significant positive band at 295 nm and a negative band at 270 nm on its circular dichroism (CD) spectrum. Also, potassium ion improves the thermal stability of TBA. The melting temperature of TBA's G-quadruplex (measuring the intensity change of the peak at 295 nm by CD) in the presence of sodium ion and potassium are 24 and 53 ℃, respectively. Compared with sodium, potassium ion fits perfectly to the cavity between two G-tetrad plane and is coordinately bound to four O6 atoms in each plane. This enhances the structural stability of TBA. In contrast, due to its small size, sodium ion can only interacts with four rather than eight oxygen atoms of two G-tetrad planes, and accordingly has two alternative position in the cavity. Thrombin shows similar influence as potassium ion. In the ion-deficient condition, thrombin helps TBA form into a stable G-quadruplex structure from a randomized coil, which results in conformational change. Some groups use this property to develop aptamer-based thrombin sensors. For this purpose, TBA is usually mounted with an additional sequence with a FRET (Förster resonance energy transfer) pair to form a transient duplex structure. Once the TBA part interacts with thrombin, the conformational change would change the distance between the FRET pair and lead to a fluorescent output. This approach provides nano-molar sensitivity and is capable of sensing thrombin in the spiked serum. mTBA A modified TBA with chain polarity inversion was reported in 1996, which is known as mTBA. A 5'-5' inversion was designed between T3 and T4 in mTBA sequence (3′-GGT-5′-5′TGGTGTGGTTGG-3′). This improves the thermal stability of G-quadruplex structure, and increases the melting temperature by 4 ℃. In spite of this, the anticoagulant activity is affected and reduced by the inversion design. Interactions between TBA and thrombin TBA is bound to the exosite I of thrombin majorly via its two TT loops (T3, T4 and T12, T13) through polar and hydrophobic interactions. The residues His71, Arg75, Tyr76, Arg77, Asn78, Ile79, Tyr117 in the exosite I epitope are involved in the interaction with TBA. Exosite 1, being a positively charged motif, engages in these interactions with the negatively charged backbone of HD1. Importantly, T3 interacts with His71, which plays a critical role for fibrinogen recognition, both through hydrogen bonding and hydrophobic interaction. However, in the presence of sodium ion, the hydrogen bonding between T3 and His71 is lost, and the intermolecular distance is longer than