Abstract: This disclosure provides, among other things, a microfluidic device for detecting an analyte in a liquid, comprising: a substrate; a fluidic channel on a surface of the substrate; and a nanosensor at a location of the channel, the nanosensor comprising: a nanostructure, the nanostructure comprising at least one nanostructure element, each element comprising at least two metallic structures that are separated by a gap, and a capture agent deposited on a surface of the nanostructure, wherein the capture agent specifically binds to the analyte. The nanosensor amplifies a light signal to and/or from the analyte or a light label attached to the analyte, when the analyte is bound or in proximity to the capture agent.