Improved Nitrate Sensing Using Solid Contact Ion Selective ...

Journal of The Electrochemical Society, 162 (10) B257-B263 (2015) 0013-4651/2015/162(10)/B257/7/$33.00 © The Electrochemical Society

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Improved Nitrate Sensing Using Solid Contact Ion Selective Electrodes Based on TTF and Its Radical Salt Magdalena Piek, Robert Piech, and Beata Paczosa-Batorz  AGH-UST University of Science and Technology, Faculty of Material Science and Ceramics, PL-30059 Cracow, Poland The possibility of improving the selectivity of solid-state nitrate-selective electrodes by the application of the tetrathiafulvalene (TTF) or its nitrate salt (TTF(NO3 )) as an intermediate layer between an electrical conductor and a polymeric membrane is demonstrated. The analytical performance of electrodes was investigated during potentiometric measurements. Fabricated sensors displayed a Nernstian slope (−58.85 mV/decade in the range from 10−5.0 to 10−1 M and −59.36 mV/decade in the range from 10−6.0 to 10−1 M for TTF− and TTF(NO3 )−modified electrodes respectively), repeatable and reproducible standard potential and detection limit of 2.5 μM (0.16 mgL−1 ) and 0.63 μM (0.039 mgL−1 ) for TTF− and TTF(NO3 )−based electrodes, respectively. The selectivity was considerably improved compared to a coated disc electrode or electrodes with intermediate layer based on carbon nanomaterials. Electrical parameters of the proposed sensors were tested by carried out current-reversal chronopotentiometry. In the case of electrodes with the use of TTF or TTF(NO3 ) the potential drift decreases to 167 μVs−1 or 16.6 μVs−1 and the capacitance is 5.99 μF or 60.3 μF. The proposed sensors were successfully applied in analyzing nitrate concentration in water samples. © 2015 The Electrochemical Society. [DOI: 10.1149/2.0631510jes] All rights reserved. Manuscript submitted April 7, 2015; revised manuscript received June 12, 2015. Published August 5, 2015.

Nitrates are widely used in the fabrication of fertilizers, drugs, explosives and many products, which significantly contributes to their high concentration in the sewage.1 The contamination of surface and ground water by nitrate ions is a material problem around the world and their removal is important for two main reasons. This type of water pollution disturbs ecological balance and causes hazard to people health. Nitrates are a source of nitrogen for microbial protein synthesizing, nevertheless excess amount of these ions causes eutrophication in water bodies manifested through uncontrolled growth of algae. The natural level of nitrate in surface water is typically less than 1 mgL−1 due to dilution of surface runoff, plant uptake and denitrification processes. However nitrate pollution has generally increased over time as a result of intensive use of nitrogen fertilizers in agriculture. The maximum acceptable concentration for nitrate has been established at 10 mgL−1 , to be protective of the health.2,3 Drinking water containing a high concentration of nitrates induces cancer or other health problems, such as the ‘blue baby syndrome’ in infants.4 Therefore, it has been necessary to introduce selective, simple and reliable method that could be used for the monitoring the concentrations of nitrates. Several analytical methods for the nitrate determination have been reported including spectrophotometry with the detection limit of 0.031 mgL−1 and 40 μgL−1 ,5,6 chromatography with the detection limit of 0.001 mgL−1 and 0.01 mgL−1 ,7,8 capillary electrophoresis with the detection limit of 0.099 mgL−1 ,9 amperometry with the detection limit of 6.2 ngL−1 ,10 polarography11 and voltammetry with the detection limit of 5.4 mgL−1 .12 However, most of these methods require a complex equipment and the processes of analysis are time-consuming. In this case, the potentiometric detection using ion-selective electrodes (ISEs) is an unrivalled solution. Potentiometric detection based on the relationship between the activity of the analyte and the potential of an indicator electrode measured toward to a reference electrode.13 The analysis conducted with the use of ion-selective electrodes has several advantages, such as speed and ease of measurement, short response time, simple instrumentation and time-efficiency. Additionally, method is nondestructive and the results of measurements are not dependent on the color or turbidity of the sample.14 Ion-selective electrodes, due to their unusual properties, for example low construction costs, selectivity, small size and portability,15 are widely used in determination of both inorganic and organic ions in industrial,13 environmental16 and clinical analysis.17 Nevertheless, the sensors sensitive to nitrate ions which would provide a good selectivity to NO2 − and Cl− ions are still

