An ion-selective membrane is an essential element in the construction of ion-selective electrodes. In this work, some investiga- tions concerned on influence of an ion-selective membrane composition on sodium-selective electrode properties are presented. Several coated-disc sodium-selective electrodes were prepared with the use of sodium ionophore III or sodium ionophore VI, or mixture of these two selective carriers. Membranes with the same compositions were also used to obtain sensors with solid contact transducer layer consisted of 7,7,8,8-tetracyanoquinodimethane (TCNQ) and its sodium radical salt (NaTCNQ). In other polymeric membranes, a whole or some part of ionophore was replaced by NaTCNQ in order to assess the effect of its presence on sodium single-piece electrode characteristics. All kinds of prepared electrodes were tested using potentiometric and chronopotentiometric studies, and significant impact of membrane composition on electrode parameters was observed. Among investigated sensors, TCNQ/NaTCNQ-contacted all-solid-state electrodes exhibited the best analytical and electrical performance due to the presence of the intermediate layer, which simplifies the ion-to-electron transduction process between the ion-sensing membrane and the glassy carbon electrode. Exemplary, these electrodes with sodium ionophore III had a close-to- −5 −1 –5.2 Nernstian slope (58.63 mV/pNa) in the range from 10 –10 -M NaCl and revealed detection limit of 10 M. However, the mixing of sodium ionophores III and VI in one polymeric membrane led to improved sensitivity and limit of detection –5.4 (58.79 mV/pNa and 10 M, respectively). The highest capacitance observed for electrodes with TCNQ/NaTCNQ based on intermediate layer was 139 μF. . . . Keywords Ionophore Selectivity change Sodium-selective electrode 7,7,8,8-Tetracyanoquinodimethane Introduction compound with them. During complexation process, ions are transported between the water phase (sample) and the organic The most important element of an ion-selective electrode phase (membrane). This phenomenon enables to notice the (ISE) is an ion-selective membrane (ISM) responsible for potential change at the membrane/sample interface, which forming the potentiometric response. A typical cation- forms grounds for potentiometric measurements. However, selective liquid membrane consists of an ionophore, lipophilic the rest of membrane ingredients affect the formation of the salt, water-immiscible plasticizer, and a polymer matrix. A life complex compound as well, thus the membrane composition span of electrode depends largely on the lipophilicity of the should be optimized [1, 2]. Nevertheless, the ionophore has membrane, because during its use, the slow elution of mem- the greatest influence on the binding of primary or interfering brane components into aqueous solution is observed . ions, thereby defining the electrode selectivity, which is one of The ionophore, which is the carrier of the ions, selectively the most important sensor parameter [3, 4]. and reversibly binds the determined ions forming a complex In conventional ISEs, the sensing membrane is placed at the end of a plastic tube filled with an internal solution and inner reference electrode is immersed therein. However, such electrodes with liquid contact have some inconveniences that * Beata Paczosa-Bator firstname.lastname@example.org limit their practical use and render impossible applications in mobile analytical systems. The polymer-based membrane Faculty of Materials Science and Ceramics, AGH University of separates sample solution from the internal reference system, Science and Technology, Mickiewicza 30, thus differences in the ionic strength between the sample and PL-30059 Krakow, Poland Ionics the inner filling can result in the osmotic pressure and water electron acceptor and may form radical anion salts and charge transport between these solutions, and in consequence, signal transfer complexes. One of the most interesting properties of interferences. Moreover, conventional ISEs have to be fabri- such compounds is high electrical conductivity . cated and used with care, and their miniaturization is restricted Therefore, TCNQ and its compounds have been successfully