  Nucleic acid-based electrochemical sensors (NBEs) have emerged as a promising
approach to continuous molecular monitoring in vivo. NBEs consist of
electrically conducting gold surfaces coated with self-assembled monolayers of
a mixture of electrode-passivating alkylthiols and functional
alkylthiol-modified oligos. These oligos also display binding sites for the
target analyte and redox reporters able to transfer electrons to the underlying
gold electrode. Although sufficiently robust for continuous, multi-hour sensing
of small molecules and proteins in biological fluids both in vitro and in vivo,
NBEs decay over periods longer than 12 hours of continuous operation in these
fluids. To address this issue, here we report a biofluid mimetic that can be
leveraged to specifically study competitive displacement of oligonucleotides
from NBEs, a critical sensor degradation pathway. Using this mimetic, we
demonstrate three strategies that drastically mitigate competitive displacement
and improve sensor stability in vitro. A combination of these strategies also
improves sensor stability in vivo, demonstrated here via sensors emplaced in
the brain cortex of live rats.
