Development of combined scanning electrochemical optical microscopy with shear force feedback using a tuning fork and current feedback
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A technique that combines scanning electrochemical microscopy (SECM) and optical microscopy (OM) was developed. To accomplish SECM/OM, the most important aspect is the conception, design and fabrication of a special probe tip, which can serve as a light source and microelectrode. Once fabricated, the tip must then be characterized to validate all future experimental measurements. One particular probe tip that was investigated for SECM/OM contained a ring ultramicroelectrode. Theoretical SECM tip current–distance (approach) curves for all ring electrodes studied were calculated by numerical (finite element) analysis. The SECM curves obtained were a function of the geometry of the tips including the thickness of the ring and the insulating sheath. Comparison of experimental and theoretical SECM curves provided a good method of evaluating the size and shape of ring electrodes. Out of the numerous tips designed and fabricated, the most reliable tip for SECM/OM was constructed by electrochemically depositing electrophoretic paint onto a gold metal film instead of an aluminum film used for a typical NSOM tip. The development of valiadation techniques for the optical and electrochemical characterization of such tips is an important part of this work. The reliable probe tip exhibited stable steady-state current and well-defined SECM approach curves for both conductive and insulating substrates. We consistently fabricated quite durable tips whose geometry was a ring with < 1 µm as an inner ring diameter. Simultaneous electrochemical/optical images of an interdigitated array (IDA) electrode were obtained with a resolution on the micrometer scale, demonstrating good performance of the tip as both an optical and electrochemical probe for imaging microstructures. Another key point of SECM/OM is that the tip must be positioned within nanometers above the substrate. The application of a quartz crystal tuning fork (32.768 kHz) for sensing shear force provided a feedback to regulate tip-substrate distance as well as simultaneous topography with electrochemical and optical images. The capacity of this technique was confirmed by obtaining simultaneous topographic, electrochemical, and optical images of an IDA electrode in a constant distance mode. Imaging in this mode based on a tuning fork allowed a closer proximity between a tip and a substrate than in a constant height mode. Thus, a better spatial resolution was obtained in terms of both electrochemical and optical imaging. Application of SECM/OM to the imaging of soft biological samples was accomplished with SECM tip current rather than shear force as a feedback signal to control tip-sample proximity. Imaging in a constant current mode was an excellent imaging tool for soft materials because it preserves the benefits of the constant distance mode but eliminates the strong interaction between a tip and samples, which may damage the samples.