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Article

Determination of Ascorbic Acid in Pharmaceuticals and Food Supplements with the New Potassium Ferrocyanide-Doped Polypyrrole-Modified Platinum Electrode Sensor

by
Ancuța Dinu
and
Constantin Apetrei
*
Faculty of Science and Environment, Dunărea de Jos University of Galati, 47 Domnească Street, 800008 Galati, Romania
*
Author to whom correspondence should be addressed.
Chemosensors 2022, 10(5), 180; https://doi.org/10.3390/chemosensors10050180
Submission received: 21 April 2022 / Revised: 29 April 2022 / Accepted: 6 May 2022 / Published: 9 May 2022
(This article belongs to the Special Issue Nanotechnology for Sensing, Medical and Environmental Application)
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Abstract

:
This paper reports the results obtained from the determination of ascorbic acid with platinum-based voltammetric sensors modified with potassium hexacyanoferrate-doped polypyrrole. The preparation of the modified electrodes was carried out by electrochemical polymerization of pyrrole from aqueous solutions, using chronoamperometry. Polypyrrole films were deposited on the surface of the platinum electrode, by applying a constant potential of 0.8 V for 30 s. The thickness of the polymer film was calculated from the chronoamperometric data, and the value was 0.163 μm. Cyclic voltammetry was the method used for the Pt/PPy-FeCN electrode electrochemical characterization in several types of solution, including KCl, potassium ferrocyanide, and ascorbic acid. The thin doped polymer layer showed excellent sensitivity for ascorbic acid detection. From the voltammetric studies carried out in solutions of different concentrations of ascorbic acid, ranging from 1 to 100 × 10−6 M, a detection limit of 2.5 × 10−7 M was obtained. Validation of the analyses was performed using pharmaceutical products with different concentrations of ascorbic acid, from different manufacturers and presented in various pharmaceutical forms, i.e., intravascular administration ampoules, chewable tablets, and powder for oral suspension.

1. Introduction

Ascorbic acid (AAs), also known as Vitamin C, is found in many foods and beverages, medicines, and food supplements, and has been studied since 1928, when biochemist Albert Seent Ceyorghi received the Nobel Prize for isolating this compound from red pepper [1].
AAs is the compound most used by the human population, with or without the recommendation of a medical-pharmaceutical specialist; being considered a primary supplement for the prevention of cold and flu symptoms [2,3]. Most often, individuals resort to self-medication with vitamin C, not only for the normal functioning of the immune system [4] but also to protect cells against oxidative stress [5], it being a powerful antioxidant [6], for the normal functioning of blood vessels [7], the bone system, and the absorption of iron, in case of anemia treatment [8]. Moreover, being an essential water-soluble vitamin for the human body, its lack can be the cause of the condition called scurvy, manifested by various symptoms, such as bleeding gums, anemia, bone pain, bleeding skin [9,10], and fatigue [11]. In this respect, the World Health Organization (WHO) has established the usual dose of AAs for an adult, administered orally via supplements, and between 50 and 200 mg/day [12]. Scientific research mentions many preventive and curative actions of AAs, including for infertility, cancer [13], and central nervous system disorders [14]. Moreover, since the emergence of the new SARS-CoV-2 virus, Vitamin C is part of the mandatory treatment regimen for infected people, with the maximum recommended daily dose, both during infection and in the pre- and post-COVID periods, for prevention and rapid immunization [15,16,17,18]. On the other hand, gastrointestinal tract diseases caused by excessive administration of AAs have also been reported [19]. For all these reasons, this paper highlights the need for rapid and accurate measurement of the concentration of AAs, both in biological fluids and in the pharmaceutical products available on the market for consumption by the population.
The detection of this molecule can be performed by numerous analytical techniques, such as spectrophotometry [20,21], chromatography [22,23], chemiluminescence [24,25], Fourier transform infrared spectrometry (FT-IR) [26], capillary electrophoresis [27,28], and Raman spectrometry [29], with numerous studies already reported in the literature. The results are very good in terms of sensitivity and selectivity, but these techniques have a number of drawbacks, related to their lengthy analysis time, high costs of procedures and equipment, and use of large amounts of solvents and various chemicals [30,31].
The novelty of the present study is the fabrication of a sensor based on a platinum (Pt) electrode, modified by electropolymerization with a conductive polymer with remarkable properties, polypyrrole (PPy), and analyzed using an electrochemical method that has drawn the attention of many researchers, namely cyclic voltammetry (CV) [32], due to its advantages: easy and fast operation, fast response, high accuracy and selectivity, ability to detect low concentrations in a short time, and low cost [33,34,35,36]. Studies with electrodes modified with different conductive polymers, such as polyaniline (PANi) [37], polypyrrole [38], polythiophene (PTh) [39,40], and poly(3,4-ethylenedioxythiophene) (PEDOT) [41] have been reported in the literature [42,43]. During electrochemical polymerization, conductive polymers acquire a number of desirable sensitive characteristics through the incorporation of various anions from the electrolyte solution, a process that relies on the attraction between the positively charged polymer and negatively charged dopant ions [44].
The polymer selected for this purpose was PPy, a conductive polymer that brought many benefits to the sensor prepared in this study, and improving a number of parameters compared the devices mentioned in the literature (Table 5), namely: limit of detection (LOD), sensitivity and selectivity (LOQ), accuracy, reproducibility, and repeatability [1,2]. Furthermore, PPy was selected for its good conductivity, low cost, and easy synthesis, and its immobilization by electropolymerization on a noble metal, such as Pt, was easily achieved in one step [45,46]. The presence of Pt as a support for the polymer matrix proved to be important, with Pt being a good electrically conductive metal that facilitates electron transfer in electrochemical processes [47]. Studies have been reported in the literature in which Pt was deposited on other metals, namely Au [48,49], Pd [50,51], Ag [52], and Cu [53,54].
In essence, this study presents a rapid and simple way to prepare a new electrochemical sensor based on a PPy-modified Pt electrode for electrocatalytic oxidation of AAs by CV. The very good sensitivity properties of the PPy conducting polymer led to sensitive and accurate determination of vitamin C in pharmaceuticals. The reason for the development of these realized devices was to perform quality control of drugs and OTC (over-the-counter drugs) containing AA, checking the accuracy of the concentrations reported by manufacturers, the quality of AAs, and its interferences with other compounds existing in real samples. Thus, on the one hand, these instruments are used in the monitoring and quantitative and qualitative analysis of medicines and can be transformed into portable instruments made available to inspectors working in National Drug Quality Control Agencies. On the other hand, the devices can be useful to any individual who intends to measure the level of AAs in his biological fluids, so that he can monitor whether the drugs administered have a good bioavailability for the body and whether they prevent various diseases caused by a lack or excess of AAs.