z

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desirable. As we showed in our previous work18 there is a possibility to influence on the sensors selectivity by applying an intermediate layer based on TCNQ and its ion-radical salts. In this paper the possibility of improving the selectivity of sensor response to the nitrate ions by the application of the tetrathiafulvalene (TTF) or its nitrate salt (TTF(NO3 )) is demonstrated. Conventional ISEs consist of an internal electrode, internal solution and an ion-selective membrane. However such construction limited the working conditions (pressure, temperature and positions) and requires supplement of the internal solution. Coated-wire electrodes (CWEs),19 where the metal conductor is coated directly by the ion-selective membrane, are used only in special applications due to lack of long-term potential stability of these sensors. In order to solve this problem conducting polymers (CPs) were introduced as ion-to-electron transducers between the membrane and the electronic conductor.20,21 Although CPs improved analytical performance of electrodes, they also suffer from sensitive to gases or light and formation of water film between the intermediate layer and the membrane.22 Recently, nanomaterials has attracted a lot of attention due to their unique mechanical, chemical and electric properties,13 and also have wide range of applications as an intermediate layer in solidcontact ion-selective electrodes (SC-ISEs), for example platinum nanoparticles,23 graphene,22 carbon nanotubes24 or carbon black.25,26 Though the usage of nanomaterials improves the analytical parameters of sensors for example the potential stability, capacity and reduces the resistance of electrodes, however has only slight effect on the selectivity of the electrodes, which is still comparable to coated-disc electrode. Since the discovery in 1972, the first organic conductor TTFCl, tetrathiafulvalene (TTF) and its salts are among the most studied compounds.27 This kind of organic compounds may have conductive and superconductive properties and have been tested for example as single crystals, thin layers, molten salts or in polymer films by means of electrochemical methods.28 TTF has been frequently used as redox mediator29 and widely applied in ion-selective voltammetric electrodes,30,31 biosensors32 and electro-optical sensors.33 As regards the potentiometric detection, it was shown by Fibbioli et al.34 that redox-active lipophilic self-assembled monolayers (SAMs) based on TTF derivative (namely 2,3-bis-({[5-(1,2-dithiolan3-yl) pentanoyl]oxy}methyl)-6,7-(ethylenedithio) tetrathiafulvalene), can be used as transducers for the process of switching between ionic and electronic conductivity in order to eliminate undesirable water layer of solid-contact potassium selective electrodes. This paper presenting the analytical parameters of sensors contained TTF or TTF(NO3 ) layer between glassy carbon and nitrateselective membrane. The introduction of TTF and its nitrate salts significantly improves the selectivity of sensors, especially toward NO2 − ions.

