Silica-Based Nanoparticles: Design and Properties - Advanced Fluorescence Reporters in Chemistry and Biology - page 245

3.6 Reaction Kinetics of Doped Molecules
The amorphous silica matrixes are porous network structures that allow other
species to penetrate [ 44 ]. Thus, the doped dye molecules have the ability to react
with targets. However, the reaction kinetics is significantly different than the
molecules in a bulk solution. In the synthesis of DDSNs, commonly used silicon
alkoxides including TEOS and TMOS have tetrahedron structures, which allow
compact polycondensation. As a result, the developed silica nanomatrix can be very
dense. The small pore sizes provide limited and narrow pathways for other species
to diffuse into the silica matrix.
In a bulk silica matrix that differs from the silica nanomatrix regarding only the
matrix size but has a similar network structure of silica, several kinetic parameters
have been studied and the results demonstrated a diffusion controlled mechanism
for penetration of other species into the silica matrix [ 89 - 93 ]. When the silica is
used as a catalyst matrix in the liquid phase, slow diffusion of reactants to the
catalytic sites within the silica rendered the reaction diffusion controlled [ 90 ]. It
was also reported that the reduction rate of encapsulated ferricytochrome by sodium
dithionite decreased in a bulk silica matrix by an order of magnitude compared to its
original reaction rate in a homogeneous solution [ 89 ]. In gas-phase reactions in the
silica matrix, diffusion limitations were observed occasionally [ 93 ].
In the silica nanomatrix, the low diffusion was also reported in both liquid and
gas phases. In the gas-phase reaction, it was found that the luminescence signal of
Ru(bpy) 3 2+ doped in DDSNs remained stable when the air pressure increased from
1 to 8 psi, showing no quenching by oxygen in the air. When the air pressure was
further increased to above 8 psi, a decrease in fluorescence emission intensity was
observed. The results suggested a slow diffusion of quencher oxygen in the silica
nanomatrix.
The liquid-phase reaction kinetics of doped molecules in silica nanomatrixes was
conducted using the metalation of meso -tetra (4- N , N , N -trimethylanilinium) porphy-
rin tetrachloride (TTMAPP) with Cu(II) as a model. To demonstrate the effect of the
silica nanomatrix on the diffusion, pure silica shells with varied thickness were coated
onto the same silica cores, which doped the same amount of TTMAPP molecules.
The Cu(II) from the suspension could penetrate into the silica nanomatrixes and bind
to the TTMAPP. The reaction rate of TTMAPP metalation with Cu(II) was signifi-
cantly slower than that in a bulk solution. The increase in the thickness of the silica
resulted in a consistent decrease of reaction rates (Fig. 8 ).
The diffusion limitations appeared to be the primary reason for the decreased
reaction rate (Fig. 9 ).
The measurements of the reaction activation energies indicated that the reaction
mechanism in the nanomatrix was different than in the bulk solution. Both adsorp-
tion-based diffusion and simple diffusion appeared to control the reaction rate in the
nanomatrix. The adsorption-based diffusion corresponded to the relatively fast
reaction of the doped TTMAPP, which were close to the particle surfaces. The
simple diffusion correlated to the slow reaction of the deeply embedded TTMAPP.