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toxicity properties of nanoparticles [
80
]. Eventually, the cytotoxicity of differently
functionalized nanoparticles (including typical ligands) should be systematically
assessed using previously standardized procedures.
Even though nanoparticles have extremely promising and advantageous (opti-
cal) properties, at present, they cannot be recommended for routine applications,
due to the problems discussed in this review. In very specific cases, such as single
molecule/single particle imaging and tracking applications, QDs are superior to
most luminescent dyes due to their photostability, in principle allowing single-
particle tracking for a much longer time span compared with organic fluorophores.
However, blinking that is observed for all QDs is a major drawback even for these
specialized applications. Nevertheless, there is hope that quantum dot blinking can
be overcome, making them eventually the ideal labels for all applications in need
of exceptional photostability [
148
]. On the other hand, blinking, as well as other
QD-specific features, may be even exploited for advanced techniques such as
superresolution microscopy [
82
,
149
]. Here, further exciting potential applications
of QDs are expected to appear in the near future.
5 Conclusions
Nanocrystals have been exploited in several areas of biosensing and -imaging,
including immunohistochemistry, microarray technologies as well as advanced
fluorescence techniques such as FISH, and in vivo fluorescence imaging using
conventional techniques and multiphoton microscopy. Despite many superior opti-
cal properties of these particles, such as tunable absorption and emission bands and
extremely broad and intense absorption, high fluorescence quantum yields even in
the NIR region, and large two-photon action cross sections as well as unique
spectroscopic prerequisites for spectral multiplexing in the case of QDs, or sophis-
ticated optical effects such as upconversion luminescence in the case of rare-earth
doped nanocrystals, until now, nanocrystals failed to be routinely used on a large
scale. The fact that these materials behave like colloids but not like molecules
complicates their application in biological environments. Practitioners must con-
sider the costs of finding a solution to the challenges of their particular experimental
system against the benefits of their advanced spectroscopic features. However, it
is anticipated that advances in nanosciences combined with the attractive features
of many nanoparticle systems will render these particles increasingly attractive for
bioanalytical applications in the future.
References
1. Resch-Genger U, Grabolle M, Cavaliere-Jaricot S, Nitschke R, Nann T (2008) Quantum dots
versus organic dyes as fluorescent labels. Nat Methods 5:763-775
2. Lavis LD (2008) Bright ideas for chemical biology. ACS Chem Biol 3:142-155
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