Dc amperometric flow-injection analysis of ions and neutral molecules

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Dc amperometric flow-injection analysis of ions and neutral molecules transduced by electroactive (conductive) polymers Marina D Zavolskova, Vita N. Nikitina, Ekaterina D. Maksimova, Elena E. Karyakina, and Arkady A. Karyakin Anal. Chem., Just Accepted Manuscript • Publication Date (Web): 22 May 2019 Downloaded from http://pubs.acs.org on May 23, 2019

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Analytical Chemistry

Dc amperometric flow-injection analysis of ions and neutral molecules transduced by electroactive (conductive) polymers Marina D. Zavolskova, Vita N. Nikitina, Ekaterina D. Maksimova, Elena E. Karyakina, Arkady A. Karyakin* Chemistry faculty of M.V. Lomonosov Moscow State University, 119991, Moscow, Russia ABSTRACT We first report on constant potential (dc) amperometric flow-injection analysis (FIA) transduced by electroactive (conductive) polymers. Amperometric response is caused by the polymer recharging in order to maintain the electrode potential at constant level when (i) ions are crossing the film|solution interface and polarizing electrode|film interface or (ii) ions or neutral molecules are specifically interacting with the polymer recharging it. The response under constant solution flow is a current peak, and, respectively, in flow-injection mode is a couple of current peaks directed oppositely, with the first sharp, analytically valuable peak. In both constant flow and flow-injection regimes peak current is dependent on analyte concentrations, obviously FIA mode provides more advantageous analytical characteristics. Dc amperometric flow-injection analysis is shown for boronate- and sulfate-functionalized polyanilines, as well as for Prussian Blue, a member of inorganic polymer family. As a proof of concept the successful dc amperometric lactate detection in human sweat on the basis of boronate-functionalized polyaniline has been shown. The proposed approach would revolutionize the field of conductive/electroactive polymer supported ion sensing introducing reliable and robust amperometry as a valuable alternative to existing potentiometry.

*

Corresponding author, e-mail: [email protected] ACS Paragon Plus Environment

Analytical Chemistry 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

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2 INTRODUCTION Conductive polymers since the discovery of polyacetylene 1 attract great interest of scientists in different fields including analytical chemistry. A search in WoS for conducting polymer based sensors returns > 6000 citations. Of particular importance is the so-called “solid contact” for ionselective electrodes 2-4, also referred to as “ion-to-electron interface” 5. In addition to ion-selective electrodes, conductive polymers can also serve as transducers for neutral molecules. Among synthetic receptors the phenylboronic acid is particularly attractive due to binding selectivity to compounds possessing 1,2- or 1,3-diol functionalities, common structural elements of saccharides and hydroxy acids

6-9.

Involving phenylboronic acid in conductive

polyaniline, the reagentless sensor, which is able to generate an increase in its conductivity as a result of binding with polyols, has been elaborated 10. The flow-injection analysis (FIA) 11 is of particular importance providing simple, cost effective and express analysis of multiple samples. In addition, FIA protocol involves the transducer washing step after each injection, which is important for its regeneration if it is inhibited by the analyte or the product of its transformation. The most reliable and robust detection principle for FIA is constant potential (dc) amperometry. However, for conducting (electroactive) polymer based transducers this detection principle has not been reported yet. Despite the first amperometric FIA of electro-inactive ions on the basis of conductive polymers was carried out 33 years ago 12, in it and the following article

13

the potential

step techniques were used making the detection scheme voltammetric rather than true amperometric. The attempts to realize dc amperometric transduction principle for conductive polymer supported ion-selective electrodes introduced by Bobacka’s group

14-16

resulted in coulometric

rather than amperometric read-out 14, 15, most probably, to improve the reproducibility of the chosen batch rather than flow-injection mode. However, even integrating amperometric response (coulometric read-out) the only stepwise dilution calibration graphs over wide concentration range ACS Paragon Plus Environment

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3 have been obtained

15.

Stepwise addition for chloride-selective electrode resulted in the extremely

narrow calibration range (0.01 – 0.02 M) 16 obviously hardly applicable for analysis. Flow-injection analysis in known to provide significant advantages over batch mode even for amperometry of electroactive compounds reaching steady-state current under continuous flow or stirring. For electroactive material based transducers, which current-time dc response in similar conditions is a peak of current (see

15, 16),

the advantages of FIA are even more substantial. The

attractive performance characteristics of the proposed constant potential (dc) amperometric flowinjection analysis transduced by electroactive (conductive) polymers over the mentioned dc coulometry in batch mode are: the dramatically improved precision and signal-to-noise ratio, resulted in wider calibration range, better reproducibility, short response time. As a proof of principle, the analysis of real objects, not shown by the way for coulometry in batch mode 14-16, has been demonstrated through a successful amperometric lactate detection in human sweat. Dc amperometric FIA would obviously open new horizons for electroactive/conductive polymer based transducers in analytical chemistry.