2. Materials and Methods

2.1. Chemicals and Solutions

AAs, KCl (potassium chloride), K4Fe(CN)6 (Potassium ferrocyanide), HCl (hydrochloric acid), KIO3 (potassium iodate), KI (potassium iodide), starch, and pyrrole purchased from Sigma-Aldrich (St. Louis, MO, USA). Solutions were prepared with ultrapure water and analyzed immediately after preparation (18.3 MΩ × cm, Milli-Q Simplicity® Water Purification System from Millipore Corporation (Millipore, Bedford, MA, USA).

2.2. Real Samples

The prescription drug Vitamin C ARENA 750 mg intravascular injectable solution and the OTCs: vitamin C+ Bioland 180 mg chewable tablet and Molekin Imuno Vitamin C 1000 mg powder for oral suspension were purchased from local pharmacies and studied with the new sensor developed in this study, in order to validate the results obtained in solutions prepared with pure AAs. Samples were dissolved in 0.1 M KCl solutions prepared in volumetric flasks.

2.3. Equipment and Software

The AAs study was performed with a Pt working electrode modified with the conductive polymer PPy. Electropolymerization was performed with an EG&G potentiostat/galvanostat apparatus (Princeton Applied Research, Oak Ridge, TN, USA), model 263A. This apparatus was controlled by ECHEM software (Princeton Applied Research, Oak Ridge, TN, USA). An electrochemical cell, with a capacity of 50 mL, was used to study the prepared solutions, and the electrode system that was immersed in the cell and connected to the apparatus, as follows: the working electrode (Pt/PPy-FeCN), the reference electrode Ag/AgCl/KCl 3.5 M, and the counter electrode was a Pt wire.
Obtaining homogeneous solutions in the bubble-coated flasks and complete dissolution of the active compounds was possible with the help of an Elmasonic S10H ultrasonic bath.
Origin and Microsoft Excel software were used for the analysis of the experimental data obtained from the chronoamperometric and voltammetric curves.

2.4. Analytical Methods

The present research involved a number of steps, involving a range of methods, namely: chronoamperometry (CA) and cyclic voltammetry (CV).
CA is the method applied for the modification of Pt electrodes with the conductive polymer PPy in a monomer/doping agent (pyrrole/FeCN) solution by electropolymerization. CV, a versatile method, has been intensively applied and developed in the scientific literature, and allowed characterizing the voltammetric responses obtained with the newly prepared electrodes and the quantitative determination of AAs [42]. The quantitative results obtained with the new sensor were compared with those obtained by the reference method STAS 1682/15-87 for the validation of the electroanalytical method. The European Pharmacopoeia mentions the iodometric method as a reference method [55]. The principle of the method is based on the oxidation of ascorbic acid with molecular iodine in a hydrochloric acid medium. Iodometric titrimetry was carried out, finally calculating the amount of ascorbic acid present in the analyzed samples [56]. Molecular iodine, which plays the role of oxidant, was generated in situ by the reaction of potassium iodate with potassium iodide in the presence of hydrochloric acid, as shown in Figure 1:

