The coffee‐ring effect denotes the accumulation of particles at the edge of an evaporating drop pinned on a substrate, as one can observe in the black ring of a coffee stain. Because it can be detected by simple visual inspection, this mechanism can be used to develop robust and cost-effective diagnostic tools. In the first part of the talk, I will present the analysis of the deposit morphology of drying drops containing polystyrene nanoparticles and proteins. Deposit patterns reveal information on both the adsorption of proteins on the particles and their reorganization following adsorption. This method is sensitive enough to detect a single point mutation in hemoglobin, as shown by the distinct patterns formed by human native hemoglobin or the mutant responsible for sickle cell anemia.
In the second part, I will be dealing with protein adsorption at the air/water interface. We investigate protein charge at the air/water interface by interface-selective nonlinear vibrational spectroscopy. The protein net charge at the interface is deduced from the orientational flip-flop of water molecules revealed by heterodyne-detected vibrational sum frequency generation spectroscopy (HD-VSFG). Our results show that the isoelectric point of hemoglobin is significantly lowered at the air/water interface compared to that of the bulk. It suggests that the structural modifications of proteins at interfaces can dramatically affect their charge profile.