I have tried to synthesize copper nanoparticles by Chemical synthesis method by using CuSO4.5H2O, Ascorbic acid, and NaBH4. The Mole ratio of taken precursors is 1:2:10 (reported in some papers). CuSO4.5H2O is mixed with ascorbic acid and stirred. when NaBH4 has added the colour of the solution changed from light blue to green & then gradually became light brown. it is translucent brown. But if we stir for more time the colour change reverses back from brown to initial blue. now if again NaBH4 is added the colour once again starts changing from blue to brown. this cycle of colour change continuously occurs. anyone, please help with some of my doubts:

1) why this cycle repeats? Please explain the whole procedure taking place here.

2) tell me how this change can be stopped. i.e., once the reaction changes from blue to brown, no further change should occur. so that I can collect the copper nanoparticles from the solution.