A novel dibasic phosphate-selective electrode based on Ferrocene-bearing macrocyclic amide compound

Sensors and Actuators B 126 (2007) 609–615

A novel dibasic phosphate-selective electrode based on Ferrocene-bearing macrocyclic amide compound Wei Liu, Xia Li, Maoping Song ∗ , Yangjie Wu Department of Chemistry, Zhengzhou University, Zhengzhou, Henan 450052, China Received 28 November 2006; received in revised form 6 April 2007; accepted 10 April 2007 Available online 20 April 2007

Abstract A new PVC membrane electrode for dibasic phosphate anion based on Ferrocene-bearing macrocyclic amide compound (L) has been prepared. The construction, response characteristic, selectivity and application of the electrode are investigated in detail. The best performance is exhibited by the electrode membrane containing 20 mol% TDMACl with o-NPOE as a plasticizer, and the electrode shows good selectivity towards HPO4 2− over a wide concentration range (1.0 × 10−2 to 1.0 × 10−5 M) with a Nernstian slope of 29.8 mV per decade at pH 9.0, and a detection limit of 2.2 × 10−6 M. It has a very short response time, less than 20 s, and can be used for at least 2 months without any divergence in response slope and linear range. The electrode displays high selectivity towards dibasic phosphate with respect to many other anions including Cl− , Br− , I− , NO3 − , OAc− , and SO4 2− . The proposed electrode has been used as the indicator electrode in titration of phosphate with Ba(NO3 )2 solution. © 2007 Elsevier B.V. All rights reserved. Keywords: Ferrocene-bearing macrocyclic amide; Dibasic phosphate; Sensor

1. Introduction Phosphate is an essential nutrient and energy carrier for both human and plants. The products containing phosphate are wildly used in many areas. A fast and direct measurement system for phosphate is necessary in many fields such as pharmacology, biomedical research, clinical chemistry, industrial process monitoring, environmental, etc. [1]. Up to now, various methods including ion-pairing high performance liquid chromatography (HPLC), standard spectrophotometry and potentiometry based on ion-selective electrodes (ISEs), have been adopt for determination of phosphate concentration in various samples. Among these methods, ISEs attracted more interest because of their advantages, such as simple, fast, low cost, portable and nondestructive, etc. It is well known that the ionophore was the center of an electrode membrane. Therefore, the design and synthesis of new ionophores for phosphate has become a research area of great interest. During the past few decades, remarkable improvement has been achieved for phosphate ISEs. Several phosphate

∗

Corresponding author. Fax: +86 371 67767753. E-mail address: [email protected] (M. Song).

0925-4005/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2007.04.024

ISEs based on variety of carrier have been reported. For example, many organotin compounds [1–6] have traditional been employed successfully in developing phosphate ISEs. In addition to these, polyamines [7,8], guanidinium [9], uranyl salphen [10,11], thiourea [12], thioxamide [13] or organometallic compounds like Co(II)phthalocyanine derivatives [14] and Ni(II)diketonate-ethylene [15] are also effective ionophores for phosphate-selective electrodes. It is well documented in literature that many anion electrodes were prepared based on the organometallic complexes [16–21]. In all of the mentioned electrodes, a specific interaction is assumed to occur between the anions of interest with the central metal of the ionophores. In addition, some complexes containing functional group, such as urea, thiourea, amide, thioamide can be used as ionophores for anion electrodes because of the hydrogen-bond forming [12,13,22,23]. Besides, the macrocyclic compound [7] also can bind some anions whose sizes are fit for the ring size. Taken these advantages into consideration, a newly synthesized ligand, which had the three characters above mentioned, was used as PVC membrane ionophore and anticipates developing a new anionselective electrode with anti-Hofmesister behavior. In this paper, we reported a dibasic phosphate-selective electrode incorporating Ferrocene-bearing macrocyclic complex as ionophore

