Induced Effect of Gold Nanoparticles (AuNPs) and Halide Ions on Pyridoxine Molecule Stability

Abstract

The electrochemical behavior of pyridoxine was studied in 0.1 mol × L⁻¹ NaX (X = F, Cl, Br) support electrolyte on a gold electrode using cyclic voltammetry and UV–Vis spectrophotometry. The influence of gold nanoparticles (AuNPs) on the electrochemical behavior of pyridoxine was studied. The experimental results obtained by both cyclic voltammetry and UV–Vis spectrophotometry indicate strong interactions in the B₆ and NaBr/NaCl_AuNP systems, while in the NaF_B₆_AuNP ternary system the results indicate a mechanism of direct electrochemical degradation of vitamin B₆. The experimental results obtained for the electrochemical degradation of pyridoxine, in the presence of chloride and bromide ions, indicate strong interactions in the NaCl_B₆_AuNP and NaBr_B₆_AuNP systems associated with the spectrophotometric identification of the electrogenerated intermediates, while in the presence of fluoride ions no such products are identified. The development of the mechanism of electrochemical degradation of the pyridoxine molecule predicts both the formation of the corresponding electrogenerated intermediates and the steps of electro-incineration in a direct mechanism.

1. Introduction

Vitamin B₆ (other names: pyridoxine, pyridoxol, pyridoxine hydrochloride) belongs to the group of soluble B vitamins that are involved in metabolic processes and plays an important role in human health and especially in DNA repair, for which it is an essential cofactor [1]. It is involved in the pathways of methyl-group metabolism as a cofactor for different important enzymes, thus being a protective nutrient for pancreatic cancer by preventing the alteration of the DNA methylation mechanism [2]. Precisely because of its importance in the body, this vitamin has been introduced in food supplements to combat its deficiency as a food additive with an antioxidant role and even in drugs, alone or in various multivitamin complexes. B₆ vitamers with common 2-methyl-3-hydroxypyridine structures (pyridoxine—P, pyridoxal—PO, pyridoxamine—PN and the corresponding 5`-phosphorylated derivatives; pyridoxine 5`-phosphate—P5P, pyridoxal 5`-phosphate—PO5P and pyridoxamine 5`-phosphate—PN5P) are biologically active compounds, known since the early 1900s and comprising six interconvertible members under the action of specific enzymes [1]. Pyridoxal-5′-phosphate is the form of B₆ vitamin that is biologically active, being produced in human organisms from the vitamers. It is involved in the regulation mechanisms of the glucose, lipid and amino-acid metabolism as a co-enzyme for about 160 different enzymatic reactions. The survival rate of non-small cell lung cancer patients was correlated with high levels of pyridoxal kinase, while the vitamin B₆ intake was inverse associated with colorectal cancer risk [3]. The antagonistic properties of B₆ vitamers have been discovered over time, so they are powerful antioxidants (inactivate reactive oxygen species) but can also act as endogenous photosensitizers that lead to the formation of reactive oxygen species [1].

Along with vitamin B₁ and B₁₂, pyridoxine is involved in peripheral nerve repair processes through mechanisms that are yet to be fully understood. The evidence for this role was provided by numerous animal studies over time, and now these elements are referred to as “neurotropic” vitamins. Pyridoxine plays a very important role in the synthesis of neurotransmitters and in inhibition of neurotoxic glutamate [4].

