Stöber silica1
Ø : 40 - 1000 nm SD: 20 – 5% Batch: 20 g

	Microemulsion silica2
	Ø : ca. 50 nm SD: ca. 6% Batch: 2-3 g (after purification) Dumbbells: 4-10%

These two synthesis eventually result in charged dispersions of silica particles which can be dispersed in polar solvents like ethanol, water or dimethylformamide. The microemulsion silica synthesis, however, first results in a silica dispersion in
cyclohexane which is only moderalely stablilized by the large excess of surfactant (the surfactant concentration is higher than the silica concentration), but is transferred into the polar solvents after the synthesis and then purified from the surfactant.

Additional possibilities:

• Add fluorescent groups and grow them into the particles³, e.g. fluoresceinisothiocyanate (FITC) or Rhodamine-B-isothiocyanate (RBITC).
  
  

  FITC:

  RBITC:

  
  
• Additional growth steps, e.g. to grow a layer of silica around a fluorescent core particle.
  
  
• Coatings with stearyl alcohol (C18H37OH)⁴, 3- methacryloxypropyltrimethoxysilane(TPM)⁵ or polydimethylsiloxane (PDMS)⁶.
  
  

  TPM:

  
  The TPM coating yields silica dispersions which are still charged, but are more apolar than uncoated silica so that they can be dispersed in an ethanol/toluene mixture with upto 80% toluene. This allows for the refractive index matching of the silica particles and their use in light scattering studies. Stearylalcohol and PDMS coated silica particles can be dispersed in non-polar solvents like cyclohexane, chloroform or toluene. This results in an uncharged hard sphere-like interaction potential for the former particles and in a more soft interaction potential for the latter particles.
  

References

1. W. Stöber, A. Fink and E. Bohn, J. Colloid Interface Sci. 1968, 26, 62.
2. K. Osseo-Asare and F.J. Arriagada, Coll. Surf. 1990, 50, 321; F.J. Arriagada and K. Osseo-Asare, J. Colloid Interface Sci. 1999, 211, 210.
3. A. van Blaaderen and A. Vrij, Langmuir 1992, 8, 2921.
4. A.K. van Helden, J.W. Jansen and A. Vrij, J. Colloid Interface Sci. 1981, 81, 354.
5. A.P. Philipse and A. Vrij, J. Colloid Interface Sci. 1989, 128, 121.
6. P. Auroy, L. Auvray and L. Léger, J. Colloid Interface Sci. 1992, 150, 187; P.A.
7. Nommensen et al., Progr. Colloid Polym. Sci. 1998, 110, 144.