Anal. Chem. 1993, 65, 1533-1536
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Vitamin BI2 Derivatives as Anion Carriers in Transport through Supported Liquid Membranes and Correlation with Their Behavior in Ion-Selective Electrodes Cristina Palet: Maria MuiiozJ Sylvia Daunert,*Leonidas G. Bachas3 and Manuel Valiente**+ Qulmica Analltica, Universitat Autbnoma de Barcelona, 08193 Bellaterra, Barcelona, Spain, and Department of Chemistry and Center of Membrane Sciences, University of Kentucky, Lexington, Kentucky 40506
Two derivatives of vitamin B12,aquocyanocobalt(111) heptapropylcobyrinate (HPC) and aquocyanocobalt(II1) heptamethylcobyrinate (HMC), were synthesized and used as anion carriers in supported liquid membranes. More efficient transport was observedwhen the more lipophiliccarrier (Le., HPC) was used. The effect of several parameters such as the carrier concentration and the composition of the stripping solution on the permeabilityof the membraneswere investigated. The properties of these transport systems were related to the response behavior of I$Es that are based on the HPC carrier.
INTRODUCTION The response characteristics of ion-selective electrodes (IS&) and the selectiveand efficient transport of ions through supported liquid membranes are controlled by carriermediated phenomena.’ These ion carriers are usually lipophilic natural or synthetic molecules.2*3To date most of the work in selective transport through liquid membranes deals with the transport of cations.4 There are only a few examples on the transport of anions through liquid membranes, and these mainly involve the use of quaternary ammonium or phosphonium salts as anion carriers. These carriers,however, are essentially nonselective because the observed transport selectivity is determined by the hydration energy of the ions; i.e., quaternary ammonium or phosphonium salts are better transport agents for hydrophobic ions.5 It has been demonstrated that the incorporation of selective interactions between ions and carriers can result in the selective carriermediated transport of anions.1+11 Likewise, although there are several functional cationselective electrodes that are currently employed in a variety of analyses, there is only a limited number of selective
* To whom correspondence should be addressed.
Universitat Autbnoma de Barcelona. University of Kentucky. (1)Tsukube, H. InLiquidMembranes: Chemical Applications;Araki, T.. Tsukube. H.. Eds.: CRC Press: Boca Raton. FL. 1990.DD 26-76. ’(2)Izatt, R. hi.;Lamb, J. D.; Bruening, R. L. Aep. bci. Telhnol. 1988, 23.1645-1658. ~. , - - - -- - - (3)Meyerhoff,M. E.;Opdycke, W. N. Adu. Clin.Chem. 1986,25,1-45. (4)McDowell, W.J. Sep. Sci. Technol. 1988,23,1251-1268. (5)Behr, J.-P.; Lehn, J.-M. J. Am. Chem. SOC.1973,95,6108-6110. (6)Lamb, J. D.; Christensen, J. J.; Izatt, S. R.; Bedke, K.; Astin, M. S.; Izatt, R. M. J . Am. Chem. SOC.1980,102,3399-3403. (7)Tabushi, I.; Kobuke, Y.; Imuta, J.-I. J. Am. Chem. SOC.1981,103, 6152-6157. (8)Maruyama, K.; Tsukube, H.; Araki, T. J. Am. Chem. SOC.1982, 104,5197-5203. (9)Tsukube, H. J. Chem. SOC.,Perkin Trans. 1 1983,104,29-34. (10)Morf, W. E.; Huser, M.; Lindemann, B.; Schulthess, P.; Simon, W. Helu. Chim.Acta 1986,69,1333-1342. (11) Kokufuta, E.; Nobusawa, M. J . Membr. Sei. 1990,48,141-154. +
