Dear Maryam Zulfiqar, after drying you have no more nanoparticles, either fine powder or solid mass are the result. Thus, washing has no alteration on them. My Regards

Surely, after the treatment in oven it is not a problem to wash again the nanoparticles. But you write that humidity is present in your powder, so it seems to me that it could be more useful to treat again in oven the sample, to remove H2O...

You can wash the nanoparticles with organic solvent like ether , filter and then try to dry keeping in an oven above 110 deg centigrade for one hour to remove moisture.

Dear Maryam Zulfiqar thank you for sharing this interesting technical question on RG. Did you follow a literature procedure? If you are uncertain about the right order of washing and drying, why don't you just repeat the experiments and follow exactly the protocol described in the literature? Birmuth salt precursors are nit that expensive...

Nowadays, bismuth oxide nanoparticles are often prepared via so-called "green synthesis" using environmentally benign plant extracts. For a typical literature refernce describingthis method, please have a look at the following potentially useful article:

Green Synthesis and Characterization of Bismuth Oxide Nanoparticle Using Mentha Pulegium Extract

Article Green Synthesis and Characterization of Bismuth Oxide Nanopa...

This and other closely related articles have been posted by the authors as public full texts on RG. Thus the paper can be freely downloaded as pdf file.

I hope this helps. Good luck with your research and best wishes, Frank Edelmann