Nanoparticles as adsorbents for hydrophobic molecules: Exploring size, pH, and structural dependencies

Understanding the adsorption mechanism of hydrophobic molecules, such as fluorescent xanthene dyes is crucial for applications in sensing, imaging and nanoparticle characterization. Using molecular dynamics, we investigate the adsorption of Rhodamine 6G, Rhodamine B and anthracene on α-cristobalite, α-quartz, gold, and graphene under varying pH conditions. Our results show that van der Waals forces, rather than electrostatic interactions, are responsible for adsorption. Furthermore, we have found that adsorption stability increases with nanoparticle size, while higher pH levels promote surface deprotonation and hydrogen bonding, thereby suppressing adsorption. Hydrophobic adsorbents such as gold and graphene exhibit the strongest adsorption due to their nonpolar nature. These insights provide a deeper understanding of xanthene dye adsorption and, more broadly, the adsorption of hydrophobic molecules, which is critical for optimizing nanoparticle-based applications.