Substituted metalloporphyrin derivatives as anion carrier for PVC membrane electrodes

ANALmcA CHIMICA

ACIYA ELSEVIER

Analytica

Chimica Acta 302 (1995) 263-268

Substituted metalloporphyrin derivatives as anion carrier for PVC membrane electrodes De Gao a, Jun Gu a, Ru-Qin Yu ay*, Guo-Dong Zheng b a Department of Chemistry and Chemical Engineering, Hunan Unkersity, Changsha 410082, China b Department Received

of Chemistry, Jilin University, Changchun 130023, China

18 May 1994; revised manuscript

received 30 August 1994

Abstract The lipophilic metalloporphyrin derivatives containing electron donating or withdrawing groups are used as anion carriers with selectivity characteristics deviated from classical Hofmeister series. The electron effect of the carrier on the selectivity characteristics has been interpreted in terms of the selective coordination action along the axial position of metalloporphyrins. An metalloporphyrin derivative with electron-donating groups is preferred for the preparation of this type of anion-sensitive membrane electrodes. Keywords;

Substituted

metalloporphyrins;

Coordination

action

1. Introduction Chemical sensors have emerged as viable tools to traditional methods of analysis. The active materials used in electrochemical sensors should be capable of analyte recognition. Because the ion recognition process involves both reversed and selective binding of ions, the design and synthesis of new carriers for interesting species currently becomes a very interesting area of sensor research. The classical liquid membrane electrodes based on quaternary ammonium salts used to display the Hofmeister selectivity pattern (i.e., ClO; > SCN- > I- > NO; > Br- > NO, > Cl-) [l]. The Hofmeister selectivity sequence actually follows the order of the

* Corresponding

author.

0003-2670/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved .SSDI 0003-2670(94)00447-l

hydration energy of anion species. Searching for lipophilic anion carriers, which can more effectively destroy the hydration shell of the anions located at the end of the sequence, would deviate the selectivity pattern of the analyte anions from the Hofmeister series and some interesting anion species can be sensed accurately. Recent studies have shown that the anti-Hofmeister behaviour can be achieved by using metalloporphyrins as membrane active components [l-9]. The interaction mechanism between the ionophore and the analyte anion has been related to the coordination action along the fifth or sixth positions of the metalloporphyrins [2]. As the analyte anion reversely binds with the ionophore along the axial direction, the structure of the metalloporphyrin used, the steric and electron effect or porphyrin ring will obviously affect the selectivity of the carrier for anions. Chaniotakis et al. [2] have demonstrated that the steric effect of the metalloporphyrin ring has

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Chimica Acta 302 (1995) 263-268

significant effect on the selectivity observed. In this paper, some substituted metalloporphyrin derivatives with electron-donating or electron-withdrawing groups were used and compared as neutral carriers for the preparation of anion-sensing electrodes. The effect of the carrier structure on the selectivity characteristics and the interaction mechanism involved have been investigated.

trioctylammonium iodide (MTOAI) was synthesized according to the literature method [15]. All ionophores synthesized were purified by chromatography using a neutral Al,O, column and identified by UV-visible, IR spectrometry and elemental analysis. Poly(viny1 chloride) (PVC) powder of chromatographic grade, tetrahydrofuran (THF) and dodecyl sebacate (DDS) of analytical purity grade were purchased from Shanghai Chemical Co. Redistilled deionized water was used throughout.

2. Experimental 2.2. Apparatus 2.1. Reagents 5,10,15,20-Tetraphenyl porphyrin (TPP) was prepared according to the method of Alder et al. [lo]. 5,10,15,20-Tetra(p-methoxyphenyl) porphyrin [(pOCH,)TPP] was synthesized according to the method described by Walker [ll]. 2-Nitro-5,10,15,20-tetraphenyl porphyrin [(2-NO,)TPP] was prepared from TPP by following the procedure of Hombrecher et al. [12]. Chloro(5,10,15,20-tetraphenyl porphyrinato) manganese (MnTPPCl), chloro[5,10,15,20-tetra(pmethoxyphenyl) porphyrinato] manganese[(pOCH,)MnTPPCl] and chloro(2-nitro-5,10,15,20-tetraphenyl porphyrinato) manganese [(2-NO, )MnTPPC11 were synthesized according to the method of Alder et al. [13]. The structures of these metalloporphyrins are shown in Fig. 1. Bis(imidazole)5,10,15,20-tetraphenyl porphyrinato manganese [MnTPP(Im),Cl] was prepared from MnTPPCl according to the method of Epstein et al. [14]. Methyl-

