Analyst, February 1996, Vol. 121 (127-131) 127

Caesium Ion-selective Electrodes Based on Crowned Benzoquinones*

Michael G. Fallon, David Mulcahy, William S. Murphy and Jeremy D. Glennont Department of Chemistry, University College Cork, Ireland

Three new derivatives of benzo-15-crown-5 were examined as potential caesium ion-selective ionophores in PVC membrane electrodes. The ionophores were 2,3-benzoquino[ 15lcrown-5, 2-bromo-1,4-dihydroxybenzo[15]crown-5 and 5-bromo- 2,3-benzoquino[ 151crown-5. PVC membranes plasticized with 2-nitrophenyl octyl ether, and incorporating the secondary ion exchanger potassium tetrakis(4-chloropheny1)borate were used throughout this study. Interferent studies for each electrode were carried out to assess electrode selectivity in the presence of ammonium ion, alkali and alkaline earth ions and selected transition metals. A graphical method of representing selectivity coefficients is provided together with the separate solution technique for selectivity determination. Dynamic response, precision, lifetime and temperature studies was also reported. The benzoquinone crown ethers proved to be good caesium ionophores, displaying near-Nernstian responses to this ion, in the range 10-1-10-4.5 mol 1-1. The electrode incorporating the 2,3-benzoquino[ 15]crown-5 ionophore had the highest selectivity for caesium ions over alkali and alkaline earths studied. Keywords: 2,3-Benzoquino[l5]crown-5; 2 -bromo-l,4-dihydroxybenzo[l5]crown-5; 5- bromo-2,3-benzoquino[l5]crown-5; potentiometry ; caesium ion-selective electrode

Introduction Crown ethers and related macrocyclic compounds have attracted great attention as artificial ionophores.' Their struc- tural properties have also led them to be successfully applied in areas as diverse as ion chromatography,* near-IR redox-active fluoroionophores3 and as supported liquid membrane (SLM)4 modifiers. A number of model systems have been developed as enzyme mimic^,^ in which several functional groups, appro- priately cited, cooperate in their specific reactions. The effect of structural modification to model crown compounds has been extensively investigated, particularly in the areas of optimizing solvent extraction efficiencies6 and electrode ionophore selectivities toward target ana ly t e~ .~

Crown ethers have been used for some time as neutral ionophores in Cs+ ion-selective electrodes (ISEs). Those based on 15-crown-5 functionality have produced excellent Nernstian response slopes, limits of detection (LODs), and short response times of less than 1 min. However, their selectivities for Rb+, K+, and Na+ were quite low.8 The bis(benzo-18-crown-6) derivatives seem to produce the best Cs+ electrodes,g since the cis-form is capable of forming sandwich-type Cs+ complexes. The electrode developed by Attiyat et al. 10 which incorporates

* Presented at The SAC '95 Meeting, Hull, UK, July 11-15, 1995. + To whom correspondence should be addressed.

a TMC-crown formazane produced (16,17-dihydro-5H, 15H- dibenzo[b,i] [ 1,11,4,5,7,8]dioxatetra-azacylotetradecin-7-car- bonitrile) a five- and twenty-fold selectivity over Rb+ and K+, respectively. More recently a dibenzo-24-crown-8 has been proposed as a promising Cs+ PVC-based ISE.11 The plasticizing agent was dibutyl phthalate (DBP) as opposed to the more frequently employed 2-nitrophenyl octyl ether (2-NPOE). Moreover, the influence of membrane components such as secondary ion exchangers,12 plasticizers l 3 and polymer base l4

on electrode performance has been extensively examined in the literature.

Crowned natural products15 are of interest in biological and pharmacological studies. Coenzyme Q (also called ubiquinone- 10, n = 10, Fig. 1) is an important redox carrier in the mitochrondrial respiratory chain. Crown ether derivatives of ubiquinone-0, n = 0 can be synthesized, by replacement of the two methoxy groups by oligo-ethylene glycol bridges.167'7 Such crowned benzoquinones are capable of multisite interactions via ion-binding and charge-transfer complexation, in addition to redox activity.

