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# Modification of Glassy Carbon Electrode by Electrografting of In Situ Generated 3-diazopyridinium Cations

J. Agullo, Mario Morin, Daniel Bélanger

► To cite this version:

J. Agullo, Mario Morin, Daniel Bélanger. Modification of Glassy Carbon Electrode by Electrografting of In Situ Generated 3-diazopyridinium Cations. Journal of The Electrochemical Society, 2012, 159 (9), pp.H758-H764. 10.1149/2.054209jes. hal-02301322

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https://hal.science/hal-02301322v1

Submitted on 11 Oct 2024

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Distributed under a Creative Commons Attribution 4.0 International License
Abstract

The effect of various parameters on the grafting of pyridine, by electrochemical reduction of 3-diazopyridinium cations generated in-situ, on a glassy carbon electrode was investigated. More specifically, the influence of 3-aminopyridine and NaNO₂ concentration used for the diazotization reaction and the influence of the grafting conditions such as time and potential used for reduction of the diazonium cations, were investigated by cyclic voltammetry, electrochemical impedance spectroscopy and X-ray photoelectron spectroscopy. The presence of immobilized pyridine at the electrode surface was demonstrated by X-ray photoelectron spectroscopy measurements and indirectly, by the blocking effect of the grafted layer. The best blocking effect is obtained for a diazotization reaction of 1 min. and 1 equivalent of sodium nitrite. As a general observation, when the time is increased and the reduction potential is made more negative, the higher blocking effect that is observed indicates a more compact layer and a higher surface coverage of the glassy carbon electrode. The apparent pKa of the immobilized pyridine was determined by cyclic voltammetry in the presence of Fe(CN)₆³⁻/₄⁻ solution of different pHs and found to be equal to 5, which is close to the pKₐ value (5.25) of pyridine in homogeneous solution.

Keywords: Pyridine; Glassy carbon; Diazonium; Electrografting; pKa
**Introduction**

The functionalization of surfaces by reduction of diazonium salts is currently
attracting a lot of interest due to their potential applications (1) in chemical sensors
and biosensors, (2) catalysis, (3) energy storage, (4) and surface stabilization. (5) The
reduction of diazonium salts is a simple and rapid process which involves the
generation of the corresponding radicals at the electrode surface (carbon (C), metal
(M) or semiconductor (SC)) and the formation of a covalent bond (6-7) (Eq. 1).

<figure><img src="image_3.png" /></figure>

The diazonium cations can be generated either in the electrochemical cell or in a
reaction flask by reaction of the corresponding amine with an appropriate
diazotization reagent. The procedure is versatile because it can be used on different
substrates (eg. carbon, (8-12) metal, (5, 13-18) and semiconductor (19-23)) and in
different media (eg. water, (24-27) organic, (28-32) and ionic liquids (33-35)).
Several substituted phenyl groups have been grafted on carbon (24, 28, 30-32, 36-37)
or gold electrodes. (15-16) However, the number of studies dealing with the formation
of diazopyridinium cations and their stability for the electrochemical or chemical
modification of an electrode surface are more limited. (25-27, 38-39) Interestingly,
modified electrodes with pyridine groups and other nitrogen-based heterocycles,
allow further chemical modification on the attached groups (25, 40), complexation of
metallic ions (39, 41-45) and could be used to develop electrocatalytic sites. (42, 45-
47)
Pyridine has been chemically grafted on single walled carbon nanotubes (SWNTs) by reaction with 4-diazopyridinium cations generated in-situ and the resulting pyridine-functionalized SWNTs have been shown to act as a gelator to form hydrogels of poly(acrylic acid). (25) The reactivity of 3-aminopyridine, 2-aminopyridine and 2-amino-3-methylpyridine in a diazonium-based functionalization with SWCNTs has been investigated and it has been found that only 3-aminopyridine was successful grafted to yield functionalized SWCNTs. (27) Alternately, pyridine has been recently electrografted on glassy carbon using the diazonium modification method with 3-aminopyridine in acetonitrile at 0°C. (39) The surface concentration of the pyridine groups and the barrier properties of the modified glassy carbon electrodes in the presence of redox probes were determined before and after complexation with ruthenium metal cations. Recently, we reported the modification of glassy carbon by electrochemical reduction of in-situ generated 3-diazopyridinium cations in acid media at room temperature. (26, 38) The formation and reactivity of 3-diazopyridinium cations and their stability under experimental conditions used for electrografting of pyridine groups were investigated. (26)