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electrodes for anions.12 As in transport studies, the classical ion carriers that have been used for the developmentof anionselective electrodes are quaternary ammonium salts. The anion response of these ISEs follows the Hofmeister selectivity series, which reflects the hydrophobicity of the anions.3 Therefore, the anions for which the electrodes have the greatest selectivityarelarge-sized hydrophobic anions. There are some examples of “truly” anion-selective electrodes that are based on a specific interaction between the ion carrier and the anion. Examples of this type of electrode include those that employ as ion carriers metallomacrocyclic (such as porphyrins, corrins, and phthalocyanines) and organometallic compounds.12 Vitamin BIZ is a corrin, and its coordination properties with anions have been extensively studied. Several hydrophobic vitamin BIZderivatives (cobalaminsand cobyrinates) have been employed in the development of anion-selective electrodes.13-18 Morf et al. have used aquocyanocobalt(II1) heptakis(2-phenylethy1)cobyrinatefor the transport of nitrite through a plasticized poly(viny1chloride) membrane.10 This was coupled to either a thiocyanate or chloride countertransport. However, no information was provided on other parameters that control the behavior of this transport system. Given the potential usefulness of cobyrinate-basedsystems in the selective transport of anions, it was decided to investigate further the transport properties of these compounds as related to the parameters that control the efficiency of the transport. In that respect this article describes the properties of transport systems based on supported liquid membranes containing the cobyrinates aquocyanocobalt(II1) heptapropylcobyrinate (HPC) and aquocyanocobalt(II1)h e p tamethylcobyrinate (HMC) (Figure 1). The observed transport properties are related to the intrinsic coordination chemistry of the cobyrinatesand correlate with the selectivity behavior of ISEs that are based on the HPC carrier.
EXPERIMENTAL SECTION Reagents and Membranes. Isopropylbenzene was obtained from Carlo Erba (RPE,Milan, Italy). Sodium nitrite (ACSgrade) and sodium thiocyanatewere purchased from Aldrich (Steinheim, Germany). Acetic acid, sodium acetate, and sodium benzoate were from Panreac (PA,Barcelona,Spain). Tris(hydroxymethy1)(12)Wotring, V. J.; Johnson, D. M.; Daunert, S.; Bachas, L. G. In Immunochemical Assays and Biosensor Technology for the 1990s; Nakamura, R. M., Kahasara, Y.,Rechnitz, G. A., Eds.; American Society of Microbiology: Washington, DC, 1992; pp 355-376. (13)Schulthess,P.; Amman, D.; Simon, W.; Caderas, C.; Stephek, R.; Krlutler, B. Helu. Chim. Acta 1984,67,1026-1032. (14)Schulthess, P.; Amman, D.; Krlutler, B.; Caderas, C.; Stephek, R.; Simon, W. Anal. Chem. 1985,57,1397-1401. (15)Stephek, R.; Krlutler, B.; Schulthess, P.; Lindemann, B.; Amman, D.; Simon, W. Anal. Chim. Acta 1986,182,83-90. (16)Daunert, S.;Witkowski, A,; Bachas, L. G. h o g . Clin.Biol. Res. 1989,292,215-225. (17)Daunert, S.;Bachas, L. G. Anal. Chem. 1989,61,499-503. (18)Florido, A.; Daunert, S.;Bachas, L. G. Electroanalysis 1991,3, 177-182. 0 1993 American Chemical Soclety
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 11, JUNE 1, 1993
A
