All solid state chronopotentiometric ion-selective electrodes based on ferrocene functionalized PVC

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Article

Reference

All solid state chronopotentiometric ion-selective electrodes based on ferrocene functionalized PVC

JAROLIMOVA, Zdenka, et al.

Abstract

An all solid contact ion-selective electrode based on poly(vinyl chloride) covalently modified with ferro- cene moieties allows one to operate the membrane in a chronopotentiometric sensing mode. The mem- brane is considered as initially non-perm-selective towards anions, and an applied anodic current provokes a defined anion flux in direction of the membrane.

With this protocol, a variety of anions can be depleted at the membrane surface. Since this model system does not yet contain an ionophore, their order of preference follows the expected Hofmeister selectivity sequence. The all solid-state configura- tion tolerates an

dis- solved alkyl ferrocene derivative for an expected upper detection limit of 17.0 mM.

Numerical simula- tions are performed in order to establish the fundamental basis of the mechanism that takes place in this all solid-state membrane electrode. The oxidation of bound Fc and the ion-transfer process are con- [...]

JAROLIMOVA, Zdenka, et al . All solid state chronopotentiometric ion-selective electrodes based on ferrocene functionalized PVC. Journal of Electroanalytical Chemistry , 2013, vol.

709, p. 118-125

DOI : 10.1016/j.jelechem.2013.10.011

Available at:

http://archive-ouverte.unige.ch/unige:31464

Disclaimer: layout of this document may differ from the published version.

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All solid state chronopotentiometric ion-selective electrodes based on ferrocene functionalized PVC

Zdenˇka Jarolímová, Gastón A. Crespo, Majid Ghahraman Afshar, Marcin Pawlak, Eric Bakker

^⇑

Department of Inorganic and Analytical Chemistry, University of Geneva, Quai Ernest-Ansermet 30, CH-1211 Geneva, Switzerland

a r t i c l e i n f o

Article history:

Received 26 July 2013

Received in revised form 30 September 2013 Accepted 8 October 2013

Available online 19 October 2013

Keywords:

Chronopotentiometry Permselective membranes Ion-selective membranes Solid contact transducer Ferrocene

a b s t r a c t

An all solid contact ion-selective electrode based on poly(vinyl chloride) covalently modified with ferrocene moieties allows one to operate the membrane in a chronopotentiometric sensing mode. The membrane is considered as initially non-perm-selective towards anions, and an applied anodic current provokes a defined anion flux in direction of the membrane. With this protocol, a variety of anions can be depleted at the membrane surface. Since this model system does not yet contain an ionophore, their order of preference follows the expected Hofmeister selectivity sequence. The all solid-state configuration tolerates an imposed current density of 1.4lA mm ², which translates into an upper detection limit of ca. 1.2 mM. Higher current densities of up to 31.2lA mm ²are possible with addition of freely dissolved alkyl ferrocene derivative for an expected upper detection limit of 17.0 mM. Numerical simulations are performed in order to establish the fundamental basis of the mechanism that takes place in this all solid-state membrane electrode. The oxidation of bound Fc and the ion-transfer process are considered in the simulation. In view of developing an analytical sensor, different anions are tested. A linear range of two orders of magnitude from 0.01 to 1 mM is found. The membranes are evaluated over several days, displaying practically the same slopes and intercepts, with a RSD of less than 2%. Electrochemical limitations of free Fc and bound Fc are critical evaluated. This approach should allow one to develop a new family of solid-state chronopotentiometric ion sensors that require relatively high current densities.

1. Introduction

Polymeric ion selective membranes play an important role in ﬁelds such as clinical diagnostics and environmental monitoring for the detection of small and hydrophilic ions such as sodium and chloride[1–3]. These membranes are frequently interrogated by the simplest electrochemistry technique, potentiometry, and are typically sensitive to ion activity changes in the sample solution[4]. While traditional membrane electrodes are backside contacted with an aqueous electrolyte [5,6], recent efforts aimed at replacing the inner solution by a solid contact ion-to-electron transducer [7,8], which includes conducting polymers [6,9,10]

and nanostructured materials [8,11–13] as potential candidates to replace the inner ﬁlling solution and the Ag/AgCl element.

