Optimization of the Coupling of Target Recognition and Signal Generation - Advanced Fluorescence Reporters in Chemistry and Biology - page 83

result of the ability of the CP's delocalized electronic structure (i.e., energy bands)
to facilitate efficient energy migration over large distances. To rationalize how
energy migration can amplify fluorescence-based sensory events, consider a conju-
gated polymer with a receptor attached to every repeating unit. If energy migration
is rapid with respect to the fluorescence lifetime, then the excited state can sample
every receptor in the polymer, thereby allowing the occupation of a single binding
site to dramatically change the entire emission. Once a single receptor site is
occupied by a quenching agent, energy along the wire is funneled toward the sink
resulting in enhanced deactivation (Fig. 32b ).
A first proof-of-principle for the signaling amplification phenomena in CPs was
reported by Zhou and Swager in 1995. They used poly(propylene ethylene) as CP
with 34-crown-10 groups attached to the CP as receptors for the recognition of the
well-known quencher paraquat (PQ 2+ ). The authors could observe how PQ 2+ binding
produced a trapping site for excitons, which were effectively deactivated by ET
[ 193 ]. In several early works, Swager employed the concept of CPs for the amplified
detection of explosives such as 2,4,6-trinitrotoluene (TNT) and 2,4-dinitrotoluene
(DNT) by fluorescence quenching, using pentiptycene derived CPs [ 194 ]. As is
obvious from this section so far, CPs are predestined for the amplification of
quenched signals and thus most of the systems developed until today show these
features. However, few examples reporting “turn-on” signal amplification have also
been realized, e.g., a pH-sensitive fluorophore using energy harvested from a CP.
System design includes a cationic poly( p -phenylene ethynylene) (PPE) polymer and
an anionic polyacrylate polymer doped with fluoresceinamine dyes (PA-FA) which
were deposited in an LbL type of fashion to form a pH sensor. PPE emission overlaps
with the absorption band of the fluorescein moieties of the PA-FA, facilitating
F¨rster resonance energy transfer (FRET; for an introduction to resonance energy
transfer, see Sect. 4.5 ). At each pH, excitation in the absorption band of PPE produced
an approximately tenfold higher PA-FA emission signal than direct excitation in the
fluorescein absorption band of PA-FA. The authors thus attributed the amplification
to an energy transfer from PPE to FA [ 195 ]. For this system, the overall fluorescence
pH response depends on the p K a of the pH-responsive FA fluorophore, of which
the latter, however, is shifted to higher values because of the highly negatively
charged environment in the PA-FA domains. A comprehensive overview of
further examples and mechanistic descriptions of CP-based signaling systems can
be found in [ 196 ].

4.4.2 In Nano- and Microparticles

One of the first examples describing the idea of fluorescence signal amplification in
NPs was reported by Montalti et al. [ 197 ]. In an early work, silica NPs were covered
with a layer of dansyl (Dns) moieties and were evaluated for their pH response.
Photophysical studies indicated that protonation had a dramatic effect on the
fluorescence of the NPs, leading to the quenching of both, the protonated Dns and