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.