*2.12. Drug Loading*

Methotrexate (MTX) (Sigma Aldrich, St. Louis, MO, USA) was dissolved in PBS (1 mg/10 mL) and mixed with bi-HNTs (200 mg) and keep stirring for 12 h, then, the mixture was vacuumed for 24 h. After centrifugation, the supernatant liquid was collected and stored at −20 ◦C for drug loading efficiency determination, and the bottom deposited bi-HNTs were washed by PBS for 3 times and air-dried, the final product was collected for further study.
