Continuous Small-Molecule Monitoring with a Digital Single-Particle Switch

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.