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Introducing cobalt(II) porphyrin / cobalt(III) corrole containing transducers for improved potential reproducibility and performance of allsolid-state ion-selective electrodes Ewa Jaworska, Mario Naitana, Emilia Stelmach, Giuseppe Pomarico, Marcin Wojciechowski, Ewa Bulska, Krzysztof Maksymiuk, Roberto Paolesse, and Agata Michalska Anal. Chem., Just Accepted Manuscript • Publication Date (Web): 08 Jun 2017 Downloaded from http://pubs.acs.org on June 8, 2017

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Analytical Chemistry

Introducing cobalt(II) porphyrin / cobalt(III) corrole containing transducers for improved potential reproducibility and performance of allsolid-state ion-selective electrodes Ewa Jaworska1, Mario L. Naitana2, Emilia Stelmach1, Giuseppe Pomarico2, Marcin Wojciechowski1, Ewa Bulska1, Krzysztof Maksymiuk1, Roberto Paolesse2, Agata Michalska*1 1)

Faculty of Chemistry, University of Warsaw, Pasteura 1, 02-093 Warsaw, Poland

Fax: +48 22 8225996 email: [email protected] 2)

Dipartimento di Scienze e Tecnologie Chimiche, Università di Roma Tor Vergata, Via della

Ricerca Scientifica 1, 00133 Roma, Italy

Abstract A novel solid contact type for all-solid-state ion-selective electrodes is introduced, yielding high stability and reproducibility of potential readings between sensors as well as improved analytical performance. The transducer phase herein proposed takes advantage of the presence of porphyrinoids containing the same metal ion at different oxidation states. In contrast to the traditional approach, the compounds of choice are not a redox pair, although they have different oxidation states, they cannot be electrochemically driven one to another. The compounds of choice were cobalt(II) porphyrin and cobalt(III) corrole – both characterized by a high stability of the coordinated metal ions in their respective redox states and electrical neutrality, as well as relatively high lipophilicity. The porphyrinoids were used together with carbon nanotubes to yield transducer layers for ion-selective electrodes. As a result, we obtained a high stability of potential readings of the resulting ion-selective electrodes together with good reproducibility between different sensor batches. Moreover, advantageously the presence of porphyrinoids in the transducer phase results in improvement of the analytical performance of the sensors: linear response range, selectivity due to interactions with membrane components, resulting in tailoring of ion fluxes through the membrane phase. Thus carbon nanotubes with cobalt(II) porphyrin/ cobalt(III) corrole system are promising alternative for existing transducer systems for potentiometric sensors.

Key words: all-solid-state potentiometric sensor, transducers, porphyrinoids, high potential stability, high potential reproducibility

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All-solid-state potentiometric sensors have been reaching significant research attention for more than thirty years.1,2 Over this time different approaches have been proposed, all aiming to achieve internal solution free sensors of high stability of potential readings, easy to miniaturize, potentially disposable, and ideally calibration free, due to high electrode-toelectrode reproducibility. The research has focused on the transducer, a material that is placed, typically, between an electronically conducting substrate and an ionically conducting membrane to facilitate reversible charge transfer leading to all-solid-state potentiometric sensors. Different transducer systems have been proposed, including hydrogels,3 redox polymers,4 conducting polymers – from relatively hydrophilic oxidized structures like polypyrrole or polyaniline,5,6,7 modified ones,8 to hydrophobic conjugated polymers in neutral state, e.g. poly(3-octylthiophene).9 Nanostructures with special emphasis on carbon materials, including carbon nanotubes,10 three-dimensionally ordered macroporous carbon11 or reduced graphene oxide,12 as well as others systems, e.g. thiols on gold surface13 have also been used as transducers in all-solid-state potentiometric sensors. In recent years carbon nanotubes (CNTs) have reached attention as highly attractive transducer material that can serve as electrical lead in disposable sensors, due to their high electronic conductivity.

14,15,16,17,18,19

Preparation of a CNTs transducer most often requires application of a stabilized CNTs suspension, thus the effect of additives required to prepare these suspensions on the potential readings of resulting sensors needs to be taken into account,19 and carboxymethylcellulose seems to be attractive solution in this respect. Generally it is quite clear that, from the practical point of view, key parameters of transducer layer are high surface area, high redox capacity, high lipophilicity, and some ion-exchange with the membrane phase.1,2 Clearly finding a single material of above properties is not an easy task, opening the possibility of application of systems being in fact a mixture of different chemicals, yielding solid contact of desired properties. It has been observed that the presence of a redox couple at the back side of the membrane, e.g. silver/silver chloride system3 or metallic silver/silver ions compounds,20,21 mixture of hexacyanoferrate(II) and (III) in polypyrrole,6,22 or in the membrane, as in the case of cobalt(II) and cobalt(III) complexes containing redox couples,23,24,25 is beneficial for high stability of potential readings of all-solid-state potentiometric sensors. The advantageous effect is in this case attributed to the presence of an all-solid-state analog of a redox buffer. Thus the redox potential is well defined and a high redox capacity is assured, provided that the concentration of oxidized and reduced form is sufficiently high. The high lipophilicity of the redox system seems to be necessary in order to prevent partitioning of the redox system to 2 ACS Paragon Plus Environment

