Solid-Contact Ion-Selective Electrodes with Highly Selective

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Solid-Contact Ion-Selective Electrodes with Highly Selective Thioamide Derivatives of p-tert-Butylcalix[4]arene for the Determination of Lead(II) in Environmental Samples Marcin Guziński,†,‡ Grzegorz Lisak,† Tomasz Sokalski,† Johan Bobacka,† Ari Ivaska,† Maria Bocheńska,*,‡ and Andrzej Lewenstam†,§ †

Laboratory of Analytical Chemistry and Centre for Process Analytical Chemistry and Sensor Technology “ProSens”, Process Chemistry Centre, Åbo Akademi University, Biskopsgatan 8, 20500 Abo, Finland ‡ Department of Chemical Technology, Chemical Faculty, Gdansk University of Technology, ul. Narutowicza 11/12 80-233 Gdansk, Poland § Faculty of Material Science and Ceramics, AGH University of Science and Technology, Mickiewicza 30, 30-059 Cracow, Poland S Supporting Information *

ABSTRACT: Thioamide derivatives of p-tert-butylcalix[4]arene were used as ionophores in the development of solid-contact ion-selective electrodes based on conducting polymer poly(3,4-ethylenedioxythiophene)/polystyrene sulfonate (PEDOT/PSS) which was synthesized by electrodeposition on the glassy carbon electrodes. The typical ion-selective membranes with optionally two different plasticizers [bis(2-ethylhexyl)sebacate (DOS) and 2-nitrophenyl octyl ether (NPOE)] were investigated. The potentiometric selectivity coefficients were determined by separate solution method (SSM) for Pb2+ over Cu2+, Cd2+, Ca2+, Na+, and K+. High selectivity toward Pb2+ was obtained. By applying two conditioning protocols, a low detection limit log(aDL) ≈ −9 was achieved. The fabricated ion-selective electrodes were used to determine Pb2+ concentration in environmental samples. The obtained results were compared to analysis done by inductively coupled plasma mass spectrometry (ICPMS).

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complicated to perform online and in-line determination of lead in drinking water. For drinking water suppliers it is important that the concentration of lead is below the highest allowed concentration. In this case inexpensive, simple, and fast chemical sensors should be used to check whether the level of the analyte is below the highest allowed concentration. One of the promising types of chemical sensors are ion-selective electrodes (ISEs), which have been tested as a lead sensor in drinking water.8 From one point of view drinking water is a very challenging sample due to the low concentration of Pb2+ ions (below 10 μg/L, which is 4.8 × 10−8 mol/L) requiring low detection limit of the analytical method. In environmental analysis the selectivity of the method to be used over alkaline metal ions must be much better than over heavy and transition metals since the alkaline metals usually are present at higher concentration levels. On the other hand the drinking water sample is not a very complex medium, does not contain (or should not contain) organic impurities like humic acids, solid particles, biological matter, or any other components that may influence and interfere with the measurements. The drinking

norganic lead is extensively studied as a hazardous metal. Lead is widespread in the environment, and its monitoring is essential for human health. Lead has toxic effects on a series of organs and tissues. Lead poisoning may impact the nervous system (encephalopathy, ataxia, coma, convulsion), kidneys, cardiovascular system (increase of blood pressure, heart disease), endocrine system, immune system, reproduction system as well as may cause cancer.1 Currently, atomic absorption spectroscopy (AAS) is used as a reference method to determine lead concentrations. Another method of choice is inductively coupled plasma mass spectrometry (ICPMS). AAS and ICPMS are very reliable methods with low detection limits, which is the key factor in lead determination in drinking water. However, both of those methods need rather expensive equipment and maintenance. According to WHO (World Health Organization) drinking water should contain less than 10 μg/L Pb; however, WHO pointed out that this value is based on the current analytical achievability for AAS that has a detection limit of 1 μg/L Pb.2 There are various techniques to determine lead concentration such as voltammetry,3−7 potentiometric analysis,8−10 spectrophotometry 11−17 as well as the traditional volumetric method.10,18−20 To measure the concentration of lead in drinking water AAS or ICPMS is usually used, which makes it expensive and © 2013 American Chemical Society

Received: September 24, 2012 Accepted: January 2, 2013 Published: January 2, 2013 1555

