Ion-Selective Electrodes Based on a Pyridyl-Containing

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Ion-Selective Electrodes Based on a Pyridyl-Containing Triazolophane: Altering Halide Selectivity by Combining Dipole-Promoted Cooperativity with Hydrogen Bonding Elsayed M. Zahran,† Yuran Hua,‡ Semin Lee,‡ Amar H. Flood,‡ and Leonidas G. Bachas*,†,§ †

Department of Chemistry, University of Kentucky, Lexington, Kentucky 40506, United States Department of Chemistry, University of Miami, Coral Gables, Florida 33124, United States ‡ Department of Chemistry, Indiana University, 800 East Kirkwood Avenue, Bloomington, Indiana 47405, United States §

ABSTRACT: Triazolophanes are ionophores, with preorganized cyclic cavities that have tunable selectivities for halides. The interaction with halides is based on hydrogen bonding between the eight CH hydrogen atoms of the cavity and the halide anion. The rigidity of the cavity in tetraphenylene triazolophane along with the hydrogen bonding favors planar 1:1 complexation of “snugly” encapsulated chloride and bromide. Manipulating the triazolophane’s structure by introducing two pyridyl moieties into the cavity alters the receptor’s binding mode. This change adds a dipole-promoted driving force that combines with hydrogen bonding to favor the formation of 2:1 sandwich complexes around halides. The potentiometric response of electrodes based on this new ionophore was evaluated for optimal halide selectivity. The new triazolophane-based electrode showed an anti-Hofmeister selectivity toward iodide with a submicromolar detection limit. The stoichiometry of complexation and the stability constants with different halides were evaluated using a segmented sandwich membranes method. The pyridyl-triazolophane demonstrated a response consistent with a 2:1 sandwich-type complex with iodide, in polyvinyl chloride (PVC) membranes.

he field of anion sensing and recognition has experienced an exponential growth over the last two decades.1
6 This expansion is partially driven by the vital role that anions play in industry, the environment, and biology. For example, chloride, the most abundant anion in the body, plays a critical role in maintaining a cellular osmotic balance, which regulates normal blood pressure, blood volume, and the pH of body fluids.7,8 Although an increasing number of anion-selective ionophores have been developed in recent years,9
12 designing anion-selective ionophores is an ongoing challenge in comparison with the well-established cation-selective ionophores. This situation arises because most of the anions are larger in size with higher degrees of geometrical diversity than cations. Consequently, large macromolecules with high degrees of geometrical rigidity and strong interactions with the targeted anion should be considered in designing anion-selective ionophores. Success with this strategy has increased the number and diversity of receptors examined in anion supramolecular chemistry. Various types of interactions have been utilized in designing anion-selective ionophores including coordination to a metal center, ion-pairing, electrostatics, and hydrogen bonding.3,13
16 The latter is widely used with anions that possess relatively high charge densities, such as halides and hydrogen sulfite, which can form hydrogen bonds to the NH and OH donors in amide, urea, guandinium, and thiourea functionalities.17
22 Although CH H-bond donors were recognized at the same time as NH H-bond donors,23 far fewer anion-selective ionophores have been based on pure CH hydrogen bonding.11,15,24,25 In an early example,26 the highly withdrawing CF2 group was used to increase the acidity of the methylene CH

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r 2011 American Chemical Society

hydrogen leading to the serendipitous discovery of a hydrogen bonding based fluoride macrocyclic receptor. This macrocycle was used successfully to prepare a fluoride selective electrode.25,26 Very recently, computational studies have shown that the aryl CH 3 3 3 anion hydrogen bond strength is about half of that of the NH 3 3 3 anion bond.27,28 These studies have drawn attention to the possibility of preparing receptors that stabilize anions based on pure CH hydrogen bonding. A number of CH 3 3 3 anion binding receptors with cyclic,10,11,15,29,30 acyclic,15,31,32 or foldamer11,33 and switchable34,35 architectures have been developed recently. Triazolophanes are a new class of macrocycles that are prepared using click chemistry. They consist of four 1,2,3-triazole units and four phenylene units arranged alternately in a planar cyclic structure in order to provide, respectively, four strongly and four weakly polarized CH donors in the internal cavity.10,11,36 The preorganization of the triazolophane’s central cavity and the structure’s rigid planarity favors 1:1 binding with encapsulated spherical anions, such as those halides that have the best size match with the cavity. Such a triazolophane (I, Figure 1) has been used as a halide-selective ionophore in polyvinyl chloride (PVC) membrane ion-selective electrodes.24 The triazolophane-based electrode demonstrated enhanced selectivity toward chloride with near-Nernstian response. By manipulating the membrane composition, anti-Hofmeister selectivity to bromide with submicromolar detection limit was achieved Received: January 7, 2011 Accepted: February 26, 2011 Published: April 01, 2011 3455