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4 EXPERIMENTAL Materials Experiments were carried out with Millipore Milli-Q water. Potassium L-lactate, D-fructose, 3aminophenylboronic acid hydrochloride were purchased from Sigma Aldrich (Germany). Inorganic salts and acids were obtained of the highest purity from Reachim (Moscow, Russia). 1% pilocarpine chloride was purchased from Ferein (Russia). Planar screen-printed three-electrode sensor structures (Rusens Ltd, Russia) had carbon working electrode (Ø 1.8 mm) encircled with carbon auxiliary electrode. Instrumentation Cyclic voltammetry was carried out using µAUTOLAB III (Metrohm, The Netherlands), constant potential amperometry – using PamlSens 3 (PalmSens, The Netherlands). Skin electrophoresis was made using Potok-1 (Russia). The flow-injection setup consisted of a Perfusor Compact syringe pump (Braun, Germany), homemade flow-through wall-jet cell with 0.5 mm nozzle, and injector (IDEX Health & Science LLC). The flow rate used was of 0.67 mL min−1. Methods Cyclic voltammetry was carried out in three-compartment electrochemical cell (with separated compartments of all electrodes) containing a glassy carbon auxiliary and Ag|AgCl in 1 M KCl reference. Self-doped polyaniline was synthesized from a solution containing 0.1 M total aniline (aniline and m-aminobenzenesulphonic acid) in 0.1 M sulfuric acid. Boronate functionalized polyaniline was synthesized by electropolymerization of 3-aminophenylboronic acid from its 0.1 - 0.15 M solution in 0.1 – 0.3 M sulfuric acid containing potassium L-lactate (0.9 M) or sodium fluoride (0.04M). The electropolymerization was carried out by cycling the applied potential at a sweep rate of 40 mV s−1, the anodic switching potential was in the range 0.85 – 0.9 V. Interfacial synthesis of Prussian Blue was made by dipping a droplet of 2–4 mM K3[Fe(CN)6] and 2 – 4mM FeCl3 in 0.1 M HCl and 0.1 M KCl and initiating by addition of H2O2 to a final ACS Paragon Plus Environment

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5 concentration of 50 – 200 mM. After deposition modified electrodes were annealed at 100◦C during 1 h. Biosensors were made according to 17 by casting an enzyme containing mixture (lactate oxidase suspended by 2% γ-aminopropyltriethoxysilane in 90% isopropanol) onto Prussian Blue modified electrode with subsequent drying at a room temperature for one hour. Sweat samples were collected from healthy human volunteers using Macroduct Sweat Collector (USA) during 30 min after activation of skin spot with 1% pilocarpine solution by means of electrophoresis. The informed content was obtained from all subjects. Samples were stored frozen at -18° C. For standardization of the analytical procedure, sweat samples were 50 times diluted with buffer to final phosphate concentration of 100 mM (pH 6.0) and 0.7 M NaCl.



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6 RESULTS AND DISCUSSION An initial aim of this study was to improve non-enzymatic lactate detection in human sweat. Accordingly, lactate imprinted boronate-functionalized polyaniline

18

has been deposited onto the

screen-printed electrode surface. Cyclic voltammograms of the polymer growth in the potential range from 0.00 V to 0.85 V, Ag|AgCl, (Figure S1, Supporting Information) display an increase in current at the anodic switching potential indicating the deposition of conductive polymer. Cyclic voltammograms of the resulting modified electrode in monomer-free solutions (Figure S1, inset, Supporting Information) indicate that elecroactivity of the resulting polymer is similar to it of conventional conducting polyaniline. Dc amperometric flow-injection analysis of saccharides and hydroxy acids After injection of lactate the FIA system with the integrated boronate-functionalized polyaniline (BFPAn) modified electrode in constant potential mode generates a couple of current peaks: the anodic sharp peak followed with the cathodic peak (Figure 1A). Injection of fructose, which is also able to interact specifically with boronate-functionalized polyaniline, results in a similar couple of peaks, but directed oppositely: the first sharp cathodic peak followed by anodic peak (Figure 1A). To understand the nature of the observed FIA responses the system has been studied in continuous flow mode. To minimize the hydrodynamic disturbance when one solution is changed to another, the injection loop has been disconnected from the injector, instead the latter inlet has been connected to the second pump (Figure S2, Supporting Information). Accordingly, switching injector positions in the ‘two-pumps’ setup results in changing of the solution flows between the continuously working pumps.


