four connection loops: T9-A10, T18-T19, G13-C14-A15 and a one-nucleotide loop (T6). In the core of G-quadruplex motif, two G-tetrad planes are formed by G5-G7-G12-G16 and G8-G11-G17-G20. The upper plane (G5-G7-G12-G16) is not a typical G-tetrad with the chain topology of anti-syn-anti-syn alternation. Instead, three guanines (G5, G7 and G16) adopt syn conformation, and only one guanine (G12) adopts anti conformation. Additionally, the one-nucleotide loop inserted between G5 and G7. These make G-tetrad formed not through a typically cyclic pattern. This unusual G-tetrad plan is formed by four hydrogen bonds: one on N2:N7(G5-G16), two on O6:N7(G12-G7; G16-G12) and one on O6:N2 (G7-G5). Some other interactions could be found in the G-quadruplex motif: two Watson-Crick base pairs (T6-A15 and A10-T19) and a G-fork (G5-G21). Importantly, because of the interaction between G5 and G21, there is a 90-degree turn between the G-qudruplex and duplex motifs. Interactions between HD22 and thrombin The nucleotides G23, T24, G25, A26, C27 in the duplex and T9, T18, T19, G20 in G-quadruplex contribute to the interaction with the exosite II of thrombin. On the protein side, the residues Tyr89, His91, Pro92, Arg93, Tyr94, Asn95, Trp96, Arg97, Arg126, Leu130, Arg165, Lys169, His230, Arg233, Trp237, Val241 and Phe245 in thrombin are involved in the interaction. Since the exosite II is a positively charged motif, it creates many ion pairs with the HD22 backbone especially in the duplex region. Hydrophobic interactions are mainly observed in the G-quadruplex region (T9, T18 and T10), and this stabilizes the complex formation. Moreover, Interacting with thrombin improves the thermal stability of HD22 structure, and results in the increase of melting temperature (from 36 to 48 ℃). Calculated binding energy of HD22 to thrombin exosite II is -88.37 -kcal.mol-1. Avidity effect of TBA and HD22 Similar to antibody, aptamers TBA and HD22 show avidity effect against thrombin after dimerization. When TBA and HD22 are conjugated with an optimal linker or co-printed on the sensor surface with an optimal density, the affinity against thrombin could be significantly enhanced by 100 to 10,000 fold. Furthermore, the dimerization improves the anticoagulant activity as well. The TBA-HD22 construct (linked with 16-mer polyA) shows significant improvement both in the assay of activated partial thromboplastin time, clotting time and thrombin-induced platelet-aggregation. TBA-HD22 construct shows comparable efficacy compared with bivalirudin, but much more potent than argatroban. In addition, the TBA-HD22 avidity can be examined by ecarin clotting time. Ecarin activates prothrombin and accordingly