3. Results and Discussions

3.1. Modification of Platinum Electrodes by Electropolymerization

For the study of AAs detection, the surfaces of a series of Pt electrodes were modified in the laboratory with PPy film doped with FeCN, which was deposited by chronoamperometry, applying a constant potential of 0.8 V for 30 s. The platinum electrode was physically cleaned with very fine-grain abrasive paper, and rinsed with ultrapure water. The solution used for the polypyrrole deposition was a homogenous aqueous solution containing the monomer pyrrole and doping agent potassium ferrocyanide. The coating step consisted in immersion of the electrode in a homogeneous solution of 0.1 M pyrrole/0.1 M K4[Fe(CN)6] and applying a constant potential 0.8 V for 30 s. In this way, the polypyrrole doped with FeCN anion was obtained in a single stage, with the important advantage of electropolymerization. The thickness of the polymer film was estimated to be 0.163 μm, according to Equation (1):
d = QMw/nFAρ,
where Q is the oxidation charge, Mw is the molecular mass of the monomer solution (Mw. = 67.0892 g/mol), n is the number of electrons, F is the Faraday constant (F = 96485.33 C/mol), A is the surface area of the electrode, and ρ is the polymer density [58].
The technique used in this synthesis/doping step was CA [59]. The chronoamperogram recorded when the electrode was modified by immersing it in the monomer/doping agent solution (0.1 M pyrrole/0.1 M K4[Fe(CN)6]) for 30 s, applying a constant potential of 0.8 V, as illustrated in Figure 2.
After electrodeposition of the polymer matrix, the modified electrode was rinsed with bi-distilled water and then immersed in the solution to be analyzed, where it was characterized by CV.

3.2. Electrochemical Responses of Pt/PPy/FeCN Immersed in 0.001 M AAs-0.1 M KCl Double Solution

In order to observe the oxidation–reduction processes of the material with which the electrode surface was modified, an electroinactive KCl solution was used. Thus, the prepared sensor was initially immersed and characterized in a 0.1 M KCl solution. In the first step, the stabilization signal of the Pt/PPy-FeCN electrode was recorded in the potential range from −1 to +1.3 V at a scan rate of 100 mV/s (Figure 3A). The three cycles recorded in the stabilization stage demonstrated a very good reproducibility of the electrode response. Thus, cycle 5 is the stable electrode signal, and the cyclic voltammogram is illustrated in Figure 3B.
From Figure 3A,B, it can be seen that the potential range was too wide, so for clear observation of the redox processes of Ppy a narrower potential range was chosen, i.e., between −1.0 and +0.5 V, keeping the same scan rate of 100 mV/s (Figure 4A,B).
From Figure 4A,B can be easily observed the two pairs of peaks that correspond, on one hand, to the redox processes of PPy and, on the other hand, to the redox process of FeCN immobilized in the polymer matrix. The electrochemical parameters corresponding to the peaks observed in this 0.1 M KCl electroinactive solution are listed in Table 1.
It is, thus, found that the peaks on both the anodic and the cathodic side show high intensities, and are well defined, their characteristics (shape, potential, intensity) being closely related to the polymer and the dopant included in the polymer matrix.

3.3. Stable Electrochemical Response and Behavior of Modified Electrodes Immersed in 0.001 M K4[Fe(CN)6]-0.1 M KCl Double Solution

Compared with the electroinactive KCl solution, the double solution of K4[Fe(CN)6]-KCl showed a redox activity; therefore, at this stage of the study, the electroactivity of the double solution is highlighted with the realized sensor. The working technique was CV, after which the reversible oxidation of the ferrocyanide ion to ferricyanide at the active surface of the sensor was characterized.
The stabilization of the sensor signal was achieved by recording three cycles in 0.001 M K4[Fe(CN)6]-0.1 M KCl solution, in the potential range from −1 to +1.3 V, and at a scan rate of 100 mV/s (Figure 5A). Figure 5B shows the stable cyclic voltammogram of the electrode recorded under the above-mentioned working conditions.
Since, in this case, neither of the oxidation-reduction systems could be clearly observed, a narrower potential range of −1.0 and +0.5 V was applied, and the electrochemical response of the electrode is illustrated in Figure 6A,B.
In this case, it is important to highlight the redox processes occurring at the surface of the modified electrode, as the voltammetric responses obtained in this 0.001 M K4[Fe(CN)6]-0.1 M KCl solution were different compared to those obtained in the 0.1 M KCl solution. Therefore, at the Pt/PPy-FeCN sensor level, in addition to the redox activity of the PPy and FeCN modifiers present on the Pt surface, the ferrocyanide/ferricyanide redox process in the solution to be analyzed was also observed. Table 2 shows the values of the electrochemical parameters determined from the voltammogram in Figure 6B.
The oxido-reduction activity was enhanced in this double solution of 0.001 M K4[Fe(CN)6]-0.1 M KCl, with both redox systems exhibiting higher anodic and cathodic peak intensities. Moreover, on the cathodic side, a third reduction peak was observed, proving once again the electroactivity of the solution.
As a result, this sensor could be successfully applied for the determination of a compound affecting electrochemical processes, such as AAs, by modifying the material of the sensitive element.

