The disclosure includes: a coating composition, comprising a powdered cold-water-soluble gelling agent and surface-modified nanoparticles disposed in the powdered cold-water-soluble gelling agent; a coated film that includes a transparent film coated with the coating composition; and a device for growing microorganisms, including the coated film releasably attached to at least a portion of a body member that includes a self-supporting and water-proof substrate.