The present work builds on recent studies on pyridine-modified electrodes by electrochemical reduction of diazonium salts (26, 38-39) and focuses on the influence of the grafting reaction parameters, such as time and reduction potential, amine and nitrite concentrations, on the electrografting efficiency and the resulting grafted layer by the modification of glassy carbon from the reduction of 3-diazopyridinium cations generated in-situ in aqueous acidic media at ambient temperature. The electrografting process and the resulting modified electrodes were characterized by cyclic voltammetry, electrochemical impedance spectroscopy and X-ray photoelectron spectroscopy. Preliminary results related to the present study have been published
elsewhere. (38) Here, a correlation is also established between electrochemical and X-
ray photoelectron spectroscopy data and the influence of grafting conditions on the
nature of grafted moieties is discussed. The apparent pKa of grafted pyridine groups
was determined and compared to that of the solution-phase value.
## Experimental

**Reagents.** 3-aminopyridine was purified by column chromatography (silica/ethyl acetate). Sodium nitrite, potassium ferricyanide, potassium ferrocyanide, and potassium chloride were used as received from Aldrich, (99% +).

**Preparation and modification of glassy carbon electrodes.** The glassy carbon electrode was obtained from Bioanalytical Systems Inc. (d = 3 mm). All potentials were reported versus the Ag/AgCl reference electrode ($E^0 = 0.199$ V vs. NHE) and a platinum gauze was used as counter electrode for all experiments. The glassy carbon electrode surface was cleaned by polishing with Buehler 1 µm alumina slurry (Tech-Met Canada). Then, the electrode was rinsed with Nanopure water followed by 5 min. of sonication to remove impurities. Glassy carbon electrodes were modified by cyclic voltammetry from 3-diazopyridinium cations in a 0.5 M HCl solution. In order to generate in-situ diazonium cations, an appropriate amount of $NaNO_2$ was added to the 3-aminopyridine/0.5 M HCl solution under vigorous stirring. To avoid the diazonium decomposition, the glassy carbon electrode was immersed in the reagent solution exactly 1 min. after $NaNO_2$ addition. (26) For modification by cyclic voltammetry, a potential of 0.8 (or 0.6) V was applied for 10 s and then the glassy carbon electrode was cycled 3 times between 0.8 (or 0.6) and -0.8 V at 50 mV s⁻¹. For potentiostatic modification, a potential of -0.5 or -0.8 V was applied for either 30 or 240 s. The stirring was turned off a few seconds before the electrochemical experiment. After grafting, the electrodes were abundantly rinsed and ultrasonication (5 min. in Nanopure water) was performed to remove all adsorbed molecules, which might remain on the glassy carbon electrodes. Finally, the electrodes were characterized in a solution containing a redox system (5 mM potassium ferricyanide/5 mM potassium
ferrocyanide, 0.1 M KCl, adjusted to various pH with KOH and HCl). All solutions were prepared using Nanopure water (ρ > 18.2 MΩ cm⁻¹) and degassed by bubbling with nitrogen (grade 4.8) for 30 min. before each experiment.

*Instrumentation and procedures.* Electrochemical measurements were performed in a one-compartment cell using a three-electrode configuration. Cyclic voltammetry was carried out using an electrochemical interface SI1480 (Solartron Instruments). Electrochemical impedance spectroscopy (EIS) measurements were performed in aqueous solutions containing the electroactive probes at open circuit potential in the frequency range 100 kHz to 0.5 Hz using a 10 mV sine-wave amplitude.

