Anion recognition through modified calixarenes: a highly selective sensor for monohydrogen phosphate

Analytica Chimica Acta 538 (2005) 213–218

Anion recognition through modified calixarenes: a highly selective sensor for monohydrogen phosphate Vinod K. Gupta a,∗ , Rainer Ludwig b , Shiva Agarwal a b

a Department of Chemistry, Indian Institute of Technology, Roorkee 247667, Uttaranchal, India Institute for Inorganic and Analytical Chemistry, Freie Universitaet, Berlin 14195, Berlin, Germany

Received 2 November 2004; received in revised form 30 January 2005; accepted 1 February 2005 Available online 3 March 2005

Abstract PVC-based membranes of 5,11,17,23,29,35-hexa-tert-butyl-37,38,39,40,41,42-hexakis(carbamoylmethoxy)calix[6]arene (I) as electroactive material with dioctyl phthalate (DOP), 1-chloronaphthalene (CN), dibutyl phthalate (DBP), o-nitrophenyl octyl ether (NPOE) and tris(2-ethylhexyl) phosphate (TEP) as plasticising solvent mediators have been prepared and used for phosphate determination. The membrane having composition PVC:I (150:5.7) exhibited the best result with linear potential response in the concentration range of 1.77 × 10−5 to 1.0 × 10−1 M of HPO4 2− with a slope of 33.0 mV/decade. The membrane worked satisfactorily in non-aqueous medium up to 5% (v/v) non-aqueous content. The selectivity coefficient values for mono- and divalent anions indicate good selectivity for phosphate over a large number of anions. The proposed sensor has been successfully used in the real sample analysis. © 2005 Elsevier B.V. All rights reserved. Keywords: PVC; Phosphate; Potentiometric sensor; Calix[6]arene

1. Introduction Phosphate levels in freshwater bodies have increased in the past 50 years [1], which may have a negative effect on aquatic ecology and water quality. Recent studies show that leaching of P as small as 10 ␮g l−1 (2–3 kg ha−1 per year P) from agricultural land can contribute to eutrophication [2]. Hence, phosphate determination assumes importance. Nowadays, measurement of phosphate in water is achieved by sophisticated automated flow apparatus, which is an expensive technique and produces toxic waste. Thus, an electrochemical method would be more suitable for onsite analysis of phosphate as ISEs have the advantage of being portable and low cost instruments. Many phosphate selective potentiometric sensors have been reported in the literature. Organic tin compounds have traditionally been used as phosphate selective ionophores ∗

Corresponding author. Tel.: +91 1332 285801; fax: +91 1332 273560. E-mail addresses: [email protected], [email protected] (V.K. Gupta). 0003-2670/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.aca.2005.02.008

[3–7], together with cobalt phthalocyanine [8] or polyamine [9] while uranyl salophene [10,11], vanadyl salene complex [12], well-designed guanidinium [13] or thiourea compounds [14,15] are some recent examples. Ion selective electrodes for lipophilic anions are usually based on ion exchangers and in this case their selectivity pattern follows the Hofmeister series. This series is related to low hydration energy, thus, favoring lipophilic anions [16,17]. Due to high hydration energy of hydrophilic anions (such as phosphate and sulfate), membranes containing tetraalkylammonium salts have poor selectivity for these ions. In order to prepare ISEs for these ions, the membranes should contain ionophore molecules, which are able to bind specific ions [10]. Neutral anion ionophores containing hydrogen-bonddonating moieties or immobilized Lewis acidic binding site can be applied for this purpose, as summarized recently [18]. Simple calixarenes do not deprotonate under high pH and thus cannot transport anions. Further, anions cannot be complexed with such calixarenes because the OH oxygen atom and the anion are electron donors and thus, repulse each other. We have, therefore, modified the lower rim of the

