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 14

these Ru(II) complexes (as well as other MLC), their luminescence quantum yields
and their lifetimes can be elegantly tuned via the ligand [ 52 ].
Luminescent lanthanide complexes (Tb 3+ ,Eu 3+ , etc.) are of growing interest,
e.g., as fluorescent reporters for biological applications. Since the lanthanide f-f
transitions have low absorption coefficients (symmetry-forbidden transitions), typ-
ically sensitized emission is used to rationalize more intense luminescence, thereby
exploiting energy transfer (via intersystem crossing) from the triplet state of the
initially excited sensitizer or antenna (ligand with an integral or appended chromo-
phore like phenanthroline) to the emissive lanthanide ion. Accordingly, applica-
tion-relevant compounds present multicomponent systems, in which the active
components, the metal cation, the antenna, and the coordination site are organized
in a supramolecular structure. The ligand is commonly also chosen to protect the
rare earth ion (chelates in the case of DELFIA and cryptates for the compounds
from CISBio International) from potential quenching by the environment (water
molecules in the coordination sphere etc.) [ 54 ]. The optical properties of lumines-
cent lanthanide complexes are thus determined by the absorption properties of the
antenna ligand, the efficiencies of intersystem crossing in the ligand within
the complex, triplet-mediated energy transfer from the excited state of the ligand
to the lanthanide ion yielding the excited lanthanide, and the quantum yield of the
lanthanide emission [ 55 ]. The most remarkable features of luminescent lanthanide
complexes, that are typically only excitable in the short wavelength region (com-
monly at ca. 365 nm, sometimes at longer wavelength like 405 nm or even longer),
are their narrow and characteristic emission bands in the visible (Tb 3+ : 490,
545 nm; Eu 3+ : 580, 613, 690 nm; Sm 3+ : 598, 643 nm; Dy 3+ : 575 nm), in the NIR
region (Yb 3+ : 980 nm; Nd 3+ : 880, 1,065 nm; Er 3+ : 1,522 nm) and their long
luminescence lifetimes (e.g., Eu 3+ : 300-1,500 s, Tb 3+ : 100-1,500 s; Sm 3+ :

m m

20-50 s) [ 10 , 56 , 57 ]. Maximum luminescence quantum yields are in the order

of 0.25 found for Eu 3+ - and 0.15 for Tb 3+ -complexes in aerated solution and
decrease for all the other rare earth ions. Although criteria for the choice of the
lanthanide ion and the antennae have been reviewed [ 11 , 55 , 58 ], the complicated
mechanism of light generation renders the design of highly luminescent lanthanide
reporters still a challenge.

m

2.1.4 Comparison of Chromophores

In comparison to organic dyes as well as metal-ligand and lanthanide complexes,
nanocrystal labels offer a wide variety of spectroscopic properties which are often
scalable, optically stable, and not achievable in these molecular fluorophores (e.g.,
size-controllable spectroscopic properties and continuous absorption below the first
excitonic absorption band in the case of QDs, see Fig. 1a-c ; upconversion lumines-
cence). With values in the range of 100,000 to 1,000,000 M 1 cm 1 , the (size-
dependent) molar absorption coefficients at the first excitonic absorption band of
QDs are generally large as compared to organic fluorophores [ 33 ] (Table 1 ) and
strongly excelling the e M values obtained for MLC (in the order of a few 10,000 M 1