2 Aptamer-Based Probes for Molecular Imaging
41
b
a
Fig. 2.4 a The PEG-stabilized, thrombin-activated microbubble (PSTA-MB). Phospholipid DSPE
is conjugated with poly acrylic acid (PAA) and functionalized with the complementary DNA strand
to a longer thrombin aptamer cross-linking sequence (TACS). Neighboring phospholipid groups are
crosslinked because of the TACS, which increases the rigidity of the shell. In the presence of an acute
blood clot, the TACS unhybridized ends will bind to the free thrombin. Then it will dislodge the
strand from the bubble surface, which results in the number of crosslinks on the surface of the bubble
and restoring acoustic ability. b The ultrasound imaging of rabbit deep venous thrombosis after
injection of TACS (a) crosslinked microbubbles and the scrambled aptamer crosslinking sequence
(SACS) (b) crosslinked microbubbles at several specified time points. Probable clot site is pointed
out by blue arrows. Outlines indicate vena cava walls. (c) Over time, TACS (red) within the clot site
or SACS (blue) within the inferior vena cava generate acoustic signal. (d) Active acoustic signal is
generated when bubble encounter the clot. Reprinted from [54], Copyright 2013, Elsevier