The protein solution was filtered with a 0.1 μm polyvinylidene fluoride membrane filter (Millipore Billerica, MA, USA). Nanoparticle self-assembly was performed by dialysis into buffer containing 8 M urea, 20 mM Tris pH 7.5, 150 mM NaCl, and 5% glycerol, at a protein concentration of 0.1 mg/mL. This was followed by dialysis into the same buffer containing decreasing concentrations of urea: 6 M, 4 M, 2 M, 1 M, and two changes of the same buffer without urea. Following self-assembly, the nanoparticle solution was again filtered with a 0.1 μm filter.