Recently, ion-selective membranes have started to be operated with controlled current and potential techniques. Attractive analytical sensing characteristics were obtained by applying such dynamic electrochemistry approaches and some analytical problems, such as polyion detection in undiluted human blood have been only successfully solved in this manner[14]. Stripping ion voltammetric [15–18], thin layer coulometric [19], pulsed

chronopotentiometric and chronopotentiometric flash titration protocols[14,20,21]have been developed on the basis of such approaches. In a number of cases, operationally unstable potentiometric sensors can be made much more reproducible by controlling ion fluxes by instrumental means[22]. This class of ion sensors has its roots in the field of ion-transfer voltammetry between two immiscible electrolyte solutions (ITIES)[23,24]. Most experiments at the ITIES used non-permselective membranes (so-called ideally polarizable interfaces). Here, the interfacial ion- transfer at the sample side must be coupled to the charge transfer at the backside of the solution in order to fulfill the electroneutral- ity condition, either by the use of a common ion in the membrane and inner solution, or the extraction of a counterion[25–27].

Because ion-selective membranes have shown attractive analytical characteristics in open circuit potentiometry as well as in dynamic electrochemistry, the application of solid contact ion-to- electron transducers may allow one to achieve a new generation of ion sensors, as achieved earlier with potentiometric ion-selective electrodes. Conducting polymers[6], nanostructured materials [28] and other materials that display high redox capacity have chieﬂy been used in potentiometry so far[8]. Recently, conducting polymers such as PEDOT or POT have been also explored as solid contact ion-to-electron transducer for the detection of small hydrophilic ions by ion-transfer voltammetry[29–32]. Very low 1572-6657/$ - see front matterÓ2013 Elsevier B.V. All rights reserved.

http://dx.doi.org/10.1016/j.jelechem.2013.10.011

⇑Corresponding author. Tel.: +41 22 3796431.

E-mail address:[email protected](E. Bakker).

Contents lists available atScienceDirect

Journal of Electroanalytical Chemistry

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / j e l e c h e m

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limits of detection on the order of 1 nM were obtained for potassium and perchlorate with all solid-state thin layer stripping ion voltammetry in rotating disk electrode mode[15–17]. The reports have focused on the lower limit of detection and therefore em- ployed relatively mild applied currents or potentials.

In another direction, Langmaier et al. used freely dissolved dim- ethylferrocene, DMFc, a well-established electroactive material that exhibits excellent redox capacity[33]. DMFc was embedded in a polymeric non-permselective film containing PVC, an apolar plasticizer (DOS) and a lipophilic salt. The oxidation of DMFc at the electrode creates anion-exchanging groups at the metal-membrane interface during the amperometric pulse. To fulfill electro- neutrality, anions are concurrently extracted from the sample into the film in order to compensate for the unbalanced charge [34]. This methodology was successfully used to determine heparin (a polyanion with about 70 charges per molecule) in the phys- iological concentration range (1–10 U mL ¹).

Recently, Pawlak et al. introduced ferrocene groups covalently attached to the PVC backbone by ‘‘click chemistry’’ (Huisgen cyclo- addition)[35]. This prevents the possible loss of the ferrocene derivative by leaching into the sample solution and provides a materials approach to conﬁne the ferrocene functionality to a layer close to the contacting metal electrode at the inner side of the sensing ﬁlm.

That work introduced the synthesis of Fc-PVC as well as preliminary electrochemical characterization of the transducer using cyclic voltammetry and normal pulse voltammetry. The amount of Fc in the polymer was optimized and the electrochemical stability of Fc- PVC was evaluated. In subsequent work, it was shown that pulsed chronopotentiometry can be used to interrogate membranes based on Fc-PVC as ion-to-electron transducer[36]. The observed calibration curves exhibited near-Nernstian behavior, in analogy to open circuit potentiometry. Very recently the interfacial electrochemistry of a Fc-PVC membrane in a contact with an aqueous electrolyte was studied by cyclic voltammetry, synchrotron radiation-X-ray photoelectron spectroscopy and near edge-X-ray absorption ﬁne structure[37]. After a chronoamperometric pulse of 180 s duration, neither Fe²⁺ nor Fe³⁺remained present in the ﬁrst 80

l

m of the membrane. This demonstrates that Fc-PVC is capable of diffusion to and accumulation at the buried interface as the ferrocene moiety in Fc-PVC is oxidized to ferrocenium-PVC during electrolysis, although other work using optical methods suggests that such mass transport is normally very slow[38].

