Living cells, SECM

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. 

The first micro-ITIES were introduced in 1986, using a glass micropipette which was pulled down to a fine tip of around 25 pm to support the interface [66-71]. The smaller size of micropipettes or microcapillaries is advantageous for sensor applications, providing the possibility of studying microenvironments as living cells, and it can also be used as a probe in scanning electrochemical microscopy (SECM) [72]. 

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. 

Thongh SECM stndies on live cells can be performed without the use of an inverted microscope, the rongh positioning of the UME over the cells is greatly facihtated if one is used. Also, the use of fluorescent viabihty dyes is often possible when the inverted microscope has flnorescent capabilities. This allows for simultaneous determination of cellnlar viability before and after the experiment. Optical micrographs of the UME positioning over the cells can also be acquired. They provide independent proof that the... 

Since these early days, SECM has been widely used to study charge-transfer reactions, and excellent reviews have been published recently [270-272]. Among the recent work, one should mention the possibility of generating electrochemiluminescence at ITIES using SECM [273,274], or of exploring cell metabolism and detect Ag" toxicity in living cells [275]. 

The hybrid technique of SECM-SICM, first reported concurrently by two separate research groups, combines the capability of SICM to attain topographical images of samples within biologically relevant media with the ability of SECM to measure localized electrochemistry. Like SICM, the ion current flow between bulk solution and a nanopipette is utilized in this system as a feedback control for the probe position so that a topographical image of the sample can be obtained. Simultaneously, an independent electrode on the probe measures the faradaic current to provide localized chemical information of redox-active species. This hybrid SPM technique has been utilized to measure the transport of redox probes through polymer nanoporous membranes, immobilized enzymes, and live cells. ... 

Bauermann, L. R, Schuhmann, W., Schulte, A. 2004. An advanced biological scanning electrochemical microscope (Bio-SECM) for studying individual living cells. PCCP 6 4003-4008. 

Besides the DC or transient amperometric techniques described earlier, several research groups have also explored and shown that AC impedance measurements at solid-liquid interfaces with SECM (AC-SECM or IMP-SECM) could provide information on topography and conductivity of substrates. The distance dependence of the AC feedback signal can be employed for constant distance imaging of insulating substrates immersed in dilute electrolyte solutions and of live cells in a growth medium. ... 

Human breast epithelial (MCE-lOA) cells are the last cell type reviewed in this section. As predominant as the HeLa cells, they were used in early studies to probe the redox activity of single living cells using SECM. ° From these early studies, it became clear that there are several ways by which a redox mediator can interact with a live cell (Figure 12.24). 

One of the earliest SECM studies was performed on living cell of plant origin." Using the feedback mode, Lee et al. obtained topographical images of the upper surface of a grass leaf and the lower surface of Ligustrum sinensis leaf under different illumination conditions. 

Diakowski, R M., Ding, Z. Interrogation of living cells using alternating current scanning electrochemical microscopy (AC-SECM). PCCP 2007, 9, 5966-5974. 

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