Continuous molecular monitoring of human dermal interstitial fluid with microneedle-enabled electrochemical aptamer sensors.

The ability to continually collect diagnostic information from the body during daily activity has revolutionized the monitoring of health and disease. Much of this monitoring, however, has been of physical "vital signs", with the monitoring of molecular markers having been limited to glucose, primarily due to the lack of other medically relevant molecules for which continuous measurements are possible in bodily fluids. Electrochemical aptamer sensors, however, have a recent history of successful in vivo demonstrations in rat animal models. Herein, we present the first report of real-time human molecular data collected using such sensors, successfully demonstrating their ability to measure the concentration of phenylalanine in dermal interstitial fluid after an oral bolus. To achieve this, we used a device that employs three hollow microneedles to couple the interstitial fluid to an ex vivo, phenylalanine-detecting sensor. The resulting architecture achieves good precision over the physiological concentration range and clinically relevant, 20 min lag times. By also demonstrating 90 days dry room-temperature shelf storage, the reported work also reaches another important milestone in moving such sensors to the clinic. While the devices demonstrated are not without remaining challenges, the results at minimum provide a simple method by which aptamer sensors can be quickly moved into human subjects for testing.

Solution-Phase Electrochemical Aptamer-Based Sensors.

Electrochemical aptamer-based sensors (EABs) using self-assembled monolayers on gold working-electrodes have now been in-vivo demonstrated for multiple-analytes, demonstrating their sensitivity and specificity even in a continuous sensing format. However, longevity has been demonstrated for only 24 hours and sensitivity has been challenging for highly dilute analytes (nM regime). A novel approach is reported here using electrochemical aptamer-based sensing that is not covalently-bound to a gold-working electrode but where aptamers are freely mobile in solution. This alternative approach has the potential to improve longevity by reducing electrode surface degradation and improving sensitivity using aptamer binding constructs that are not available for aptamers when covalently bound to the electrode. Specifically, a molecular-beacon (fluorescent) cortisol aptamer was adapted into an amperometry solution-phase cortisol EAB sensor, demonstrating ∼5% signal gain starting at only 10 nM and a saturated signal gain of ∼70% at several µM. A robust signal was achieved due to use of methylene-blue redox-tagged aptamer that was measured through amperometry with interdigitated electrodes. While this result demonstrates the basic feasibility of solution-phase EAB sensors, the result also required a self-assembled monolayer alkylthiolate blocking-layer on the gold working electrode which restricts potential device longevity. These results cumulatively suggest that initial significance of solution-phase EAB sensors may be strongest for point-of-care type testing applications and further development would be required for long-lasting continuous sensing applications.