Fig. 1. Basic principle of continuous biomarker monitoring based on measuring diffusional motion of biofunctionalized particles hovering over a substrate.

The particles exhibit reversible target-induced molecular interactions with the substrate. a Microparticles (Dynabeads) are functionalized with particle-side binders (blue). The particles diffuse in the vicinity of a substrate functionalized with substrate-side binders (red). The binders (e.g. ssDNA or antibodies) have a specific affinity to target molecules (green; ranging from small molecules to macromolecules). Target-induced sandwich complexes are reversibly formed and cause the particles to switch between unbound and bound states. The particles exhibit free Brownian motion in the unbound state and confined Brownian motion in the bound state. The right panel shows a microscopy image of ~500 particles in the field of view (single frame). The inset shows the reconstructed in-plane trajectories of a random subset of particles (n = ~25) tracked for 300 s (1800 frames). In this experiment, the particles have a diameter of 2.8 μm. b Experimental data for a sandwich system with oligonucleotide binders and target. Left: Trajectories of single particles in absence (top) and presence (bottom) of target molecules in solution. The orange traces in the bottom panel indicate bound states caused by target-induced sandwich bonds. Right: Effective diffusivity D as a function of time based on the in-plane displacements derived from the particle trajectories. In the absence of analyte (top) the particles typically exhibit free Brownian motion. In the presence of analyte (bottom) particles show transitions from unbound (blue) to bound (orange) states and back. Attributed state transitions are indicated by binary step functions (black line at top). c Distributions of D of ~500 particles showing unbound state (blue) and bound state (orange) populations in absence (top) and presence (bottom) of target molecules in solution. Source data are provided as a Source Data file.