Cyclic voltammetry studies of 2,3-benzoquino[ 15lcrown-5 interaction with alkali metals in N,"-dimethylformamide (DMF) has been reported.18 Anodic shifts of the Q/semi-Q peak (El) for K+ and Na+ were approximately +lo0 mV, while the shift for Li+ was +60 mV. Corresponding shifts for 1,4-benzo- quinone resulted in E l potential shifts of +5, +20 and +40 mV, respectively.19~20 Therefore, cation-dependent E l potential shifts observed for it are consistent with complexation of the metal cations to the crown ether moiety, which in turn makes the quinone more labile towards reduction to the corresponding semi-quinone. It was also shown that ion-binding by the crown group will tend to increase the electron affinity of the quinone moiety and conversely, charge transfer complex formation (i.e., partial reduction of quinone) will tend to enhance the ion- binding ability of the crown group in a co-operative manner reminiscent of the positive effectors of allosteric enzymes.18 Macrocycles, such as those capable of such coorperative interactions, could lead to the development of novel potentio- metric sensors for ion and organic analysis. In this work, novel crowned ubiquinone derivatives are examined as ionophores in caesium-selective PVC membrane electrodes.

Experimental Materials The ionophores synthesized and employed in this work are shown in Fig. 2. The membrane components, PVC (ISE grade),

0 /I = 6-10 Chemical structure of ubiquinone. Fig. 1

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128 Analyst, February 1996, Vol. 121

potassium tetrakis(4-chloropheny1)borate (KTpClPB), 2-NPOE and analytical-reagent grade tetrahydrofuran (THF) were ob- tained from Fluka AG (9470 Buchs, Switzerland). The nitrate salts of silver(I), cobalt(r~), nickel(rr), copper(II), lead(II), cadmium(II), mercury(I1) were of analytical-reagent grade as were the chloride salts of the alkali and alkaline earth metals. All solutions and standards were made up in doubly distilled water.

Synthesis of Benzo-15-crown5 Derivatives The crown-ether I was prepared according to slight modifica- tion of the procedures of Merz and Rauschel17 and Hayakawa et al.18,21 mp 122-124 "C (from propan-2-01) (literature value, 121-123 "C). Treatment of the quinone (I) with hydrobromic acid (47% aq, 1 .5 equiv.) gave the 2-bromo-l,4-dihydroxy- benzo[lS]crown-5 (11) as a white solid in 80% yield mp 123-1 25 "C (from hexane). This was readily oxidized to 5-bromo-2,3-benzoquino[ 151crown-5 (111) by stirring for 5 min with a 20% dispersion of potassium dichromate on silica with methylene chloride as solvent.22 The crowned bromobenzo- quinone was obtained as a red oil in 77% yield.

For ionophore 11, found: C, 44.49; H, 5.16; Br, 21.22. C14H19Br07 requires: C, 44.33; H, 5.01; Br, 21.1 1; vmaX (KBr)/ cm-1 3385.8, 2865.5, 1485.4, 1447.8; 6H (270 MHz; CDC13) 3.71-3.76 (8H, crown H), 3.88-3.97 (4H, M, crown H), 4.254.30 (4H, M, crown H), 5.87 (2H, br s, D20 exchangeable 2 X ArOH), 6.87 (IH, S, ArH); bC (67.5 MHz; CDC13) 70.27

71.04 (CHz), 73.08 (CHz), 73.37 (CHT), 103.29 (C-Br), 113.32 (CHz), 70.42 (CHZ), 70.51 (CHz), 70.60 (CH2), 70.82 (CH2),

(C-H), 139.40 (C-0), 139.86 (C-0), 140.42 (C-0), 143.17 (C-0).

For ionphore 111, found: M+, 376.0079. C14H1779Br07 requires M, 376.0157; m/z 379, 378, 377, 376, 246, 231, 218, 216, 192, 165, 134, 82; Y,,, (film)/cm-' 2868.5, 1660.5, 1585.5, 1451.5; aH (270 MHz CDC13) 3.62-3.72 (8H, M, crown H), 3.80-3.84 (4H, M, crown H), 4.46-4.49 (2H, M, crown H), 4.54-4.58 (2H, M, crown H), 7.08 (IH, S, quinone-H); 6c (67.5 MHz; CDC13) 70.20 (CH2), 70.31 (CHZ), 70.60 (CHZ), 70.66 (CHZ), 70.71 (CH2), 70.91 (CHZ), 73.17 (CH;?), 73.30 (CHZ),

B P O ?

I

134.98 (=CBr), 135.58 (HC=C), 144.16 (=C-0), 145.15 (0-C=), 176.62 (C--O), 181.59 (C=O).

