Photonic micro- and nanostructures provide the opportunity to
manipulate photons within small volumes for biophotonic applications11–
15. Photonic crystals, first proposed in the 1980s, can be
useful for controlling the density of photon states by creating
a medium with artificially designed periodic structures16.
Biocompatible photonic crystals can extend the utility of these structures
to applications such as reflection-based or label-free biosensing17,18.
In the context of this work, previous reports focusing on
the mechanical properties of silk have demonstrated silk monoliths
with porous nanoscale inverse structures at scales appropriate for
photonic applications19. Here, we bring together silk and photonic
crystals and report the manufacture of robust, free-standing,
three-dimensional photonic crystals with different lattice constants
and a structure in the form of an inverse opal entirely composed
of silk fibroin (a silk inverse opal, SIO). Theoretical calculations
are paired with experimental characterization of the spectral
responses of the device and reveal the existence of a pseudo-photonic
bandgap (pPBG) in the visible spectral range. Tuning of the colorimetric
response is shown by immersing the SIOs in a liquid, and
their potential use as a chemical-free contrast agent is demonstrated
by embedding the SIO within scattering biological tissue in vitro. In
addition, by mixing gold nanoparticles into the silk solution, we
show the simple fabrication of functionalized SIOs and demonstrate
photonic crystal-enhanced absorption and associated patterned
laser heating