Vitamin B₆ is obtained industrially through fermentation processes, so it was necessary to develop more and more efficient methods to ensure maximum production efficiency and, on the other hand, a simple and accurate method of determining the concentration of this vitamin. Application of the liquid chromatography (LC) method using different detection techniques for determination of vitamin B₆ in pharmaceutical formulations, plant foods, human plasma and serum demonstrated a high degree of sensitivity, precision and linearity [5,6,7,8,9,10]. Determination of vitamin B₆ in tablets by LC with electrochemical detection using a carbon fiber electrode showed a detection limit of 2.5 ng for P and 1.0 ng for PO and PN [5]. LC coupled to mass spectrometry (MS) provided a good linearity between 5.0 and 400.0 ng × mL⁻¹ [6]. High-performance LC (HPLC) with coulometric electrochemical and ultraviolet detection using C18 column type indicated detection limits of 2.7 [7] and 0.18 ng × mL⁻¹ [8]. Simultaneous determination of vitamin B₆ in the presence of co-administrated compounds such as other vitamins or antiepileptics showed a good predictability with reversed-phase HPLC (RP-HPLC) [9,10]. Limits of quantification of 0.39 µg × mL⁻¹ and 0.11 ng × mL⁻¹ were obtained in the presence of levetiracetam and B₁/B₁₂, respectively.

Experimental determination of vitamin B₆ can involve some analytical impediments due to its complex structure that can lead to the appearance of intermolecular interferences. Some studies involved vitamin B₆ separation and determination in food, dietary supplements and pharmaceutical tablets using various spectrometric techniques [11,12,13,14,15,16]. UV–Vis spectrophotometry applied in the analysis of samples containing vitamin B₆, showed an increase in sensitivity followed by a decrease in detection limits to values of about 60 ng × mL⁻¹ [11], 5–25 µg × mL⁻¹ [12] and 2–21 µg × mL⁻¹ [13]. The use of fluorescence spectrophotometry, without or with a CdSe quantum dot sensor, also led to detection limits of 33–450 µg × mL⁻¹ [14,15]. UV–Vis spectrophotometry, fluorescence spectrophotometry and FTIR spectroscopy allowed the individual physicochemical study of pyridoxine as well as its interactions with bovine serum albumin [16].

Although these methods have advantages such as selectivity and sensitivity, they also have certain limitations, such as high costs, elaborate sample preparation processes and long analysis time. Compared to these methods, electrochemical sensors have been introduced and developed as a modern alternative for determining vitamins [17,18,19,20,21,22,23,24,25,26,27,28,29,30]. Electrochemical sensors have a much greater simplicity, miniaturization, reproducibility, low cost and high sensitivity, and among them, vitamin sensors based on carbonaceous materials, namely glassy carbon electrodes [18,19,20,21], carbon ceramic electrodes [22], graphite, [23,24], screen printed carbon [25], multiwalled carbon nanotubes [26,27], graphene [28] and boron-doped diamond [29,30] have been used. The performance of the electrochemical sensors for detecting water-soluble vitamins (B₁, B₂, B₆, B₉, B₁₂, C) and fat-soluble vitamins (A, D, E, K) varies from the nano to micro scale and takes into account the electrochemical processes of vitamins on the electrode surface [17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34]. Sensitive hybrid heterostructures such as SnO₂ nanoparticles supported mesoporous TiO₂ over glassy carbon electrodes [31], Au–CuO core–shell/multiwalled carbon nanotubes [32], calix-8-arene/phosphotungstic acid/di isooctyl phthalate [33] and β-cyclodextrin/carboxylated poly(vinyl chloride) [34] are reported for pyridoxine determination.

Nanomaterials incorporating electrochemical sensors for vitamin B₆ determination are an excellent class of materials due to their unique multifunctionality [35]. Synthesis, characterization and application of gold and silver nanoparticles are revolutionizing all the fields to which they are applied [36,37,38,39]. Probably the most important applications are the detection, diagnosis and therapy of cancer with gold nanoparticles [40,41]. Despite their valuable properties and bio-applications, gold and silver nanoparticles also have negative aspects, such as toxicity [42,43,44]. The synthesis and use of gold and silver nanoparticles and even gold–silver bimetallic nanoparticles in the removal of pollutants from polluted waters have been reported [44,45,46,47,48,49].

