The sensitivity and responsiveness of living cells to environmental changes are enabled by dynamic protein structures, inspiring efforts to construct artificial supramolecular protein assemblies. However, despite their sophisticated structures, designed protein assemblies have yet to be incorporated into macroscale devices for real-life applications. We report a 2D crystalline protein assembly of ^C98/E57/E66L-rhamnulose-1-phosphate aldolase (^CEERhuA) that selectively blocks or passes molecular species when exposed to a chemical trigger. ^CEERhuA crystals are engineered via cobalt(II) coordination bonds to undergo a coherent conformational change from a closed state (pore dimensions <1 nm) to an ajar state (pore dimensions ~4 nm) when exposed to an HCN(g) trigger. When layered onto a mesoporous silicon (pSi) photonic crystal optical sensor configured to detect HCN_(g), the 2D ^CEERhuA crystal layer effectively blocks interferents that would otherwise result in a false positive signal. The 2D ^CEERhuA crystal layer opens in selective response to low-ppm levels of HCN_(g), allowing analyte penetration into the pSi sensor layer for detection. These findings illustrate that designed protein assemblies can function as dynamic components of solid-state devices in non-aqueous environments.