in polar solvents
  • Applications

    • Biodiagnostic and nanomedicine applications
    • Photothermal applications
    • Color applications
    • Controlled aggregation