Under one aspect, a system (100) for sensing the presense of an analyte in a fluid includes a nanotube sensor element including a plurality of nanotubes and positioned for exposure to a fluid; an optical source capable of generating optical radiation (102), the radiation having a source frequency and a fluence selected to generate a nonlinear optical response by the nanotube sensor element; an optical detector (110) capable of measuring the nonlinear optical response by the nanotube sensor element; and logic in electrical communications with the optical detector to sense the presense of an analyte in the fluid based on the nonlinear optical response measured by the optical detector.