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the results determined by ICPMS. The uncertainty of the potentiometric measurements was estimated according to current recommendations.

water sample has also a quite predictable composition which allows one to design a proper analytical system. In order to use ISEs in common environmental analysis a low detection limit has to be achieved. This was explored by Sokalski and co-workers using complexing agent in the internal filling solution (IFS), which eliminates the undesired leaching of primary ions from the membrane into the sample solution. That method also enables determination of unbiased selectivity coefficients.21−23 Low detection limit can also be achieved by using solid-contact ISEs based on, e.g., conducting polymers (CP) as ion-to-electron transducers.24−27 The use of CP as the solid contact27 eliminates the main drawback of the coated-wire electrodes (CWE),28 which is poor potential stability resulting from the blocked interface that is formed between the electronic conductor (metal, carbon) and the ionic conductor (ion-selective membrane). A crucial role in the ion-selective membrane is played by the ionophore, which provides selectivity by selective ion binding. There is a constant need to develop novel selective ionophores that improve selectivity of ISEs. One of the most studied types of ionophores are functionalized p-tert-butylcalix[4]arene. Due to the remarkable and versatile binding properties, they may act as hosts for cations or anions. Currently some of the derivatives of the p-tert-butylcalix[4]arenes are commercially available for cations such as silver ionophore IV,29 lead ionophore IV,30 calcium ionophore VI,31 and sodium ionophore X.32−34 Recently, there has been also development in the theory of ISEs. Lewenstam and co-workers developed a nonequilibrium response model by applying Nernst−Planck (NP) and Poisson (P) coupled equations where the NP equation describes the transport of ions due to diffusion and migration and the P equation governs the electrical interaction of the species.35,36 The NPP model allows one to find optimal conditions to achieve optimal selectivity and detection limit by taking into account physical parameters such as response time and finite charge-transfer rates and membrane parameters such as permittivity, thickness, and mobility of ions.37−39 In this paper we present solid-contact ISEs based on conducting polymer poly(3,4-ethylenedioxythiophene) doped with polystyrene sulfonate anion PSS (PEDOT/PSS) as an ionto-electron transducer and two thioamide derivatives of p-tertbutylcalix[4]arene (1, 2) acting as the lead selective ionophores which are the analogue of the commercially available lead ionophore IV that differs by the attached thioamide moieties (Figure 1). The electrodes based on ionophores 1 and 2 were used in ion-selective membrane and further applied in the determination of Pb2+ lead in environmental samples by direct potentiometry. The potentiometric results were compared with

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EXPERIMENTAL SECTION

Materials. For membrane preparation, high molecular weight poly(vinyl chloride) (PVC), bis(2-ethylhexyl)sebacate (DOS, ≥97%), 2-nitrophenyl octyl ether (NPOE, >99%), tetrahydrofuran (THF), potassium tetrakis(p-chlorophenyl)borate (KTpClPB) were Selectophore grade from Fluka (Buchs, Switzerland). 3,4-Ethylenedioxythiophene (EDOT, >97%) and poly(sodium 4-styrenesulfonate) (NaPSS, average Mw ∼70 000) were obtained from Sigma-Aldrich (Steinheim, Germany). For standard solutions preparation, Pb(NO3)2, Cd(NO3)2, Cu(NO3)2, Ca(NO3)2, NaNO3, and KNO3 were purchased from Fluka (Buchs, Switzerland). Aqueous solutions were prepared with freshly deionized water (18.2 MΩ cm) obtained with the ELGA purelab ultra water system (High Wycombe, United Kingdom). Electrodeposition of PEDOT/PSS on Glassy Carbon Electrodes. Poly(3,4-ethylenedioxythiophene) (PEDOT/ PSS) was chosen to serve as the intermediate layer in solidcontact ISEs because of its good stability in the oxidized state.40−42 Prior to polymerization, the glassy carbon (GC) working electrode was polished using 0.3 μm Al2O3 and rinsed with deionized water. To prepare the GC electrode with PEDOT/PSS film a conventional one-compartment threeelectrode electrochemical cell was used with a GC disc as the working electrode (area = 0.07 cm2), a GC rod as the counter electrode, and the double-junction Ag/AgCl reference electrode. Electrodeposition of PEDOT/PSS was achieved using an Autolab general purpose electrochemical system (AUT20.FRA2-Autolab, Eco Chemie, B.V., The Netherlands) by galvanostatic electropolymerization. PEDOT/PSS films were obtained from aqueous solution containing 0.01 M EDOT and 0.1 M NaPSS by applying a constant current 0.01414 mA (0.2 mA/cm2) for 714 s.42 After polymerization, GC/PEDOT electrodes were rinsed with deionized water and dried overnight at room temperature. Solid-contact sensors were prepared by coating the GC/PEDOT electrodes with the membranes (2 × 20 μL) of the following composition: 0.8% ionophore, 0.2% KTpClPB (60 mol % relative to ionophore), 33% PVC, 66% plasticizer (w/w) which was dissolved in 2 mL of THF. The electrodes were left overnight to evaporate THF. Two kinds of plasticizers, DOS and NPOE, were used. Electrode Characteristics and Selectivity Coefficients. The potentiometric measurements of heavy and transition metal ions were performed at pH 4, adjusted by 0.1 M HNO3; other metal ions were performed at pH 7. Activity coefficients were estimated using the semiempirical Pitzer’s model which enables one to describe the nonideal behavior of the electrolytes up to high concentration.43−46 The single-ion activities were calculated using software PHREEQC version 2.17 (David Parkhurst, U.S. Geological Survey). The ion activities were calculated taking into account the complexation with Cl−, NO3−, and OH− (hydrolysis). The selectivity coefficients were determined by the separate solution method (SSM) according to the procedure described by Bakker et al. according to the following formula using the experimental slope.47,48

