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# Modification of Glassy Carbon Electrode with Pyridine

Julia Agullo, Mario Morin, Daniel Bélanger

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Julia Agullo, Mario Morin, Daniel Bélanger. Modification of Glassy Carbon Electrode with Pyridine. ECS Transactions, 2011, 35 (36), pp.19-26. 10.1149/1.3652841 . hal-04600873

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Distributed under a Creative Commons Attribution 4.0 International License
# Modification of Glassy Carbon Electrode with Pyridine

J. Agullo, M. Morin, D. Bélanger

Département de Chimie, Université du Québec à Montréal, Case postale 8888,
succursale centre-ville, Montréal, Québec, H3C 3P8, Canada

Glassy carbon electrodes were modified with pyridine by electrografting of the corresponding in situ generated diazonium cations. The diazonium cations formation from the 3-aminopyridine in acid media at room temperature was confirmed by UV spectroscopy. The presence of immobilized pyridine at the electrode surface was demonstrated by XPS measurements. The influence of the diazotization reaction conditions on the grafting process was investigated by determination of the blocking properties of the grafted layer by electrochemical impedance spectroscopy in presence of $Fe(CN)_6^{3-/4-}$ as redox probes. The best blocking properties of the grafted layer are obtained for a diazotization reaction of one min. and 1 equivalent of sodium nitrite.

## Introduction

The modification of surfaces by reduction of diazonium salts has attracted a lot of interest in the past decade. This simple and rapid process was used in diverse areas such as chemical sensors and biosensors, catalysis, energy conversion, surface stabilization (1). This method is attractive because several molecules can be used to form a covalent attachment between aryl groups bearing a desired functionality and different substrates. In this study, the procedure developed by Pinson and coworkers is used to immobilize pyridine molecules at a glassy carbon electrode surface (2). In the first step, 3-aminopyridine is transformed into the corresponding diazonium by a standard diazotization procedure (Eq. 1). In the second step, a solution containing the diazonium cations is used to modify the electrode by electrochemical reduction (Eq. 2). In fact, from a diazonium, it is possible to generate the corresponding radical at the electrode surface and create a covalent bond according to following reactions:

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The immobilization of the pyridine units makes it possible to perform further chemistry on the attached group (3), to complex metallic ions (4) or to provide electrocatalytic activity (5). Depending on the targeted applications, it might be important to control the compacity and thickness of the grafted layers. This work focuses on the influence of the diazotization reaction parameters on the electrografting efficiency and consequently on the obtained grafted layer (6).

# Experimental

## Reagents

3-aminopyridine (3AMP) was purified by column chromatography with 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

All solutions were prepared using Nanopure water ($pH > 18.0 MΩ.cm^{-1}$) and degassed by bubbling with nitrogen (grade 4.8) for 30 min. The working electrode made of glassy carbon was obtained from Bioanalytical Systems Inc. ($d = 3 mm$). A platinum gauze was used as counter electrode. All potentials were reported versus the Ag/AgCl reference electrode ($E° = 0.199 V$ vs. NHE). 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. The electrode was kept in water to prevent oxidation and contamination of the surface.

Electrochemical modification of glassy carbon electrode was carried out in 0.5 M HCl containing 1 mM diazonium cations. The modification of the glassy carbon electrode surface, by using in situ generated diazonium cations, was performed in two steps. Firstly, the adequate number of sodium nitrite equivalent was added to the electrolytic solution containing 1 mM 3-aminopyridine and 0.5 M HCl under vigorous stirring at room temperature. Secondly, the mixture was left to react for 1 min prior to the electrochemical modification. After modification, the electrode was rinsed abundantly with Nanopure water and sonicated in Nanopure water for 5 min to remove all molecules, which could be only adsorbed but not grafted. The solution containing the electroactive probes was potassium ferricyanide/ferrocyanide (5 mM each, 0.1 M KCl, adjusted $pH=5$ with HCl).

## Instrumentation and Procedure

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 at open circuit potential in aqueous solution containing the electroactive probes.

The XPS spectra were collected on AXIS ULTRA spectrometer (Kratos Analytical) using a glassy carbon plate from Tokai Carbon USA as a substrate. Monochromatic Al
Kα source (hν = 1486.6 eV) was used at a power of 210 W. The binding energy scale was calibrated with respect to the C1s binding energy of 284.5 eV.

UV spectra were recorded with a UV-visible spectrophotometer from Varian.

## Results and Discussion

### UV Spectroscopy Characterization of Diazonium Cations Formation

The formation of the diazonium salts was monitored by UV spectroscopy (Figure 1). In hydrochloric acid, the 3-aminopyridine spectrum presents two characteristic absorption bands at 317 and 249 nm (7). After a one min reaction between 3-aminopyridine and sodium nitrite, the 3-aminopyridine absorption band at 317 nm decreases significantly and a characteristic band at 281 nm corresponding to the generated diazonium cations appears. This indicates that 3-aminopyridine is readily diazotized. This reactivity is mainly attributed to the inductive effect of the amino group in the 3 position of the pyridine ring. The inductive effect is effective in depleting the amino group of its electron pair which confer it good electron-donor capacity (8).