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the solution.23,24,25 However, the high redox capacity of the transducer phase usually generates considerable ion fluxes through the transducer/ membrane interface, which result in relatively high detection limit of these ion-selective electrodes type, unless the flux it tailored to accumulate the analyte in the transducer. It was previously reported that the increase in stability of potential readings can be also achieved by incorporation of noble metal nanoparticles, as observed for ion-selective electrodes with platinum nanoparticles dispersed in the membrane phase.26 The stabilizing potential influence resulted from both lowered membrane resistance and the effect of oxygen and platinum/platinum oxide redox system presence.26 Nevertheless, this heterogeneous system is probably not optimal for large scale sensors manufacturing process. In this work we propose a novel approach to stabilize and define potential of all-solid-state ion-selective electrodes that can be applicable for classical sensors, as well as potentially disposable low cost solutions. The idea of introducing redox reagents to the transducer phase can be further explored by incorporation to the transducer phase of electrically neutral, stable compounds that can affect the redox potential of the transducer, although they are not complementary as a redox pair. As one compound cannot be transformed to the other one in course of redox reaction, the ion-flux between the membrane and the transducer phase is limited, nevertheless the redox potential is stabilized, e.g. due to formation of two independent irreversible redox system, resulting in mixed potential formation. To the best of our knowledge this is a novel approach that has not been studied yet. One of the possibilities of such approach is application of cobalt porphyrinoids in the transducer phase. Porphyrinoid complexes have been considered in context of ion-selective membrane sensors, however, with the primary interest so far on exploring ionophore properties of these compounds, especially on using them as anion-selective ionophores.27,28,29 To the best of our knowledge these compounds have not been incorporated into the transducer phase to improve stability and reproducibility of sensors potential readings. Thus it can be expected that porphyrinoid complexes can not only stabilize the potential readings, but also help to control ion fluxes through the membrane, leading to improved analytical parameters of sensors. Porphyrins are known to form highly stable and electrically neutral complexes with cobalt(II) ions, whereas corroles form stable, electrically neutral complexes with cobalt(III) ions. Both Co(II)

porphyrin,

[TbuPPCo(II)],

and

e.g.

[5,10,15,20-tetrakis(4-tert-butylphenyl)porphyrinato]cobalt(II)

Co(III)

corrole,

e.g.

(triphenylphosphine)[5,10,15-tris(4-tert-

butylphenyl)corrolato]cobalt(III) [TbuPCCo(III)(PPh3)], are characterized with high stability



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in solid state as well as they support the respective redox state of the cobalt ions in the complexes formed. Moreover, the structure of porphyrinoid complexes results in spontaneous adsorption on the surface of carbon nanotubes – thus the obtained system can be advantageously used as transducer to yield improved ion-selective electrodes. Similarly, as previously discussed for platinum nanoparticles,26 cobalt porphyrins exert catalytic influence on oxygen reduction,30,31 which can also result in potential stabilizing effect. On the other hand, a redox reaction between Co(II) porphyrin and Co(III) corrole, although generally difficult to achieve,32,33 is probable. If it occurs spontaneously, even in extremely small fraction, it can result in a stable mixed potential, leading to stabilized potentiometric response of the sensor. Thus the presence of two compounds mixture in the transducer phase is an attractive alternative to known systems as a method to stabilize and tailor the open circuit potential at the back side of the membrane, ultimately to achieve high reproducibility of potential values acquired from multiple exemplars of nominally the same type of sensor (in the same sample). This approach can be attractive for disposable, calibrationless, sensors, especially those using CNTs as transducer and electrical lead.14,18 The novel approach herein proposed was studied on the example of a model sensor – potassium-selective electrodes with poly(vinyl chloride) membranes using glassy carbon support electrodes.

Experimental Reagents, electrodes and apparatus Reagents and apparatus used were previously described (12,19,26,36). The detail description of used chemicals and apparatus is given in Supplementary information.

[TbuPPCo(II)]/[TbuPCCo(III)(PPh3)] synthesis Synthesis

of

5,10,15,20-tetrakis(4-tert-butylphenyl)porphyrin

and

5,10,15-tris(4-tert-

butylphenyl)corrole and their Co complexes [TbuPPCo(II)] and [TbuPCCo(III)(PPh3)] was carried out according to literature procedures34,35 and is described in details in Supporting Information.

