Anal. Chem. 1994,66, 2245-2249
Metalloporphyrin Derivatives as Neutral Carriers for PVC Membrane Electrodes De Gao, Jun-Zhong LI, and Ru-Qin Yu' Department of Chemistry and Chemical Engineering, Hunan University, Changsha, 4 10082 China
*
Guo-Dong Zheng Department of Chemistry, Jiln University, Changchun, 130023 China
The anion-sensitive membrane electrodes based on lipophilic metalloporphyrin derivatives (FeTPP)20 show similar antiHofmeister selectivity sequence: SCN- > I- > C104- > N02> B r > C1- > NOj-. The electrodewith an optimum membrane composition has a linear response for SCN- between and 10-l mol/L, with a slope of 53.0 mV/pSCN- (25 "C).The origin of the anti-Hofmeisterresponsecharacteristicshas been discussed in view of the coordination chemistry of the metalloporphyrins. The interaction mechanism between (FeTPP)zO and SCN- is studied by UV/vis and IR spectroscopy. The transfer process of SCN- across the membrane interface is investigated by ac impedance measurements. Anion membrane electrodes based on classical liquid ion exchangers have the same Hofmeister selectivity sequence: C104- > SCN- > I- > NO3- > B r > N02- > C1-. Searching for carriers with the selectivity sequence, which is different from the Hofmeister pattern, is the subject of a series of investigations.I-l2 Among the anti-Hofmeister sensing materials, the vitamin Bl2 derivatives and the metalloporphyrins were supposed to havea certain interaction between the central metal and response anions.'-* The alkyl tin compounds show selectivity sequence deviated from the Hofmeister pattern, which is due to the selective extracting ability of these compounds for some anion~.~-lI The lipophilic diquaternary ammonium salt with closely located nitrogen atoms show the anti-Hofmeister character, which is related to the interaction between the sensing material and the anion species to be sensed.12 Inthis paper, wereport the resultsoftheinvestigation of oxo bridge iron porphyrin (FeTPP)20 as a new neutral carrier sensing the thiocyanate ion. The origin of the antiHofmeister selectivity sequence observed is discussed; the UV/ vis and IR spectra of (FeTPP)20 interacting with SCN- as well as the transfer process of SCN- across the organic/water interface are studied. The preliminary application of the (1) Schulthess, P.; Ammann, D.; Simon, W.; Caderas, C.; Stepanek, R.; Krautler, B. Helv. Cfiim.Acta 1984, 67, 1026. (2) Ammann, D.; Huser. M.; Krauther, B.; Rusterholz, B.; Schultness, P.; Lindemann, B.; Halder, E.; Simon, W. Hela Cfiim. Acta 1986, 69, 849. (3) Chaniotakis, N. A.; Chasser, A. M.; Meyerhoff, M. E.; Groves, J. T. Anal. Chem. 1988,60, 188. (4) Hodinar, A,; Jyo, A. Cfiem. Left. 1988, 993. ( 5 ) Hodinar, A.; Jyo, A. Anal. Cfiem. 1989, 61, 1171. (6) Chang, Q.; Meyerhoff, M. E. Anal. Cfiim.Acta 1988,186, 81. (7) Chaniotakis, N. A.; Park, S.B.; Meyerhoff, M. E . Anal. Cfiem.1989,61,566. (8) A h , H.; Kokufuta, E. Bull. Cfiem.Soc. Jpn. 1990, 63, 1360. (9) Glazier, S.A.; Arnold, M. A. Anal. Cfiem. 1988, 60, 2540. (10) Glazier, S.A.; Arnold, M. A. Anal. Left. 1989, 22. 1075. (1 1) Glazier, S.A.; Arnold, M. A. Anal. Cfiem. 1991, 63, 754. (12) Wotring, V. J.; Johnson, D. M.; Bachas, L. G. Anal. Cfiem. 1990.62, 1506. 0003-2700/94/0366-2245$04.50/0 0 1994 American Chemlcal Society
e>/
\
-N'N-Pe - N /
I '
w
Figure 1. Structure of (FeTPP)*O.
