Roles of entropic and solvent damping forces in the dynamics of polymer tethered nanoparticles and implications for single molecule sensing.

Tethering a particle to a surface with a single molecule allows detection of the molecule and analysis of molecular conformations and interactions. Understanding the dynamics of the system is critical to all applications. Here we present a plasmonic imaging study of two important forces that govern the dynamics. One is entropic force arising from the conformational change of the molecular tether, and the other is solvent damping on the particle and the molecule. We measure the response of the particle by driving it into oscillation with an alternating electric field. By varying the field frequency, we study the dynamics on different time scales. We also vary the type of the tether molecule (DNA and polyethylene glycol), size of the particle, and viscosity of the solvent, and describe the observations with a model. The study allows us to derive a single parameter to predict the relative importance of the entropic and damping forces. The findings provide insights into single molecule studies using not only tethered particles, but also other approaches, including force spectroscopy using atomic force microscopy and nanopores.