Figure 1.

Nucleic acid-based probes can be broadly classified into three types: (i) hybridization-based probes, (ii) aptamers, and (iii) DNAzymes. Hybridization-based probes are single-stranded and recognize complementary targets through Watson-Crick base pairing. For ease of indentification, complementary regions are colored the same in this figure. Aptamers are oligonucleotides evolved through combinatorial selection strategies that can bind to analytes of interest including ions, small molecules, and proteins. Aptamers often adopt complex tertiary structures that enable target recognition. Alternately, target binding can induce conformational changes in their structure. DNAzymes consist of a substrate strand and an enzyme strand (containing a catalytic core) hybridized to each other. The substrate strand contains a single RNA nucleotide which can be catalytically cleaved in the presence of a target.