Durable phosphate-selective electrodes based on uranyl salophenes

Analytica Chimica Acta 432 (2001) 79–88

Durable phosphate-selective electrodes based on uranyl salophenes Wojciech Wróblewski a , Kamil Wojciechowski a , Artur Dybko a , Zbigniew Brzózka a,∗ , Richard J.M. Egberink b , Bianca H.M. Snellink-Ruël b , David N. Reinhoudt b b

a Department of Analytical Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland Laboratory of Supramolecular Chemistry and Technology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands

Received 10 April 2000; received in revised form 20 November 2000; accepted 28 November 2000

Abstract Lipophilic uranyl salophenes derivatives were used as ionophores in durable phosphate-selective electrodes. The influence of the ionophore structure and membrane composition (polarity of plasticizer, the amount of incorporated ionic sites) on the electrode selectivity and long-term stability were studied. The highest selectivity for H2 PO4 − over other anions tested was obtained for lipophilic uranyl salophene III (with t-butyl substituents) in poly(vinylchloride)/o-nitrophenyl octyl ether (PVC/o-NPOE) membrane containing 20 mol% of tetradecylammonium bromide (TDAB). Moreover, phosphate-selective electrodes based on this derivative exhibited the best long-term stability (2 months). The electrode durability can be improved decreasing the amount of the ammonium salt in membrane to 5 mol%. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Anion-selectivity; Uranyl salophenes; Phosphate ISE

1. Introduction In recent years, there have been several reports on phosphate-selective electrodes [1]. Most electrodes were based on membranes containing organotin compounds, which were also used as a reagent for phosphate extraction. Among alkyltin and benzyltin derivatives, the latter were the most suitable for ion-selective electrodes and generally exhibited better phosphate-selectivity [2,3]. Only the selectivity of the alkyl multidentate derivative with four tin centers [4] was comparable to dibenzyltin [2], tribenzyltin [5] or distannyl ionophores [6]. Despite high selectivity for phosphate, sensors based on membranes containing these ionophores exhibited in many ∗ Corresponding author. Tel.: +48-22-660-5427; fax: +48-22-628-2741. E-mail address: [email protected] (Z. Brz´ozka).

cases sub-Nernstian slow response and especially limited functional lifetime (not exceeding few weeks). Metallocomplexes like Co(II) phthalocyanine derivatives, Ni(II) diketonate–ethylene complexes were also used as ionophores in phosphate-selective membranes. High phosphate-selectivity (but with strong sub-Nernstian response) was measured for electrodes based on cobalt phthalocyanine [7] (used previously as a nitrite carrier). Nernstian responses and the best durability (9 months) were obtained for hydrogen phosphate-sensitive electrodes based on macrocyclic polyamines [8]. The electrode selectivity was attributed to the size and charge of the N-cyclic amine relative to the size and charge of H2 PO4 2− anions. A novel class of neutral metallocomplexes, containing uranyl cation as Lewis acid center, for selective phosphate recognition has been synthesized. Introduction of the uranyl cation into the salophene moiety gives stable metalloreceptors–uranyl salophenes,

0003-2670/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 3 - 2 6 7 0 ( 0 0 ) 0 1 3 5 8 - 1

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which exhibit strong interactions and selective binding of hydrophilic anions. The rigid planar structure of the receptors is enforced by the uranyl cation (pentagonal bipyramidal coordination of UO2 2+ ). Two oxygen atoms of uranyl center occupy perpendicular positions to the aromatic rings plane and there is one equatorial position able to form a Lewis acid–base coordinative bond. High association constants of uranyl salophenes with phosphate anions were determined conductometrically in the MeCN–DMSO solutions [9,10]. Thus, these receptors can be successfully applied as phosphate-selective ionophores in polymeric membranes of electrochemical sensors. Recently, uranyl salophenes derivatives were incorporated in poly(vinylchloride) (PVC) membranes of ion-selective electrodes (ISEs) [11] and chemically modified field effect transistors (CHEMFETs) [12,13]. Application of the lipophilic uranyl salophene II (Fig. 1) induced a high selectivity for H2 PO4 − anions (the values of selectivity coefficients for phosphate increased by a factor of 105 comparing to the blank membrane). However, the long-term stability of the designed ISEs and CHEMFETs was not investigated. This paper describes durability studies of phosphateselective electrodes based on lipophilic uranyl salophenes II and III. The influence of the ionophore structure and membrane composition (polarity of plasticizer, the amount of incorporated ionic sites) on

the membrane selectivity and long-term stability were also studied.

