Elucidation of the electron transfer mechanism of marker ions at SAMs with charged head groups

Self assembled monolayers (SAMs) of 11-mercaptoundecanoic acid (MUA) and 11-amino-1-undecanethiol (AUT) were prepared on gold electrodes. The SAMs were investigated by differential pulse voltammetry (DPV), cyclic voltammetry (CV) and rotating disk voltammetry (RDV) using [Fe(CN)6]3−, [Fe(CN)6]4− and Ru ( NH 3 ) 6 3 + as electroactive marker ions, and La3+, Ca2+, 1,3,7-naphthalene trisulfonate (NTS3−), N,N′-dihydroxyethyl-4,4′-bipyridinium (Vio++), 1-ferrocenylethanol and ferrocenealdehyde as ion-gate promoters, surface site competing ions, and/or electrocatalysts. The current densities at the SAM-modified electrodes correlate with three types of simple interactions, i.e., charge–equal charge, charge–uncharged, and charge–opposite charge. More detailed investigations revealed: (i) strong counter ion binding of multiply charged electroactive or non-electroactive ions to the oppositely charged SAM head groups; (ii) competition between equally charged ions for such surface sites; (iii) electrocatalysis of electron transfer by surface confined electroactive marker ions to redox species in solution including quantitative electron transfer rates by analysis of Koutecký–Levich plots; (iv) apparent charge inversion of the SAM head groups by surface confinement of a multiply charged counter ion; (v) build-up of a second layer of multiply charged electroactive ions at the charge-inverted SAM; (vi) competing electron transfer through the inner and outer layer at the charge inverted SAM. This insight allows for a more detailed understanding of the principles of signal amplification in ion gate sensors.