Structure-Switching Aptamer-Based Molecular Pendulum for Profiling Melatonin Circadian Dynamics in Humans

Continuous electrochemical monitoring of small molecules remains difficult due to their small size and lack of enzymatic sensing strategies. Here, we develop a structure-switching aptamer molecular pendulum platform for reagentless electrochemical detection of small molecules, including melatonin and cortisol. Target binding induces a conformational change in the receptor, which increases the hydrodynamic diameter of the construct, thereby slowing electron transfer kinetics and resulting in a measurable time-resolved electrochemical current. To explore the feasibility of melatonin sensing in vivo, we validated the presence and circadian dynamics of melatonin in human interstitial fluid using mass spectrometry. We then demonstrated quantitative electrochemical detection of melatonin directly in human ISF, with signals that mirror the circadian profile observed by mass spectrometry. This work provides a foundation for continuous, minimally invasive monitoring of circadian biomarkers using electrochemical sensors.