Selectivity mechanisms at self-assembled monolayers on gold: Implications in redox recycling amplification systems

Redox recycling systems enable marked improvement of electrochemical detection capabilities. Enhanced sensitivity is achieved by employing a sacrificial redox species that recycles the analyte back to its original oxidation state through a catalytic homogeneous electron transfer. Repetition of the cycle leads to multiple heterogeneous electron transfer events for each analyte molecule, serving to enhance the transduced signal. The success of redox recycling is intimately linked to the selectivity of heterogeneous electron transfer: the analyte should undergo a fast reaction while the sacrificial species should ideally be excluded from contributing directly to the current. This requirement stems from the relationship between selectivity and detection limit in that the direct heterogeneous electrolysis of the sacrificial additive can increase the background current which can degrade detection capabilities. Earlier work has shown that electrodes with suitable selectivity can be constructed using alkanethiolate monomolecular films on gold, with various ferrocenes (FcX) serving as a model analyte and ferrocyanide (Fe(CN)64-) acting as the recycling agent [A.J. Bergren, M.D. Porter, J. Electroanal. Chem. 585 (2005) 172, A.J. Bergren, M.D. Porter, J. Electroanal. Chem. 591 (2006) 189, P.T. Radford, M. French, S. E. Creager, Anal. Chem. 71 (1999) 5101, P.T. Radford, S.E. Creager, Anal. Chim. Acta 449 (2001) 199, S.E. Creager, P.T. Radford, J. Electroanal. Chem. 500 (2001) 21]. The work herein investigates the origins of the selectivity for this system by analysis of the different pathways (e.g., electron transfer kinetics, size-exclusion, and partitioning) that can suppress the heterogeneous electrolysis of Fe(CN)64-, while maintaining that for FcX at a rapid level. Comparisons of experimental data to expectations derived from model assessments are used to evaluate the relative importance of each possibility. The properties (i.e., size and hydrophobicity) of several FcX molecules and Fe(CN)64- are also examined to provide additional insight into the processes that are important in creating a potent redox recycling system. These results show how the inherent differences in the heterogeneous electron transfer reaction rates can dictate the kinetic selectivity of the system. These findings indicate that partitioning augments the kinetic selection for the FcX/Fe(CN)64- system, leading to the high level of observed selectivity.