**1. Introduction**

All genetic information with regards to every living organism is stored in the deoxyribonucleic acid, DNA. This is the fundamental application of DNA that makes it the basic building block of life. However, DNA is a very dynamic molecule whereby its function is not confined only to information storage and delivery. DNA also has the ability to form a number of spatial arrangements such as single-stranded hairpins, homoduplexes, triplexes and quadruplexes with high-order complexity. In nature, the formation of these structures has been found to be involved in many cellular mechanisms such as DNA recombination, regulation of gene expression and possibly the proliferation of tumor cells [1, 2].

One of these DNA structures that are well studied is the G-quadruplex (G-quad) structure. The G-quad structure is made up of a stack of nucleic acid sequence that is rich in guanine (G)

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

[3]. Besides, G-quad structure is found to be polymorphic as it forms many different structural arrangements depending on the variation in DNA composition and environment. Different formation of G-quad structure has different yet specific functions in nature. Thus, the study of G-quad has driven the structure into many applications, especially in the field of medicine, biology and material sciences.

The biological functions of the G-quad structure are well documented and the principles of it can be exploited for use as biosensors and therapeutics. One of the functions of the structure is the formation of DNA enzyme or catalytic enzymes, in short, DNAzyme, that can exhibit catalytic capabilities such as exhibiting peroxidase-like activity. G-quad can also serve as internal fluorescent probes in which its nucleobases are modified or attached with fluorescent dyes for sensing. In addition, G-quad can be combined with some electrochemical methods to produce signal readout. The discovery of such remarkable advantages of G-quad have given rise to the development of assays exploiting the G-quad structure such as DNA detection assay, protein detection assay and even the detection of molecules and ions [4-7].

Taken together, the advantages and flexibilities accorded to DNA G-quad have made G-quad very useful for the development of a variety of reporter systems for sensing applications. Consequently, DNA-based assay has now become a potential alternative to the conventional diagnostic platforms that use enzymes. In this chapter, the focus of our discussion would be on the structural features and application of DNA G-quad structures for the development of various sensing platforms.
