Use of Redox Enzymes for the Electrochemical Detection of Sequence-Specific DNA and Immunochemical Entities

Abstract

There is a host of methods for the detection of immunoassays and sequence-specific DNA on a microarray chip (or biochip); see earlier chapter. Many detection technologies are optical methods, such as surface plasmon resonance, luminescence, fluorescence, and visible detection modes (absorbance or reflectance). They do require optical systems that are somewhat expensive. As shown in previous chapters, electrochemical detection is a viable option for immunochemical and sequence-specific DNA detection. The electrochemical detection methods vary greatly, with anything from impedance measurements, to oxidation of specific nucleotdides within the duplex, to conductive interacalators, redox-intercalators, metal tags, and redox enzyme systems. Many have specific niches and amplification modes. The bottom line is that electrochemical detection is sensitive, the system footprint is small, and the system is inexpensive. To date only a few electrochemical detection systems have been commercialized and succeeded. These include (1) the CombiMatrix ElectraSense system, utilizing HRP as a redox enzyme (1–6); (2) the Osmeotech Esense (Motorola Life Sciences) system, utilizing a Ferrocene tag (see Chapter 12 in this book); and (3) the Toshiba gene analyzer, utilizing a redox active dye (7). The CombiMatrix system is the only commercial system that uses a true microarray concept, up to 12,000 individual electrodes. The latter E-chem detection systems utilize a microarray that requires spotting onto gold electrodes and the number of electrodes is in the neighborhood of 15–25 per chip. In these cases, the voltage is scanned and current recorded. A substantial increase in the peak-to-peak current reflects the presence of the redox species present, and hence a duplex being formed. In the case where redox enzymes (or products that are redox active) are utilized for the detection of specific DNA sequences and immunoassays, there is a variety of enzymes from which to choose. Many have been used before on simple electrode