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calix[6]arene by introducing a carbamoyl methoxy group as it combines hydrogen-bond donor/acceptor functions, electrostatic and dipole–dipole interactions. The modified calixarene thus imitates the function of natural receptors for nucleosides where the phosphate is also complexed by similar interactions. The synthesized calixarene is used to design a monohydrogen phosphate selective electrode and the results are presented herein. 2. Experimental 2.1. Reagents Di-sodium hydrogen phosphate was obtained from Merck (Germany). High molecular weight poly(vinyl chloride) (PVC) was obtained from Fluka (Switzerland) and used as such. All other reagents used were of analytical grade purity (BDH, UK). Aqueous solutions were prepared in doubly distilled water. Dioctyl phthalate (DOP) and dibutyl phthalate (DBP) from Reidal (India), and 1-chloronaphthalene (CN) and tris(2-ethylhexyl) phosphate (TEP) from Merck (Germany) and o-nitrophenyl octyl ether (NPOE) from Acros Organics (Belgium) were used. Solutions of different concentrations were prepared by diluting the stock solution of 0.1 M concentration. 2.2. Apparatus The potential measurements were carried out at 25 ± 0.1 ◦ C on digital pH meter/millivoltmeter (Toshniwal Inst. Mfg. Pvt. Ltd., Ajmer, India). pH measurements were made on a digital pH meter (LabIndia pH Conmeter, India). Spectra and absorbance were recorded on UV–vis spectrophotometer (Specord 200 Analytik Jena, Germany). 2.3. Synthesis The synthesis of 5,11,17,23,29,35-hexa-tert-butyl-37,38, 39,40,41,42-hexakis(carbamoylmethoxy) calix[6]arene (I) (Fig. 1), was first reported by Wolf et al. [19]. However, we prepared this compound as follows: tert-butyl-calix[6]arene (1.25 g, 1.3 mmol) was dissolved in 0.1 L of dry CCl4 and treated with (COCl)2 (12 mL, 0.14 mol) for 4 h under reflux. The solvent and excess (COCl)2 were removed in vacuo. The intermediate acid chloride was dissolved in dry tetrahydrofuran and treated with gaseous NH3 at room temperature for 6 h, until NH3 was no longer absorbed by the solution. The white precipitate was filtered off and washed with water followed by tetrahydrofuran. It was dissolved in CH2 Cl2 , washed twice with 0.1 M NH3 and then dried. Yield 84%; calc. for I·2H2 O: C 69.31%, H 7.90%, N 6.22%, found: C 69.5%, H 7.93%, N 6.06%; m.p. (Gallenkamp, in vacuo) 325 ◦ C (dec.), 13 C NMR (CDCl3 , δ in ppm), 31.3 (CH3 ), 34.2 (Ar–CH2 ), 71.2 (OCH2 ), 171.8 (C O), CAr: 126.3, 132.7, 147.3, 151.1. These values are in conformity of those reported for this calixarene earlier [19].

Fig. 1. Structure of 5,11,17,23,29,35-hexa-tert-butyl-37,38,39,40,41,42hexakis(carbamoylmethoxy)calix[6]arene (I).

Calixarene (I) with its terminal NH2 as hydrogen bond donor groups is capable of binding phosphate anion by multiple H-bonding interactions. Examples for this type of interaction are found in the literature [20,21] for calix[4]arenes bearing NH2 groups, which coordinate to the phosphate oxygen atoms. In addition, dipole–dipole interaction may contribute to the binding. The carbamoyl groups protonate only at pH < 1, thus reducing the influence of ligand protonation on the electrode performance. Therefore, electrostatic interactions are considered less likely at pH > 1. The H-bonding capability was confirmed: A 0.08 M CDCl3 solution of the ligand, magnetically stirred with a D2 O solution of D3 PO4 (pD ca. 1) within 1 min forms a solid–gel which is stable up to 100 ◦ C. The gelation is interpreted in terms of a hydrogen-bond network in solution. The solvents were removed in vacuo and the residual complex was subjected to NMR (20 ◦ C, CDCl3 /thf-d8 ). The 1 H NMR spectrum shows a rigidified conformation, compared with the uncomplexed ligand which was flexible on the NMR-timescale: the Ar–CH2 bridges now appear as two broad singlets at 3.44 (equatorial position) and 4.16 ppm (axial), as compared with one broad singlet in uncomplexed state. The O CH2 signal appears at 4.4 ppm. The t Bu region indicates a distorted cone conformation. The IR-spectrum (KBr) of this sample after drying in vacuo showed broad absorptions between 3450 and 3115 cm−1 (NH2 ), the lowered ν˜ and the shape indicate a Hbonding network. A small absorption at 2347 indicates a fraction of NH2 was protonated. C O absorption 1665 cm−1 showed overlap of amides I and II. From these observations, we conclude that phosphate is complexed to more than one NH2 group in a calixarene molecule, thus rigidifying it. 2.4. Membrane preparation PVC-based membrane incorporating ionophore I was prepared by the method of Craggs et al. [22]. Varying amounts

