We have developed glucose and lactate ultramicroelectrode (UME) biosensors based on glucose oxidase and lactate oxidase (with enzymes immobilized onto Pt UMEs by either electropolymerization or casting) for scanning electrochemical microscopy (SECM), and have determined their sensitivity to glucose and lactate, respectively. The results of our evaluations reveal different advantages for sensors constructed by each method: improved sensitivity and shorter manufacturing time for hand-casting, and increased reproducibility for electropolymerization. We have acquired amperometric approach curves (ACs) for each type of manufactured biosensor UME, and these ACs can be used as a means of positioning the UME above a substrate at a known distance. We have used the glucose biosensor UMEs to record profiles of glucose uptake above individual fibroblasts. Likewise, we have employed the lactate biosensor UMEs for recording the lactate production above single cancer cells with the SECM. We also show that oxygen respiration profiles for single cancer cells do not mimic cell topography, but are rather more convoluted, with a higher respiration activity observed at the points where the cell touches the Petri dish. These UME biosensors, along with the application of others already described in the literature, could prove to be powerful tools for mapping metabolic analytes, such as glucose, lactate and oxygen, in single cancer cells.

In a recent paper [9] we reported the manufacturing and performance of miniaturized reference electrodes (MREs) with low sensitivity to chloride ions and pH. Here, we demonstrate the wide range applicability of a MRE based on an Ag/Ag 2 S internal reference element (IRE), imbedded in a photopolymerized hydrogel of improved composition, which contains the supporting electrolyte. Exchange current density, temperature coefficient, impedance value, and the voltammetric and potentiometric use of the Ag/Ag 2 S-based MRE are discussed relative to the previously reported Ag/AgSCN, Ag/Ag 3 PO 4 , and Ag/AgCl-based MREs. No special or extensive conditioning is required when moving these MREs from aqueous supporting electrolyte to an organic solution or from one organic medium to another, and the equilibration time in a new medium is very rapid ( < 6 min). The new Ag/Ag 2 S MRE has a highly stable potential in various media, including aqueous solutions (salt buffers and 20 wt.% H 2 SO 4 ), biological samples (bovine serum albumin), mixed aqueous-organic, and organic supporting electrolytes (methanol, ethanol, acetonitrile, propylene carbonate, methylene chloride, and DMSO). This is particularly advantageous when in the course of the electrochemical analysis an organic solution is being added to an aqueous supporting electrolyte. Such MREs are suitable for analyses of mL sample volumes and for use in protein-containing media.

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Voltage-gated biological ion channels were simulated by insertion of the peptaibol antibiotic alamethicin into reconstituted phosphatidylcholine bilayer lipid membranes (BLMs). Scanning electrochemical microscopy (SECM) was utilized to probe initial BLM resistivity, the insertion of alamethicin pores, and mass transport across the membrane. Acquired SECM images show the spatial location of inserted pore bundles, the verification of voltage control over the pore conformational state (open/closed), and variations in passive mass transport corresponding to different topographical areas of the BLM. SECM images were also used to evaluate overall BLM integrity prior to insertion as well as transport (flux in open state) and leakage (flux in closed state) currents following insertion.

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Scanning electrochemical microscopy (SECM), a non-invasive variant of scanning probe microscopy, can be used to measure oxygen uptake or insulin secretion of isolated murine pancreatic islets in the presence of low and high physiological glucose concentrations levels. SECM imaging of islet topography and islet oxygen uptake is accomplished by the use of a redox mediator that cannot penetrate into the cell membranes of the islet coupled to direct reduction of oxygen at the SECM tip. An insulin sensing ultramicroelectrode (UME), adapted from a macro-scale insulin sensor incorporating a multiwalled carbon nanotube/dihydropyran film developed in our lab previously, was used for the direct electrochemical detection of insulin levels within extracellular media in real-time. We were able to stimulate and record increased insulin production in a single pancreatic islet by raising the concentration of extracellular glucose to the physiological 'high level.' These experiments generated probe scan curves (PSCs) for insulin production in the extracellular media surrounding single pancreatic islets extracted from male mice. This research is a part of an ongoing attempt to increase the scope of application for Bio-SECM, both through the adaptation of specialized biosensors and application of existing techniques for biological analysis of single cells and micro-organs.

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