The XPS spectra obtained on glassy carbon plates were collected on AXIS 165 spectrometer (Kratos Analytical). The base pressure in the analytical chamber was lower than 3 x 10⁻⁸ Pa. Monochromatic Al Kα source (hv = 1486.6 eV) was used at a power of 210 W. The analysis spot was 300 x 700 µm. The survey scans were collected for binding energy spanning from 1100 eV to 0 with analyzer pass energy of 160 eV and a step of 0.35 eV. For the high-resolution spectra, the pass-energy was 20 eV with a step of 0.1 eV. No charge neutralization was required. The binding energy scale was set relative to the C 1s binding energy at 284.5 eV. Core level spectra were used to evaluate the atomic concentrations of the species present at the glassy carbon electrode surface. The atomic concentrations (at. %) of each individual element were determined from the relative peak areas of the spectra and the corresponding sensitivity factors according to:

$$ \text{at. \%} = \frac{A_i/S_i}{\sum_i A_i/S_i} \qquad (2) $$
where Aᵢ is the peak area of the element i and Sᵢ is the sensitivity factor for this element. Sᵢ values of 1.0, 1.71 and 2.8 and 3.2 were used for C 1s, N 1s, O 1s and Cl 2p, respectively. The surface coverage of the electrode by the grafted layer was estimated from the pyridine N1s peak area/C1s peak area ratio. (29) The ratio takes into account deprotonated and protonated pyridine. Assuming the simplest case of a carbon atom surface density equal to that of basal plane graphite (7.3 x 10⁻⁹ mol cm⁻²), the surface coverage was estimated by using the following equation:

$$ \Gamma = \frac{R}{C} \cdot 7.3 \times 10^{-9} \qquad (3) $$

where $\frac{R}{C}$ is the ratio of the total pyridine area under the N 1s peak with respect to the C 1s peak.
## Results and discussion

Electrochemical modification of surface by reduction of in-situ generated diazonium cations has become very popular in the recent years. (1) However, the influence of diazotization conditions on the grafting process has not yet been investigated in great details. Such studies would be especially important in the case of pyridine diazonium ions that have been found to be relatively unstable. (26) Accordingly, the objective of the present study is to get more insight in the effect of the diazotization conditions on the nature of the resulting grafted pyridine layers.

Electrochemical grafting from in-situ generated 3-diazopyridinium cations by cyclic voltammetry. The first negative going cyclic voltammetry scan provides information concerning the potential necessary to reduce in-situ generated 3-diazopyridinium cations (Eq. 4). Figure 1 shows that when a glassy carbon electrode is scanned from 0.8 to -0.8 V, a typical irreversible reduction wave is observed at approximately – 0.45 V during the first negative scan corresponding to the reduction of 3-diazopyridinium cations to pyridinium radicals and subsequently to the formation of a covalent bond (6-7) between a carbon atom of the substrate and the pyridinium cation (Eq. 5) as already demonstrated for electrodes modified by using diazonium ions. (1) During the following scans, this cathodic peak decreases and disappears indicating that the surface of the glassy carbon electrode is blocked by a grafted organic layer.

(6)

