The understanding of interfacial interactions and reactivities in the assembly of nanoparticles mediated by DNAs, proteins, and amino acids is essential to the exploration of nanotechnology in biomedical applications. This thesis presents findings of an investigation of the effect of salt on the assembly of gold nanoparticles mediated by thiol-containing amino acids. Gold nanoparticles of different sizes (10-30 nm) were used as a model system. The unique surface plasmon (SP) resonance absorption of gold nanoparticles allows the nanoparticle assembly to be monitored by UV-Vis spectrometric methods. The observed evolution of the SP bands was found to be highly dependent on salt concentration. A systematic investigation of the effects of the salts on the nanoparticle assembly kinetics was carried out. The findings were explained based on the electrical double layer model for gold nanoparticles in the presence of different electrolyte concentrations. The interfacial binding reactivity of thiol-containing amino acids at gold nanoparticles was demonstrated to be tunable by controlling salt concentration and manipulating the nature of the salt in the solution. These findings have broad implications for the precise control of interfacial interactions and reactivities between nanoprobes and biological molecules.