Simultaneously with pyridoxine, the halide ions can be ingested from food or by administering sodium halide-containing drugs. In particular cases, the gold nanoparticles can be an option for antiviral treatment, inducing immunostimulatory activity. In this context, it is important to study the concerted action of these biologically active compounds. Various interactions between halide ions and vitamin molecules or AuNPs can occur, leading to the inhibition or stimulation of vitamin degradation reaction. Consequently, the present study aimed to highlight the induced effects by gold nanoparticles on the decomposition/transformation reaction of B₆ vitamin molecules. Concomitantly, the influence of B₆ vitamin molecules and halide ions on the on the activity of gold nanoparticles was investigated.

Given all these findings, we conducted studies related to the competition between vitamin B₆, gold nanoparticles and halide anions by exploiting the sensitivity of their electrochemical properties. We performed electrochemical and spectrophotometric characterization of their binary and ternary systems (NaX/AuNPs; NaX/B₆; NaX/B₆/AuNPs) to determine if there is a strong interaction between the species present in the solution.

2. Materials and Methods

2.1. Materials

Vitamin B₆ was purchased (ZENTIVA SA ROMANIA) as 5 mL vials containing the active substance pyridoxine as pyridoxine hydrochloride at a concentration of 250 mg/5 mL with pure water. Single use vials of 250 mg/5 mL vitamin B₆ solution for muscularly or intravenous use only was used as stock solutions. NaF, NaCl and NaBr used as supporting electrolyte were of AR grade and acquired from Fluka. Gold nanoparticle (AuNP) suspension was purchased from AquaNano (Aghoras Invent srl, Bucharest) as 480 mL bottles with a concentration of 110 mg × L⁻¹.

Four working solutions were used as follows:

(i)

0.1 mol × L⁻¹ NaX (NaX curves);

(ii)

0.1 mol × L⁻¹ NaX, 2.9 × 10⁻⁴ mol × L⁻¹ B₆, (NaX_B₆ curves);

(iii)

0.1 mol × L⁻¹ NaX, 75 mg × L⁻¹ AuNPs, (NaX_AuNP curves);

(iv)

0.1 mol × L⁻¹ NaX, 2.9 × 10⁻⁴ mol × L⁻¹ B₆, 75 mg × L⁻¹ AuNPs, (NaX_B₆_AuNP curves).

All solutions were freshly prepared and used on the same day.

2.2. Methods

Cyclic voltammetry and electrolysis at constant current density (50 mA × cm⁻²) were performed using a VoltaLab 40 controlled by a PC using VoltaMaster 4 software.

Cyclic voltammograms were recorded at a scan rate of 100 mV × s⁻¹, using a traditional one-compartment three electrode electrochemical cell consisting of two gold electrodes (work and counter electrode) and an AgǀAgCl, KClsat reference electrode. The working electrode and the counter electrode were made of glossy gold and had a geometric surface of 1 × 1 × 0.1 cm.

All voltammetric experiments were performed in stirred conditions at stirring rate of 300 rpm (with a magnetic bar) in order to prevent any diffusion problems.

Controlled current density electrolysis (50 mA × cm⁻²) was performed in tandem with UV–Vis spectrophotometry. UV–Vis spectra changes for each of the four working solutions were evaluated with a VarianCary spectrophotometer (CaryWin UV software) using quartz cuvettes with 1 cm optical path.

The experimental conditions and methods are presented in previous articles of the authors [39,48,50,51,52,53,54,55].

3. Results and Discussion

Cyclic voltammograms of the four systems (NaX; NaX_B₆; NaX_AuNPs and NaX_B₆_AuNPs) were produced to study the redox behavior of pyridoxine at a glossy gold electrode in dynamic experimental conditions.

3.1. The Influence of Fluoride Ions on Electrochemical Behavior of Pyridoxine

Figure 1 presents a comparison of the voltammograms recorded on the gold electrode in the supporting electrolyte of 0.1 mol × L⁻¹ NaF, in the absence and in the presence of 75 mg × L⁻¹ AuNPs and 2.9 × 10⁻⁴ mol × L⁻¹ pyridoxine (vitamin B₆).