due to the indispensable liquid contact volume. These disad- applied in electrochemical sensors with voltammetric  vantages have been overcome by replacing conventional ISEs or potentiometric detection . Herein, the effect of with all-solid-state ion-selective electrodes (ASS-ISEs). In TCNQ/NaTCNQ presence was tested with the use of two this group of potentiometric sensors, liquid contact was sodium-selective ionophores, which were applied sepa- substituted by solid contact (SC) layer placed between the rately, as well as simultaneously in one polymeric mem- ISM and an electrical substrate. Such construction comes to brane solution. Potentiometric and chronopotentiometric meet the demand for ISE application in various fields due measurements were carried out in order to evaluate ana- to ability of miniaturization, easy maintenance, low cost lytical and electrical parameters of studied sensors, respec- of production and analysis processes, and small sample tively. Infrared (IR) spectroscopic studies were performed volume [5, 6]. to characterize prepared ISMs. Materials that are used as effective ion-to-electron transducers should be characterized by a high redox or a double-layer capacitance [7, 8]. There are many examples Experimental of conducting polymer (CP) [9–12], metal nanoparticle , or carbon nanomaterial (CNM) [14–19] applications Materials as SC layers. Noteworthy is also the intermediate layer consisting of a redox buffer based on the Co(III) and TCNQ 98%, sodium ionophore III (N,N,N′,N′-tetracyclohexyl- Co(II) complexes of 1,10-phenanthroline, the use of which 1,2-phenylene-dioxyacetamide, ETH 2120), sodium ionophore allowed to obtain high reproducibility of the ISEs potential VI (bis[(12-crown-4)methyl]dodecylmethylmalonate), potassi- . Although CPs generally improve sensor sensitivity, lin- um tetrakis(p-chlorophenyl)borate (KTpClPB), sodium ear range, and detection limit, they suffer from light, gases, or tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (NaTFPB), sodi- pH interferences . Introduction of nanomaterial-based SC um tetraphenylborate (NaTPB), o-nitrophenyl octyl ether (o- significantly changes for the better not only analytical but also NPOE), poly(vinyl chloride) (PVC) of high molecular weight, electrical parameters of electrodes and especially stability of tetrahydrofuran (THF), methanol, and acetone were the the potentiometric response [13–19]. This desirable effect is selectophore reagents purchased from Sigma-Aldrich. All other caused by the unique properties of nanomaterials, e.g., high chemicals used were of analytical-reagent grade. Distilled and hydrophobicity, good conductivity, and large surface area. deionized water was used to prepare the aqueous solutions. The high double-layer capacitance of the polymeric NaTCNQ was obtained by chemical synthesis carried out ac- membrane/nanomaterial based on layer interface increases cording to the method proposed by Melby et al. . stability of the measured response, electrodes reproducibility, and capability for long-term use . Despite of influence on Electrode preparation sensor characteristics, the application of CNMs leaves elec- trode selectivity unchanged. For instance, potassium sensors In order to assess the impact of NaTCNQ presence and mem- modified with platinum nanoparticles (PtNPs) , carbon brane composition on the parameters of sodium sensors, four black (CB) , or CB supporting platinum nanoparticle groups of electrodes were prepared: coated disc electrodes (CB-PtNP)  intermediate layer, presented similar values (CD-ISEs), solid-state single-piece electrodes, all-solid-state of selectivity coefficients. Although the dissolution of the CP electrodes, and ionophore-free solid-state single-piece elec- in ISM in the solid-state single-piece electrode (SP-ISEs) can trodes. All studied electrodes were obtained applying a drop- change electrode selectivity , the CP-contacted potentio- casting method by dropping the appropriate membrane solu- metric sensor shows selectivity close to conventional ISEs or tion onto bare or modified electrodes surface. In the first stage, slightly better . Nevertheless, the application of ionic liq- the glassy carbon disc (GCD) electrodes (Mineral, Poland) uids