9 Aptamers for the Diagnosis of Malign Tumors
249
detection, using the aptamer-functional quantum dots (QDs) for fluorescence signal
amplification. The reported LOD was 7 zmol (10
−21 mol) of CRP in serum in this
study [55].
Surface-enhanced Raman Scattering (SERS) is also a label-free spectroscopy
technique. Dependent on the narrow spectral width of distinctive Raman peaks,
SERS could offer the information of the fingerprint of the analyte [56]. With the help
of metallic nanostructures for Raman signal enhancement, the aptamer-based SERS
sensor exhibits extreme sensitivity for the detection of biomarkers at a very low level
in biological samples [57].
9.3.2.2 Fluorescence-Based Platform
Fluorescence is a commonly used strategy to establish the aptasensors for
protein biomarkers sensing for its high sensitivity, operation simplicity, and costeffectiveness. Using covalently labeled or non-covalently fluorescent probes, the
target-induced transitions of the secondary structures of aptamers could be monitored
conveniently. And the change of signal intensity reflects the extent of the aptamerligands binding event, resulting in quantitative measurements of the concentration.
Fluorophores-labeled and label-free aptamers correspond to two categories of fluorescence aptasensors, typically using the fluorescence resonance energy transfer
(FRET). Till now, several studies have reported the aptamer-based fluorescence
biosensors for cancer-related proteins identifying and sensing.
Thrombin is an important protein for many coagulation-related processes,
including hemostasis, thrombosis, and has been known as a universal cancer
biomarker [58]. Using the thrombin-binding aptamers (TBAs) to conjugate the fluorophore (6-carboxyfluorescein, 6-FAM) and the quencher (4,4’dimethylaminophenylazo benzoic acid, DABCYL), a fluorescent molecular beacon
had been constructed. The structure of oligonucleotide within this molecular beacon
transforms to an intramolecular G-quadruplex from random coil after the binding
process, resulting in a concentration-dependent “turn-off” of the fluorescence
signal with the LOD of 112 pM [59]. Alternative approaches for this “structure
switch” strategy was employed for aptasensor design as from DNA-DNA duplex
to DNA/analyte complex. This molecule aptamer beacon consists of a tripartite
duplex structure, containing a fluorophore (5
-fluorescein-labeled oligomer, FDNA),
a quencher (3
-DABCYL-labeled oligomer, QDNA), and a long oligonucleotide
comprising Stem-1 and Stem-2. Stem-1 and Stem-2 are complementary to FDNA
and QDNA, respectively, and Stem-2 also contains the TBA sequence in the partial
overhang. The presence of thrombin could trigger the TBA-target secondary structure formation, destroying the close proximity of FDNA and QDNA and restoring
the fluorescence emission [60] (Fig. 9.5a).
The human platelet-derived growth factor (PDGF) is a protein present in serum
that takes part in the regulation of embryogenesis, cell growth, and migration [61].
And one of the homodimers, PDGF-BB has been considered as a cancer biomarker for
early diagnosis [62]. For example, a label-free aptasensor was established for PDGF
249
detection, using the aptamer-functional quantum dots (QDs) for fluorescence signal
amplification. The reported LOD was 7 zmol (10
−21 mol) of CRP in serum in this
study [55].
Surface-enhanced Raman Scattering (SERS) is also a label-free spectroscopy
technique. Dependent on the narrow spectral width of distinctive Raman peaks,
SERS could offer the information of the fingerprint of the analyte [56]. With the help
of metallic nanostructures for Raman signal enhancement, the aptamer-based SERS
sensor exhibits extreme sensitivity for the detection of biomarkers at a very low level
in biological samples [57].
9.3.2.2 Fluorescence-Based Platform
Fluorescence is a commonly used strategy to establish the aptasensors for
protein biomarkers sensing for its high sensitivity, operation simplicity, and costeffectiveness. Using covalently labeled or non-covalently fluorescent probes, the
target-induced transitions of the secondary structures of aptamers could be monitored
conveniently. And the change of signal intensity reflects the extent of the aptamerligands binding event, resulting in quantitative measurements of the concentration.
Fluorophores-labeled and label-free aptamers correspond to two categories of fluorescence aptasensors, typically using the fluorescence resonance energy transfer
(FRET). Till now, several studies have reported the aptamer-based fluorescence
biosensors for cancer-related proteins identifying and sensing.
Thrombin is an important protein for many coagulation-related processes,
including hemostasis, thrombosis, and has been known as a universal cancer
biomarker [58]. Using the thrombin-binding aptamers (TBAs) to conjugate the fluorophore (6-carboxyfluorescein, 6-FAM) and the quencher (4,4’dimethylaminophenylazo benzoic acid, DABCYL), a fluorescent molecular beacon
had been constructed. The structure of oligonucleotide within this molecular beacon
transforms to an intramolecular G-quadruplex from random coil after the binding
process, resulting in a concentration-dependent “turn-off” of the fluorescence
signal with the LOD of 112 pM [59]. Alternative approaches for this “structure
switch” strategy was employed for aptasensor design as from DNA-DNA duplex
to DNA/analyte complex. This molecule aptamer beacon consists of a tripartite
duplex structure, containing a fluorophore (5
-fluorescein-labeled oligomer, FDNA),
a quencher (3
-DABCYL-labeled oligomer, QDNA), and a long oligonucleotide
comprising Stem-1 and Stem-2. Stem-1 and Stem-2 are complementary to FDNA
and QDNA, respectively, and Stem-2 also contains the TBA sequence in the partial
overhang. The presence of thrombin could trigger the TBA-target secondary structure formation, destroying the close proximity of FDNA and QDNA and restoring
the fluorescence emission [60] (Fig. 9.5a).
The human platelet-derived growth factor (PDGF) is a protein present in serum
that takes part in the regulation of embryogenesis, cell growth, and migration [61].
And one of the homodimers, PDGF-BB has been considered as a cancer biomarker for
early diagnosis [62]. For example, a label-free aptasensor was established for PDGF