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and o-nitrophenyloctyl ether (o-NPOE) as a plasticizer. This is the first use of a Ferrocene-bearing macrocyclic complex as a suitable carrier for the preparation of a highly selective PVC membrane electrode. 2. Experimental 2.1. Reagents o-Nitrophenyloctyl ether (o-NPOE) and tridodecylmethylammonium chloride (TDMACl) were purchased from Fluka. Dibutyl phthalate (DBP), bis(2-ethylhexyl) sebacate (DOS), tetrahydrofuran (THF) (dried by sodium and distilled prior to use), poly(vinyl chloride) (PVC) of high molecular mass were obtained from Shanghai Chemical Company and used as received. Analytical-grade potassium salts of test anions were used. All other reagents were of analytical reagent grade and doubly deionized, distilled water was used throughout. The H2 SO4 and KOH were used to adjust the pH. 2.2. Ionophore synthesis 2.2.1. General remarks The synthetic routes are shown in Scheme 1. IR spectra were recorded on a Brucke-VECTOR22 spectrophotometer as KBr pellets in the 400–4000 cm−1 regions. 1 H NMR and 13 C NMR spectra were obtained on a Brucke DPX400 MHZ superconductive NMR spectrometer using CDCl3 as solvent and tetramethylsilane as an internal standard. Element analyses were performed with a Carlo-Erba 1106 elemental analyzer. All chemical reagents were obtained from commercial sources and solvents were purified by standard method. 2.2.2. Synthesis 1,1-Bis-(chlorocarbonyl)ferrocene (1). 1,1-Ferrocenedicarboxylic acid (1.1 g, 0.004 mol) were suspended in 10 ml of CH2 Cl2 and 0.5 ml of pyridine at room temperature, then 2.0 ml of oxalyl chloride were added dropwise with vigorously stirring. The mixture was refluxed for 6 h, cooled down to room temperature, and the solvent were removed by evaporating under reduced pressure. The dark-red residue was then extracted with anhydrous petroleum (bp: 60–90 ◦ C). Evaporation of the solvent afforded 0.98 g of a dark-red solid, yield: 78.8%.

1,2-Bis(o-nitrophenoxy)ethane (2). A mixture of onitrophenol (13.9 g, 0.1 mol), 1,2-dibromoethane (10.3 g, 0.055 mol), 20 ml DMF and anhydrous K2 CO3 (10.4 g, 0.075 mol) was heated for 6 h at 140 ◦ C, then concentrated under reduced pressure. The mixture was diluted with water and the precipitate was collected, washed with 10% aqueous NaOH, dried and recrystallised from AcOH, yield: 62%, mp 170–171 ◦ C. 1,2-Di-(o-aminophenoxy)ethane (3). 1,2-Bis(onitrophenoxy)ethane (2) (6.1 g, 0.02 mol) was added in portions to a solution of SnCl2 ·2H2 O (31.9 g, 0.125 mol) in 40 ml concentrated hydrochloric acid at 75 ◦ C during 30 min, then the mixture was refluxed for 7 h. The precipitate was gathered and dispersed in 20% aqueous NaOH, then the white solid was collected and recrystallised from ethanol to give the white crystal of diamine (3) 4.6 g, yield: 94%, mp 131–132 ◦ C. 1,1 -[1,2-Ethanediylbis(oxy-2,1-phenyleneiminocarbonyl)] Ferrocene (L). A solution of 1,1 -bis-(chlorocarbonyl)ferrocene (0.5 g, 0.0016 mmol) in CH2 Cl2 (20 ml) was added dropwise to a vigorously stirred solution of the diamine(0.4 g) in CH2 Cl2 (40 ml) containing pyridine (1 ml). The stirred reaction mixture was maintained at room temperature for 12 h. Removal of the solvent afforded the crude amide (L), which was chromatographed on neutral alumina. Elution with dichloromethane–acetic ester (4:1) gave the pure amide (L), yield: 81.3%, IR: 3425(s), 1665(vs), 1601(s), 1533(vs), 1459(m), 1443(m), 1248(s), 1111(m), 936(m), 747(m). NMR: δH (ppm) 4.73 (s, 4H, Fc-H), 4.53 (s, 4H, Fc-H), 4.56 (s, 4H, OCH2 ), 7.04–7.12 (m, 6H, Ar-H), 8.41 (s, 2H, NH), 8.56–8.58 (m, 2H, Ar-H). δC (ppm) 66.88, 69.53, 74.17, 77.68, 111.04, 119.79, 122.29, 123.42, 128.27, 146.25, 167.13. m/z (HRMS): 505.0810 (M+ + Na), 521.0582 (M+ + K). Anal. Found (%): C, 64.78; H, 4.64; N, 5.79. Calc. for C26 H22 FeN2 O4 : C, 64.75, H, 4.68, N, 5.81. 2.2.3. X-ray structure determination of L Crystals of L suitable for X-ray analysis were obtained by slow evaporation of a 1:1 (V/V) dichloromethane–hexane solution to afford orange crystals. The crystal structure was determined on a Rigaku RAXIS-IV imaging plate area detector with graphite monochromated Mo K␣ radi˚ The data were collected using the ation (λ = 0.71073 A). ω–2θ scan technique and corrected for Lorentz-polarization