(FeTPP)zO-based membrane electrode for body fluid is investigated.
EXPERIMENTAL SECTION Reagents. Tetraphenyl porphyrin (TPP) was prepared according to the method of Alder et al.13 Tetraphenyl porphyrinatoiron (FeTPPCl) was synthesized according to the method of Rothemund et al.14 p-Oxotetraphenyl porphyrinatoiron [(FeTPP)20] was prepared by treating the chloroform solution of FeTPPCl with aqueous NaOH solution for 2 h,15 and then the organic layer was refined by chromatography using the neutral AI203 column. The structure of (FeTPP)20 is shown in Figure 1. Bis(imidazo1e)p-oxotetraphenyl porphyrinatoiron [(Im)2(FeTPP)20] was prepared from (FeTPP)20 according to the methodof Epstein et a1.I6 All synthesized ionophores were identified by UV/ vis, IR, and elemental analysis. 2-Nitrophenyl octyl ether (o-NPOE) was prepared according to the literature method.17 Tetrahydrofuran (THF), didecyl sebacate (DDS), and dibutyl phthalate (DBP) of pure analytical grade and poly(viny1 chloride) (PVC) of chromatographic grade were purchased from Shanghai Chemical Co. Redistilled deionized water was used throughout. Apparatus. The potentiometric and pH measurements were carried out on a Model 901 microprocessor Ionalyzer (Orion, Cambridge, MA). Cell assemblies of the following type were (13) Alder, A. D.; Longo, F. R.; Finarelli, J. D. J . Org. Cfiem. 1%7, 32, 476. (14) Rothemund, P.; Menotti, A. R. J. Am. Cfiem.Soc. 1948, 70, 1808. (15) Strauss, H.; Pawlik, M. J.; Skowyra, J.; Kennedy, J. R.; Anderson, 0. P.; Spartalian, K.; Dye, J. L. Inorg. Chem. 1987, 26. 724. (16) Epstein, L. M.;Straub, D. K.;Maricondi, C. Inorg. Cfiem. 1967, 6, 1721. (17) Homing, E. E. OrganicSynthesis, John Wiley &Sons Inc.: New York, 1955; Collective Vol. 3, p 140.
AnalflicalChemistty, Vol. 86, No. 14, July 15, 1994 2245
used:
Table 1. Potontlomotrlc 8.kctMty C o M c h t a b g K-,W#
Hg, HgZCl,, KCl (satd.)(samplesolution)membrane)0.1mol/L of KClIAg, AgCl The solution was buffered with 0.01 mol/L of H3P04, and the pH was adjusted with a NaOH solution. The membrane composition was 1%(w/w) ionophore, 33% (w/w) PVC, and 66% (w/w) solvent mediator. The potentiometric selectivity coefficients,log K s c N - , ~ ~ x were obtained by the separate solution method. The solution was buffered with 0.01 mol/L of H3P04 and adjusted to pH 3.01 with a NaOH solution. The single ion activity was calculated by the extended Debye-Huckel equation. The ac impedance was recorded with a PAR M 368-2 system (EG&G Princeton Applied Research, Princeton, NJ) in 0.01 mol/L of H3PO4 solution buffered to pH 3.01; SCNconcentration ranged from 10-6to 10-l mol/L. The frequency range used was 105-10-3 Hz (25 "C). UV/vis and IR Spectra. The chloroform solution of (FeTPP)20 was treated with aqueous 1.0 mol/L NaSCN solution for 30 min; the organic layer was eliminated in a rotaevaporator. The UV/vis spectra were recorded on a PE X 17 spectrophotometer (Bodenseewerk Perkin-Elmer & Co. GMBH, D-7770 Ueberlingen, Germany). The IR spectra were recorded on a AQS-20 (Analect Instruments, a Division of Laser Precision Corp.). The colorimetric experiment was recorded at 457.5 nm based on the SCN- - Fe3+reaction according to the method of Butts et ale1*
RESULTS AND DISCUSSION Selection of Sensing Materials. Because of the specific characteristics of metalloporphyrins, some conditions are important for the incorporationof the ionophores as membrane active components. First, under atmosphericconditions,some metalloporphyrins exposed to air, moisture, or light might cause demetallation or polymeri~ation.