2. Experimental 2.1. Chemicals and membrane materials All sodium salts used, 1-morpholinoethanesulfonic acid (MES), sulfuric acid were of analytical grade and were purchased from Fluka. The solutions of sodium salts (0.1 M) and MES buffer solution (0.5 M) were prepared with redistilled water. The pH was adjusted by the addition of sulfuric acid. The synthesis of ionophore II was described previously [14], ionophores I and III were synthesized in Warsaw University of Technology according to the procedure given in [10]. High-molecular-weight PVC, plasticizers: o-nitrophenyl octyl ether (o-NPOE), 2-fluorophenyl 2-nitrophenyl ether (FPNPE) and 1-chloronaphtalene (Cl-Napht); lipophilic salt: tetradecylammonium bromide (TDAB) were obtained from Fluka. Freshly distilled tetrahydrofuran (THF) (Fluka) was used as a solvent for the membrane components. 2.2. Membrane and electrode preparation The membranes contained: 2 wt.% ionophore, 65 wt.% plasticizer, 33 wt.% PVC and 5–50 mol% (versus ionophore) TDAB. The membrane components (150 mg in total) were dissolved in 1 ml of THF. The membranes were prepared according to the procedure described previously [11]. Membrane discs were mounted in electrode bodies (type IS 561, Philips) for electromotive force (EMF) measurements. Solution of NaCl (0.1 M) was used as an internal filling. The electrodes were conditioned overnight in a dilute solution of internal electrolyte (0.01 M NaCl) or in 0.01 M NaH2 PO4 . For each membrane composition three electrodes were prepared. 2.3. EMF measurements

Fig. 1. Structures of uranyl salophenes I–III.

All measurements were carried out with cells of the following type: Ag, AgCl; KCl 1 M/CH3 COOLi 1 M/sample solution//membrane//internal filling solution; AgCl, Ag.

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The EMF values were measured using a custommade 16-channel electrode multiplexer. Details of this equipment were described previously [15]. The performances of the electrodes were examined by measuring the EMFs of the primary ion solutions (increasing the activity in steps of 0.5 log a (H2 PO4 − )), concentration range 10−7 to 10−1 M) stirred with a magnetic stirrer. The experimental data points were fitted to the Nikolski–Eisenman equation by non-linear least squares fitting according to the method proposed by Diamond [16] and introducing the formalism proposed by Bakker et al. [17]. The fitting procedure determines values of the sensor parameters: cell constant and potentiometric selectivity coefficient (theoretical slope was assumed), which best describe the experimental data. Moreover, the quality of the sensor can be characterized not by the value of response slope but through the χ 2 -value (sum of the squares of the deviations of the theoretical curve from the experimental points), which quantified the goodness of the experimental data to theoretical Nikolski–Eisenman model. The non-linear regression method was based on the Levenberg–Marquardt algorithm. Origin 4.1 (microcal origin) was used for calculations.

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Potentiometric selectivity coefficients (log K (NO3 − ), X− )) were determined by the separate solution method (SSM) using 0.1 M solutions of sodium salts [18]. Nitrate anion was chosen as the reference anion in order to clarify the deterioration of the phosphate-selectivity of membranes in the selectivity pattern. The measurements were carried out in solutions buffered with 0.01 M MES adjusted to pH = 4.5. The activities of anions in aqueous solutions were calculated according to the Debye–Hueckel approximation.

3. Results and discussion The selectivity of the electrodes based on four uranyl salophene derivatives was previously evaluated in [11]. The introduction of the lipophilic uranyl salophene II in the PVC/o-NPOE membranes containing 20 mol% of quaternary ammonium induced phosphate-selectivity (see Fig. 2). Theoretical performances in 0.1 M solutions of interfering anions were observed for these sensors. Although, high association constants of uranyl salophene I with phosphate anions were determined [9], electrodes based on

Fig. 2. Selectivity coefficients of phosphate-selective electrodes based on PVC/o-NPOE membranes containing ionophores I–III in comparison to blank membrane (20 mol% of TDAB).