As can be seen, the supporting electrolyte leads to a higher peak corresponding to the potential range between 1.5 and 2.0 V. However, the addition of gold nanoparticles in the electrolyte solution leads to the appearance of a new wide peak of the current density recorded between 0.8 and 1.5 V vs. Ag|AgCl reference electrode. At high values of the anodic overvoltage, oxides/hydroxides (AuOH, AuO, Au₂O₃) are formed on the surface of the gold electrode [56]. The electrogeneration of oxides on the surface of the gold electrode modifies the catalytic properties of the surface [56], which can lead to various mechanisms of electrochemical degradation of pyridoxine molecules. The onset potential for initiating the growth of the mixed oxide/hydroxide layer is about 1.35 V. The irreversible feature in the gold response is the huge anodic peak that extends in the range of 1.5 to 2.0 V.

In terms of electrochemical properties, the potential range from 1.5 to 2.0 V is very important because, in addition to oxygen depolarization, there is also the continuous change of the electrode/electrolyte interface. In this range of potential, gold nanoparticles oxidize (NaF_AuNP curve). The properties of gold nanoparticles are highlighted so that the anodic current densities are considerably reduced in their presence, decreasing from 37 (NaF curve) to 22 mA × cm⁻² (NaF_AuNP curve). According to the cyclic voltammogram NaF_B₆, lower values of anodic current densities are recorded, compared to that of the supporting electrolyte. This is due to the strong adsorption processes of organic vitamin molecules on the surface of the gold electrode. The electrochemical analysis highlights the heterogeneous oxidation processes corresponding to the oxidation of organic molecules by the appearance of an anodic peak, well defined by low intensity at anodic potential of 0.9 V. In the detail of the NaF_B₆ curve, shown in Figure 2, there is a clear increase in current densities starting with potential greater than 0.8 V. This is also the onset potential corresponding to the charge transfer in which the pyridoxine molecules are involved. The simultaneous presence in the electrolyte solution of pyridoxine and nanoparticles (NaF_B₆_AuNP curve) has an antagonistic effect, thus reducing the anodic current densities corresponding to oxygen depolarization (NaF curve), but at the same time has higher current densities compared to the NaF_AuNP curve. The characteristics and appearance of cyclic voltammograms also changes during cathodic polarization. The cathodic polarization highlights the appearance of a peak of the cathodic current density in the support electrolyte, with low intensity (0.7 mA × cm⁻²), due to the reduction of oxides/hydroxides on the surface of the gold electrode. Characteristic to this peak is that, in the presence of gold nanoparticles, the peak intensity increases to 7.5 mA × cm⁻², and in the ternary system (NaF_B₆_AuNPs) it has the highest value of 12.6 mA × cm⁻².

A comparison of the UV–Vis molecular absorption spectra of freshly prepared solutions, as well as after the recording of cyclic voltammograms, is presented in Figure 2.

The UV–Vis spectra corresponding to the support electrolyte solution (NaF) show a great similarity before (b) and after (a) the performance of cyclic voltammetry. This means that the supporting electrolyte influences the electrode processes only to a very small extent, for example through its high polarizability.

The UV–Visible spectrophotometer provides qualitative and quantitative information about the participation of AuNPs in electrode processes and implicitly about the modification of their properties. The presence of AuNPs in the electrolyte solution is confirmed by the absorption maximum recorded at the wavelength of 570 nm [37]. As can be seen in Figure 2 (NaF_AuNP spectra), during the recording of the cyclic voltammogram, the gold nanoparticles undergo both chemical and physicochemical changes. Thus, the spectrum recorded after the CV is located at slightly lower values of the absorbance, being the proof of their consumption in the electrode processes.

The spectral properties of the pyridoxine molecule are highlighted by three absorption maxima, in the UV range, corresponding to the wavelengths λmax = 326, 256 and 221 nm [1]. Cyclic voltammetry is a “short-time” method, and therefore the processes of electrochemical degradation of pyridoxine cannot ensure a total consumption of pyridoxine molecules from the electrolyte solution. However, the UV–Vis spectrum of pyridoxine recorded after CV (NaF_B₆, −a) is below the initial spectrum (NaF_B₆, −b), the electrocatalytic properties of the gold electrode being effective for the electrochemical degradation of this vitamin.