V.K. Gupta et al. / Analytica Chimica Acta 538 (2005) 213–218

of the ion active phase 5,11,17,23,29,35-hexa-tert-butyl37,38,39,40,41,42-hexakis(carbamoylmethoxy)calix[6]arene (Fig. 1) and an appropriate amount of PVC were dissolved in a minimum amount of THF. The solvent mediators DOP, DBP, TEP, NPOE and CN were also added to improve the response of the membranes. The solutions were poured into acrylic rings placed on a smooth glass and allowed to evaporate at room temperature. After 24 h, transparent membranes were obtained which were then cut to size and attached to a ‘Pyrex’ tube with the help of Araldite (Vantico Performance Polymers Pvt. Ltd., India). Since calixarene (I) is not soluble in water, the various membranes prepared do not bleed and generate stable and noiseless potentials.

to the molybdenum blue method [26] and other halves were maintained at pH 7.0 using Bis–Tris–H2 SO4 buffer prior to potentiometric analysis.

3. Results and discussion 3.1. Membrane characteristics All the membranes were dipped in a solution of 0.5 M HPO4 2− for 4 days to achieve proper equilibration. Of the various membranes prepared, one having I and PVC in the ratio 5.7:150 (w/w) gave satisfactory results. The membranes without ionophore (dummy membranes; containing only PVC and plasticizers) were also studied and potentials generated with these membranes were insignificant (5–10 mV).

2.5. Potential measurements Phosphorous exists in water exclusively as P(V) species, particularly in forms of orthophosphate and their distribution in aqueous phase is pH dependent [23]

3.2. Effect of plasticizers

H3 PO4 ⇔ H2 PO4 − ⇔ HPO4 2− ⇔ PO4 3− −

215

The effect of addition of plasticizers, viz., dioctyl phthalate (DOP), dibutyl phthalate (DBP), tris(2-ethylhexyl) phosphate (TEP), o-nitrophenyl octyl ether (NPOE) and chloronaphthalene (CN) was also studied for optimizing the composition of the membranes for obtaining best response characteristics and the same are listed in Table 1.

2−

The H2 PO4 prevails in acidic solution and HPO4 is dominant in basic solution [24] while at pH 7.0 mono- and dibasic forms coexist and from the slope of the sensor one can conclude whether the electrode is responding for mono- or dianion [3]. The slope in our case confirmed that the sensor is responding to dianion at pH 7.0. Thus, all sample solutions were prepared from sodium salts with 0.1 M Bis–Tris–H2 SO4 buffer solution at pH 7.0 and the studies were carried out at this pH only. Proper equilibration of membranes is essential to have a sensor showing good response characteristics. All the membranes were equilibrated in different concentrations of sample solutions. The time of contact in each case was also varied. Potentials were measured by direct potentiometry at 25 ± 0.1 ◦ C with the help of ceramic junction calomel electrodes and the cell set up was the same as reported earlier [25]. An amount of 1.0 × 10−1 M disodium hydrogen phosphate was taken as inner reference solution and saturated calomel electrodes (SCE) were used as reference electrodes.

3.3. Working concentration range and slope The potentiometric response characteristics of the membrane sensors were carried out with the varying concentration range of 1.0 × 10−6 to 1.0 × 10−1 M HPO4 2− . Table 1 depicts the results of the working concentration range, slope of each membrane. It was observed that membrane which contained only I in PVC matrix (membrane number 1) exhibited a working concentration range of 1.77 × 10−5 to 1.0 × 10−1 M of HPO4 2− . This electrode has a slope of 33.0 mV/decade of concentration, which predicts that the proposed electrode is responding for dibasic orthophosphate. As evident from Fig. 2 and Table 1, the addition of plasticizers did not show any improvement in the working of the electrode. The working concentration range is shortened to 2.5 × 10−4 to 1.0 × 10−1 , 7.5 × 10−4 to 1.0 × 10−1 , 5.0 × 10−4 to 1.0 × 10−1 , 1.9 × 10−4 to 1.0 × 10−1 and 1.0 × 10−3 to 1.0 × 10−1 M with the addition of DOP, DBP,

2.6. Treatment of real samples Water samples were collected from local agricultural field and filtered off. One half of the filtrates were treated according

Table 1 Composition and response characteristics of PVC-based calixarene (I) membranes selective to HPO4 2− Sensor/membrane number

Composition of the membrane (w/w)

1 2 3 4 5 6

5.7 5.7 5.7 5.7 5.7 5.7

I

DOP

DBP

CN

NPOE

TEP

PVC

150

150 150 150 150 150 150

150 150 150 150

Working concentration range (M)

Slope (±1.0 mV/decade of activity)