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OR Flgure 1. Structuresof (a)aquocyanocobaR(II I) heptamethylcobyrinate (HMC),R = CH3,and (b) aquocyanocobait(II1) heptapropylcobyrinate (HPC),R = CHzCHzCH3. aminomethane (Tris) and sodium salicylate were obtained from Merck (Darmstadt, Germany). Doubly distilled water was used for all experiments. Two vitamin BIZ derivatives, dicyanocobalt(II1) heptapropylcobyrinate and dicyanocobalt(II1)heptamethylcobyrinate, were synthesized as described earlier.1Y."' In order to remove one of the axially coordinated cyano groups, both cobyrinates were treated in the following manner. A 2.5 X 10 M solution of either of the cobyrinates was prepared in ethanol. A volume of 10.0 mL of this solution was placed in a round-bottom flask, and concentrated acetic acid [enough to achieve a 10s';(viv) acetic acid solution] was added. The solvent was eliminated in a rotaevaporator while heating at 45 O C . The residue was redissolved in 10 mL of a 10% acetic acid-ethanol solution, and the procedure described above was repeated for a total of three times to yield HPC and HMC. Different amounts of HPC and HMC were dissolved in isopropylbenzene to prepare solutions with varying concentrations of these cobyrinates (ranging from 1 to 10 mg/mL). The solutions were used to impregnate the microporous solid supports and form the liquid polymeric films employed in the studies. Durapore GVHP 047 00 (Millipore, Bedford, MA), a laminar microporous poly(viny1idene fluoride) film of 125 Fm thickness, was used as solid support for the liquid membranes. The nominal porosity of the film was 75%, and the effective pore size was 0.22 pm. In the transport experiments, the effective surface area of the supported liquid membrane was 8.5 cm-'. This value was determined as a product of the nominal porosity coefficient (0.75) and the membrane surface area being in contact with the feed and stripping solutions. Apparatus. Experiments were carried out by using a membrane cell that contained two separate compartments (for the aqueous feed and stripping solutions) divided by a window, where the impregnated membrane was placed (Figure 2)." D.G. stirring motors (JVC; 0-15 V, 0-0.5 A) were used to stir the contents of the two compartments. A tachometer (IKA-TRON,Model DZM 1; IKA-WERK, Staufen, Germany) was employed to control the stirring motors. The concentration of nitrite was determined by using a UV-visible spectrophotometer (Perkin-Elmer, Model 554). A diode-array spectrophotometer (Hewlett-Packard,Model HP-8451) was employed for benzoate, thiocyanate, and salicylate determinations. Procedure. A 1.00 X 10- M nitrite solution in 0.50 M acetate buffer, pH 5.25, was used as the feed solution. Different conditions for the preparation of the stripping solution were investigated. These included varying the concentration and the pH of a solution of thiocyanate in 0.50 M Tris-HC1. The concentration of thiocyanate was varied from 1.00 X 10 to 1.00 X M, and the pH of the stripping solution ranged between 4 and 10. (19) Murakami, Y.; Hisaeda, Y.; Ohno, 'r.Bull. Chem. Soc. J p n . 1984, 57, 2091-2097. (20) Wetherman, doctoral thesis, ETH-Zurich, 1968. (21) Salvad6, V.; Masana, A,; Hidalgo, M.; Valiente, $1,; Muhammed, M. Anal. Lett. 1989. 22. 2613-2626.
Flgure 2. Schematic of the permeability cell used in these studies. The Teflon stirrers are symbolized as A. B is the window that divides the two separate compartments for the feed (1) and strlpping (2) solutions, and in which the impregnated membrane is placed.
