Surface-confined redox-active monolayers of a multifunctional anthraquinone derivative on nanoporous and single-crystal gold electrodes

We have investigated self-assembled molecular monolayers (SAMs) of a novel thiol-bearing anthraquinone derivative AQ(OMe) (1-amino-4-((3-mercapto-5-(methoxycarbonyl)phenyl)amino)-9,10-dioxo-9,10-dihydroanthracene-2-sulfonic acid) with multifarious protonation sites, on nanoporous gold (NPG) electrode surfaces. We employ cyclic voltammetry, electrochemical impedance spectroscopy and X-ray photoelectron spectroscopy (XPS). Single-crystal Au(1 1 1)-electrode surfaces area used as a reference. XPS exhibits S2p3/2 peaks at 161.1 and 162.1 eV, verifying well-defined AQ(OMe) Au-S bound SAMs. The mid-point potential (E1/2) of the SAMs shifts negatively with increasing pH with a slope of −55 mV pH−1, indicative of a 2e2H redox reaction. The anthraquinone group exhibits quasi-reversible behavior over a wide range of pH values (from 2.8 to 9.0) and a sigmoidal electron transfer (ET) rate constant (kapp) with a maximum value at pH 5.5, which is likely to be close to the isoelectric point. The electrochemically addressable surface coverage, determined from the voltammetric peak areas, decreases with increasing pH. The apparent surface coverage on NPG is more than 13 times higher than that on Au(1 1 1), attributed mainly to the larger surface area and different structural packing modes, and possibly also to favorable terrace edges on NPG, but – somewhat unexpectedly – with little effect on kapp. The new anthraquinone derivative offers perspectives for gentle immobilization and efficient mediated interfacial electrochemical ET of redox proteins and other complex biomolecules.