as polymer-sensing membrane additives or SC elements were carefully polished using 0.3-μm alumina slurry. Then [23, 24] or recently reported combination of organic crystals electrodes were sonicated several times with water and meth- with their radical salt in SC layer [25–27] positively affects the anol and in the end rinsed with intense water stream. All selectivity of ISEs. studied electrodes were prepared using the same GCD sub- In order to shed the light on the last group, in this work, the strate electrodes consisting of GC rods enveloped in PEEK influence of 7,7,8,8-tetracyanoquinodimethane (TCNQ) and (polyetheretherketone) bodies (GC area = 7 mm ). its sodium salt (NaTCNQ) application on single-piece and Next, above-mentioned groups of electrodes were prepared all-solid-state ISE parameters is studied. TCNQ is strong π- and three various membrane compositions were proposed for Ionics each group. Two different sodium ionophores or their mixture the picture. After coverage of electrodes with the ISM, they were used to prepare coated disc electrodes by dropping suit- were left to complete solvent evaporation at room temperature able membrane solution onto the bare GCD surface. In the for 24 h. Three identical electrodes of each kind were prepared second group, membrane cocktails contained sodium iono- and investigated. Before every measurement, sensors were phore (III, VI or III, and VI), NaTCNQ, lipophilic salt, plasti- conditioned in 0.01-M NaCl solution for 1 day. All electrodes cizer, and PVC. NaTCNQ was added to ISMs as a supplemen- were separately stored and conditioned. tary ingredient in order to obtain solid-state single-piece elec- trodes. The mass percentages of ionophores in these mem- Apparatus branes were smaller than in preceding membranes due to the presence of NaTCNQ with unchanged content of lipophilic The measurements of the electromotive force (EMF) of an salt. Similarly to the previous group, sodium-selective mem- indicator electrode, a reference electrode (Ag/AgCl/3-M KCl brane was deposited on unmodified glassy carbon electrodes. electrode type 6.0733.100-Ω Metrohm, Switzerland), and an Other sensors—all-solid-state sodium-selective electrodes auxiliary electrode (platinum electrode) cell were performed were prepared with the use of primary membrane solutions with a 16-channel mV-meter (Lawson Labs., Inc., Malvern, (applied for the group of CD-ISEs) but dropped onto the SC PA). The activity coefficients necessary to convert the concen- transducer layer, which were obtained by casting GCD surface trations to activities were calculated according to the extended with a 15 μL of TCNQ/NaTCNQ acetone suspension. And Debye-Hückel equation . finally in the last group of sensors (ionophore-free solid-state The chronopotentiometry measurements were carried out single-piece electrodes), no ionophore was used to prepare using an Autolab General Purpose Electrochemical System membrane solution, but only NaTCNQ and different lipophil- (AUT302N.FRA-2-AUTOLAB, Eco Chemie, ic salts were added to polymer matrix and plasticizer. As for The Netherlands) connected to a one-compartment, three elec- the other SP-ISEs and CD-ISEs, no intermediate layer was trode cell. Tested ISE was used as a working electrode. An Ag/ introduced and membrane was dropped directly on the GCD AgCl/3-M KCl electrode type 6.0733.100-Ω Metrohm, electrodes. All precisely defined membrane compositions and Switzerland was connected as the reference electrode and a electrode symbols are listed in Table 1. glassy carbon rod as the auxiliary one. The bare GCD electrodes or GCD electrodes modified with The composition of the ISMs applied in studied sensors TCNQ/NaTCNQ layer were twice coated using 30-μLTHF was investigated by IR spectroscopic studies. Spectra of solutions of membrane components. The process of preparing all types of membranes were measured on a Bruker sensors with sodium ionophores is schematically shown in VERTEX 70 v vacuum FT-IR spectrometer using attenuated Fig. 1. Ionophore-free single-piece electrodes were prepared total reflectance (ATR) technique. They were collected in the −1 in the same way as shown for SP-ISEs, but using membrane mid-infrared (MIR) region (4000–500 cm ) after 128 scans at −1 solutions without ionophores and therefore were omitted in 4-cm resolution. Table 1 Composition of studied polymer-based membranes No electrode SC Membrane composition (wt%) Ionophore NaTCNQ Ionic additive o-NPOE PVC III VI NaTFPB KTpClPB NaTPB CD-ISEs 1a − 1.35 –– 1.15 –– 66.50 31 1b − – 1.62 – 1.15 –– 66.23 31 1c − 0.67 0.81 – 1.15 –– 66.37 31 SP-ISEs 2a − 0.67 – 0.28 1.15 –– 66.90 31 2b − – 0.81 0.28 1.15 –– 66.76 31 2c − 0.45 0.54 0.18 1.15 –– 66.68 31 ASS-ISEs 3a + 1.35 –– 1.15 –– 66.50 31 3b + – 1.62 – 1.15 –– 66.23 31 3c + 0.67 0.81 – 1.15 –– 66.37 31 ionophore-free 4a − –– 1.3 0.65 –– 65.05 33 SP-ISEs 4b − –– 1.3 – 0.65 – 65.05 33 4c − –– 1.3 –– 0.65 65.05 33 Ionics Fig. 1 Schematic illustration of studied sodium-selective electrode preparation using drop casting method (CD-ISEs— coated disc ion-selective electrodes, ASS-ISEs—all-solid- state ion-selective electrodes, SP- ISEs—single-piece ion-selective electrodes) Results and discussion hence, only the spectra for membranes removed from the electrodes 1a, 2a, 3a, and 4a are shown. Mid-infrared spectroscopic studies of ion-selective Comparison and analysis of the presented spectra do not membranes indicate any significant differences between them except the spectrum 3a collected for the membrane taken off from elec- ATR-FTIR spectroscopy was used in this work in order to trode together with TCNQ/NaTCNQ layer. In this case, the verify the composition of all polymeric membranes used to additional vibrational bands specific to TCNQ appeared at −1 prepare studied sodium sensors, and achieved spectra of mem- 2190 and 2159 cm and are associated with nitrile group −1 branes with and without sodium ionophore III are presented in (C☰N) stretching. The presence of band at 823 cm related Fig. 2. The results obtained for membranes with sodium ion- to C〓C▬H bending vibration also confirms the presence of ophore VI as well as the ionophore mixtures are analogous; NaTCNQ . It would be expected that such bands should as well occur for membranes in which sodium salt of TCNQ was added instead of ionophore (2a and 4a). Nonetheless, the amounts of NaTCNQ used in SP-ISEs were much lower than in ASS-ISEs, thus the tested membranes contained much less NaTCNQ than the sample obtained from electrode 3a. Although wherever the NaTCNQ completely replaced the −1 ionophore, slight bands at ca. 2000 cm were observed. Similarly, due to a very low content of ionophore in the polymer matrix, it may not seem possible to deduce explicitly the presence of ETH 2120 individually from the obtained spectra. However, small absorption bands in the range from −1 1620 to 1750 cm for spectra 1a and 3a may be connected with C〓O stretching vibration derived from sodium iono- phore III. And what is more, in the band of C▬Ostretching −1 which appears at ca. 1164 cm associated with the presence of plasticizer (o-NPOE) in the membrane matrix, additional −1 shoulder is appeared at 1125 cm but only for spectra 1a, 2a, and 3a, i.e., samples containing ETH 2120. Since the iono- Fig. 2 FT-IR analysis of the PVC membranes with (1a, 2a, 3a) and phore molecule has such C▬O bond as well, it can be as- −1 without (4a) ETH 2120 in the range of 4000–500 cm (the range −1 sumed that this shoulder originated from sodium ionophore between 4000 and 3500 cm was omitted in the picture due to lack of bands) III (ETH 2120). Ionics It is noteworthy to mention that a small amount of lipophilic Na activity, standard potential values, and detection limits salt in investigated membranes and overlapping bands make evaluated as the intersection of the two slope lines are listed difficult a specific description the bands related to NaTFPB in Table 2. Three identical sensors of each type were prepared, presence. Both lipophilic salt and plasticizer showed the com- so the electrode-to-electrode reproducibility of analytical pa- mon bands derived from C〓C aromatic stretching vibration at rameters was also investigated and presented as a standard −1 ca. 1607 and 1581 cm . On the other hand, the presence of o- deviation (SD). NPOE in samples is confirmed unequivocally by the strong The sensor sensitivity corresponding to the slope of the −1 stretching N▬O bands observed at 