Equal volumes (200 mL) of the feed and stripping solutions were placed in the respective compartments of the membrane cell. The concentration of nitrite present in the stripping solution was determined by following the method of Griess.jL For this purpose, 1-mL samples were withdrawn from the stripping solution during a 4-h period. The data obtained were used to calculate the corresponding concentration of nitrite in the feed and then to determine the permeability coefficient for nitrite. A similar procedure was developed to measure the permeabilities of benzoate, salicylate, and thiocyanate. For these anions, d multivariate approach using a diode-array spectrophotometer was employed to determine the concentration of the anions.23
RESULTS AND DISCUSSION It is well established t h a t an ion carrier can function both as an active component in an ISE a n d as a selective carrier of chemical species in transport processes. For example, a diquaternary ammonium salt has been used in both transport studies; and ISEs.L4 In both cases, the ion carrier is placed between two aqueous solutions of different composition, namely, t h e sample solution and t h e internal reference solution (in ISEs) or t h e feed solution and t h e stripping solution (in transport studies). In this work a n attempt was made t o correlate t h e results obtained with electrodes based on hydrophobic vitamin B12 derivatives (previously described in ref 16) with permeability studies performed in supported liquid membranes containing the same derivatives as ion carriers. T h e transport data obtained as described in t h e Experimental Section were transformed into permeability coefficient values defined by the following equation:
where P is t h e permeability coefficient of the membrane (cmi min), Cf is t h e concentration of t h e ion of interest in the feed compartment (M), t is t h e time (min), V i s t h e volume of t h e feed solution (mL), and Q is t h e effective areaof the membrane (cm'). T h e parameter P was used t o determine the influence of the chemical variables studied. Two cobyrinates ( H P C and HMC) that have different lipophilicities were tested as selective carriers in t h e countertransport system described here, and their transport efficiencies were compared. H P C is a hydrophobic cobyrinate t h a t has been used for the development of ion-selective electrodes for nitrite.16 Therefore, it was thought t h a t this (22) Marczenko, 2. In Separation and Spectrophotometric Determination of Elements, 2nd ed.; Ellis Horwood Limited-John Wiley ((r Sons: New York, 1986; pp 418-420. (23) Palet, C.; Riba, J.; Mufioz, M: Maspoch, S.;Valiente, M. Reuni6n del Grupo de Espectroscopia Analitjca, Sociedad Espafiola de Quimica Analhica, Girona, Spain, May 1992; Paper A9. ( 2 4 ) Wotring, V. J.;Johnson, D. M.; Bachas, L. G. Anal. C h e n . 1990, 62, 1506-1510.
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Table I. Influence of the Concentration of Thiocyanate in the Stripping Solution on the Nitrite Permeability Coefficient
ap
P,”cm/min NaSCN concn, M 0 1 x 10-4 1 x 10-3 1 x 10-2
u
I
6.95-
6.85 80
120
160
200
240
280
time ( m i d Influence of the HPC canler concentration on nitrite permeationthrough the supported llquldmembrane. The concentratlon 10.2mg/mL. ofthecarrlerwas(O)O,(O)1.3,(A)2.6,(0)5.1,and(U) The paxls refers to the logarithm of the concentration of nltrite In the stripping solution.
HPC 0,0096 f 0.0007 0.0125 f 0.0002 0.0194 f 0.0007 0.0362 f 0.0003
HMC 0.0040 f 0.0003 0.0063 f 0.0003 0.0096 f 0.0004 0.0055 i 0.0001
OThe uncertainty in the value of P was calculated from the uncertainty in the slope obtained by linear regression of the ln[nitrite] vs time plota.
Flgm 3.
0
2
4
6
8
1
0
1
2
Carrier (mg/mL) Figure 4. Effect of the carrler concentrationon the nitrite permeability coefficient, R (W) HPC, ( 0 )HMC. The error bars were calculated from the uncertainty in the slope obtained by linear regression of the In[nitrite] vs tlme plots. compound could also be employed for the selective transport of anions across liquid membranes. HMC contains seven methyl ester groups in ita molecule rather than the propyl ester groups of HPC and, consequently, is more hydrophilic in nature than HPC. The first set of experiments that was performed involved studying the effect of the carrier concentration on the nitrite permeation. In these experiments, the initial feed solution was 1.00 X M NaN02, pH 