Preparation of all-solid-state electrodes with [TbuPPCo(II)]/[TbuPCCo(III)(PPh3)]MWCNTs transducer layers 4 ACS Paragon Plus Environment

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1.06, 5.01 or 7.8 µmol of [TbuPCCo(III)(PPh3)] and equal amounts of [TbuPPCo(II)] together with 0.9 mg of MWCNTs were dissolved in 600 µL of chloroform. The dispersion was prepared using a tip sonicator. The receptor layers were prepared by drop casting 45 µL (in 9 portions) on the top of glassy carbon electrodes and left for the evaporation of chloroform at room temperature, resulting in 0.08, 0.38 or 0.6 µmol of [TbuPPCo(II)] and [TbuPCCo(III)(PPh3)], and 0.06 mg MWCNTs per electrode, denoted as A, B or C systems, respectively. For

control

experiments

only

one

of

the

porphyrinoids

compounds

(either

[TbuPCCo(III)(PPh3)] or [TbuPPCo(II)]) was applied together with MWCNTs. These suspensions were prepared similarly as described above, but using only one compound corresponding to the amount used to obtain transducer type B. For

control

experiment

the

layer

of

equimolar

mixtures

of

[TbuPPCo(II)]/

[TbuPCCo(III)(PPh3)] (in the absence of MWCNTs) – prepared as used for cyclic voltammetry experiments, was covered with ion-selective membrane and tested under potentiometric conditions.

Ion-selective membranes for classical glassy carbon substrate based all-solid-state ionselective electrodes Potassium-selective, K - ISE, membranes contained (by weight): 1.2% sodium tetrakis[3,5bis(trifluoromethyl)phenyl]borate (NaTFPB), 2.8% valinomycin, 64.3% bis(2-ethylhexyl sebacate) (DOS) and 31.7% poly(vinyl chloride) (PVC). Total 100 mg were dissolved in 1 mL of tetrahydrofuran (THF). 30 µL of the membrane cocktail was applied on the top of electrodes prepared as described above (in 10 µL aliquots), when the electrode was placed in up-side down position and left for 5 h to evaporate THF. Sensors were conditioned overnight before measurements and were stored in-between measurements in 10-3 M KCl solution. The sensors to be tested by Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) were prepared in the same way as all-solid-state sensors tested in potentiometric measurements using glassy carbon substrate electrodes, however the substrate electrodes were of special type to fit the LA-ICP-MS chamber as described in our previous work36 and were conditioned for 16 h in 10-3 M KCl solution and well rinsed before LA-ICPMS measurements.

UV/Vis measurement



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Samples of [TbuPPCo(II)]/[TbuPCCo(III)(PPh3)] suspensions containing MWCNTs type A, B or C were prepared as described above. The obtained suspensions were then centrifuged (18000 rpm) to separate MWCNTs from the mixture. The obtained supernatants were diluted with chloroform (in proportions 10 µL of supernatant to 3 mL of chloroform) and the absorbance of the resulting solutions was measured.

Electrochemical studies of transducer layers Samples of [TbuPPCo(II)]/[TbuPCCo(III)(PPh3)] suspensions containing MWCNTs type A, B or C were prepared as described above. The amounts of mixtures corresponding to the ones used to obtained transducer layers were coated on glassy carbon (GC) electrode (when in the position with GC surface facing up) and left to dry at room atmosphere.

Results and Discussion The main goal of this work was to prepare sensors characterized by stable potential readings and reproducible potential between sensors, using a novel type solid contact transducer - a pair of two different redox reagents (but not a redox couple). The system of choice was composed of equimolar mixture of [TbuPPCo(II)] and [TbuPCCo(III)(PPh3)] together with CNTs. The advantage of herein applied redox reagents lies in the high stability of cobalt(II) and cobalt(III) compounds in respective porphyrinoids, high lipophilicity and electrical neutrality of these compounds, resulting in the formation of a system stabilizing the open circuit potential of the whole sensor. Moreover, the benefits for the performance of ion-selective electrodes can be also expected, taking into account the properties of the porphyrinoids, including cobalt complexes, to interact with anions present in the membrane phase of cation-selective sensors, i.e. anions of lipophilic salt (cation-exchanger).27-29 Thus the porphyrinoids present in the transducer phase, due to solubility in membrane components (plasticizer) and solvent used in membrane cocktail, can be spontaneously transferred to the membrane phase. Thus, they can contribute to tailoring ions fluxes through the membrane due to formation of cation-exchanger gradient in the membrane, ultimately resulting in lower cation exchange at the membrane sample interface. This effect is expected to improve analytical parameters, e.g. sensor detection limits and/or selectivities.


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The effect of [TbuPPCo(II)]/[TbuPCCo(III)(PPh3)] equimolar system incorporation in the MWCNTs transducer layers of all-solid-state sensors was studied using typically glassy carbon supporting electrodes. Potassium-selective electrodes were chosen as model sensors.

