Week-long operation of electrochemical aptamer sensors: New insights into self-assembled monolayer degradation mechanisms and solutions for stability in biofluid at body temperature

Conventional wisdom suggests that widely-utilized self-assembled alkylthiolate monolayers on gold are too unstable to last more than several days when exposed to complex fluids such as raw serum at body temperature. Demonstrated here is that these monolayers can not only last at least one week under such harsh conditions, but that significant applied value can be captured for continuous electrochemical aptamer biosensors. Electrochemical aptamer biosensors provide an ideal tool to investigate monolayer degradation, as aptamer sensors require a tightly-packed monolayer to preserve sensor signal versus background current and readily reveal fouling by albumin and other solutes when operating in biofluids. Week-long operation in serum at 37 ˚C is achieved by: (1) assembling monolayers on gold with roughness that promotes small alkylthiolate monolayer defect sizes; (2) increasing van der Waals interactions between adjacent monolayer molecules to increase the activation energy required for desorption; (3) optimizing electrochemical measurement to decrease both alkylthiolate oxidation and electric-field induced desorption; (4) mitigating fouling by using protective zwitterionic membranes and zwitterion-based blocking layers with antifouling properties. This work further proposes origins and mechanisms of monolayer degradation in a logical stepwise manner that was previously unobservable over multi-day time scales. Several of the observed results are surprising, revealing that short term improvements to sensor longevity (i.e., hours) actually increase sensor degradation in the longer term (i.e., days). The results and underlying insights on mechanisms not only push forward fundamental understanding of stability for self-assembled monolayers, but demonstrate an important milestone for continuous electrochemical aptamer biosensors.