Nanocomposite films are prepared by layer-by-layer (LBL) assembly of cationic genetically engineered rodlike nontoxic viruses and anionic spherical Au nanoparticles. The cationic viruses electrostatically interact with the anionic Au nanoparticles to drive the LBL assembly. The nanocomposite films exhibit humidity-dependent surface plasmon resonance spectra (see figure).