Electrode Preparation The membrane components outlined in Table 1 were mixed and dissolved in THF overnight. The resulting homogeneous syrup was poured into a 25 mm diameter ground glass casting ring,23 and the solvent was allowed to evaporate off at room temperature, over a period of 48 h. An approximately 0.5 mm thick semi-transparent flexible membrane was obtained from which a working membranes of 7 mm diameter were cut using a cork borer. These disks were then pasted, using THF, to an interchangeable PVC tip which was clipped on to the end of the electrode body. This was in turn connected to a silver wire (1.63 mm diameter, Merck, Poole, Dorset, UK) contact which was previously chloridized by immersion in 40% m/m sodium hypochlorite for 30 min. Each electrode was stored in 0.1 moll-' CsCl solution when not in use.

Measurement of Electrode Potentials All measurements were carried out in a thermostated poten- tiometric cell. The potential readings were measured using a Metrohm (Herisau, Switzerland) 654 millivolt/pH-meter rela- tive to a Metrohm 6.0702.100 SCE reference electrode. The electrochemical systems for the study were as follows:

Ag/AgCl I 10-1 moll-' CsCl I PVC membrane 1 sample I KCl,,. I Hg2C12/Hg for alkali and alkaline earth metals, and Ag/AgCl I 10-1 mol 1-I CsCl I PVC membrane I sample I salt bridge I 1 mol 1-1 KN03 I KCl,,,, I Hg2Clflg for heavy metals. The salt bridge was prepared by dissolving 10 g of KN03 and 1.5 g of agar in 100 ml of doubly distilled water by heating. This was poured into a glass salt bridge to which two rubber tubes were attached, which on retraction allowed surface renewal on a daily basis. A dynamic method was used to construct calibration graphs of each analyte by performing regular injections at 1 min intervals. Absolute as opposed to relative potentials were used to construct each calibration graph. Dynamic responses were measured by injecting 90.9 pl of 1 moll-' CsCl into 10 ml of 10-3 moll-' CsCl thus producing a ten-fold increase in concentration while monitoring the potential change at a chart speed of 12 cm min-I. This plot was used to calculate the slope for a particular electrode. The LOD was taken at the point of intersection of the extrapolated linear segments of the caesium calibration curve. Repeatability was estimated by immersing the electrode alternatively into 1 0-2 and 10-3 mol 1-1 CsCl solutions at 25 "C. Potential readings were noted after 2 min, and the relative standard deviation (s,) was calculated. Selectivity coefficients were determined using the separate solution method24 at a concentration of lo-' moll-1 unless otherwise stated. The calibration plots were used in order to determine the change of KpotCsm as a function of concentration using the rearranged form of the Nicolsky equati0n:~5

I L 0 - J OH

I1

Table 1 Membrane components used in ISE membrane construction

111

Fig. 2 Ionophores employed in this work: I, 2,3-benzoquino[ 151crown-5; 11, 2-bromo- 1 ,bdihydroxy[ 151crown-5; and 111, 5-bromo-2,3-benzo- quino[ 151crown-5.

Component Amount (% m/m) Masslg Ionophore 0.66 0.0034 KTpClPB 0.17 0.0009 2-NPOE 65.84 0.3357 PVC 33.33 0. I700 THF - 3 ml

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Analyst, February 1996, Vol. 121 129

r 1

2.303 RT

zcs+ F where S =

Electrode lifetimes were examined by monitoring the slope of Cs+ calibrations periodically. The temperature dependence of each electrode was investigated by noting the potential change in solutions of lO-3,10-2, and 10-1 moll-' CsCl taken over a temperature range of 5-55 "C. These values were rearranged to construct isotherms for each electrode at selected temperatures of 25 and 45 "C. The intercept of these isotherms represented the isopotential point of each respective electrode.

Discussion Three crown ether derivatives containing quinone or dihy- droxybenzo groups were incorporated into PVC membrane electrodes and were shown to give acceptable linear response for E versus log (Cs+ activity) (Fig. 3). Previous electrodes based on 15-crown-5-phosphotungstic acid precipitates give Nernstian responses also in a wide Cs+ activity range.' The alkali metal ion calibration plot of the electrode based on ionophore I, i.e., 2,3-benzoquino[ 15lcrown-5, is given in Fig. 4. The corresponding plots for this electrode for alkaline earths and selected transition metal ions indicate high Cs+ selectivity over divalent ions. There is however a slight curvature for Cs+ response at approximately - 1.5 of log concentration. All cations with the exception of Rb+ produced responses with slopes and LOD values worse than the Cs+ response. The cation responseorder for alkalimetalsisCs+ > Rb+ > K+ > NH4+ > H+ > Na+ > Li+. This series is similar to that for the 2-NPOE plasticized PVC electrodes incorporating the KTpClPB secon- dary ion exchanger, where selectivity is inversely proportional to the cation hydration enthalpy. The effect of functional group changes adjacent to the crown ether ring on the ionophores used in this work on cation response order would thus be important to study.