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Influence of the electroreduction conditions on the efficiency of the electrochemical grafting. To further investigate carbon modification by pyridine, potentiostatic depositions were performed at –0.5 V, which is near the potential of the cathodic peak observed during the first negative scan (Figure 1) and arbitrarily for a potential 300 mV more negative at –0.8 V. (24, 28) Glassy carbon electrodes were also modified for 30 and 240 s in order to study the influence of time. The modified electrodes were characterized by cyclic voltammetry (Fig. 2a) and electrochemical impedance spectroscopy (EIS) (Fig. 2b) in order to evaluate their blocking properties. Indeed, the presence of a grafted layer on a glassy carbon surface can be demonstrated by these two techniques in the presence of the Fe(CN)₆³⁻/₄⁻ redox system and the relative amount of grafted materials at the electrode surface can be indirectly evaluated. (31) Figure 2a shows that the bare glassy carbon electrode presents a quasi reversible redox behaviour attributed to the reduction and oxidation of the Fe(CN)₆³⁻/₄⁻ redox probes. In the case of the pyridine-modified electrodes, the electrochemical responses for the Fe(CN)₆³⁻/₄⁻ redox system is affected. Figure 2a shows that if potential is set close to the value of the peak corresponding to the reduction of 3-diazopyridinium cations (–0.5 V), the blocking effect is not significant indicating that the grafting is quite inefficient. If the potential is set at a more negative value (–0.8 V), the grafting efficiency is slightly improved for 30 s and for 240 s the redox waves for the Fe(CN)₆³⁻/₄⁻ redox system almost disappear indicating that the glassy carbon surface is almost entirely covered by a layer of chemisorbed pyridine molecules. The behaviour of the electrode modified by cyclic voltammetry is also shown for comparison purpose but its blocking effect is less important than that of the electrode modified at –0.8 V for 240 s. To support these conclusions, EIS measurements were performed.
Figure 2b shows the Nyquist plot for the unmodified electrode that presents a semicircle corresponding to a charge transfer resistance for the Fe(CN)₆³⁻/₄⁻ redox reaction for the high frequencies and for the low frequencies, a slope of 45° (Warburg diffusion) which corresponds to diffusion limitation of the system. (30) This response is the typical one of a simple redox reversible reaction. The diameter of the semicircle is used to determine the charge transfer resistance (R<sub>CT</sub>) of the redox reaction and the blocking properties of the grafted layer. (30) A high value of R<sub>CT</sub> indicates a strong blocking effect of the film. The evolution of the R<sub>CT</sub> values is consistent with the cyclic voltammetry data (Fig. 2a).

*Influence of the parameters of the diazotization reaction on the efficiency of the electrochemical grafting.* Figure 3a shows the cathodic branch of the first scan for the reduction of the diazonium cations formed after one min. for several concentrations of 3-aminopyridine. The increase of the current density at –0.45 V can be explained by the fact that a higher amine concentration involves a higher diazonium cations concentration (Eq. 4) and the current density of the cathodic peak is related to this concentration. Figure 3b shows the R<sub>CT</sub> values calculated from EIS measurements in the presence of Fe(CN)₆³⁻/₄⁻ for the different modified electrodes. It can be observed that the R<sub>CT</sub> values and thus the blocking effect of the grafted layer increases with the 3-aminopyridine concentration indicating that the grafting is more important for higher diazonium cations concentrations. The R<sub>CT</sub> values reach a plateau, which suggests that the growth of the layer might be limited to a given thickness due to its low electronic conductivity. (37)

