Chapter 7 - Detection of charged macromolecules by means of field-effect devices (FEDs): Possibilities and limitations

Abstract

This chapter evaluates the possibilities and limitations of a direct electrical detection of charged macromolecules by field-effect devices (FED). Field–effect–based DNA chips are used for the simultaneous analysis of a multitude of nucleic acids, representing a wide field of possible applications in biomedical research, clinical diagnostics, drug screening, genomics, environmental monitoring, and food analysis. The main advantage of FED is that they combine the possibility of a direct electrical detection of the DNA hybridization without labels with their large integration ability as active microelectronic devices. Some of the factors which influence the DNA immobilization and hybridization detection by FED include the gate material of the FED—such as metal, insulator, polymer—and its surface charge or surface modification and the immobilization method of the ssDNA. The immobilization; hybridization; as well as measuring conditions such as pH, ionic strength, and temperature; the packing density and length of the immobilized ss-DNA; and the length of the spacer or linker molecule are some other factors which influence the detection. A new mechanism for a direct label-free DNA detection using an FED that involves the ion concentration redistribution in the intermolecular spaces of the immobilized ssDNA (due to the DNA-hybridization event) and the alteration of the ion sensitivity has been proposed. Various experiments with capacitive EIS sensors used for DNA immobilization and hybridization detection as well as for monitoring the layer-by-layer adsorption of polyelectrolytes anionic poly(sodium 4-styrene sulfonate):PSS and cationic poly(allylamine hydrochloride: PAH) are also presented. Both theoretical calculations and preliminary experiments with synthetic ssDNA and polyelectrolyte multilayers demonstrate a substantial sensor signal on the order of several tens of millivolts.