Silica-Based Nanoparticles: Design and Properties - Advanced Fluorescence Reporters in Chemistry and Biology - page 235

Fig. 2 TEM images of variable sizes of silica NPs obtained from a quaternary water-in-oil
microemulsion using different organic solvents: (a) cyclohexane, (b) n -pentane, (c) n -hexane,
(d) n -heptane, (e) n -decane, and (f) n -hexadecane. Reproduced with permission from Ref. [ 56 ]
leaking out of the silica matrix, which is a major challenge for making DDSNs. Due
to this advantage, the covalent binding method has been widely used since the first
report of DDSNs. Currently, various succinimide ester or isothiocyanate modified
dyes are commercially available including most commonly used dyes, such as
fluorescein [ 5 , 13 ], tetramethylrhodamine [ 13 , 60 ], rhodamine B [ 58 ], Cy5 [ 61 ],
etc. With abundant dye molecule supplies, most of the reported DDSNs have been
prepared using this doping method. However, covalent binding is not a universal
method. The modification is difficult for some dye molecules, resulting in the
impracticality of doping these dyes into silica based on the covalent bonds. Therefore,
other doping methods are needed.
2.2.2 Electrostatic Interaction
Electrostatic interaction is a relatively weak association force compared to covalent
bonds, but strong enough to keep dye molecules within a silica matrix [ 8 , 21 ]. If a
dye molecule is positively charged, it can be doped within a negatively charged
silica matrix via the electrostatic interaction. The first example of employing
electrostatic interactions to make DDSNs was reported in 2001 [ 8 ]. In this work,
a positively charged tris(2,2 0 -biprridyl)dichloro-ruthenium(II) hexahydrate (Ru
(bpy) 3 2+ ) was added to a reverse microemulsion, producing Ru(bpy) 3 2+ doped
silica nanoparticles. Electrostatic interactions are feasible doping forces only for
the positively charged dye molecules, such as transition-metal-ligand complexes,
quaternary ammonium organic compounds, etc.