Quantifying particle coatings using high-precision mass measurements.

We present a general method to quantify coatings on microparticle surfaces based on the additional mass. Particle buoyant mass is determined in a solution with a density that is nearly equivalent to that of the core particle, reducing the magnitude and uncertainty of the measurement. Under these conditions, added material with a different density than that of the core is a larger fraction of the total buoyant mass of the coated particle. This method can resolve a buoyant mass difference between uncoated and coated particles of ~1 fg. For the protein layer on the 3 μm polystyrene spheres measured herein, this is equivalent to 1/10th of a full layer.