Electrochemical atomic-force microscopy using a tip-attached redox mediator. Proof-of-concept and perspectives for functional probing of nanosystems.
This paper presents the first steps toward the development of a new type of high-resolution AFM-SECM microscopy which relies on the use of tip-attached redox-labeled polymer chains as mediators to probe the local electrochemical reactivity of a planar substrate at the nanoscale. Submicrometer-sized combined gold AFM-SECM probes were functionalized by linear, nanometer-sized, flexible, PEG3400 chains bearing a ferrocene (Fc) redox label at their free end. Analysis of the force and current approach curves recorded when such Fc-PEGylated probes (tips) were approached to a bare gold substrate allowed the presence of the Fc-PEG chains at the very tip end of the combined probes to be specifically demonstrated. It also allowed the chain coverage, configuration, and dynamics to be determined. When the Fc-PEGylated probe is positioned some approximately 5 nm above the substrate, only a few hundred chains are actually electrochemically contacting the surface, thus reducing the size of the tip-substrate interaction area to 20-40 nm. Most importantly, we have shown that the tip-borne PEG chains are flexible enough to allow their Fc heads to efficiently "sense" locally the electrochemical reactivity of the substrate, thus validating the working principle of the new AFM-SECM microscopy we propose. This innovative microscopy, we label Tarm (for tip-attached redox mediator)/AFM-SECM, should be particularly suitable for probing the activity of slowly functioning nanometer-sized active sites on surfaces, such as individual enzyme molecules, because it is, by design, free of the diffusional constraints which hamper the characterization of such nanosystems by classical SECM.
Anne
Agnès
A
Laboratoire d'Electrochimie Moleculaire, Unité Mixte de Recherche Université-CNRS No. 7591, Université Paris Diderot (Paris 7), 15 rue Jean-Antoine de Baïf, 75205 Paris Cedex 13, France.
Demaille
Christophe
C
Goyer
Cédric
C