The concept by Pawlak et al. is here extended and characterized in more detail with ﬂash chronopotentiometry. Using this technique, the ion-target is locally depleted at the membrane interface due to the fact that the ionic ﬂux is limited in the aqueous phase.

An initially non-perm-selective membrane, which contains the reduced form of Fc-PVC, plasticizer and ETH 500 is interrogated with a current period of 1–5 s[39]. In this short period, a transition time (E vs time) is observed for several anions indicating the local depletion at the membrane. This work aims to elucidate the materials limitations of this approach and to characterize maximum current densities achievable for the realization of all-solid-state ion sensors that are operated by controlled current techniques. Furthermore, analyte concentration proﬁles outside and inside the membrane as well as the concentration proﬁle of Fc-PVC at the metal electrode (during the applied current interval) are simulated numeri- cally in order to clarify the transduction mechanism.

2. Experimental section 2.1. Reagents and solutions

Sodium ﬂuoride, sodium nitrate, sodium perchlorate, sodium thiocyanate, anhydrous tetrahydrofuran (THF), tetrakis(4-chloro-

phenyl)borate tetradodecylammonium salt (ETH 500), bis(2-ethyl- exyl)sebacate (DOS), high molecular weight poly(vinyl chloride) (PVC), sodium azide, ethynylferrocene, ascorbic acid and copper sulfate pentahydrate were purchased from Sigma Aldrich with analytical grade (used without further puriﬁcation). Aqueous solutions were prepared by dissolving the appropriate salts in Milli-Q- puriﬁed distilled water.

2.2. Electrochemical equipment

A double – junction Ag/AgCl/3M KCl/1M LiOAc reference electrode was used in chronopotentiometric and potentiometric measurements (Metrohm Autolab, Utrecht, The Netherlands). A platinum-working rod (3.2 cm²surface area) was used as a counter electrode. Potential responses of the electrodes were measured with an EMF16 interface (Lawson Laboratories Inc., Malvern, The USA). Chronopotentiometric measurements were performed with an Autolab PGSTAT302N (Metrohm Autolab, Utrecht, The Nether- lands). A Faraday cage was used to protect the system from unde- sired noise. Glassy carbon rods of (3.0 ± 0.1) mm electrode diameter (7.06 mm²surface area) were used as working electrodes and were purchased from Metrohm Autolab (model 6.1204.300).

2.3. Synthesis of poly(vinyl chloride) covalently modiﬁed with ferrocene groups (Fc-PVC)

The procedure of preparation of Fc-PVC was described by Pawlak et al.[35]7% Fc-PVC was prepared by the click chemistry reaction between azide-modified PVC and ethynylferrocene (1114.29 mmol kg ¹) with CuSO₄. 5H₂O and ascorbic acid as cata- lysts in THF medium. Ferrocene modified PVC was filtered and washed with methanol and then dissolved again in THF. This solution was filtered to remove insoluble impurities. The product was precipitated with methanol, filtered and dried under reduced pressure.

2.4. Composition of membrane cocktails

The ﬁnal composition of membrane cocktails is shown in Table 1. All components were dissolved in 1 ml of THF. The lipophilic electrolyte ETH 500 was added to provide counterions for the extracted analyte in the membrane and was also used to reduce the electrical resistance of the membrane. Electrodes were prepared by drop casting 13.5

l

L of cocktail and letting THF evap- orate under ambient conditions. The thickness of membrane was 25 ± 5

l

m. The calculated concentration of bound ferrocene in membrane M1 was 0.167 M.

2.5. Electrochemical experiments

Potentiometric experiments were performed in Milli-Q puriﬁed distilled water. The appropriate volume of salt solution was grad- ually added to reach the desired concentration.

Chronopotentiometric experiments were performed with a galvanostatic pulse of 5-s duration using aqueous 10 mM NaF as a background electrolyte. The potential was recorded as a function

Table 1

Composition of membrane cocktails.

