Methods and compositions for extracting nucleic acids such as microRNAs (miRNAs) from biological samples are provided. Aspects of the methods include contacting a biological sample with proteinase K followed by contact with ferric oxide particles under acidic conditions to induce binding between the ferric oxide particles and nucleic acids (e.g., miRNAs) of the sample. In some cases, the ferric oxide particles are provided as part of a dissolvable film, which releases the ferric oxide particles upon solvation. In some embodiments, after nucleic acids bind to the ferric oxide particles, the particles are magnetically separated from the sample and are contacted with an alkaline elution buffer to release the nucleic acids.