Response time (s)

1.77 × 10−5 to 1.0 × 10−1 2.5 × 10−4 to 1.0 × 10−1 7.5 × 10−4 to 1.0 × 10−1 5.0 × 10−4 to 1.0 × 10−1 1.9 × 10−4 to 1.0 × 10−1 1.0 × 10−4 to 1.0 × 10−1

33 36 44.4 42.5 34 40

20 25 50 30 10 25

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V.K. Gupta et al. / Analytica Chimica Acta 538 (2005) 213–218 Table 3 Selectivity coefficient KPot 2− values for HPO4 2− selective sensor as HPO4 ,B obtained by fixed interference method (FIM) and matched potential method (MPM) (interfering ion concentration 1.0 × 10−2 M) Interfering ion (B)

Pot Selectivity coefficient, KA,B

FIM Cl− NO2 − NO3 − SCN− SO4 2− HCO3 − ClO4 −

Fig. 2. Variation of membrane potential of PVC-based membranes of (I) with varying concentrations of HPO4 2− ions: (1) without plasticizer; (2) with DOP; (3) DBP; (4) CN; (5) NPOE and (6) TEP.

CN, NPOE and TEP, respectively. The membrane without plasticizer exhibited a response time of 20 s while those containing plasticizers achieved stable potentials in 10–50 s (Table 1). Since the best response characteristics, i.e. working concentration range, slope and response time, are exhibited by membrane number 1, thus the same was chosen for further studies. The membranes were stored in 0.1 M HPO4 2− solutions, when not in use and were re-equilibrated before use. Periodic monitoring of potentials showed that the lifetime of the electrode is about 1 month. 3.4. Solvent effect Sometimes in real situation the sample may contain the non-aqueous content, the functioning of the sensor system was, therefore, also investigated in partially non-aqueous media using methanol–water and acetone–water mixtures (Table 2). It was found that the membranes do not show any appreciable change in working concentration range or slope in mixtures up to 5% (v/v) non-aqueous contents. However, above 5% potentials show drift with time. The drift in potentials in the organic phase may be probably due to leaching of the ionophore at higher organic content.

MPM

With superscript

Without superscript

9.0 8.8 9.0 9.6 5.0 × 10−2 3.0 50

9.0 × 10−2 8.8 × 10−2 9.0 × 10−2 9.6 × 10−2 5.0 × 10−2 3.0 × 10−2 5.0 × 10−1

0.56 0.45 0.50 0.58 0.26 0.25 0.31

Table 4 Comparison of results of membrane electrode and molybdenum blue method (colorimetry) for determination of phosphate (M) in water samples collected from agricultural field Sample number

Molybdenum blue method (M)

Electrode method (M)

1 2 3

2.3 × 10−5 1.6 × 10−4 9.0 × 10−5

2.5 × 10−5 1.58 × 10−4 8.9 × 10−5

3.5. Potentiometric selectivity The selectivity is the most important characteristic of a sensor as it defines the nature of the device and the range to which it may be successfully employed. This is Pot and measured in terms of the selectivity coefficient KA,B may be evaluated by fixed interference method (FIM) using Nicolsky–Eisenman equation under limiting conditions (Eq. (1)) Pot KA,B =

aA

(1)

(aB )ZA /ZB

Further, Sa’ez de Viteri et al. [27] have proposed a modification in the Nicolsky equation (Eq. (1)) because this equation is valid only if the charges on the primary ion aA and interfering ion aB are the same. Thus, Sa’ez de Viteri and Diamond neglected the power term from the equation for calculating the selectivity coefficients Pot KA,B =

aA aB

(2)

Table 2 Performance of phosphate sensor (number 1) in non-aqueous media Non-aqueous content (%, v/v)

Slope (±1.0 mV/decade of activity)

Working concentration range (M)

33

1.77 × 10−5 to 1.0 × 10−1

Methanol 5 10

32.5 31

1.9 × 10−5 to 1.0 × 10−1 7.5 × 10−5 to 1.0 × 10−1

Acetone 5 10

33 30

2.0 × 10−5 to 1.0 × 10−1 6.8 × 10−5 to 1.0 × 10−1

0

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217

Table 5 Comparison of proposed HPO4 2− selective electrode with reported electrodes S. no.