Scheme 1.

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Prior to potentiometric measurements, all the electrodes were conditioned in a 0.01 M primary anion solution overnight. For each membrane composition, three electrodes were prepared. 2.4. EMF measurements

Fig. 1. X-ray crystal structure of compound (L).

effects. The crystal structure of compound L is shown in Fig. 1. It belongs to monoclinic system P2(1)/n ˚ b = 12.765(3) A, ˚ c = 14.512(3) A, ˚ space group, a = 11.860(2) A, ◦ 3 −3 ˚ β = 94.07(3) . V = 2191.5(8) A , Z = 4, Dc = 1.462 Mg m , Completeness: 91.0%, R1 [I > 2σ(I)] = 0.0564, ωR2 (all data) = 0.1362. All calculations were performed using the shelxl-97 crystallographic software package [24]. Selected bond lengths and angles are list in Table 1.

The following cell assembly: Hg/Hg2 Cl2 /KCl (saturated)/0.1 M LiAc/sample solution//PVC membrane//0.01 M K2 HPO4 /Ag/AgCl were used for measurements of all emf. Potentials and pH values were measured by using an ion meter of model pXS-215 (Leici Instruments Corporation, Shanghai) and a pH meter of model 6071 (JENCO Electronics, Ltd., Shanghai). All emf measurements were made relative to an Hg/Hg2 Cl2 double junction reference electrode with 0.1 M lithium acetate in the outer compartment and saturated potassium chloride in the inner compartment. The performance of each electrode was investigated by measuring its potential of the primary ion solutions in the range of 1.0 × 10−1 to 1.0 × 10−6 M. The data were plotted as observed potential versus the logarithm of the anions activity. The activities were calculated according to the modified Debye-H¨uckel equation [25]. According to the suggestion of Carey and Riggan [7], at different pH value, the activities of dibasic phosphate were calculated using the total phosphate, the standard solution pH, the equilibrium constant of the different phosphate species and the ionic strength. The detection limit was determined according to IUPAC recommendations [26]. 3. Results and discussion 3.1. Crystal structure of ionophore L

2.3. Preparation of electrodes The general procedure to prepare the PVC membrane was dissolving the mixture of 1 wt% (2.8 mg) ionophore, 33 wt% (92.8 mg) PVC, 66 wt% (185.5 mg) plasticizer and 0–30 mol% versus ionophore TDMACl in 5 ml of THF. The resulting mixture was transferred into an 18 mm diameter glass ring and the solvent was allowed to evaporate at room temperature for 2 days. The resulting membrane was cut into small diameter disks and was sealed onto the end of the Ag/AgCl electrode barrel with a 5% THF solution of PVC (wt%). For all electrodes, a solution of 0.01 M K2 HPO4 was used as the internal reference solution. Table 1 ˚ and angles (◦ ) Selected bond lengths (A) N1–C1 N1–C26 N2–C7 N2–C13 C1–O1 C1–C2 C7–O2 C7–C8 C18–O4 C19–O4 C20–O3 C21–O3

1.341(6) 1.407(6) 1.352(6) 1.404(6) 1.235(5) 1.488(6) 1.216(5) 1.485(6) 1.375(6) 1.419(5) 1.414(5) 1.379(6)

C1–N1–C26 C6–C2–C1 C7–N2–C13 C9–C8–C7 O1–C1–N1 O1–C1–C2 N1–C1–C2 O2–C7–N2 O2–C7–C8 N2–C7–C8 C21–O3–C20 C18–O4–C19

129.7(5) 128.0(4) 130.3(5) 128.9(4) 123.4(5) 120.2(4) 116.3(4) 123.7(5) 122.4(5) 113.9(4) 119.2(4) 117.9(4)

