Dear Yen Thi My Nguyen, solubility is not of sole importance, though methanol is the most casually used solvent, other consideration on the influence of the solvent on the quality of analysis. Please have a look at the following links. My Regards

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8017072/

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6699328/

https://www.researchgate.net/post/Can-we-use-some-other-solvent-for-DPPH-assay-instead-of-methanol

https://link.springer.com/article/10.1007/s00217-013-1982-1

The most common solvents used to test the antioxidant activity of polysaccharides using the DPPH assay are methanol, ethanol, and acetone. In addition, water, dimethylsulfoxide (DMSO), and dimethylformamide (DMF) may also be used.

Methanol

We are using 80% methanol