The invention relates to a measuring device comprising a device for forming a liquid optical waveguide, comprising a substrate (1) having a closed micro-channel (2) which is curved at least in sections and having a low refractive index coating (13). At least one supply line (6) for supplying liquid is formed in the substrate (1), and a device for axially injecting light into the closed micro-channel (2) and/or for axially extracting light from the closed micro-channel is provided on at least one end of the closed micro-channel (2). Said device also comprises a light source (4), a light detector (5) and a first liquid pump (9) which supplies a sample liquid (7) to the closed micro-channel (2) via the at least one supply line (6, 6a).