the structure. This can be achieved either by doping the silk or by
filling the structure. Spectral responses were measured using an
optical set-up under white-light illumination by collecting visible–
near-infrared reflectance spectra. By decreasing the refractive
index contrast, the wavelengths corresponding to the peak reflected
light are redshifted. To illustrate this concept simply, we exposed the
SIO to acetone, which has a low enough surface tension to infiltrate
the nanostructured lattice and is volatile enough not to cause any
swelling in the protein matrix. The acetone (n ≈ 1.36) causes a
decrease in index contrast from the original Dn ≈ 0.54 to Dn ≈
0.18, thereby causing shift of the pPBG to a lower frequency.
These results are presented in Fig. 2. Clear changes in structural
colour can be observed in Fig. 2a, as well as in the calculated
band structures in Fig. 2b and the experimental results in Fig. 2c.
In Fig. 2c, two reflectance peaks are clearly visible. The first
appears in the blue, centred at l ¼ 438 nm for a corresponding
SIO lattice constant of L ¼ 240 nm, and the second in the green,
centred at l ¼ 540 nm, with an SIO lattice constant of L ¼ 300 nm.
The positions of the peaks and the magnitude of the shift are in
agreement with the theoretical estimation obtained from the
pPBG calculation