Ensemble average TIRM for imaging amperometry

• The use of colloidal particles as probes of electrochemical current is explored.
• Theory is developed to account for use of ensemble average measurements.
• Derived corrections allow for quantitative amperometry.
• The impact of assumed electrode kinetics on inferred currents is explored.
• The method is demonstrated on a nonhomogeneous ITO/gold electrode.

Colloidal particles can function as probes of local electrochemical current density if a functional relationship between the response of the particles and the electric field in the vicinity of the particles can be established. The nanometer scale movement of a single colloidal particle during cyclic voltammetry can be observed with the aid of total internal reflection microscopy. The intensity of scattered light can be related back to the current density local to that particle, and hence the method is called imaging amperometry. Data acquisition and optical constraints, however, make a single-particle method impractical for analysis of macro-scale (∼1 cm2) surfaces covered by several hundred thousand particles. Subdivision of the electrode into small patches, each containing an ensemble of particles, solves this problem if the scattering from the ensembles can be related to the local electric field. For example, a 100 × 100 array of square 100 μm patches each containing approximately two dozen particles would form a mosaic of electrochemical activity with 0.01% area resolution on a 1 cm2 electrode having location-dependent electrocatalytic properties. The focus of this contribution, therefore, is adaptation of the method from single particles to particle ensembles. The algebraic relationship between current density and scattering intensity for single particles holds for ensembles if the mean scattering intensity is corrected to its mode. Currents calculated from particle light scattering at different locations on a single ITO/gold patterned electrode agree well with currents measured on these two electrode materials, which have quite different electrocatalytic properties.

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