/qR1 Rl

C----R1 A

P b

RI

Fig. 1. Structure of manganeseporphyrins. (1) MnTPPCI, R, = H, R, = H; (2) (p-OCH,)MnTPPCI, R, = OCH,, R, = H; (3) (2NO,)MnTPPCI, R, = H, R, = NO,.

Potentiometric and pH measurements were carried out on a Model 901 microprocessor ionalyzer (Orion, Cambridge, USA). Cell assemblies of the following type were used: Hg IHg,Cl 2IKCl(sat.)]lsample solution Imembrane(0.1 mol/l KC1kg (AgCl. The solution was buffered with 0.01 mol/l H,PO,, the pH was adjusted with NaOH solution. The membrane composition was typically 1% (w/w) ionophore, 33% (w/w) PVC and 66% (w/w) solvent mediator. The membranes were prepared by using 165 mg PVC, 330 mg solvent mediator and 5 mg of ionophore. First, the ionophore was dissolved in the solvent mediator. Then the PVC and sufficient amount of THF were added and mixed to obtain a transparent solution. This mixture was transferred onto a glassplate of 20 cm2, and the THF was allowed to evaporate at room temperature leaving a though flexible membrane trapped in a PVC matrix. As the concentration of the ionophore is at a higher concentration, the solubility of the carrier in the membrane is exceeded and this results in a heterogeneous membrane phase. The potentiometric selectivity coefficients, log KS&-X- were obtained by the separate solution method [16]. The solution was buffered with 0.01 mol/l H3P0, and adjusted to pH 5.0 with NaOH solution. The single ion activity was calculated by the extended Debye-Huckel equation. 2.3. W-visible

spectra

The chloroform solution of a metalloporphyrin was treated with a blank buffered solution or a buffered 0.01 mol/l NaSCN solution for 2 min, the UV-visible spectrum of the organic layer was

D. Gao et al. /Analytica

recorded on a PE Lambda (Perkin-Elmer, oberlingen).

Chimica Acta 302 (1995) 263-268

17 spectrophotometer

3. Results and discussion A very useful property of the metalloporphyrin derivatives used as anion carriers is their ability to coordinate with some analyte anions at the fifth and sixth axial positions of the carrier molecule producing selective interaction and inducing the selectivity sequence for anions deviated from the Hofmeister pattern. The coordination action of metalloporphyrins thus plays the central role in the search for carriers with anti-Hofmeister selectivity sequence. For the conjugated nature of the porphyrin ring, the electron donating or withdrawing groups on the periphery of the molecule have been shown to affect the basicity of porphyrin nitrogens [17]. This, in turn, often affects the axial ligation reaction of the metalloporphyrins. It is interesting to investigate the electron effect of the porphyrin ring on the response characteristics of the electrodes for anions. 3.1. EMF response characteristics

and selectivity

Table 1 gives the selectivity coefficients of the electrodes based on different carriers and the anionic hydration energy. The selectivity sequence of MnTPPCl, ( p-OCH,)MnTPPCl and (2-NO,)MnTPPCl apparently differs from the Hofmeister selectivity pattern of the classical membrane ionophore MTOAI under comparable experimental conditions. This be-