~~ For instance, Mo(IV) and Mn(II1) porphyrinscan polymerize to (MoOTPP)20 and (MnTPP)20, respectively, in basic solution.20g21When an electrode incorporating MnTPPCl was conditioned in a basic solution, the membrane active component MnTPPCl would convert into (MnTPP)20. Chaniotakis et al.3 had reported the potentiometric response characteristics of MnTPPCl as the ionophore, but the real active component might be (MnTPP)20. Recently, the potentiometric characteristics of binuclear and mononuclear Mn(II1) porphyrins were discussed by the coordinationchemistry of metalloporphyrins as well as by quantum chemistry in more detail.22 Anyhow, the stability of membrane active components based on the metalloporphyrins must be considered. As FeTPPCl tends to polymerization,formingstable binuclear (FeTPP)20, the latter is a favorite candidate to be selected as the electro-active material (vide infra). Due to the instability, up to date, there (18) Butts, W. C.; Kuehneman, M.;Widdowson, G. M.Clin. Chem. 1974, 20, 1344. * (1 9) Dolphin, D., Ed. The Porphyrins;Academic Press: New York, 1978;Vol. 3A, p 417. (20)Johnson, J. F.; Scheidt, W. R. J . Am. Chem. Soe. 1977.99, 294. (21) Fleischer, E. B.: Palmer, J. M.; Srivastava, T. S.; Chatterjee, A. J. Am. Chem. SOC.1971, 93, 3162. (22) Gao, D.; Liu, D.; Yu, R. Q.;Zheng, 0. D. Submitted toSci. China, Ser. B.
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AnalyficalChemistry, Vol. 66, No. 14, Ju3/ 15, 1994
DDS O-NPOE DBP a
solvent mcdiator/anions ClO4- NOy B r
SCN-
I-
0.00
-2.17
-2.34
-2.73
-2.92
0.00 0.00
4.58
4 3 0 -1.52 4.60 -2.00
-1.88
-1.30
-2.50
C1-
NO,-
-3.04 -2.02 -2.90
-3.70 -1.37 -2.30
Measured by separate solution method.
is no literature reports on electrodes based on FeTPPCl. Secondly, as for vitamin Bl2 derivativesand metalloporphyrins, the coordination or interaction between the carriers and the analyte anions is one of the essential conditions for getting the anti-Hofmeister response pattern; such interaction is determined by the coordination sites in the carrier and the coordination affinity between the ionophore and the analyte anions. In vitamin B12 derivatives and metalloporphyrins, a counterion occupies the fifth axial coordination position,'" and the analyte anions can coordinate with the carrier only at the sixth axial coordination site. Introducing more than one coordinationposition in the carrier moleculewould improve the potentiometric response characteristics of the electrodes for anions. (FeTPP)20 is a very stable binuclear metalloporphyrin and has two coordination sites. Beside the lipophilicity of (FeTPP)20 which is better than that of the mononuclear metalloporphyrins, it is expected that the electrodes based on (FeTPP)20 have better potentiometric characteristics. EMF Response Characteristics and Selectivity. Table 1 summarizes the potentiometric selectivity characteristics of the membranes containing different solvent mediators. The selectivity sequence clearly differs from the classical Hofmeister pattern. This behavior is determined by the characteristics of both the sensing material and the anions. For classical liquid membrane electrodes,the responseof the anion is based on the ion-exchange properties between analyte anions and the c o ~ n t e r i o n . ~ The ~ electrostatic interaction plays the dominate role for the transfer of theanion across the organic/ water interface. The hydration energy of the analyte anions must be overcome by the electrostatic affinity, and the selectivity sequenceis determined by the order of the hydration energy or by the hydrophilicity of the analyte anions. For the ionophores based on vitamin B12 derivatives or metalloporphyrins, beside the electrostaticinteractionbetween the central metal and the analyte anions, there is a coordination action between both species. The anion hydration energy is conquered by both electrostatic and coordination forces. As the charges of carriers and analyte anions are fixed, the Hofmeister selectivity sequence of classical liquid membrane electrodes can only be altered, for instance, by the coordination affinity. Taking the anions Clod- and SCN-as examples,the hydration energy of SCN- is larger than that of CI04-, when the lipophilic quaternary ammonium salts are used as the sensing materials, the Hofmeister selectivity sequence is realized, and the C104preferentially enters the membrane phase. For (FeTPP)20 membrane, however, beside the electrostatic interaction between the membrane active component and analyte anions, (23) Morf, W. E. The Principle of Ion-Selecrive Electrodes and of Membrane Transport; Elscvicr Scientific Publishing Co.: Amsterdam, 1981; p 214.
