In this Letter, we adopt a template method based on the use of
poly(methyl methacrylate) (PMMA) submicrometre spheres to fabricate
SIOs20. The fabrication procedures are described in Fig. 1.
Layers of submicrometre PMMA spheres with diameters of 250,
320 and 350 nm were stacked in a face-centred cubic (fcc) structure
onto a silicon substrate (Supplementary Fig. S1a). An aqueous solution
of fibroin extracted from B. mori silkworm cocoons was
poured into the PMMA template and allowed to solidify into an
amorphous free-standing silk film. To obtain the inverse silk opal
structure, the silk film was detached from the silicon substrate and
immersed in acetone to dissolve the PMMA spheres. This process
led to the appearance of iridescence (otherwise known as ‘structural
colour’) within the transparent silk film (Supplementary Fig. S1b).
The resulting free-standing silk film with an inverse opal structure
is shown in Fig. 1. A scanning electron microscopy (SEM) image
of a section of the sample (Fig. 1) shows a nanostructure with
ordered voids (where the PMMA spheres were originally located),
templated for both green and blue iridescence. Inspection of the
SEM images of the sample reveals a slight reduction in the lattice
constant, with the measured centre-to-centre distances of the airvoid
diameters appearing to have become smaller (240, 280 and
300 nm, respectively) than the diameters of the original PMMA
spheres. This is consistent with prolonged exposure to acetone,
which ultimately induces some contraction of the film.