Figure 1. The studied thioamide derivatives of p-tert-butylcalix[4]arene acting as lead selective ionophores. 1556

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Analytical Chemistry log

pot KPb,M

Article

0 (E 0 − E Pb ) = M SPb

Kpot Pb,M

10−3 M Pb(NO3)2 (first procedure); for sample 4 the electrodes were conditioned in 10−6 M Pb(NO3)2 (second procedure) overnight. The electrodes were separately calibrated in the range of pPb (6−3) (samples with lead concentration above cPb2+ > 10−6 M, first procedure). The electrodes for the sample 4 were calibrated in the range of pPb (9−6) (sample with lead ion concentration below cPb2+ < 10−6 M, second procedure). The measurements were done in 10−3 M KNO3 as background electrolyte using freshly deionized water (18.2 MΩ cm). Calibration was done by automatic dilution of Pb(NO3)2 stock solution using two Metrohm Dosino 700 instruments equipped with burets of 50 mL volume. The pumps were programmed to dilute the sample solutions. After each dilution the potential was recorded for 10 min (sampled every 10 s). The final potential was taken as the mean of the last five recorded potential values for each electrode. After the calibration the electrodes were washed with deionized water, immersed in the target sample, and the potential was measured as described above. Subsequently, the electrodes were washed again with deionized water and the calibration curves were again recorded. The Pb2+ concentration was calculated using the data from two calibrations (before and after sample being measured). The measurements were repeated once per day, three days in a row. The electrodes were kept in a dark place immersed in conditioning solution (10−3 or 10−6 M Pb(NO3)2). Each day the electrodes were washed with deionized water and then the calibration curves were recorder as described above. Estimation of Uncertainty Measurement. The uncertainty was estimated according to Guide to the Expression of Uncertainty in Measurement (GUM) recommendation.49,50 The suitable informations are placed in the Supporting Information.

(1)

