Microdroplet surfaces interface organic molecule formation and the origins of life

The primitive Earth, starting from the evolution of primitive chemistry, gradually developed into a habitable world. In prebiotic chemistry, a central question is how nature’s simplest molecules (e.g., carbon dioxide [CO2], water [H2O], and ammonia [NH3]) could form organonitrogen molecules, which serve as precursors to amino acids, nucleotides, and peptides. Urea, a representative organonitrogen molecule, typically requires extreme conditions in the industrial Bosch-Meiser process, including high pressure (125–250 bar) and high temperature (170°C–220°C).1 Those conditions are hardly simultaneously plausible on the primitive Earth.

In this remarkable recent study, Mercede et al. demonstrate that urea can spontaneously form within water droplets exposed to CO2 and NH3 gases under ambient temperature and pressure, without catalysts or external energy input.2 At the droplet interface, a microscopic flow reactor is established,3 where protons catalyze the coupling of CO2 and NH3 to generate urea. This nonequilibrium interfacial mechanism provides a plausible abiotic pathway for early organonitrogen chemistry. On the primitive Earth, geological processes such as volcanic eruptions would have released large amounts of H2O vapor and CO2, with the resulting droplets (H2O vapor) potentially enabling the spontaneous formation of urea from CO2 and ambient NH3 (Figure 1). Such findings reveal how simple, spontaneous physicochemical processes may have preceded enzymatic catalysis in the emergence of life.

To probe urea formation in a thousandfold-higher surface-to-volume ratio droplets, the authors employed in situ single-droplet Raman spectroscopy. In this setup, aerosol droplets were immobilized in a gaseous environment, and the focused Raman laser enabled time-resolved monitoring of interfacial species (e.g., urea and bicarbonate) over longer periods (many hours). This approach directly links the spectral evolution to interfacial chemistry within a microscopic reactor. Under a CO2 and NH3 atmosphere, a new Raman band appeared at 1,003 and 1,031 cm−1 after 30 min, corresponding to urea and bicarbonate species, respectively. Remarkably, the urea concentration reached 42 ± 10 mM, which is exceptionally high for an abiotic, catalyst-free system.4 Complementary ex situ experiments, including gas chromatography-mass spectrometry (GC-MS) and carbon-13 nuclear magnetic resonance (13C-NMR), further confirmed the successful synthesis of urea. In control experiments where CO2 was replaced by nitrogen (N2) gas, in the absence of dissolved ammonia or with bulk water, no urea was detected.