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1 minute

Figure 1. A – flow-injection and B – constant flow current responses towards 5 mM L-lactate and 20 mM D-fructose; 0.1M phosphate buffer, pH 6.0; Edc = 0.00 V. Figure 1 B displays that changing the carrier solution to lactate-containing one results in reproducible anodic peak, after which the current falls to the background. Changing the lactatecontaining solution back to the carrier one results in cathodic peak. On the contrary, the flow of fructose solution causes cathodic peak, and its change to the buffer solution results in anodic peak. The peak height in continuous flow mode is concentration dependent for both lactate and fructose (Figure S3, Supporting Information). Accordingly, the couple of peaks in flow-injection response (Figure 1 A) can be explained as follows. An appearance of analyte in solution flow causes the current increase in the corresponding direction. When after emptying the injection loop carrier solution is changed back to the analytefree buffer, the polymer has to be recharged causing current response in the opposite direction. Hence, the first peak of the FIA response (Figure 1 A) has to be considered for analysis. Intuitively the dc potential for amperometric flow-injection analysis has been chosen close to the polymer redox potential. Indeed, conductivity window of polyaniline with the rise of solution pH is narrowed

19,

and in neutral solutions polyaniline behaves as electroactive material

20, 21.

As

seen the highest sensitivity of the dc amperometric FIA response is coincided with the peak of square wave voltammogram (Figure 2). ACS Paragon Plus Environment


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1 0

-0.4

-0.2

0.0

0.2

0.4

0.6

0

E, V (vs. Ag/AgCl)

Figure 2. Sensitivity of amperometric flow-injection lactate detection as a function of the applied (dc) potential (■) in comparison with square wave voltammogram (Estep = 10 mV, dE = 20 mV, f = 0.3 Hz) of BFPAn; 0.1M phosphate buffer, pH 6.0. As mentioned, the first peak of the FIA response (Figure 1 A) has been considered for calibration. The resulting plot (Figure 3) displays the calibration range prolonged over 3 orders of magnitude of lactate concentration: from 0.1 mM to 0.15 M, the signal-concentration function is linear up to 0.1 M. The linear current dependence on analyte concentration known for electroactive compounds is observed here for electro-inactive ones due to ion-to-electron interface properties of conducting polymers. Sensitivity evaluated as a slope of calibration graph when analyte concentration tends to zero, is of 17±4 mA M-1 cm-2 (Figure 3, inset). Comparing with impedimetric lactate detection on the basis of similar sensor 22, the detection limit of dc amperometric FIA (0.01 mM) is 150 times decreased causing the two orders of magnitude wider calibration range. The proposed dc amperometric FIA has been compared with coulometry in batch mode introduced by Bobacka’s group 14-16. The stepwise addition seems to be the only possible procedure to record the calibration graph. The main problem with batch mode is that the current is very noisy because of the use of magnetic stirrer (Figure S4, Supporting Information). The corresponding calibration graph of dc amperometric detection in batch mode is prolonged from 5 mM to 70 mM of lactate concentration (Figure S5, Supporting Information), the calibration range is 100 times narrower as compared to dc amperometric FIA (Figure 3). In contrast to 14-16 the use of coulometry ACS Paragon Plus Environment

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9 instead of amperometry does not provide any improvements, on the contrary, the sensitivity and,

100

15 10

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E, mV

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|j|, A cm

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respectively, the precision is even decreased (Figure S5, Supporting Information).

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10 5

1 0.1

1

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100

0

Concentration, mM Figure 3. Calibration plots of BFPAn based sensor for L-lactate (■) and D-fructose (●) in amperometric flow-injection mode, Edc = 0.00 V and for L-lactate potentiometric detection in flow-injection (∆) and batch () modes; 0.1M phosphate buffer, pH 6.0. Since it is hard to compare the detection limits due to the dramatically different calibration ranges, we present the noise level relatively the slope of the calibration graph at the particular analyte concentration. The latter referred to as the analyte measurement error at 5 mM lactate concentration is 25 - 30 times higher for both dc batch coulometry and amperometry compared to the proposed dc amperometric FIA. The latter at similar analyte concentration is characterized also by 20 – 25 times higher signal-to-noise ratio. As known, for electroactive compounds flow-injection analysis provides certain advantages over batch mode

11.

In case of electro-inactive compounds detection supported by

conductive/electroactive polymers, which response in steady-state conditions is current peak (see Figure 1 B) rather than constant limiting current, is even more dramatic. The proposed dc amperometric FIA as compared to both dc batch coulometry and amperometry upon lactate detection displayed: (i) the two orders of magnitude broader calibration range, (ii) 25 – 30 times lower analyte measurement error and (iii) 20 – 25 higher signal-to-noise ratio.



