Chemical Modification of Carbonaceous Stationary Phases by the Reduction of Diazonium Salts

Abstract

This paper describes a new strategy for the creation of chemically modified carbonaceous stationary phases. The strategy exploits the electroreduction of arenediazonium salts as a means for functionalizing the surface of glassy carbon (GC) and porous graphitic carbon (PGC) stationary phases. The one-electron reduction of these salts forms an arene radical which then couples via a carbon-carbon linkage to the carbon framework at the surface of the stationary phase. Two arenediazonium-based modifiers were used in evaluating the potential utility of this strategy: 4-nitrobenzenediazonium tetrafluoroborate for the GC and PGC phases and 4-hexylbenzenediazonium tetrafluoroborate for only the PGC phases. Modifications were carried out by packing the phases into a column used for electrochemically modulated liquid chromatography. The effectiveness of the modifications was assessed by X-ray photoelectron spectroscopy and by comparing the liquid separation of a series of mixtures before and after coating deposition. For the nitrobenzyl-modified GC phase, the test mixture contained both anisole and fluoranthene. The performance of the nitrobenzyl- and hexylbenzyl-modified PGC stationary phases was characterized by the separations of substituted phenols (i.e., nitrophenol and resorcinol) and a few important pharmaceutical agents (i.e., hexobarbital, oxazepam, and nitrazepam). The potential utility of this modification procedure to form stationary phases that are stable upon extended exposure to aggressive mobile phases is discussed and briefly examined.

Covalent modification of graphitic carbon substrates by non-electrochemical methods

“…Edge sites were assumed to be H-terminated and in all cases, binding of a phenyl group converted a sp 2 -carbon to sp 3 .…”

“…Fig. 7 Proposed mechanism for the addition of alkenes to a H-terminated sp 3 carbon surface. Figure 7 shows the type of mechanism proposed for thermal reactions of alkenes at H-terminated diamond, germanium and silicon surfaces.…”

“…The amount of Hterminated sp 3 carbon at the surface of these materials is unknown but is generally considered to be low. Hydrogen abstraction from sp 2 edge sites is also a possibility but would be considerably less energetically favourable than abstraction from sp 3 carbon.…”

“…Graphitic carbons have many attractive characteristics, including ease-ofhandling, low-cost, high mechanical stability and good electrical conductivity. The material is readily available in a number of forms including high surface area felts and cloths suitable as supports for combinatorial chemistry [1,2], carbon powders useful for chromatography [3], glassy carbon (GC) [4], a convenient material for planar electrodes and very smooth films of pyrolysed photoresist [5] suitable for applications in molecular electronics [6,7,8]. Another advantage of graphitic carbon is that the surface can be easily modified by methods which give very stable molecular coatings, attached by strong C-C [9] or C-N covalent bonds.…”

Valence Discriminative Detection of Metal Cations by a Chromotropic Acid-grafted Glassy Carbon Electrode

“…This change in voltammogram was already observed in previous studies for the reduction of isolated diazonium salts dissolved in acetonitrile and attributed to the growth of a grafted and passivating layer on the electrode surface. [14][15][16] The mechanism was shown to involve the intermediate aromatic radical obtained upon one-electron reduction of the diazonium cation; this radical reacting with the carbon surface to form a covalent bond. The formation of an organic layer significantly limits further reduction of diazotized PV, although it is not completely inhibited.…”