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Y. Cheng and H. Jin
11.2.5 Cortisol-Related Aptamers
Cortisol is a stress hormone released from the adrenal gland; it is a kind of glucocorticoid hormone which has a considerable impact on human’s daily life. Cortisol
increases heart rate and blood pressure and represses growth, digestive and reproductive activities [24]. It can regulate human’s physiological functions [49] via multiple
methods, and also has immunosuppressive and anti-inflammatory effects. Cortisol
can regulate the body’s energy metabolism, electrolyte balance, and cognitive level
[50]. The detection of cortisol has conventionally been performed by chromatographic separation methods [51] coupled with fluorescence [52] or mass spectrometric quantification, which require multiple sample preprocessing steps. However,
these methods require complicated laboratory techniques, and they may not be as
specific as predicted. To monitor the level of cortisol, researchers also use radioimmunoassays or ELISA [53] with relatively large sample volumes (μL), prolonged
incubation periods (hours) and specialized lab equipment (plate readers, for instance),
which are not well-suited for point-of-care monitoring applications [53]. Since
2010, electrochemical immunoassays have emerged as a promising alternative [53].
In contrast with former analytical methods, electrochemical immunoassays need
smaller sample volume, and can be integrated with standard antibody immobilization
schemes. More importantly, they do not need such complicate equipment, whereas
both detection sensitivity and efficiency are improved.
To further improve detection sensitivity and selectivity using aptasensors,
aptamers are successively developed in recent years. Unlike antibodies, Aptamers,
isolated in vitro, are single-stranded DNAs or RNAs that have high affinity and specificity against the corresponding target [2], they not only exhibit no batch-to-batch
variation [54], but also have reversible denaturation properties. Because of the superior properties of aptamers over antibodies, aptamers can be elegant candidates to
replace the counterpart antibodies in diagnostic assays of cortisol.
In 2014, Martin et al. [24] firstly selected a DNA aptamer for the stress biomarker
cortisol using a modified SELEX method. Among selected aptamers, the aptamer
15-1(KD = 6.9 ± 2.8 μM by equilibrium dialysis; 16.1 ± 0.6 μM by microscale
thermophoresis) presents unique specificity for cortisol without interacting with its
analogs such as epinephrine, norepinephrine, cholic acid, and P4. The aptamer 15-1
also shows an excellent response in a gold nanoparticle assay as monoclonal cortisol
antibody investigated elsewhere [55].
In 2016, Bankim J group [53] developed a quantitative detection system for
cortisol in a microfluidic device. This system is based on aptamer15-1 and does
not require capture probe immobilization on device surfaces, target labeling, or
washing steps before electrochemical readout. Researchers used high concentrations
of aptamer functionalized gold AuNPs, and carried out the detection on an alternate graphene-modified electrode. The aptamer assay exhibited good signal linearity
from 30 pg/ml to 10 μ g/mL. Compared to ELISA and radio labeling, this detection
method requires a shorter detection time (2.5 min) and a smaller sample volume (0.1
μL) [53].
Y. Cheng and H. Jin
11.2.5 Cortisol-Related Aptamers
Cortisol is a stress hormone released from the adrenal gland; it is a kind of glucocorticoid hormone which has a considerable impact on human’s daily life. Cortisol
increases heart rate and blood pressure and represses growth, digestive and reproductive activities [24]. It can regulate human’s physiological functions [49] via multiple
methods, and also has immunosuppressive and anti-inflammatory effects. Cortisol
can regulate the body’s energy metabolism, electrolyte balance, and cognitive level
[50]. The detection of cortisol has conventionally been performed by chromatographic separation methods [51] coupled with fluorescence [52] or mass spectrometric quantification, which require multiple sample preprocessing steps. However,
these methods require complicated laboratory techniques, and they may not be as
specific as predicted. To monitor the level of cortisol, researchers also use radioimmunoassays or ELISA [53] with relatively large sample volumes (μL), prolonged
incubation periods (hours) and specialized lab equipment (plate readers, for instance),
which are not well-suited for point-of-care monitoring applications [53]. Since
2010, electrochemical immunoassays have emerged as a promising alternative [53].
In contrast with former analytical methods, electrochemical immunoassays need
smaller sample volume, and can be integrated with standard antibody immobilization
schemes. More importantly, they do not need such complicate equipment, whereas
both detection sensitivity and efficiency are improved.
To further improve detection sensitivity and selectivity using aptasensors,
aptamers are successively developed in recent years. Unlike antibodies, Aptamers,
isolated in vitro, are single-stranded DNAs or RNAs that have high affinity and specificity against the corresponding target [2], they not only exhibit no batch-to-batch
variation [54], but also have reversible denaturation properties. Because of the superior properties of aptamers over antibodies, aptamers can be elegant candidates to
replace the counterpart antibodies in diagnostic assays of cortisol.
In 2014, Martin et al. [24] firstly selected a DNA aptamer for the stress biomarker
cortisol using a modified SELEX method. Among selected aptamers, the aptamer
15-1(KD = 6.9 ± 2.8 μM by equilibrium dialysis; 16.1 ± 0.6 μM by microscale
thermophoresis) presents unique specificity for cortisol without interacting with its
analogs such as epinephrine, norepinephrine, cholic acid, and P4. The aptamer 15-1
also shows an excellent response in a gold nanoparticle assay as monoclonal cortisol
antibody investigated elsewhere [55].
In 2016, Bankim J group [53] developed a quantitative detection system for
cortisol in a microfluidic device. This system is based on aptamer15-1 and does
not require capture probe immobilization on device surfaces, target labeling, or
washing steps before electrochemical readout. Researchers used high concentrations
of aptamer functionalized gold AuNPs, and carried out the detection on an alternate graphene-modified electrode. The aptamer assay exhibited good signal linearity
from 30 pg/ml to 10 μ g/mL. Compared to ELISA and radio labeling, this detection
method requires a shorter detection time (2.5 min) and a smaller sample volume (0.1
μL) [53].