2.1.3 Aptamer recognition
Due to its small size, high stability, high affinity, and high
specificity, aptamers can not only separate and combine with any
selected molecule (including metal ions, small molecules, peptides,
proteins, and even materials surfaces), but also can be easily modified
in any position. Therefore, they are suitable for use as recognition
elements of sensors [36, 37]. Aptamers are single-stranded DNA or
RNA molecules, most of which are obtained by a combinatorial biology
technique called SELEX. In the cell-free environment, the
single-stranded region of the aptamer is easily regulated by phage
polymerase, which regulates the transcription and translation of the
aptamer sequence through the T7 promoter. For example, Iyer et
al . used nucleic acid aptamers to identify thrombin. They inserted
thrombin-bound ssDNA aptamers in the downstream of the T7 promoter,
creating a ”bubble” structure downstream that binds to the aptamers in
the presence of thrombin, blocking transcription of the T7 RNA
polymerase (Fig. 2C). Finally, the presence of thrombin can be detected
by the expression of the reporter gene [38, 39].
Although this method has high stability and specificity, its sensitivity
needs to be further improved. In addition, the identification mechanism
lacks high-quality aptamers for clinically important targets in medical
applications. The aptamer technology must be extensively tested in
clinical sample substrates to establish the reliability and accuracy.