Reference

Working concentration range (M)

Selectivity, log KPot

HPO4 2− ,B

Response time (s)

Life time (month)

NM NM 2 min 20

NM 1 40 days >1

(where B is SO4 2− ) 1 2 3 4

[7] [13] [27] Proposed electrode

5.0 × 10−6 to 1.0 × 10−3 1.0 × 10−6 to 1.0 × 10−2 1.0 × 10−6 to 3.9 × 10−3 1.77 × 10−5 to 1.0 × 10−1

−3.0 −1.8 NM −1.3

where aA is the activity of the primary ion and aB the activity of interfering ion and ZA and ZB are their respective charges. Further, if the charges are not the same or if the response to interfering ion is not Nernstian, then matched potential method (Eq. (3)) is to be used [28] Pot KA,B =

aA aB

(3)

Here aA is the increment in the activity of primary ion and aB the increment in activity of interfering ion which gives the same potential change as obtained by adding aA . Pot values for a range of common In the present studies, KA,B interferents were calculated using fixed interference method (FIM) (Eqs. (1) and (2)) as well as by matched potential method (MPM) (Eq. (3)). The resulting values are summarized in Table 3. The data given in Table 3 indicate that electrode has good selectivity of HPO4 2− with respect to other anions except for ClO4 − , which registers slight interference. To know the extent to which this ion may be tolerated, mixed run studies were carried out (Fig. 3). It can be seen from Fig. 3 that ClO4 − at concentration ≤1.0 × 10−4 M do not cause any deviation in the original plot obtained in pure HPO4 2− solution which depicts that the electrode can tolerate ClO4 − at concentration ≤1.0 × 10−4 M over the entire working concentration range. When present at higher concentrations, the electrode can be used over reduced concentration ranges. Fig. 3 shows that in the presence of 1.0 × 10−3 and 1.0 × 10−2 M ClO4 − ,

the sensor assembly can be used to determine HPO4 2− in the reduced concentration ranges of 1.25 × 10−4 to 1.0 × 10−1 and 1.99 × 10−3 to 1.0 × 10−1 M, respectively. The order of selectivity differs significantly from the Hofmeister [16] series, e.g. sulfate and hydrogen carbonate are well discriminated, while ClO4 − and SCN− are less. We interprete this deviation from the order of lipophilicity with a combination of macrocyclic effect, size exclusion, hydrogen bonding and charge density, e.g. among the tetrahedral SO4 2− and HPO4 2− the latter fits the ligand H-bond donor/acceptor groups ( NH2 and >CO) better. Previously, [29] a macrocyclic ditopic ligand showed an opposite order in complex stability. Compared with substituted aza-crown ether, [30] the selectivity over sulfate and nitrate is better. On the other hand, the weaker hydration of SCN− and ClO4 − may contribute to the lower selectivity. Further, certain cations such as potassium and ammonium can enter into the deeper cavity of the calix[6]arene. In the ligand design, we tried to prevent it by avoiding cation-exchange groups. However, there may still a possibility of co-binding of potassium and phosphate, due to different coordination sites. Such co-binding phenomena were previously observed by the authors for complexed cation–cation combinations. In the current membrane system, it could lead to improved sensitivity towards phosphate, due to ligand molecular preorganization. 4. Analytical application The proposed electrode has been successfully used for onsite determination of phosphate in the agricultural fields. The water samples were collected from three different parts of a local agricultural field and were maintained at pH 7.0 using 0.1 M Bis–Tris–H2 SO4 buffer solution. These were then analyzed potentiometrically using the proposed electrode. The phosphate ion concentration was also determined by molybdenum blue method [26] (colorimetry) for the sake of comparison. The results (Table 4) showed that the proposed sensor could be used for the determination of phosphate in real samples.

5. Conclusions Fig. 3. Variation of cell potential with varying concentrations of HPO4 2− at different concentration levels of ClO4 − for sensor number 1.

The membrane sensor with 5,11,17,23,29,35-hexa-tertbutyl-37,38,39,40,41,42-hexakis(carbamoylmethoxy)calix

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[6]arene (I) as electroactive material in a PVC matrix in the ratio 5.7:150 (I:PVC) is suitable for HPO4 2− estimation. It gives a near Nernstian slope (33 ± 1.0 mV/decade of activity) in the concentration range of 1.77 × 10−5 to 1.0 × 10−1 M of HPO4 2− . The lifetime of the electrode is about 1 month in aqueous as well as in non-aqueous medium. The response characteristics of the proposed sensor are the best amongst other sensors reported so far (Table 5). We are currently testing the tert-octyl analogue for prolonged membrane stability.

Acknowledgements The authors are highly thankful to the Department of Science and Technology (DST), Government of India and DAAD, Germany for providing financial help.

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