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Analytical Chemistry

’ EXPERIMENTAL SECTION Reagents. The pyridyl-triazolophane II was prepared according to previously reported procedures.37 The plasticizer 2-nitrophenyl octyl ether (NPOE) along with poly(vinyl chloride) were obtained in Selectophore grade from Fluka (Ronkonkoma, NY). Tridodecylmethylammonium chloride (TDMACl), sodium nitrate, sodium salicylate, sodium bromide, sodium perchlorate, and tetrahydrofuran (THF) were purchased from Sigma (St. Louis, MO). The buffers, N-(2-hydroxyethyl)piperazine-N-(2-ethanesulfonic acid) (HEPES) and 2-(N-morpholino)ethanesulfonic acid (MES)

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for the same electrode. On account of the performance of the triazolophane-based electrode, it was hypothesized that manipulation of the triazolophane’s central cavity by different functionalities could alter the selectivity of the triazolophane to halides. To investigate the possibility of cooperative binding of the triazolophane around larger anions, a new pyridyl-triazolophane (II, Figure 1) was prepared with the phenylenes on the east and west side of the structure replaced by pyridyls.37 The negative electrostatic potential of the pyridyls inside the triazolophane cavity along with the lone-pair repulsion destabilized the 1:1 complex with halides in favor of the 2:1 complex. UV and NMR binding studies showed that the new pyridyl-triazolophane forms a sandwich-type complex with iodide with high association constant (βIL2 = 8  1010 M
2, dichloromethane). It was also shown that the sandwich form of the pyridyl-triazolophane exhibits high selectivity for the larger iodide over the chloride and bromide, which better fit inside the cavity in a 1:1 stoichiometry. This selectivity makes the pyridyl-triazolophane a potential iodide-selective ionophore. Herein, we report pyridyltriazolophane II as a new ionophore for halide-selective electrodes. An optimized membrane for iodide selectivity was obtained. To aid in correlating this selectivity to the formation of sandwich, the complexation stoichiometry with various halides was characterized using a segmented sandwich membranes method.