M1 M2 M3 M4

7% FcPVC (mg) 7.5 (0.167 M) 7.5 (0.143 M) – 7.5

Free Fc (mg) – 7.5 – –

PVC (mg) – – 7.5 –

DOS (mg) 37.5 37.5 37.5 37.5

ETH 500 (mg) 5 5.7 5 –

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of time during the constant current pulse. After each chronopotentiometric determination the membrane was regenerated by a potentiostatic pulse, which was applied for 50-s at the open circu- ital potential determined before the current pulse.

Normal pulse voltammograms were performed between 0.6 and 0.8 V potential range using 10 mM solutions of NaNO3. The potential pulse (1 s) with a 20 mV potential increasing was followed by a 5 s regeneration pulse (baseline potential) at the open circuit potential (measured before each voltammetric experiment).

3. Results and discussion

Fig. 1schematically illustrates the operating principle of this membrane electrode, which contains a solid contact ion-to-electron transducer bound to the PVC polymeric membrane and back side contacted with a glassy carbon electrode. The membrane contains a very lipophilic salt (R⁺R ), PVC–bound Fc⁰ and an apolar plasticizer such as DOS. Before imposing any electrochemical per- turbation, the phase boundary potential at the sample-membrane interface is not clearly defined. As expected, the open circuit potentiometric response did not exhibit Nernstian behavior. Electrode slopes in the range of 20–30 mV dec ¹were obtained for nitrate, thiocyanate and perchlorate in a wide range of concentrations (10 ⁴ to 10 ¹M, see Fig. S1 in the supporting information for calibration curves) and potentiometric time traces). An appreciable potential drift was observed for the time response (E vs t). In order to evaluate whether Fc⁰is spontaneously oxidized and eventually stabilized by the extraction of ions from the solution, different membrane compositions were considered. Membranes containing ETH 500-Fc⁰-PVC (M1) and ETH 500-PVC (M3) showed essentially the same potentiometric behavior (Fig. S2). On the other hand, very low slopes were observed in the absence of ETH 500 (M4) (Fig. S3) and one may argue that the observed slopes originate chiefly from the presence of ETH 500 in the membrane (arguably by incomplete metathesis of the salt), and not from the influence of the ferrocene moieties.

Once an anodic current is applied to the membrane-coated electrode, Fc will start to oxidize to Fc⁺at the glassy carbon (GC) electrode. As a result, anion-exchanging functionalities are rapidly generated at the electrode–membrane interface. To compensate this unbalanced charge in the vicinity of the electrode, R (from ETH 500) migrates inside the membrane to form the counterions of the oxidized Fc⁺species, seeFig. 1. This re-arrangement leaves an excess of anion-exchanging quaternary ammonium groups at the membrane surface, which results in the extraction of sample ions into the membrane.Fig. 2shows the normal pulse voltammo- gram for four anions in the range of 0–0.8 V (vs. Ag/AgCl). The

Hofmeister selectivity sequence is clearly obtained. In choosing a constant current value, the potential shifts by about 300 mV when chloride is replaced by the more lipophilic perchlorate, suggesting a 5 orders of magnitude discrimination of chloride relative to perchlorate. In contrast, a control experiment containing all components except Fc does not show any current response at all (labeled as PVC inFig. 2).

The electrochemical characteristics of the ion-to-electron transducer were elucidated by applying a range of current amplitudes to the membrane in contact with a concentrated solution of 100 mM NO₃. With this high concentration in the sample, concentration polarization phenomena in the aqueous phase are suppressed.

Since the concentration of ETH 500 is much higher than that of the bound ferrocene, and the ﬁlms studied here contain no ionophore, the electrode reaction kinetics should be limited by the maximum ferrocene oxidation rate.Fig. 3a shows the observed potential as a function of time (chronopotentiogram) at different current amplitudes, increasing from 1 to 60

l

A (0.14–8.5

l

A mm ²).

The inﬂection points in the chronopotentiograms are interpreted to indicate the exhaustive depletion of Fc at the electrode surface.

