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.