At ambient temperature, the cyclic-amide L was obtained via directly condensation reactions. Different acid scavengers (including pyridine and triethylamine) were examined, and the pyridine gave the better chemical yield 81.3% than the triethylamine 60.7%. When a large excess amount of acid scavengers was employed, slightly improving in chemical yield was noticed. To investigate the solid-state structure of L and also to examine the effect of a ferrocenyl moiety on the conformational structure of the macro cycle, single-crystal molecular structure of L was determined. Herein a distorted macrocyclic has formed due to the amazing sandwich structure of ferrocenyl moiety, the cavity dimensions for the receptor L ˚ C6· · ·O3 4.400 A, ˚ C6· · ·O4 are as follows: N1· · ·N2 5.040 A, ˚ C9· · ·O3 4.305 A, ˚ C9· · ·O4 4.268 A. ˚ Other princi4.738 A, ˚ C1 O1 pal dimensions for L are amide C1–N1 1.341(6) A, ˚ N1–C1 O1 123.4(5)◦ and C2–N7 1.352(6) A, ˚ 1.235(5) A, ˚ N2–C7 O2 123.4(5)◦ . Additionally the two C7 O2 1.216(5) A, pentagonal ring of the ferrocenyl moiety are planar, parallel and the Cp rings are across arranged. Though the cavity size of L seems small to wrap the HPO4 2− completely according to the solid-state structure, the size can be micro-tuned by the mutual rotation of the Cp units. The vital hydrogen-bonding donor and receptor groups (–NH– and –CH2 O–, respectively) may work efficiently in binding process since it has been postulated by Beer and others [27] that hydro-

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Table 2 Optimization of membrane ingredients Membrane number

1 2 3 4 5 6 7 8 9

Membrane composition (mg) Ionophore

Plasticizer

PVC

Additive (mol%)

2.8 2.8 2.8 3.9 5.1 3.9 3.9 3.9 0

185.5,NPOE 185.5,DBP 185.5,DOS 185.5,NPOE 185.5,NPOE 185.5,NPOE 185.5,NPOE 185.5,NPOE 185.5,NPOE

92.8 92.8 92.8 92.8 92.8 92.8 92.8 92.8 92.8

0 0 0 0 0 0.28 (10) 0.56 (20) 0.84 (30) 0.56 (20)

gen bonding plays an important role in anions sensing process. It is postulated that the –NH– of L can act as a Lewis acid donating a proton to the HPO4 2− , and the –CH2 O– group can act as a base accepting a proton from the HPO4 2− . Moreover, the intriguing distorted structural feature has shaped a 3D cavity, which may be better able to accept the HPO4 2− than a planar one. Additionally, the specific interaction between iron center and HPO4 2− give an extra enhancement in sensing process. 3.2. The PVC membrane dibasic phosphate electrode In preliminary experiments, the potentiometric response of the electrodes having membrane composition (w/w) L:PVC:NPOE (1:33:66) was tested for several anions. In these studies, the internal solution was 0.01 M the corresponding anion solution, and each electrode was conditioned in the same solution overnight. As seen from Fig. 2, among these anions, the best response was observed for HPO4 2− ion. Therefore, the ionophore L was selected as a suitable ionophore for dibasic phosphate-selective electrode. 3.2.1. The influence of membrane composition Besides the critical role of the nature of the ionophore in preparing membrane electrode, the nature of the plasticizer used,

Fig. 2. Potentiometric responses of an o-NPOE-plasticized membrane composed of 1% ionophore L to various anions.

