I have a protocol for silica coating magnetic microparticles that has been relatively reliable in the past. The particles are coated with physisorbed Na2SiO3 in 2 steps (to displace the SDS they are stabilised with). Then the TEOS reaction uses the following parameters (for 50 mg of 800 nm particles):

Reaction volume: 450 ml

EtOH:Water: 2:1

TEOS: 1.5 ml dissolved in 15 ml EtOH

Base: Ammonium hydroxide 8 ml.

Reaction time: 20 hours.

From this protocol I have achieved coatings of approximately 30 nm. However I recently tried this protocol with amounts modified (I used 20% of everything above) to try and coat 10 mg of larger particles (~1.5 um). And the coating was not very successful. Could there be TEOS depositing on the glassware too? Should this reaction be done in plastic?