Developed at the INL, the microelectrode array microscope allows “chemical images” to be made in real-time. The method, based on the established technique of scanning electrochemical microscopy (SECM), uses many microelectrodes to form chemical images of a reactive surface in solution. We like to think of this as chemistry in four dimensions, as a signal is measured as a function of spatial position and time. This instrument could find use in the study of surface reactions where the reactivity is not uniform in spatial position and time. A classic example of this type of reaction, localized corrosion, has been studied as will be described below.
The figure above shows profile diagram of the MEAM. The array is carefully placed over the sample using a positioning system. These are placed in an electrochemical cell where the reactions are performed. The reactions are performed in solution and require various electrodes to control. The multiple potentiostat is used to measure the signals at the array microelectrodes simultaneously. A sample potentiostat electrochemically controls the reaction at the sample, a corrosion reaction in this case. The insets show a magnified view of what happens at the microelectrodes, placed within 20mm of the surface.
Microelectrode Array Design
To simultaneously generate a chemical image in real-time, many sensors must be employed. The image above (courtesy of Cyberkinetics, Inc.) shows the array of microelectrodes used in the current MEAM instrument. The array is carefully positioned in a parallel fashion over a flat surface to perform the measurement. While the shaft of each microelectrode is 1mm in length as shown here, only the final 50 m are coated with Pt. The remainder of the shaft is insulated and does not provide signal. Each microelectrode has a wire that connects to the multi-channel microelectrode Analyzer (Scribner Associates, Inc).
The MEAM Instrument
The picture above shows the MEAM instrument located in the INL Research Center. The Multi-channel Microelectrode Analyzer (Obtained from Scribner and Assiociates), located on the left, is where the microelectrodes of the array are controlled potentiostatically and the current for each measured at a rate up to 100 HZ. The signal measured can be tuned to the system of interest, with potentiometric sensors such as pH possible given the right array design. The MEAM has been used to measure amperometric (current) signals using a redox mediator in solution. The motor system on the right positions the array with respect to the sample located in the electrochemical cell (center).
The data shown above were collected with the array positioned over a Stainless steel 304 sample held at a potential where localized corrosion is known to occur. This experiment was performed over 24 hours. The sample current (A) and the total array current (B) appear to correlate with each other in time. Individual signals for the array are also shown (C). The spikes labeled in the plot of the individual microelectrode signals correlate to times of activity as shown in the selected images (D). The dark blue shows areas of localized corrosion and pinpoints when the corrosion is occurring. The best representation of this data can be made using movies, where a frame by frame account of the events is shown.