Slope (mV decade−1 )

Linear range (M)

Detection Limit (M)

23.3 ± 18.9 ± 16.3 ± 26.9 ± 24.6 ± 27.6 ± 29.8 ± 23.4 ± –

1.0 × 10−2 1.0 × 10−2 1.0 × 10−2 1.0 × 10−2 1.0 × 10−2 1.0 × 10−2 1.0 × 10−2 1.0 × 10−2 –

7.0 × 10−6 1.0 × 10−4 1.0 × 10−4 4.2 × 10−6 5.1 × 10−6 5.7 × 10−6 2.2 × 10−6 6.4 × 10−6 –

0.3 0.5 0.5 0.4 0.3 0.2 0.3 0.4

to 1.0 × 10−5 to 1.0 × 10−4 to 1.0 × 10−4 to 1.0 × 10−5 to 1.0 × 10−5 to 1.0 × 10−5 to 1.0 × 10−5 to 1.0 × 10−5

the amount of ionophore, and the nature of additives are also significantly influence the sensitivity, linear range and selectivity of ion-selective electrodes. At first, the effect of the plasticizers used on the performance of membrane electrodes was investigated. Table 2 illustrates the response properties of PVC membrane electrodes (membrane nos. 1–3) containing different plasticizers including DBP, DOS and o-NPOE. It can be seen that the membrane electrode preparing with o-NPOE showed the lowest detection limit, the widest linear range and most Nernstian slopes for HPO4 2− anion. This is due to the high tendency of L and ability of o-NPOE for extraction of HPO4 2− anion from aqueous phase to the organic membrane phase. Therefore, we used o-NPOE as a suitable plasticizer for further studies. The influence of the amount of ionophore incorporated in the membrane on the electrode characteristics was also investigated (membrane nos. 1, 4, 5). As seen, the sensitivity of electrode response was improved on increasing the ionophore concentration to 1.4%. Further addition of the ionophore concentration to 1.8%, resulted in diminished response of the electrode, most probably due to some inhomogeneities and possible saturation of the membrane [28]. It is well established in literature that lipophlic ionic additives should promote the inter facial ion-exchange kinetics and decrease the membrane resistance by providing mobile ionic sites in membranes [29]. Table 2 gives the influence of the amount of additives TDMACl (0–30 mol% versus ionophore) on the performance of the electrodes (membrane nos. 6–8). It is found that the membrane incorporating 20 mol% TDMACl (membrane no. 7) shows the best performance characteristics and selectivity, and further increasing the site ratio to 30 mol% resulted in a deterioration of potentiometric response to HPO4 2− . The deterioration can be explained by the low concentration of the free ionophore in the membrane at test site ratios [30]. The response of the blank membrane (membrane no. 9) containing additives but no ionophore was also studied. As the results show there is no potentiometric response for this composition of membrane. Thus, the optimized membrane (membrane no. 7) containing 1.4% ionophore (L), 20 mol% additive TDMACl, and o-NPOE as aplasticizer was obtained to design a dibasic phosphateselective electrode. The electrode shows a linear response to the concentration of HPO4 2− in the range 1.0 × 10−2 to

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Table 3 The lifetime of the proposed electrode

Fig. 3. Calibration graphs for HPO4 2− at different values of pH: () 7.0, () 8.0, (䊉) 9.0, () 10.0.

1.0 × 10−5 M (Fig. 2). The slope of calibration graph was 29.8 mV per decade. The limit of detection was 2.2 × 10−6 M that determined from the intersection of the two extrapolate segments of the calibration graph. 3.2.2. The effect of pH The potentiometric response of the optimized membrane towards solution pH was studied over the range from 7 to 10. All the pH values of the tested solutions were adjusted with KOH or H2 SO4 solution. At each pH, a calibration graph for HPO4 2− was obtained. As shown in Fig. 3, it is found that while pH 9, the electrode gave the best slope and wide linear range. At other pH value tested, the slope was lower than the Nernstian slope, especially at pH 7. This could be attributed to the decreasing of the amount of HPO4 2− . 3.2.3. Response and characteristics of the electrode The stability of the electrode was tested. Continuous monitoring of potentials for 8 h at a fixed concentration (1.0 × 10−3 M K2 HPO4 ) gave a potential drift of 1.0 mV. The response time is an important factor in analytical application. In this study, the response time (t95% ) was tested by measuring the time to achieve a 95% of the final potential when the concentration of K2 HPO4 solution was rapidly increased from 10−5 to 10−4 , 10−4 to 10−3 , 10−3 to 10−2 M (by fast injection of a ␮l amount of a concentrated solution). The electrode has a fast response time of <20 s. The lifetime was studied by periodically measuring the electrode response slope and linear range in standard K2 HPO4 solution. The results are summarized in Table 3. As seen, after 2 months, no significant change in the performance of the electrode was observed (29.0 ± 0.2 mV decade−1 and 4.2 × 10−6 M, respectively). Therefore, the lifetime of the electrode is at least 2 months without obvious change in its response characteristics towards HPO4 2− . 3.2.4. Selectivity Selectivity is the most important characteristic of any ion selective electrode. It is this characteristic which defines the