Voltammetry of [TbuPPCo(II)] and/or [TbuPCCo(III)(PPh3)]) on MWCNTs Figure S1 presents comparison of cyclic voltammetric curves recorded on GC electrodes with MWCNTs layer in the presence or absence of [TbuPPCo(II)] or [TbuPCCo(III)(PPh3)]. In the absence of cobalt compounds, a typical quasi-capacitive curve was recorded in deoxygenated solution, resulting from MWCNT properties. However, in the presence of dissolved oxygen, a cathodic peak at potential close to -0.5 V and anodic one above 0.9 V were recorded. In the presence of [TbuPPCo(II)] the MWCNT related background current was lower, due to the inhibiting influence of insoluble and non-conducting cobalt containing compound. On the other hand, the cathodic peak corresponding to oxygen reduction was exposed and was slightly higher, compared to MWCNT background level, than in the absence of [TbuPPCo(II)], resulting from catalytic influence of the Co(II) compound. However, in the presence of [TbuPCCo(III)(PPh3)] on the electrode, the currents corresponding both to MWCNTs and oxygen reduction were significantly diminished compared to the electrode without porphyrinoids. This result suggests inhibiting influence of [TbuPCCo(III)(PPh3)] on charge transfer and no catalytic influence on oxygen reduction. On the other hand, in the presence of equimolar amount of porphyrinoid cobalt compounds, [TbuPPCo(II)] and [TbuPCCo(III)(PPh3)], the influence on oxygen reduction was practically the same as in the presence of [TbuPPCo(II)] only, however the background current corresponding to MWCNTs was lower and the reduction peak was shifted to lower potentials, showing inhibiting effect of both cobalt compounds.

Potentiometric responses of all-solid-state potassium selective electrodes For potassium-selective sensors with PVC based membranes and the MWCNTs transducer layer containing [TbuPPCo(II)]-MWCNTs (no corrole added to the transducer phase), the potentiometric responses obtained were characterized with similar potential values (for each concentration tested, potential differences close or smaller than 10 mV) between different (nominally the same) sensors. However, the absolute potential values recorded (vs. Ag/AgCl reference electrode) were relatively high and were reaching 600 mV for 0.1 M KCl solution, Figure S2. The slopes of the potentiometric dependences recorded were lower than Nernstian and close to 50 mV/dec. The detection limit of sensors with MWCNTs and [TbuPPCo(II)] 7 ACS Paragon Plus Environment


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containing transducers was close to 10-5.3 M, slightly higher compared to sensors containing only MWCNTs in the transducer phase.19 It was also observed for these sensors that with consecutive calibrations the potential values recorded for different potassium ions concentrations were gradually increasing. For sensors containing in the transducer phase [TbuPCCo(III)(PPh3)] and MWCNTs (porphyrin free transducer phase) the potential values recorded for different, but nominally similar sensors were also comparable (potential differences close or smaller than 10 mV). On the contrary, the slope of recorded dependences on potential vs. logarithm of aK+ was 54.1 ± 0.3 mV/dec, and the detection limit of the dependence was reaching 10-6.2 M, Figure S2. The potential values recorded were stable in time and the absolute potential values recorded were about 0.2 V mV lower compared to those observed for sensors with transducer phase containing only [TbuPPCo(II)]. The potentiometric responses of all-solid-state potassium ion-selective electrodes with MWCNTs transducer containing both [TbuPPCo(II)] and [TbuPCCo(III)(PPh3)] equimolar mixture, using different absolute amounts of porphyrinoids are shown in Figure 1. It should be stressed that the layers of MWCNTs modified with cobalt(II) porphyrin/cobalt(III) corrole (uncovered with PVC based ion-selective membranes) were insensitive to the change of logarithm of concentration of KCl, KNO3 or NaClO4 within the concentration range from 10-9 to 0.1 M, showing under these experimental conditions neither anionic nor cationic sensitivity (results not shown). As it can be seen in Figure 1 the potentiometric dependences recorded for sensors containing in the transducer phase both [TbuPPCo(II)]/[TbuPCCo(III)(PPh3)] and MWCNTs were characterized by almost Nernstian slopes, being equal to 60.8 ± 1.2 mV/dec (R2 = 0.999); 60.5 ± 0.5 mV/dec (R2 = 0.999); 59.2 ± 2.0 mV/dec (R2 = 0.993) for increasing contents of porphyrinoids in the transducer phase from A to C, respectively. The intercept values, Eo, for the electrodes containing different absolute amounts of porphyrinoids (A, B or C) were equal within the range of experimental error (496.2 ± 6.7 mV, (508.6 ± 5.8 mV, 503.7 ± 8.4 mV, for sensor type A, B and C, respectively), being promising for the preparation of disposable all-solid-state sensors with reproducible Eo. The obtained potential value falls between Eo obtained for sensors with transducer phase containing either [TbuPPCo(II)] or [TbuPCCo(III)(PPh3)] and MWCNTs, discussed above. The change of the amounts of cobalt porphyrinoid complexes in the transducer phase led to a substantial change in the detection limit of the sensors, Figure 1. The all-solid-state ionselective electrode containing the smallest amount of porphyrinoids – MWCNTs mixture in 8 ACS Paragon Plus Environment

