Scanning electron microscopy
The morphology and structure were observed and recorded by
scanning electron microscope (SEM,
S-3400N, Hitachi, Tokyo, JP). The MTSs were collected and washed with
PBS twice, then fixed with 2.5% glutaraldehyde in deionized water for 1
h and washed with deionized water for 5 times. The fixed spheroids were
subsequently dehydrated with a graded ethanol series (25%, 50%, 75%,
95% and 100%) for 1 h, respectively, then washed with tert-butanol for
30 min (three times) and freeze-dried at -80 ℃ overnight. The
freeze-dried MTSs were fixed on the sample post with double-sided carbon
conductive tape, and coated with palladium alloy to make the sample have
good conductivity for the SEM analysis.