The effect of NaNO₂ concentration on the diazotization reaction was also investigated and its influence on the grafting efficiency by electrochemical reduction of the corresponding diazonium cations was discussed. Figure 4a shows the cathodic
branch of the first scan for the reduction of the diazonium cations formed after one
min. for various equivalents of NaNO₂. As in the case of an increasing amine
concentration, when the number of NaNO₂ equivalents used to form the
corresponding diazonium cation is increased, the current density at –0.45 V becomes
more important. The significant increase of the current density when the nitrite
concentration is changed from 1 to 10 equivalents could be due to a higher diazonium
ions concentration in the solution. However, a more likely explanation is that the
current observed at –0.45 V is not only due to the reduction of diazonium cations but
also to the reduction of the excess of sodium nitrite that has not reacted with the
amine. The inset of Figure 4.a shows the cyclic voltammograms for the reduction of
the amine with 50 mM of nitrite and for a 50 mM nitrite solution. The onset of nitrite
reduction occurs at around 0.4 V and the second wave for the reduction of nitrite is
close to that of the diazonium cations. (26) This allows to conclude that both the
reduction of the diazonium cations and nitrite anions contribute to the observed
current. Figure 4b shows the R<sub>CT</sub> values calculated from EIS measurements in the
presence of Fe(CN)₆³⁻ for the different corresponding modified electrodes. It can be
observed that an increase of the number of NaNO₂ equivalents leads to a decrease of
R<sub>CT</sub> values and thus to a weaker blocking effect. This behavior was also previously
observed for concentrations five times smaller of both reactants (38) and this result
can be explained as follows. The addition of an excess of NaNO₂ in the grafting
solution can insure the complete transformation of the starting amine into the
diazonium ions and it can be expected that the diazonium cations concentration
slightly increased when the number of nitrite equivalent is changed from 1 to 2. (26)
However, the diazonium cations concentration is limited by the amine concentration
(5 mM) and a larger excess leaves unreacted NaNO₂ in solution. It appears that the
reduction of the excess of unreacted $NaNO_2$ contributes more to the observed current and since a smaller fraction of the current is used for the reduction of the diazonium cations, this leads to a lower grafting efficiency. To conclude, the data of Figure 4 demonstrate that it is not necessary to have an excess of $NaNO_2$ to have a good grafting efficiency.

**XPS characterization of modified glassy carbon electrodes.** In order to characterize further the pyridine-modified electrodes, XPS measurements were performed to determine their electronic properties and to estimate their surface concentration of pyridine moieties. The inset of Figure 5 presents XPS survey spectra for unmodified and modified glassy carbon electrode, characterized in both cases by the presence of C 1s (285 eV) and O 1s (530 eV) peaks and for the modified electrode only, an additional N 1s peak (400 eV). This is a good indication that pyridine was successfully grafted at the glassy carbon substrate. A high-resolution spectrum of the N region was recorded and one example is shown in Figure 5. The N 1s core level spectrum for the pyridine-modified electrode can be separated into four contributions. The main peak at 398.5 eV is attributed to the neutral nitrogen of pyridine (48) which accounts for 59.1% of the total nitrogen and the peak at 401.5 eV corresponds to protonated pyridine (49) representing 9.5% of the total nitrogen. In HCl, the nitrogen atom in the ring of 3-aminopyridine (pyridinium) is protonated and the nitrogen of the amino group (anilinium) remains deprotonated because the pKa are 6.03 for the pyridinium form and -1.38 for the pyridinium and anilinium moieties at 20 °C. (26, 50) Thus, the pyridine is originally protonated in our modification conditions but a significant fraction of the surface-bound pyridine groups are neutralized while the electrode is rinsed with Nanopure water. The fact that chloride ions are detected by XPS, and for which the atomic concentration is given in Table 1, indicates the
presence of pyridinium cations. However, the low Cl concentration relative to that of the pyridinium suggests the presence of other counter anions, presumably carbonate which is detectable in the C 1s core level spectra (not shown). The binding energy of the third peak at 399.6 eV corresponds to azo bridges (51) which are formed during the electrode modification (30, 52-53) in a proportion of 22.8 % of the total nitrogen. This percentage is comparable to the one of 20 % found for this type of molecule (39) and to the one of 25 % for 4-nitrophenyl groups electrografted on a glassy carbon electrode by using the corresponding diazonium cations. (28)

Equations 6 a and b illustrate two pathways to the formation of azo bridges in the film by the reaction of the diazonium species directly on the surface in a coupling reaction (a) (30, 53) or on the first grafted layer taking account preferential attack on the grafted ring which is controlled by activating and ortho/para-directing effects of the nitrogen of the ring (b). (52-53) The origin of the higher binding energy peak in the N1s region (at ca. 406.6 eV) is unclear. Its proportion of 8.6 % of the total nitrogen is comparable to the one of the protonated nitrogen of the pyridine. This peak might be due to some nitro type (54) from chemisorbed nitrogen oxides. (55) The pyridine-N-oxide species could be formed by pyridine structures combined with oxygen resulting from oxidation of the pyridine functional groups during storage in air before XPS analysis.