3.4. Influence of Scanning Rate of the Modified Electrodes Immersed in 0.001 M K4[Fe(CN)6]-0.1 M KCl Double Solution

The recording of cyclic voltammograms with different scanning rates, ranging from 100 to 1000 mV/s, in the potential range −1.0 and +0.5 V, in 0.001 M K4[Fe(CN)6]-0.1 M KCl double solution with the prepared sensor was performed, with the aim of determining the phenomenon controlling the kinetics of electrochemical processes at the electrode surface (Figure 7).
To determine which factor limited the rate of redox processes, the dependencies between the intensity of the highest anodic peak and the scan rate or the square root of the scan rate were plotted. A better fit was obtained in the case of I vs. ν1/2, as shown in Figure 8.
From this dependence, it can be concluded that the redox processes are influenced by the diffusion phenomenon, and for the calculation of the active area the most intense pair of peaks was considered, using the Randles-Sevcik Equation (2), according to which:
Ip = (2.69 × 10−6) × n3/2 × A × C × D1/2 × ν1/2,
where Ip is the current corresponding to the peak (A), n—number of electrons involved in the redox reaction, A—electrode area (cm2), D—diffusion coefficient (cm2 × s−1), ν—scan rate (V × s−1), and C—concentration in mmol × L−1 [60].
The coefficient of determination in the case of the anodic peak was greater than 0.98, demonstrating the linear dependence between I and ν1/2. Thus, in agreement with the Randles–Sevcik equation, the oxidation process of the ferrocyanide ion at the electrode is controlled by diffusion, with the electrochemical parameters (intensity, potential) being influenced, not only by the properties of the electrode surface modifying element, but also by the scan rate. In addition, the value of the roughness factor was obtained from the ratio of active area/geometric area [61]. Thus, from the value of the slope of the Ipa line as a function of ν1/2, the active area of the electrode under focus was calculated, with the results being shown in Table 3.
Since the value of the active area is much larger than the geometrical area, this aspect demonstrates a high sensitivity, with well-defined and intense peaks. In conclusion, this sensor can be used for the detection of AAs, because AAs influence the electrochemical processes of polypyrrole, and it also could be directly detected, with the peaks characteristic to AAs redox processes also being observed.

3.5. Electrochemical Responses of Modified Electrodes Immersed in 0.001 M AAs-0.1 M KCl Double Solution and the Influence of Scan Rate on Sensor Response

AAs is a substance intensively studied using various methods of analysis, including by the CV method with different sensors and in various materials [37]. This study reports the preparation of a new Pt sensor using a PPy and FeCN dopant deposited on the electrode surface.
The study of the modified electrode continued with the analysis of the 0.001 M AAs solution in the presence of 0.1 M KCl in the potential range −1.0 and +1.0 V and at scanning rates between 100 and 1000 mV/s. The recording of the stable electrochemical behavior of the sensor in this solution at a scan rate of 100 mV/s is illustrated in Figure 9.
We, thus, can observe two pairs of peaks in both the anodic and the cathodic part, considered the most intense and well-defined peaks; their intensities and potentials obtained by CV are shown in Table 4.
Comparing the voltammetric responses of the sensor obtained in inactive KCl solution with those obtained in AAs-KCl double solution (Table 1 and Table 4, Figure 9), it can be seen that there are more intense peaks in the solution containing AAs, and the Ipc/Ipa ratio is much closer to the ideal value.
Ascorbic acid was oxidized on platinum electrodes at Epa 0.223 V, with the loss of two electrons, and reduced to Epc 0.166 with the loss of two protons, transforming into dehydroascorbic acid. The mechanism of the electrochemical reaction is illustrated in Figure 10.
The influence of scan rate on the voltammetric responses of the modified electrode was also studied. Thus, cyclic voltammograms at scanning rates between 100 and 1000 mV/s were recorded in the double solution of 0.1 M KCl and 0.001 M AAs with the modified sensor, as shown in Figure 11a.
According to the graph shown in Figure 11b, the linear dependence between the cathode peak current and the scanning rate demonstrates that the oxidation process is controlled by the electron exchange and that this sensor detects AAs, since the rate determining step of the electrochemical process is the absorption of AAs on the active electrode surface.
The degree of coverage of the electrode surface with electroactive centers was calculated according to Laviron’s Equation (3), starting from the equation of the linear fit between the highest cathode peak current and the scanning rate.
I pc = n 2 F 2 Г Av 4 RT
where
Г—surface coverage, mol × cm−2; Ipc—current corresponding to the peak, A; A—electrode surface area, cm2; n—number of electrons transferred during redox processes, 4; F—Faraday’s constant, 96.485 C × mol−F; R—universal gas constant, 8.314 J × mol−u × K−×; T—absolute temperature, K [63,64].
The result obtained was 1.29 × 10−10 mol × cm−2 and it can be seen that the prepared and modified Pt/PPy-FeCN electrode was very sensitive to the presence of AAs in the solution to be analyzed, compared to other sensors reported in the literature; the differences can be observed in Table 5.

