Spontaneous formation of urea from carbon dioxide and ammonia in aqueous droplets

Urea is a key molecule in the search for the origin of life and a basic chemical produced in large quantities by industry. Its formation from ammonia and carbon dioxide requires either high pressures and temperatures or, under milder conditions, catalysts or additional reagents. Here we report the spontaneous formation of urea under ambient conditions from ammonia and carbon dioxide in the surface layer of aqueous droplets. Single optically-trapped droplets were probed using Raman bands as markers. We found the surface layer to act like a microscopic flow reactor with chemical gradients providing access to unconventional reaction pathways. This reveals a general mechanistic scheme for unique droplet chemistry. Interfacial chemistry is a possible non-energetic route for urea formation under prebiotic conditions.