1523 cm . Moreover, plas- calibration curve was dependent on the type of electrode. ticizer and PVC reveal the common aliphatic stretch bands of Taking into account the first group of electrodes, it can be −1 C▬H at 2925 and 2855 cm and band related to C▬Hbend- noted that mixing of two ionophores (1c) allowed to obtain −1 0 inginthe rangefrom1350to1250cm , though the presence sensors with better sensitivity and E potential stability than those −1 of C▬Cl stretching vibration in 700 to 600-cm region with ionophores used separately (1a and 1b). Nevertheless, coat- definitely indicates the matrix made of poly(vinyl chloride). ed disc electrodes behaved quite poorly due to lack of interme- diate layer with ionic and electronic conductivity. However, re- placing some amount of ionophore by NaTCNQ salt in the mem- Analytical and electrical parameters of sensors brane composition improved the reproducibility of the elec- trodes. In the case of membrane with ionophore III, and iono- The potentiometric response of prepared electrodes was ex- −6 amined in the sodium ion concentration range from 10 to phore III mixed with VI (2a and 2c), the SP-ISE sensitivity was −1 þ also improved compared to coated-disc electrodes (without 10 M. The dependency of measured potential on loga for Na all studied electrodes is presented in Fig. 3. Some significant NaTCNQ, 1a and 1c, respectively) in the same range of linearity. Electrodes with solid-contact transducer layer exhibited the best differences in electrode behavior can be observed. The obtain- analytical parameters compared to the other sensors due to the ed calibration plot slope values calculated for linear range of Fig. 3 The potentiometric responses of each type of prepared electrodes tested sensors (1—CD-ISEs, 2—SP-ISEs, 3—ASS-ISEs, 4—ionophore- recorded in sodium chloride solutions after 72 h conditioning in 0.01-M free SP-ISEs), while the particular electrodes are marked as: ■ (a), ● (b), and ▲ (c) NaCl. The chart number is simultaneously the number of the group of Ionics Table 2 Metrological parameters of all studied electrodes determined after 72-h conditioning time (number of electrodes n =3) Electrode Parameter ± SD 0 −1 Slope (mV/dec) E (mV) Linear range (M) Detection limit (M) R (kΩ) dE /dt (μVs ) C (μF) total dc –4.5 −1 –5.1 ± 0.1 CD-ISEs 1a 56.97 ± 0.64 371 ± 16 10 –10 10 1177 ± 56 738 ± 63 1.36 ± 0.12 –4.5 −1 –4.8 ± 0.2 1b 57.78 ± 0.77 409 ± 20 10 –10 10 780 ± 15 832 ± 78 1.21 ± 0.13 –4.5 −1 –4.9 ± 0.1 1c 57.90 ± 0.34 401 ± 11 10 –10 10 1053 ± 31 785 ± 38 1.28 ± 0.06 –4.5 −1 –5.0 ± 0.2 SP-ISEs 2a 58.41 ± 0.11 316 ± 12 10 –10 10 774 ± 45 169 ± 11 5.95 ± 0.40 –4.5 −1 –4.7 ± 0.1 2b 57.16 ± 0.07 341 ± 15 10 –10 10 850 ± 11 593 ± 63 1.70 ± 0.18 –4.5 −1 –4.9 ± 0.1 2c 58.19 ± 0.19 326 ± 3 10 –10 10 1319 ± 8 305 ± 46 3.28 ± 0.14 –5.0 −1 –5.2 ± 0.1 ASS-ISEs 3a 58.63 ± 0.05 262.1 ± 0.5 10 –10 10 328 ± 8 7.24 ± 0.57 139 ± 11 –5.0 −1 –5.3 ± 0.1 3b 58.77 ± 0.09 233.6 ± 0.6 10 –10 10 424 ± 13 16.1 ± 1.0 62.3 ± 3.9 –5.0 −1 –5.4 ± 0.1 3c 58.79 ± 0.15 253.5 ± 0.4 10 –10 10 254 ± 7 13.5 ± 1.0 74.7 ± 5.7 –4.0 −1 –4.7 ± 0.2 ionophore-free SP-ISEs 4a 51.03 ± 0.59 193 ± 41 10 –10 10 1912 ± 98 853 ± 53 1.18 ± 0.07 –4.5 −1 –5.1 ± 0.1 4b 54.33 ± 0.55 283 ± 32 10 –10 10 859 ± 37 800 ± 40 1.25 ± 0.06 –3.0 −1 –3.7 ± 0.2 4c 49.11 ± 2.24 238 ± 43 10 –10 10 3562 ± 223 1322 ± 56 0.76 ± 0.03 presence of TCNQ/NaTCNQ intermediate layer and good- Na activity. For other electrodes, the linear range has been defined phase boundary potential. Analytical parameters of other shortened, especially for sensors denoted as 4c. Here, the previously reported sodium-selective electrodes with ion-to- membrane was consisted on PVC, o-NPOE, NaTCNQ, and electron transducer layer are presented in Table 3. It can NaTPB. As it was shown in , if the membrane solution be observed, that TCNQ/NaTCNQ-contacted electrodes with similar composition is dropped directly onto glassy car- showed comparable parameters to earlier presented sodium bon substrate, the leaching of the lipophilic salt is observed sensors. The SD of E represents the reproducibility from resulting in the decline of potentiometric response. The diver- electrode to electrode. It is worth noting that the studied sity in