Micropipette Contact Technique as a Tool to Reveal, Characterize, and Modify Nanopore Electrodes

This paper reports on the use of a micropipette contact method (MCM) to reveal and characterise single submicrometer- sized pores randomly distributed within a thin insulating alumina layer (250 nm thickness) that coats a platinum film substrate (Pt/Al2O3). Moveable micropipette probes, with orifice radii of 40–50 um, filled with electrolyte solutions, are brought in contact with the sample surface to form a microcell and allow electrochemical reactions to be confined within a limited contact region. The aqueous solutions inside the micropipettes contain Ru(NH3)6 3+ or Ag+ ions and cyclic voltammograms (CVs) are obtained for several locations of 500-500 um2 above the Pt/Al2O3 sample. The presence of nanoscopic pores is revealed by sigmoidally shaped volatmmograms, which are recorded when Ru(NH3)6 3+ is used as redox probe. From the steady-state limiting current and the theoretical equation that applies for recessed microelectrodes, the radius of each platinum disk at the bottom of the pore is evaluated. In this way, pore sizes spanning from about 50 nm to 350 nm are revealed. Similar information is obtained by the use of the MCM with the pipette filled with Ag+ ions. In this case, CVs typical for nucleation and growth of metallic silver are recorded. The volume of a single pore is evaluated from the charge involved in the anodic stripping process of the metal deposited. Potential step measurements and relevant current (I) against time (t) transients allow establishing that only a single nucleus can be grown in each submicometer-sized pore. Moreover, a diffusion coefficient value of 1.41_10_5 cm2 s_1 for Ag+ is evaluated from the experimental I vs. t1/2 linear plot. Finally, the measurements performed at the Pt/Al2O3 sample are compared with those obtained by using a plain platinum disk electrode 3 mm diameter as substrate for MCM measurements.