Silver nanoparticles were successfully synthesised at room temperature from silver nitrate using unripe Carica papaya fruit extract as a reducing and capping agent. PH of the reaction solution was adjusted with the addition of sodium hydroxide from pH 5.6 (original pH of the broth) to pH 6.5, pH 7.5 and pH 8.5. Quasi spherical silver nanoparticles were obtained across the studied pH range. The average particle size obtained decreased from 34 nm to 15 nm as the pH of the solution increased from 5.6 to 8.5. From the observations and ultra violet-visible spectrophotometer analysis, the rate of reduction and formation of silver nanoparticles increased as the pH increased. The crystalline nature of the as-synthesised silver nanoparticles in a cubic structure was confirmed by the peaks in X-ray diffraction and selected area electron diffraction. From Fourier transform infrared spectra, it was found that proteins in unripe papaya fruit extract were strongly bounded to the surface of silver nanoparticles as a capping agent. The thin layer of protein coating also further confirmed under high resolution transmission electron microscope. A plausible formation mechanism has been briefly proposed. Copyright © 2017 Inderscience Enterprises Ltd.