The interaction between an essential amino acid L-tryptophan (TRP) with semiconductor zinc oxide (ZnO) nanoparticles and amino acid L-aspartic acid (ASP) is investigated by steady state and time resolved spectroscopic techniques. In both the cases static mode of fluorescence quenching occurs indicating the formation of ground-state complex. Binding constants and the number of binding sites were determined for both the complexes. The observed thermodynamic parameters suggest that the key interacting forces involved are van der Waals interaction and hydrogen bonding in case of TRP and ZnO nanoparticles whereas hydrophobic interaction is responsible in formations of TRP-ASP complex. (C) 2009 Elsevier B.V. All rights reserved.