2.2 Protein suspension preparation
Four biological macromolecules, bovine serum albumins (BSA; Sigma-Aldrich), ubiquitin (Sigma-Aldrich), low-density lipoproteins (LDL; Sigma-Aldrich), and high-density lipoproteins (HDL; Sigma-Albumin), were selected for testing. 10 mg proteins were suspended in 10 ml 20 mM NH4CH3COO solution in an ultrasonic both for 15 minutes for the good dispersion of proteins. Protein suspension samples with different concentrations were prepared by serial dilutions. For the stability monitoring, protein suspensions were stored at 4 ˚C and sonicated for 15 minutes at 20 ˚C.