were also obtained from Sigma (St. Louis, MO). Sodium chloride, potassium chloride, and sodium bicarbonate were obtained from EMD Chemicals (Gibbstown, NJ). Sodium iodide and sodium sulfate were purchased from Mallinckrodt Baker (Phillipsburg, NJ). Deionized water of 18 MΩ (Milli-Q water purification system; Millipore, Bedford, MA) was used throughout the experiments. Membranes and Electrode Preparation. The PVC-based ion-selective electrodes were prepared as described previously.24 The membrane cocktails were prepared by dissolving the specific amounts of the pyridyl-triazolophane, TDMACl, and NPOE in 1 mL THF. The different compositions of the membranes used in this study are shown in Table 1. The solution was sonicated for 30 min to ensure complete dissolution of all components, left at rest for 2 h to release any air bubbles, and poured into a 22-mm-i.d. glass ring located on a glass slide. The solvent was allowed to evaporate at room temperature overnight leaving the PVC membrane with a thickness of about 150
200 μm. To prepare the electrodes used for studying the performance of the pyridyl-triazolophane-based electrode, small disks of each membrane were punched using a cork borer and mounted onto Philips IS-561 (Gl€asblaserei M€oller, Zurich, Switzerland) electrode bodies. A solution of 1  10
3 M NaCl was used as the internal filling solution. To prepare the sandwich membranes, a set of three disks from membranes with different concentrations of the ionophore or from membranes with the same composition but no ionophore were conditioned separately for two days in 1  10
2 M solution of a specific halide. The sandwich membrane was then assembled by first blotting two individual disks (one containing a specific concentration of the pyridyl-triazolophane and the other having the same composition but no ionophore) dry with filter paper. The two disks were then fused together by pressing them between two metal pistons, and the sandwich membrane was mounted immediately in the electrode body. The time span between fusing the membranes and recording the potential was less than 3 min. A single Philips IS-561 electrode body was used throughout the sandwich membrane experiment to avoid any potential difference attributed to installation. A 1  10
2 M halide solution was used both as the internal filling solution and the sample solution; however, in the case of iodide or bromide, instead of using the internal electrode of the Philips body, a small salt bridge was used to connect an external Ag/AgCl electrode to the internal solution. This was necessary to avoid precipitation of AgBr or AgI on the reference electrode. Potentiometric Measurements. All potentiometric measurements were performed at 23 °C, controlled by a Fisher Isotemp Circulator bath (model 9500), using the following cell assembly: Ag/ AgCl|3.0 M KCl 1.0 M CH3COOLi sample solution|ISE membrane|1  10
3 M NaCl|Ag/AgCl. The change in the electrode potential was measured using an 8-channel SC-2345 interface (SCCLP01, National Instruments, Austin, TX). A PCI-6036E multifunction I/O data acquisition device was used to connect the interface to a computer running Lab View 7.0 (National Instruments) software with a custom-designed program for data acquisition and analysis. The potential of the sandwich membranes were recorded as average readings over a period of 10 min after placing the sandwich membrane between two identical 1  10
2 M halide solutions. Each data point was obtained as the average of the potential of three sandwich membrane disks prepared from the same parent membrane. The pH studies and buffer preparations were performed with the aid of a Fisher Scientific Accumet 915 pH/Ion meter, which was equipped with an Orion pH glass electrode (Beverly, MA). The performance of the pyridyl-triazolophane-based membrane electrode at different pH values (2
12) was determined by adding small aliquots of )

Figure 1. Structures of (I) triazolophane and (II) pyridyl-triazolophane.

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Analytical Chemistry

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Table 1. Membrane Composition and Response Characteristics of Different Triazolophane-Based Electrodes TDMACl membrane

pyridyl-triazolophane wt %

mol %a

wt %

NPOE wt %

PVC wt %

slope mV/decade

detection limit, mol/L

A

2.0

60

0.8

64.7

32.5


58.1

2.5  10
6

B

3.0

20

0.5

64.0

32.0


62.0

2.5  10
7

C

3.0

10

0.2

64.5

32.3


51.6

3.0  10
6

D

2.0

65.3

32.7

þ21.1

2.0

65.3

32.7


72.2

E b

a

1

0.3

0.02

66.3

33.2

2

0.9

0.2

65.6

33.3

3 4

2.0 2.8

0.2 0.2

65.2 64.6

32.6 32.4

5

5.4

0.2

62.9

31.5

6

9.0

0.2

60.5

30.3

6.3  10
6

Mole ratio relative to the ionophore. b Membranes with Arabic numbers were devoted to the segmented sandwich membranes study.

Table 2. Selectivity Coefficients, log KI
pot ,J , of Pyridyl-Triazolophane-Based Electrodes A, B, and C Doped with Various Mole Ratios of TDMACl (Relative to the Ionophore) in Comparison with the Anion-Exchanger-Based Membrane, Electrode D (2 wt % TDMACl), and That of Triazolophane I Electrode electrode anion

A

B

C

TDMACl

I Br


0.00
2.84

0.00
2.38

0.00
1.30

0.00
1.34

0.00 0.54

Cl



3.66


5.21


2.24


2.69


1.86

ClO4



0.60


1.39

0.69

2.32

1.86

SCN


0.24


1.46

0.66

0.82

1.00

Sal



0.39


1.52


0.04

0.79

0.18

NO3



1.56


2.02


0.12


0.65


1.09

HCO3



4.64


5.39


3.95


3.87


2.66

SO42



5.23


4.90


4.76


3.20


3.83




triazolophane I

sodium hydroxide or sulfuric acid to solutions containing 0, 1  10
4, or 1  10
2 M iodide. The effect of different buffers was evaluated by monitoring the change in the electrode’s potential with incremental additions of the buffer solution to deionized water. The selectivity coefficients were determined by the separate solutions method.