The UV–Vis spectrum of the electrolyte solution for the ternary system (NaF_B₆_AuNPs) corroborates the results obtained both by cyclic voltammetry and by spectrophotometric analysis. Thus, this spectrum highlights both strong chemical interactions between ionic and molecular species present in the electrolyte solution and their electrochemical degradation.

The electrochemical behavior of pyridoxine over time has been studied in order to identify the electrochemical stability of the vitamin and how its stability is influenced by gold nanoparticles (Figure 3).

Figure 3 shows the UV–Vis spectra during the electrode processes (current density = 50 mA × cm⁻²) for pyridoxine and gold nanoparticles, individually (NaF_B₆ and NaF_AuNPs) as well as together (NaF_B₆_AuNPs).

The UV–Vis spectrum NaF_B₆ indicates a gradual decrease in absorbance over time. In this electrolyte solution, vitamin molecules participate in redox processes, but there is only a minor decrease in absorbance corresponding to pyridoxine. An important indication is also given by the fact that no new absorption maxima are recorded. This indicates an electrochemical degradation of the pyridoxine to the final products that are not active in the analyzed spectrophotometric range (200–800 nm); pyridoxine molecules are most likely degraded to final products such as carbon dioxide, nitrogen oxides and water.

During electrolysis at a constant current density, the gold nanoparticles are also oxidized in the NaF support electrolyte solution. Under an electrical impulse, the UV–Vis spectra (NaF_AuNPs) recorded every 10 min show a total degradation at the end of the electrolysis of 60 min.

The NaF_B₆_AuNP ternary system has two important features of evolution over time: (i) the absorbance values decrease over time, both those corresponding to pyridoxine and those corresponding to nanoparticles, and (ii) the absorption maxima show a slight shift towards longer wavelengths (bathochromic effect), both those corresponding to pyridoxine and those corresponding to nanoparticles.

The study of the processes that take place was continued by applying the kinetic models of the zero- and first-order reactions. The experimental data obtained by recording the variation of UV–Vis absorption spectra over time, of electrolyte solutions at constant current density, were used in the kinetic approach of the processes (Figure 4). Thus, the rate of electrochemical degradation processes of pyridoxine (λ = 326 nm) and gold nanoparticles (λ = 570 nm) was calculated (

Table 1. Kinetic parameters for B₆ and AuNP electrochemical degradation, individual and mixed.

Experimental Conditions	Kinetic Model	k
B6 individual	zero-order reactions	0.0006 u.Abs. × min−1
B6 in the presence of AuNPs	first-order reactions	0.005 min−1
AuNPs individual	zero-order reactions	0.0048 u.Abs. × min−1
AuNPs in the presence of B6	first-order reactions	0.004 min−1

).

According to Figure 4a and Table 1, it is observed that the rate of electrochemical degradation of pyridoxine is higher in the presence of gold nanoparticles (there is a 7-fold increase). At the same time, a change in the initial absorbance value is highlighted, which proves a spontaneous chemical interaction between the pyridoxine molecules and the gold nanoparticles. Another important feature is that, in the presence of gold nanoparticles, it fits a first-order reaction model, which means that the rate is proportional to the concentration of a single reactant [57].

Gold nanoparticles exhibit opposite electrochemical behavior, both in the absence and in the presence of pyridoxine molecules. The difference between the value of the initial absorbance is very small (0.73 ± 0.04). In this situation, gold nanoparticles are characterized by high stability, their spectrum being modified to a very small extent. The presence of pyridoxine molecules in the electrolyte solution leads to a decrease in the rate of AuNP degradation.

In conclusion, it can be said that pyridoxine molecules delay the oxidation of AuNPs, while AuNPs accelerate the oxidation of pyridoxine molecules.

