Luminescent Dendrimers as Ligands and Sensors of Metal Ions - Advanced Fluorescence Reporters in Chemistry and Biology - page 255

light harvesting [ 13 ], stepwise complexation [ 14 ], and reversible metal complex
assembly [ 15 ].
In most cases, metal ion coordination by a dendrimer takes place by units that are
present along the dendrimer branches (e.g., amine, imine, or amide groups) or
appended at the dendrimer periphery (e.g., terpyridine, cathecolamide ligands).
When multiple identical coordinating units are present, dendrimers give rise to
metal complexes of variable stoichiometry and unknown structures. Luminescent
dendrimers with a well defined metal-coordinating site have been reported so far
[ 16 , 17 ], and the most used coordination site is 1,4,8,11-tetraazacyclotetradecane
(cyclam).
A very interesting feature of luminescent dendritic ligands is the possibility of
their performing as sensors of metal ions with signal amplification. The advantage
of using a dendrimer for sensory applications is related to the fact that a single
analyte can interact with a great number of units and change their properties, which
results in signal amplification. In the case of luminescent dendrimers, one analyte
can quench the luminescence of all the peripheral luminescent units appended at the
dendrimer periphery (Fig. 2a ). This is a significant advance compared to a conven-
tional luminescent sensor, in which the ratio between quenched units and analyte is
1:1 (Fig. 2b ). This behavior is made possible by the well-ordered and flexible
dendritic structure in which any excited fluorophore can “feel” the presence of
the coordinated metal ion. Signal amplification effects have already been obtained
with polymeric chains of sensors [ 18 ], and nanoparticles [ 19 ]. Because of their well
defined and fully programmable structures, dendrimers are the more promising
a
b
Fig. 2 Schematic representation of (a) a luminescent dendritic sensor with signal amplification
and (b) a conventional luminescent sensor. The curved arrows represent interaction processes
which changes the luminescence properties (from empty to filled circles ). Analyte is represented
by a solid hexagon , while the recognition site is by an empty hexagon