I repeated the experiment again and again burnt getting the peak at 420nm for silver Nanoparticles synthesized using sodium citrate. The peak is coming at 450 nm everytime. what can be the mistake?
I think you have Ag NPs with different sizes. On changing the size of Ag NPs the wavelength of the absorption band is changed. There is good explanation in RG for this observation discussed by other people. Please do a search for " The relationship between absorption band and size of Ag NPs"