Silica-Based Nanoparticles: Design and Properties - Advanced Fluorescence Reporters in Chemistry and Biology - page 230

3 Properties of DDSNs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 238
3.1 Fluorescence Intensity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 239
3.2 Quantum Yield and Lifetime . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 240
3.3 Photostability of DDSNs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241
3.4 Fluorescence Enhancement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241
3.5 Solubility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 244
3.6 Reaction Kinetics of Doped Molecules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245
3.7 Toxicity of DDSNs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 246
4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 248
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 248

1 Introduction

Fluorescence imaging and sensing are powerful techniques in a wide variety of
biological studies. The use of fluorophores as biomarkers or indicators, for in vitro
and in vivo monitoring of biological processes and determinations of targets in
biosamples has been achieved [ 1 , 2 ]. However, two major limitations exist in
molecular fluorophores, photobleaching and limited fluorescence intensity. When
the amount of targets present is very low, these two limitations become significant.
Additionally, poor water solubility of some organic fluorophores makes their
applications in biosamples a challenge.
To overcome these limitations, dye molecules have been doped into a silica
matrix to form a dye-doped silica nanoparticle (DDSN). According to an old story,
firefly-lamps were made to light the night by collecting a large number of fireflies in
a small glass bottle. Similarly, by encapsulating thousands of fluorophore mole-
cules into a silica nanoparticle, DDSNs are produced to illuminate the dim views of
biological samples [ 3 , 4 ]. The first report on developing DDSNs was published in
1992 [ 5 ]. In this pioneering work, a fluorescein derivative was doped into a silica
nanosphere. Since then, there has been a growing interest in DDSNs because of
their unique optical properties. In contrast to free dye molecules, DDSNs exhibit
two major features, dramatically improved photostability and high fluorescence
intensity. Using DDSNs as fluorescence labeling agents, many biological processes,
previously invisible, have now been observed.
In addition to the two features mentioned above, good biocompatibility of
DDSNs is significant for their bioapplications. Their silica surface can be modified
by various functional groups, through which they can be immobilized onto a wide
variety of analytes. The hydrophilic surface of the silica matrix provides good water
solubility. The low toxicity further favors the biological applications of the DDSNs.
As a result, biolabeling and biosensing are the most important application areas of
the DDSNs [ 3 , 6 , 7 ]. With recognizing groups on their surfaces such as antibodies
and aptamers, the DDSNs can recognize and specifically bind to biotargets. The
high signal intensity of the DDSNs greatly improves the detection limit. The silica
nanomatrix protects the doped luminescent indicators from interference by the