Double helical silica fibrils by sol–gel transcription of chiral aggregates of gemini surfactantsElectronic supplementary information (ESI) available: Fig. S1: TEM image of double stranded silica obtained by sol–gel transcription of l-1/d-1 gel (2∶1 mol/mol, 33% ee l-1 excess). See http://www.rsc.org/suppdata/cc/b2/b202799m/