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Materials.— Tetrathiafulvalene 97% (TTF), nitrate ionophore V, tridodecylmethylammonium chloride (TDMACl), o-nitrophenyl octyl ether (o-NPOE), poly(vinyl chloride) (PVC) of high molecular weight, acetone, dimethylformamide (DMF) and tetrahydrofuran (THF) were the selectophore reagents obtained from Fluka. All others chemicals were of analytical-reagent grade. Distilled and deionized water was used to prepare the aqueous solutions. Scanning electron microscopy.— The chemical and morphologies analysis of the TTF layer before and after electrochemical synthesis of nitrate salt TTF(NO3 ) were examined using a scanning electron microscope (SEM), model LEO 1530 from LEO Electron Microscopy Ltd. equipped with the Image and X-ray Analysis System, model Vantage, from ThermoNoran for energy-dispersed X-ray Spectroscopy (EDS). Potentiometric measurements.— The potentials were measured with the use of a 16-channel mV-meter (Lawson Labs, Inc., Malvern, PA). The reference electrode was an Ag/AgCl electrode with 3 M KCl solution as a bridge electrolyte (model 6.0733.100 Metrohm, Switzerland) or an Ag/AgCl/3 M KCl (type 6.0729.100 Metrohm, Switzerland) with 1 M CH3 COOLi in a bridge cell. Electrochemical measurements.— The chronopotentiometry, voltammetric and impedance measurements were carried out using an Autolab General Purpose Electrochemical System (AUT32N.FRA2AUTOLAB, Eco Chemie, The Netherlands) connected to a conventional, three-electrode cell. An Ag/AgCl/3 M KCl electrode (type 6.0733.100 Metrohm, Switzerland) was the reference one, a glassy carbon rod was used as the auxiliary electrode and the SC-ISE was connected as a working electrode. In the case of voltammetric measurements a stream of nitrogen was used to degassing the aqueous solutions. All measurements were performed at room temperature. Electrodes preparation.— First the glassy carbon disc (GCD) electrodes consisting of GC rods enveloped in PEEK (polyetheretherketone) polymer bodies were polished with 0.3 and 0.05 μm alumina powder, then rinsed with water and subsequently cleaned ultrasonically with water and methanol. The GCD/TTF electrodes were prepared by the drop casting method. The 10 μL solution containing 20 mg of TTF dispersed in 1 mL of acetone was added on the top of the GCD electrodes. After that, the GCD/TTF electrodes were left to dry. In order to obtain electrodes denoted as GCD/TTF(NO3 ) electrochemical synthesis of TTF(NO3 ) was carried out. The TTF layer was deposited by coating GCD electrodes by 15 μL of mixture containing 10 mg of TTF dispersed in 0.5 mL of DMF. After evaporation of the solvent electrochemical oxidation of TTF was performed in the presence of nitrate ions in the 0.1 M KNO3 by cycling the potential at a scan rate of 100 mVs−1 between +0.40 V and −0.30 V (vs. Ag/AgCl/3 M KCl) during 5, 10, 15, 20 or 25 cycles. The TTF(NO3 )-modified electrodes have been denoted in addition number corresponds to number of cycles. In the next step, to prepare the nitrate selective electrodes, the solid contact layers (GCD/TTF and GCD/TTF(NO3 )) were subsequently coated with 60 μL NO3 − -ISM solution containing 1.1% (w/w) nitrate ionophore V, 0.7% (w/w) TDMACl, (the molar ratio of ionophore : TDMACl was 2 : 1) 65% (w/w) o-NPOE and 33.2% (w/w) PVC dissolved in THF. As the TTF is soluble in THF, a solution of the membrane was added in portions of 15 μL. In order to compare the results obtained by electrodes contain TTF or TTF(NO3 ) intermediate layer with coated disc electrodes, bare GCD electrodes were coated only with above mentioned membrane. All the membrane electrodes were left to dry for 24 h at room temperature to evaporation of THF and then conditioned in 0.01 M KNO3 water solution for at least 1 day before further measurements were performed. The approximate thickness of the PCV membrane was 65–70 μm.

The examined electrodes were also conditioned before every measurements and stored separately. Five identical electrodes for both kind of SC-ISEs were prepared. All of them were studied. Results and Discussion The morphology of TTF and TTF(NO3 ) were investigated by SEM and shown in Figure 1. In contrast to TTF (Fig. 1a), TTF(NO3 ) obtained by electrochemically synthesis present characteristic needleshaped crystals (Fig. 1b). The EDS analysis of the TTF and TTF after electrochemical oxidation in 0.1 M solution of KNO3 confirmed the presence of molecules of TTF(NO3 ). In the case of TTF in the spectrum only peaks corresponding to C and S has appeared. In analysis of the TTF(NO3 ) additional N peaks were observed. In addition, Fig. 1c, shows two definite layers in the modified electrodes, i.e., a PVC membrane layer and a layer of TTF or TTF(NO3 ) needles. As TTF and TTF(NO3 ) are good soluble in THF, same amount of them are partly dissolved into the PVC membrane. Figure 2 shows exemplary cyclic voltammograms obtained at a scan rate of 0.1 Vs−1 over the potential range of +0.40 V and −0.30 V with the GCD/TTF electrode when placed in 0.1 M KNO3 solution. According to,31 TTF is oxidized to the TTF+ and NO3 − anions are incorporated into the crystal lattice. Potentiometric measurements.— The exemplary dependency of the electromotive force (emf) on log aNO3- recorded for the developed electrodes in the concentration range 10−8 – 10−1 M KNO3 is presented in Figure 3a. The slopes calculated from the linear range of calibration plots of the nitrate-selective electrodes with TTF or TTF(NO3 )(25) were −58.85 mV/decade and −59.36 mV/decade respectively and they are closer to theoretical value compared to coated wire electrode (−56.08 mV/decade). The GCD electrodes coated only by TTF or TTF(NO3 ) layer showed the slopes of calibration curves considerably lower than Nernstian value (Figure 3b).