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Fig. 3. Time dependence of selectivity coefficients of phosphate-selective electrodes based on PVC/o-NPOE membranes containing ionophore II (20 mol% of TDAB).

this ionophore exhibited worse phosphate-selectivity (Fig. 2). This can be attributed to the insufficient solvation of the ionophore I by the components of polymeric matrix (mainly the plasticizer). Moreover, the hydrophilic water–UO2 –salophene complex precipitates (the membranes became opaque after their conditioning), which lowers the effective concentration of the ionophore inside the membrane. This results in a decrease of phosphate-selectivity of sensors based on the ionophore I. After a few days of conditioning the phosphate-selectivity was lost. The functionalization of UO2 –salophenes with long alkyl chains (ionophore II) improves their lipophilicities and thus, increases the solubilities in the organic membrane phase. The membranes containing salophene II showed no visible changes during their conditioning in a chloride solution. A deterioration of the phosphate-selectivity was measured during their long-term conditioning (see Fig. 3). High phosphate-selectivity of the sensors was maintained only during 1–2 weeks and after more than 2 months the electrode selectivity was lost. The degradation of the phosphate-selectivity of the electrodes based on derivative II was caused by the slow precipitation of hydrated ionophore, insoluble in the membrane (not

visible in the membrane). Detailed studies [19] showed that the formation of the water–UO2 –salophene complex can be followed by the decomposition of the ionophore molecules due to their interactions with phosphate anions. Although, the deterioration process occurred, the durability of the electrodes was improved in comparison with the sensors based on salophene I, due to the higher lipophilicity of the ionophore II. The durability of the phosphate-selective sensors can be effectively improved using uranyl salophene derivatives that are better solvated in the membrane phase. Thus, the uranyl salophene III, containing t-butyl substituents, was synthesized and investigated as an ionophore. The initial phosphate-selectivity of the electrodes based on this derivative (20 mol% TDAB) was slightly better in comparison to ISEs of the same composition but with ionophore II (see Fig. 2). The electrode performances determined in 0.1 M solutions of various interfering anions are presented in the Fig. 4. There is a good correlation (χ 2 < 10−5 ) between experimental data points and the Nikolski–Eisenman model. Moreover, the incorporation of the uranyl salophene III in the polymeric membrane enhanced the lifetime of the phosphate-selective

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Fig. 4. H2 PO4 − -responses of ISEs based on PVC/o-NPOE membranes containing ionophore III (20 mol% of TDAB) in 0.1 M solutions of interfering anions. Solid lines represent non-linear least squares fitting.

electrodes (see Fig. 5a). During 2 months of the permanent electrode conditioning in 0.01 M NaCl, high phosphate-selectivity was maintained and repeatable performances were observed. This leads to the conclusion that the ionophore III is better solvated in PVC/o-NPOE membrane phase than the derivative II. In this case the rate of water–UO2 –salophene complex formation decreases. If the rate of precipitation of the water–ionophore complex would be a function of solvation of the ionophore, the use of plasticizers that better solvate uranyl salophenes should result in better durability of phosphate-selective electrodes. Therefore, the influence of the polarity and structure of the plasticizer on the electrode long-term stability was investigated. Plasticizers Cl-Napht, less polar than o-NPOE and FPNPE more polar were incorporated in the membranes containing the derivative III and 20 mol% of lipophilic

ionic sites. Comparable initial values of selectivities (except of log K (NO3 − , SO4 2− )) for sensors based on membranes containing various plasticizers were determined (see Fig. 5). The application of the non-aromatic plasticizers (e.g. bis(2-ethylhexyl) sebacate) induced worse initial phosphate-selectivity of the electrodes, because the plasticizer was not able to dissolve the ionophore sufficiently (data not shown). The results may indicate that the plasticizer structure (not the plasticizer polarity) governs the ionophore solubility process due to the “␲–␲ stacking”. However, the durability studies of sensors showed, that the electrodes based on the less polar plasticizer i.e. Cl-Napht exhibited the worse long-term stability (Fig. 5b). The highest phosphate-selectivity was observed during 2 months for electrodes containing o-NPOE plasticizer. The lowest phosphate-selectivity was determined with the most polar plasticizer FPNPE. However, the