A better visualization of the transition is observed inFig. 3b where the time derivate of the potential is shown. Higher applied current amplitudes result in shorter observed transition times, as expected by the Sand equation. De Marco and co-workers recently found that Fc-PVC exhibits signiﬁcant mobility within the membrane [37]. Since Fc-PVC appears not to be immobile and the ferrocene electrochemistry is known to be reversible, the Sand equation is used to quantify the chronopotentiometric data. For high sample concentrations, this assumes that the maximum current is limited by Fc-PVC diffusion in the membrane. A current of at most 10

l

A (1.4

l

A mm ²) can be applied for up to 5-s with this configuration, which corresponds to the trace shown in red color inFig. 3a. One finds a linear relationship between the square root of transition time and the applied current density, which is compatible with diffusion of bound ferrocene being the predominant mass transport process. With the Sand equation and the apparent molar concentration of ferrocene groups in the membrane of 0.167 M, the diffusion coefficient of covalently bound ferrocene is found to be small, consistent with a polymer-bound functionality. Depending on the current amplitude (see Fig. S5a), two different diffusion coeffi- cientsD= (2.410 ¹²cm²s ¹and 8.210 ¹²cm²s ¹) are identi- fied. Freely dissolved membranes of this composition (PVC-DOS

GC MEMBRANE

0 PVC

Fc Fc+ ^PVC

SAMPLE CX*-

X-

CX- 0

R-R⁺

Fig. 1.Schematic illustration of the all solid state membrane electrode mechanism.

The membrane contains a lipophilic salt (R⁺R ), attached Fc⁰to PVC and no-polar plasticizer (DOS). An applied current provokes an oxidation of Fc at the electrode surface and a high number of positives sites (Fc⁺) are generated at the metal- membrane junction. The accumulation of positive charge during the pulse causes the diffusion of R inside the membrane and is stabilized by ion-pairing. As a result, the anion (X ) is transferred from the sample to the membrane. Diffusional concentration proﬁles of X are shown in the aqueous phase. Localized anion depletion is eventually achieved at a transition time (green line). (For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.)

PVC Cl-

NO3-

SCN-^-

SCN ClO4-

Fig. 2.Voltammograms performed in 10 mM solutions of the indicated anions for Fc-PVC membrane (M1). A control experiment (labeled as PVC) containing all components except Fc is shown for comparison (M3).

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1:5 by mass) tend to exhibit diffusion coefﬁcients of nearly 10 ⁷cm²s ¹[39], a dramatic difference of 4 orders of magnitude.

The multiple calibration curves shown inFig. S5aare from separate measurements involving different batches of Fc-PVC of nominally equal composition. The experimental variability may have its ori- gin in the polydispersity of the polymer. For numerical calcula- tions, the larger of the two values was utilized because it corresponds to the (lower) applied current used in the remainder of this study.

In a different experiment, freely dissolved ethynylferrocene was also incorporated into the polymeric membrane while maintaining the remainder of the components (M2 in the experimental part). As expected, a higher current (ca. 145

l

A, 20.5

l

A mm ²) was required to achieve the depletion of free Fc after 5-s (see Fig. 4).

Although the ferrocene concentration is different for M1 and M2, this experiment provides evidence on the possibility to further ex- tend the upper limit of detection if desired. The estimated diffusion coefﬁcient ((1.2 ± 0.2) 10 ⁸cm²s ¹) for free Fc is smaller than

other values reported for calcium ionophore and chromoiono- phores for PVC-membranes (1:5 mass ratio of PVC–DOS) (see Fig. S5b)[39,40]. Chemical heterogeneities at the interface of the glassy carbon electrode and the polymeric membrane may perhaps explain this result.

The maximum depleted concentration in the aqueous phase can be estimated by the Sand equation. Considering a diffusion coefﬁ- cient of 10 ⁵cm²s ¹ in the sample solution and a 5-s measure- ment time, the maximum measurable concentration is 1.2 mM for 10

l

A (1.4

l

A mm ²) and 17.0 mM for 145

l

A (20.5

l

A mm ²), for bound and freely dissolved ferrocene, respectively. As expected, the upper limit of detection for free-ferrocene membranes is higher than for bound ferrocene by one order of magnitude. Evidently, a compromise between upper limit, leaching and lifetime is required to achieve a sensor with the desired characteristics.