Figure 1. SEM images of (a) TTF and (b) TTF(NO3 ) crystals and a crosssection of the nitrate-selective electrode with TTF or TTF(NO3 ) layer.

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Figure 2. Cyclic voltammograms obtained for 25 cycles of the potential over the range of +0.40 V and −0.3 V at a scan rate of 0.1 Vs−1 for the TTFmodified GCD electrode.

The limit of detection was calculated as the activity of the NO3 − at intersection of the two slope lines and collected with another metrological parameters for all investigated electrodes in Table I. The prepared electrodes display a stable response over time and showed a linear response in the same range of nitrate ions activity for 4 weeks. The exemplary dependency log aNO3- vs. electromotive force (emf) for presented electrodes is shown on Figure 4. Reversibility test.— In Figure 5 the reversibility test of studied electrodes is presented. As can be seen, developed electrodes showed very reversible potential response toward nitrate ions. The times of response of the GCD/TTF/NO3 − -ISM and GCD/TTF(NO3 )/NO3 − ISM electrodes for the progressive additions of different amounts of nitrate ions were very short (about 4 – 5 s) and were similar to the results obtained for sensors with carbon black26,35 or carbon black supporting platinum nanoparticles (PtNPs-CB)36 and better than nitrate electrodes based on graphene.22

Figure 3. (a) The emf dependence on NO3 − activities for GCD/NO3 − -ISM (i), GCD/TTF/NO3 − -ISM (ii), GCD/TTF(NO3 )(5)/NO3 − -ISM (iii) and in turns GCD/TTF(NO3 )(10)/NO3 − -ISM (iv), GCD/TTF(NO3 )(15)/NO3 − -ISM (v), GCD/TTF(NO3 )(20)/NO3 − -ISM (vi) and GCD/TTF(NO3 )(25)/NO3 − ISM (vii) electrodes conditioned in 0.01 M KNO3 solution over 24 h. (b) Exemplary potentiometric response for the GCD/TTF and GCD/TTF(NO3 )(25) electrodes recorded in KNO3 .

Redox sensitivity test.— Redox sensitivity measurements were performed for the developed electrodes as well as GCD covered with the TTF or TTF(NO3 ) layer. In order to verify if studied electrodes reveal redox response the solutions contain redox couple FeCl3 and FeCl2 of the total concentration a 10−3 M with a ratio of log of Fe3+ /Fe2+ equal to −1, −0.5, 0, 0.5 and 1 at constant ionic strength (10−2 M KNO3 ) were used. As expected, the GCD/TTF and GCD/TTF(NO3 ) layers are separated from the sample by the NO3 − -ISM and in the case of GCD/TTF/NO3 − -ISM and GCD/TTF(NO3 )/NO3 − -ISM electrodes no redox response is observed as is presented in Figure 6.

according to37 using potassium salts of different anions are listed in Table II. Measurements were carried out in solutions of interfering anions in a concentration range from 10−5 M to 10−1 M. The values calculated on the basis of the standard potential of electrodes were given. It is worth remarking that the directly presence of TTF(NO3 ) or TTF in PVC membrane and as an intermediate layer significantly improves selectivity of nitrate electrodes compared to the conven-

Selectivity coefficients.— The potentiometric selectivity coefficients of proposed sensors obtained with a separate solution method

Table I. Potentiometric parameters of studied nitrate sensors (where SD is standard deviation, n = 5). Parameter Slope ± SD (mV/decade)