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the transducer phase, A, was characterized with detection limit close to 10-6.3 M. The responses obtained for sensor of transducer type A were comparable within the range of experimental error to that of sensor containing MWCNTs in the transducer phase in the absence of porphyrinoids, Figure S2C, however the absolute potential values of sensor containing porphyrinoids in the transducer were shifted about 90 mV to higher potential values, Increase in the amount of porphyrinoids in the transducer phase, B system, led to significant lowering of the detection limit of the sensor to 10-7.6 M. When the amount of [TbuPPCo(II)]/ [TbuPCCo(III)(PPh3)] (1:1 molar ratio) and that of MWCNTs was further increased in C, a lower 10-8.3 M detection limit of the sensor was obtained. It should be stressed that a similar effect was observed for sensors in which different amounts of [TbuPPCo(II)]/ [TbuPCCo(III)(PPh3)] (1:1 molar ratio) without CNTs were placed at the glassy carbon substrate, Figure S2. The sensor of this type containing the same amount of [TbuPPCo(II)]/[TbuPCCo(III)(PPh3)] as used in sensor type A was characterized with 54.9 ± 0.4 mV/dec (R2 = 0.999) close to Nernstian slope and 10-6.1 M detection limit, while the sensor containing the same amounts of porphyrinoids as the sensor type B, yet without MWCNTs, was characterized with 55.7 ± 0.7 mV/dec (R2 = 0.999) close to Nernstian slope and 10-6.9 M detection limit. Interestingly also the Eo values of these (unoptimized) sensors were comparable between each other (potential differences close or smaller than 10 mV), similarly as observed for all-solid-state sensors with MWCNTs and porphyrinoids, too. In accordance with the observed lowering of the potassium ion-selective electrodes detection limit, with herein proposed solid contact, Figure 1, the increase of the amount of porphyrinoids in the MWCNTs containing transducer was leading to the improvement of the obtained selectivity coefficients, Table S1, with the exception of Ca2+ ions, as expected for gradually lowered detection limit of the sensors in range from A to C. The values obtained were well comparable (or improved) compared to MWCNTs used as the transducer (in the absence of porphyrinoids). To explain this effect, additional studies concerning possible incorporation of the lipophilic cobalt compound to the ion-selective membrane were required. These studies were carried out using electrochemical impedance spectroscopy, UV/Vis spectroscopies, and LA-ICP-MS experiments.

EIS and chronopotentiometric results



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Electrochemical impedance spectra (EIS) of all-solid-state potassium sensors with MWCNTs contact containing different amounts of cobalt(II)porphyrin/cobalt(III) corrole, are shown in Figure 2. As it can be seen in Figure 2. the impedance spectra of sensors with transducer type A were different from those typically observed for ion-selective electrodes with MWCNTs transducer.14,18 In addition to single semicircle typical for the presence of PVC based ionselective membrane placed on the MWCNTs transducer19 (MWCNTs layer unmodified with porphyrinoids, results not shown), a second semicircle of smaller diameter was also observed at lower frequencies. The two observed semicircles point out to the presence of another parallel resistance-capacitance system, connected in series to the PVC based ion-selective membrane parallel resistance/capacitance element. The increase of the [TbuPPCo(II)]/ [TbuPCCo(III)(PPh3)]-MWCNTs amount, contact type B, resulted in the increase of the second semicircle diameter, to be comparable with the resistance/capacitance of the ionselective membrane. Further increase of the amount of porphyrinoids present in the system (sensor with transducer type C) resulted in further increase of the diameter of the second semicircle. The low frequency semicircles recorded suggest the presence of an inhibiting step in the charge transfer reaction. This assumption is confirmed by voltammetric curves (Figure S1), showing that the presence of non-conducting cobalt(II) porphyrin or cobalt(III) corrole together with MWCNTs on the electrode surface results in current lowering and charge transfer inhibition. However, although the inhibition of charge transfer reaction is observed, it does not affect adversely the analytical parameters of resulting ion-selective electrodes, since under potentiometric (open circuit) conditions fast charge transfer is not the crucial factor determining the analytical properties of the sensor. Chronopotentiometric studies performed for herein proposed all-solid-state potassium sensors with MWCNTs contact containing different amounts of cobalt(II)porphyrin/cobalt(III) corrole, supplement EIS results and confirm above drawn conclusions. The capacitance of sensors containing equimolar mixture of [TbuPPCo(II)] and [TbuPCCo(III)(PPh3)] together with MWCNTs in transducer phase calculated from experiment performed using anodic and cathodic current 10-8 A, in 0.1 M KCl, was equal to: 1.4.10-5 F, 1.5.10-5 F and 2.4.10-5 F for sensor type A, B and C, respectively. The obtained capacitance values are typical for solid state transducers used in ion-selective electrodes. The resistance determined from chronopotentiometric experiments was equal to: 3.4.106 Ω, 6.2.106 Ω and 7.5.106 Ω, for sensor type A, B and C, respectively, and is consistent with the data obtained from EIS spectra. These resistance values are quite high, pointing to significant inhibition in charge transfer. 10 ACS Paragon Plus Environment

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From the chronopotentiometric experiment performed for sensor containing in transducer phase MWCNTs (in the absence of porphyrinoids in the transducer phase) capacitance and resistance values determined were 2.7.10-5 F and 1.8.106 Ω. These results clearly show that if porphyrinoids, regardless their amount, are used together with MWCNTs as transducer, the capacitance and resistance of the prepared sensors is not altered significantly compared to MWCNTs used without porphyrinoids, the obtained parameters are rather typical for allsolid-state sensors.

