Shear force SECM

SECM associated with scanning ion conductance microscopy (SICM) requires a double tip, on one side of which is a conventional microdisc electrode and on the other side is a narrow pipette filled with electrolyte and an electrode that measures ionic conductance through the mouth of the pipette with respect to another electrode in the bulk solution. When the pipette mouth is within one pipette tip radius away from the sample surface, the conductance varies sufficiently to be used as a control signal to maintain the z-position of the tip during the scans, thereby affording constant-distance SECM operations [133,134]. This methodology is fast and apparently less-challenging to implement than shear force SECM, but it requires the fabrication of double-barrel tips in which one channel is left empty and the other is filled with a conventional microdisc. 

Shear Force SECM with Vibrating Needle UMEs... 

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]. 

SECM instrumentation (see footnote 1). Nevertheless, the use of a shear force system adds another level of complexity to the experiment because the mechanical properties of the UME, the sample, and the entire setup become important and have to be optimized. Shear force systems have been used to investigate electrode arrays [22,114,116] and detect metabolic activity of living cells [115] or following cell-cell communication processes [112] by the group of Schuhmann. 

FIG. 8 Tip mounts used for implementing shear-force feedback in SECM (A) the tuning fork mount (B) proposed piezo tube mount. 

In some experiments, such as TPM SECM or shear-force mode, the probe is intentionally vibrated. The TPM mode is a vertical oscillation that can be treated as outlined above. Experimenters should be wary of intentional vibration applied to the probe to avoid excitation of additional me-... 

An AC voltage can be applied to the UME and a counter electrode (AC-SECM). The AC current response can be evaluated and it can provide information about local surface conductivity of the surface under investigation [123-125]. This setup has been applied to interrogate living cells [126]. Enhanced spatial resolution may be obtained by using a shear force-based distance control to operate the UME at submicrometer distance. 

Adapted from the technology implemented in scanning near field optical microscopy, SECM with shear-force detection involves vibrating the SECM tip with a piezoelectric acmator and recording variations in resonance frequency when the tip is within a few hundred nanometers from the sample surface [124,125]. This is a fast method when operated in real time during the scan, but it is tricky to implement in practice. The tip must be long and narrow to be appropriately flexible but, more importantly, the control loop parameters need to be frequently adjusted as the resonance and shear force properties vary with the tip dimensions, solution viscosity, and sometimes with the elasticity of the sample surface. Moreover, the parameters need to be readjusted if the tip is removed for polishing. 

Increase in the shear force in close proximity to the sample surface leads to damping of the amplitude of oscillation. Using the amplitude of the oscillations as feedback signal, the tip-to-sample surface distance is maintained constant during the SECM measurements. 

Fig. 11 Shear force in SECM. a Illustration of a microelectiode horizontally vibrating at its resonance frequency. Detection of the vibration by b optical, c piezo, and d tuning fork methods. Reproduced from [30] with permission of the Royal Society of Chemistry...

Mezour et al. successfully attached cobalt (II) 5,10,15,20-tetraphenyl-21 H, 23 h-porphine (CoTPP) onto gold and glassy carbon surfaces via its coordination with 4-aminothiophenol (ATP) self-assembled monolayers as depicted in the scheme of Fig. 26 [104]. The electrocatalytic activity of the resulting film, Au-ATP-CoTPP was investigated for oxygen reduction in H2SO4 (0.5 M) using SECM in the SG-TC mode. The tip was a Pt nanoelectrode with a diameter of 460 nm. The measurements were recorded at a constant distance of 6 pm which was achieved by means of shear force-based constant distance SECM [105]. 

Ballesteros Katemann B, Schulte A, Schuhmann W (2003) Constant-distance mode scanning electrochemical microscopy (SECM)-Part 1 adaptation of a non-optical shear-force-based positioning mode for SECM tips. Chem Eur J 9(9) 2025-2033... 

The powerful combination of SECM with ITIES electrochemistry has enabled the spatially controlled deposition of metal particles, which is potentially extendable to nanoparticle (NP) deposition by using a nanotip. Eor instance, Ag particles were locally electrodeposited on a conductive substrate by employing a micropipet-supported ITIES tip in the egress IT mode (Eig. 9c). The spatial resolution of the tip-induced electrodeposition is controlled by the tip size and the tip-substrate distance. A shorter distance can be maintained by monitoring a shear force between the micropipet tip and the substrate to improve the spatial resolution. An even higher spatial resolution can be achievable by the shear-force-based control of a submicrometer-sized ITIES tip. ... 

Etienne, M. Layoussifi, B. Giornelli, T Jacquet, D. 2012. SECM-based automate equipped with a shear-force detection for the characterization of large and complex samples. Electrochem. Commun., 15, 70-73. 

Similarly, a commercial Ca + ionophore-based micropipette electrode was used by Etienne et al. to investigate the applicability of the potentiometric mode of SECM to the study of dissolution kinetics at solid-liquid interfaces. Using the shear force technique, they were able to precisely control the tip-substrate distance and perform imaging of the ion activity at constant distance over the substrate. The local dissolution of crystal plates of different orientations, as well as that of different parts of calcium carbonate shells of Mya arenaria, was obtained. 

Pitta Bauermann et al. have applied constant distance shear force-controlled SECM to elec-trochemically image fibroblasts and adrenal chromaffin ceUs. In the case of the catecholamine-releasing chromaffin cells, the authors succeeded in amperometrically detecting the release of neurotransmitters from a single secretory vesicle, following chemical stimulation. 

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