3.2. The Influence of Chloride Ions on Electrochemical Behavior of Pyridoxine

Chloride ions are part of the active electrolytes, so they participate in the electrode processes [38,39]. Under these conditions, the processes are different from the case of fluoride ions, ions that are electrochemically inactive.

Cyclic voltammetry responses of gold electrode in binary electrolyte solutions, NaCl_B₆, NaCl_AuNPs and ternary NaCl_B₆_AuNPs are shown in comparison to those of the individual support electrolyte (NaCl) in Figure 5.

The cyclic voltammograms in Figure 5 show an intensification of the electrode processes in the field of high anodic overvoltage. At a working electrode potential greater than 0.8 V, the current densities increase significantly. The cyclic voltammogram indicates a wide, clearly contoured peak of the anodic current density, between 0.8 and 1.7 V, specific to the oxidation of the gold electrode surface. This anodic peak retains its shape in the absence and presence of pyridoxine molecules but shows small changes in its characteristics. This anodic oxidation peak, although very wide, includes both the processes of the metal surface and the processes of organic molecules, clearly highlighted by the spectrophotometric analysis associated with cyclic voltammetry. The very high hysteresis of the cyclic voltammogram, recorded in the presence of chloride ions, indicates a very high susceptibility to oxidation and is due to the much higher aggressiveness of these ions. In this potential field, the oxidation of chloride ions also takes place, with the electrochemical generation of active chloride species, for example hypochlorite, chlorite, chlorate and perchlorate anions [50,51,55]. Anodic current densities increase 10 times more than in the case of fluoride ions, but in both cases, the current densities are reduced by the addition of gold nanoparticles.

The presence of AuNPs in the electrolyte solution leads to a decrease in both anodic and cathodic current densities. The presence of pyridoxine molecules in the electrolyte solution results in increased anodic current densities, associated with the electrooxidation of organic molecules on the surface of the gold electrode.

The cathodic peak recorded at the potential of 0.0 V is observed only in the absence of gold nanoparticles. This peak is of the irreversible type and, consequently, depends on the nature and composition of the electrolyte solution. Under these conditions, the structure and composition of the layer formed on the surface of the gold electrode may be different.

The results of the spectrophotometric analysis of the electrolyte solutions, before—b and after—a, the recording of cyclic voltammograms are represented in Figure 6.

The UV–Vis spectrum of the support electrolyte solution, 0.1 mol × L⁻¹ NaCl, shows an increase in absorbance at wavelengths less than 270 nm due to the presence of oxygenated chlorine compounds.

The presence of gold nanoparticles reduces absorbance; in other words, the nanoparticles prevent, to a greater or lesser extent, the electrooxidation of chloride ions. The absorption peaks corresponding to the vitamin are modified so that pyridoxine can no longer be identified qualitatively.

Spectral changes are identified for both the supporting electrolyte and the pyridoxine; however, the maximum absorption corresponding to gold nanoparticles does not undergo significant changes. This is a common feature of the electrochemical behavior of pyridoxine in the presence of chloride and fluoride ions.

The electrolyte solutions NaCl_B₆, NaCl_AuNPs and NaCl_B₆_AuNPs were subjected to electrolysis at constant current density in association with UV–Vis spectrophotometric analysis. The UV–Vis spectra recorded every minute, over a period of 10 min, are shown in Figure 7.

Figure 7 shows a strong solvatochromic effect in the NaCl_B₆ system, compared to the NaF_B₆ system, due to the interaction of chloride ions with the pyridoxine ring. It is observed that pyridoxine molecules are electrochemically degraded from the first minutes of electrolysis. In the presence of chloride ions, gold nanoparticles are also degraded from the first minutes of electrolysis. Thus, the processes take place at a much higher speed, and no effect of delay or catalytic activation of AuNPs can be observed.

The three spectra in Figure 7 are characterized by the appearance of a maximum absorption, of increasing intensity, at the wavelength of 293 nm, attributed to the formation of chlorinated oxygen species [51].