UV/Vis spectroscopy and LA-ICP-MS Porphyrinoids compounds mixed with MWCNTs can be non-covalently bound to the CNTs surface, e.g. via π-π stacking interaction, however, the excess of unbound compounds due to solubility in organic solvent of ion-selective membrane cocktail, can partition to the ionselective membrane during its application. As a result, the porphyrinoids present in the membrane can affect ion fluxes through the phase and thus contribute to the observed potentiometric responses. To estimate the amount of porphyrinoids unbound to the MWCNTs, absorption spectra of the supernatant, obtained after the centrifugation of MWCNTs from the mixtures, were recorded, Figure 3. The results shown in Figure 3. clearly suggest that the mixtures applied to form solid contact layer contain unbound [TbuPPCo(II)]/ [TbuPCCo(III)(PPh3)], even in the case of composition A, which potentially can be (at least in part) dissolved in the membrane cocktail, and thus be present at the back side of the ion-selective membrane in the ready sensor. The increase of the porphyrinoids amount mixed with MWCNTs to obtain the transducer results in increased amount of [TbuPPCo(II)]/ [TbuPCCo(III)(PPh3)] not bound to the MWCNTs surface. This effect is especially clear when the amount of porphyrinoids was increased from A to B, while the further increase from B to C has clearly a lower effect on the amount of MWCNTs unbound porphyrinoids present in the sample, Figure 3. Thus the above presented results are in good correlation with potentiometric dependencies presented in Figure 1 and Table S1, where the significant improvement in performance of sensors with transducer type A and B is clearly seen, and smaller improvement in analytical parameters was achieved when the amount of porphyrinoid in the transducer phase was further increased from B to C. The release of porphyrinoids from the transducer phase in ion-selective electrodes clearly results in their partition to the ion-selective membrane. To study this effect LA-ICP-MS experiments were performed.



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As it can be seen in Figure S3, LA-ICP-MS studies of the sensor, i.e. ion-selective membrane coated on the [TbuPPCo(II)]/[TbuPCCo(III)(PPh3)]-MWCNTs layer, revealed increase of cobalt signals for the membrane phase starting from approximately the half of its thickness onwards towards the transducer phase, as well as in the transducer phase. Moreover, increasing amount of [TbuPPCo(II)]/ [TbuPCCo(III)(PPh3)] introduced to the transducer phase (change from contact type A to B) resulted in significant increase in the cobalt signals (originating from [TbuPPCo(II)]/ [TbuPCCo(III)(PPh3)]) in the membrane phase. The further increase of the porphyrinoid contents in the transducer phase, from B to C, had significantly lower effect on the cobalt signals intensities in the membrane phase. This result is in full accordance with UV/Vis absorption results presented in Figure 3. and discussed above. The 59

Co signals recorded for the ion-selective membrane clearly suggest that porphyrinoids are

able to partition to this phase (these compounds are the only source of cobalt in the tested systems). Moreover, as it can be expected taking into account results presented in Figure 3, the amount of [TbuPPCo(II)]/[TbuPCCo(III)(PPh3)] compounds present in the membrane (as confirmed by 59Co counts) increases for increasing amount of porphyrinoids introduced to the transducer phase (i.e. from A to C).

The above presented results of LA-ICP-MS studies and UV/Vis spectrometry, Figure S3 and Figure 3, respectively, suggest that the observed improvement of the all-solid-state sensors potentiometric responses for higher amount of porphyrinoids introduced to the transducer phase

can

be

attributed

to

the

increased

partitioning

of

the

[TbuPPCo(II)]/

[TbuPCCo(III)(PPh3)] to the membrane phase. Both cobalt(II) porphyrin and cobalt(III) corrole are known to interact with anions,27,29 thus it is possible that their presence at the back side of the cation-selective membrane containing neutral ionophore and lipophilic anions of cation-exchanger applied – the only anions that are present in the cation selective membrane results in the flux of ion-exchanger anions towards the back side of the membrane. As a result, the amount of anions close to the sample side of the membrane will be diminished. Assuming equal distribution of neutral ionophore through the membrane phase, due to depletion of the membrane phase close to the sample with ion-exchanger anions (on the favor of increased contents close to the back side of the membrane), the analyte cations contents at the membrane surface interface is expected to be lower compared to the membrane bulk. This in turns will result in lower out-flux of analyte cations from the membrane in the diluted solutions and in lower detections limit as well as improved selectivity of the sensors with increased amount of porphyrinoids present in the membrane phase close to the transducer, e.g. 12 ACS Paragon Plus Environment

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sensors of transducer type B or C. In consequence, the increasing amount of porphyrinoids will lead to improvement of analytical parameters of ion-selective electrodes, as indeed observed, Figure 1. As it can be seen in Figure S3, LA-ICP-MS results clearly show a profound difference in 39K signals intensities, especially close to the membrane/solution interface for sensors type A and sensors type B or C. The