5.25, and the initial stripping solution was 1.00 X M NaSCN, pH 8.75, Samples were withdrawn periodically from the stripping solution and were analyzed for nitrite concentration (Figure 3). These concentrations were then used to calculate the permeability coefficient of the membrane to nitrite as described above. Figure 4 shows a calibration curve that relates the concentration of carrier present in the membrane with the permeability of the membrane to nitrite. These data demonstrate that as the concentration of the carrier in the membrane increases, the permeability coefficient also increases. This effect can be attributed to the selective interaction of the cobyrinate carriers with nitrite. A t high concentrations of carrier the permeability coefficient for nitrite reaches a plateau, indicating a diffusion-limited process in the organic phase. When experiments were performed with blank membranes (i.e., membranes containing no carrier), the permeation of nitrite was insignificant. The more lipophilic cobyrinate, HPC, shows a higher nitrite flux across the membrane due to the decreased distribution of the carrier from the membrane to the adjacent aqueous phases. This relationship between lipophilicity and flux is
commonly observed in membrane transport system^.^^^^^ Although it is apparent that increased lipophilicity is desired and is easily achieved by increasing the length of the side chains in cobyrinates, it should be noted that this strategy is not universal. Indeed, in several instances, increased lipophilicity induced by benzyl2or long alkyl groups2’ has resulted in a reduction of the complexing power and, thus, the transport efficiency of the carrier. Incidentally, high lipophilicity is also a requirement for the development of ISEs that possess good detection limits and extended lifetirnesa28 The influence of the nature of the stripping solution on the nitrite permeation was also investigated. For that, the concentration of thiocyanate in the stripping solution was varied from 1.00 X10-4 to 1.00 X 10-2 M, while the initial feed solution was 1.00 X 10-3 M NaN02, pH 5.25, and the HPC carrier concentration in the membrane was 10.2 mg/mL. The obtained data are shown in Table I. As the concentration of thiocyanate increases, the permeability to nitrite incretuses, which indicates that thiocyanate acts as an efficient stripping agent. This effect can be explained by the slight preference of the carrier for thiocyanate anions over nitrite, which has been observed with ISE experirnents.l6 In addition, in a theoretical treatment of countertransport systems, Morf et al. have indicated that maximum permeability is expected when the selectivity of the carrier for the transported ion and the etripping agent is about the same.10 The same study has indicated that if the carrier has a high preference for either the ion to be transported or the stripping agent, a significantly reduced transport rate is to be expected. The data in Table I also indicate that in the absence of thiocyanate there is still a significant, albeit reduced, permeability to nitrite. This observation suggests that the transport of nitrite can also be induced by a pH gradient. In this case, the countertransport mechanism implies that while nitrite is being transported in one direction, hydroxide is transported in the other. In that respect, the effect of the pH of the stripping solution on the nitrite permeation was studied. In a series of experiments, the pH of the stripping solution was varied between 4.40 and 10.24. Other parameters were kept as in the experiments describedabove. The HPC carrier concentration was 10.2mg/mL. The increase of permeability observed with pH (Figure 5 ) can be attributed to the stripping activity of the hydroxide ions. Indeed, studies with ionselective electrodes demonstrated a significant effect of pH on the response of the electrodes,29which indicates a possible (25) StoIwijk,T.B.; Sudholter, E. J. R.; Reinhoudt,D. N. J. Am. Chem. SOC.1989,111, 6321-6329. (26) Lamb,J. D.; Bruening, R. L.; Izatt, R. M.; Hirashima, Y.; Tse, P.-K.; Christensen, J. J. J. Membr. Sci. 1988,37, 13-26. (27) Gehrig, P.; Morf, W. E.; Welti, M.; Pretsch, E.; Simon, W. Helo. Chim. Acta f990, 73, 203-212. (28) Dinten, O.;Spichiger,U.E.;Chaniotakis,N.; Gehrig,P.;Rusterholz, B.;Morf, W.; Simon, W.Anal. Chem. 1991,63,596-603. (29) O’Reillv, S. A.: Daunert, S.; Bachas, L. G. Anal. Chem. 1991,63, 1278-1281.
