Cyclic voltammograms obtained for 4-CBD at 2.50 mmol/L and 0.15 mmol/L are displayed in
Figure 1. The voltammograms of 2.50 mmol/L 4-CBD,
Figure 1A, resulted in two consecutive reduction peaks at 0.20 V (peak 1) and -0.01 V (peak 2). The peaks disappeared in the subsequent scans (2
nd and 3
rd). Peak 1 is reportedly attributed to the surface-catalyzed reduction of 4-CBD that can only occur on clean GCE surface [
28,
29]. Thereafter, peak 2 that corresponds to the non-catalytic reduction of 4-CBD appears, which builds more blocking layers [
29]. The reduction current decreases drastically in the 2
nd and 3
rd cycles suggesting that electron transfer to 4-CBD is completely inhibited by the multilayers formed in the previous scan in accordance to earlier reports [
15,
19,
29,
30].
For lower concentration of 4-CBD (0.15 mmol/L),
Figure 1B, a single and sharp peak was observed at more positive potential (ca. 0.30 V) compared to peak 1 (0.2 V) for the higher concentration,
Figure 1A, for which the grafted layer is thicker, resulting in increased electron transfer resistance. The single peak shifted to lower potential values after each scan and the general trend is a gradual decrease in peak current from the 2
nd to the 22
nd cycle, except for a slight increase that was seen in the middle (11th to 16th),
Figure 1B. Lowering the concentration of diazonium molecules leads to a localized grafting, which is in agreement with the explanation of Lee et al. [
28,
29] that the layer grafted at peak 1 is not thick enough to block the surface and give peak 2. The reduction of 4-CBD continues on the unreacted sites (pinholes) as well as on the already grafted layer. The shift in potential and decrease in peak current can be explained by the increase in layer thickness and increased blockage of the electrode pinholes that alters the electrode kinetic after grafting. The reduction peak continuously shifts until the peak potential is no longer clearly identifiable (i.e., from the 23
rd cycle onward as shown in
Figure 1B). This is probably because of the cathodic potential limit of the CV scan (–0.70 V). Based on these observations, for the lower concentration of 4-CBD, we can conclude that the grafting on GCE is a continuous process and there is no evidence pointing to the electrochemical reduction of aryl radical to aryl anion as suggested by [
11,
12,
30],
Scheme 1 (inset). The reduction peaks obtained during the first CV cycle for the higher concentration, 2.50 mmol/L (
Figure 1A), and lower concentration, 0.15 mmol/L (
Figure 1B) of 4-CBD were integrated to obtain the total charge which was then used for the calculation of surface concentration using Equation (1) [
31]. It should, however, be noted that this approximation is based on the assumption that 100% of the reduced 4-CBD leads to bound CP moieties as is done elsewhere [
32,
33].
where Γ is the surface concentration (mol.cm
−2),
Q is the total charge (C),
n is the number of electrons transferred (1 e
-),
F is the Faraday’s constant (96485 C.mol
−1), and
A is the electrode surface area (0.071 cm
2).
The number of layers can then be calculated by taking the ratio between surface concentration obtained from CV and the theoretical value [
34]. The theoretical surface concentration of a close-packed layer of CP was estimated from the structure of benzoic acid in which its geometry was optimized using Avogadro software. The distance between H atoms (4.28 Å) at both ortho positions, relative to the carbon that covalently binds to the surface, was used for approximation of the diameter of CP. This was then used for the calculation of the surface area occupied by a single CP molecule and thereby the surface concentration. The surface concentration was found to be 1.15 × 10
−9 mol.cm
−2, which is in good agreement with previous reports: 1.35 × 10
−9 mol.cm
−2 [
12] and 1.25 × 10
−9 mol.cm
−2 [
10].
The number of layers formed during the first cycle varied with the concentration of 4-CBD used for grafting: 100 layers for 2.50 mmol/L and 2.5 layers for 0.15 mmol/L 4-CBD. For the lower concentration, 35 layers are formed during the 22 cycles (1
st to 22
nd). The number of layers obtained for the lower concentration of 4-CBD confirm the gradual formation of the multilayer film, Step 2B in
Scheme 1.
The extent of grafting was investigated using 5.0 mmol/L Fe(CN)
6 −3/−4 in 100 mmol/L KCl as a redox probe. The signal for the redox probe obtained at bare GCE was completely suppressed after the electrode was grafted using one cycle with 2.50 mmol/L 4-CBD,
Figure 2A. This indicates that a totally blocking layer is formed on the GCE after the first scan of grafting. At lower concentration of 4-CBD (0.15 mmol/L) the signal of the redox probe decreased with the number of cycles used for grafting (1 and 3) which after 22 cycles was suppressed to almost the same extent as that observed with the higher concentration,
Figure 2B. These results are complementary to the number of layers obtained from the two concentrations used for grafting, 35 for 0.15 mmol/L (after 22 cycles) and 100 for 2.50 mmol/L (after 1 cycle), as discussed above. Although the calculation of the number of monolayers is based on the assumption that 100% of the CP radical is bound to the surface, the trend is still valid.