We report the design, fabrication, and demonstration of a chemical sensor-based on the spectral shift of organic microcavity lasers. The shape of the cavity contour is used as a parameter and is optimized to improve the sensitivity. Analytical and numerical predictions are in good agreement with experiments performed in a microfluidic environment, showing sensitivities of up to 100 nm per refractive index unit for stadium-shaped microlasers on pedestal. Selective sensing of Hg2+ at a concentration down to 200 ppb is then demonstrated with cavities functionalized by ligands that are known to bind mercuric cations.