Trace Biomolecule Detection with Functionalized Janus Particles by Rotational Diffusion

Cytokines are small proteins secreted by cells in innate and adaptive immune systems. Abnormal cytokine secretion is often regarded as an early cue of dysregulation of homeostasis due to diseases or infections. Early detection allows early medical intervention. In this study, a natural phenomenon called rotational Brownian motion was characterized by Janus particles and its potential use in detection of trace biomolecules explored. Through the functionalization of the Janus particles with an antibody, the target cytokine, that is, tumor necrosis factor-α, was measured in terms of rotational diffusion. Rotational diffusion is highly sensitive to the particle volume change according to the Stokes–Einstein–Debye relation and can be quantified by blinking signal. Accordingly, 1 μm half-gold and half-fluorescent microbeads were conjugated with 200 nm nanobeads through sandwiched immunocomplexes. The light source, lead time for stabilization, and purification were investigated for optimization. Particle images can be captured with green light at 5 Hz within 300 s. Under such conditions, the functionalized Janus particles eventually achieved a limit of detection of 1 pg/mL. The rotational diffusometry realized by Janus particles was power-free and feasible for ultrasensitive detection, such as early disease detection.