Abstract
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The
ability to continuously measure concentrations of small molecules
is important for biomedical, environmental, and industrial monitoring.
However, because of their low molecular mass, it is difficult to quantify
concentrations of such molecules, particularly at low concentrations.
Here, we describe a small-molecule sensor that is generalizable, sensitive,
specific, reversible, and suited for continuous monitoring over long
durations. The sensor consists of particles attached to a sensing
surface via a double-stranded DNA tether. The particles transiently
bind to the sensing surface via single-molecular affinity interactions,
and the transient binding is optically detected as digital binding
events via the Brownian motion of the particles. The rate of binding
events decreases with increasing analyte concentration because analyte
molecules inhibit binding of the tethered particle to the surface.
The sensor enables continuous measurements of analyte concentrations
because of the reversibility of the intermolecular bonds and digital
read-out of particle motion. We show results for the monitoring of
short single-stranded DNA sequences and creatinine, a small-molecule
biomarker (113 Da) for kidney function, demonstrating a temporal resolution
of a few minutes. The precision of the sensor is determined by the
statistics of the digital switching events, which means that the precision
is tunable by the number of particles and the measurement time.
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