todes) in this field remains an important project of research, since
the optodes can offer advantages that the analytical signal is free
of influence of an electromagnetic field and easy to transmit over
a long distance. In recent years, many effective optical chemical
sensors for amines and amino acids have been successfully
developed;18-22 however, very few have been explored for imida-
zole derivatives. Notable exceptions include the work of Fabbrizzi
and co-workers, who developed a competitive noncovalent fluo-
rophore-displacement method that selectively responds to imida-
zole and histidine in aqueous solution.23,24 Although the nonco-
valent assembly provides a simple block approach for chemo-
sensing, the resulting ensemble may be less stable than a
covalently linking sensory system.
The construction of a usefully covalent sensory molecule for
an optical sensor is dependent on two general design features.
One requirement is that a reversible recognition element be
present to provide selectivity for a given analyte. The second
general requirement is that the sensor must be able to exhibit a
measurable response signal to the recognition process. The
development of recognition sites in optical sensors has been
vigorously pursued in the past decades. One of the most important
of them that can be utilized in optical sensors involves the specific
metal-ligand interactions of metalloporphyrins.25-31 Recently,
Meyerhoff and co-workers reported a novel optode toward chloride
ion30 and amine vapors31 based on the dimer-monomer equilib-
rium reaction of indium(III) octaethylporphyrin in a polymeric film.
The ligation of the analyte to the In(III) center, concomitantly
breaks the equilibrium of the dimer and monomer of the
porphyrin, yielding changes in the absorption spectra of the
porphyrin. Indeed, it has been shown that in some cases, the
Mn(III), Co(III), Sn(IV), and Ga(III) porphyrins also exist in
monomer-dimer equilibrium interactions by hydroxide ion
bridging.32-36 These equilibrium interactions have been extensively
applied in optical sensing toward anions and neutral molecules.
It should be noted, however, that even though the number of such
reports is large, the response signals of these sensors were
confined to absorbance changes. While absorbance is sufficiently
sensitive for many routine applications, it is not sensitive enough
for microscopic measurements. The higher sensitivity of fluores-
cence over absorbance is established. Further, the use of
fluorescence in an optical sensor provides a means to significantly
reduce the required membrane volume while retaining the same
signal-to-noise ratio, which in turn will shorten the response time.
We are interested in developing new macrocyclic ligands as
potential fluorescent sensors toward cationic ions37,38 and neutral
molecules.22,39 Inspired by the success of the previous work of
Meyerhoff et al., the goal of the present work is to extend the
metalloporphyrin monomer-dimer equilibrium interaction to a
novel fluorescent sensor.
There are numerous mechanisms by which fluorescent signal
transduction may be affected. Recently, we reported a fluorescent
sensor for silver ion based on an intramolecular exciplex formed
in the monomer-dimer equilibrium system of the excited fluo-
rophore, pyrene.38 The large spectral shift for pyrene excimer
versus monomer emission and high fluorescence quantum yields
make such molecules attractive as fluorescence probes and
chemosensors.40-44 We report herein the first attempt to apply
the pyrene excimer fluorescence emission in the metalloporphyrin
monomer-dimer equilibrium reaction to signal the molecule
recognition process fluorescently. Figure 1 shows the structure
of the new design sensory molecule, Py-Zntpp. In alkaline solution,
the ligand-free state of Py-Zntpp is present in its monomer species,
and the fluorophore, pyrene, emits weak monomer fluorescence
at 378 and 397 nm due to intramolecule photoinduced electron
transfer (PET).45 In the presence of a bridging ligand such as
histidine, the ligation of the imidazole residue to the Zn(II) center
of the porphyrin, causes the monomer species to be converted to
dimer, yielding the strong excimer emission of pyrene at 454 nm
with a little increase in the monomer fluorescence. The sensor
was constructed and applied in the fluorescence assay of histidine
in aqueous solution by immobilizing the sensing material in a
plasticized poly(vinyl chloride) (PVC) membrane. The sensor was
optimized and characterized with respect to response character-
istics, membrane formulation, influence of pH, and the resultant
amino acid selectivity.
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