1. A device comprising: a head-mountable support; a light source supported by the head-mountable support, wherein the light source is positioned to illuminate a retina in an eye of a wearer of the device with incident light, wherein the incident light has a wavelength corresponding to an excitation wavelength of fluorescent nanoparticles, wherein the fluorescent nanoparticles selectively bind to a target analyte; and an imager supported by the head-mountable support, wherein the imager is positioned to image the retina using fluorescent light emitted from the fluorescent nanoparticles in response to the incident light, and wherein the imager includes a filter that substantially blocks the wavelength of the incident light and passes the fluorescent light.