# Biosensors Based on Bivalent and Multivalent Recognition by Nucleic Acid Scaffolds
## 2 Biosensors Based on Bivalent Binding Modes
### Biosensors Based on DNA Displacement  
Inspired by proximity-based assays, biosensors based on DNA displacement arising from bivalent interactions have emerged as detection methods for target proteins; this strategy is in line with the binding-induced DNA assembly principle called BINDA (Figure 7A) [53]. In 2012, [PERSON] et al. designed a protein assay based on the displacement of fluorescent oligonucleotides on AuNPs [54]. In this system, a probe sequence was hybridized with an oligonucleotide conjugated with the AuNPs; consequently, the fluorescence was quenched by the AuNPs in the absence of the target protein. However, in the presence of the target protein, the protein-ligand binding forces the probe to switch with a competing DNA strand by bringing them in close proximity to one another; this leads to restored fluorescence (Figure 7B). This concept was further applied to the detection of PDGF-BB, where the probe strand was successfully displaced because of the strong affinity between the aptamers and PDGF-BB, and the fluorescence signal increased. This method enabled the selective detection of nanomolecular concentrations of the target protein.  
More recently, in 2018, [PERSON] et al. developed a strategy using a proximity-induced DNA assembly that they used for imaging protein dimerization [55]. Specifically, when two proteins are dimerized on a cell membrane, two FAM-modified aptamer probes specific to a target membrane protein are in close proximity and thus form a stable stem; this provides an initiator strand for the displacement process. The initiator triggers DNA strand displacement of signal output probes containing a Cy5 and quencher pair to form a three-way junction structure. As a result, Cy5 is released from the quencher, which restores the fluorescence of Cy5 (Figure 8). Using this strategy, protein dimerization, such as the dimerization of the Met receptor or the transforming growth factor-\(\beta\) type II receptor, was clearly visualized on the surface of living cells in real-time.  
Figure 7: (**A**) Conceptual scheme of binding-induced DNA assembly (BINDA). Adapted with permission from _Anal. Chem._ 2012, 84, 877–884, doi:10.1021/ac203207g: Copyright (2012) American Chemical Society [53]. (**B**) Principal scheme of a biosensor based on bivalent interaction-induced DNA displacement on AuNPs. Reproduced by permission from John Wiley & Sons Ltd.: _Angew. Chem. Int. Ed._ 2012, 51, 9317–9320, doi:10.1002/anie.201202677. Copyright (2012) [54].  
To date, bivalent binding-induced DNA displacement has been used to sense streptavidin on magnetic beads [56; 57], thrombin [58; 59; 60; 61; 62], PDGF [63; 64; 65], human epidermal growth factor receptor-2 (HER2) in live cells [66], human acute lymphoblastic leukemia cells (CCRF-CEM) [67], prostate-specific antigen (PSA) [68; 69; 70], antibodies [71; 72; 73; 74], cardiac troponin [75], and other, specific cells [76]. Additionally, streptavidin has been detected using chemilluminescent detection based on a DNA walker [77], and antibodies have been identified through quantum dot doping methods [78]. Moreover, DNA displacement has been combined with other enzymatic amplification systems [79; 80]; for example, rolling circle amplification (RCA) and CRISPR/Cas12a systems were recently conjugated with DNA displacement strategies to detect femtomolar levels of thrombin [81].