The pyridine surface coverage of the electrode (Γ) was estimated from the peak area of pyridine nitrogen peaks at 398.5 and 401.5 eV and the C 1s peak. (26, 29) For the glassy carbon electrode modified by cyclic voltammetry (Fig. 1), the surface coverage can be estimated at 7.7 x 10⁻¹⁰ mol cm⁻². It is important to notice that the estimation of Γ by XPS can be considered as semi-quantitative and that it is likely to underestimate the pyridine surface coverage of the glassy carbon electrode. (56)
However, this value suggests that the equivalent of a monolayer of pyridine is immobilized at the glassy carbon surface taking into account the higher packing density for pyridine of 6.7 x 10⁻¹⁰ mol cm⁻². (57)

<figure><img src="image_6.png" /></figure>
General discussion

Grafting efficiency of 3-diazopyridinium cations related to the surface coverage of pyridine. Table 1 compares surface coverage (Γ) and charge transfer resistance values (R<sub>CT</sub>) determined by XPS analyses and electrochemical measurements, respectively, for the electrode modified by cyclic voltammetry (Fig. 1) and the two electrodes modified potentiostatically at –0.8 V for 30 and 240 s. Concerning the pyridine surface coverage (Γ), the observed values (from 7.0 x 10<sup>-10</sup> to 8.7 x 10<sup>-10</sup> mol cm<sup>-2</sup>) suggest the presence of close to a monolayer for all modified electrodes. (57) The R<sub>CT</sub> values seems to indicate the presence of a film which behaves like a more or less porous physical barrier but does not allow to confirm the presence of either a mono or multilayers. However, a correlation of the XPS and EIS measurements will be made below to get more insight in the nature of the grafted layer.

The effect of grafting time on the surface coverage values suggests that the grafting rate decreases after 30 s. (16) This is also demonstrated by the cyclic voltammogram of Fig. 1 which shows a decrease of the current density peak for the second scan. Since the grafted layer is not very conductive, the rate of the diazonium reduction at the electrode surface decreases when the first layer is deposited, contrary to some aryl groups, which lead to the formation of a conductive layer. (53, 58) Increasing the deposition time allows more molecules to be grafted but another parameter that must be considered is the fact that the 3-diazopyridinium cations are not stable in solution and their decomposition into 3-hydroxypyridine occurs quickly with a rate constant of 53 x 10<sup>-3</sup> s<sup>-1</sup> at 22°C. (26) Two other reactions can compete with pyridine grafting process; the azo bridge reaction formation (Eq. 6) and the nitrite reduction (vide supra). The percentage of azo bridges (at 399.6 eV) is between 21 and 23 % for the three modified electrodes and it is close to the one reported for a
pyridine-modified electrode by electrografting in acetonitrile. (39) This indicates that changing the modification conditions does not affect the nature of the grafted species.

It is interesting to notice that the surface coverage of the three modified electrodes determined from XPS are consistent with R<sub>CT</sub> values evaluated from EIS experiments (Table 1). The slight increase of surface coverage indicates that a thicker film is formed. (37) However, the larger increase for R<sub>CT</sub> values determined from EIS experiments in comparison to that of the surface coverage for electrodes modified by cyclic voltammetry and potentiostatically at –0.8 V for 240 s is consistent with the presence of a more porous grafted layer in the first case which allows the redox probes to reach the surface whereas a more homogenous and compact grafted layer is formed in the second case. These results demonstrate that it is possible to obtain a relative good control of surface coverage and blocking properties by changing the electrochemical grafting conditions such as the applied potential and the grafting time.