Table 1 The selectivity

coefficients

MnTPPCl SCN CIO, SalNO, NO; I_ BY Cl-

-

0.0 2.34 0.76 3.29 2.80 2.25 2.70 3.05

log Kgh-

.-

and hydration

265

haviour is determined by the properties of both the carriers and the anions. For classical liquid membrane electrodes, the anion response is based on the ion exchange properties between analyte anions and the counterion [ 181, the electrostatic interaction plays a dominant role for the transfer of the anion across the organic/water interface. The hydration energy of the analyte anions must be overcome by the electrostatic affinity and the selectivity sequence is determined by the order of hydration energy or hydrophilicity of analyte anions. For the ionophores based on metalloporphyrins, besides the electrostatic interaction between the central metal and 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 the carriers and analyte anions are fixed, the Hofmeister selectivity sequence of classical liquid membrane electrodes can be altered by the coordination affinity. Taking the anions ClO, and SCNas examples. The hydration energy of SCNis - 287 kJ/mol, larger than - 214 kJ/mol, of Cloy. When the lipophilic quaternary ammonium salt is used as the sensing material, the electrostatic force plays an important role in the realization of the Hofmeister selectivity pattern. This results in the ClO; preferentially entering the membrane phase. For the metalloporphyrin membrane electrodes, however, besides the electrostatic interaction between the membrane active component and the analyte anions, there is an unoccupied coordination site at the sixth axial position. The coordination affinity will display a unique action. For the analyte species, the larger

energy of anion species a

(2-NO,)MnTPPCI

(p-OCH,)MnTPPCI

MnTPPImzCl

MTOAI

Hydration

0.0 -2.14 - 0.36 - 3.03 - 2.49 - 2.00 - 2.60 - 2.91

0.0 - 2.40 - 1.00 - 3.26 - 2.81 - 2.44 -2.80 -3.18

0.0 -0.12 - 1.00 - 2.80 -3.10 - 1.60 - 3.00 - 4.20

0.0 1.10 -0.15 - 0.90 - 0.55 - 0.30 - 1.00 - 1.40

- 287 -214 - 306 -339 - 283 -321 - 347

a Determined relative to SCN-, ’ Taken from Ref. [19].

using the separate solution method, in a 0.01 mol/l

H,PO,-NaOH

buffered

energy

b (kJ/mol)

solution, pH 5.0.

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Chimica Acta 302 (1995) 263-268

the anion is, the more delocalized the charge and the more weakly coordinating with the carrier it will be. It is well known that the coordination ability of SCN- is stronger with respect to ClO,. The strong coordination action of SCN- to the carrier induces SCNpreferentially to coordinate with the ionophore, therefore, the selectivity sequence of response anions is deviated from the Hofmeister selectivity order. In order to verify the hypothesis that the coordination action of the membrane active component really plays the dominant role of anti-Hofmeister response behaviour, the compound bis(imidazole)-5,10,15,20tetraphenyl porphyrinato manganese, MnTPPIm,Cl, 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 6.5, imidazoles occupied the fifth and sixth axial positions, the analyte anions could not coordinate with MnTPPIm,Cl and they entered the membrane phase mainly with the aid of electrostatic affinity. The selectivity sequence essentially returned to the classical Hofmeister pattern. While the pH was adjusted to 4.05, the two imidazole molecules combined with H+ and dissociated from MnTPPCl, the two axial coordination sites became empty, then one was occupied by the counterion, and the another could be coordinated by analyte anions. The selectivity sequence became an antiHofmeister one: SCN- > Sal- > I- > ClO; > Br- >

-6

-5

-4

-3

-2

-1

-6

-5

NO, > Cl- > NO,. These results show the role of the coordination action in altering the selectivity sequence and the possible mechanism of the antiHofmeister selectivity sequence. To a certain extent, the results obtained show that the electrode based on the metalloporphyrin derivative with electron-donating groups (p-OCH,)MnTPPC1 enhances the selectivity of SCN- with respect to other anions, whereas the electrode based on (2NO,)MnTPPCl containing an electron-withdrawing group decreases the selectivity for SCN-. The result can be explained in terms of the coordination chemistry of metalloporphyrins. The coordination action between the metalloporphyrin and the analyte anion can be treated by the concept of generalized acidbase reaction [20], the acidity of the central metal determines the coordination strength between the porphyrin and the analyte anion as well as the selectivity of the primary anion with respect to other anions. For the metalloporphyrin with (2-NO,) group, the electron-withdrawing group increases the acidity of the central metal ion (Hammett substituent constant is 1.25 with respect to that of unsubstituted metalloporphyrin) [21], and then enhances the coordination action between the metalloporphyrin and all the analyte anions. This effect reduces the selectivity of (2-NO,)MnTPPCl for the primary anion SCNwith respect to other anions and makes the whole UV-visible spectrum shifted to the red in comparison with the unsubstituted metalloporphyrin MnTPPCl. On the other hand, for (p-0CH3)MnTPPCl, the

-4

-3

-2

-1

-6

-5

-4

-3

-2

bOgabSCN-

Fig. 2. PotentiometricpH response of electrodes: (a) MnTPPCI; (b) (2-NO,)MnTPPCl; (*) pH 6.5.