there are two coordination sites in (FeTPP)20 at the two sixth axial positions and SCN- can preferentially coordinate with the ionophore at the two sites, whichleads to the selectivity sequence of anion response deviating from the Hofmeister pattern. In order toverify the hypothesis that the coordination action of membrane active component really plays a main role in the anti-Hofmeister response behavior, the compound bis(imidazo1e)-p-oxotetraphenylporphyrinatoiron [(Im)2(FeTPP)20] was synthesized and used to prepare the electrode membrane. Imidazole is one of the strongest electron-donating ligands; its coordination ability toward metalloporphyrins is larger than that of OH-. As the pH of the solution was adjusted to 8.98, imidazoles occupied the two sixth axial positions, the analyte anions could not coordinate with (FeTPP)20, and they entered the membrane phase mainly with the aid of electrostatic affinity. The selectivity sequence returned to the classical Hofmeister pattern. When the pH was adjusted to 3.01, the two imidazole molecules combined with H+ and dissociated from (FeTPP)20, and the two sixth axial coordination sites became empty and could be coordinated by analyte anions. The selectivity sequence became the antiHofmeister one: SCN- > I- > C104- > NO2- > B r > C1> NO3-. These results show theimportance of thecoordination action in altering the selectivity sequence and the possible mechanism of formation of the anti-Hofmeister selectivity sequence. Effect of pH on Response Characteristics of (FeTPP)20Based Membrane Electrodes. Figure 2 shows the effect of pH on response characteristics of (FeTPP)~O-basedelectrodes. SCN- and OH- ions seem to coordinate competitively with (FeTPP)20. The coordination ability of SCN- is stronger than that of OH-. As the concentration of SCN- is relatively high (10-1-10-3 mol/L), the SCN- ion is the primary analyte ion, and OH- does not cause remarkable interference for the SCN- response. The linear response characteristics of the electrode for SCN- ion are not affected by OH- substantially. As the concentration of SCN- is relatively low (10-3-10-7 mol/L), with the increase of OH- concentration, OH- becomes the primary analyte anion. The potentiometric response characteristics of the electrode for SCN- are deviated from the linear one, especially at high pH values. Similar results are obtained for different solvent mediators. For DBP, however, the response characteristics of electrode were not affected by the variation of pH values, although the DBP version showed narrower linear response range. Table 2 shows the response characteristics of (FeTPP)~O-based electrodes for different solvent mediators. Binuclear metalloporphyrins have lower polarity and higher lipophilicity than that of mononuclear metalloporphyrins, so the binuclear metalloporphyrins are more easily solvated in a solvent mediator with lower permitivity than in that with higher permitivity. It is clear that (FeTPP)20 will havegreat mobility in the lower permitivity solvents such as DDS than in the other ones. Consequently, the response characteristics are improved by using DDS as a solvent mediator. Although the association constant between the ionophore and the analyte anions is affected by the solvent mediators, the selectivity will change with the solvent mediators.