*Influence of the solution pH in the presence of Fe(CN)₆³⁻/₄⁻ as redox probes on the blocking efficiency of the grafted layer.* Figure 6a shows the cyclic voltammograms in the presence of Fe(CN)₆³⁻/₄⁻ in solutions of different pHs for a glassy carbon electrode modified at –0.8 V for 30 s. At pH 3, a slight blocking effect is observed. When the pH is increased, this blocking effect becomes more pronounced to a point that for pH = 9 the electrochemical response of the redox probe has almost disappeared. This set of experiments was performed with the same modified electrode and obviously the pyridine surface coverage is constant. Consequently, this evolution is due to the barrier effect of the grafted layer and the absence (at high pH) and presence (at low pH) of electrostatic interactions between the modified surface and the electroactive probes. (24, 30, 36, 39, 59) It is important to keep in mind that in some cases, the pKₐ of the immobilized and the soluble forms might differ. (24, 30, 59) In order to
Conclusion

Glassy carbon electrodes were modified by electrochemical reduction of 3-diazopyridinum cations generated in-situ in acid media at room temperature. XPS, cyclic voltammetry and EIS measurements confirmed the presence of immobilized pyridine groups. It is possible to achieve a relative control of the grafted layer by changing the grafting time and the electrografting potential. An increase of the time and of the reduction potential (in the range of the grafting peak) involves higher blocking properties and surface coverage of the immobilized pyridine. The effect of amine concentration on the blocking behavior of the grafted layer shows that an increase of the amine concentration leads to more important grafting. However an excess of NaNO₂, in the grafting solution yields a decrease of the grafting efficiency. It appears that an excess of NaNO₂ does not strongly affect the grafted layer but using only 1 equivalent of NaNO₂ during the diazotization reaction of 3-aminopyridine allows an efficient electrochemical grafting. This study showed that it is possible to immobilize pyridine moieties at glassy carbon surface using simple conditions to obtain a relatively thin homogeneous grafted layer with high blocking properties. The study of influence of solution pH in the presence of Fe(CN)₆³⁻/₄⁻ as a redox probes allowed to estimate the apparent pKa of the immobilized pyridine at 5. The application of pyridine-modified electrode in selected electrochemical processes is currently investigated in our laboratory and the results will be reported elsewhere in due time.
## ACKNOWLEDGEMENTS

Financial support from the Natural Sciences and Engineering Research Council of Canada (Strategic and Discovery Grant) is acknowledged for this project and Dimitre Karpuzov (U. of Alberta) is thanked for XPS measurements.
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<figure><img src="image_7.png" /></figure>

Figure 1. Cyclic voltammograms for the reduction of 5 mM of 3-aminopyridine + 10 mM NaNO₂ in 0.5 M HCl after 1 min. of reaction, recorded at a scan rate of 50 mV s⁻¹. Cycles # 1, 2 and 3 are shown.

<figure><img src="image_8.png" /></figure>

Figure 2. a) Cyclic voltammograms of 3-aminopyridine-modified electrode modified by cyclic voltammetry, potentiostatic deposition at –0.5 V and –0.8 V for 30 s and 240 s recorded at a scan rate of 20 mV s⁻¹ in a 5 mM Fe(CN)₆³⁻/₄⁻ solution at pH = 7; b)
Complex impedance plots for the bare (inset) and 3-aminopyridine-modified
electrodes obtained by cyclic voltammetry, potentiostatic deposition at –0.5 V and –
0.8 V for 30 s and 240 s in a 5 mM Fe(CN)₆³⁻/₄⁻ solution at pH = 7.

<figure><img src="image_9.png" /></figure>

Figure 3. a) Cyclic voltammograms recorded at a scan rate of 50 mV s⁻¹ for the reduction of different concentrations of 3-aminopyridine + 2 equivalent number of NaNO₂ in 0.5 M HCl after 1 min. of reaction. The initial potential is 0.6 V and only the first scan in the negative potential direction is shown. The concentration range was from 0.5 to 10 mM; b) Plot of R<sub>CT</sub> as a function of concentration. The R<sub>CT</sub> values were estimated for a 3-aminopyridine-modified glassy carbon electrodes by EIS in a solution containing 5 mM Fe(CN)₆³⁻/₄⁻ at pH = 5.
<figure><img src="image_10.png" /></figure>

