Characterization of serum albumin nanoparticles by sedimentation velocity analysis and electron microscopy

Nanoparticles prepared by desolvation and subsequent crosslinking of human serum albumin (HSA) represent promising carriers for drug delivery. We have studied the particle size distribution and the shape of such HSA nanoparticles in aqueous solutions by sedimentation velocity analysis in the analytical ultracentrifuge and by electron microscopy (EM) of negatively stained samples. The sedimentation of the particles was approximated as ideal and with negligible diffusion, and their particle size distribution was characterized by calculating the sedimentation coefficient distribution, g*(s), by applying the computer program SEDFIT. Broad distributions were obtained; the s values within any given preparation might vary by as much as a factor of approximately 5. In addition, depending on the preparation the s 20,w values of the maxima of the distributions varied over a range of 5,000–20,000 S. Since, according to EM, the particles are spherical, the distributions of the sedimentation coefficients could be transformed into distributions of radii, with the corresponding maxima being located between 85 and 160 nm. The ordinate values in the latter distributions can be corrected for Mie light scattering, which yields true relative concentration versus particle radius plots. The broad distributions described could be fractionated into narrow distributions by preparative sucrose density gradient centrifugation. Such isolated fractions may be useful for studying the relationships between the size of the (loaded) HSA carriers and their biological activity. In addition, the procedure described for the analysis of the size distribution of the HSA nanoparticles may be of significant value for the optimization of the method of preparation.