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length of the bridge or "handle." This allows for sensing over a wide
range of oxygen concentrations. In related studies, Pretsch and coworkers
have utilized picket-fence cobalt porphyrins for optical detection of
oxygen."21 Immobilized on poly(octylmethacrylate-co-l-vinylimida-zole) or
41 / Porphyrin Materials Chemistry
n + 2
Figure 119. Synthesis of one-dimensional porphyrin-oligothiophene
polymers. Reprinted with permission from Segawa, H.; Kunimoto, K.;
Susumu, K.; Taniguchi, Ì.; Shimidzu, T. /. Am. Chem. Soc. 1994, 776,
11193. (c) 1994 American Chemical Society.
ñî- 1-vinylimidazole) polymer membranes, the porphyrin gives a readily
detectable shift in its visible absorbance spectrum upon binding of
molecular oxygen. The useful range of the sensor is 1 -1000 kPa oxygen
partial pressure, or 0.1 -100% of atmospheric pressure. Optimum response
is achieved with the fluorinated membrane, at a thickness of
20 (im, which gave a 90% response time range of 5-15 seconds.
b. Other Gases
Porphyrin-based sensors have also been used for the detection of gases
such as ammonia, hydrazine and NO.
Chou et al.
Figure 120. Schematic representation of a ferroelectric coordination
polymer and dipole moment switching in response to an external field.
Figure 121. The dipole moment of metalloporphyrins.
Narayanaswamy and coworkers have developed optical ammonia-sensing films
based upon immobilized Zn(TPP) in silicone rubber.222 Upon ligation of
the metal center, the well-known phenomena of visible and/or fluorescence
spectral shifts can be used to quantify exposure to the analyte. The
Zn(TPP)-silicone films gave a linear range of
0-8.5 ppm NH3 with a detection limit of 0.7 ppm. The equilibration time
upon exposure was found to be 4 minutes with good reversibility of the
ligation. This approach, of course, could be expanded to other amines, as
well as to other ligating vapors. Ammonia detection has also been studied
by Valli and coworkers, who have explored LB films grown from a
conjugated dimer of nickel(II) octaethylpor-phyrin 21 (Figure 129). The
films were created by combining the dimer with arachidic acid, followed
by deposition onto hydrophobic quartz substrates. Gold contacts were
sputtered onto the film ends to allow for resistivity measurements as a
function of vapor exposure. As seen in
RELATIVE POSITION OF METAL TO PORPHYRIN PLANE
Figure 122. The double-well potential of the position of the metal atom
relative to the porphyrin core.
Figure 130, the sensor demonstrates a series of equilibria as the ammonia
concentration is increased in a stepwise fashion, followed by a return to
the original resistance value upon exposure to air. At 500 ppm of NH3,
the response time is 200 seconds, with a recovery time of 500 seconds.
The authors postulate that the decreased resistivity upon analyte
exposure results from electronic holes created in the porphyrin film upon
electron transfer to ammonia molecules.
Detection of nitric oxide has become particularly important in light
of its regulatory role in many physiological processes. In an example of
NO detection via porphyrin-based sensors, Malinski and coworkers have
used microelectrode sensors consisting of layers of a polymeric
41 / Porphyrin Materials Chemistry
6 (Óñãô é
pyCN Clpyz Fpyz OpyCN 4-MelmH
V' V°~ X.
0pyC02 pyC02 ImPhO 4-Melm 4-Phlm his
o^o' ntp cnp
Figure 123. Three classes of the nonsymmetrical bridging ligands.
porphyrin and Nation deposited on a thermally sharpened carbon fiber.224
polymerized onto the fiber electro-chemically from a solution of 0.1 M
NaOH containing the monomer. The resulting sensor operates based on the
electrochemical oxidation of NO at the porphyrin-doped electrode. A 10-ms
response time and a detection limit of lOnM have been observed, and the
sensor has been applied to NO analysis from single endothelial cells in
the pulmonary artery as well as to NO quantitation in the blood.
c. Porphyrin Array Vapor Detectors
Array-based sensing has emerged as a powerful approach to vapor
detection. Combinations of chemically diverse sensing elements are
capable of responding to a variety of analytes. Materials such as
polymers, functionalized selfassembled monolayers, metal oxides and
dendrimers have been used in electronic devices via coupling with
piezoelectric, SAW and semiconductor transducers. In notable examples,
Lewis and coworkers have utilized composites of carbon black and polymers
for electronic sensing,225 while immobilization of fluorescent dyes in
polymer matrices has allowed for optical detection of nonligating vapors