majority of patients are diagnosed at a late stage, because α-fetoprotein, considered
to be the most useful HCC marker in human serum, lacks sensitivity and specificity
as increased concentrations might be due to other liver related diseases [41]. A more
specific and reliable tool for diagnostics might be the direct detection of cancer cells
as published by Kashefi-Kheyrabdi et al., who proposed an impedimetric aptasensor
for quantification of HepG2 cells which are a hepatocellular carcinoma cell line
[38]. A LoD of 2 cells/ml and a linear range of 10
2
–10
6 cells/ml were achieved based
on the covalent immobilization of aptamer molecules on a gold electrode.
Since the blood of tumor patients contains low levels of circulating tumor cells
(CTCs), screening of isolated CTCs enables diagnosis at early stage. Shen et al. [40]
presented an approach that focuses on the reuse of the biosensor surface and the
collection of viable cells after detection [40]. The authors used an aptamer that
targets an epithelial adhesion molecule that is specifically overexpressed in breast
cancer cells. The sensor design consisted of an immobilized capture probe complementary to the aptamer. The hybridized aptamers captured the CTCs and the
impedance is measured. A uracil excision enzyme specifically digested the aptamer,
leaving the DNA capture probe intact for further experiments and the isolated cells
are released for further cultivation (see Fig. 4). With regard to the CTC, a LoD of
10 cells/ml and a high linear range of 30–10
6 cells/ml were achieved.
2.2.2 Soluble Tumor Marker
The carcinoembryonic antigen (CEA) is an example of a protein tumor marker that is
quantifiable by impedimetric aptasensors. Despite its unspecificity, CEA plays an
important role in diagnostics and monitoring. CEA is expressed on the tumor
membrane and additionally secreted into the blood. Normal concentrations of
CEA in the blood range from 3 to 5 ng/ml; increased CEA levels indicate tumor
progression or recurrence [42, 43].
Different biosensors for CEA detection have recently been published as summarized in Table 3. Shekari et al. developed a selective aptasensor with a linear range of
1 pg/ml to 100 ng/ml [43]. An even lower linear range of 0.1 fg/ml to 1 pg/ml was
achieved by Wang et al. in 2015 by excluding the usual need of covalent immobilization [44]. The required sample volume was 50 μl. The working principle engages
the differences of adsorption from ssDNA and dsDNA on graphene surfaces when
target binding to the single-stranded aptamer induces dsDNA formation. The
lowered charge-transfer resistance results from dsDNA desorption. Since the linear
range falls far below the diagnostic range, the required dilution of serum is not
limiting. In 2017, Guo et al. reported about a design based on silver nanoclusters and
aptamers embedded in zirconium metal-organic clusters [45]. Achieving a linear
range of 10 pg/ml up to 10 ng/ml (LoD: 5 pg/ml) in spiked serum, the approach
potentially enables appropriate diagnostics. Good repeatabilities (n ¼ 5), selectivity,
lifetime (12 days), and possible regeneration (n ¼ 8) were achieved. Just the sample
incubation time of 3 h might be limiting with regard to rapid diagnostics.
Impedimetric Aptamer-Based Biosensors: Applications
53
to be the most useful HCC marker in human serum, lacks sensitivity and specificity
as increased concentrations might be due to other liver related diseases [41]. A more
specific and reliable tool for diagnostics might be the direct detection of cancer cells
as published by Kashefi-Kheyrabdi et al., who proposed an impedimetric aptasensor
for quantification of HepG2 cells which are a hepatocellular carcinoma cell line
[38]. A LoD of 2 cells/ml and a linear range of 10
2
–10
6 cells/ml were achieved based
on the covalent immobilization of aptamer molecules on a gold electrode.
Since the blood of tumor patients contains low levels of circulating tumor cells
(CTCs), screening of isolated CTCs enables diagnosis at early stage. Shen et al. [40]
presented an approach that focuses on the reuse of the biosensor surface and the
collection of viable cells after detection [40]. The authors used an aptamer that
targets an epithelial adhesion molecule that is specifically overexpressed in breast
cancer cells. The sensor design consisted of an immobilized capture probe complementary to the aptamer. The hybridized aptamers captured the CTCs and the
impedance is measured. A uracil excision enzyme specifically digested the aptamer,
leaving the DNA capture probe intact for further experiments and the isolated cells
are released for further cultivation (see Fig. 4). With regard to the CTC, a LoD of
10 cells/ml and a high linear range of 30–10
6 cells/ml were achieved.
2.2.2 Soluble Tumor Marker
The carcinoembryonic antigen (CEA) is an example of a protein tumor marker that is
quantifiable by impedimetric aptasensors. Despite its unspecificity, CEA plays an
important role in diagnostics and monitoring. CEA is expressed on the tumor
membrane and additionally secreted into the blood. Normal concentrations of
CEA in the blood range from 3 to 5 ng/ml; increased CEA levels indicate tumor
progression or recurrence [42, 43].
Different biosensors for CEA detection have recently been published as summarized in Table 3. Shekari et al. developed a selective aptasensor with a linear range of
1 pg/ml to 100 ng/ml [43]. An even lower linear range of 0.1 fg/ml to 1 pg/ml was
achieved by Wang et al. in 2015 by excluding the usual need of covalent immobilization [44]. The required sample volume was 50 μl. The working principle engages
the differences of adsorption from ssDNA and dsDNA on graphene surfaces when
target binding to the single-stranded aptamer induces dsDNA formation. The
lowered charge-transfer resistance results from dsDNA desorption. Since the linear
range falls far below the diagnostic range, the required dilution of serum is not
limiting. In 2017, Guo et al. reported about a design based on silver nanoclusters and
aptamers embedded in zirconium metal-organic clusters [45]. Achieving a linear
range of 10 pg/ml up to 10 ng/ml (LoD: 5 pg/ml) in spiked serum, the approach
potentially enables appropriate diagnostics. Good repeatabilities (n ¼ 5), selectivity,
lifetime (12 days), and possible regeneration (n ¼ 8) were achieved. Just the sample
incubation time of 3 h might be limiting with regard to rapid diagnostics.
Impedimetric Aptamer-Based Biosensors: Applications
53