Insight into the role of nitrogen in N-doped ordered mesoporous carbons for the spontaneous non-covalent attachment and electrografting of redox-active materials

Abstract

The role of nitrogen functional groups in nitrogen-doped ordered mesoporous carbons (OMCs) toward the spontaneous non-covalent and electrografting was investigated using two home-made ionic liquid-derived ordered mesoporous carbons having different nitrogen concentrations (guanine-rich ionic liquid-derived ordered mesoporous carbon (GIOMC) and ionic liquid-derived ordered mesoporous carbon (IOMC)). The carbonaceous materials were fabricated by the carbonization of a mixture of ionic liquid (1-methyl-3-phenethyl-1H-imidazolium hydrogen sulfate) as a carbon source using SBA-15 as a hard template. Guanine was used during the carbonization of GIOMC as a nitrogen source. The electrode was modified with either GIOMC or IOMC followed by electrochemical surface functionalization with a few electro-active precursors as redox-active molecular models bearing different substituents and electronic properties. The high surface coverage of 5.6(±0.3) × 10⁻⁹ mol cm⁻² for 4,4-biphenol was obtained for the GIOMC-modified electrode. We seek to explain whether the nitrogen content could indeed exert a dramatic impact on loading electroactive species on the electrode surface. The non-covalent anchoring studies indicated that at higher pH values the loading of electro-active moieties was significantly influenced by the content of nitrogen on the employed OMCs. The adsorption capacity (mg g⁻¹) of the OMCs was studied for catechol as a typical electro-active species in the range of 0.050–0.165 mg ml⁻¹. The adsorption capacity of 0.11 mg g⁻¹ catechol was 42(±4) and 26(±3) mg g⁻¹ for GIOMC and IOMC, respectively. In addition, our observations revealed that electro-grafting efficiency via diazonium ion was restricted by the protonation of nitrogen in the reaction media. Further, the fabricated redox-active/N-doped OMC electrodes showed sensitivity to pH, which was accompanied by either a Nernstian shift of the redox peak potentials (60(±3) mV per pH) in the pH range of 2–13 in the buffer solutions or variations of the redox peak currents (9.7(±0.3) μA per pH) in the pH range of 1–5.5 in the unbuffered situations. The resulting electrodes as voltammetric pH probes showed a simple response to pH in both buffer and unbuffered solutions. In addition, we introduced the fabricated electrode as a zero-gap generator/collector electrode system using a single electrode to recognize proton-dependent electron transfer from the proton-independent electrode process by detecting pH changes quite close to the surface of the electrode. The detailed descriptions are outlined.