Determination of Nano-sized Adsorbate Mass in Solution using Mechanical Resonators: Elimination of the so far Inseparable Liquid Contribution

Abstract

Assumption-free mass quantification of (supra)molecular adsorbates in liquid environment remains a key challenge in many branches of science. Mechanical resonators can uniquely determine the mass of essentially any adsorbate; yet, when operating in liquid environment, the liquid dynamically coupled to the adsorbate contributes significantly to the measured response. On the bases of the Navier-Stokes equation for liquid velocity in contact with an oscillating surface, we show that the liquid contribution can be eliminated by measuring the response in solutions with identical kinematic viscosity but different densities. Here we used quartz crystal microbalance (QCM), the most widely-used mechanical resonator, to demonstrate that kinematic-viscosity matching can be used to accurately quantify the dry mass of systems such as highly hydrated polymeric films as well as adsorbed rigid nanoparticles. The same approached applied to the simultaneously measured energy dissipation made it possible to quantify the mechanical properties of the adsorbate and its attachment to the surface.