E0M

E0Pb

where log = selectivity coefficient; and = values from the extrapolated to log(a) = 0 calibration curves for various metal ions (M) and for Pb2+, for the studied electrode, respectively; SPb = slope of the Pb2+ electrode. In order to obtain unbiased selectivity coefficients, the calibration of the electrodes was performed for various metal cations, starting preferably from the most discriminated ion. The following order of cations, Ca2+, K+, Na+, Cd2+, Cu2+, Pb2+, was used. Prior to determination of selectivity coefficients the electrodes were conditioned overnight in 10−3 M KNO3. The calibration curves for Pb2+, Cu2+, and Cd2+ were prepared in the 10−6−10−2 M range and for Na+, K+, and Ca2+ in the 10−6− 10−1 M range. The electrodes were kept for 10 min in 10−4 M solution of consecutive electrolyte before measurements of the next cations solution, then washed with water, and then calibration curves were recorded. After calibration for Pb2+ cations the electrodes were conditioned in 10−3 M Pb(NO3)2 (three electrodes, first conditioning procedure) and in 10−6 M Pb(NO3)2 (three electrodes, second conditioning procedure) overnight. Then the calibration curves for Pb2+ cations were again recorded using the standard addition method of the lead(II) solution. All measurements were carried out using a 16-channel LAWSON LAB potentiometer and double-junction reference electrode ORION 800500U ROSS Ultra D/J. Collection of the Environmental Water Samples. A sample of groundwater was collected in the Finnish Southwestern Archipelago from a puddle located 40 m northwest from the main shaft of a disused silver mine. The soil and stones (ore) from the bygone activities located around the main shaft, which had been rinsed with rainfall, created conditions for the uncontrolled release of various ions into the environment. Sampling was performed applying standard regulation for sampling and sample handling, according to the World Health Organization recommendation.2 The sample was carefully collected to avoid disturbing bottom sediment. One liter of sample was collected in polyethylene containers. After 1.5 h from the sampling, basic characteristics of the sample were determined. The measurement of pH was done using a pH meter (Metrohm, Herisau, Switzerland). Environmental samples were frozen between the determinations and kept under −20 °C. For every method each determination was carried out in the same way: in unfrozen sample, brought to room temperature (left overnight). Then the calibration and the measurements were performed in the same conditions. Inductively Coupled Plasma Mass Spectrometry. A Perkin-Elmer-Sciex Elan 6100 DRC Plus inductively coupled plasma mass spectrometer (ICPMS) was used to determine the total concentration of the most relevant elements in the environmental samples. Before the analysis, samples were filtered by a filter unit MF75 (Thermo Fisher Scientific, Rochester, NY, U.S.A.) on paper filters ME25 (47 mm in diameter with 0.45 μm pore size, Schleicher & Schuell, Germany). Analysis was performed at room temperature (21−22 °C). The standard deviation was calculated from three consecutive measurements. Analysis of Environmental Samples. To determine the concentration of Pb2+ in environmental samples new sets of electrodes were prepared (three electrodes for each membrane type). On the basis of ICPMS results for the studied samples the electrodes for samples 1, 2, 3, and 5 were conditioned in

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RESULTS AND DISCUSSION Electrode Characteristics and Selectivity Coefficients. The goal of this work was to prepare a series of ion-selective electrodes based on new lead selective ionophores (Figure 1), to determine the unbiased selectivity coefficients and achieve low detection limit. The prepared electrodes were used to determine Pb2+ in the environmental samples. The recorded curves are presented in Figure 2. This procedure allows a Nernstian slope to be achieved for all studied cations (Figure 2). In the case of the membrane containing NPOE as the plasticizer the sensitivity for calcium cations is slightly below

Figure 2. Potentiometric response for the electrode with the membrane 1/NPOE. 1557

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determine Pb2+ in some environmental samples. The selectivity over copper(II), the most interfering cation, is sufficient to determine lead(II) in 10 000-fold excess of copper(II), whereas in the common situations the concentration of copper(II) does not exceed 100 times the concentration of lead(II).2,8 In order to find conditions for low detection limit, two conditioning procedures were used with freshly prepared electrodes (see the Experimental Section). With the first procedure the super-Nernstian step disappears proving saturation of the membrane with lead(II) cations; however, the obtained detection limit was typical for ion-selective electrodes, about log(aDL) ≈ −6.5. Such results are commonly observed with conventional electrodes with IFS in which the constant release of the primary ions from the membrane phase to the sample occurs, dictating the detection limit.21,23 The high concentration of lead(II) cations in the conditioning solutions may lead to coextraction into the membrane leading to release of lead(II) into more dilute solutions, thereby increasing the detection limit. With the second procedure the super-Nernstian step also disappears and allows a relatively low detection limit to be obtained. The results based on the second protocol are presented in Table 1. In Figure 4 the calibration curves for two