repeatability of the other sensors is also worth empha- ASS-ISEs were characterized by the comparable reproducibility sizing. As expected, the coated disc electrodes were charac- as sensors modified with Co(II)/Co(III) redox buffer layer  terized with the poorest stability due to the lack of reversible or CNMs . Considering the ionophore-free membranes, transfer of ion and electron across the Bblocked^ interface electrodes with KTpClPB showed the best characteristics and between the ISM and glassy carbon electrode. In the first electrodes with NaTPB the worst. group of electrodes, the smallest deviation from the standard On the one hand, it seems that all studied sensors have potential value after this time was observed for sensor 1c with similar properties; nevertheless, distinct differences between two ionophores-based membrane and it was equal 9.6 mV, them have been observed only during long conditioning time. whereas for electrode 1a and 1b, 17.0 and 28.2 mV, respec- Exemplary changes in electrode response are shown in Fig. 4 tively. The standard potential values of electrodes with mem- (one sensor was selected from each tested group). After a brane where some part or all of the ionophore was replaced by month of soaking in 0.01-M NaCl solution, only 3a, 3b, and NaTCNQ were changed, and SDs of E were equal to 15.0 3c sensors still presented a linear response in the same range of (2a), 20.1 (2b), 8.3 (2c), and 15.1 (4a), 11.3 (4b), and 39.5 mV Table 3 Characteristic of various reported Na -selective solid contact electrodes Ref. Sodium ionophore Intermediate layer Slope (mV/dec) Linear range (M) Detection limit (M) −5 −1  ETH 227 Poly(pyrrole) 58.3 2 · 10 –6·10 − −6 −1 −6.3 IV TCNQ 58.68 10 –10 10 –5.8 −1 −6.1 NaTCNQ 58.78 10 –10 10 –4.5 −1 −5.1  IV TCNQ:NaTCNQ:CB (1:1:1) 58.08 10 –10 10 −6 −1 −6.2 TCNQ:NaTCNQ:CB 59.10 10 –10 10 (1:0.2:0.8) −5 −1 −6  IV CB-Printex XE2-B 57.10 10 –10 9·10 −6 −1 −6.2  ETH 2120 CB-Printex U 59.14 10 –10 10 −6 −5 −5.5 Poly(N-methylpyrrole) 58.54 10 –10 10 Ionics Fig. 4 EMF dependence on Na activities for all tested electrodes with of the group of tested sensors (1—CD-ISEs, 2—SP-ISEs, 3—ASS-ISEs, mean potential values and standard deviations recorded over 1 month of 4—ionophore-free SP-ISEs), while the particular electrodes are marked electrode conditioning. The chart number is simultaneously the number as: ■ (a), ● (b), and ▲ (c) (4c) after 1 month of conditioning in 0.01-M NaCl. The most is consistent with those previously reported for potentiometric repeatable potentiometric response was observed for sensors sensors modified with organic crystals [25, 27]. with TCNQ/NaTCNQ intermediate layer and at the same time All prepared sensors were also investigated using current- E values alerted only of 2.3, 1.4, and 3.1 mV for 3a, 3b, and reversal chronopotentiometry measurements in order to evaluate 3c electrodes, respectively. One of the reasons for the instability of electrode potential in time may be the formation of a thin water layer between the polymeric membrane and the underlying substrate, and, as a consequence membrane detachment. Possible presence of such undesirable aqueous film was assessed by potentiometric water layer test . As it was shown in Fig. 5, first the electrode response was recorded during conditioning in 0.01-M NaCl solution. After 2.5-h, solution was changed to 0.01-M KCl and potential of electrodes was measured for the same period of time. Then solution of interfering ion was replaced by solution of primary ion again. In the case of coated disc electrode (1a), the observed potential drift confirmed the formation of water layer at the Na -ISM/GCD interface. Under the same experimental conditions, SP-ISE (2a) present- ed more stable potential, but the best characteristic was achieved for ASS-ISE (3a). Such result demonstrated that Fig. 5 Water layer test determined for 1a (CD-ISE), 2a (SP-ISE), and 3a the presence of NaTCNQ suppressed the water layer in exam- (ASS-ISE) electrodes in 0.01-M NaCl and 0.01-M KCl solution. For clarity, the response curves have been shifted in relation to each other ined sensors due to its hydrophobic character. Observed effect Ionics the electric characteristic of electrodes . The potential