EMF
-7
-6
-5
-4
-3
-2
-1
Figure 2. Response characteristics of (FeTPP)@based electrodes: (a) DDS; (b) eNPOE; (0)pH 3.01; )(. pH 3.61; (A)pH 5.08; (0)pH 6.84; (*) pH 8.98. The x-axis values are Ig am Table 2. Rerponw Characterlstks of (FeTPP)&-Based Electrodes wHh Merent Solvent Medlatonr
solvent mediators slope (mV/pSCN-) linear range (mol/L) detection limit (mol/L)
DDS
0-NPOE
53.0 43.0 10-6-10-1 1W-10-1 3.98 X le7 6.31 X
DBP 58.0 10-4-10-1 6.31 X
Up to now, the best thiocyanate-sensitive electrodes based on metalloporphyrins had been reported by D a ~ n e r t The .~~ response characteristics of a (FeTPP)~O-basedelectrode such as the slope, linear range, and detection limit compared favorably with those of Daunert’s, which was expected to get application in clinical chemistry. Interaction Mechanism of (FeTPP)zO with SCN-. Although it has been supposed that there is an interaction between metalloporphyrins and anions, the interaction mechanism has not been studied in depth so far. It is interesting to investigate the interaction between (FeTPP)20 and SCN- for the elucidation of the potentiometric response mechanism. Figure 3 shows the UV/vis spectra of (FeTPP)20 with and without interacting with SCN-. A summary of electronic absorption spectral data for (FeTPP)20 and (FeTPP)+ SCN- is given in Table 3. Comparing the spectra of (FeTPP)20 and (FeTPP)20SCN-, one notices that the Soret band of 407.2 nm shifts to 409.2 nm (the precision of the spectrophotometer is of the order of 0.2 nm) with increased (24) Daunert, S.;Wallace, S.;Florido, A.; Bachas, L.G.Awl. Chem. 1991, 63, 1676.
Analytical Chemism, Vol. 66, No. 14, Ju& 15, 1994
2247
1.0
0
P3 d
f
0.8 0.8
I
0.6
-
A
0.4 0.2 .-
-340.32
430.24
520.16
-
700.00
(FeTPP)20(SCN-)2.
1 .WE4
Tabk 3. UV/vb Poakr (CHCb, nm)
319.2 320.4
610.08
Wavelength(nm1
Flgure 3. UV/vls spectra (CHCis soiutlon): (- -), (FeTPPkO (-),
(FeTPP)20 (FeTPP)ZO(SCN-)z
--.
- \\ -2
0.0 250.40
407.2 409.2
569.6 570.4
611.2 610.8
h
c
Z'(ohms)
1.200E5
1.00E4 Flgure 5. ac impedancespectra of (FeTPPhO-based electrodex 0.01 m l / L NaSCN solution, pH 3.01.
I
-200
I
1600
1200
800
100
Flgwo 4. I R spectra (KBr pellets): (a, top) (FeTPPkO; (b, bottom) (FeTPP)20(SCN-k. The x-axis values are wavenumbers (cm-l).
intensity. This is characteristic for the SCN- coordination with the central metal.25 The Soret band reflects the valence state of the central metal. The coordination of SCN- with the central iron metal leads to the delocalization of the iron d orbitals, whichcauses the red-shift of theSoret band. Figure 3 also shows that the numbers of the absorption peaks do not change after (FeTPP)ZO is coordinated with SCN-. This indicates that, after SCN- is coordinated with the central metal, the symmetry group of (FeTPP)ZO-SCN- agrees with the Cb symmetry group of ( F ~ T P P ) Z O ,and ~ ~ two * ~ ~SCNanions are coordinated with two central iron atoms in (FeTPP)20 producing (FeTPP)20(SCN-)2. Figure 4 shows the IR spectra of (FeTPP)20 and (FeTPP)20 interacting with SCN-. For (FeTPP)20, the vibrational frequency at 879.5cm-l is due to the F e O bond antisymmetry stretching;15 1003.0 cm-1 is due to the Fe-N (25) Jeffrey, H.; Helms, L. W. H.; William, E. H.; David, L. H. Inorg. Chcm. 1986, 25. 2334. (26) Hoffman, A. B.; Collins, D. M.;Day, V. W.; Fleischer, E. B.; Srivastava, T. S.;Hoard, J. L. J. Am. Chcm. Soc. 1972, 94, 3620. (27) Dolphin, D., Ed. The Porphyrins; Academic Press: New York, 1978;Vol. 3A, p 111.
