Barrier Properties of Organic Monolayers on Glassy Carbon Electrodes

Abstract

The barrier properties of phenyl layers covalently attached to glassy carbon electrodes by the aryldiazonium reduction method have been interpreted using a model of electron transfer at defect sites and closely spaced microscopic pinholes. The surface coverage of phenyl groups determines the effective average thickness of the modifying layer which is most likely less than that of a closely packed monolayer of phenyl groups. The voltammetric responses of eight redox probes in aqueous media at polished and modified glassy carbon electrodes were examined in order to evaluate the barrier properties. With the exception of the MV +/0 couple, the films are much more blocking toward electron-transfer reactions of solution species than is predicted on the basis of the average film thickness. Comparisons of pairs of redox couples show that the electron-transfer kinetics of hydrophobic probes are slowed less than those of hydrophilic probes at the modified electrodes. This finding supports the notion that hydrophobic/hydrophilic interactions between solution species and the monolayer restricts the approach of redox probes to the monolayer surface, forcing electron transfer at the modified electrodes to occur over a distance significantly greater than that defined by the monolayer. Under experimental conditions where adsorption of MV + and MV 0 is not important, electrode modification has no apparent effect on the kinetics of the MV +/0 couple, indicating that these redox species interact closely with the phenyl layer.

Electron Transfer Processes at Aryl‐Modified Glassy Carbon Electrode

“…However, with increasing layer thicknesses from 0.8 to 20 nm, a major blocking effect has been observed for both redox systems, indicating the possibility to inhibit electron transfer completely for most redox systems by extremely thick layers. A similar trend has been demonstrated by Downard and Prince, who investigated the influence of a phenyl-grafted layer on GC on Fe(CN) 3À=4À 6 , IrCl 2À=3À 6 and Ru(NH 3 ) 2þ=3þ 6 at pH 7 [4]. Even though they claimed their grafting to be a monolayer, the rather long deposition time of 10 min as well as the high concentration of diazonium salt suggests multilayer formation, indicated by the complete blocking of all three redox systems.…”

“…It is known from literature that the pKa of the grafted DEA species is at pH 10 [3]. Secondly, redox systems characterized by slow (Ru(NH 3 ) 2þ=3þ 6 and IrCl 2À=3À 6 Þ and fast (Fe(CN) 3À=4À 6 and Co(phen) 2þ=3þ 3 Þ electron transfer kinetics at edge planes of GC were used (Table 1) [4,7]. This is qualitatively consistent with a peak potential separation of about 60 mV for the cyclic voltammogram (CV) of the Ru(NH 3 ) 2þ=3þ 6 and IrCl 2À=3À 6 redox systems obtained on bare GC, indicating a reversible electron transfer mechanism.…”

“…As mentioned before, Kariuki and McDermott [11] as well as Downard and Prince [4] showed that the thickness of the grafted layer strongly depends on the deposition time, thus increasing times yield thicker layers. The former group investigated the blocking effects of the DEA film thickness of a modified GC electrode on the electrochemistry of Fe(CN) 3À=4À 6 and Ru(NH 3 ) 2þ=3þ 6 [11].…”

“…However, it is very important to assess the various factors, which affect the electron transfer at these modified electrode surfaces. The electron transfer through aryl groups, grafted at carbon electrodes, has been investigated before, and it was concluded that tunneling through the organic layer is not the only operative mechanism [4,5,11]. Indeed, other parameters such as hydrophobic/hydrophilic as well as electrostatic interactions can play a significant role, depending on the experimental conditions (such as nature of the grafting and redox species, pH value, electrolyte, and solvent).…”

“…It is performed by radical generation at the carbon surface and thus creation of covalently bound species, which are of aromatic nature. The layer thickness as well as multilayer generation can be controlled by choosing the grafting potential and time during the electrochemical grafting process [3,4,9].…”