The cation response order for alkaline earth ions was Ba2+ > Ca2+ > Sr2+ > Be2+ > Mg2+. The response for Ba2+ was 15.5 mV decade-', whereas responses for other divalent cations were considerably less. The transition metal ion responses were very poor with the response order being Pb2+ > Ni2+ =: Co2+ > Cu2+ but the slope for Ag+ (45.1 mV decade-') approaches that for Cs+ (51.9 mV decade-'). The Ag+ calibration plot has a

higher LOD (10-3.7 mol 1-1) than the Cs+ calibration (Table 2).

The cation selectivity of a second electrode incorporating ionophore 11, i.e., 2-bromo-1,4-dihydroxybenzo[ 15lcrown-5, is significantly different. The cation response order is Cs+ > Rb+ = K+ > Na+ > H+ > N&+ > Li+. Of the diverse cations, Rb+ gave the best slope and LOD, closely followed by K+. Constant slope values (in the range 3.6-13.2 mV decade-') in the concentration range investigated were obtained for alkaline earth ions, with the response order Be2+ > Ba2+ > Ca2+ > Mg2+. Unlike the electrode based on I there was a linear response for Ca2+. Transition metal ions gave better responses for this electrode. A Nernstian response was obtained for Ag+ (56.4 mV decade-') in the concentration range 10-4.5-10-2 moll-1, followed by a sharp drop in potential at 10-1 moll-'. The order of responses for the other metals is Pb2+ > Cu2+ >

The cation response order of the third electrode incorporating ionophore 111, i.e., 5-bromo-2,3-benzoquino[ 151crown-5, for alkali metals is Cs+ 2 K+ > Rb+ > N&+ > H+ > Na+ > Li+. The corresponding series for alkaline earth metals is Ba2+ > Be2+ > Ca2+ = Mg2+. All the alkaline earth metal ions with the exception of Ba2+ gave sub-Nernstian linear responses across the concentration range investigated. Transition metals fol- lowed the series Pb2+ > Cu2+ > Co2+ =. Ni2+, similar to electrode 11. Both Co2+ and Ni2+ produced no response (zero slope) across the calibration concentration range.

Co2+ = Ni2+.

I-CCS +H +Li +Na -K -Rb -NH4I

43 5 4 3 2 1 0

Log [CI

Fig. 4 Calibration of the ionophore I electrode for alkali metal and ammonium ions.

I I I I I I I

-6 -5 -4 -3 -2 -1 0 Log [CS']

Fig. 3 ionpohores 1-111.

Plot of responses for caesium ion-selective electrodes incorporating

Table 2 Response slope, LOD, and s, (%) (of potential readings) properties of each ISE

s, (%) at 25 "C (n = 10)

Slopel LOD 10-2 10-3 Ionophore mV decade-' (-Log [Cs+]) moll-' mol I-' I 51.9 4.3 1.7 2.4 I1 48.6 4.4 11.4 4.7 I11 52.2 4.6 12.5 3.1

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130 Analyst, February 1996, Vol. 121

The slope of caesium response for each electrode is presented in Table 2 and the corresponding dynamic response trace is shown in Fig. 5. Each electrode exhibits a near-Nernstian response for caesium. The response time for electrodes 1-111 is less than 3 s. Electrode response stability is in the order I = I11 > 11, with the slope of electrode I1 gradually decreasing over time to produce a stable sub-Nernstian slope of approximately 26 mV decade-' after 30 d. The LOD and repeatability for each electrode is also presented in Table 2. The LOD of the electrodes increase in the order I = I1 > 111. The high s, value for electrode I1 and I11 at high concentration was due to a positive potential response drift throughout the experiment.

