Nanotip Characterization

The shape and size of a nanotip must be accurately determined for nanoscale SECM, especially when data is quantitatively analyzed or spatial resolution is quantitatively assessed. It shonld be emphasized that, without the knowledge of tip shape, tip size can be readily nnderestimated or overestimated from a limiting current in the bulk solution, which can be conveniently measured by amperometry and voltanunetry. The shape and shape of a nanotip can be much more reliably characterized by SECM. An important advantage of the SECM-based characterization is that the effective shape and size of a nanotip can be determined in solution. This in situ SECM characterization contrasts to ex situ SEM, FIB, and TEM characterization. Recently, AFM was introduced as a unique method for both in situ and ex situ tip characterization. 

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Imperfections that are inherent to all of the discussed test structures are minimized partially in case of application of porous anodic alumina. It is known that anodic alumina has sharp pore edges [4]. Therefore it may be used as tiplike grating. In order to combine the benefits of porous anodic alumina and a nanoscale tip array a one-step electrochemical approach is used to fabricate nanoscale alumina tips (tip-like anodic alumina) [5]. However, a curvature radius of pore edges or alumina nanotips of these structures is too high for ultrasharp SPM tip characterization. 

Remarkably, the tip radius of a nanoelectrode can be -1000 times smaller than the size of a cell so that the nanotip can penetrate through the ceU manbrane for direct characterization of the intracellular redox state [65]. The penetration of a nanotip into the cell was monitored as a change in the amperometric tip current as the tip moved perpendicularly to the cell surface, that is, approach curve. The tip current based on FcMeOH oxidation was constant while the tip was moving outside of the cell (Figure 1.9c). As the tip approached within a feedback distance from the cell, tip current decreased because the diffusion of FcMeOH to the tip was... 

The SECM theories were developed for various tip shapes to enable the in situ quantitative characterization of a nanotip by approach curve measurement in solution. The fit of an experimental approach curve with a theoretical curve provides quantitative information about the shape and size of an SECM tip as originally proposed in... 

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