re- The best potential stability and the highest electrode capaci- sponses of electrodes recorded during polarization by applying tance were measured for TCNQ/NaTCNQ-contacted sensors. acurrent (I)of+1nAfor 60sand – 1 nA over another 60 s are As regards capacitance, these estimated values are slightly presented in Fig. 6. The chronopotentiometric tests were carried better or similar to those reported for carbon nanotubes out in 0.01-M NaCl solution and allowed to determine the elec- (60 μF) , CB (51 μF) , PtNPs (82 μF) , and trical parameters of sensors. The total resistance of electrode graphene (83 μF) , but lower than those obtained for CP R = ΔE /2I was calculated based on the signal jump (204 μF)  or colloid-imprinted mesoporous carbon total dc (ΔE ) observed in the potential response after the change of (1.0 mF)  used as intermediate layer. Nevertheless, dc current polarity. The potential drift of electrode was derived from TCNQ and its radical sodium salt may act as redox reaction ΔE /Δt ratio and was related with the capacitance C according transducer. The resulting capacitances were comparable to dc to ΔE /Δt = I / C equation. All electric parameters of developed those previously reported for sensors modified with TCNQ dc electrodes calculated using chronopotentiometric studies are also or NaTCNQ separately (154 and 132 μF, respectively) . presented in Table 2. As expected, all-solid-state electrodes exhibited the Selectivity smallest resistance compared to other tested electrodes, thus the presence of SC transducer facilitated sodium ions trans- Undoubtedly, one of the most important properties of ISEs port. In turn, in the case of non-ionophore membranes, the is the ability to react only to the chosen ion despite of the highest resistances were detected, due to the absence of com- presence of other accompanying ions. Selectivity describes pounds forming sodium complexes in PVC matrix. Slight the attractiveness of the practical usage of ISE and its capability differences in (1a, 1b, 1c and 2a, 2b, 2c) sensor capacitance, to distinguish primary ion from other interfering ions and is respectively, were caused by replacement the certain amount expressed by the potentiometric selectivity coefficient. Low of ionophore by small quantities of NaTCNQ conductive salt. values of selectivity coefficient are the most advantageous, Fig. 6 Exemplary chronopotentiograms for tested electrodes recorded in tested sensors (1—CD-ISEs, 2—SP-ISEs, 3—ASS-ISEs, 4—ionophore- 0.01-M NaCl by applying current + 1 and – 1 nA successively three free SP-ISEs), while the particular electrodes are marked as: □ (a), ○ (b), times. The chart number is simultaneously the number of the group of and Δ (c) Ionics because they indicate better electrode selectivity to the or raised selectivity coefficients compared to those obtained particular ion. for membranes in which these ionophores were used separate- The selectivity coefficients values were calculated using ly. The selectivity coefficients evaluated for the electrode 1c the separate solution method (SSM)  by measuring the are the resultant of those calculated for electrodes 1a and 1b. potentiometric response in the chloride solutions of main cat- Replacing a certain amount of ionophore with NaTCNQ + 2+ 2+ + salt (electrodes 2a, 2b, and 2c) caused a worsening of selec- ion (i:Na ) and interfering cations (j:Ca ,Mg ,Li ,NH , tivity. The values of selectivity coefficients increased in rela- K ). Response functions were extrapolated to the activity of tion to these calculated for the corresponding electrodes from ions equal to 1 M (a = a = 1 M). The obtained results present- i j ed in Fig. 7 indicate that the membrane composition, kind of the first group. Such change in selectivity behavior can be caused by insufficient quantity of selective carriers in ISM ionophore, as well as the introduction of NaTCNQ as mem- then the process of sodium ion-ionophore complex formation brane or SC component affects the sensors selectivity. takes place on a smaller scale. Taking into account the electrodes covered with mem- The introduction of TCNQ/NaTCNQ SC layer led to a branes not containing the ionophore, but only NaTCNQ and reduction of the selectivity coefficient