The selectivity coefficients (Log K P O t c s ~ ) determined by the separate solution method at 10-l moll-' are presented in Table 3 for a wide range of cations including alkali, alkaline earth and transition metal ions. From this tabulation, the major inter- ferents can be readly identified. For electrode I, Rb+, Ag+ if"d Hg2+ are the major interferents for the unsubstituted benzoquino crown ether. The incorporation of a bromo substituent at position 5 on the benzoquinone for electrode I11 increases the level of interference from Cu2+ significantly and Rb+ to a lesser extent. The Hg2+ and Ag+ interferences are lowered somewhat. Electrode I1 incorporating the bromo-substituted dihydroxy- benzo functional crown ether is significantly different from the

i f

Fig. 5 Dynamic responses of the three electrodes for a ten-fold increase in caesium concentration.

Table 3 Interferent selectivity values (logKPotCs~) using the separate solution method

Interferent I I1 I11 H' Li+ Na+ K+ Rb+ NH4+ Be2+ Mg2+ Ca2+ Sr2+ B a2+ Ag+ co2+ Ni2+ cu2+ Pb2+ Cd2+ Hg2+

-2.06 -3.00 -2.38 -0.99 -0.47 - 1.40 - 3.62 -4.03 -3.44 -3.10 -2.88

-2.59 -2.47 -2.42 -2.00 -2.1 1 +2.12

+0.94*

-0.45 -1.44 -0.65 +0.04 -0.10 -1.79 -1.73 -2.37 -2.21

-1.64 +1.13t -1.83 -2.11 -1.68 -0.75 -0.40

-

+2.93 * Determined at 10-2 mol 1-1.

-1.10 -2.27 - 1.94 -0.89 -0.39 -0.99 -3.17 -2.77 -2.70 -

-2.51 +0.47* -2.43 -2.56 -0.15 -3.05 -3.37 +1.83

other electrodes with increased responses to the interferent ions Na+, K+, Rb+, Ag+, Pb2+, Cd2+ and Hg2+. This is accompanied by an increase in the response to H+ ions and is attributed to the presence of phenolic groups capable of proton release and coordination to heavy metal ions.

Further insight into the level of interference by diverse ions can be obtained graphically from plots of selectivity coefficients as a function of interferent concentration. This plot for electrode I11 is illustrated in Fig. 6 for monovalent interferents. Interferents that give calibration plots of similar slopes and LOD inflection points as the primary analyte ion produce corresponding selectivity plots which are parallel to the abscissa. This is the case for both monovalent and divalent interferences. The degree of selectivity as a function of concentration is reflected in the vertical position of the plotted lines across the calibration range to 10-l mol 1-l which encompases the Nernstian range of each electrode. This graphical representation also highlights the selectivity behavi- our obtained for Ag+ at 10- moll- which is not representative of its behaviour across the calibration range 10-4-10-2 moll-'.

The result of temperature studies are outlined in Table 4. The isopotential point for electrodes I to I1 are reasonably constant and slightly above the working concentrations of the internal solutions employed throughout these studies. The remaining electrode had a significantly higher isopotential point which suggested the requirement of an almost two-fold concentration of the internal solution in order to optimize the temperature properties of the electrode.

.

+

Conclusions Caesium ion-selective electrodes incorporating novel benzo crown ether derivatives based on the important biological redox

-2.5 -1 5 4 3 2 1

Log concentration

Fig. 6 Plot of selectivity coefficients for monovalent ions as a function of concentration for the ionophore I11 electrode.

Table 4 Temperature studies on each ISE

Temperature coefficient (6E/611

Isopotential 10-1 mol 10-2 rnol 10-3 rnol point*/

Ionophore 1 - 1 Cs+ 1-1 CS' I-' CS' moll-' I -0.166 -0.353 -0.428 0.129 I1 -0.167 -0.629 -1.215 0.117

-0.681 - 1.282 0.252 I11 -0.28 1 * Determined from isotherms at 25 and 45 "C.

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Analyst, February 1996, Vol. 121 131

carrier ubiquinone-0, have been investigated. Structural varia- tions within the benzo crown are reflected in the selectivity of the electrodes which was assessed for a wide range of cations, including heavy metals. The responses obtained for Ag+ ions are worthy of further investigation, particularly in conjunction with synthesis of newer benzocrown derivatives.

The authors acknowledge D. Compagnone for his helpful comments.

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2

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4 5 6

7

8 9

10

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Paper 5107253K Received November 3,1995 Accepted December 8, I995

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