Formation and Electrochemical Characterization of Self-Assembled Monolayers of Thioctic Acid on Polycrystalline Gold Electrodes in Phosphate Buffer pH 7.4

Abstract

Formation of self-assembled monolayers (SAMs) of thioctic acid (TA) on polycrystalline gold electrodes has been investigated. The Au electrodes are polished, first mechanically and subsequently electrochemically, until the roughness of the surface is minimal. The best TA SAMs are formed onto these electropolished electrodes from stirred solutions, while a potential E = 0.0 V or E = +0.2 V versus saturated calomel reference electrode (SCE) is applied during the adsorption process. Alternating current (ac) impedance methods are used to monitor the formation of the SAMs and to characterize these. Potentiostatically formed TA SAMs have better characteristics than those formed at open circuit. The TA SAMs are stable in 100 mM phosphate buffer pH 7.4, and when protected from light, also in buffered 2 mM hexacyanoferrate(II/III) solution. However, when the hexacyanoferrate(II/III) solutions are exposed to light, the TA SAMs are not stable. Probably, damaging of the SAM proceeds via etching of the gold surface by photochemically released cyanide ions. Using potential pulses (+1.6, 0.0, −0.8 V versus SCE, each 0.1 s) for 15 min in the flow (0.5 mL min-1), the TA SAM can be removed completely, recovering the initial state of the gold electrode, as deduced from potential-step experiments. This way TA SAMs can be formed and removed repeatedly and reproducibly on a single electrode.

Characterization of self-assembled thiols monolayers on gold surface by electrochemical impedance spectroscopy

“…The exact physical meaning is still unknown. 22 The CPE was used instead of the ideal capacitor in every fit in order to improve the final fit and minimize errors. A small deviation from 1.0 (ϕ > 0.85), as observed here, suggests that the gold substrates are adequately smooth for these studies.…”

Label‐Free Impedance Biosensors: Opportunities and Challenges

“…Lai et al recently published an important report of SAM stability during dry storage; they concluded that longer SAMs are more stable and that preservatives can insure stability over one month with reproducible results [162].The hexacyanoferrate(II/III) redox couple used almost universally in faradaic impedance sensors can degrade the electrode-SAM interface over time, particularly when exposed to light [175,176], and also reduces the activity of peptide probe layers [177].…”

Voltammetry of immobilized cytochrome c on novel binary self-assembled monolayers of thioctic acid and thioctic amide modified gold electrodes

“…The disulfide-containing bases yields two gold-sulfur bonds with the surface of gold electrode, which gives an additional stability, compared with the single gold-sulfur bond. Formation and characterization of single-component SAMs of T-COOH on gold electrodes have been monitored by different methods [21][22][23]. This single-component SAMs is stable in PBS and in buffered hexacyanoferrate (2+/3+) solution [22].…”