3.3. The Influence of Bromide Ions on Electrochemical Behavior of Pyridoxine

Electrode processes are influenced by the nature and composition of the electrolyte solution. At high values of anodic overvoltage, the working electrode is active; thus, the composition and structure of the surface layer differ with the nature of the counterion.

Figure 8 shows the cyclic voltammograms of the gold electrode in the absence and presence of pyridoxine and gold nanoparticles in the supporting electrolyte represented by 0.1 mol × L⁻¹ NaBr.

A comparison of typical cyclic voltammograms, in the absence and presence of pyridoxine and AuNPs, for the gold electrode in aqueous NaBr solution is presented.

For all the considered electrolyte systems, the observed voltammetric response was in accordance with previous findings in the literature [58,59,60]. The systems are characterized by an anodic peak in the forward step and the absence of any peak on the reverse scan, indicating that the reaction is irreversible in all cases. As can be seen, pyridoxine provides smaller peaks at the same potential values relative to the Au electrode. However, the decrease is not very significant, from 21 to 16 mA × cm⁻². This is due to the very good electrocatalytic properties of gold and implicitly to the phenomena of chemisorption of pyridoxine molecules on the surface of the gold electrode. The nature of the ion present in the working solution also has a major influence on the electrochemical mechanism. It can be seen that the bromide ion shifts the onset potential of pyridoxine oxidation to lower values compared to other anions, without being able to make a clear separation between the electrogeneration processes of oxybrominated species and the electrooxidation/electrotransformation processes of pyridoxine.

A peculiarity of the cyclic voltammograms recorded in the presence of bromide is the absence of the cathodic peak attributed to the reduction of oxygen species (gold oxides/hydroxides) on the metal surface.

To investigate the effect of bromide and AuNPs on the electrochemical stability of pyridoxine, a series of spectrophotometric experiments was performed in the corresponding binary and ternary electrolyte solutions (Figure 9).

The UV–Vis spectrum of the supporting electrolyte solution, obtained after (NaBr−a) recording the cyclic voltammogram, shows a slightly contoured peak at a wavelength of 331 nm, attributed to the brominated electrogenerated species [51].

The presence of electrogenerated brominated species leads to the degradation of pyridoxine molecules both by direct mechanisms and by indirect and electromediated mechanisms. The spectra shown in Figure 9 demonstrate this, so that the spectrum recorded after performing the cyclic voltammetry shows a significant change in absorbance. The characteristic peak of AuNPs, at a wavelength of 570 nm, disappears in both cases, both in the absence and in the presence of pyridoxine molecules.

To investigate the influence of AuNPs and halogen ions on the electrochemical behavior of pyridoxine, the spectrophotometric response of solutions subjected to electrolysis at constant current density was studied in the mixed electrolyte systems mentioned above (Figure 10).

As expected, pyridoxine is practically oxidized on gold electrodes under the given experimental conditions, from the first minute of electrolysis, in either case (NaBr_B₆ and NaBr_B₆_AuNPs).

The predominant process is represented by the electrogeneration of brominated species, as can be seen in Figure 10; they indicate the appearance of an absorption peak at the wavelength of 331 nm. Regarding the stability of AuNPs, the spectrophotometric analysis indicates the disappearance of the corresponding maximum absorption from the first moments of electrolysis without being influenced by the presence of pyridoxine molecules.

The interactions between molecular species and AuNPs in the presence of iodide ions can be of both practical and theoretical interest. According to previous studies [50], the 0.1 mol × L⁻¹ NaI solution has the lowest stability compared to that of the electrolytes containing the other sodium halides, namely NaF, NaCl and NaBr. As reported in our previous study [50], in the case of the NaI electrolyte containing drug, numerous interferences, before and after the electrochemical measurements, revealed in the wavelength range of 300–500 nm. These interferences would be multiplied in the presence of gold nanoparticles that absorb between 500 and 600 nm, leading to illegible and uncommentable spectrophotograms.