Having identiﬁed the limiting current value for this sensor conﬁguration based on Fc-PVC (between 11 and 20

l

A (1.6–2.8

l

A mm ²)), different experiments aiming to determine the concentration of several anions in a more realistic range were performed. When the concentration of the sample was reduced

b)

16µA15µA

14µA 12µA 17µA

18µA 20µA 22µA

60 µA

1µA 25 µA

a) Fc-PVC

(

Fig. 3.Chronopotentiometric response for Fc-PVC membrane (M1). (a) Potential as a function of time for different current amplitudes (from 1 to 60lA (0.14–8.5lA mm ²)) in a concentrated primary analyte solution (100 mM NO3).

Inﬂection points in the chronopotentiogram, indicated as red dots, signal the depletion of Fc at the electrode surface (from right to left, at 12, 14, 15, 16, 17, 18, 20 and 22lA, respectively). The red line indicates the highest current amplitude, 10lA, that can be sustained by the Fc-PVC material for a period of 5-s. (b) Observed time derivatives of the chronopotentiometric response upon increasing current from 1 to 60lA (0.14–8.5lA mm²).

155µA 220µA

200µA 185µA

165µA 175µA

145µA

Fig. 4.Chronopotentiometric response for freely dissolved Fc (M2 composition). (a) Potential as a function of time for different current amplitudes (from 1 to 220lA (0.14–31.2lA mm²)) in 100 mM of NO3. Inﬂection points in the chronopotentiogram shows the depletion of free Fc at the electrode surface. (b) Observed time derivatives of the chronopotentiometric response upon increasing current from 1 to 220lA (0.14–31.2lA mm ²).

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from 100 mM to 0.1 mM, the current began to be limited by the mass transport of anions in the aqueous phase instead of that by Fc-PVC.

Numerical simulations were performed in order to provide a fundamental basis to the experimental results. To accomplish this, three diffusion layers were considered. The first one corresponds to the defined interface between glassy carbon and inner backside membrane. As described above inFig. 1, oxidation of Fc is the only process that occurs at this interface. The concentration profile of Fc is simulated by considering the diffusion coefficient previously calculated above (seeFig. 5, inset). At 3.5

l

A for 5 s the concentration of Fc decreases 50% of the initial concentration at the GC surface. In agreement with above, this suggests that Fc is not completely depleted at the surface and the ion-to-electrode transduction is maintained at this current density. The other two diffusion layers correspond to the interface between the membrane and the aqueous phase. By using a diffusion coefﬁcient of nitrate in the membrane and aqueous phase of 10 ⁸and 10 ⁵cm²s ¹, respectively, concentration proﬁles are obtained for 3.5

l

A, see Fig. 5. Once the ion-transfer process is established, nitrate begins to be depleted at the aqueous phase boundary and to be accumulated at the membrane phase boundary. At the transition time (

s

= 3 s, green line) the concentration of nitrate at the aqueous side of the interface drops to zero whereas at the membrane side it increases up to 13 mM. Note that after the transition time, the concentration gradient for the primary analyte decreases in the vicinity of the membrane. Since the same total flux is imposed to maintain the applied current, the difference between total flux and primary analyte flux needs to be provided by a background ion (not shown in the simulation). As a result, the concentration profile for the

primary analyte starts to approach the steady state condition for inﬁnite times, thereby increasing the aqueous diffusion layer thickness.

Having estimated all concentrations proﬁles inside and outside of the membrane, chronopotentiograms are produced by introduc- ing the calculated concentrations at the phase boundary positions into the Nernst equation.Fig. 6shows the simulated chronopotentiogram obtained as a combination of the two involved process (Fc oxidation and ion transfer). The experimental chronopotentiogram, under the same conditions, is displayed in the sameFig. 6.