’ RESULTS AND DISCUSSION Pyridyl-triazolophane II is a new halide receptor with binding properties that depend upon the combination of hydrogen bonding and dipole-promoted cooperativity. The pyridyl-triazolophane is analogous to the parent triazolophane I in which the phenylenes at the east and west sides of the structure are replaced with pyridyl groups. The negative electrostatic potential induced by the pyridyl groups along with the lone-pair repulsion changes the triazolophane to favor dimerization resulting from π
π stacking. The cooperation of these effects with the size mismatch of the guest halide destabilizes the 1:1 complex in favor of the 2:1 sandwich architecture. This type of complexation is commonly observed in cation-selective ionophores such as crown ethers.38 We hypothesized that pyridyl-triazolophane II could be used as a

Figure 2. Iodide potentiometric response of pyridyl-triazolophanebased electrodes containing 2 wt % ionophore and different mole ratios of TDMACl (relative to the ionophore): (0) 0, (9) 10, (2) 20, and (Δ) 60 mol %.

neutral carrier-type ionophore with selectivity toward halides in ion-selective electrodes. Membranes with different compositions of the pyridyl-triazolophane II and the lipophilic salt, TDMACl (in mole percent relative to ionophore) were prepared (Table 1), and their potentiometric responses toward different anions were evaluated for optimal performance. The composition of the membranes was guided by the lessons acquired from the tetraphenylene-triazolophane I based electrode.24 Thus, the high polarity plasticizer NPOE was chosen for all membranes to enhance permselectivity toward anions. However, it is also well-established that the addition of lipophilic positive sites to neutral carrier based membranes improves the Nernstian response and enhances the permselectivity toward anions.1,39 Electrodes based on membranes containing 2 wt % of II and no lipophilic positive sites (electrode D) showed a weak cationic response (Figure 2). This cationic response might be attributed to the residual negative sites in the PVC membrane.40,41 Incorporating 10 mol % TDMACl into the membrane was enough to alleviate the poor cationic response of the membranes. The electrodes demonstrated sub-Nernstian response to iodide of
51.0 mV/decade over a linear range of 1  10
5
1  10
1 M with a slight interference from highly lipophilic anions such as SCN
, salicylate, and ClO4
. 3457

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Figure 3. Potentiometric response of pyridyl-triazolophane-based electrode containing 2 wt % ionophore and 20 mol % of TDMACl (relative to the ionophore) toward different anions: (() chloride, (9) bromide, (2) iodide, (0) thiocyanate, (O) nitrate, (b) salicylate, () bicarbonate, (Δ) perchlorate, and ()) sulfate.

Figure 4. Effect of pH on potentiometric response of pyridyl-triazolophane-based electrode (2 wt % ionophore and 20 mol % TDMACl in NPOE) in the presence of different iodide concentrations: (b) iodide free, (2) 0.1 mM iodide, and (() 10 mM iodide. The pH was adjusted with sodium hydroxide and sulfuric acid.

Increasing the TDMACl concentration to 20 mol % (electrode B Figure 2) showed the optimal response toward iodide of
62.0 mV/ decade over a linear range of 8  10
7
1  10
1 M with a detection limit of 2.5  10
7 M. Further increase of the TDMACl content to 60 mol % resulted in a Nernstian response of
58.0 mV/ decade toward iodide with elevation of the detection limit by one concentration decade and a slight interference from SCN
. This elevation of the detection limit is attributed to the increase of the ion flux across the membrane with increasing ion-exchanger concentration.42,43 Electrode B, which showed a Nernstian response to iodide of
62.0 mV/decade, was considered for further studies (Figure 3). To select an appropriate buffer for evaluating the electrode, the effect of pH (2
12) on the pyridyl-triazolophane-based electrode was studied by recording the change in the electrode response (Figure 4) as a result of adding very small aliquots of 0.1 M NaOH or H2SO4 to deionized water, 0.1 mM, and 10 mM KI solutions. In iodide-free solution, the pyridyl-triazolophanebased electrode showed a response that was independent of pH in the range of 4
8. The change in the electrode potential at higher pH might be attributed to weak hydroxide interference. This range was extended when the concentration of iodide was