(c) (p-OCH,)MnTPPCl;

(0)

pH 5.0; (.) pi 5.5;

D. Gao et al, /Analytica

Wavelength

Chimica Acta 302 (1995) 263.-268

(nm)

Fig. 3. UV-visible spectrum of MnTPPCl with and without SCN- solid line: MnTPPCI; dashed line: MnTPPCl treated with blank buffered solution; -.- -: MnTPPCl treated with SCNsolution.

electron-donating effect decreases the acidity of manganeseporphyrin and enhances the selectivity for the primary anion. 3.2. The effect

of pH

on response

characteristics

of

the electrodes

Fig. 2 shows the effect of pH on response characteristics of MnTPPCl, (2-NO,)MnTPPCl and (pOCH,)MnTPPCl based electrodes. When the pH is adjusted to 6.5, the linear range of both MnTPPCl and ( p-OCH,)MnTPPCl based electrodes is 10p4lo- mol/l with a detection limit of 3.2 X lo-’ and 2.0 x 10e5 mol/l, respectively. Under the same conditions, the linear range of the (2-NO,)MnTPPCl based electrode is only 10-3-10-1 mol/l and the detection limit is 6.3 X 1O-5 mol/l. When the pH is adjusted to 5.0, the linear range of all metalloporphyrin-based electrodes becomes lo-“-lo-’ mol/l. Table 2 UV-visible

The results show the effect of pH on response characteristics of the electrodes. SCN- and OH- ions seem to coordinate competitively with manganeseporphyrin, the coordination ability of SCN- is stronger than that of OH-. As the concentration of SCN- is relatively high (10-‘-10-~3 mol/l), the SCN- ion is the primary analyte ion and OH- does not produce interference for the SCN- response, the linear response characteristics of the electrode for SCN- ion are not substantially affected by OH-. As the concentration of SCN- is relatively low (lo-‘lo-” mol/l), with the increase of OH- concentration, OH- becomes the primary analyte anion, the potentiometric response characteristics of the electrode for SCN- were deviated from the linear one, especially at high pH values. It can be seen that when the pH is raised, the effect of pH on the (2-NO,)MnTPPCl based electrode becomes more significant in comparison with other two electrodes. As the acidity of (2NO,)MnTPPCl is the highest one among these three metalloporphyrin derivatives, the coordination action between the (2-NO,)MnTPPCl and OH- is enhanced. When the SCN- concentration is relatively lower, the effect of OH- becomes quite significant. The result is in agreement with the above discussion about the phyrins.

coordination

3.3. UV-lisible

chemistry

of metallopor-

spectra

Although it has been supposed that there is an interaction between metalloporphyrins and anions, the interaction mechanism has not been investigated

spectra of manganeseporphyrins

MnTPPCl CHCI, solution treated with buffered blank solution treated with buffered SCN- solution

267

357.9, 381.0, 458.7, 561.9, 597.7 360.3, 375.5, 450.1, 552.7, 590.3 364.7, 384.0, 462.7, 557.0, 596.5

(2-NO,)MnTPPCl CHCI, solution treated with buffered blank solution treated with buffered SCN- solution ( p-OCH 7)MnTPPCl

358.3, 387.3, 443.3, 570.0, 611.5 360.0, 385.0, 458.5, 562.0, 606.0 371.9. 389.9, 466.5, 568.0, 610.0

CHCI, solution treated with buffered blank solution treated with buffered SCN- solution

368.9, 388.1, 461.1, 566.5, 605.0 366.4, 381.0, 453.5, 560.0, 603.0 370.7, 390.0,465.1, 570.0,606.5