39

K signals intensities are for sensors type B and C similar and

pointing out to gradient of potassium ions towards the inside of the membrane and/or transducer phase, i.e. to the regions where higher contents of porphyrinoids is observed. Moreover, the outermost part of the membrane is characterized with low contents of potassium ions, which is in-line with observed improvement of performance of these sensors, and limited ion-exchange achieved on the membrane/ sample interface. To the best of our knowledge achievement of this effect without instrumental control was not reported earlier for fully conditioned sensors. On the contrary, for sensor type A potassium signal intensities were similar throughout the membrane phase, despite the same pretreatment of all sensors tested using LA-ICP-MS approach. Finally, the observed improvement of analytical parameters for all-solid-state sensors type B and C can be attributed to the presence of [TbuPPCo(II)]/ [TbuPCCo(III)(PPh3)]-MWCNTs in the transducer phase and spontaneous partition of porphyrinoids to the membrane, resulting in their presence close to the back side of the membrane. To the best of our knowledge this effect has not been previously reported.

Potential stability and reproducibility The previous studies19 point out that the long-term stability of all-solid-state sensors is related to the properties of the transducer layer, such as water contact angle. For this reason the contact angle of prepared layers of MWCNTs, carboxymethylcellulose (CMC) and porphyrinoids, used in amount corresponding to B, was determined. The obtained value was 63.4o, being significantly higher than the contact angle (39.2o) reported for MWCNTs dispersed in CMC.19 The observed increase of the contact angle value can be attributed to presence of rather lipophilic porphyrinoids together with CMC on MWCNTs. This effect is generally expected to improve performance of potentiometric sensors and can be attributed to properties of the porphyrinoids used. Taking into account the responses presented in Figure 1, the B system composition of the transducer layer containing porphyrinoids was chosen – due to achieved improvement in analytical performance, but not yet showing super-Nernstian type potentiometric behavior. 13 ACS Paragon Plus Environment


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The potentiometric responses of four different exemplars of (nominally the same) sensors were compared; all the tested sensors were characterized by linear dependences of potential vs. logarithm of potassium ions activity in solution, within the concentration range from 0.1 to 10-6 M with close to Nernstian slope, as described above. All the sensor obtained were characterized with equal E0 value, within the limit of experimental error (437.3 ±1.6 mV, 437.8 ±2.0 mV, 438.9 ±1.4 mV, 438.5 ±0.6 mV). It should be stressed that for optimized measurements setup/conditions (measurements in beaker, on laboratory bench) and for “handmade” non-optimized sensor preparation procedure, excellent reproducibility between sensors was achieved: mean E0 was equal to 438.1 mV with SD equal to 0.7 mV (n = 4) (see Table S2 for details). Clearly further optimization of the measurement conditions and sensor preparation method (e.g. automatization) can result in further improvement of between sensors reproducibility. The reproducibility of the potentials recorded for the same electrode (within day reproducibility) was also high – mean E0 was equal to 439.6 mV, with SD equal to 0.8 mV (n = 4 calibrations). The potential stability of the electrodes in continuous measurement performed for as prepared in 10-2 M KCl, for electrodes conditioned overnight in 10-3 M KCl, in time was also high. The mean value of the potential recorded for an electrode continuously tested for 9 h in 10-2 M KCl was equal to 370.0 ± 0.02 mV, which is a clear evidence of high stability of potential in time, Figure S4. The transfer of the stored sensor to 10-3 M KCl solution resulted in initial increase of the measured potential of about 2 mV observed during 1 h, followed with slight decrease of measured potential (not exceeding 2 mV) value observed for about 3 h. Taking into account that herein proposed sensors contain porphyrinoids in the transducer phase, it seems vitally important to check the effect of the oxygen presence in the solution on the recorded potential for all-solid-state ion selective sensor. Figure 4 presents open potential change recorded in 10-3 M KCl for sensor containing in the transducer layer porphyrinoids in amount denoted as B (black line) and for coated wire type sensor (red line). Despite the fact that the two lines shown in Figure 4. are plotted in different scales (upper, porphyrinoids containing transducer, trace scale unit is 10 mV), a significant difference between the tested sensors is evident. The herein proposed sensor containing porphyrinoids in the transducer phase was practically insensitive for the change of oxygen contents in solution. Neither deaeration, nor opening of the cell resulted in significant change of the potential value recorded. This effect is clearly promising for practical applications of the novel type all-solid state sensors herein proposed. 14 ACS Paragon Plus Environment

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On the other hand, in a control experiments for coated wire type sensors, of known susceptibility of potential readings for oxygen concentration changes in the sample

(1,2)

, not

only de-aeration of the solution resulted in over 80 mV potential decrease, but also immediately after the end of argon purging, a gradual increase of the potential was observed, followed by a significant potential jump when the cell was open, Figure 4. The results shown in Figure 4 clearly confirm the stability of the novel type all-solid state sensors containing porphyrinoids in the transducer phase. The effect of carbon dioxide presence in the sample was also tested, by recording the potential of a sensor containing porphyrinoids in the transducer phase in 10-2 M K2CO3 solution, while pH of the sample was gradually decreased from 9.7 (pH of potassium carbonate) to 3.6 by HCl addition. 0.5 mM concentration of CO2 generated in the sample resulted in 3 mV potential change during about 1 h experiment. The observed potential increase, however, is mainly attributed to the change of the liquid junction potential, related to replacement of carbonates by the introduced chlorides. Therefore, it can be concluded that the porphyrinoids containing sensors are also not sensitive to changes of CO2 contents in the sample.

