If you use the pH between 8 to 9. You will get good results. 

For further detail read the articles below

1- M. Valodkar et al. (2012),  Journal of Hazardous Materials, 201-202, 244.

2- S. Harne et al. (2012), Colloids and Surfaces B: Biointerfaces, 95, 284.

3- 1.      N.V. Surmawar, S.R. Thakare, and N.T. Khaty. (2011),  International Journal of Green Nanotechnology, 3, 302.

Plant extract contains various organic reducing agents. However, their reducing power is weak and they did not reduce the copper ions in the aqueous solution at any pH .

Best regards.

@ Yuri..Sir do you mean that Cu Nps cannot be synthesized from plant extracts? But there have been studies conducted which state plant mediated synthesis of Cu Nps..

Noor Ain :

Give an example of the research articles.

http://www.sciencedirect.com/science/article/pii/S0021979715300230

http://pubs.rsc.org/en/content/articlelanding/2014/ra/c4ra01414f/unauth#!divAbstract

I doubt that in the first study received nanoparticles of copper rather than copper oxide. Especially in the second paper to produce copper nanoparticles applied calcination of copper oxide.

If you use the pH between 8 to 9. You will get good results. 

For further detail read the articles below

1- M. Valodkar et al. (2012),  Journal of Hazardous Materials, 201-202, 244.

2- S. Harne et al. (2012), Colloids and Surfaces B: Biointerfaces, 95, 284.

3- 1.      N.V. Surmawar, S.R. Thakare, and N.T. Khaty. (2011),  International Journal of Green Nanotechnology, 3, 302.

Easy Synthesis of Copper Nanoparticles by Leaf Extract,

http://www.instanano.com/nanomaterial-synthesis/metal-nanoparticles/synthesis-copper-nanoparticles-2/