Nanocrystals and Nanoparticles Versus Molecular Fluorescent Labels as Reporters for Bioanalysis and the Life Sciences: A Critical Comparison - Advanced Fluorescence Reporters in Chemistry and Biology - page 17

species identification from time-resolved fluorescence measurements. Neverthe-
less, for QD labels displaying a concentration-independent fluorescence decay
behavior, the quantification of these multiexponentially decaying species could be
recently demonstrated for mixtures of different chromophores [ 5 ]. The lumines-
cence lifetimes of upconversion nanocrystals are in the long microsecond to
millisecond time domain and are not sensitive to oxygen. Similarly as described
for MLC and lanthanide chelates, this can be exploited, e.g., for time-gated
emission and time-resolved FRET applications which have already been reported
for micrometer-sized upconverting phosphors.
This comparison of the spectroscopic properties of the different types of fluores-
cent reporters underlines that semiconductor QDs and upconverting nanoparticles
have no analogs in the field of organic dyes. Therefore, their unique features are
unrivaled. The different molecular labels detailed here each display unique advan-
tages that can compete with some of the favorable features of QDs and upconvert-
ing phosphors such as long lifetimes in the case of MLC systems and lanthanide
chelates or very narrow emission bands for lanthanide chelates beneficial for
spectral multiplexing.

2.2 Solubility and Aggregation

The solubility of a chromophore is one of the mayor factors governing its applica-
bility. Suitable labels and probes should not aggregate or precipitate under
application-relevant conditions. For bioanalysis and life sciences, this includes
aqueous solutions, in vitro conditions (cell cultural media), on supports such as
microarrays, in cells or in vivo conditions. Moreover, for many biological applica-
tions such as the specific labeling of cells and tissue, nonspecific binding to the cell
surface and the extracellular matrix can also play a role. Organic molecules (dyes as
well as ligands for MLC and lanthanide complexes) can be easily solubilized by
derivatization with substituents such as sulfonic acid groups. Provided that the
structure-property relationship is known for the respective dye class, the solubility
can be tuned by substitution without considerably affecting the labels' optical
properties and other application-relevant features. A whole range of organic dyes,
that are soluble in relevant media, are commercially available.
Nanoparticle dispersibility is controlled by the chemical nature of the surface
ligands (coating). Nanoparticles, which are prepared in aqueous solution, are
inherently dispersible in water. However, with the exception of CdTe, high-quality
nanocrystals with narrow size-distributions are typically synthesized in organic
solvents and must be rendered water-dispersible (i.e., aggregation of nanoparticles
in aqueous solution must be prevented). As summarized in Table 2 , this can be
accomplished electrostatically, by using small charged ligands such as mercapto-
propionic acid [ 34 ], cystamine [ 75 ], or with charged surfactants that intercalate
with the hydrophobic ligands present

from synthesis [ 36 ]. Alternatively,