The potential time trace (E vs time) that corresponds to the oxidation of ferrocene sharply increases in the ﬁrst few ms. This is due to the absence of the oxidized species at the beginning of the experiment, giving a poor redox buffer. After that, the potential becomes more stable, though a signiﬁcant drift is observed (10 mV s ¹ at 3.5

l

A). The ion-transfer chronopotentiogram displays an inflection point at 3 s as expected from the concentration profiles. Note that the sharpness of the transition depends on the selectivity coefficient. The experimental chronopotentiogram is in agreement with the simulated one. This powerful tool allows one to obtain both qualitative information about the shape and quanti- tative information about the position of the transition time (for more information about numerical simulations, see reference[20].

In view of developing a solid contact ion selective chronopotentiometric sensor, nitrate was selected as primary analyte in most experiments because of its relevance in environmental chemistry.

Fig. 7a displays a linear relationship between the square root of the transition time and concentration of nitrate at 3.5

l

A (0.50

l

A mm ²) [

s

^1/2= 7.84(±0.02)cnitrate+ 0.18 (±0.11)]. Note that the transition time inFig. 7b corresponds to the localized nitrate depletion at the aqueous side of the membrane (seeFig. 5, concentration proﬁles in the aqueous phase). After the transition time, a second anion from the sample (i.e., background electrolyte) needs to be transported to sustain the imposed current. As a result, a potential change (around 300 mV) is observed in the chronopotentiogram (Fig. S6). This suggests an important selectivity between NO₃ and the background electrolyte (F or Cl ) that is estimated as about ﬁve orders of magnitude.

The linear range of the proposed methodology could be extended to up to two orders of magnitude (from 0.01 mM up to

Fig. 5.Numerical simulation of NO3concentration proﬁle in the aqueous phase (top) and in the membrane phase (bottom). Inset corresponds to the simulation of Fc depletion at the GC surface. Parameters: D(NO3, aq) = 10⁵cm²s ¹, D(NO3, m) = 10 ⁸cm²s ¹,D(Fc-PVC, m) = 2.10 ¹²cm²s¹with a time step of 0.5 s. Green lines correspond to the time when depletion in the aqueous phase occurs (s= 3 s).

(For interpretation of the references to colour in this ﬁgure legend, the reader is referred to the web version of this article.)

0.1 V

NUMERICAL SIMULATION

Fc⁰ Fc⁺ ION TRANSFER

PREDICTED EXP

.

t / s

Fig. 6.Bottom two traces: simulated potential–time changes for the separate processes of Fc-PVC oxidation and ion-transfer, see Fig. 5 for corresponding concentration proﬁles. The predicted potential–time trace labeled with PREDICTED corresponds to the sum of these two potentials and is compared to the observed (labeled as exp.) chronopotentiograms for nitrate. The curves are shifted vertically for clarity. SeeFig. 5for details on the simulation.

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1 mM), if the magnitude of the applied current was appropriately tuned.Fig. 8a displays several calibration curves for nitrate at different current amplitudes. All calibrations showed linear behavior even at the highest current densities. As expected, the higher the applied current is, the higher the concentration of nitrate that is depleted. One notes, however, that the intercepts of the calibration curves depend on the applied current density. Since a higher current density results in a faster depletion and smaller diffusion layer thickness for a given concentration, we suspect that the observed intercept variation originates in more efﬁcient diffusion at the edges of the membranes, which is not taken into account in the Sand equation. Further work is needed to fully elucidate this effect.

Mass transport induced by additional migration process could modify the linear relationship between current, concentration and square root of transition time. This was studied by normalizing the square root of transition time by multiplying it with the applied current, and plotting these values as a function of the analyte concentration (Fig. 8b). A precise determination of the diffusion coefficient is obtained from the slope of the normalized plot. The observed diffusion coefficient for NO₃is (1.98 ± 0.08)10 ⁵cm²s ¹ and is in agreement with reported values in the literature (1.9010 ⁵cm²s ¹)[41]. Consequently, diffusion is confirmed to

be the predominant mass transport mechanism under the conditions chosen here.

The linear range of the proposed methodology can be extended by the introduction of free Fc in the membrane.Fig. S7compares free Fc and bound Fc-PVC experiments at the same applied current.