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Figure 5. Potentiometric selectivity coefficients of pyridyl-triazolophane-based electrodes doped with 20 mol % of TDMACl (relative to the ionophore) toward iodide over different anions.

increased to 0.01 M. This indicates the pyridyl-triazolophane is stable over a wide range of pH values, and there is no protonation of the ionophore taking place. The response of electrode B was studied in different buffers, HEPES, pH 7.4, acetate, pH 4.4, Tris, pH 7.4, and MES, pH 5.6. The pyridyl-triazolophane-based electrode showed optimal performance in 0.01 M HEPES, pH 7.4, with a Nernstian slope toward iodide of
62.0 mV/decade over a wider linear range of 8.0  10
7
1.0  10
1 M with a detection limit of 2.5  10
7 M. The 0.01 M HEPES, pH 7.4, buffer was used for all further studies unless otherwise stated. The selectivity pattern of electrodes based on II was studied by the separate solutions method.44 It was found that all electrodes based on triazolophane II with 10
60 mol % of TDMACl exhibit selectivity toward iodide over chloride and bromide. Specifically, the electrode based on membranes containing 3 wt % of II and 20 mol % of TDMACl showed anti-Hofmeister selectivity toward iodide with log KI
,ClO4
pot =
1.4 and log KI
,SCN
pot =
1.5 (Table 2). It is also worth noting that electrode B showed high selectivity toward iodide over chloride and bromide (Figure 5) with log KI
,Cl
pot of
5.2 and log KI
,Br
pot of
2.4. This selectivity pattern is different from that demonstrated previously for electrodes based on I.24 This change in the selectivity of the pyridyl-triazolophane can be explained based on the electronic effect of the pyridyl groups in the triazolophane macrocyclic cavity. The electronic repulsion of the pyridyl’s lone-pair to the encapsulated halide destabilizes the 1:1 complex in favor of the 2:1 sandwich-type complex. The sandwich is also enhanced by the size mismatch of the large iodide with the cavity of a single triazolophane and the increased ionophore concentration in the membrane. On the other hand, triazolophane I with an electronically attractive cavity was more selective toward anions that fit best within the macrocyclic cavity. Thus, we hypothesized that the 2:1 sandwich-type complexation is the dominating interaction of the pyridyl-triazolophane with halides in the PVC membrane. This hypothesis was further investigated by determining the stoichiometry of the pyridyl-triazolophane with halides and their stability constants within the PVC membranes. Few methods have been reported for the determination of the ion
ionophore stability constant in PVC membranes. These methods include using an additional reference ionophore such as an Hþ ionophore or chromoionophore, voltammetric measurements at the PVC membrane
sample interface, and segmented sandwich membranes.45
49 The latter method has been used successfully for the determination of both the stability constant and stoichiometry of complexation of the ion
ionophore in PVC 3458

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Figure 6. Potentiometric response of sandwich membranes containing different concentrations of pyridyl-triazolophane toward (b) chloride, (2) bromide, and (() iodide. The linear portion of the plots corresponding to 2:1 complexation is shown with a solid line. Triazolophane concentration is in units of moles per kilogram.