3.6. Pt/PPy-FeCN/AAs Sensor Calibration Curve and Detection Limit

To obtain the detection limit of the prepared sensor, AAs solutions of different concentrations in the presence of 0.1 M KCl were used. The range of concentrations used was between 1 and 100 µM. The influence of AAs concentration on the voltammetric responses obtained by the Pt/PPy-FeCN electrode is illustrated in Figure 12.
The calibration curve illustrated in Figure 12B, and the correlation coefficient, were calculated, taking into account the cathodic peak of the voltammogram obtained for each concentration of AAs solution. An increased intensity of each peak was observed as a result of increasing AAs concentration. Thus, the intensity of the cathodic peak is in direct proportion to the concentration of AAs.
Since this step was performed to show the sensitive and selective detection of AAs with the newly prepared electrode, i.e., Pt/PPy-FeCN, LOD and LOQ were calculated from the equation of the calibration line using Equations (4) and (5):
LOD = 3σ/m,
LOQ = 10σ/m,
where
σ is the standard deviation of the current recorded in the blind sample, i.e., in the inactive 0.1 M KCl solution, and m is the value of the calibration slope [64].
Thus, the Pt/PPy-FeCN sensor obtained the following values: LOD = 0.2535 μM (2.5 × 10−7 M) and LOQ = 0.8451 μM (8.4 × 10−7 M), with these values being comparable with the performance of other sensors reported in the literature; the most recent studies being mentioned in Table 5.
Since the LOD and LOQ values obtained by the new Pt/PPy-FeCN sensor for the detection of AAs were superior to the other studies carried out, the study was continued on real samples, i.e., on pharmaceutical products.

3.7. Quantitative Determination of AAs in Pharmaceutical Samples with a Pt/PPy-FeCN Sensor

This step aimed to validate the sensor prepared on real samples, i.e., on prescription drugs, but also on OTC drugs without prescription, and which can be purchased not only in pharmacies but also in supermarkets or drugstores. These products had different concentrations of AAs: 180 mg, 750 mg and 1000 mg, coming from different manufacturers in Romania: Bioland, Arena, and Zdrovit, and being presented in different pharmaceutical forms: chewable tablet, injectable vial, and powder for oral suspension.
The analysis of these products was carried out using the CV method, the same method used to study the previous solutions, but also by the iodometric method and the STAS 1682/15-87 method, which is a reference method. The method applied followed the principle of analysis set out in the European Pharmacopoeia, assaying ascorbic acid from both pharmaceutical products and pure ascorbic acid solution. The comparison of the results obtained by the two methods with the results declared by the manufacturers was one of the objectives of the study.
Similar to the study of the pure AAs solution, solutions of pharmaceuticals dissolved in 0.1 M KCl solution, used as a supporting electrolyte, were prepared and studied. Thus, the pharmaceutical product in the form of tablets containing ascorbic acid was first crushed to powder with a pestle in a mortar, weighed on an analytical balance, and then dissolved by stirring in 0.1 M aqueous KCl aqueous solution in a volumetric flask.
The contents of one vial of vitamin C solution for injection were diluted in a volumetric flask with 0.1 M KCl aqueous solution.
The powder for oral suspension was weighted and dissolved in 0.1 M KCl aqueous solution in a volumetric flask. All the solutions were filtered with blue band filter paper before electrochemical measurements. The working parameters were the same: potential range between −1 and 1, at a scan rate of 100 mV/s. As a result, for the quantitative determination of AAs, the cathodic peak of Figure 13a–c, at a potential −0.443 V, was considered, with the peak intensities being different, as shown in Table 6.
The response of the sensor exposed to the solutions of pharmaceutical products was similar; differences were observed in the intensity of the electrochemical peaks, which was related to the AAs concentration in the analyzed solutions. The cathodic peak currents are presented in the Table 6.
The quantitative determinations on real samples were repeated five times for each sample, with the table showing averaged results. The relative standard deviation of the measurements were lower than 3.4%.
From the results obtained, it was proven that the method of AAs study proposed in this work, namely CV, is efficient, the results were accurate, and the quantity of AAs obtained was in agreement with that reported by the manufacturers, and also with that obtained by the standard method.