Period (week)

Slope (mV decade−1 )

1 2 3 4 5 6 7 8

29.8 29.8 29.6 29.4 29.3 29.3 29.2 29.0

± ± ± ± ± ± ± ±

Detection limit (M) 2.2 × 10−6 1.6 × 10−6 2.8 × 10−6 2.6 × 10−6 3.2 × 10−6 4.0 × 10−6 4.6 × 10−6 4.2 × 10−6

0.3 0.2 0.2 0.4 0.3 0.2 0.4 0.2

nature of the device and the range to which it may be successfully employed. Conventionally, the selectivity coefficients are measured by either the fixed interference or the separate solution methods as recommended by IUPAC. These methods work well when the ions under investigation have the same charge. They are not well suited, however, when ions of different charge are involved [31,32]. In our study, the activity ratio method was adopted to determine the selectivity coefficients according to the suggestion of Glazier, etc. [2]. In this method, the electrode response to the individual anions was measured over an ion activity range from approximately 10−5 to 10−2 M, and the date were modeled by the following function: f (x) = E + S log(x + B) by using a nonlinear least-squares curve-fitting routine for the three parameters E, S, and B. Values of f(x) and x were the potential changes and ion activity or concentration, respectively. The resulting equations were then used to calculate the activity of dibasic phosphate that corresponds to the potential change predicted for an activity of 10−2 M for the interfering ion. The selectivity coefficient was then calculated as the ratio of these pot activities (i.e. Ki/j = ai /aj , where ai and aj , are the activities of the primary and interfering ions, respectively). The resulting selectivity coefficients are listed in Table 4. The selectivity coefficient values indicate that the electrode revealed the best selectivity to HPO4 2− over other anions, especially for Cl− , which is the most important interference in the practical application. In other aspect, the present electrode showed better selectivity than the other electrodes previously reported [1,2,6,8,9]. Table 4 Values of selectivity coefficient for proposed electrode pot

Anion

log Ki/j

Cl− Br− OAc− I− NO3 − SO4 2− CO3 2− ClO4 − SCN−

−2.92 −3.30 −1.66 −1.26 −2.19 −1.45 −0.08 −0.69 −0.50

± ± ± ± ± ± ± ± ±

0.15 0.13 0.08 0.13 0.06 0.10 0.07 0.08 0.06

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References

Fig. 4. Potentiometric titration curve of 20.0 ml 1.0 × 10−3 M solution of HPO4 2− with 1.0 × 10−2 M Ba(NO3 )2 .

3.2.5. Application The proposed electrode was successfully used as the indicator electrode in the titration of a 20 ml K2 HPO4 (1.0 × 10−3 M) solution with the Ba(NO3 )2 (1.0 × 10−2 M) at pH 9.0 (Fig. 4). As seen, the titration curve is of sigmoid shape showing perfect stoichiometry that indicates the amount of the Ba2+ ion can be accurately determined with the electrode. 4. Conclusions The result presented in this study demonstrates that Ferrocene-bearing macrocyclic amide compound, as a new ionophore, can be used in the development of a PVC membrane HPO4 2− selective electrode. The electrode responds to HPO4 2− in a Nernstian slope of 29.8 mV per decade and presents a wide working concentration range (1.0 × 10−2 to 1.0 × 10−5 M) and good limit detection (2.2 × 10−6 M). The electrode reveals the best selectivity to HPO4 2− over other anions, especially for Cl− , which is the most important interference in the practical application. Thus the present electrode can be put to analytical use both by direct potentiometry as well as potentiometric titration. Supplementary information Crystallographic data for the structural analyses have been deposited with the Cambridge Crystallographic Data Centre, CCDC no. 632689 for L. Copies of this information may be obtained free of charge from The Director, CCDC, 12 Union road, Cambridge, CB2 1EZ, UK (fax: +44 223 336033; e-mail: [email protected]). Acknowledgment This work was financially supported by the National Natural Science Foundation of China (no. 20572102).