membranes.50
52 In this method, a sandwich membrane composed of two segments, one containing a specific concentration of the ionophore with ionic sites while the other having the same composition but no ionophore, is placed between two electrolyte solutions of identical composition. The measured potential is a function of the relative activities of the ion in both sides of the sandwich membrane. Mi and Bakker50 discussed the detailed theory of the sandwich membrane phase boundary potential, where the stability constant, βILn (I = ion; L = ionophore; n = stoichiometry), can be calculated based on the membrane composition and the measured membrane potential according to the following equation: !
n   LT
nRTþ EM zI F exp ð1Þ βILn ¼ RT zI where LT is the total concentration of ionophore in the membrane segment, Rþ T is the concentration of lipophilic ionic site additives, n is the ion
ionophore complex stoichiometry, and R, T, and F are the gas constant, absolute temperature, and Faraday constant. This equation can be rearranged to52 EM ¼ n

RT RT ln CL ex þ n ln βILn F F

ð2Þ

ex þ where Cex L is the free ionophore concentration (CL = LT
nRT ). According to eq 2, the stoichiometry of the ionophore
ion interaction can be estimated from the slope of the linear plot of the logarithm of ionophore concentration vs the membrane potential. Therefore, a series of membranes with different concentrations of the pyridyl-triazolophane were prepared (membranes with Arabic numbers 1
6, Table 1) to investigate the stoichiometry of the complex between triazolophane II and iodide, bromide, and chloride by the segmented membranes sandwich method. Sandwich membranes were prepared as described in the Experimental Section. The average potential of three freshly prepared sandwich membranes was plotted against the logarithm of the pyridyl-triazolophane concentration in the membrane for iodide, bromide, and chloride (Figure 6). The concentration of TDMACl in all membranes shown in Figure 6 was 3 mmol/kg, whereas the ionophore concentration

range used was 22
100 mmol/kg. Because LT . RT, Cex L ≈ LT at the chosen concentrations irrespective of whether n has a value of 1 or 2. It is evident from Figure 6 that the potentiometric response of the pyridyl-triazolophane-based sandwich membranes toward iodide, bromide, and chloride varies linearly with the concentration of the pyridyl-triazolophane in the membrane in the range of 22
100 mmol/kg, which indicates that these halides are being complexed by the ionophore. The slopes of the linear best fit lines were calculated to be
118,
127, and
107 mV for iodide, bromide, and chloride, respectively. On the account of eq 2, these values can be interpreted as a 2:1 sandwich type complex is the prevailing interaction of pyridyl-triazolophane with these halides in the membrane. This is consistent with the prior finding that the pyridyl-triazolophane can dimerize by π
π stacking at high concentrations and in the solid state.37 Thus, although the preorganized triazolophane cavity is more compatible with the size of chloride and bromide, it is still possible to form a sandwich-type complex with these anions at high concentrations of ionophore in the membrane. Membrane 4 was used to calculate the stability constant of pyridyl-triazolophane toward I
, Br
, and Cl
based on eq 1. To circumvent the effect of the ion-exchanger, the membrane potential EM was calculated by subtracting the potential of the membrane with no ionophore from the sandwich membrane potential. The stability constants (n = 2), log βIL2, were found to be 10.4 ( 0.3, 6.7 ( 0.4, and 2.3 ( 0.4 for I
, Br
, and Cl
, respectively. The higher stability constant, β, of the pyridyl-triazolophane-iodide complex in comparison to those with chloride and bromide could be explained by the size-mismatch between the cavity and the too large iodide that favors a 2:1 complexation. This explains the high selectivity of the electrodes toward iodide over chloride and bromide. The trend in the stability constants of pyridyl-triazolophane complexes with halides is consistent with the experimentally determined halide selectivity order of the ion-selective electrodes. The difference between the log βIL2 values for iodide and chloride is about 8, whereas log KI
pot =
5.2. This ,J difference could be explained based on the coextraction and ion-exchange processes (due to the presence of small amounts of ion-exchangers within the membrane) that generate ion flux across the membrane.44 For membranes that are based on ionophores that bind strongly the primary ion, this ion flux generates a bias between the measured and theoretical selectivity of the electrode.53
55 The stability of the pyridyl-triazolophane-based electrode was investigated by monitoring the slope and baseline potential of electrode B over a period of four months. The pyridyl-triazolophane-based electrode exhibited minimal change in the slope of (0.4 mV/decade over the first month and dropped by 2 mV/ decade over 4 months. The baseline potential of the electrode changed by