Figure 1.

Strategies for the construction of aptamers acting as smart contrast agents: (A) Response to a biochemical stimuli: An oligonucleotide is equipped with a Gd³⁺-DOTA complex. This oligonucleotide is complementary to part of the aptamer and upon binding to the target, the structural reorganization causes the Gd³⁺-DOTA-labeled strand to dissociate from the duplex, which in turn increases the relaxation time and thus the brightness of the MRI signal [46]; (B) Vectoring to intended target: An aptamer is equipped with a contrast agent and will vector the probe directly to the intended target.