SEM and layer-by-layer sample preparation
This protocol is extracted from research article:
Unusual packing of soft-shelled nanocubes
Sci Adv, May 17, 2019; DOI: 10.1126/sciadv.aaw2399

The sample deposited on a cleaned silicon substrate was measured using a Hitachi S-4800 scanning electron microscope with typical 1-kV voltage and 10-μA emission current. A standard polyelectrolyte-assisted layer-by-layer method was applied to prepare the diluted NP-assembled clusters for SEM characterization (44). Silicon wafers were used as substrates for SEM characterization. The substrates were sonicated for 10 min in water and then in ethanol, subsequently thoroughly cleaned using piranha solution (H2SO4:H2O2, 7:3), rinsed with deionized water, and dried under an air stream. The wafers were stored in water until use. Before use, the wafers were first immersed in an aqueous solution of positively charged poly(diallydimethylammonium chloride) [PDDA; weight-average molecular weight (Mw) = 200,000, 1 mg/ml in 0.5 M NaCl aqueous solution] for 20 min, then in an aqueous solution of the polyanion poly(acrylic acid, sodium salt) (Mw = 15,000, 1 mg/ml in 0.5 M NaCl aqueous solution) for 10 min, and lastly, in PDDA solution for 10 min. At this stage, the wafers were positively charged, favoring the electrostatic interaction with negatively charged DNA in the assembled aggregates. To obtain a monolayer of NP-assembled clusters, the pretreated wafers were immersed in the corresponding solution with diluted aggregates and kept for a suitable period of time. After enough absorption, the substrates were rinsed with deionized water and dried under an air stream for further SEM characterization.

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