Electrode − -ISM

GCD/TTF/NO3 GCD/TTF(NO3 )(5)/NO3 − -ISM GCD/TTF(NO3 )(10)/NO3 − -ISM GCD/TTF(NO3 )(15)/NO3 − -ISM GCD/TTF(NO3 )(20)/NO3 − -ISM GCD/TTF(NO3 )(25)/NO3 − -ISM GCD/NO3 − -ISM

−58.85 ± 0.15 −59.60 ± 0.40 −59.60 ± 0.17 −59.32 ± 0.22 −59.62 ± 0.24 −59.36 ± 0.21 −56.08 ± 0.29

E0

± SD (mV)

−36.7 ± 0.5 −103.0 ± 1.6 −119.1 ± 0.7 −136.3 ± 0.8 −146.6 ± 1.0 −151.6 ± 0.9 61.6 ± 1.9

Linear Range (M)

Detection Limit ± SD (μM)

10−5.0

2.51 ± 0.58 1.58 ± 0.76 1.00 ± 0.23 1.00 ± 0.23 0.63 ± 0.30 0.63 ± 0.15 6.31 ± 2.1

10−5.5 10−5.5 10−5.5 10−6.0 10−6.0 10−4.5

10−1

– – 10−1 – 10−1 – 10−1 – 10−1 – 10−1 – 10−1

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Figure 4. The potentiometric response values with standard deviation recorded over 14 calibrations performed during 4 weeks for GCD/TTF/NO3 − ISM (i line), GCD/TTF(NO3 )(25)/NO3 − -ISM (ii line) and GCD/NO3 − -ISM (iii line) electrodes.

Figure 6. The redox sensitivity (i), GCD/TTF(NO3 )(25)/NO3 − -ISM GCD/TTF(NO3 )(25) (iv) electrode.

of (ii),

a GCD/TTF/NO3 − -ISM GCD/TTF (iii) and a

tional ISEs38 and solid state sensors without (coated-wire electrode) and with intermediate layer made with graphene,22 carbon black26 or the PtNPs-CB,36 as shown in Table II. Furthermore, the selectivity increases with the number of cycles performed for the GC/TTF electrode during electrochemical oxidation of TTF as described above.

Figure 5. The reversibility test of GCD/TTF(NO3 )(25)/NO3 − -ISM (ii).

the

GCD/TTF/NO3 − -ISM

(i),

Chronopotentiometry.— In order to evaluate the potential stability and electric capacity of the solid contact the current-reversal chronopotentiometry was used.40 An exemplary change in potential vs. time during applied to studied electrodes the currents of +1 nA for 60 s and −1 nA over next 60 s is shown in Figure 7. The potential drift of the electrodes (E/t) was derived from the slope of the recorded curves. The related to the low-frequency capacitance was calculated according to equation of E/t = I/C. The resulting values are presented in Table III. The TTF(NO3 )-modified electrodes presented significantly lower potential drift and higher capacity in comparison with coated disc electrode developed under similar condition. In the case of GCD/TTF/NO3 − -ISM electrode values of these parameters have also changed but not significantly. Although electrical parameters of electrodes with TTF or TTF(NO3 ) have been improved, they are little worse than those obtained for sensors with an intermediate layer made of nanomaterials.26,36

Table II. Comparison of the potentiometric selectivity coefficients of the proposed and other electrodes described in literature (where SD is standard deviation, n = 5). pot NO− 3 ,Y

log K Electrode − -ISM

GCD/TTF/NO3 GCD/TTF(NO3 )(5)/NO3 − -ISM GCD/TTF(NO3 )(10)/NO3 − -ISM GCD/TTF(NO3 )(15)/NO3 − -ISM GCD/TTF(NO3 )(20)/NO3 − -ISM GCD/TTF(NO3 )(25)/NO3 − -ISM GCD/NO3 − -ISM PtNPs-CB based SC-ISE [36] Beckman 39618 graphene-based SC-ISE [22] carbon black-based SC-ISE [26] conventional ISE [38] THTDPCl-based ISE [39]

± S D, Y:

Cl−

NO2 −

acetate

HCO3 −

salicylate

ClO4 −

−3.0 ± 0.2 −3.3 ± 0.2 −3.3 ± 0.1 −3.4 ± 0.2 −3.5 ± 0.2 −3.5 ± 0.2 −2.6 ± 0.2 −2.2 ± 0.2 −2 −1.9 ± 0.1 −2.1 ± 0.2 −1.46 −2.2

