Quantitative Estimation of metal NPs using UV-Vis only, is highly tricky job as I tried but couldn't get it through. Instead, UV-Vis can be used to get the desired peak in its range for identification purpose only.

Instead, nowadays the best way to calculate the metallic NPs are:

1. ICP -MS / ICP-OES (i followed that)

2. SEM with EDAX data

Better to follow this with your best optimized formulations. Hoping that this would help.

See also the answers to a very similar (recent) question by Aminul Islam. UV/VIS is not anymore a suitable method for determining metals.

If you really want to use a uv/vis spectrometer there are several colorimetric tests that will work with a simple spectrometer.  Perhaps the best book for this would be "Standard Methods For the Examination of Water and Wastewater" (ISBN 978-087553-013-0) but it is not the only source that will contain instructions. For metal analysis in water and wastewater most will suggest using ICP but if you don't have one (or don't want to take on the costs associated with the purchase and constant maintenance that ICPs require) I've found that the Standard Methods tests are quite cost effective ways as long as you keep in mind the limitations from the sample and the method.  If you have certain metals in mind I might be able to provide links or copies of papers that can get you started.

kindly follow instruction manual which is provided with UV/Vis spectrophotometer. Usually Procedure and other details are well described.  

UV-Vis spectrophotometer not a suitable instrument in determination or  quantitative determination of metals in water/wastewater sample. Most appropriate instruments are; Atomic Absorption Spectrometer (AAS), Inductive Couple Plasma ICP)-MS.

AAS or ICP-OES are the best method to determine heavy metal content in a samples. But if you insist want to use UV-Vis spectrophotometer, maybe you can refer to this journal:

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&cad=rja&uact=8&ved=0ahUKEwiJ2p__jIrLAhWIRI4KHXkQA6kQFggjMAE&url=http%3A%2F%2Fjournal.hibiscuspublisher.com%2Findex.php%2FBSTR%2Farticle%2Fdownload%2F245%2F287&usg=AFQjCNFOuEQNl23swVtwHt1ArmKGgaIaGg&bvm=bv.114733917,d.c2E

UV / vis methods are subject to many interferences. But if you have no other alternative equipment, the best source of information is the "Standard Methods For the Examination of Water and Wastewater", as indicated by Christopher Lloyd