Generator-collector experiments at a single electrode: exploring the general applicability of this approach by comparing the performance of surface immobilized versus solution phase sensing molecules

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

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.

Voltammetric detection of vitamin B1 (thiamine) in neutral solution at a glassy carbon electrode via in situ pH modulation

Pretreatment of glassy carbon electrode at an appropriate negative potential provide hydroxide ion which contributes to the in situ pH modulation of the electrode for thiamine detection in neutral solution.

Voltammetric characterization of DNA intercalators across the full pH range: anthraquinone-2,6-disulfonate and anthraquinone-2-sulfonate

The use of anthraquinone and its derivatives, notably the sulfonate and disulfonate salts, for the detection of DNA via electrochemical techniques, has been the focus of a number of recent articles. This study provides a quantitative model of the two redox systems of anthraquinone-2,6-disulfonate and anthraquinone-2-sulfonate, over the full aqueous pH range (0-13); the model is based upon the theoretical "scheme of squares" for a 2H(+), 2e(-) system, as first proposed by Jacq (Jacq, J. J. Electroanal. Chem. 1971, 29, 149-180). The effect of pH and ionic strength on the observed cyclic voltammetry was investigated experimentally. The variation of the electrochemical response with proton concentration was modeled through use of the commercially available simulation software, DIGISIM; the system was successfully fitted with attention to voltammetric peak height, position, width, and shape. The model demonstrates how the pK(a) values of the anthraquinone intermediates dominate the observed pH dependence of the voltammetry. At high pH (above pH 12), a simple EE process is found to occur. As the pH decreases, the formation of other protonated species becomes possible; this not only causes a Nernstian shift in the measured electrochemical potential for the redox couple but also results in changes in the mechanistic pathway. At pH 10, an EECC process dominates, as the pH is further lowered into the range 4-7, the overall mechanism is an ECEC process, and finally a CECE mechanism operates at around pH 1 and below. This work provides physical insight into the complex mechanistic pathways involved and will aid the future development of more sophisticated and accurate anthraquinone based DNA sensors.