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10 Since the dc amperometric FIA is proposed as an alternative for potentiometry, analytical performance characteristics of BFPAn based sensor for L-lactate have been investigated also in potentiometric mode (Figure 3). Potentiometric lactate detection in batch mode mainly suffering from baseline drift (Figure S6, Supporting Information), is characterized by the narrow calibration range from 5 mM to 120 mM (Figure 3). Flow-injection mode allows potentiometric lactate detection in wider range (0.5 mM – 50 mM, Figure 3), however, baseline drift is even more significant (Figure S7, Supporting Information). In addition to one – two orders of magnitude wider calibration range the proposed dc amperometric FIA provides 5 – 20 times lower measurement error compared to potentiometry even in flow-injection mode. Figure 3 also displays the calibration plot for fructose. As seen, besides the oppositely directed response, FIA fructose detection is characterized by lower sensitivity (6±2 mA M-1 cm-2, Figure 3, inset). We note, that the affinity of both phenylboronic acid

6

and boronate functionalized

polyaniline 10, 18 towards fructose is higher compared to lactate. This indicates that the mechanisms of the dc amperometric FIA responses to saccharides and hydroxy acids are different. Operational stability is a crucial point for application of any sensor-based system. The response of the boronate functionalized polyaniline based sensor in dc amperometric FIA is completely stable within 10 injections, and after 100 injections of 100 mM lactate the peak current remains at the level of 80% from its initial value (Figure S8, Supporting Information). Hence, dc amperometric flow-injection analysis on the basis of conducting polymer is stable enough for applications. On the mechanism of dc amperometric response To understand the response mechanism, first, the response of the boronate functionalized polyaniline towards inorganic anions has been investigated. As found, injection of chloride ions (Cl¯) generates a couple of peaks similar to those shown in Figure 1 A with the first sharp cathodic peak (Figure S9, Supporting Information). It is necessary to prove, that the observed response is indeed towards Cl¯ rather than to the corresponding cation (Na+). First, the responses of boronatefunctionalized polyaniline in dc amperometric FIA mode to KCl, KBr, NaNO3, Na2SO4 are all ACS Paragon Plus Environment

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11 cathodic (data not shown) similarly to it for NaCl (Figure S9, Supporting Information). Second, on the contrary, NaF generates FIA response in the opposite (anodic) direction (Figure S10, Supporting Information). Third, similar experiment with polyaniline containing sulfonate rather than boronate ring substituents has shown cathodic responses to F¯ (Figure S11, Supporting Information), Cl¯ and other inorganic anions (data not shown). This undoubtedly proves that the observed cathodic response to NaCl (Figure S9, Supporting Information) is indeed towards chloride ions (Cl¯). The opposite FIA response of BFPAn to F¯ is due to ability of the latter to interact specifically with phenylboronic functionality. Let’s consider the one-dimensional model for conductive polymer modified electrode. Obviously, in case of negligible current the electrode potential is determined by the sum of the electrode|polymer (Ee/p) and polymer|solution (Ep/s) potentials. In the simplest case ions from solution are able to pass the polymer|solution interface and, thus, to polarize the electrode|polymer interface. On the contrary, electrons passing the electrode|polymer interface are able polarize the polymer|solution interface. When concentration of the counter-anions, unable to interact specifically with the polymer (Cl¯) in solution is sharply increased, they cross the polymer|solution interface polarizing the electrode|polymer interface. To keep the total electrode potential at constant level, the flux of electrons has to similarly polarize the polymer|solution interface causing cathodic current response. On the contrary, anions, able to interact specifically with the polymer (lactate and fluoride in case of BFPAn), charge polymer chain and polarize the electrode|polymer interface. Hence, for maintaining the electrode potential it is enough to depolarize the latter interface removing electrons from the polymer film. Accordingly, the increase in lactate or fluoride ion concentrations results in anodic current response. The case of neutral molecules specifically binding to BFPAn (saccharides) is more complicated. Despite negative charges appear in polymer chain as a result of their interaction with boronate functionality, this interaction is coupled with the consumption of hydroxyl, thus, decreasing local pH in polymer film. Such pH shift increases redox potential of the polymer, hence ACS Paragon Plus Environment