−2.0 ± 0.2 −3.0 ± 0.1 −3.1 ± 0.2 −3.2 ± 0.2 −3.4 ± 0.1 −3.4 ± 0.2 −1.8 ± 0.3 −1.3 ± 0.1 −1.2 −1.4 ± 0.2 −1.63 −1.8

−4.2 ± 0.1 −4.4 ± 0.2 −4.5 ± 0.1 −4.5 ± 0.2 −4.6 ± 0.1 −4.6 ± 0.2 −3.5 ± 0.2 −3.2 ± 0.2 −2.2 −3.1 ± 0.2 −1.2 −3.3

−4.1 ± 0.1 −4.2 ± 0.1 −4.2 ± 0.2 −4.3 ± 0.1 −4.4 ± 0.2 −4.4 ± 0.2 −3.9 ± 0.1 −3.1 ± 0.3 −3.2 ± 0.2 −1.5 -

1.4 ± 0.2 1.0 ± 0.1 1.0 ± 0.2 0.9 ± 0.1 0.9 ± 0.2 0.9 ± 0.2 2.0 ± 0.2 1.4 ± 0.1 1.3 ± 0.2 -

3.3 ± 0.2 3.0 ± 0.1 2.9 ± 0.1 2.9 ± 0.2 2.9 ± 0.1 2.9 ± 0.1 3.7 ± 0.3 2.5 ± 0.2 2 2.6 ± 0.1 3.1

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Figure 7. Exemplary chronopotentiograms for the (a) GCD, GCD/TTF and GCD/TTF(NO3 )(25) electrodes and (b) GCD/NO3 − -ISM (i curve), GCD/TTF/NO3 − -ISM (ii curve), GCD/TTF(NO3 )(5)/NO3 − ISM (iii curve) and in turns GCD/TTF(NO3 )(10)/NO3 − -ISM (iv), GCD/TTF(NO3 )(15)/NO3 − -ISM (v), GCD/TTF(NO3 )(20)/NO3 − -ISM (vi) and GCD/TTF(NO3 )(25)/NO3 − -ISM (vii) electrode recorded in 0.01 M KNO3 .

Electrochemical impedance spectroscopy.— Impedance measurements were carried out in 0.1 M KNO3 solution for the GCD, GCD/TTF and GCD/TTF(NO3 ) electrodes before (Fig. 8) and after (Fig. 9) covering with PCV membrane. The complex plane impedance plot for each system (Figs. 8–9) was fitted to an equivalent circuit and fitted values are presented in Table IV.

Table III. Electrical parameters of studied sensors calculated as the average of five different electrodes (where SD is standard deviation). Electrode

E/t ± SD (μV/s)

C ± SD (μF)

GCD GCD/TTF GCD/TTF(NO3 )(25) GCD/TTF/NO3 − -ISM GCD/TTF(NO3 )(5)/NO3 − -ISM GCD/TTF(NO3 )(10)/NO3 − -ISM GCD/TTF(NO3 )(15)/NO3 − -ISM GCD/TTF(NO3 )(20)/NO3 − -ISM GCD/TTF(NO3 )(25)/NO3 − -ISM GCD/NO3 − -ISM

295 ± 5 23.1 ± 2.0 0.62 ± 0.22 167 ± 2 57.4 ± 0.8 24.7 ± 0.6 21.2 ± 0.6 16.5 ± 0.4 16.6 ± 0.5 692 ± 15

3.39 ± 0.06 43.5 ± 3.8 1615 ± 78 5.99 ± 0.07 17.4 ± 0.3 40.5 ± 0.8 47.2 ± 1.4 60.6 ± 1.5 60.3 ± 1.8 1.45 ± 0.03

