Shear Force SECM with Vibrating Needle UMEs

1 Shear Force SECM with Vibrating Needle UMEs 

Shear force constant distance scanning potentiometry has been employed to map the dissolution of calcite crystals and shells [74] and to map spatial variations in pH at high resolution over complex microcavity-pattemed substrates [75]. The ability to approach capillaries very close to a surface, using shear force feedback, was also used for the local deposition of silver microstructures on conducting surfaces using potential-assisted ion transfer at the tip of a micropipette [76]. 

The shear force constant distance mode SECM was later mounted onto an inverted optical microscope, in a so-called Bio-SECM configuration, in order to study individual living cells. The Bio-SECM instrument was notably used to detect nitric oxide released from single cells [78]. In parallel, the shear force setup was modified by replacing the optical detection system, used to monitor the tip vibrating motion, by a piezoelectric element [79]. Electrical detection of the tip vibration was shown to be much easier and more convenient than optical detection, partly because the delicate laser alignment on the tip was made unnecessary. Further developments to shear force SECM have seen the implementation of high-resolution constant distance mode AC-SECM, which was used for the visualization of corrosion pits on stainless steel samples [80,81]. 

reduction in tip size is necessary for better control of the imaging conditions and to improve both the topographical and electrochemical resolution of constant distance mode SECM imaging. To this end, needle-type nanoelectrodes were fabricated by pulling Pt wires in glass capillaries. This method, described in detail in Chapter 3, results in submicron-sized disk-shaped Pt electrodes encased in long, thin glass capillaries [82]. 

Even though shear force feedback has been successfully used for constant distance mode SECM imaging in many instances, it nevertheless has several shortcomings. One of the problems is that the imaging scan rate has to be low in order for the electronic feedback loop to reestablish the set-point value (hence distance) every time the tip is moved to a new position, otherwise tip crash occurs. Another problem is that the range of the shear force interaction is very small (a few hundreds of nanometers) and shear force-based feedback can thus only position tips at this distance or closer to the substrate. For very small tip-substrate separations, the presence of the tip will block diffusion of species toward/away from the surface thus potentially interfering with the 

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