the theoretical value (for ligand 1 S = 26.1 and for ligand 2 S = 24.9 mV/dec) only within the concentration range of 10−1− 10−2 M. In the case of the plasticizer DOS the slope is S = 27.3 for ligand 1 and S = 23.9 mV/dec for ligand 2 in the concentration range of 10−4−10−1 M. A sub-Nernstian slope suggests that the interaction between the ionophore and calcium cations is weak and Ca2+ is highly discriminated. The calibration curves for K+ and Na+ cations show almost theoretical sensitivity, usually S ≥ 55 mV/dec for all the cases that are shown in Figure 2. The recorded curves for Cd2+, Cu2+, and Pb2+ (as primary cations) show a super-Nernstian step at the concentration range between 10−5 and 10−4 M. This indicates that mass transport limits the counterdiffusion flux of the studied cation into the membrane as earlier reported.26,51 The super-Nernstian response is a common phenomenon for unconditioned membranes. The super-Nernstian slope for interfering cations might be the result of significant interaction with the studied ligands which is confirmed by the value of stability constant within the membrane with similar compounds also containing thiocarbonyl groups in their structures.52 The pot are calculated selectivity coefficients given as log KPb,M presented in Figure 3.

Table 1. Observed Electrodes Slope and Detection Limit Based on the Second Conditioning Protocol ionophore

plasticizer

1

DOS NPOE DOS NPOE

2

slope (σn−1) 31.2 30.6 32.5 28.4

(0.5) (0.3) (0.4) (0.3)

DL [log(aDL)] −8.7 −8.9 −8.7 −9.1

Figure 3. Determined selectivity coefficients for studied electrodes.

The determined selectivity coefficients proved that the prepared ion-selective electrodes are highly selective toward lead(II) cations. The selectivity coefficients for the electrodes with ionophore 1 are similar to those obtained by Sokalski et al. using the commercially available lead ionophore IV, in classical electrodes with internal filling solutions (IFS) containing Na2EDTA as a complexing agent.23 The most interfering cation for membranes containing ionophore 1 is copper(II) with the DOS in the ion-selective membrane, log Kpot Pb,Cu = −4.4 and with NPOE log Kpot = −3.95 The most discriminated is Pb,Cu pot the calcium cation, log Kpot Pb,Ca = −14.4 (DOS) and log KPb,Ca = −13.1 (NPOE). The determined selectivity coefficients for the electrodes with ionophore 2 reveal a high selectivity toward lead(II). The selectivity for the membrane containing ionophore 2/DOS is similar to the selectivity coefficients for the membrane 1/DOS, but the selectivity for lead(II) over copper(II) is about 1 order of magnitude better, log Kpot Pb,Cu = −5.70. The electrodes with the 2/NPOE membrane show a selectivity which is about 3 orders of magnitudes better than the electrodes with the 2/DOS membrane: log Kpot Pb,Ca = −9.22, pot pot log Kpot Pb,Ca = −16.8, log KPb,Na = −9.90, log KPb,K = −10.0. The selectivity over copper(II) compared to the membrane with DOS is log Kpot Pb,Cu = −4.78. On the basis of the determined selectivity coefficients those electrodes may be used to

Figure 4. Calibration curves for Pb2+ cations solution based on two conditioning procedures, for the electrodes with the membrane 1/ DOS and 2/NPOE: dashed line, after conditioning in 10−3 M Pb(NO3)2; solid line, after conditioning in 10−6 M Pb(NO3)2.

procedures are presented. The detection limit is decreased down to nanomolar level which indicates that ion leaching from the membrane is not significant or is significantly diminished and the detection limit is dictated by lead(II) impurities in the solution or by the interfering H+ ions. The electrodes which were conditioned in more diluted solution were working 2 weeks with low detection limits if the calibration was done for concentrations below pPb < 5 three times per day. If those electrodes were calibrated at higher concentration, pPb ≤ 3, the detection limit goes up to log(aDL) ≈ −6.5 which is a typical value for a fully saturated membrane as it is in the first conditioning protocol. 1558