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As can be seen, the imaginary part of the impedance (-Z”) is about 160 times lower in the spectrum of GCD/TTF than in that of the GCD and further reduced by 60 times in the case of the GCD/TTF(NO3 ) electrode, which means higher capacitance of GCD/TTF(NO3 ) electrode compared to bare GCD electrode. This finding is in good agreement with the results obtained from the chronopotentiometric results. As can be seen form Fig. 8, impedance spectra are mostly govern by the processes occurring at the interfaces between the aqueous phase and the electrode phase. Impedance of this interface can be approximated by a charge-transfer resistance (Rct ), which is inversely proportional to the rate of charge transfer and consequently provides us information about the easiness for electron/ion transfer at the electrode interface (estimation of Rct is related with the diameter of the semicircle) in parallel with a double-layer capacitance and can be obtained from the maximum value of impedance data at the semicircle (Cdl ). Rs is the uncompensated series resistance, mainly electrolyte resistance and can be extracted from impedance data at the higher frequencies and Zw is the Warburg impedance and is identified with the linear portion of the impedance spectra that appears at the lower frequencies. The Warburg impedance arises from mass-transfer limitations.41,42 The impedance spectra obtained for GCD, GCD/TTF and GCD/TTF(NO3 ) are characteristic for porous materials. In the case of GCD electrode characteristic impedance for polarizable electrode was observed and charge-transfer processes can take place through the exchange of electrons. The charge-transfer processes in the case of TTF based electrodes are very quickly due to its high electron kinetics. The GCD/TTF(NO3 ) electrode presenting also charge transfer with electrolyte due to ion-exchange processes (as nitrate ions present in the layer and in the solution can participate in ion-exchange processes). In such cases the Rct is the dominant component of the total internal resistance.41,42 The impedance spectra of TTF- and TTF(NO3 )-modified electrodes with polymeric membrane are shown in Figure 9. After covering the electronic substrate with polymeric membrane a high –frequency semicircle related to a bulk resistance (Rb ) and a geometric capacitance (Cb ) has appeared on the impedance spectrum.43 The bulk resistance decreases from 412 k (GCD/NO3 − -ISM) to 271 k (GCD/TTF/NO3 − -ISM), 218 k (GCD/TTF(NO3 )(10)/NO3 − 162 ISM), 188 k (GCD/TTF(NO3 )(15)/NO3 − -ISM), k (GCD/TTF(NO3 )(20)/NO3 − -ISM) and to 147 k (GCD/TTF(NO3 )(25)/NO3 − -ISM). This is obviously due to the increasing amount of TTF and TTF(NO3 ) in the PVC membrane, a similar effect was previously observed after adding another electron-conductive material (CNT) to the membrane.44 At low frequency impedance plots obtained for TTF(NO3 )modified electrodes are different from that observed for the electrode with TTF or the coated-disc electrode and low-frequency semicircles for GCD/TTF(NO3 )/NO3 − -ISM are significantly smaller. The low frequency parts of these spectra are due to processes occurring at the interfaces between the aqueous phases and the membrane phase. Since these electrodes have the same ion-selective membrane, it is clearly seen that the presence of NO3 − ions in a mediation layer facilitate the process of ion-to-electron transduction. Low-frequency part in impedance spectra of the GCD/TTF(NO3 )/NO3 − -ISM electrodes was more than 600 times lower than that of the GCD/NO3 − -ISM electrode. In the case of GCD/TTF/NO3 − -ISM electrode such significant change was not observed. This indicate larger capacitance in the GCD/TTF(NO3 )/NO3 − -ISM electrodes. Analytical applications.— In order to demonstrate the feasibility of the proposed GCD/TTF/NO3 − -ISM and GCD/TTF(NO3 )/NO3 − ISM electrodes for environmental analysis, the nitrate contents in ground water, well water, tap water, and river samples were detected by the direct potentiometry. The ion concentrations in water were measured as soon as collected. Analysis was conducted using a calibration curve method. The spectrophotometric method

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Figure 8. Impedance plots of a GCD (a), GCD/TTF (b) and GCD/TTF(NO3 )(25) (c) electrode recorded in 0.1 M KNO3 at the open circuit potential. The frequency range was 100 kHz–10 mHz. Eac = 10 mV.

was the control method for the determination of nitrate in water. As shown in Table V, the concentrations of NO3 − determined by the proposed NO3 − -ISEs are in good agreement with those obtained by spectrophotometry. Conclusions

Figure 9. Impedance plots of a GCD/TTF(NO3 )(25)/NO3 − -ISM (×), GCD/TTF(NO3 )(20)/NO3 − -ISM (), GCD/TTF(NO3 )(15)/NO3 − -ISM (), GCD/TTF(NO3 )(10)/NO3 − -ISM (♦), GCD/TTF/NO3 − -ISM (∇) and GCD/NO3 − -ISM (◦) recorded in 0.1 M KNO3 .