Conclusions In this work a novel type transducer for ion-selective electrodes was proposed and tested. The approach herein proposed is based on the application of two different compounds containing metal ions in different oxidation states, but not being a redox couple. The compounds applied were cobalt(II) porphyrin /cobalt(III) corrole, together with MWCNTs. Taking the advantage of properties of the compounds used, namely the interactions of porphyrin cobalt(II) with oxygen and probably formation of a mixed potential due to cobalt(II) porphyrin and cobalt(III) corrole reactions, a beneficial effect of stabilization of potentiometric sensor potentials is achieved. On the contrary to the redox buffer used previously to stabilize the potential of the all-solid state potentiometric sensors, the new system does not create ionfluxes through the transducer - membrane interface. Due to the presence of cobalt(III) corrole the obtained potentials are falling into range of typical values achieved for potentiometric sensors, moreover the corrole presence helps to improve the analytical parameters of the sensors. The partial release of porphyrinoids to the membrane phase helps to control the analyte ions fluxes through the membrane phase, resulting in additional beneficial effects on the sensor performances. The obtained sensors are characterized with high potential stability 15 ACS Paragon Plus Environment


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both between days and within day, as well as high resistivity towards changes of oxygen or carbon dioxide contents in the sample.

Acknowledgements Authors are grateful to Żaneta Pławińska MSc for help with potentiometric experiments. Financial support from National Centre of Science project, based on decision DEC2013/09/B/ST4/00098 is gratefully acknowledged (EJ, ES, AM, KM).


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400

300

E (mV)

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200

100

0 -10

-8

-6

-4

-2

0

log a K+

Figure 1. Potentiometric responses of all-solid-state potassium-selective electrodes (glassy carbon substrates electrodes) with transducer layer applied containing MWCNTs and different amounts of [TbuPPCo(II)]/ [TbuPCCo(III)(PPh3)], in 1:1 mole ratio: () A, (●) B and (▲) C recorded in KCl solutions.



6

2.0x10

6

-Z"/ ohm

1.5x10

6

1.0x10

5

5.0x10

0.0 0.0

5

5.0x10

6

1.0x10

6

1.5x10

6

2.0x10

Z'/ohm

Figure 2. EIS spectra of potassium ion-selective sensor (glassy carbon substrates electrodes) with transducer layer applied containing MWCNTs and different amounts of [TbuPPCo(II)]/ [TbuPCCo(III)(PPh3)], in 1:1 mole ratio: () A, (●) B and (▲) C recorded in 0.1 M KCl at 0.3 V using amplitude 50 mV.

1.0 0.8 Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0.6 0.4 0.2 0.0 450

500

550

600

650

700

Wavelength (nm)

Figure 3. UV/Vis absorption spectra of the supernatant post separation of MWCNTs from the mixture used to prepare transducer layers, colors denote contents of [TbuPPCo(II)]/ [TbuPCCo(III)(PPh3)] in the initial mixture: blue line – A, red line – B, black line - C.


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480 10 mV

470 460 Deaeration On

450 E (mV)

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Deaeration Off

440

Cell open

100 mV

200

100 0

1000

2000

3000

4000

Time (s)

Figure 4. Potential changes recorded in 10-3 M KCl purged with argon: (black line) sensor containing in the transducer layer porphyrinoids in amount denoted as B and for (red line) coated wire type senor, note the scale difference.



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Supporting Information Supporting Information for Publication.docx Reagents, electrodes and apparatus, [TbuPPCo(II)]/[TbuPCCo(III)(PPh3)] synthesis. Cyclic voltammetric curves recorded for layers containing MWCNTs with TbuPPCo(II)], TbuPCCo(III)(PPh3) or both cobalt complexes, potentiometric responses of all-solid-state potassium-selective

electrodes

with

transducer

layer

applied

containing

different

porphyrinoids, potentiometric responses of sensors containing different amount of equimolar porphyrinoids mixtures; potential changes in time, selectivity coefficients of tested sensors and actual potential values recorded in parallel calibration in KCl solutions performed for four different exemplars of nominally the same type sensors. Intensities of 59Co or 39K counts for sensors with porphyrinoids in the transducer layer obtained in LA-ICP-MS experiments, potential trace recorded during continuous measurement potential stability test for sensor containing porphyrinoids in the transducer phase.


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