Measuring Ions with Scanning Ion Conductance Microscopy

47 T he opportunity to position a small electrode at an interface with high accuracy and precision has led to important advances in surface electrochemical measurements and has made scanned probe microscopies (SPMs) an indispensable tool for the electrochemist. Scanning electrochemical microscopy (SECM) 1-8 has been a key SPM for electrochemists, and has found wide implementation in subjects that range from battery science 9 to bacterial communication. 10 A similar technique, scanning ion conductance microscopy (SICM), 11 was introduced by Hansma and coworkers around the same time as SECM was first reported, but wasn't really appreciated by the electrochemical community to the same degree. The lack of adoption of SICM stemmed from several issues, including the lack of chemical specificity exhibited and the less quantitative mathematical descriptions of the processes involved. Seminal efforts of Korchev and Klenerman to develop SICM further, notably in hardware and imaging protocols for biological systems, provided quantum leaps in methods available. 12 More recently, other groups have taken advantage of electrochemical measurements that might be afforded by SICM. Here, in context of the larger body of work, we describe some of our recent efforts in electroanalysis with SICM. In their original report, Hansma and coworkers realized the possibilities of measuring ion transport with SICM, and stated: " The most promising application for the SICM is not, however, just imaging the topography of surfaces at submicrometer resolution. The SICM can image not only the topography, but also the local ion currents coming out through pores in a surface. " 11 Measuring local ion transport at small scales can lead to insight for both fundamental studies and technological applications. For instance, in biological measurements, studies of ion transport through protein channels in cells have informed understanding of biological processes such as cell signaling and regulation of cell volume. Further applications of ion transport measurements include microfluidic separations, biosensors, and lithium-ion batteries. In SICM (shown as a cartoon in Fig. 1, left) a pipette with a small tip diameter (Fig. 1, middle) is scanned over the surface of a sample in electrolyte solution. A potential difference is applied between an electrode inside the pipette and an electrode in the outside bath solution, which results in an ion current that flows through the pipette. As the pipette is moved to the sample surface, resistance that develops in the gap between the pipette tip and sample affects the ion current …