4. Conclusions

Electrode coating with PPy/FeCN was found to be highly efficient, the electrocatalytic effect on the oxide reduction of AAs being very important. In addition, in the pharmaceutical industry, it is considered that a decrease in the diffusion coefficient implies a decrease in the absorption of AAs in the body, i.e., a decreased bioavailability of the drug. For this reason, the proposed sensor for the detection of AAs obtained very good results on real samples consisting of pharmaceuticals. A linear response of the electrode was demonstrated over a wide range of AAs concentrations, between 1 and 100 μM. The detection limit for the determination of pure AAs was 2.5 × 10−7 M. The electrochemical sensor demonstrated good analytical performance, representing an efficient future platform for monitoring AAs in both drugs and other types of real samples: biological fluids and food products.

Author Contributions

Conceptualization, C.A. and A.D.; methodology, C.A.; validation, C.A. and A.D.; formal analysis, A.D.; investigation, C.A. and A.D.; data curation, C.A. and A.D.; writing—original draft preparation, A.D.; writing—review and editing, C.A.; supervision, C.A. All authors have read and agreed to the published version of the manuscript.

Funding

The contribution of the author Ancuta Dinu was supported by the project ANTREPRENORDOC, in the framework of Human Resources Development Operational Programme 2014–2020, financed from the European Social Fund under the contract number 36355/23.05.2019 HRD OP/380/6/13–SMIS Code: 123847.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The authors confirm that the data supporting the findings of this study are available within the article.

