Electrochemical detection of cortisol on graphene quantum dots modified electrodes using a rationally truncated high affinity aptamer

Stress nowadays is one of the major causes of various pathologies in humans. Monitoring of stress associated biomarker levels in biological fluids on demand can help in effective management of stress. Since cortisol has been recognised as a potential biomarker for stress, its detection can help in an appropriate therapeutic intervention or stress management. Various types of biosensors such as immunosensors, aptasensors, molecularly imprinted polymers and enzyme-linked based biosensors have been reported till now for the cortisol sensing. Out of all, aptasensors have shown various advantages over other sensors such as specificity, high room temperature stability, reproducibility, etc. Among various reported aptasensors, full-length aptamer (> 60-mer) has been used. However, only a portion of aptamer interact with its target, therefore, establishing its structure-activity relationship is vital for a biosensor to achieve an optimized performance. In the current work, we have rationally truncated already existing 61-mer aptamer sequence into its smaller variant with only 14 bases. Parent and truncated aptamers were immobilized on the graphene quantum dots modified electrodes for electrochemical biosensing of cortisol. Detection limits and cross-reactivities of both the aptamers were compared and it was found that truncated aptamer showed better results in terms of specificity. Though the limit of detection is same for both parent and truncated aptamers, i.e. 0.1 pg/ml but cross-reactivity from various structural analogues in sensing is reduced drastically in case of truncated variant. In brief, we are reporting a label-free electrochemical aptasensor utilizing the truncated aptamer (14-mer) that has remarkable limit of detection (0.1 pg/ml), high level of selectivity and reproducibility for the direct analysis of the cortisol.