  Microemulsion (water-in-oil) methods enable the encapsulation of individual
nanoparticles into SiO2 spheres. The major drawbacks of this method, when
applied for silica encapsulation of anisotropic nanorods (NRs), are a spatially
unequal silica growth and long reaction times (24 h at least). In this work,
various tetra-alkoxysilanes (tetramethyl orthosilicate (TMOS), tetraethyl
orthosilicate (TEOS) and tetrapropyl orthosilicate (TPOS)) with different
alkyl-chain lengths were used as the silica precursors in attempt to tune the
silanization behavior of CdSe/CdS NRs in a microemulsion system. We find an
enhanced spatial homogeneity of the silica growth with decreasing alkyl-chain
length of the tetra-alkoxysilanes. In particular, by using TMOS as the
precursor, NRs can be fully encapsulated in a continuous thin ($\le$ 5 nm)
silica shell within only 1 h reaction time. Surprisingly, the thin silica shell
showed a superior shielding ability to acidic environment even compared to the
30 nm thick shell prepared by using TEOS. Our investigations suggest that the
lower steric hindrance of TMOS compared to TEOS or TPOS strongly promotes the
homogeneous growth of the silica shells while its increased hydrolysis rate
leads decreases the porosity of these shells.