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12 maintaining electrode potential at the constant level requires cathodic current. An independent evidence of the local pH shift in the presence of saccharide has been obtained from square wave voltammograms of boronate functionalized polyaniline recorded at different solution pH and in the presence of fructose (Figure S12, Supporting Information). As found, an addition of 0.5 M fructose is similar to a decrease of solution pH for 0.5 units. For analytical purposes the pH-sensitivity of BFPAn indicates that the corresponding sensor may suffer from pH sensitivity. However, the right choice of the carrier solution for flow-injection analysis obviously overcomes this problem, which is clearly shown below upon successful lactate detection in human sweat. Dc amperometric FIA based on inorganic polymer In order to generalize the approach shown for polyanilines (boronate and sulfate substituted) the dc amperometric FIA on the basis of Prussian Blue (ferric hexacyanoferrate), being a member of inorganic polymer family, has been investigated. Since reduction of Prussian Blue to Prussian White requires an intercalation of alkali metal ion for charge compensation 23, the dc amperometric FIA of both potassium and sodium cations have been investigated. As found, injection of KCl, as well as of NaCl causes cathodic peak current response; the peak height is proportional to concentration of the injected cations (Figure S13, Supporting Information). It is, thus, possible to conclude that the dc amperometric FIA discovered for polyaniline is applicable for any electroactive polymer material. Dc amperometric detection of lactate in human sweat In order to validate the concept of dc amperometric FIA on the basis of electroactive polymer the detection of lactate in human sweat has been carried out. This analytical tool is particularly required as non-invasive approach for accessing training level of sportsmen. The possibility for detection of sweat lactate on the basis of boronate functionalized polyaniline is provided by the knowledge that its concentration is 100 times higher compared to glucose content and 1000 times


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13 higher than fluoride ion content

24.

Other sweat components either appear in much lower

concentrations or are unable to bind to phenylboronic functionality. In contrast to the previously reported impedimetric lactate sensor based on the same transducer 22,

which was inactivated in course of sweat analysis and thus used only as disposable one, the

reported

approach provides complete transducer regeneration. Validation of the proposed dc

amperometric FIA based on boronate-functionalized polyaniline has been carried out using the previously reported highly specific biosensor based on Prussian Blue and the enzyme lactate oxidase 17 as a reference. The measurements using both methods are in a good agreement (Table S1, Supporting Information), Pearson correlation coefficient (r) exceeds 0.9. Hence, the proposed dc amperometric FIA on the basis of electroactive polymers is suitable for analysis of real objects.



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14 CONCLUSIONS Despite the 30 years history of using conducting polymers as transducers, it is possible to propose the measurement protocol able to improve analytical performance characteristics of the corresponding sensors. Indeed, still the most widely used potentiometry suffers from electromagnetic strays dramatically affecting its performance characteristics. Aimed to avoid this problem, chemical sensitive field effect transistors, popular 20 years ago, still haven’t found wide applications, probably, because of their complexity. Constant potential (dc) amperometry is apparently the most universal electroanalytical tool because of its simplicity, robustness and advantageous analytical performance characteristics. With the use of electroactive (conductive) polymer as a transducer the dc amperometric flow-injection analysis of ions and neutral molecules is possible. Neutral analytes obviously require specific interaction with the polymer. The proposed dc amperometric FIA is expected to revolutionize the field of ion sensing introducing more robust and reliable amperometry as a valuable alternative to existing potentiometry. The desired selectivity can be achieved by specific analyte interaction with electroactive polymer, as shown here. In addition, a variety of perm-selective membranes can be used for this aim.

SUPPORTING INFORMATION The Supporting Information is available free of charge on the ACS Publications website at DOI: Figure S1, Electropolymerization of 3-APBA; Figure S2, Scheme of the two-pumps setup; Figure S3, Calibration graphs for L-lactate and fructose under continuous flow; Figure S4, Сurrent responses in amperometric batch mode; Figure S5, Calibration plots of BFPAn-based sensor in batch mode; Figure S6, Potentiometry in batch mode of BFPAn-based sensor; Figure S7, Potentiometry in flow-injection mode; Figure S8, Operational stability; Figure S9, FIA of BFPAn to chloride; Figure S10, FIA responses of BFPAn to fluoride; Figure S11, FIA responses of self-doped


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15 polyaniline to fluoride; Figure S12, Square wave voltammograms of poly(APBA); Figure S13, FIA responses of Prussian Blue; Table S1, Validation of BFPAn based dc amperometric FIA.

ACKNOWLEDGEMENTS Financial support through Russian Science Foundation grant #19-13-00131 is greatly acknowledged. We thank Mrs. Elena V. Karpova for her help with lactate-sensitive biosensors.



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TOC graph 82x44mm (300 x 300 DPI)


Dc amperometric flow-injection analysis of ions and neutral molecules

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