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A m M h I m k t t y , Vd. 66, No. 14, Ju& 15, 1994
characteristic stretching.2* After (FeTPP)zO is coordinated with SCN-, the characteristic vibration of H20 at 3461.8 cm-l disappears. At the same time, the characteristic vibrational frequencies of SCN- at 2067.8 and 2016.3 cm-I appear. This indicates that two SCN- anions occupy the positions of two H2O and coordinatewith the nitrogen atomsz9 The electron delocalization on iron metal reduces the charge density in the vicinity of the Fe atom, which weakens the F e O and Fe-N bond strength. The vibrational frequencies of Fe-O and F e N shift to 874.4and 992.7cm-I, respectively. The UV/vis and IR spectra confirm that there is a direct interaction between the central metal and the analyte anions. Transfer hocess of SCN- across Organic/Water Interface. Figure 5 shows the ac impedance spectra of an (FeTPP)20based electrode. The ac impedance measurement shows that the bulk resistance decreases with the increasing SCNconcentration. Negatively charged SCN- anions coordinate with the neutral carrier (FeTPP)zO and move across the membrane interface, reducing the membrane impedance. The process is controlled by diffusion. heliminaryApplicationfor ClinicalCbemistry. A chemical test to distinguish smokers and nonsmokers is important in many epidemiologic studies. Experimental results have shown that the urinary thiocyanate concentration is higher for smokers than for nonsmokers, and the assay can be used to detect whether or not a person is smoking. Table 4 lists thiocyanate concentration in human urine measured by the ~~~
(28) Boucher, L. J.; Katz, J. J. J. Am. Chrm. Soc. 1%7,89, 1340. (29) Nakamoto, K.Infrared and Ramon Spectra of Inorganic and Coordination Compounds, 3rd ed.;John Wiley & Sons Inc.: New York, 1978; p 270.
T W 4. ol SCN- Concmlratbn In Ulkn m 8mokm and "nokm
no. 1 2 3 4
o
l
component, the response mechanism of electrodes sensing SCN- is of the following type:
nonsmokers (mmol/L) sample smokers (mmol/L) electrode colorimetric+ no. electrode colorimetri@ 0.25 0.18 0.24 0.26
0.25 0.19 0.25 0.27
5 6 7 8
0.62 0.46 0.68 0.84
0.63 0.46 0.70 0.85
H\ /H
0
I
Urine samples aredilutcd 10-foldwith 0.01 m d Lof H3PO-NaOH solution, pH 3.01. Urine samples are diluted 10- old according to the method of Butts et al.Ia io\
(FeTPP)~O-basedelectrode as well as the colorimetric results for smokers and nonsmokers. As shown in the table, there is a fair correlation between the values obtained by the electrode and the colorimetric methods.
CONCLUSIONS For plane macrocyclic metallorganic compounds such as vitamin B12 derivatives and metalloporphyrins, there is a coordination action between the central metal and the analyte anions. Under certain conditions, the coordination ability determines the selectivity sequence of anions to be sensed. When the (FeTPP)20 is used as the membrane active
H
H
ACKNOWLEDGMENT The project was supported by the National Natural Science Foundation of China and partially by the Laboratory of Electroanalytical Chemistry,Changchun Institute of Applied Chemistry, Chinese Academy of Science. Recehred for revlew November 10, 1993. Accepted March 28, 1994.. Abstract published in Advance ACT Abstracrs, May
IS, 1994.
AMlyticelChedby, Vd. 66, No. 14, Ju& 15, 1994
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