Electrochemical Substrates and Systems for Enzyme-Based Bioassays

Enzymatic substrates are widely used for bioassays. For example, enzyme-modified antibodies are used for immunoassays, and the reaction of enzymatic substrates with the enzyme releases products from the substrate. By measuring the products, the amount of analytes can be quantified. In medical applications, biological samples, such as blood and cells, are stained with antibodies labeled with enzymes. The targeted antigens are monitored via the enzymatic reaction. In general, monitoring is done by optical methods. As an alternative approach, electrochemical methods have been proposed owning to their simplicity and high sensitivity. For electrochemical applications, it is important to prepare electrochemical substrates. Some electrochemical substrates are commercially available and are widely used for electrochemical detection of alkaline phosphatase, β-galactosidase, and other enzyme activities. New substrates have been synthesized for detection of caspase-3, endotoxin, and other molecules. In this review, we summarize these electrochemical substrates, their products, electrochemical reactions for detection, and their applications in bioassays, such as immunoassay, reporter gene assays, single-cell analysis, cell analysis of threedimensionally cultured cells, and others. In addition, we discuss state-of-the-art electrochemical devices/systems for signal amplification, high throughput analysis, and highresolution electrochemical imaging. Finally, conclusions and perspectives are presented.