Achieving nanometer scale tip-to-substrate gaps with micrometer-size ultramicroelectrodes in scanning electrochemical microscopy

Scanning electrochemical microscopy (SECM) tips with rounded glass insulation around the metal wire (radius a = 5 μm) were fabricated (apparent RG < 1.1, where RG is the ratio of the radius of the insulation sheath divided by the electrode radius), and their SECM feedback approach curves were studied in solutions of tris(2,2′-bipyridine)ruthenium(2+) (Rubpy) in acetonitrile and ferrocenemethanol in water with a platinum disk as the substrate electrode (radius a_s = 1 mm). Considerable enhancement of the normalized feedback current, I_T(L) = i_T/ i _T,∞, where L = d/a and d is the distance traveled by the SECM tip, was observed in both systems (e.g., I_T(L) = 15 in organic solutions and I_T(L) = 30 in aqueous solutions) with good electrode alignment. This shows that tip-to-substrate gaps of ca. d = 110 nm can be achieved. To account for any deviations from the usual disk UME behavior and currents caused by possible changes in the tip electrode geometry, simulations of the feedback response were performed for a 2D axisymmetric environment. All simulated results match in a point-to-point comparison with experimental values (average relative standard deviation (RSD) = 0.01 ± 0.005).