Aptamer-based lateral flow assay for point of care cortisol detection in sweat

A new aptamer-based lateral flow strip assay has been designed and developed for on-site rapid detection of cortisol in sweat. Cortisol in sweat has been identified as a key biomarker to monitor physiological stress. A highly sensitive and specific cortisol sensor was achieved by conjugating cortisol-selective aptamers to the surface of gold nanoparticles (AuNPs). Aptamer-functionalized AuNPs are stable against salt-induced aggregation. When cortisol molecules are present in the sample, they interact with the designed aptamers causing their desorption from the AuNP surface. Free AuNPs can then be captured by reaction with cysteamine immobilized on the test zone of the lateral flow test strip. This enables the visual detection of cortisol within minutes. Important parameters that affect the detection sensitivity in both solution and lateral flow assays, such as the loading density of aptamers per AuNP, salt and cysteamine concentrations, were investigated to provide the optimum assay performance. This hand-held device successfully exhibited a visual limit of detection of 1 ng/mL, readily covering the normal range of free cortisol in sweat (8-140 ng/mL). No significant cross reactivity to other stress biomarkers was observed. The advantages of this paper-based biosensor over previously reported test strips include the use of aptamers (which are more stable, simpler to use and lower cost than antibodies) and a simplified lateral flow assay (LFA) strip design (without the use of complementary aptamers in the test line). The resulting LFA aptasensor provides a rapid, sensitive, user-friendly and cost-effective point of care device for cortisol detection in sweat and other biofluids.