Optimization of the Coupling of Target Recognition and Signal Generation - Advanced Fluorescence Reporters in Chemistry and Biology - page 84

a number of unprotonated Dns units in the close vicinity, the amplified response
arising from charge-transfer interactions. The signal modulation involved a number
of units much larger than those effectively affected by the chemical modification so
that the chemical input was translated into an amplified signal. Dansylated silica
NPs were also employed for the detection of metal ions (Cu 2+ ,Ni 2+ ,Co 2+ )by
incorporation of a polyamine chain as receptor. Again, enhanced quenching of the
fluorescence was observed and the authors estimated that each ion could quench up
to 13 dansyl units [ 198 ]. Recalling the features of CPs, such hybrid NP design also
seems much better suited for generating amplified quenching. However, also in this
type of system, amplified “off-on” signals have been achieved. For instance, a
6-methoxy-8- p -toluene sulfonamidequinoline (TSQ) was used as ligand for Zn 2+
ions (Fig. 33 ). The complexation process takes place in concert with the deprotona-
tion of the sulfonamide group and leads to a red shift of the fluorescence band and a
strong increase in its intensity (cf. the effects discussed for the very first ligand
systems in Sect. 2 ). Due to the fact that the TSQ units are organized in a dense
multichromophoric net on the particle surface, not only is the fluorescence of
the moieties directly involved in the coordination of Zn 2+ switched on, but the
surrounding uncomplexed units also generate an intense fluorescence signal by
transferring their excitation energy to the luminescent complexes [ 199 ].

4.4.3 In Dendrimers

Dendrimers containing photoactive components can exhibit particularly interesting
properties [ 9 ] since cooperation among the photoactive units can allow the den-
drimer to perform specific functions, e.g., light harvesting through antenna effects
(see also the following chapter [ 23 ]). Properly designed dendritic materials can
show efficient migration of energy from the dendrons or peripheral groups to the
more conjugated units or core, leading to dramatically enhanced fluorescence
intensity changes (Fig. 32d ). One of the first works realizing the application of
dendrimers in optical sensing and signal amplification was described by V ¨ gtle,
Balzani et al. [ 200 ]. A fourth generation (4D) poly(propylene amine) dendrimer
decorated with 32 dansyl units at the periphery and containing 30 aliphatic amine
units in the core was used for the detection of Co 2+ . The fluorescence of all dansyl
units was quenched when a single Co 2+ ion was coordinated in the dendrimer's core
by the aliphatic amine units. In a further work, the authors described a polylysin
dendrimer branched at the 1, 3, and 5 positions. Each branch carries eight fluores-
cent dansyl units in the periphery and six aliphatic amide units in the interior. The
behavior of a monodansylated reference compound was also investigated for
comparison. Addition of Ni 2+ or Co 2+ to a basic solution of the monodansyl
compound did not cause any effect in the absorption or emission properties of the
molecule. However, in the case of the dendrimer, a strong fluorescence quenching
of the dansyl emission at 514 nm was observed. At low metal ion concentration,
each metal quenches about nine dansyl units [ 201 ]. The effect of dendrimer size