Behavior of ZnO nanoparticles in glycine solution: pH and size effect on aggregation and adsorption

Growing interest in nanoparticles' (NPs) behavior in the environment needs deeper purport of interactions between NPs and amino acids. In this work, we evaluated particle size distribution, aggregation degree, zeta potential, adsorption efficiency, and specific adsorption of three ZnO NPs with average sizes 30, 135, and 210 nm in 1 M glycine solution at different pH. The interaction of ZnO NPs and glycine is stimulated by the pH growth from 4 to 8 (an increase of glycine adsorption and decrease of particle aggregation), whereas at pH > 8 colloidal and adsorption properties are weakly affected by pH change. The addition of glycine to water promotes the aggregation of 30 nm NPs, while the aggregation of bigger NPs is weakened. For smaller particles aggregation and adsorption efficiency are higher, and specific adsorption is lower, meanwhile, the size effect is stronger in aggressive medium: alkaline - in water, and acid - in glycine.