Scanning electrochemical microscopy is a scanning probe technique that is based on faradaic current changes as a small electrode is moved across the surface of a sample. The images obtained depend on the sample topography and surface reactivity. The response of the scanning electrochemical microscope is sensitive to the presence of conducting and electroactive species, which makes it useful for imaging heterogeneous surfaces. The principles and instrumentation used to obtain images and surface reaction-kinetic information are discussed, and examples of applications to the study of electrodes, minerals, and biological samples are given.

The scanning electrochemical microscope (SECM) can be used to detect electron-transfer reactions of tip-generated species that occur at nonconductlve surfaces containing a redox-active enzyme. Experiments were carried out with glucose oxidase Immobilized on several substrates and the apparent kinetics of the enzyme catalysis were measured for several mediator oxidants under conditions of high o-glucose concentration. Theory for the SECM feedback current was developed to model the limiting zero-and first-order electrontransfer kinetics expected for such surface catalysis. Working curves relating the SECM feedback current to surface rate constants are presented. For substrates with glucose oxidase covalently attached to the surface or trapped within a porous membrane, the enzyme reaction was readily detected and the SECM feedback currents measured at low mediator concentration (ca. 50 µ ) were found to exclusively fit the model for zero-order heterogeneous kinetics. However, at low enzyme surface concentration or with extensive chemical cross-linking of Immobilized glucose oxidase, It was very difficult to use the SECM to detect and quantify the enzyme reaction. General guidelines for studying enzyme surface reactions with the SECM are given, and the prospects for detecting and klnetlcally assaying enzymes of cellular, and even subcellular, samples are discussed.

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Controlling the size of silica nanoparticles (NPs) on a continuously variable scale was achieved by systematically varying the organic solvent(s) used in water-in-oil microemulsion synthesis. A number of individual as well as binary solvent mixtures were investigated for tuning silica NP size. The results demonstrated that the size of a silica NP was continuously tunable over as range of 20-100 nm by varying the alkane chain length of the organic solvent(s) being used. A simple physical model was proposed to describe the size effect and identify the principle factors needed for precisely controlling the size of a silica NP. In the model, the alkane chain length(s) of the organic solvent(s) primarily determined the average size of a silica NP when other synthetic conditions were fixed, whereas variance in size was affected by water droplet percolation with the precursor microemulsion. The significance and utility of these tunable silica NPs was evaluated for bioimaging applications. Specifically, dye-doped silica NPs of variable but precise size were used for in vitro contrast imaging of cells. The results demonstrated that precise control of silica NPs size can be used to reduce cytotoxicity, optimize luminescence signal intensity, and selectively discriminate between structures both inside and outside of cellular membranes.

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