2.3. Electrochemical Behavior of AA over a CPE/Poly-β-CD Electrode
Figure 4 shows the family of CVs recorded in the system CPE/poly-β-CD/1 mM AA, 0.1 M NaCl at pH 3.0, at different potential scan rates. Just like in the previous case, only one oxidation peak is observed, which corroborates that the process is still irreversible, although the oxidation peak is located at about 200 mV, a comparatively smaller potential. When plotting the
jpa as a function of
v, (see the inset in
Figure 4), a linear trend is obtained in the 10 to 100 mV s
−1 potential range, which indicates that the process is now adsorption-controlled [
28].
Figure 4.
(a) CVs obtained from the system CPE/poly-β-CD/1 mM AA, 0.1 M NaCl, at pH 3.0, at different potential scan rates, as indicated: (a) 10, (b) 20, (c) 30, (d) 40, (e) 50, (f) 60, (g) 70, (h) 80, (i) 90, mV s−1. The inset shows the variation of the peak’s current density, jp, as a function of ν, the circles correspond to experimental data (○) of the anodic peak, while the line corresponds to the linear fitting. The broken line corresponds to the cyclic voltammogram recorded in the same system but in the absence of AA.
Figure 4.
(a) CVs obtained from the system CPE/poly-β-CD/1 mM AA, 0.1 M NaCl, at pH 3.0, at different potential scan rates, as indicated: (a) 10, (b) 20, (c) 30, (d) 40, (e) 50, (f) 60, (g) 70, (h) 80, (i) 90, mV s−1. The inset shows the variation of the peak’s current density, jp, as a function of ν, the circles correspond to experimental data (○) of the anodic peak, while the line corresponds to the linear fitting. The broken line corresponds to the cyclic voltammogram recorded in the same system but in the absence of AA.
The AA adsorption on the CPE/poly-β-CD can be explained by the formation of an inclusion complex between the AA and the β-CD molecules that constitute the poly-β-CD polymer, see
Scheme 1. Notwithstanding that
Scheme 1 is a representation of the adsorption process of AA on the surfaces of the poly-β-CD modified CPE, it is based on our previous study [
32] regarding a quantum chemical study of the stability of the complex formed by the neutral form of AA, that predominate at pH 3.0, where the pKa value of AA is 4.17 [
33], and β-CD. In this study [
32] we found that two different structures are energetically favored for this inclusion complex namely that formed by the interaction through the functional hydroxyl groups of the lactone in AA and the primary hydroxyl groups of the β-CD or the β-CD-AA complex formation through the alcohol group of AA, as is represented in
Scheme 1. Furthermore, it should be added that the value of the thermodynamic constant of this inclusion complex formation has been reported by our group from both spectrophotometric and electrochemical calculations [
18].
Scheme 1.
Surface inclusion complex formation between AA and β-CD immobilized on the surface of a CPE.
Scheme 1.
Surface inclusion complex formation between AA and β-CD immobilized on the surface of a CPE.
Considering that the species adsorbed are electroactive [
21,
32,
34,
35], Equation (1) has been proposed to describe the
i-E experimental plots [
34,
35]:
where
A is the electrode surface area,
n is the number of electrons transferred during the heterogeneous reaction,
v is the potential scan rate,
R,
T and
F are the universal gas constant, absolute temperature and Faraday constant, respectively.
E0 is the formal potential,
Γо* is the surface coverage,
bO and
bR are related with the Gibbs’ standard free energy of the surface inclusion complex formation, ∆
G0inclusion, see
Scheme 1, of the AA’s oxidized (O) and reduced (R) species, respectively.
The peaks’ potentials and the currents are given as follows:
Substituting Equations (2) and (3) in (1) Equation (4) results in:
Parameterizing Equation (4) Equation (5) results in:
where:
Performing a non-linear fitting procedure to Equation (1), allows one to obtain theoretical CVs that can be compared to the experimental ones of the system CPE/poly-β-CD/1 mM AA, 0.1 M NaCl, at pH 3.0 and at 10 mV s
−1 (
Figure 5), where the adsorption model observed, derived from Equation (1), adequately describes the results from the experimental electrochemical oxidation taking place on the poly-β-CD-modified CPE. It is important to note that the fitting was quite good, regardless of the scan rate used. From this analysis, the values of the best fit parameters
P1 to
P4 were obtained. In particular, from the value of the parameter
P2 obtained at 50 mV s
−1 (1.7) and the thermodynamic constant value obtained through electrochemical techniques, reported in [
18], for the inclusion complex between the reduced form of AA and β-CD, (ln
Kincl. = 8.52), the ∆G
0inclus. of the oxidation product of AA was obtained as −36.4 kJ/mol. To the best of the authors’ knowledge, this is the first time that the Gibbs’ standard free energy of formation for this inclusion complex has been determined.
Figure 5.