The evolution of UV spectra recorded during 60 min revealed an increase of diazonium concentration with time. This indicates that the reaction involves the formation of rather stable diazonium ions.

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Figure 1. UV spectrum of 1 mM 3-aminopyridine in a 0.5 M HCl solution (- - -) and 1 mM 3-aminopyridine in a 0.5 M HCl solution containing 2 equivalents of NaNO₂; a first spectrum was recorded after reaction for 1 min. (black solid line) and another after 60 min. (grey solid line). The absorption spectrum of NaNO₂ is also shown (---).
# Grafting of 3-aminopyridine by Electroreduction of the Corresponding Diazonium Cations

Once the diazonium is formed, the carbon surface is immersed in the solution containing the in-situ generated diazonium cations and a cyclic voltammetry is recorded. The cyclic voltammogram shows an irreversible reduction wave at –0.4 V (not shown) during the first negative scan whose intensity decreases during the following scans. This peak corresponds to the reduction of diazonium cations to radicals and consequently to the formation of a covalent bond between a carbon atom of the substrate and pyridine (Eqs. 1 and 2). The formation of a grafted layer passivates the electrode surface and explains why the main cathodic peak at around –0.4 V, observed during the first scan, almost disappears during the following cycles (2).

## Blocking Properties of the Grafted Layer

The presence of a grafted layer on the electrode surface can be indirectly demonstrated by its blocking properties. Electrochemical impedance spectroscopy measurements are used to evaluate blocking behavior of the grafted layer in the presence of Fe(CN)₆³/₄⁻ redox probes (9). The Nyquist plot for a bare glassy carbon electrode is characterized by a semicircle at high frequency and at low frequency straight line (with an angle of 45°) corresponding to Warburg diffusion. The equivalent circuit contains a parallel combination of the charge transfer resistance (R<sub>CT</sub>) with the double layer capacitance (C<sub>DL</sub>). The value of the charge transfer resistance can be estimated by the diameter of the semicircle. For modified electrodes, only a semicircle is observed and the R<sub>CT</sub> value in order 10⁴ Ω demonstrates the presence of a grafted layer (10).

## XPS Characterization

Glassy carbon electrodes were characterized by X-ray photoelectron spectroscopy (XPS). XPS survey spectra for unmodified and modified glassy carbon electrodes show the presence of C1s and O1s at around 285 and 530 eV, respectively. For the modified electrode, a significant N1s peak appears at 400 eV. Core level spectrum for N1s can be separated into three contributions. The main peak at 398.5 eV can be attributed to the neutral nitrogen of pyridine (11) whereas the peak at 401.5 eV is assigned to the protonated pyridine (12). The modification occurs in acidic media (pyridine is protonated) but during the rinsing with water (pH around 7) a fraction the pyridine should be neutralized. The third peak at 399.6 eV corresponds to azo bridges in the film which is a consequence of the electrode modification (13). For the later contribution, the diazonium species can react with the surface or the first grafted layer in a coupling reaction.

## Effects of Diazonium Formation Conditions on the Blocking Properties of the Grafted Layer

**Effects of Diazotization Reaction Time.** The influence of the diazotization reaction time on the grafting efficiency was investigated. The UV spectra of Figure 1 suggest that the concentration of the diazonium cations increases slightly with time after the first minute of reaction. As shown in Figure 2, increasing the time for the formation of the
diazonium cations affects the reduction peak current density. The well-defined irreversible reduction wave is observed at -0.4 V for a 1 min.-reaction decreases when the diazonium formation time is increased to the point that it almost vanishes after 60 min (Figure 2 and inset). This decrease of the wave at -0.4 V despite the presence of diazonium cations in the solution (as demonstrated by UV spectroscopy) is unclear for the moment. Experiments carried out in presence of only nitrite (2 mM) in 0.5 M HCl and without amine demonstrated that the onset of nitrite reduction occurred at around -0.3 V (not shown). Since, the reduction potential of nitrite is close to that of the diazonium species, the current for the reduction peak at -0.4 V has also a contribution from the reduction of the excess of sodium nitrite that did not react with the amine. It is well known that hydrochloric acid reacts with sodium nitrite to form unstable nitrous acid, which is then converted to nitrosyl cation used for the diazotization reaction. However, nitrous acid is a weak acid and the disproportionation reaction can occur to form nitrate (14). Consequently, with time, the nitrite concentration decreases. A given amount of nitrite is consumed as it reacts with the amine to form the diazonium cations and the excess of nitrite is transformed into nitrate. Thus, even if the diazonium concentration is increasing slightly after one min. (see Figure 1) a decrease of reduction peak current density with time is observed in Figure 2 because the NaNO₂ concentration is significantly decreasing.

