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by Walt et al.226 Much of the work in this area has been driven by a
desire to mimic the mammalian sense of smell, which is based upon a large
family (approximately 1000 members) of receptor cells.227 More than half
of the receptors respond to
any given odorant exposure228 and create a temporal-response signature
that is mapped in the olfactory bulb and interpreted by the brain.
While much work has been done for the detection of nonligating organic
odorants, detection of metal-ligating vapors via array-based sensing has
been less explored. Included in this class of analytes are noxious
ligands such as amines, phosphines and thiols, as well as more toxic
substances, for example, nerve toxins such as Tabun. Due to their
synthetic versatility and excellent chemical and thermal stability,
metalloporphyrins are a natural choice for the detection of such species.
Natale and coworkers have utilized metalloporphyrin films in
piezoelectric-sensing arrays, where they have shown some success in
of ligating vapors and determination of food quality." ' Their approach
has been to coat quartz crystalline microbalances with an array of
porphyrins, consisting of Ru(TPP)(CO), Rh(TPP)Cl, Mn(TPP)Cl, Co(TPP),
Sn(TPP)Cl2, Co(TpN02PP), Co(Tp-OCH3PP) and Mn-octamethylcorrole.
Responses to different ligating species occur upon ligand binding, which
induces mass changes detectable by the microbalances. As seen in Figure
131, the array shows high sensitivities for a series of ligating vapors,
though the response is limited for alcohols and sulfur-based ligands.
This type of array has been applied to the quality evaluation of several
types of foods such as fish, meats and wine.
Suslick et al. have recently exploited the colorimetric properties of
metalloporphyrin arrays for detection of
Chou et al.
? P -f-
* . I .
Figure 124. Three alignments of bridging ligands in metalloporphyrin
polymers and ORTEP plots of the packing diagrams.
various ligating vapors.231 Metalloporphyrin-visible spectra are known to
exhibit shifts in wavelength and intensity upon binding of axial ligands.
These shifts produce color changes that are often dramatic. The color
changes are dependent upon a variety of factors, but vary in large part
Figure 125. Response of Pt(OEP)-doped sol-gel glass on switching between
(a) 100% nitrogen and (b) 100% oxygen. Reprinted with permission from
Lee, S.; Okura, I. Analyst 1997, 122, 81.
the degree of polarizability of the analyte.232-211 Upon exposure to a
given analyte, therefore, an array of metalloporphyrins is expected to
give a unique color-change signature. A series of metalated
tetraphenylporphyrins was prepared and spotted onto a reverse-phase
silica gel plate to give the detector shown in Figure 132. Subtraction of
scanned images taken before and after exposure to analytes gives the
color-change profiles observed. All of the vapors studied reveal unique
color-change patterns. Particularly notable is the fact that ligands of
similar functionality, such as pyridine / hexylamine, or n-
tributylphosphine/triethyl-phosphite, are readily distinguishable.
Analyte concentrations of less than 1 ppm can be detected by the array.
Figure 126. A metalloporphyrin ketone, M(OEPK), R = C2H5.
41 / Porphyrin Materials Chemistry
Figure 127. Variation ot (a) pi,v(Cb) and (b) l4o with TBP plaslicizer
content. Reprinted with permission from Mills, A.; Lepre, A. Anal. Chem.
1997, 69, 4653. (c) 1997 American Chemical Society.
Ongoing work with this system involves incorporation of functionalized
metalloporphyrins, such as dcndrimer-metal-loporphyvins, which will he
2. Sensors for Solution Species
Porphyrin-based materials have been widely applied toward the analysis of
several types of solution species, including ions and organic analytes.
Much of the research in these
Figure 129. Structure of a conjugated Ni(OEP) dimer.