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Analysis of Environmental Water Samples. One of the most known publications which describes the determination of lead in water samples at low concentration was published by Ceresa et al.8 In that paper it was shown for the first time how to use practically an ISE with low detection limit for determination of lead at nanomolar level. The results obtained in the above work were compared with those obtained by ICPMS. The ISE results, however, deviated to some extent from the ICPMS results. Interferences from H+ and Cu2+ cations may explain this deviation. Despite that, it was proved that ISEs may be used to control whether lead concentration exceeds the highest allowed concentration in aqueous samples. In the discussed work, the used ISEs were conventional electrodes with IFS of common composition. Their calibration procedure is suitable for laboratory conditions but may be difficult to apply in field conditions. The designed automatic/ remote/portable sensors system is also unpractical to be used in on/in-line analysis. It has been shown that the toxicity of metals is related rather to concentration (activity) of unbound cations and not to the total concentration of metals.53−55 Here, we would like to show determination of Pb2+ concentration in water samples containing heavy and transition metals as well as organic compounds such as humic-like acids. The proposed procedure is to be as simple as possible, without any need for special sample treatment. It is also fast and user-friendly in order to apply the procedure to portable/remote sensor systems. The total concentration of lead was determined by ICPMS. The obtained values of total concentration of lead in each of the four samples are above 10−5 M (samples 1, 2, 3, 5), and one sample is below 10−7 M (sample 4). Therefore, two procedures are proposed. The procedures are based on the experience from the conditioning procedures and are described in the Experimental Section. The measured pH values of each sample are presented in Table 2. All samples were from yellowish to yellow-brown in color and together with the pH data indicate that the samples contain humic/fulvic acids or in general humus substances.

The results obtained by the solid-contact ISEs and the ICPMS method are given in Table 3 for the five collected samples with the estimated uncertainty for the Pb 2+ determination. As expected, the concentration of Pb2+ is lower than the total concentration. In drinking water concentration of the Pb2+ is fairly equal to the total concentration and by simple acidification this can be achieved (pH = 4). In the case when substances like humic acids (and other organic compounds) are present in the samples such acidification is not enough (pH should be even below 2). There have been extensive studies of humic-like acids. However, the number of the results, used method, types, conditions etc. make it difficult to show complex stability constants as conclusive values. The main conclusion is that those natural compounds bind metal cations, especially Pb2+, Cu2+, Fe3+, which explains the decrease in the concentration of Pb2+.56−59 Slaveykova et al. used the ISE to follow Pb2+ concentration as a function of the toxicological response of an aquatic algae in the presence of fulvic acid.53 It was shown that Pb uptake is decreased in the presence of fulvic acid (meaning lower concentration of Pb2+). Another reason why Pb2+ concentration is lowered might be biological matter and suspended particles on which lead(II) cations might be adsorbed. Despite the fact that ISEs showed lower lead concentration there is correlation between ISE and ICPMS. The samples 1, 2, 3, and 5 contain similar total concentration of lead, and the same way they contain the similar amount of Pb2+ which was determined by ISEs. The differences in Pb2+ concentration among those samples are influenced by total lead concentration, concentration of chelating compounds (like fulvic, humic acids), pH, individual sample composition (cations which compete in complexation, anions which may form associates with cations), and other specific parameters of each sample (organic sample, place of origin of the sample, etc.). This comparison gives the opportunity not only to study the concentration of Pb2+ but also gives information about the specific environment of the sample. When comparing the ISEs results we see that the electrodes with ionophore 1 show slightly lower concentration of Pb2+ than the electrodes with ionophore 2. That may be surprising considering the selectivity coefficients which suggest stronger influence of metal cations by the ISEs with ionophore 1. However, organic and biological matter present in the sample may affect the signal of the ion-selective membrane as well as the reference electrode in an unexpected way. It may be possible that coextraction of lead salts with various organic anions by membranes with ionophore 1 is stronger with 2, and in this way the Pb2+ concentration measured by ISEs with ionophore 1 appears lower (anion interference). The obtained

Table 2. Obtained pH Values and Places of Origin for the Environmental Water Samples sample

origin

1 2 3 4 5

20 10 35 40 55

m m m m m

pH

east from the main shaft south from the main shaft west from the main shaft north-west from the main shaft north-east from the main shaft

3.75 3.44 3.57 5.04 4.61

Table 3. Pb2+ Concentration Determined (with Estimated Uncertainty) by Direct Potentiometry in Environmental Water Samples and Comparison to the ICPMS Method (Total Concentration of Lead) ionophore plasticizer

