Silica-Based Nanoparticles: Design and Properties - Advanced Fluorescence Reporters in Chemistry and Biology - page 244

An additional approach to enhance the fluorescence signal is to increase
the coupling efficiency of the fluorescence emission to the far field (Fig. 6e ).
The excited dye molecules can transfer theirenergytothemetal,andthenthe
metal starts emission of radiation. The overlap between the emission of dye
molecules and the metal plasmon resonance band is more effective producing
enhancement (Fig. 7b ). In a recent review, it was discussed that the fluorophor-
es-metal complex is the real emitting species and gave the complex the name
“plasmonphore” or “fluoron” for distinguishing them from fluorophores and
plasmonics [ 77 ].
The fluorescence quenching occurs when dye molecules are close to the metal.
The energy from the first excited fluorophores can be consumed through a non-
radiative path to the metal. A spacing layer is usually required to avoid this energy
transfer process. In addition, the concentration of the dispersed dye molecules
should be suitable to avoid self quenching [ 34 , 81 ].
The plasmon decay time is adjusted by controlling the geometry of the metal
nanoparticles. However, so far it is uncertain whether a longer or a shorter plasmon
decay time is preferable for the fluorescence enhancement [ 77 ]. Shorter plasmon
decay times can prevent energy conversion to thermal energy and thus produce
more emission signals while longer decay time allows more time to radiate and thus
enhance the emission. Currently, the effect of geometry of nanoscaled metal on the
fluorescence enhancement is still under the study [ 77 ].
3.5 Solubility
The solubility of a DDSN is determined by its surface groups [ 24 ]. Without any
surface modifications, the amorphous silica nanomatrix provides DDSNs good
water solubility [ 3 ]. This water solubility can be further enhanced by surface
modification of DDSNs with carboxyl groups. This surface can be obtained
by adding 3-(trihydroxysilyl)propylmethylphosphonate or carboxyethylsilane-
triol as a cocondensation agent with silicon alkoxide in a post coating process
after the DDSNs have been synthesized [ 24 ]. The resultant carboxyl-group
modified DDSNs have excellent water solubility, which can be stably suspended
in aqueous solution for a long time period. The hydrophilic surface of the DDSNs
may cause nonspecific binding to the biological samples, which challenges
the applications of DDSNs in the biological area. A solution to this problem is
to introduce some hydrophobic groups to the surface of the DDSNs. For exam-
ple, the octadecyl groups can be integrated with carboxyl groups in the post
coating process by adding octadecyltriethoxysilane in the reverse microemul-
sion [ 24 ]. The obtained octadecyl and carboxyl modified DDSNs exhibited
minimal nonspecific binding to a DNA modified glass surface. Similarly,
poly(lipid)-coated DDSNs avoided nonspecific adsorption in determination of
analytes [ 88 ].