Cover Picture: Assembly of Wiseana Iridovirus: Viruses for Colloidal Photonic Crystals (Adv. Funct. Mater. 8/2006)

Assembly of colloids is a versatile tool for micro- and nanofabrication. Natural and artificially engineered viruses offer the opportunity to expand the functionality and versatility of such assemblies. The cover shows optically iridescent, thin polycrystalline arrays (background) as well as bulk pellets (inset right) that exhibit reversible hydration-dependent reflection spectra, as reported by Vaia and co-workers on p. 1086. The films and pellets were created in vitro with classical colloid-assembly techniques from Wiseana iridescent virus (inset, center) harvested from infected Wiseana spp larvae (inset, left). In vitro assembly of Wiseana iridescent virus (WIV) yields iridescent pellets and films with structural color more vivid than in the native insect. WIV is icosahedral in shape, 140 nm in diameter, with 30 nm long fibrils attached to the outer surface, and exhibits a surface charge ca. 1/6th that of a comparable polymer colloid. The low surface charge and tethered chains on the virus surface allow the facile modification of the interparticle distance. Directed sedimentation yields predominantly an amorphous liquid-like packing of the virus. Such samples exhibit a broad reflection band that is angle independent and for which the broad maximum can be reversibly shifted from blue towards red with increased hydration. Slow sedimentation and flow-assisted assembly methods produce thin films with a polycrystalline morphology that exhibit narrower, more intense reflectivity peaks, which are hydration and angle dependent. This study points toward the potential of viral particles for photonic crystals where their unique structural features (icosahedral symmetry, extreme monodispersity, precise surface functionalization, and tethered surface chains of low surface-charge density) may lead to superior control of optical properties of their assembled arrays.