Comparison of the experimental CVs from the system CPE/poly-β-CD/1 mM AA, 0.1 M NaCl, at pH 3.0 (points) and theoretical plots (lines) generated through linear non-linear fitting of Equation (1), at different potential scan rates as indicated, in mV s−1.
Figure 5.
Comparison of the experimental CVs from the system CPE/poly-β-CD/1 mM AA, 0.1 M NaCl, at pH 3.0 (points) and theoretical plots (lines) generated through linear non-linear fitting of Equation (1), at different potential scan rates as indicated, in mV s−1.
In order to corroborate further that the AA molecules were indeed adsorbed on the surfaces of the poly-β-CD-modified CPE, this electrode was immersed in an aqueous solution containing AA for a few minutes, after which the electrode was withdrawn and immediately placed in another aqueous solution containing solely NaCl to perform a CV experiment.
Figure 6 depicts two experimental CVs recorded in the system CPE/poly-β-CD/0.1 M NaCl, at pH 3.0, where it is possible to note that when the CPE/poly-β-CD electrode was previously immersed in the AA solution an oxidation peak becomes apparent due to AA oxidation, otherwise there were none.
Figure 6.
Experimental CVs recorded in the system CPE/poly-β-CD/0.1 M NaCl, at pH 3.0 (points) at 120 mV s−1 scan rate. In one case (solid line) the CPE/poly-β-CD was previously immersed in 1 mM AA, 0.1 M NaCl, at pH 3.0 for 5 min. whereas in the other case (broken line) the electrode was not exposed to the AA solution.
Figure 6.
Experimental CVs recorded in the system CPE/poly-β-CD/0.1 M NaCl, at pH 3.0 (points) at 120 mV s−1 scan rate. In one case (solid line) the CPE/poly-β-CD was previously immersed in 1 mM AA, 0.1 M NaCl, at pH 3.0 for 5 min. whereas in the other case (broken line) the electrode was not exposed to the AA solution.
Figure 7a shows a comparison of CV plots recorded during AA oxidation using the electrodes considered in this work. It is possible to note that both the current density and the anodic peaks’ potential vary drastically as a function of the electrode used.
Figure 7.
(a) Comparison of experimental CV plots recorded in the system electrode/0.1 mM AA, 0.1 M NaCl at pH 3.0, for different electrodes as indicated in the Figure, in both cases at 80 mV s−1 potential scan rate. (b) Family of Differential Pulse Voltammetry plots recorded in the system CPE/poly-β-CD/0.1 M NaCl, at pH 3.0 and different AA concentration: (a) 0.07, (b) 0.1. (c) 0.3, (d) 0.6, (e) 0.8 and (f) 1 mM, at a scan rate of 20 mVs−1, the inset show the calibration plot for AA quantification.
Figure 7.
(a) Comparison of experimental CV plots recorded in the system electrode/0.1 mM AA, 0.1 M NaCl at pH 3.0, for different electrodes as indicated in the Figure, in both cases at 80 mV s−1 potential scan rate. (b) Family of Differential Pulse Voltammetry plots recorded in the system CPE/poly-β-CD/0.1 M NaCl, at pH 3.0 and different AA concentration: (a) 0.07, (b) 0.1. (c) 0.3, (d) 0.6, (e) 0.8 and (f) 1 mM, at a scan rate of 20 mVs−1, the inset show the calibration plot for AA quantification.
When modified electrodes were used, the respective peak potential,
Ep,m, moves towards less positive potential values. This change makes the difference between the peaks’ potentials of the bare carbon paste electrode,
Ep,b, and
Ep,m, ∆
Ep, 324 mV, see
Table 1. Moreover, the ratio between the peak’s current density recorded with the modified electrodes,
jp,m and that obtained with the bare CPE,
jp,b is greater than 3. The change in the AA oxidation potential towards less positive values can be attributed to the formation of the surface inclusion complex, that apart from provoking its strong adsorption it may give rise to an increase of the heterogeneous reaction rate constant as was observed in the case of the interaction of dopamine (DA) with the same modified electrode [
35]. Moreover, using a similar treatment to the one described in the present work, Palomar-Pardavé
et al. [
35], showed that the CPE/poly-β-CD/electrode can be successfully used as working electrode during the electrochemical determination of DA, in the presence of ascorbic acid at pH 3.0. Notwithstanding, in this work we shown that due to the negative value found for the ∆G
0inclus., this electrode can also be adequately use for the electrochemical quantification of AA, see
Figure 7b, with a sensitivity of 3.41 Am
−2 mM
−1 and a detection limit of 0.22 µM.
Table 1.
Variation of the voltammetric parameters for AA oxidation as a function of the electrode used.
Table 1.
Variation of the voltammetric parameters for AA oxidation as a function of the electrode used.
Electrode | Ep/mV | jp/Am−2 | ∆Ep = Ep,b − Ep,m/mV | jp,m/jp,b |
---|
CPE | 564 | 2.96 | 0 | 1 |
CPE/poly-β-CD | 240 | 9.19 | 324 | 3.14 |