Either the reducing agent NaBH4 may not be pure or the concentration of NaBH4 is more than the required amount which leads to aggregation might be a reason for the observed phenomenon. I have synthesized silver nanoparticles in different ways that described in few of my publications. If you are interested, please go through:

https://www.researchgate.net/publication/229103842_Hydroxy_propyl_cellulose_capped_silver_nanoparticles_produced_by_simple_dialysis_process

https://www.researchgate.net/publication/223553321_Synthesis_characterization_and_mechanical_properties_of_nylon-silver_composite_nanofibers_prepared_by_electrospinning_Curr_Appl_Phys_104_1005-1008

https://www.researchgate.net/publication/276313764_Fabrication_and_Characterization_of_Functionally_Graded_Polyvinylidine_fluoride-Silver_Nanocomposite_Hollow_Fibers_for_Sustainable_Water_Recovery?ev=prf_pub

Cheers

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It may be due to aggregation or aerial oxidation.

when get oxidized the silver become black. use ice cold solution of NaBH4. Or try to use strong reducing agent instead of sodium borohydride

Hello Wang, the reason for appearance of black color is due to aggregation, Polymyxin B isn't able to stabilize the silver nanoparticles effectively. Try decreasing the concentration of NaBH4. When you were getting a yellow color dispersion why did you increase the concentration of NaBH4?

I am very grateful for the answers. Thank you very much! I changed the concentration of NaBH4 and I have got a red color and yellow brown color solution. I shall analyze the solution later.

Well I have a few more question. I checked the article, when NaBH4 is used to synthese silver nanopaticles, as long as the NaBH4: AgNO3 is above 1.4:1, we will get silver nanoparticle. That is to say the NaBH4 is able to stabilize the silver nanopaticles. Why in my experiment, this silver nanoparitles aggregated?

Sorry I am a student for medicine, I am not quite familiar to the materials. I am very sorry about this. Is there any book for me to study the foundation for silver nanoparticle synthesis? Like some handbook or sth else?

Thank you!

It also may depends upon the temperature and inert condition.

I think if you carry your reaction in N2 or argon atmosphere still you will get stable AgNPs.

Also, you may use several stabilizing agents like PEG, PVP or PEI.

Can you upload the paper you are following or maybe the link?

Yes. I am following two different article.

The first one is published in Biomaterials. The link is: http://www.sciencedirect.com/science/article/pii/S0142961213011289.

Another one: https://www.dovepress.com/facile-biofunctionalization-of-silver-nanoparticles-for-enhanced-antib-peer-reviewed-article-IJN

Thank you!

In the second article they have varied the concentration of Polymyxin B, you're varying that?

I am agree with Lijo you are using concentrated solution of NaBH4 please do optimise the concentration to be used

Yes, I checked the concentration of Polymyxin B. But even the highest concentration used (100ug/ml), the solution turns black. Now I changed the concentration of NaBH4 and I have got brown yellow solution. But in their article, the concentration of NaBH4 is much higher than that in my experiment.

Or maybe the synthesis of silver nanoparitcles is influenced by two many factors. I shall check my own concentration.

Please get the rid off from aerial oxidation as I earlier mentioned. Also you can use some protective agents to secure the stability.