# Biosensors Based on Bivalent and Multivalent Recognition by Nucleic Acid Scaffolds
## 4 Conclusions and Outlook
Since the goal of this strategy is mainly focused on the construction of supramolecules bearing many ligands, nucleic acid techniques capable of making long stretches of nucleic acids from small nucleic acid units, such as HCRs and RCA, are the subject of consideration for building multivalent constructs  
Figure 18: General strategies for designing (**A**) bivalent or (**B**) multivalent biosensors. (Figure 18B, top). On the other hand, when receptors are positioned in a specific array, a DNA nanostructure would be advantageous for the construction of multivalent interactions; various architectures have been constructed using nucleic acids. Likewise, a nanostructure can be designed with a geometry in which its vertices can serve as possible positions of target receptors (Figure 18B, bottom). Once this geometry is determined, biosensors for multivalent interactions could be readily constructed by adopting the same signaling strategies utilized for fabricating bivalent-based biosensors (e.g., common fluorescent-labeling or steric strain-induced structural change). Based on these general strategies, as shown in this review, a wide range of nucleic acid-based multivalent biosensors have been developed to possess advantageous features such as strong binding affinity to targets, high target specificity, and strong signal amplification. However, many challenges remain, and various aspects of this field still require exploration. First, numerous biological molecules capable of bivalent or multivalent interactions have not yet been explored in regard to their use in the design of biosensors [130; 131; 132; 133; 134; 135]. Second, precisely designed nanostructures are required for biosensors, particularly those used for detecting viruses; this is because they express unique patterns of antigen arrays. Consequently, biosensors employing multiple ligand arrays with controlled ligand-ligand distances and shapes are greatly desired. However, to date, only a few examples of such biosensors have been reported; these use DNA nanostructures with complex shapes [129; 136]. Finally, multivalent recognition systems are rarely conjugated with other nanodevice systems for amplification. Even if the multivalent interactions of a biosensor provide it with strong binding affinity and selectivity to its targets, the one-to-one binding between a ligand display and a target still requires extremely high sensitivity. Therefore, the combination of activation strands resulting from multivalent recognition with a wide range of DNA nanodevices could be a promising solution for such limitations. Accordingly, efforts should be directed toward constructing precisely designed, multivalent, nucleic acid scaffold-based biosensors that feature rapid responses, high sensitivities, and good selectivities. With such improved multivalent biosensors, it is expected that a wide range of disease-related biological interactions will be understood in-depth and also that development of more effective drugs, inhibitors, vaccines, and diagnostic kits for multivalent pathogens (e.g., viruses) will be accelerated in the next decade. H.K., H.C. and Y.H. contributed equally to this work. H.K., H.C., C.K. and K.T.K. summarized biosensors based on bivalent binding modes. Y.H., M.K. and K.T.K. summarized biosensors based on multivalent binding modes. All authors helped prepare and write the manuscript. All authors have read and agreed to the published version of the manuscript. This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) (No. 2021R1C1C1009199 for K.T.K.). Not applicable. The authors declare no conflict of interest.