The development of the in vitro selection technique has allowed the isolation of functional nucleic acids, including catalytic DNA molecules (DNAzymes), from random-sequence pools. The first-ever catalytic DNA obtained by this technique in 1994 is a DNAzyme that cleaves RNA. Since then, many other RNase-like DNAzymes have been reported from multiple in vitro selection studies. The discovery of various RNase DNAzymes has in turn stimulated the exploration of these enzymatic species for innovative applications in many different areas of research, including therapeutics, biosensing, and DNA nanotechnology. One particular research topic that has received considerable attention for the past decade is the development of RNase DNAzymes into fluorescent reporters for biosensing applications. This paper provides a concise survey of the most significant achievements within this research topic. 1. Introduction A biosensor is an analytical device composed of two key components: a molecular recognition element (MRE) that seeks a target of interest for binding and a signal transducer that works to translate the target-MRE interaction into a detectable signal. Proteins, particularly antibodies, receptors, and enzymes, have been the traditional choice of MREs in the design of biosensors for many decades. However, nucleic acids that possess a defined function, such as ligand binding and/or catalysis, have emerged as very attractive MREs over the past 20 years [1–3]. These “functional nucleic acids (FNAs)” include DNA and RNA aptamers, ribozymes (RNA-based enzymes), DNAzymes (DNA-based enzymes), and aptazymes (ribozyme-aptamer or DNAzyme-aptamer conjugates in which the catalytic activity of the enzyme domain is regulated by the ligand binding to the aptamer domain). FNAs, and particularly DNA aptamers and DNAzymes, are inherently more stable than proteins, resulting in more robust biosensors that can function for an increased period of time. FNAs can be created to recognize a broad range of targets by a simple test-tube evolution technique known as SELEX (Systematic Evolution of Ligands by EXponential Enrichment) or in vitro selection [4–6], a process that is short, does not require the use of animals or cells, and has little restriction on the nature of targets and the choice of experimental conditions. FNAs can be chemically synthesized at a relatively low cost with excellent batch-to-batch consistency. They can be facilely immobilized onto a solid matrix. They are easy to modify with sensing probes to allow the detection by many different methods. Binding of targets
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