When P. guajava leaf/fruit extract was mixed with CuSO4.5H2O solution, heated at 60 o C for 10 minutes, and acidified, a color change from green to pale blue occurred, indicating the synthesis of copper nanoparticles. The Surface Plasmon Resonance phenomenon in copper nanoparticles and electrostatic interactions and hydrogen bonds between biogenic capping molecules were responsible for the synthesis . This color change was consistent with literature reports indicating the completion of the reduction process within 24 hours .