Role of phenol red in the stabilization of the Sabin type 2 inactivated polio vaccine at various pH values. 
To analyze the effects of phenol red at various pH values on the Sabin type 2 inactivated polio vaccine (sIPV2), several biophysical techniques were used to evaluate the particle size and capsid protein for conformation.  sIPV2's size was assessed via transmission electron microscopy and dynamic light scattering.  The effects of various pH values (from 4.0 to 7.0) on the biophysical characters of sIPV2 particles in solution were determined by dynamic light scattering and zeta potential.  The results clearly indicated that aggregation and instability occurred in the solution of sIPV2 particles at a pH of 6.0.  Under similar conditions, by dynamic light scattering and zeta potential, the virus particles in solution showed more dispersion and were stable with the addition of 0.05 mM phenol red.  According to circular dichroism and intrinsic tryptophan fluorescence data, it was observed that the secondary and tertiary structures of the sIPV2 particles were more stable with the protection of phenol red.  At a pH below 6.0, the sIPV2 solution with phenol red had more D-antigen content, which was confirmed by enzyme-linked immunosorbent assay and rat experiments.  These results strongly suggested that phenol red improved the pH stability of the sIPV2.  The study indicated the potential of phenol red in preserving vaccine potency of the sIPV2 at various pH values.