Scanning electrochemical microscopy (SECM) is an electroanalytical scanning probe technique capable of imaging substrate topography and local reactivity with high resolution. Since its inception in 1989, it has expanded into a wide variety of research areas including biology, corrosion, energy, kinetics, instrumental development, and surface modification. In the past 25 years, over 1800 peer-reviewed publications have focused on SECM, including several topical reviews. However, these reviews often omit key details, forcing readers to search the literature. In this review, we provide a comprehensive summary of the experimental parameters (e.g., solvents, probes, and mediators) used in all SECM publications since 1989, irrespective of the application. It can be used to rapidly assess experimental possibilities and make an informed decision about experimental design. In other words, it is a practical guide to SECM.

A simple, fast, and reproducible method for the fabrication of disk ultramicroelectrodes (UMEs) with controlled geometry is reported. The use of prepulled soda-lime glass capillaries allows one to bypass the irreproducible torch-sealing and experimentally challenging tip-sharpening steps used in conventional fabrication protocols. A micron-sized electroactive wire is sealed inside this capillary producing UMEs with a highly reproducible geometry. Total fabrication time (1 h) and experimental difficulty are significantly reduced. Disk UMEs with various diameters and cores were fabricated, including carbon fiber (7 and 11 μm), gold (10 and 25 μm), platinum (10 and 25 μm), silver (25 μm), and mercury (25 μm). The ratio of the insulating sheath to the electroactive core of the UMEs was 2.5-3.6. Silver UMEs were also used to produce a Ag/AgCl microreference electrode. This general fabrication method can readily be applied to other electroactive cores and could allow any research group to produce high quality disk UMEs, which are a prerequisite for quantitative scanning electrochemical microscopy.

Hg/Pt hemispherical ultramicroelectrodes (UMEs) (25-microm diameter) were prepared either by electrodeposition from a mercuric ion solution or by simple contact of the Pt disk to a hanging mercury drop electrode. The two methods produced equivalent tips. Optical inspection and electrochemical characterization of these Hg tips with methyl viologen, cobalt sepulchrate trichloride, and hexamineruthenium(III) chloride confirm the hemispherical nature of the UME. The scanning electrochemical microscopy approach curves for all three redox couples over a conductive substrate fit theoretical plots for hemispherical electrodes. The numerical solution of the diffusion equations for substrate generation-tip collection (SG-TC) transients for a finite Pt disk and Hg/Pt hemispherical UME are reported and compared to experimental results. A diffusion layer approximation is presented, and diffusion coefficients are extracted from the simulation. The SG-TC results reveal the enhanced sensitivity of hemispherical UME to radial diffusion.

The emergence of resistance to multiple unrelated chemotherapeutic drugs impedes the treatment of several cancers. Although the involvement of ATP-binding cassette transporters has long been known, there is no in situ method capable of tracking this transporter-related resistance at the single-cell level without interfering with the cell's environment or metabolism. Here, we demonstrate that scanning electrochemical microscopy (SECM) can quantitatively and noninvasively track multidrug resistance-related protein 1-dependent multidrug resistance in patterned adenocarcinoma cervical cancer cells. Nonresistant human cancer cells and their multidrug resistant variants are arranged in a side-by-side format using a stencil-based patterning scheme, allowing for precise positioning of target cells underneath the SECM sensor. SECM measurements of the patterned cells, performed with ferrocenemethanol and [Ru(NH 3 ) 6 ] 3+ serving as electrochemical indicators, are used to establish a kinetic "map" of constant-height SECM scans, free of topography contributions. The concept underlying the work described herein may help evaluate the effectiveness of treatment administration strategies targeting reduced drug efflux.ancer cells, such as lung cancer or leukemia, acquire resistance to multiple unrelated drugs in response to treatment with chemotherapeutic agents (1, 2). Resistance impedes therapeutic effectiveness, which in turn, reduces the long-term survival rate of cancer patients (2). The emergence of multidrug resistance (MDR) involves the overexpression of transmembrane proteins P-glycoprotein (P-gp) and MDR-related protein 1 (MRP1), which both belong to the family of 5′-triphosphatebinding cassette transporters (known as ABC transporters). P-gp and MRP1 act as molecular "pumps," actively removing therapeutic agents from the cancer cells, thereby preventing the drug from inducing the desired effect on the cell nucleus or cytoplasm. MDR based on P-gp is relatively well understood and involves binding of hyaluronan to the cell surface glycoprotein CD44. The resulting up-regulation of the transcriptional cofactor p300 expression and therefore the NFkappaB-specific transcriptional up-regulation lead to the production of P-gp, and with that chemoresistance in cells (3). However, the mechanism that causes MRP1-mediated MDR remains unclear.Currently, quantification of MDR relies on immunohistochemical analyses, such as real-time PCR, focusing mostly on P-gp or other members of ABC transporters (4-9). Fluorescent MRP1-specific studies were also conducted, revealing that resistant and nonresistant cancer cells have differential intracellular content of glutathione and GST, which affect their cell death mechanism during hyperthermia (10). Microvoltammetry was also used to measure the efflux of chemotherapeutic drugs from normal and MDR cancer cells on the single-cell level, with detection limits in the nanomolar range (11). Finally, flow cytometry has routinely been used to quantify and compare expression levels and activity of dif...