The effect of diazotization time before the electrochemical reduction, on the blocking effect of the grafted layer, was investigated. Figure 3 shows that an increase of the diazotization reaction time led to a decrease of R<sub>CT</sub> and thus a weaker blocking effect. This could be explained by a degradation of diazonium cations although this hypothesis is not supported by UV data, which seems to indicate that the diazonium cations are stable. Further work is in progress in our laboratory to have a better understanding of this phenomenon.

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Figure 2. Cyclic voltammograms for the reduction of 1 mM 3-aminopyridine + 2 eq. of NaNO₂ in HCl (0.5 M) recorded at a scan rate of 50 mV s⁻¹. The initial potential is 0.6 V and only the first scan in the negative potential direction is shown. The diazotization reaction was carried out during period of times ranging from 1 to 60 min. prior to recording the cyclic voltammogram. Inset: Cyclic voltammogram for the reduction of 1 mM 3-aminopyridine + 2 equivalents of NaNO₂ after one min. of reaction in HCl (0.5 M) recorded at a scan rate of 50 mV s⁻¹.
<figure><img src="image_7.png" /><figcaption>Figure 3. Plot of R<sub>CT</sub> as a function of the reduction current density at –0.4 V of the cyclic voltammogram for different diazotization reaction time for 1 mM 3-aminopyridine and 2 equivalents of NaNO<sub>2</sub>. The R<sub>CT</sub> were estimated for a 3-aminopyridine-modified glassy carbon electrode by EIS in a solution containing 5 mM Fe(CN)<sub>6</sub><sup>3-/4-</sup> at pH=5.</figcaption></figure>

**Effects of Sodium Nitrite Concentration.** The effect of the diazonium cations concentration was investigated and related to the grafting efficiency. Figure 4 shows the cathodic branch of the first scan for the reduction of the diazonium formed after one min. for various equivalents of NaNO<sub>2</sub>. The increase of the current density at –0.4 V when the number of NaNO<sub>2</sub> equivalents used to form the corresponding diazonium is increased can be explained as follows. Firstly, the diazonium cations concentration is slightly increased when the concentration of NaNO<sub>2</sub> is increased (confirmed by UV spectroscopy). Secondly, the reduction potential of nitrite is close to the diazonium cations reduction potential. Consequently, as mentioned above, the reduction current density observed at –0.4 V is the due to the reduction of diazonium cations and the excess of unreacted nitrite. This explains the significant increase of the current density when the nitrite concentration is increased from 1 to 10 equivalents.

It is observed that the current density increases with number of NaNO₂ equivalents whereas the blocking effect (eg. RCT) of the grafted layer decreases as shown in Figure 5. This indicates that the grafting is less efficient in the presence of an excess of nitrite. The reduction of nitrite at the glassy carbon substrate may involve a significant change in local pH at the electrode/solution interface (14). As the acidic solution close to the electrode surface becomes less acidic it would greatly reduce the grafting efficiency. Thus, for the grafting of pyridine involving the diazotization of 3-aminopyridine, an excess of NaNO₂ is not desirable. The slightly higher diazonium concentration formed in presence of an excess of nitrite is insufficient to compete with the excess of unreacted NaNO₂.
<figure><img src="image_8.png" /></figure>

Figure 4. Cyclic voltammograms for the reduction of 1 mM 3-aminopyridine + increasing number of equivalents of NaNO₂ in 0.5 M HCl recorded at a scan rate of 50 mV s⁻¹. The initial potential is 0.6 V and only the first scan in the negative potential direction is shown. The diazotization reaction time prior to cyclic voltammogram measurements was 1 min.

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

Figure 5. Plot of R<sub>CT</sub> as a function of the reduction peak current density for the cyclic voltammogram of diazonium reduction for different equivalents of NaNO<sub>2</sub> and 1 mM 3-aminopyridine. The R<sub>CT</sub> were estimated for a 3-aminopyridine-modified glassy carbon electrode by EIS in a solution containing 5 mM Fe(CN)<sub>6</sub><sup>3-/4-</sup> and adjusted to pH=5.
**Conclusion**

Glassy carbon electrodes were modified by electrochemical reduction of diazonium cations in situ generated in aqueous media at room temperature. XPS and EIS measurements confirm the presence the immobilized pyridine. UV spectroscopy seems to indicate that the diazonium cations that are formed are stable. An excess of NaNO₂ competes with the diazonium reduction and led to a decrease of the grafting efficiency. A decrease of the grafting peak current density corresponding to the diazonium reduction is observed for diazotization time longer than one min. The effects of diazotization time on the blocking properties of the grafted layer are not fully understood, mainly because the (surface) electrochemical observations are not simply related to the (solution) UV spectroscopy experiments.

**Acknowledgements**

Financial support from the Natural Sciences and Engineering Research Council of Canada is acknowledged for this project. Professor Sylvain Canesi (UQAM) and Dimitre Karpuzov (U. of Alberta) are thanked for their help in 3-aminopyridine purification and XPS measurements, respectively.

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