**4. Conclusions**

Fe3O4\_PDA\_Lipase nano-biocatalyst was successfully synthesized and utilized to produce biodiesel using waste cooking oil as feedstock. Using a best fitted quadratic model, different reaction conditions of biodiesel production process were optimized, and it was revealed that Fe3O4\_PDA\_Lipase actively catalysed the reaction, resulting in a highlight biodiesel yield i.e., 92%. The synthesized nano-biocatalyst proved to have better adoptability/stability and was easily recoverable using external magne<sup>t</sup> and retained most of its catalytic activity for four cycles. The properties of synthesized biodiesel were in agreemen<sup>t</sup> with the ASTM D standards. Conclusively, Fe3O4\_PDA\_Lipase has been proved to be a sustainable and cost-e ffective catalyst for biodiesel production.

**Author Contributions:** The conceptualization of this journal article is from T.T., M.W.M. and U.R.; the methodology was designed by T.T., M.W.M. and U.R.; the writing—original draft was prepared by T.T.; S.A., A.S. and M.W.M. helped with the editing and supervision. H.M. provided technical assistance regarding the synthesis and characterization of nano-biocatalyst, A.I., T.S.Y.C. and I.A.N. reviewed the manuscript for final version improvement. All authors have read and agreed to the published version of the manuscript.

**Funding:** The authors acknowledge their gratitude to King Saud University (Riyadh, Saudi Arabia) for the support of this research through Researchers Supporting Project number (RSP-2019/80).

**Conflicts of Interest:** The authors declare no conflict of interest.
