Poly(bromocresol purple)-Based Voltammetric Sensor for the Simultaneous Quantification of Ferulic Acid and Vanillin

Natural phenolic antioxidants are extensively studied compounds due to their positive health effect and wide distribution in human diets. The simultaneous occurrence in samples requires selective methods for their determination. Electrochemical sensor based on the polyaminobenzene sulfonic acid functionalized single-walled carbon nanotubes (f-SWCNT) and electropolymerized bromocresol purple has been developed for the simultaneous quantification of ferulic acid and vanillin. The electrode has been characterized by scanning electron microscopy (SEM) and electrochemical methods, and the effectivity of the developed modifier has been confirmed. Thus, the novel sensitive voltammetric sensor is simple to fabricate, reliable, cost-effective, and can be applied for foodstuff screening.


Introduction
Natural phenolic antioxidants are extensively studied compounds in modern electroanalysis due to their positive health effect and wide distribution in the human diet [1]. Simultaneous occurrence in samples requires selective methods for their determination. Among a wide range of natural phenolics, vanillin and its biological precursor ferulic acid [2] are of practical interest. High-performance liquid [3,4] and thin-layer chromatography [5] are usually applied for this purpose. Both phenolics under consideration are electrochemically active, which makes it possible to use electrochemical methods for their quantification. Although various types of electrochemical sensors have been developed for the simultaneous quantification of natural phenolics of different classes [6][7][8], ferulic acid and vanillin are not considered as analytes.
Thus, the current work is focused on the development of an electrochemical sensor based on a poly(bromocresol purple)-modified electrode for the simultaneous quantification of ferulic acid and vanillin.

Materials and Methods
Bromocresol purple (90% purity), 99% vanillin from Sigma-Aldrich (Steinheim, Germany), and 99% ferulic acid from Aldrich (Steinheim, Germany) were used. Their standard 10 mM solutions were prepared in ethanol (rectificate). The exact dilution was used for the preparation of less concentrated solutions.
All reagents had chemical-grade purity. Double distilled water was used for the measurements. The experiments were carried out at laboratory temperature (25 ± 2 • C).
Electrochemical measurements were carried out on the potentiostat/galvanostat Autolab PGSTAT 302N with FRA 32M module (Eco Chemie B.V., Utrecht, The Netherlands) and NOVA 1.10.1.9 software. The 10 mL glassy electrochemical cell with a working glassy carbon electrode (GCE) with a 7.07 mm 2 geometric surface area (CH Instruments, Inc., Bee Cave, TX, USA) or modified electrode, a silver-silver chloride saturated KCl reference electrode, and a platinum wire as the counter electrode was used.
An "Expert-001" pH meter (Econix-Expert Ltd., Moscow, Russian Federation) equipped with the glassy electrode was applied for pH measurements.
Scanning electron microscopy (SEM) was carried out on the high-resolution field emission scanning electron microscope MerlinTM (Carl Zeiss, Oberkochen, Germany) at the accelerating voltage of 5 kV and emission current of 300 pA.

Characterization of the Electrodes
Bromocresol purple forms a nonconducting film which is confirmed by the disappearance of the oxidation peak with an increase of the cycles number and which is typical for the electropolymerization of phenolics [9].
The conditions of bromocresol purple potentiodynamic electropolymerization (monomer concentration, number of scans, supporting electrolyte pH, electrolysis parameters) have been optimized in order to find the best voltammetric response of the co-existed ferulic acid and vanillin. The peak potential separation of 170 mV on the polymer-based electrode is not affected by electropolymerization conditions, while oxidation currents change significantly. The best response has been obtained for the poly(bromocresol purple) obtained by 10-fold potential cycling from 0.1 to 1.2 V with a scan rate of 100 mV s −1 from the 25 µM monomer solution in 0.1 M phosphate buffer pH 7.0.
The electrodes have been characterized by SEM ( Figure 1). The data obtained confirm the successful immobilization of the nanomaterial on the electrode surface. All reagents had chemical-grade purity. Double distilled water was used for the measurements. The experiments were carried out at laboratory temperature (25 ± 2 °C).
Electrochemical measurements were carried out on the potentiostat/galvanostat Autolab PGSTAT 302N with FRA 32M module (Eco Chemie B.V., Utrecht, The Netherlands) and NOVA 1.10.1.9 software. The 10 mL glassy electrochemical cell with a working glassy carbon electrode (GCE) with a 7.07 mm 2 geometric surface area (CH Instruments, Inc., Bee Cave, TX, USA) or modified electrode, a silver-silver chloride saturated KCl reference electrode, and a platinum wire as the counter electrode was used.
An "Expert-001" pH meter (Econix-Expert Ltd., Moscow, Russian Federation) equipped with the glassy electrode was applied for pH measurements.
Scanning electron microscopy (SEM) was carried out on the high-resolution field emission scanning electron microscope MerlinTM (Carl Zeiss, Oberkochen, Germany) at the accelerating voltage of 5 kV and emission current of 300 pA.

Characterization of the Electrodes
Bromocresol purple forms a nonconducting film which is confirmed by the disappearance of the oxidation peak with an increase of the cycles number and which is typical for the electropolymerization of phenolics [9].
The conditions of bromocresol purple potentiodynamic electropolymerization (monomer concentration, number of scans, supporting electrolyte pH, electrolysis parameters) have been optimized in order to find the best voltammetric response of the co-existed ferulic acid and vanillin. The peak potential separation of 170 mV on the polymer-based electrode is not affected by electropolymerization conditions, while oxidation currents change significantly. The best response has been obtained for the poly(bromocresol purple) obtained by 10-fold potential cycling from 0.1 to 1.2 V with a scan rate of 100 mV s −1 from the 25 μM monomer solution in 0.1 M phosphate buffer pH 7.0.
The electrodes have been characterized by SEM ( Figure 1). The data obtained confirm the successful immobilization of the nanomaterial on the electrode surface.

Simultaneous Quantification of Natural Phenolic Antioxidants
The well-resolved oxidation peaks of the ferulic acid and vanillin at 0.732 and 0.903 V, respectively, with a potential separation of 170 mV have been obtained on the created sensor (Figure 2).

Simultaneous Quantification of Natural Phenolic Antioxidants
The well-resolved oxidation peaks of the ferulic acid and van respectively, with a potential separation of 170 mV have been sensor (Figure 2).

Simultaneous Quantification of Natural Phenolic Antioxidants
The well-resolved oxidation peaks of the ferulic acid and vanillin at 0.732 and 0.903 V, respectively, with a potential separation of 170 mV have been obtained on the created sensor ( Figure 2).    Thus, the novel sensitive voltammetric sensor is simple to fabricate, reliable, costeffective, and can be applied for the foodstuff screening.
Funding: This research received no external funding.

Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.

Conflicts of Interest:
The authors declare no conflict of interest. Thus, the novel sensitive voltammetric sensor is simple to fabricate, reliable, costeffective, and can be applied for the foodstuff screening.
Funding: This research received no external funding.

Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.

Conflicts of Interest:
The authors declare no conflict of interest.