Determination of Nanosized Adsorbate Mass in Solution Using Mechanical Resonators: Elimination of the so Far Inseparable Liquid Contribution
Journal article, 2021
Assumption-free mass quantification of nanofilms, nanoparticles, and (supra)molecular adsorbates in a 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 a liquid environment, the liquid dynamically coupled to the adsorbate contributes significantly to the measured response, which complicates data interpretation and impairs quantitative adsorbate mass determination. Employing the Navier-Stokes equation for liquid velocity in contact with an oscillating surface, we show that the liquid contribution for rigid systems can be eliminated by measuring the response in solutions with identical kinematic viscosity but different densities. Guided by this insight, we used the quartz crystal microbalance (QCM), one of the most widely employed mechanical resonators, to experimentally demonstrate that the kinematic-viscosity matching can be utilized to quantify the dry mass of rigid and in many cases also nonrigid adsorbate systems, including, e.g., rigid nanoparticles, tethered biological nanoparticles (lipid vesicles), as well as highly hydrated polymeric films. For all the adsorbates, the dry mass determined using the kinematic-viscosity matching was within the uncertainty limits of the corresponding mass determined using complementary methods, i.e., QCM in air, scanning electron microscopy, surface plasmon resonance, and theoretical estimations. The same approach applied to the simultaneously measured energy dissipation made it possible to quantify the mechanical properties of the adsorbate and its attachment to the surface, as demonstrated by, for example, probing the hydrodynamic stabilization induced by nanoparticle crowding. In addition to a unique means to quantify the liquid contribution to the measured response of mechanical resonators, we also envision that the kinematic-viscosity-matching approach will open up applications beyond mass determination, including a new means to investigate orientation, spatial distribution, and binding strength of adsorbates without the need for complementary techniques.