Current and Potential Developments of Cortisol Aptasensing towards Point-of-Care Diagnostics (POTC)

Abstract

Anxiety is a psychological problem that often emerges during the normal course of human life. The detection of anxiety often involves a physical exam and a self-reporting questionnaire. However, these approaches have limitations, as the data might lack reliability and consistency upon application to the same population over time. Furthermore, there might be varying understanding and interpretations of the particular question by the participant, which necessitating the approach of using biomarker-based measurement for stress diagnosis. The most prominent biomarker related to stress, hormone cortisol, plays a key role in the fight-or-flight situation, alters the immune response, and suppresses the digestive and the reproductive systems. We have taken the endeavour to review the available aptamer-based biosensor (aptasensor) for cortisol detection. The potential point-of-care diagnostic strategies that could be harnessed for the aptasensing of cortisol were also envisaged.

Keywords: anxiety; aptamer; aptasensor; cortisol.

Figure 1

Chemical structure of cortisol. It has the chemical formula C₂₁H₃₀O₅ and a molar mass of 362.460 g/mol.

Figure 2

Systematic Evolution of Ligands by EXponential enrichment (SELEX). The binding molecules are selected from a randomized nucleic acid library. Initially, the nucleic acid library is incubated with the target molecule. Then, the unbound nucleic acids are washed away, leaving only the molecules that have bound to the target molecule. The target-bound nucleic acid molecules will be eluted and amplified. Following amplification, the resulting double stranded DNA (dsDNA) will be converted to ssDNA in the case of DNA aptamer generation. For RNA aptamer generation, the dsDNA will be subjected to in vitro transcription to generate RNA pool. The resulting ssRNA/DNA pool will be used for the subsequent round of SELEX. Several rounds of SELEX will be carried out till the isolation of nucleic acid molecules that have high affinity and specificity against the target. The binding affinity of the putative aptamers will be estimated.

Figure 3

The mechanism of the colorimetric detection of a small target molecule using aptamers. In the presence of the target, aptamers are desorbed from the surface of AuNPs forming aptamer-target complex. As a result, Na⁺ neutralizes the negatively charged citrate ion on the surface of AuNP. This causes aggregation of the AuNPs and results in blue colour formation. In the absence of the target, aptamers on the surface of AuNPs stabilizes these nanoparticles against the NaCl-induced salt aggregation, causes the production of red colour.

Figure 4

(a) Complex of triamcinolone-aptamer-AuNPs, (b) cortisol displaces triamcinolone from aptamer-AuNPs, (c) triamcinolone are electrochemically reduced on the graphene-modified electrodes producing signal.

Figure 5

Essential illustration of the LFA integrated with an aptamer.

Figure 6

Schematic of the surface plasmon resonance system. The changes in the reflected light I and II influence the response of the system and can be used to determine the specimen. SPR-based aptasensing can be employed for the detection of cortisol. Biotinylated aptamer against cortisol can be immobilized on the surface of the Sensor chip streptavidin (SA). Reflectivity can be used to measure the interaction of the aptamer with the cortisol.

Figure 7

Schematic diagram of the potential smartphone-based cortisol measurement system. Anti-cortisol aptamer can be adopted in the gold nanoparticles-based detection of cortisol and embedded into the smartphone.