Observation of Nanometer-Sized Electro-Active Defects in Insulating Layers by Fluorescence Microscopy and Electrochemistry

Abstract

We report a method to study electro-active defects in passivated electrodes. This method couples fluorescence microscopy and electrochemistry to localize and size electro-active defects. The method was validated by comparison with a scanning probe technique, scanning electrochemical microscopy. We used our method for studying electro-active defects in thin TiO2 layers electrodeposited on 25 μm diameter Pt ultramicroelectrodes (UMEs). The permeability of the TiO2 layer was estimated by measuring the oxidation of ferrocenemethanol at the UME. Blocking of current ranging from 91.4 to 99.8% was achieved. Electro-active defects with an average radius ranging between 9 and 90 nm were observed in these TiO2 blocking layers. The distribution of electro-active defects over the TiO2 layer is highly inhomogeneous and the number of electro-active defect increases for lower degree of current blocking. The interest of the proposed technique is the possibility to quickly (less than 15 min) image samples as large as several hundreds of μm(2) while being able to detect electro-active defects of only a few tens of nm in radius.

Photochemical, Electrochemical, and Photoelectrochemical Water Oxidation Catalyzed by Water‐Soluble Mononuclear Ruthenium Complexes

“…143 nm) with current leakage 14. Oxide films deposited by ALD tend to contain many pinholes unless they grow very thick (hundreds of nanometers);15 in contrast, the electrodeposited TiO 2 film described here is amorphous and thin, which allows for tunneling electron transfer 16. Our Pt‐NP‐decorated TiO 2 /n‐Si electrode was first subjected to a 1 m M solution of FcDM in 50 m M phosphate buffer (pH 7).…”

Nanoscale Electrochemistry Revisited

“…Care must also be taken, especially when using polymers for insulation, to avoid flaws such as pinholes or cracks which can expose underlying electrode surfaces to the working solution and thus significantly and unknowingly increase the electrode’s electroactive area. Optical imaging techniques, such as electrochemical precipitation of fluorophores at active sites followed by fluorescent imaging, can be useful in identifying these defects . Additionally, one must ensure that the electrode surface is free from contaminants, as they can also distort the electrode’s response.…”

“…Optical imaging techniques, such as electrochemical precipitation of fluorophores at active sites followed by fluorescent imaging, can be useful in identifying these defects. 28 Additionally, one must ensure that the electrode surface is free from contaminants, as they can also distort the electrode's response. While polishing can be used for larger electrodes, nanoelectrodes are more delicate and polishing can significantly change the size and morphology of the electrode and surrounding insulator.…”

Real‐time Conversion of Electrochemical Currents into Fluorescence Signals Using 8‐Hydroxypyrene‐1,3,6‐trisulfonic Acid (HPTS) and Amplex Red as Fluorogenic Reporters

“…Various optical platforms have been developed enabling such fluorescence based imaging of electrode processes. [4][5][6][7][8][9][10][11][12] The most "direct" experimental approach relies on using an electroactive fluorogenic species whose fluorescence emission is switched on/off upon conversion of its redox state at the electrode. [1,9] In an alternative, "indirect" strategy the emission of a non-electroactive, but pH-sensitive, fluorophore species can be used to transduce an electrode reaction affecting local proton concentration.…”

“…In essence, sensitive fluorescence signal is used to reveal spatially‐resolved features about surface reactivity, or to image mass transport at the electrode‐solution interface. Various optical platforms have been developed enabling such fluorescence based imaging of electrode processes [4–12] . The most “direct” experimental approach relies on using an electroactive fluorogenic species whose fluorescence emission is switched on/off upon conversion of its redox state at the electrode [1,9] .…”