values compared to lipophilic salt, it can be assumed that in the case of ions carrier other groups of sensors. In this case, the improvement of se- absence the lipophilic component acts as ion exchanger . lectivity may be due to the properties of intermediate layer. Due to its ion-exchange properties, the lipophilic salt causes The applied SC can accumulate and release primary ions and the extraction of all cations between solution and membrane therefore can inhibit the flow of primary ions from the mem- phases. Merely results determined for electrode 4a are shown brane into interfering ions solution. The presence of sodium in the Fig. 7 due to the usage of NaTFPB in ionophore-based membranes also, however, similar characteristics were ob- ions at the sample/membrane interface allows an undisturbed ion exchange process with interfering ion participation to be served for the remaining lipophilic salts. The obtained selec- observed. This way, the potentiometric response in such sam- tivity coefficients sequence for each of these electrodes was + þ + + 2+ 2+ ples is not overvalued, which significantly influences the se- K >NH >Na >Li >Ca >Mg , and this order is in a lectivity behavior. Similar dependence was previously ob- good agreement with the Hofmeister series . served for sodium sensors modified with SC layer based on Only the addition of the ionophore to the ISM causes dis- CNMs mixed with TCNQ and NaTCNQ and covered with tinct differences in electrode selectivity. Electrode behavior ISM-containing sodium ionophore IV . depends on the type of ionophore used, therefore 1a and 1b TCNQ/NaTCNQ-contacted electrodes showed the most re- sensors exhibited diversity of selectivity coefficients values producible potentiometric response during selectivity mea- [3, 4]. High sensitivity of 1a electrode to calcium ions can surements. The SD of the standard potential of electrodes be explained by the fact that ETH 2120 added to membrane determined on the basis of the calibration performed in NaCl in which o-NPOE is used as plasticizer shows a clear lack of solutions between the successive measurement in interfering selectivity for sodium cations . Nevertheless, such effect ions was 2.2, 2.6, and 1.1 mV for electrodes 3a, 3b, and 3c, was reduced by introducing another sodium ions carrier to this respectively. Whereas, the same parameter obtained under the ISM. Mixing of two ionophores in one ISM solution reduced Fig. 7 The selectivity coefficients pot log K of investigated sensors i; j determined by separate solution method (1a, 1b, and 1c—CD- ISEs; 2a, 2b, and 2c—SP-ISEs; 3a, 3b and 3c—ASS-ISEs; 4a— ionophore-free SP-ISE with NaTFPB) Ionics 3. Maruizumi T, Wegmann D, Suter G, Ammann D, Simon W (1986) same experimental conditions for the remaining sodium sen- Neutral carrier-based Na -selective electrode for application in sors was in turn 14.8 (1a), 16.0 (1b), 10.2 (1c), 11.2 (2a), 12.6 blood serum. Microchim Acta 1:331–336 (2b), and 8.1 mV (2c). 4. Shono T, Okahara M, Ikeda I, Kimura K, Tamura H (1982) Sodium-selective PVC membrane electrodes based on bis(12- crown-4)s. J Electroanal Chem 132:99–105 5. 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TrAC Trends the best characteristics. Such sensors exhibited better detec- Anal Chem 76:102–114 tion limits, more reproducible standard potential values, and 9. Bobacka J, Ivaska A, Lewenstam A (2008) Potentiometric ion sen- sors. Chem Rev 108:329–351 lower potential drift compared to coated disc or single-piece 10. Michalska A, Hulanicki A, Lewenstam A (1997) All-solid-state electrodes. potentiometric sensors for potassium and sodium based on poly(- Nevertheless, type of ionophore influences not only the pyrrole) solid contact. Microchem J 57:59–64 sensor resistance but also capacitance. Elimination of ion car- 11. Bobacka J (1999) Potential stability of all-solid-state ion-selective rier from ISM resulted in the weakest analytical and metro- electrodes using conducting polymers as ion-to-electron transduc- ers. Anal Chem 71:4932–4937 logical parameters of electrodes. Moreover, partially replace- 12. 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Published: May 29, 2018
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