9 Aptamers for the Diagnosis of Malign Tumors
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emission peak, making them a satisfactory candidate for fluorescence-guided surgery
[149]. This strategy can be easily applied to other tumors for in vivo imaging, due to
the cancer-targeting universality of AS1411 aptamers.
Several other techniques have also been used in tumor in vivo imaging by aptamers
such as ultrasound (US) [150], photoacoustic (PA) imaging [151], and multimodal
imaging [152]. Each imaging technique has various advantages; however, their disadvantages still exist. For instance, the nuclear imaging methods as SPECT and PET
have high resolution and longitudinal assessment for deep tissues, but suffering
from the expensive cost and isotypes handling. In contrast, fluorescence imaging is
quite cost-effective with relatively high sensitivity, but lack of tissue penetration and
susceptibility to autofluorescence. Although the aptamer-guided in vivo molecular
imaging is still in the preclinical stage, it has shown attractive prospects.
9.7 Exosome-Sensing Aptasensors
Exosomes are endocytic, heterogeneous membrane-derived vesicles, secreted by
various types of cells and playing an important role in intercellular communications
[153]. Exosomes have the ability to carry biological cargo as protein, lipids, and
nucleic acids, and escape from the phagocyte system and circulate in body fluids
by the small size (30–100 nm) [154]. It has become increasingly convincing that
exosomes can modulate the tumor microenvironments for angiogenesis, and propagate the tumor messages to normal cells to increase metastatic potential. Therefore,
the growing interest has led to the identification of cancer-derived exosomes as
novel cancer biomarkers for early diagnosis [155]. The exosome-based diagnostic
platforms were fabricated for many types of tumors and cancers [156]. Exosometargeting aptamers were employed as recognition elements for sensor development. In this section, we will discuss many aptamer-based strategies for exosome
identification and detection.
Fluorescence-based aptasensors offer several advantages such as high sensitivity,
non-invasive, fast, and only small sample volume needed, which made it possible
for clinical application. The aptamers used in the platform target the CD63 marker
on the surface of exosomes. For example, the CD63 aptamers were immobilized
onto magnetic beads, hybridizing with a Cy3-labeled complementary ssDNA strand
to keep the resting state. The added A549-derived exosomes could compete with
the Cy3-labeled strands and then release them for fluorescence intensity recovery in
the supernatant. This analysis method offered the LOD of 1.0×10
5 exosomes/mL
[157]. The same aptamer was employed for exosome sensing in human plasma using
fluorescence polarization, with the LOD of 500 exosomes/mL [158].
Nanomaterials are applied to combine with fluorescent probes for increasing
sensitivity [159, 160]. For example, graphene oxide (GO) has the ability to absorb
DNA and RNA molecules via π-π stacking interactions, and has high fluorescence quenching efficiency. Based on these properties, a turn-on-type aptasenosor
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emission peak, making them a satisfactory candidate for fluorescence-guided surgery
[149]. This strategy can be easily applied to other tumors for in vivo imaging, due to
the cancer-targeting universality of AS1411 aptamers.
Several other techniques have also been used in tumor in vivo imaging by aptamers
such as ultrasound (US) [150], photoacoustic (PA) imaging [151], and multimodal
imaging [152]. Each imaging technique has various advantages; however, their disadvantages still exist. For instance, the nuclear imaging methods as SPECT and PET
have high resolution and longitudinal assessment for deep tissues, but suffering
from the expensive cost and isotypes handling. In contrast, fluorescence imaging is
quite cost-effective with relatively high sensitivity, but lack of tissue penetration and
susceptibility to autofluorescence. Although the aptamer-guided in vivo molecular
imaging is still in the preclinical stage, it has shown attractive prospects.
9.7 Exosome-Sensing Aptasensors
Exosomes are endocytic, heterogeneous membrane-derived vesicles, secreted by
various types of cells and playing an important role in intercellular communications
[153]. Exosomes have the ability to carry biological cargo as protein, lipids, and
nucleic acids, and escape from the phagocyte system and circulate in body fluids
by the small size (30–100 nm) [154]. It has become increasingly convincing that
exosomes can modulate the tumor microenvironments for angiogenesis, and propagate the tumor messages to normal cells to increase metastatic potential. Therefore,
the growing interest has led to the identification of cancer-derived exosomes as
novel cancer biomarkers for early diagnosis [155]. The exosome-based diagnostic
platforms were fabricated for many types of tumors and cancers [156]. Exosometargeting aptamers were employed as recognition elements for sensor development. In this section, we will discuss many aptamer-based strategies for exosome
identification and detection.
Fluorescence-based aptasensors offer several advantages such as high sensitivity,
non-invasive, fast, and only small sample volume needed, which made it possible
for clinical application. The aptamers used in the platform target the CD63 marker
on the surface of exosomes. For example, the CD63 aptamers were immobilized
onto magnetic beads, hybridizing with a Cy3-labeled complementary ssDNA strand
to keep the resting state. The added A549-derived exosomes could compete with
the Cy3-labeled strands and then release them for fluorescence intensity recovery in
the supernatant. This analysis method offered the LOD of 1.0×10
5 exosomes/mL
[157]. The same aptamer was employed for exosome sensing in human plasma using
fluorescence polarization, with the LOD of 500 exosomes/mL [158].
Nanomaterials are applied to combine with fluorescent probes for increasing
sensitivity [159, 160]. For example, graphene oxide (GO) has the ability to absorb
DNA and RNA molecules via π-π stacking interactions, and has high fluorescence quenching efficiency. Based on these properties, a turn-on-type aptasenosor