Reactions of isatins and indoles using nano-SiO₂ and HNO₃@nano-SiO₂ under stirring and grinding techniques.

Catalyst	Nano-SiO2 (0.1 mol%)52				HNO3@nano-SiO2 (ref. 53)
	Symmetrical trisindoline		Unsymmetrical trisindoline		Symmetrical trisindoline		Unsymmetrical trisindoline
	Stirring	Grinding	Stirring	Grinding	Stirring	Grinding	Stirring	Grinding
Time (min)	15–75	1–6.5	120–150	7–10	20–40	3–5	5–60	5–10
Isolated yields (%)	80–98	63–98	80–83	70–81	80–96	80–95	71–80	72–78