Nano-Electrochemistry and Nano-Electrografting with an Original Combined AFM-SECM

Abstract

This study demonstrates the advantages of the combination between atomic force microscopy and scanning electrochemical microscopy. The combined technique can perform nano-electrochemical measurements onto agarose surface and nano-electrografting of non-conducting polymers onto conducting surfaces. This work was achieved by manufacturing an original Atomic Force Microscopy-Scanning ElectroChemical Microscopy (AFM-SECM) electrode. The capabilities of the AFM-SECM-electrode were tested with the nano-electrografting of vinylic monomers initiated by aryl diazonium salts. Nano-electrochemical and technical processes were thoroughly described, so as to allow experiments reproducing. A plausible explanation of chemical and electrochemical mechanisms, leading to the nano-grafting process, was reported. This combined technique represents the first step towards improved nano-processes for the nano-electrografting.

Keywords: AFM; AFM-SECM; SECM; interface; nano-electrochemistry; nano-electrografting; nano-functionalization; nano-process; surface.

Scheme 1

Chemical structure of agarose.

Figure 1

(a) Scanning electron microscopy image of the homemade AFM-SECM-electrode after insulation with electrophoretic paint. Beam energy was 15 kV and working distance 31 mm; (b) Scanning electron microscopy image of the tip apex. The apex radius was around 130 nm. Beam energy was 5 kV and working distance 8 mm.

Figure 2

Steady-state voltammogram of the AFM-SECM-electrode in aqueous solution containing 1 mM ferrocenedimethanol and 0.1 M KH₂PO₄. The forward and backward traces are superimposed. The potential scan rate was 50 mV s⁻¹.

Figure 3

Schematic drawing of AFM setups (a) for the AFM-SECM/Agarose system; (b) for the nano-electrografting system.

Figure 4

Steady-state voltammogram of the PtIr-tip in contact with the agarose surface. Agarose was previously immersed in aqueous solution containing 5 mM ferricyanure and 0.1 M KCl, during 1 h. The potential scan rate was 50 mV s⁻¹.

Figure 5

Scanning electron microscopy image of the homemade AFM-SECM-electrode after collision with the surface during nano-electrochemical grafting attempts. Beam energy was 15 kV and working distance 16 mm.

Figure 6

(a) Topographic AFM image in tapping mode of the line pattern drawn with the AFM-SECM-electrode on the gold surface with direct aryl diazonium salt/acrylic acid reduction (chronoamperometry: −0.8 V); (b) Horizontal cross section of the electrografted line. Line width and height are about 180 nm and 40 nm, respectively.

Figure 7

Schematic representation of the electric field between the AFM-SECM-electrode and the substrate.

Figure 8

A simplified schematic representation of chemical and electrochemical reactions leading to the line grafting. (1) Reduction of the diazonium salt on the substrate to form a polynitrophenylene-like film; (2) Water oxidation at the AFM-SECM-electrode; (3) Concomitant reduction of protons on the substrate and formation of a localized grafted coating onto the top of the primer polynitrophenylene (PNP) film by radical reaction. (3a) Formation of a phenyl radical and reaction with PNP film; (3b) Phenyl radical initiates the first vinylic (plausible) radical polymerization. Reaction of the formed macroradicals with the grafted layer; (3c) Formation of radical monomer, initiation of the second (plausible) vinylic radical polymerization and reaction of the formed macroradicals with the grafted layer.