3.4. Mechanism of Electrochemical Degradation of Pyridoxine

The detailed electrooxidation mechanism of pyridoxine on the gold electrode may involve complex multistep/multielectron reactions (Scheme 1).

Taking into account the electrocatalytic properties of the gold electrode, the pyridoxine molecules are adsorbed by chemisorption phenomena on the metal surface. In the pyridoxine molecule, position 4 has a high reactivity and, participating in successive electrochemical and chemical processes, oxidizes to pyridoxic acid [58,59]. This mechanism corresponds to a process of electrooxidation by a direct path. In other words, the transfer of electrons takes place directly on the surface of the working electrode.

This mechanism applies to experimental observations obtained in the presence of an inert support electrolyte, such as a fluoride ion. At the same time, the mechanism ensures the electrochemical degradation of pyridoxine to obtain inorganic final products such as nitrogen dioxide, carbon dioxide and water.

The pyridoxine molecule contains both heteroatoms with unbound electrons and π-electrons in the pyridine ring. Wang et al. showed that the interactions between the π-electrons of pyridoxine and the gold metal surface are insignificant [59]. The results of FT-IR vibrational spectroscopy showed that the pyridoxine molecule is fixed on the surface of the gold electrode by means of the nitrogen atom. These studies also highlight the intermediate formation in the electrooxidation of pyridoxic acid [59]. Subsequent decomposition of pyridoxic acid indicates intermediates containing C=O and C-O groups. Electrochemical oxidation of pyridoxine through the pyridoxal intermediate takes place on different electrodes, such as pyrolytic graphite electrodes, single-walled carbon nanotubes and multi-walled carbon nanotubes, glassy carbon electrodes, carbon ceramic electrodes and reduced graphene oxide [35,58]. Electrochemical oxidation occurs through unstable intermediates with a very short lifespan. Even the oxidation of pyridoxine to pyridoxal or pyridoxic acid results in indeterminacy that is not specified by in situ analysis [59].

Thus, the most stable intermediate species are pyridoxal and pyridoxic acid, which electrochemically decompose in final compounds, step-by-step passing through radicals, ionic or more complex species with short lifespan.

In the presence of active anions, such as chloride and bromide ions, the electrogeneration of oxidizing active species also involves a homogeneous process of electrochemical oxidation (indirect pathway).

In the presence of gold nanoparticles, the oxidation mechanism of pyridoxine may also involve the formation of gold ions, as well as the possibility of the formation of mixed gold compounds with pyridoxine or intermediate degradation compounds.

4. Conclusions

Halogen ions are assimilated in the human body from many food sources, and they play many metabolic roles. The administration of gold nanoparticles as a tonic and/or for antibacterial purposes can lead to unknown interactions with consequences that are difficult to understand. By the action of halide ions, gold nanoparticles can dissolve; chemical interactions between them can be spontaneous leading to the formation of potentially toxic chemical species.

This study demonstrates both the chemical and electrochemical interaction between pyridoxine molecules, halide ions and gold nanoparticles.

In the presence of inert support electrolytes, such as fluoride ions, gold nanoparticles catalyze the electrochemical degradation of pyridoxine molecules. At the same time, pyridoxine decreases the electrooxidative dissolution of gold nanoparticles.

Active electrolytes, chloride and bromide, lead to active electrogenerated species, so the electrochemical degradation of pyridoxine and nanoparticles takes place more easily. As a general conclusion, gold nanoparticles most easily degrade electrochemically in the presence of bromide ions and pyridoxine molecules in the presence of chloride ions.

Regarding the current trend in consuming foods and food supplements without rigorous information, it should be noted that there can be biological conditions conducive to adverse interactions having a negative impact. The novelty of this study is attributed to experimental results that introduce new evidence of spontaneous or practically induced interactions, which can occur in binary and ternary systems consisting of pyridoxine, AuNPs and halide ions.

This study should be continued with clinical trials to identify the benefits or, conversely, the adverse effects of the simultaneous administration of these compounds.