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 12

long-lived excited states followed by the emission of a photon of higher energy
than each of the exciting photons. Accordingly, upconverting materials absorb
light in the near infrared (NIR) part of the spectrum and emit comparatively sharp
emission bands blue-shifted from the absorption in the visible region of the
spectrum yielding large antiStokes shifts [ 46 ]. Nanoscale manipulation can
lead to modifications of, e.g., the excited state dynamics, emission profiles, and
upconversion efficiency [ 47 ]. For instance, the reduction in particle size can allow
for the modification of the lifetime of intermediate states and the spatial confine-
ment of the dopant ions can result in the enhancement of a particular emission.
The most frequently used material for the design of upconverting nanocrystals is
NaYF 4 :Yb, Er. The attractiveness of upconverting nanocrystals lies in the fact
that the NIR excitation light does not excite background fluorescence and can
penetrate deep into tissue, in the large antiStokes shifted, narrow, and very
characteristic emission, and in their long emission lifetimes. Despite their obvious
potential as fluorescent reporters for the life sciences, upconverting nanoparticles
are not commercially available yet. Moreover, in comparison to other longer
existing fluorophores, many application-relevant properties have not been thor-
oughly investigated yet for nanometer-sized upconverting phosphors due to
difficulties in preparing small particles (sub-50 nm), that exhibit high dispersi-
bility and strong upconversion emission in aqueous solution.
Precious metal nanoparticles show strong absorption and scattering of visible
(vis) light, which is due to collective oscillation of electrons (usually called loc-
alized surface plasmon resonance, LSPR) [ 48 ]. The cross section for light scattering
scales with the sixth power of the particle diameter. Consequently, the amount of
scattered light decreases significantly when the nanoparticles become very small.
Fluorescence of metal nanoparticles was observed in the late 60s of the last century
[ 49 ]. Even though this effect is often very small, it becomes increasingly interesting
for small nanoparticles or clusters (the properties and applications of silver and gold
nanoclusters are discussed in chapters of Diez and Ras [ 150 ] and of Muhammed and
Pradeep [ 151 ] in this volume), since the absorption cross section scales only with
the third power of the nanoparticle diameter. Quantum yields of Au 5 clusters as
high as 0.7 have been reported [ 50 ]. At present, the major field of application of
metal particles like gold involves Raman spectroscopy.
2.1.2 Organic Dyes
The optical properties of organic dyes (Fig. 1d-f ,Table 1 ) are controlled by the nature
of the electronic transition(s) involved [ 4 ]. The emission occurs either from an
electronic state delocalized over the whole chromophore (the corresponding fluor-
ophores are termed here as resonant or mesomeric dyes) or from a charge transfer
(CT) state formed via intramolecular charge transfer (ICT) from the initially excited
electronic state (the corresponding fluorophores are referred to as CT dyes )[ 4 ].
Bioanalytically relevant fluorophores like fluoresceins, rhodamines, most 4,4 0 -
difluoro-4-bora-3a,4a-diaza- s -indacenes (BODIPY dyes), and cyanines (symmetric