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ANALYTICAL CHEMISTRY, VOL. 65, NO. 11, JUNE 1, 1993
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0.ooo-l
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I
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I
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7 9 11 PH Figure 5. Effect of the pH of the stripping solution on the nitrite permeability coefficient, P (0) HPC, (e)HMC. The error bars were calculatedfrom the uncertainty in the slope obtained by linear regression of the in[nRrite] vs time plots. 3
5
coordination of hydroxide ions with the HPC. This is also consistent with ISE studies that used a different vitamin BIZ ion carrier." It should be noted that transport of nitrite is considerably less efficient when hydroxide alone is the stripping agent. This observation can be explained by a significant difference in the selectivity of the carrier for nitrite and hydroxide in accordance with the theoretical suggestions of Morf et a1.I0 The effect of stripping conditions on nitrite permeation was also investigated by using HMC as the carrier. The HMC concentration was 10.1 mgimL, while all other parameters were kept as in the experiment with HPC. Initially, an increase in the permeability coefficient is evident with increasingthiocyanate concentration in the stripping solution (Table I). However, at 1.00 X M NaSCN, a decreased permeability coefficient was observed. This is different from the behavior of the HPC carrier, and a possible explanation for this difference is given below. The next set of experiments involved studying the influence of the pH of the stripping solution on the nitrite permeation when HMC was used as the carrier. In these experiments, the pH of the stripping solution was varied between 3.98 and 9.95, the HMC concentration was 10.1mg/mL, and the other parameters were kept as above. The results obtained show an increase in the permeability coefficient for nitrite until ca. pH 9, at which point the permability starts decreasing (Figure 5). Such a phenomenon has been observed before by Tohda et al. in a cation-transport ~ystem.3~ In particular, a decrease was observed in the sodium transport, which was attributed to backward diffusion and to a reduced pH differencebetween the feed and stripping phases.30 In our case, a different (30) Tohda, K.; Suzuki, K.; Inoue, T.; Minatoya, R.; Inoue, H.; Shirai, T. Anal. Lett. 1989,22, 2167-2174.
situation occurs and the lower transport efficiency could be explained by a reduction in the concentration of carrier in the membrane that is induced by the high concentration of the stripping agent. Given the reduced lipophilicity of HMC compared to HPC, this explanation may also account for the different behavior of the HPC-mediated transport system. Ion-selective electrodes based on plasticized poly(viny1 chloride) membranes impregnated with the HPC carrier were selective to nitrite, salicylate, and thiocyanate.I6 In addition, it was observed that the nature of the plasticizer affected the selectivity of the electrodes. An anion for which this change in selectivity was significant was benzoate. Consequently, the transport properties of liquid membranes loaded with 10.2mg/mL HPC in isopropylbenzene were investigated with these four anions. The permeability coefficients of the membranes were 0.0096 f 0.0007,0.0109 f 0.0008, 0.0214 f 0.0024,and 0.0247 f 0.0008 cm/min for nitrite, thiocyanate, salicylate, and benzoate, respectively. The concentration of the anions in the feed solution was 1.00 X M and there was no thiocyanate present in the stripping solution, which was a 0.50 M Tris-HC1, pH 8.75 buffer. Although ISEs based on the same carrier presented good response to these four anions, they were more selective to nitrite, salicylate, and thiocyanate than to benzoate. Interactions between the isopropylbenzene and benzoate (a-T interactions) may facilitate the extraction of this ion into the liquid membrane, which in turn may explain the high permeability coefficient for benzoate. This is further supported by the fact that when o-nitrophenyl octyl ether (NPOE) was employed as the plasticizer in the ISE experiments instead of dioctyl sebacate (DOS),better selectivity toward benzoate was observed. This is consistent with the existence of a-a interactions between NPOE and benzoate. In conclusion, lipophilic cobyrinates can be used in the efficient transport of selected anions. Further, this work provides a basis for the design and optimization of aniontransport systems using cobyrinate-loaded liquid membranes by taking advantage of data obtained from ion-selective electrode studies. Subsequently, the transport conditions through liquid membranes may also contribute to a better design and optimization of the corresponding ISEs. Thus, a specific relationship between sensing properties and transport phenomena has been observed in the present study.
ACKNOWLEDGMENT This work was supported by grants from the National Science Foundation (EHR-9108764) (to L.G.B.), the NATO ScientificAffairsDivision (CRG.890610) (toL.G.B. and M.V.), and the Spanish Commission for Research and Development, CICYT (MAT 886190-C02-01)(to M.V.). RECEIVEDfor review November 5, 1992. February 5, 1993.
Accepted