This study presents that the organic material (TTF) and its ionradical salt (TTF(NO3 )) can be used in solid-state ion-selective electrodes prepared with the use of simple drop-casting procedure. The constructed electrodes showed a Nernstian slope (−59.36 mV/decade of nitrate concentrations), low detection limit (0.63 μM) and short response time. What is important, modification of the solid-contact electrodes using TTF or TTF(NO3 ) greatly improves the selectivity of sensors. Additionally the presence of the TTF and TTF(NO3 ) intermediate layer significantly reduces the resistance of membrane and improves potential stability compared to coated wire electrode. Ionto-electron transduction between the glassy carbon and ion-selective membrane has been improved. The above-mentioned results clearly show that the introduction of the TTF or TTF(NO3 ) in ISEs improves their analytical parameters. Therefore the TTF or TTF(NO3 ) -based ISEs could be used as an effective tool for monitoring nitrate in environmental samples.

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Table IV. Electrical parameters of studied sensors calculated according equivalent circuit. Rs uncompensated series resistance, Rb bulk resistance, Cb bulk capacitance, CPE constant phase element (Y0 initial value for the admittance for the CPE element and N- parameter showing how well CPE is similar to ideal capacitance, if N = 1 then CPE is an ideal capacitance), W Warburg impedance, Rct charge-transfer resistance, Cdl double-layer capacitance.

Electrode

Rs (k)

Rct (k)

GCD

1.14

366

GCD/TTF

1.42

137

GCD/TTF(NO3 )(25)

1.16

2.25

GCD/NO3 − -ISM

−40

431 000

GCD/TTF/NO3 − -ISM

−38

3180

GCD/TTF(NO3 )(5)/NO3 − -ISM

−40

1020

GCD/TTF(NO3 )(10)/NO3 − -ISM

−45

978

GCD/TTF(NO3 )(15)/NO3 − -ISM

−42

919

GCD/TTF(NO3 )(20)/NO3 − -ISM

−42

833

GCD/TTF(NO3 )(25)/NO3 − -ISM

−12

713

Table V. Comparison of the results obtained by the proposed sensor and spectrophotometry for determination of nitrate in water samples (average value of three determinations ± standard deviation). Nitrate concentration ± SD (mgL−1 ) Sample

TTF/ NO3 − -ISM

TTF(NO3 )/ NO3 − -ISM

Spectrophotometry

Tap water 1 Tap water 2 River water Ground water Well water

1.01 ± 0.01 1.10 ± 0.02 15.45 ± 0.03 8.01 ± 0.02 9.41 ± 0.01

0.99 ± 0.01 1.12 ± 0.01 15.47 ± 0.02 8.10 ± 0.02 9.42 ± 0.01

1.00 ± 0.03 1.13 ± 0.03 16.15 ± 0.07 8.18 ± 0.04 9.45 ± 0.03

Acknowledgment This work was supported by AGH University of Science and Technology grant (Project No. 11.11.160.799).

Cdl (μF) 1.CPE Y0 (N) 3.31 50.5 (0.994) 864 (0.997) 0.036 (0.998) 2.19 (0.997) 16.3 (0.990) 17.3 (0.990) 18.1 (0.994) 19.09 (0.990) 22.3 (0.992)

15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33.

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34.

35. 36. 37. 38. 39. 40. 41. 42. 43. 44.

Rb (k)

Cb (pF) 2.CPE Y0 (N)

3.CPE (μF) Y0 (N)

-

-

1.01 (0.731)

-

-

53.1 (0.510)

-

-

-

412

10.1 10.8 9.8 (0.998) -

-

271 241 218

188 162 147

9.9 (0.998) 10.8 (0.998) 11.38 (0.998) 13.9 -

-

-

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