[1] S.A. Glazier, M.A. Arnold, Phosphate-selective polymer membrane electrode, Anal. Chem. 60 (1988) 2540–2542. [2] S.A. Glazier, M.A. Arnold, Selectivity of membrane electrodes based on derivatives of dibenzyltin dichloride, Anal. Chem. 63 (1991) 754–759. [3] N.A. Chaniotakis, K. Jurkschat, A. Ruehlemann, Potentiometric phosphate-selective electrode based on a multidentate-tin(IV) carrier, Anal. Chim. Acta 282 (1993) 345–352. [4] J.K. Tsagatakis, N.A. Chaniotakis, K. Jurkschat, Multiorganyltin compounds. Designing a novel phosphate-selective carrier, Helv. Chim. Acta 77 (1994) 2191–2196. [5] D. Liu, W.C. Chen, R.H. Yang, G.L. Shen, R.Q. Yu, Polymeric membrane phosphate sensitive electrode based on binuclear organotin compound, Anal. Chim. Acta 338 (1997) 209–214. [6] S. Sasaki, S. Ozawa, D. Citterio, K. Yamada, K. Suzuki, Organic tin compounds combined with anionic additives-an ionophore system leading to a phosphate ion-selective electrode? Talanta 63 (2004) 131–134. [7] C.M. Carey, W.B. Riggan Jr., Cyclic polyamine ionophore for use in a dibasic phosphate-selective electrode, Anal. Chem. 66 (1994) 3587–3591. [8] T.L. Goff, J. Braven, L. Ebdon, D. Scholefield, Phosphate-selective electrodes containing immobilized ionophores, Anal. Chim. Acta 510 (2004) 175–182. [9] M. Fibbioli, M. Berger, F.P. Schmidtchen, E. Pretsch, Polymeric membrane electrodes for monohydrogen phosphate and sulfate, Anal. Chem. 72 (2000) 156–160. [10] W. Wroblewski, K. Wojciechowski, A. Dybko, Z. Brzozka, R.J.M. Egberink, B.H.M. Snellink-Ruel, D.N. Reinhoudt, Uranyl salophenes as ionophores for phosphate-selective electrodes, Sens. Actuators B 68 (2000) 313–318. [11] W. Wroblewski, K. Wojciechowski, A. Dybko, Z. Brzozka, R.J.M. Egberink, B.H.M. Snellink-Ruel, D.N. Reinhoudt, Durable phosphateselective electrodes based on uranyl salophenes, Anal. Chim. Acta 432 (2001) 79–88. [12] S. Nishizawa, T. Yokobori, R. Kato, K. Yoshimoto, T. Kamaishi, N. Teramae, Hydrogen-bond forming ionophore for highly efficient transport of phosphate anions across the nitrobenzene–water interface, Analyst 128 (2003) 663–669. [13] A.K. Jain, V.K. Gupta, J.R. Raisoni, A newly synthesized macrocyclic dithioxamide receptor for phosphate sensing, Talanta 69 (2006) 1007–1012. [14] J.H. Liu, Y. Masuda, E. Sekido, Response properties of an ion-selective polymeric membrane phosphate electrode prepared with cobalt phthalocyanine and characterization of the electrode process, J. Electroanal. Chem. 291 (1990) 67–79. [15] N. Sato, Y. Fukuda, Anion-sensing electrodes based on nickel(II) mixed ligand complexes, Chem. Lett. 3 (1992) 399–402. [16] C.S. Pedreno, J.A. Ortuno, D. Martinez, Anion selective polymeric membrane electrodes based on cyclopalladated amine complexes, Talanta 47 (1998) 305–310. [17] M. Shamsipur, S. Ershad, N. Samadi, A.R. Rezvani, H. Haddadzadeh, The first use of a Rh(III) complex as a novel ionophore for thiocyanate-selective polymeric membrane electrodes, Talanta 65 (2005) 991–997. [18] N.A. Chaniotakis, J.K. Tsagatakis, K. Kurkschat, R. Willem, Organometallic complexing agents as carriers in polymer-based electrodes, React. Funct. Polym. 34 (1997) 183–188. [19] R. Stepanek, B. Kraeutler, P. Schulthess, B. Lindemann, D. Ammann, W. Simon, Aquocyanocobalt(III)-hepta(2-phenylethyl)-cobyrinate as a cationic carrier for nitrite-selective liquid-membrane electrodes, Anal. Chim. Acta 182 (1986) 83–90. [20] D. Gao, J. Gu, R.Q. Yu, G.D. Zheng, Substituted metalloporphyrin derivatives as anion carrier for PVC membrane electrodes, Anal. Chim. Acta 302 (1995) 263–268. [21] S. Daunert, L.G. Bachas, Anion-selective electrodes based on a hydrophobic vitamin B12 derivative, Anal. Chem. 61 (1989) 499–503. [22] H.K. Lee, H. Oh, K.C. Nam, S. Jeon, Urea-functionalized calix[4]arenes as carriers for carbonate-selective electrodes, Sens. Actuators B 106 (2005) 207–211.