Conflicts of Interest

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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Figure 1. Oxidation of ascorbic acid in iodometric analysis. Adapted from [57].
Figure 1. Oxidation of ascorbic acid in iodometric analysis. Adapted from [57].
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Figure 2. Electrode chronoamperogram during PPy electropolymerization. The applied potential was 0.8 V, time 30 s, in 0.1 M pyrrole, and 0.1 M (K4[Fe(CN)6]) solution.
Figure 2. Electrode chronoamperogram during PPy electropolymerization. The applied potential was 0.8 V, time 30 s, in 0.1 M pyrrole, and 0.1 M (K4[Fe(CN)6]) solution.
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Figure 3. (A) Pt/PPy-FeCN sensor stabilization in the potential range −1 and +1.3 V at scan rate 100 mV/s in 0.1 M KCl solution; (B) Modified stable electrode signal in 0.1 M KCl solution.
Figure 3. (A) Pt/PPy-FeCN sensor stabilization in the potential range −1 and +1.3 V at scan rate 100 mV/s in 0.1 M KCl solution; (B) Modified stable electrode signal in 0.1 M KCl solution.
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Figure 4. (A) Pt/PPy-FeCN sensor stabilization in the potential range −1.0 to +0.5 V at 100 mV/s scan rate in 0.1 M KCl solution; (B) Pt/PPy-FeCN sensor stable signal in 0.1 M KCl solution.
Figure 4. (A) Pt/PPy-FeCN sensor stabilization in the potential range −1.0 to +0.5 V at 100 mV/s scan rate in 0.1 M KCl solution; (B) Pt/PPy-FeCN sensor stable signal in 0.1 M KCl solution.
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Figure 5. (A). Stabilization of Pt/PPy-FeCN electrode in 0.001 M K4[Fe(CN)6]-0.1 M KCl double solution in the potential range from −1 to +1.3 V at scan rate 100 mV/s; (B). Stable sensor signal in 0.001 M K4[Fe(CN)6]-0.1 M KCl double solution in the potential range −1 and +1.3 V at scan rate 100 mV/s.
Figure 5. (A). Stabilization of Pt/PPy-FeCN electrode in 0.001 M K4[Fe(CN)6]-0.1 M KCl double solution in the potential range from −1 to +1.3 V at scan rate 100 mV/s; (B). Stable sensor signal in 0.001 M K4[Fe(CN)6]-0.1 M KCl double solution in the potential range −1 and +1.3 V at scan rate 100 mV/s.
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Figure 6. (A). Stabilization of Pt/PPy-FeCN electrode in 0.001 M K4[Fe(CN)6]-0.1 M KCl double solution in the potential range −1.0–+0.5 V at a scan rate 100 mV/s; (B). Stable sensor signal in 0.001 M K4[Fe(CN)6]-0.1 M KCl double solution in the same working parameters.
Figure 6. (A). Stabilization of Pt/PPy-FeCN electrode in 0.001 M K4[Fe(CN)6]-0.1 M KCl double solution in the potential range −1.0–+0.5 V at a scan rate 100 mV/s; (B). Stable sensor signal in 0.001 M K4[Fe(CN)6]-0.1 M KCl double solution in the same working parameters.
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Figure 7. Cyclic voltammograms of the Pt/PPy-FeCN sensor in 0.001 M K4[Fe(CN)6]-0.1 M KCl double solution, recorded at scanning rates between 100 and 1000 mV/s.
Figure 7. Cyclic voltammograms of the Pt/PPy-FeCN sensor in 0.001 M K4[Fe(CN)6]-0.1 M KCl double solution, recorded at scanning rates between 100 and 1000 mV/s.
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Figure 8. (A). Linear fit plot between Ipa and scan rate for the Pt/PPy-FeCN electrode; (B). Linear fit plot between Ipa and the square root of scan rate for the Pt/PPy-FeCN electrode.
Figure 8. (A). Linear fit plot between Ipa and scan rate for the Pt/PPy-FeCN electrode; (B). Linear fit plot between Ipa and the square root of scan rate for the Pt/PPy-FeCN electrode.
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Figure 9. Stable electrochemical behavior of the Pt/PPy-FeCN sensor in a double solution of 0.1 M KCl and 0.001 M AAs, at a scan rate of 100 mV/s and in the potential range −1.0 and +1.0 V.
Figure 9. Stable electrochemical behavior of the Pt/PPy-FeCN sensor in a double solution of 0.1 M KCl and 0.001 M AAs, at a scan rate of 100 mV/s and in the potential range −1.0 and +1.0 V.
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Figure 10. Scheme of the mechanism of AAs detection at the Pt/PPy-FeCN electrode surface. Adapted from [62].
Figure 10. Scheme of the mechanism of AAs detection at the Pt/PPy-FeCN electrode surface. Adapted from [62].
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Figure 11. (a). Catalytic oxidation of ascorbic acid at various scan rates. Voltammetric responses of Pt/PPy-FeCN electrode in 0.1 M KCl and 0.001 M AAs double solution, recorded at scanning rates between 100 and 1000 mV/s; (b). Linear dependence between cathodic peak and scan rate.
Figure 11. (a). Catalytic oxidation of ascorbic acid at various scan rates. Voltammetric responses of Pt/PPy-FeCN electrode in 0.1 M KCl and 0.001 M AAs double solution, recorded at scanning rates between 100 and 1000 mV/s; (b). Linear dependence between cathodic peak and scan rate.
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Figure 12. (A) CVs of the Pt/PPy-FeCN sensor immersed in AAs solutions in the range 1 and 100 × 10−6 M; (B) Dependence of cathodic current intensity on AAs concentration in the range 1 and 100 × 10−6 M.
Figure 12. (A) CVs of the Pt/PPy-FeCN sensor immersed in AAs solutions in the range 1 and 100 × 10−6 M; (B) Dependence of cathodic current intensity on AAs concentration in the range 1 and 100 × 10−6 M.
