The safety assessment of nanomaterials requires an accurate determination of their surface hydrophobicity. The fact is that hydrophobicity contributes in determining the biological/environmental fate of nanoparticles as well as their potential toxicity.

But, have you ever tried to measure the hydrophobicity of nanoparticles? One possibility is to use the standard water/octanol approach, mixing your particle solution in a beaker with the two phases. If the particles are hydrophobic they will probably segregate at the interface and you will not be able to measure the concentration in the two phases! But then how do you measure how hydrophobic the particles are (namely their polar component of the surface free energy)? We have solved this problem by developing a new method that is presented in this paper https://www.nature.com/articles/s42004-018-0054-7

The method is able to quantitatively determine the surface energy components of the nanoparticles by measuring their binding affinity to the collectors’ surfaces, via analysis of their adsorption kinetics. The adsorption kinetics is calculated by measuring the number of nanoparticles binding to the different collector as a function of time, by Dark-Field microscopy. The surface energy potential acting between the nanoparticles and the collector(s) is then calculated using the XDLVO (eXtended Derjaguin Landau Verwey Overbeek) theory. The energy barrier potential between the nanoparticle and the collector(s) represents the potential energy at which the particles are repelled by the collector surface. This energy barrier potential is inversely proportional to the binding affinity: the larger (is) the energy barrier potential, the lower (is) the affinity. In some conditions, the nanoparticles may be completely repelled by the collector due to very high energy barrier potential. Electrostatic forces are primarily responsible for the formation of the energy barrier; the surface energy of the nanoparticles (and in particular the degree of hydrophobicity) is then calculated by comparing the binding affinity of two or more collectors. The method has been tested with Au and SiO2 NP with different degrees of hydrophobicity to assess the validity of the approach. Despite the heavy theoretical background needed to explain the results, the method presented is very simple to implement for the rapid screening of the hydrophobicity of nanomaterials and it shows potential to become a standard method or an international test guideline.