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As a general practice the porphyrin-based potentiometric sensors needs to
be equilibrated at least 2 hours before calibration curves are obtained.
1. Salicylate-Selective Sensors
The salicylate-sensing membrane is prepared by incorporating 5,10,15,20-
tetraphenyl (porphinato) tin (IV) dichloride (TPP)Sn into a platicized
poly-(vinyl chloride) membrane (1 wt. % metalloporphyrin, 66 wt. % DBS
and 33 wt. % PVC).112 The resulting sensor exhibits an anti-Hofmeister
selectivity pattern, with high specificity over lipophilic inorganic
anions (selectivity coefficient log XTaiy -3.4 for Ρή4, -3.6 for IO4, -
2.5 for SCN , -3.7 for I " and -3.8 for Cl ) and biological organic
anions (-3.8 for citrate, - 3.9 for acetate and -4.0 for lactate).
(TPP)Sn-based sensors shows near-Nerstian response toward salicylate (55-
60 mV/decade) response time 2-10min and detection limit about 5 x 10 6 M.
The response mechanism of the salicylate sensor is complex. While
salicylate can bind at both axial-ligand coordination sites of Sn(IV),
water is also a veiy strong ligand that can compete for these inner-
sphere sites. It appears that salicylate complexation via hydrogen
bonding and perhaps some n-n interactions with the porphyrin ring is
responsible for the initial extraction of salicylate into the polymeric
membrane. However, at a higher salicylate concentration, a combination of
salicylate interactions, both outer- and inner-sphere ligands (displacing
the water) probably occurs.
The salicylate sensor responds to pH changes and its detection limit
deteriorates as the pH increases. The selectivity coefficient for the
salicylate relative to OH-, log ^Tfai. OH - = 4.2 (at pH 7.2), limits
practical applications of the salicylate sensor for in situ monitoring of
salicylate ions in biological systems.
Salicylate and its analogues, including acetylsalicylate (aspirin),
are commonly used as effective analgesics.
Despite their utility as pain relievers and antithrombotic agents,
salicylate can be quite toxic if taken in large doses. Recommended
therapeutic levels in plasma range from 0.15 to 2.1 mM. Enzymatic,
chromatographic and spectroscopic techniques measure "tota" not "free,"
salicylate although it is known that the latter correlates more closely
to the biological activity of the drug. Salicylate potentiometric sensors
are able to detect "free" salicylate without significant interference
from physiological chloride; however, pH sensitivity restricts its
practical applications to samples buffered below pH 6.0. But, the
practical utility of this sensor to measure salicylate levels in
biological samples (like serum samples diluted (1:10) with pH 5.5
buffer), and urine (buffered at pH 5.5) has been demonstrated.
2. Nitrite-Selective Sensors
Bromo(pyridine)5,10,15,20-tetraphenylporphyinato cobalt (pyTPP)Co is used
as a NO2 selective ion carrier.108 In dioctyl adipate plasticized poly-
(vinylchloride) membranes, the carrier induces NOy sensitivity with a
slope of -57 mV/decade and selectivity coefficients for Cl-, Br-, H2P04
andHPO4- of 5 x 10-4, 1.6 x 10-2, 2.1 x 10-4, and 6.1 x 10-4,
respectively. These membranes respond to pH changes above pH 6 with a
slope of -25 to -34 mV/pH unit. The performance of the (pyTPP)Co-based
sensor is comparable to the Co corrinoids. The membrane is prepared by
dissolving in tetrahydrofuran PVC (34 wt %), bis(2-ethylhexyl) adipate
(65.5 wt %) and (pyTPP)Co (0.5 wt %). The membrane is mounted in a
Phillips electrode body. A solution of 0.01 M NaN02 and 0.1 M NaCl is
used as the internal filling solution, and a silver/ silver chloride
electrode is used as an internal reference electrode. The sensor responds
logarithmically to changes in NO2 activity over a range of 10~1 to 10-5 M
with a slope of -57 mV/ decade (Figure 20).
3. 2-Hydroxybenzyhydroxamate Sensors
2-Hydroxybenzyhydroxamate (HBHA) is a therapeutic agent and its action is
based on inhibition of the bacterial enzyme urease.123 It has been
reported that bacterial urease elevates ammonia levels in the urinary
tract by catalyzing the decomposition of urea to ammonia, leading to the
concommitant precipitation of Ca2 + and Mg2 + salts and formation of
kidney stones. Polymeric membrane sensors formulated with either
tetraphenylporphyrin ((TPP)Sn) or tetrakis[p-fluorophenyl) porphyrin
(TFPP)Sn] with Sn(IV) as a central metal were shown to exhibit high
potentiometric anion selectivity toward HBHA and usually high selectivity
over lipophilic anions (such as salicylate, CIO 4, SCN , I-).114 The
addition of lipophilic anionic sites in the form of tetrakis[bis 3,5-
trifluoromethyl, phenyl]borate within the membranes was shown: (1) to
further enhance the observed potentiometric selectivity oxo-ligands with
higher pKa values, such as HBHA; and (2) to bind more strongly to the
central Sn(IV) of the porphyrin. Hence these are the preferred anions for