W. Liu et al. / Sensors and Actuators B 126 (2007) 609–615 [23] S. Nishizawa, P. Buhlmann, K.P. Xiao, Y. Umezawa, Application of a bis-thiourea ionophore for an anion selective electrode with a remarkable sulfate selectivity, Anal. Chim. Acta 358 (1998) 35–44. [24] G.M. Sheldrick, Shelxl-97: Program for Crystal Structure Refinement, University of G¨ottingen, G¨ottingen, Germany, 1997. [25] S. Kamata, A. Bhale, Y. Fukunaga, A. Murata, Copper(II)-selective electrode using thiuram disulfide neutral carriers, Anal. Chem. 60 (1988) 2464–2467. [26] G.G. Guilbault, R.A. Durst, M.S. Frant, H. Freiser, E.H. Hansen, T.S. Light, E. Pungor, G. Rechnitz, N.M. Rice, T.J. Rohm, W. Simon, J.D.R. Thomas, Pure Appl. Chem. 48 (1976) 127–132. [27] P.D. Beer, J. Cadman, Electrochemical and optical sensing of anions by transition metal based receptors, Coord. Chem. Rev. 205 (2000) 131–155. [28] D. Ammann, E. Pretsch, W. Simon, E. Lindner, A. Bezegh, E. Pungor, Lipophilic salts as membrane additives and their influence on the properties of macro- and micro-electrodes based on neutral carriers, Anal. Chim. Acta 171 (1985) 119–129. [29] J. Bobacka, M. Maj-Zurawska, A. Lewenstam, Carbonate ion-selective electrode with reduced interference from salicylate, Biosens. Bioelectron. 18 (2003) 245–253. [30] E. Bakker, P. Buhlmann, E. Pretsch, Carrier-based ion-selective electrodes and bulk optodes. 1. General characteristics, Chem. Rev. 97 (1997) 3083–3132. [31] V.P.Y. Gadzekpo, G.D. Christian, Determination of selectivity coefficients of ion-selective electrodes by a matched-potential method, Anal. Chim. Acta 164 (1984) 279–282.

615

[32] Y. Umezawa, M. Kataoka, W. Takami, E. Kimura, T. Koike, H. Nada, Potentiometric adenosine triphosphate polyanion sensor using a lipophilic macrocyclic polyamine liquid membrane, Anal. Chem. 60 (1988) 2392–2396.

Biographies W. Liu graduated from Henan University, Department of Chemistry in 1999. Now he is a PhD student at Zhengzhou University. His research interests have been in the areas of developing chemical sensors based on novel ferrocenebearing compounds. X. Li received her MS degree in the field of ion-selective electrodes in 2006 in Zhengzhou University, Department of Chemistry. She is now a PhD student in the same university. M. Song received his PhD degree from Nanjing University, Department of Chemical Engineering in 1992. He is a professor of organic chemistry in Department of Chemistry at Zhengzhou University. His research interests have been in the areas of supermolecular chemistry and physical organic chemistry. Y. Wu is an academician of the Chinese Academy of Science. He is a professor of organic chemistry in Department of Chemistry at Zhengzhou University. His research interests have been in the areas of physical organic chemistry and organometallic chemistry.