Evidently, free Fc membrane allows one to apply higher current levels without any obvious limitation in the range of 20–50

l

A (2.8–7.1

l

A mm ²), while Fc-PVC displays a limitation above 2 mM. Note, however, that the calculated diffusion coefﬁcient for this data set for both systems is four times higher than the expected value, which suggests a contribution of migration processes to mass transport. While currents higher than 20

l

A (2.8

l

A mm ²) are not generally encouraged for this particular conﬁguration, this issue might be overcome by increasing the concentration of both of ETH 500 and a background electrolyte such as NaCl or NaF.

In an attempt to evaluate the feasibility to determine other anions, analogous experiments were performed for thiocyanate and perchlorate.Fig. 9shows the normalized plots for nitrate, thiocyanate and perchlorate. The diffusion coefﬁcients for thiocyanate and perchlorate were found as (1.67 ± 0.05) 10 ⁵cm²s ¹ and (1.72 ± 0.05) 10 ⁵cm²s ¹, respectively, and agree with the

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(b)

Fig. 7.Chronopotentiometric response for Fc-PVC membrane (M1). (a) Obtained linear calibration curve of the square root of the transition times^1/2as a function of nitrate concentration at 3.5lA (0.50lA mm²). (b) Observed time derivatives of the chronopotentiometric response upon successively increasing the ﬁnal nitrate concentration from 0.025 to 0.25 mM.

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(b)

Fig. 8.(a) Observed linear calibration curve of the square root of the transition time s^1/2as a function of nitrate concentration at the indicated current amplitudes (1.5–

18lA (0.21–2.5lA mm ²)) for M1. (b) Linearized response curve of the square root of transition times multiplied by the applied current as a function of the nitrate concentration.

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reported values (1.7610 ⁵ and 1.7910 ⁵cm²s ¹) [41]. The observed potential change in the chronopotentiograms is between 200 and 300 mV depending the anion (seeFig. S8). This suggests between three and ﬁve orders of selectivity with respect to the background electrolyte. On the other hand, one may note that the resulting calibration curves (square root of transition time vs.

concentration) are independent of ion selectivity, which is in contrast to potentiometric sensors.

In order to characterize the stability of the Fc-PVC in chronopotentiometry, ten calibration curves for nitrate were performed at applied currents of 3.5

l

A (0.50

l

A mm ²),Fig. S9shows the calibration curve with their respective error bars [

s

^1/2= (7.84 ± 0.02) cnitrate+(0.18 ± 0.11)]. The relative standard deviation was less than 1%. Medium-term stability studies revealed that the same membranes can be used for up to one week without substantial deteri- oration of the readout signal (RSD increased to at most 3%).

4. Conclusions

An initially non-perm-selective membrane based on Fc covalently bound to PVC is here operated by flash chronopotentiometry. Ferrocene is shown to exhibit a dual function in the polymeric membrane: it acts as ion-to-electron transducer and is the initiator of the ion-transfer process from the aqueous to the membrane phase due to the generation of cationic sites. When an external current is applied to the membrane, perm-selectivity properties are induced. A Hofmeister selectivity pattern is observed for chloride, nitrate, thiocyanate and perchlorate. The observed linear range from 0.01 mM to 1 mM is attractive in view of practical applications, such as environmental analysis. Owing to the high concentration of bound ferrocene, it can effectively serve as ion to electron transducer in all-solid-state chronopotentiometric sensors for the detection of up to millimolar sample concentrations, even though the observed ferrocene diffusion coefficients are only ca. 10 ¹²cm²s ¹. While the emphasis of this work was placed on fundamental electroanalytical aspects of this material, it forms the basis of a future family of all-solid-state chronopotentiometric sensors that will involve the incorporation of additional ionophores and other sensing components for realizing non-Hofmeister selectivity. This approach will allow one to more easily miniaturize this class of sensors for broader applicability. Specifically, solid contact membranes controlled by dynamic electrochemistry, for example for a multitude of ions and polyions such as heparin

and protamine, are being developed in our group based on this principle.

Acknowledgments

The authors thank the Swiss National Foundation and the Uni- versity of Geneva for supporting this research. ZJ thanks the Eras- mus program for supporting her scientiﬁc stay in Geneva.

Appendix A. Supplementary material

Additional experiments such as potentiometric and chronopotentiometric responses can be found as supplementary data, in the online version, at http://dx.doi.org/10.1016/j.jelechem.2013 10.011.

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