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Figure 13. Quantitative determination of ascorbic acid from (a) chewable tablet 180 mg; (b) injectable ampoule 750 mg; (c) powder for oral suspension 1000 mg.
Figure 13. Quantitative determination of ascorbic acid from (a) chewable tablet 180 mg; (b) injectable ampoule 750 mg; (c) powder for oral suspension 1000 mg.
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Table
Table 1. Peak intensities and potentials of the Pt/PPy-FeCN electrode immersed in 0.1 MKCl solution at a 100 mV/s scan rate.
Table 1. Peak intensities and potentials of the Pt/PPy-FeCN electrode immersed in 0.1 MKCl solution at a 100 mV/s scan rate.
Pt/PPy-FeCN Electrode
Epa 1 (V)Epc 2 (V)ΔE 3 (V)Ipa 4 (µA)Ipc 5 (µA)Ipc/Ipa
Redox system I−0.129−0.1690.0423.07−15.680.67
Redox system II0.3220.2390.0818.35−14.450.78
1 Potential of the anodic peak; 2 potential of the cathodic peak; 3 ∆E = Epa − Epc; 4 current of the anodic peak; 5 current of the cathodic peak; Pt/PPy/FeCN–platinum/polypyrrole/potassium hexacyanoferrate (II).
Table
Table 2. Peak intensities and potentials of Pt/PPy-FeCN electrode immersed in 0.001 M K4[Fe(CN)6]-0.1 M KCl solution with 100 mV/s scan rate.
Table 2. Peak intensities and potentials of Pt/PPy-FeCN electrode immersed in 0.001 M K4[Fe(CN)6]-0.1 M KCl solution with 100 mV/s scan rate.
Pt/PPy-FeCN Electrode
Epa 1 (V)Epc 2 (V)ΔE 3 (V)Ipa 4 (µA)Ipc 5 (µA)Ipc/Ipa
Redox system I−0.171−0.1010.06921.07−42.111.99
Redox system II0.4180.2870.131101.76−48.990.48
Cathodic peak III-−0.375--−21.98-
1 Epa—potential of the anodic peak; 2 Epc—potential of the cathodic peak; 3 ΔE = Epa − Epc; 4 Ipa—current of the anodic peak; 5 Ipc—current of the cathodic peak.
Table
Table 3. Active area of the Pt and Pt/PPy-FeCN sensor.
Table 3. Active area of the Pt and Pt/PPy-FeCN sensor.
ElectrodePtPt/PPy-FeCN
Active area (cm2)0.544.1
Geometric area (cm2)0.12650.1265
Roughness factor4.2632.64
Table
Table 4. Electrochemical parameters of Pt/PPy-FeCN sensor immersed in 0.1 M KCl and 0.001 M AAs double solution at a 100 mV/s scan rate.
Table 4. Electrochemical parameters of Pt/PPy-FeCN sensor immersed in 0.1 M KCl and 0.001 M AAs double solution at a 100 mV/s scan rate.
SensorEpa 1 (V)Epc 2 (V)ΔE 3 (V)Ipa 4 (µA)Ipc 5 (µA)Ipc/Ipa
Pt/PPy-FeCN0.2230.1160.10689.14−79.970.89
1 Epa—potential of the anodic peak; 2 Epc—potential of the cathodic peak; 3 ΔE = Epa − Epc; 4 Ipa—current of the anodic peak; 5 Ipc—current of the cathodic peak.
Table
Table 5. The main sensors developed for the detection of AAs.
Table 5. The main sensors developed for the detection of AAs.
ElectrodeMethod of DetectionLOD (M)Real SampleReferences
PPy/FCN/Fe and PPy/FCN/Fe3O4/Fe)
(a polypyrrole containing ferrocyanide ions)		CV0.15 × 10−3for the determination
of H2A in aqueous solution		[65]
Ppy/FCNMCPEs
(polypyrrole/ferrocyanide films modified carbon paste electrode)		CV5.82 × 10−5-[66]
AuNPs@GO/PPy/CFP
(a gold nanoparticle decorated polypyrrole/graphene oxide composite on carbon fiber paper)		CV2.43 × 10−6urine sample[67]
Cu-PPy/Si
(copper-polypyrrole/silicon)		CV, CA0.20 × 10−3-[68]
rGO/Pd@PPy NPs
(palladium nanoparticles supported on polypyrrole/reduced graphene oxide)		CV4.9 × 10−8serum sample[69]
PtNCs-MWCNTs-GNPs
(platinum nanochains- multi-walled carbon nanotubes- graphene nanoparticles)		CV, DPV10.0 × 10−6vitamin C tablets[70]
CuxO-ZnO/PPy/RGO
(copper oxide-Zinc oxide/polypyrrole/reduced graphene oxide)		CV, DPV22 × 10−9human plasmatic serum[71]
3DCu(x)O-ZnO NPs/PPy/RGO
(a three-dimensional porous nanocomposite of reduced graphene oxide decorated with polypyrrole nanofibers and zinc oxide-copper oxide)		CV0.024human blood serum[72]
Pt/PPy-FeCN
(platinum/polypyrrole- hexacianoferat de potasiu)		CV, CA2.5 × 10−7pharmaceuticalsthis work
Table
Table 6. Results of the quantitative determination of AAs in comparative pharmaceutical products obtained using the CV method and iodometric reference method.
Table 6. Results of the quantitative determination of AAs in comparative pharmaceutical products obtained using the CV method and iodometric reference method.
The Pharmaceutical Form in which AAs Is FoundIpc (V)Epc (V)Manufacturer’s Reported Concentration (mg)AAs Concentration (mg)
CV TechniqueIodometric Method STAS 6182/15-87
Chewable tablet−6.02−443.58180180 ± 6180 ± 7
Injectable ampoule−5.38750750 ± 20750 ± 23
Powder for oral suspension−6.6610001000 ± 251000 ± 31
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Dinu, A.; Apetrei, C. Determination of Ascorbic Acid in Pharmaceuticals and Food Supplements with the New Potassium Ferrocyanide-Doped Polypyrrole-Modified Platinum Electrode Sensor. Chemosensors 2022, 10, 180. https://doi.org/10.3390/chemosensors10050180

AMA Style

Dinu A, Apetrei C. Determination of Ascorbic Acid in Pharmaceuticals and Food Supplements with the New Potassium Ferrocyanide-Doped Polypyrrole-Modified Platinum Electrode Sensor. Chemosensors. 2022; 10(5):180. https://doi.org/10.3390/chemosensors10050180

Chicago/Turabian Style

Dinu, Ancuța, and Constantin Apetrei. 2022. "Determination of Ascorbic Acid in Pharmaceuticals and Food Supplements with the New Potassium Ferrocyanide-Doped Polypyrrole-Modified Platinum Electrode Sensor" Chemosensors 10, no. 5: 180. https://doi.org/10.3390/chemosensors10050180

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