Silica Encapsulation. The silica encapsulation of the encoded
nanoparticles was achieved through a modified Stö
ber method using
the MUA carboxylic group to promote the growth of silica as follows:
The proper concentrations of H2O, NH4OH, and EtOH for silica
growth on the MUA/SERS-encoded particle solution were previously
adjusted, during the codification step, to yield final concentrations of
7.94, 0.128, and 14.60 M, respectively (EtOH/H2O molar ratio of
1.84). Then, TEOS (13.2 μL) was added, and the solution was
energetically shaken and left undisturbed at room temperature for 14
h. Finally, the resulting core−shell nanoparticles were cleaned to
remove excess reactants by centrifugation (3 × 6000 rpm, 20 min) and
redispersed in ethanol. To concentrate the solution (10−3 M) to
perform the SERS characterization, 15.3 mL of this solution was
centrifuged again (2 × 6000 rpm, 20 min) and redispersed in water.
After the final centrifugation step, everything was resuspended in a
final volume of 1 mL.