1 DOS

NPOE

DOS

pPb ± U (k = 2)

sample 1 2 3 4 5

2

5.41 5.316 5.12 8.01 5.38

± ± ± ± ±

0.10 0.082 0.12 0.42 0.15

5.34 5.36 5.07 7.99 5.263

NPOE

ICPMS

± ± ± ± ±

4.716 4.606 4.549 7.064 4.754

pPb ± U (k = 2) ± ± ± ± ±

0.10 0.15 0.11 0.34 0.089

5.104 5.075 4.956 8.10 5.18 1559

± ± ± ± ±

0.061 0.086 0.089 0.27 0.10

5.088 5.149 4.945 7.93 5.184

pPb 0.095 0.096 0.097 0.22 0.094

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values for sample 4 show the same effect. But when the estimated uncertainty is considered the results coincide. The uncertainty of the determination of Pb2+ was estimated according to GUM.49,50 We assume that the main sources of uncertainty are the calibration curves and repeatability of each measurement. It is also assumed that the uncertainty of values of standard solutions is considerably smaller than the uncertainty of the measured potential. We have also used two calibration curves for specific samples. From that point of view the big advantage of the ISE is the broad linear range, which is not used here. However, as it is shown here the separate calibration for lower and for higher concentrations give other advantages, like prolonged electrode lifetime at lower concentration. On the other hand the uncertainty for the low concentrations is usually higher, and thus, using one calibration (broad concentration range) the uncertainty for higher concentrations would be overestimated and for lower concentrations it would be underestimated. So far the main metrological work has been focused on pH measurements60 as well as some work has been done with fluoride ion-selective electrodes.61 One of the problems with ion-selective electrodes is traceability of activity. Recently, this issue has been studied by Wunderli and co-workers in mixed electrolytes for clinical and medical diagnostics.62,63 There is no reference material for ISEs with Pb2+ and complex matrixes with which the analytical results obtained with ISEs could be checked and validated. The ISEs results could be compared with ion-selective optodes which also deliver data on the Pb2+ concentration (activity). Estimation of Uncertainty Measurement. The estimated uncertainty of the obtained results is included in Table 3. The electrodes with ionophore 1 show slightly higher uncertainty. For the most diluted samples the uncertainty is higher for the membrane with DOS as the plasticizer. This is most likely related to the reproducibility of the measured potential in the standard solution which was lower than in the case when NPOE was used as the plasticizer especially for samples with lower concentration. The calibration points with the calibration uncertainty range are presented in Figures 5 and 6 for two concentration ranges.

Figure 6. Calibration curves for the ISE with the 2/NPOE membrane for the concentration range of pPb = 6−9; the dotted line represents the calibration uncertainty. The inset shows the magnification of the region of interest.

The fabricated electrodes showed remarkable selectivity for lead(II) cations. The low detection limit down to pPb ≈ 9 was successfully achieved by proper conditioning the electrodes in diluted lead(II) nitrate solutions. Those electrodes were used to determine the concentration of Pb2+ in environmental samples containing also heavy and transition metal ions as well as humus substances (humic/fulvic acids) and biological matter. The uncertainty was estimated, and the obtained results were compared with the results obtained by the ICPMS technique. In all samples analyzed the concentration of Pb2+ was lower than that obtained by ICPMS (total concentration of lead). The presence of chelating agents in water samples such as humus substances may be the cause of the differences in the obtained results. Despite the differences obtained by the two methods, it would, however, be possible to use ion-selective electrodes in automatic/remote sensor systems to monitor Pb2+ at low concentration. For the samples with a complex matrix containing chelating agents it should be possible to develop a model based on chemometric tools to find a correlation between unbound and total concentration of lead for specific types of samples.

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CONCLUSIONS The solid-contact ion-selective electrodes based on conducting polymer (PEDOT/PSS) and novel ionophores 1 and 2 being thioamide derivatives of p-tert-butylcalix[4]arene are presented.

ASSOCIATED CONTENT

S Supporting Information *

Additional information as noted in text. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Polish Ministry of Science and High Education, Grant No. N N209060040, the development of interdisciplinary doctoral studies at the Gdansk University of Technology in modern technologies, project no: POKL.04.01.01-00-368/09. The authors also thank the Åbo

Figure 5. Calibration curves for the ISE with the 2/DOS membrane for the concentration range of pPb = 3−6; the dotted line represents the calibration uncertainty. The inset shows the magnification of the region of interest. 1560

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Analytical Chemistry

Article

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Akademi University Process Chemistry Centre for the Johan Gadolin Scholarship for funding and invitation.

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dx.doi.org/10.1021/ac302772v | Anal. Chem. 2013, 85, 1555−1561