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Fig. 5. Time dependence of selectivity coefficients of phosphate-selective electrodes based on PVC membranes containing ionophore III (20 mol% of TDAB) and various plasticizers: (a) o-NPOE; (b) 1-chloronaphtalene and (c) FPNPE.

selectivity coefficients towards another anions did not change during 5 months of conditioning (Fig. 5c). The presented results indicate that the choice of the membrane composition for practical sensor applications is determined by the two measurement parameters i.e. high phosphate-selectivity or high long-term stability. The influence of the amount of lipophilic salt in the membrane on the selectivity and durability of phosphate-selective electrodes was also studied. Fig. 6 presents the selectivity pattern of electrodes based on membranes containing ionophore III and

different amounts of the ammonium salt. The presence of 20–30 mol% of ionic sites in the membrane containing ionophore III gave the highest selectivity for phosphate over other anions. Strongly decreased membrane selectivity is observed when the amount of ammonium salt exceeds 50 mol% versus ionophore. The decrease of the TDAB amount in membrane gave comparable electrode selectivity (except log K (NO3 − , SO4 2− )) and enhanced the sensor durability. The incorporation of only 5 mol% of the ammonium salt to the membrane composition gave an almost stable electrode

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Fig. 6. Selectivity coefficients of phosphate-selective electrodes based on PVC/o-NPOE membranes containing ionophore III and different amounts of ammonium TDAB.

selectivity during 5 months of sensor conditioning (Fig. 7). In this case the excess of the ionophore, due to the higher ionophore/TDAB ratio, ensures proper durable performances of phosphate-selective electrodes.

Since the potentiometric sensors were designed for phosphate determination, the effect of permanent contact of the electrodes with solution containing phosphate was verified. The conditioning of electrodes

Fig. 7. Time dependence of selectivity coefficient (log K (NO3 − , H2 PO4 − )) of phosphate-selective electrodes based on PVC/o-NPOE membranes containing ionophore III and different amounts of ammonium salt-TDAB.

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Fig. 8. Time dependence of selectivity coefficients of phosphate-selective electrodes based on PVC/o-NPOE membranes containing ionophores III and II (20 mol% of TDAB). The electrodes were conditioned in 0.01 M NaH2 PO4 .

in 0.01 M solution of NaH2 PO4 resulted in a reduced phosphate-selectivity over time. This effect was observed for sensors based on the ionophore II as well as the ionophore III (Fig. 8). However, the membrane with ionophore III maintained the initial phosphate-selectivity in a longer period i.e. during 2 weeks. The acceleration of the electrode deterioration can be explained by the formation of water–UO2 – salophene complex followed by the decomposition of the ionophore due to their interactions with phosphate

anions. Uranyl cations form more stable complexes in membrane with hydrogen phosphate than with Schiff base moiety, which can lead to the decomposition of the ionophore (formation of the uranyl hydrogen phosphate and release of the unstable Schiff base). A more detailed discussion of the proposed mechanism can be found in reference [19]. The durability of electrodes exposed to the permanent conditioning in phosphate solution can be improved by decreasing the TDAB concentration in membrane (Fig. 9).

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Fig. 9. Time dependence of selectivity coefficient (log K (NO3 − , H2 PO4 − )) of phosphate-selective electrodes based on PVC/o-NPOE membranes containing ionophore III and different amounts of ammonium salt–TDAB. The electrodes were conditioned in 0.01 M NaH2 PO4 .

The presence of 5 mol% of ammonium salt in membrane gave the highest durability (1 month) of the phosphate-selective electrodes.

Acknowledgements

4. Conclusions

References

The influence of the uranyl salophene structure and membrane composition on the electrode selectivity and long-term stability were studied. The deterioration of the phosphate-selectivity of electrodes containing uranyl salophenes seems to be caused by the insufficient solvation of the ionophore by the polymeric matrix. The water–UO2 –salophene complex precipitates, which lowers the effective concentration of ionophore inside the membrane. The differences in ionophore solubility in a membrane determine the lifetime of electrodes with membranes based on uranyl salophenes. Phosphate-selective electrodes based on uranyl salophene III (the best solvated in membrane) exhibited the highest selectivity for H2 PO4 − and the best long-term stability (2 months). The electrode durability can be improved decreasing the amount of the ammonium salt in membrane to 5 mol%.

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This work was supported by the State Committee for Scientific Research, Project No. 3 T09A 114 19.

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