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Chimica Acia 302 (1995) 263-268

carefully so far. It is interesting to investigate the interaction between metalloporphyrin and SCN- for elucidation of the potentiometric response mechanism. Fig. 3 shows the UV-visible spectra of manganeseporphyrin derivatives with and without interacting by SCN-. A summary of electronic absorption spectral data is given in Table 2. Comparing the spectra of manganeseporphyrin interacting with SCN-, one notices that when the organic phase containing manganeseporphyrin was treated with a blank buffered solution, a blue shift of the Soret band of porphyrin was observed. This may be correlated to the solvation of metalloporphyrins in different solvents (221. When the organic phase was treated with buffered SCN- solution, the red shift of the Soret band of porphyrin took place. This has been quite characteristic for the SCN- coordination with the central metal. For manganeseporphyrin, the Soret band reflects the valence state of the central metal. The coordination of SCN- with the central manganese metal leads to the delocalization of the manganese d orbitals, and induces the red shift of the Soret band [9]. This indicates that there is a direct interaction between the central metalloporphyrin and the anion species involved. In conclusion, one realizes that although the electron effect of the porphyrin ring on the selectivity of the electrode is not as significant as that of the steric effect, but it enhances the interaction between the carrier molecule and the analyte anions, reducing the selectivity for the primary anion with respect to other anions. The use of electron-donating metalloporphyrin derivatives is preferred also in terms of reduced pH-dependence of the potentiometric response of the electrodes prepared. Acknowledgements

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. The authors thank professor M.E. Meyerhoff for sending the reprint of Ref. [7]. References [l] D. Ammann, M. Huser, B. Krauther, B. Rusterholz, P. Schulthess, B. Lindemann, E. Halder and W. Simon, Helv. Chim. Acta, 69 (1986) 849. [2] N.A. Chaniotakis, A.M. Chasser and M.E. Meyerhoff, Anal. Chem., 60 (1988) 188. [3] A. Hodinar and A. Jyo, Chem. Lett., (1988) 993. [4] N.A. Chaniotakis, S.B. Park and M.E. Meyerhoff, Anal. Chem., 61 (1989) 566. [5] A. Hodinar and A. Jyo, Anal. Chem., 61 (1989) 1171. [6] S. Daunert, S. Wallace, A. FIorido and L.G. Bachas, Anal. Chem., 63 (1991) 1676. [7] S. Park, W. Matuszewski, M.E. Meyerhoff, Y.H. Liu and K.M. Kadish, Electroanalysis, 3 (1991) 909. [8] A. Jyo, P. Minakmi, Y. Kanda and H. Egawa, Sensors Actuators B, 13/14 (1993) 200. [9] D. Gao, J.Z. Li, R.Q. Yu and G.D. Zheng, Anal. Chem., 66 (1994) 2245. [lo] A.D. Alder, F.R. Longo and J.D. Finarelli, J. Org. Chem., 32 (1967) 476. [ll] F.A. Walker, J. Am. Chem. Sot., 92 (1970) 4235. [12] H.K. Hombrecher, V. Gherdan and S. Ohm, Tetrahedron, 49 (1993) 8569. [13] A.D. Alder, F.R. Longo, F. Kampas and J. Kim, J. Inorg. Nucl. Chem., 32 (1970) 2443. [14] L.M. Epstein, D.K. Straub and C. Maricondi, Inorg. Chem., 6 (1967) 1721. [15] K. Hartman, S. Luterotti, H.F. Osswald, M. Oehme, P.C. Meier, D. Ammann and W. Simon, Mikrochim. Acta, II (1978) 235. [16] G.G. Guilbault, R.A. Durst, M.S. Frant, H. Freiser, E.H. Hansen, T.S. Light, E. Pungor, G.A. Rechnitz, N.M. Rice, T.J. Rohm, W. Simon and J.D.R. Thomas, Pure Appl. Chem., 48 (1976) 127. [17] F.A. Walker, D. Beroiz and K.M. Kadish, J. Am. Chem. Sot., 98 (1976) 3484. [18] W.E. Morf, The Principle of Ion-Selective Electrodes and of Membrane Transport, Elsevier, Amsterdam, 1981. 1191 Y. Marcus, Ion Salvation, Wiley, Chichester, 1985. [20] R.G. Pearson, J. Am. Chem. Sot., 85 (1963) 33. 1211 A. Giraudeau, H.J. Callot, J. Jordan, I. Ezhar and M. Grass, J. Am. Chem. Sot., 101 (1979) 3857. [22] C. Reichardt, Solvent Effects in Organic Chemistry, Verlag Chemie, Weinheim, 1979.