Combined Scanning Electrochemical Microscopy - Scanning Ion Conductance Microscopy (SECM-SICM)

Imaging and Probing Water and Ion Transport at the Nanoscale

Supervised by Prof. Pat Unwin

Background

High-resolution electrochemical microscopy techniques have become increasingly attractive as a means of mapping the function and topography at surfaces and interfaces. Such techniques have proven useful for interpreting a wide range of biological and chemical systems, from the adsorption and transport of species across cell membranes,¹ to the dissolution and growth processes of materials such as ionic crystals.² Scanning electrochemical microscopy (SECM) has developed as the prevailing technique to quantitatively visualise interfacial functional processes.³ However, the main drawback of SECM is that it lacks distance (feedback) control, which makes it difficult to distinguish whether an observed change in current is due to a functional or structural attribute. Variants of SECM have been developed to combat this problem including SECM-atomic force microscopy (AFM),⁴ and intermittent contact (IC)-SECM.⁵ Scanning ion conductance microscopy (SICM) is another electrochemical microscopy technique that uses similar technologies to SECM, but measures the ionic conductance between two electrodes.⁶ Combining SECM and SICM (SECM-SICM) would provide a powerful new imaging technique, utilising the feedback control of SICM to map out topography, whilst simultaneously using SECM to scan for chemical activity on the surface.

In my project, I developed a method to fabricate combined SECM-SICM tips and used them to image, with nanoscale resolution, the surface of model conducting/insulating substrates (Fig. 1) and maize root cells.

Fig. 1: SECM Current as the SECM-SICM probe scans over the edge of a conducting gold band (red region) and an insulating glass substrate (blue region).

References

^{[1] Matsue, T.; Shiku, H.; Yamada, H.; Uchida (1994) Permselectivity of voltage-gated alamethicin ion channel studied by microamperometry. J Phys Chem 98: 11001}

^{[2] Macpherson, J. V.; Unwin, P. R.; Hillier, A. C.; Bard, A. J. (1996) In-situ imaging of ionic crystal dissolution using an integrated electrochemical/AFM probe. J Am Chem Soc 118: 6445}

^{[3] Edwards, M. A.; Martin, S.; Whitworth, A. L.; Macpherson, J. V.; Unwin, P. R. (2006) Scanning electrochemical microscopy: principles and applications to biophysical systems. Physiol Meas 27: R63}

^{[4] Macpherson, J. V.; Unwin, P. R. (2000) Combined scanning electrochemical-atomic force microscopy. Anal Chem 72: 276}

^{[5] McKelvey, K.; Edwards, M. A.; Unwin, P. R. (2010) Intermittent Contact- Scanning Electrochemical Microscopy (IC- SECM): A New Approach for Tip Positioning and Simultaneous Imaging of Interfacial Topography and Activity. Anal Chem 82: 6334-6337}

^{[6] Hansma, P.; Drake, B.; Marti, O.; Gould, S.; Prater, C. (1989) The scanning ion-conductance microscope. Science 243: 641}