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TEM images of the PS hollow beads that had been encapsulated with saline solutions of different concentrations: a,b) 5.9 wt%, c,d) 11.1 wt%, and e,f) 23.1 wt%. The holes were closed by annealing the samples in the corresponding saline solutions at 95 °C for 45 min. The samples were collected by centrifugation and thoroughly washed with water. The samples supported on copper grids were further dried at 75 °C for 5 min to completely remove water prior to TEM characterization.