Figure 4. a) Cyclic voltammograms recorded at a scan rate of 50 mV s⁻¹ for the reduction of 5 mM of 3-aminopyridine + different equivalent numbers of NaNO₂ in 0.5 M HCl after 1 min. of reaction. The initial potential is 0.6 V and only the first scan in the negative potential direction is shown. The concentration range of NaNO₂ was from 5 to 50 mM; Inset: Cyclic voltammograms recorded at a scan rate of 50 mV s⁻¹ for the reduction of 5 mM 3-aminopyridine + 50 mM (or 10 eq) NaNO₂ (solid line) and for the reduction of 50 mM NaNO₂ (dashed line) in 0.5 M HCl after 1 min. of reaction ; b) Plot of R<sub>CT</sub> as a function of equivalent number of NaNO₂. The R<sub>CT</sub> were estimated for a 3-aminopyridine-modified glassy carbon electrodes by EIS in a solution containing 5 mM Fe(CN)₆³⁻/₄⁻ at pH = 5.
<figure><img src="image_11.png" /></figure>

Figure 5. N1s core level spectrum of a glassy carbon electrode modified by cyclic voltammetry for 3 cycles at a scan rate of 50 mV s⁻¹ between 0.8 and −0.8 V in 5 mM 3-aminopyridine + 10 mM NaNO₂ / 0.5 M HCl after 1 min. of reaction. Inset: XPS survey spectra of bare and pyridine-modified glassy carbon electrodes.

<figure><img src="image_12.png" /></figure>

Figure 6. a) Cyclic voltammograms of 3-aminopyridine-modified electrode by deposition (−0.8 V for 30 s), after 1 min. of reaction between 5 mM of 3-
aminopyridine and 10 mM of NaNO₂ in 0.5 M HCl recorded at a scan rate of 20 mV s⁻¹ in a 5 mM Fe(CN)₆³⁻/₄⁻ solution at different pH and for a bare electrode at pH = 7;

b) Plot of Iₚc as a function of pH.
<table><caption>Table 1. Pyridine surface coverage of the electrode surface estimated by XPS measurements and charge transfer resistance estimated by EIS for different conditions of modification of glassy carbon electrodes after 1 min. of reaction between 5 mM of 3-aminopyridine and 10 mM of NaNO₂ in 0.5 M HCl. Deposition by cyclic voltammetry (3 cycles at a scan rate of 50 mV s⁻¹) and at a constant potential of -0.8 V for 30 s and 240 s.</caption><thead><tr><th rowspan="2">Electrografting conditions</th><th>C 1s total</th><th>O 1s total</th><th>Cl 2p total</th><th>N 1s total</th><th colspan="4">N 1s peak (eV)</th><th rowspan="2">Γ</th><th rowspan="2">R</th></tr><tr><th>at. %</th><th>at. %</th><th>at. %</th><th>at. %</th><th>at. %</th><th>at. %</th><th>at. %</th><th>x 10<sup>-10</sup> mol cm<sup>-2</sup></th><th>x 10<sup>4</sup> Ω cm<sup>-2</sup></th></tr></thead><tbody><tr><td>CV</td><td>77.4</td><td>10.4</td><td>0.2</td><td>12.0</td><td>59.1</td><td>22.8</td><td>9.5</td><td>8.6</td><td>7.7</td><td>22 ± 5</td></tr><tr><td>-0.8 V for 30 s</td><td>77.8</td><td>11.1</td><td>0.3</td><td>10.8</td><td>57.5</td><td>21.7</td><td>10.9</td><td>9.9</td><td>7.0</td><td>4 ± 1</td></tr><tr><td>-0.8 V for 240 s</td><td>78.2</td><td>8.6</td><td>0.2</td><td>13.0</td><td>65.6</td><td>20.9</td><td>6.4</td><td>7.1</td><td>8.7</td><td>75 ± 10</td></tr></tbody></table>