Carbon dioxide (CO 2) assisted synthesis of water-soluble silver nanoparticle with a narrow particle size distribution is reported here based on the phase-inversion procedure. Bio-derived chitosan (CS) is used to stabilize the metal nanoparticles according to its abundant functional groups. Formic acid is employed as both a solvent (for the polymer) and a reductant for in-situ reducing the silver precursor along with the solvent evaporation. CO 2 is utilized to combine with the amino groups of CS, reducing the viscosity of chitosan/formic acid solution and limiting the formation of hydrogen bonds. This promotes the stabilization and reduction efficiency of silver nanoparticles. In particular, 100% of Ag metal nanoparticles with the size of 7.5 ± 2.3 nm is successfully synthesized with the assistance of CO 2. Interestingly, the synthesized CS/Ag nanocomposites are water-soluble owing to the formation of carbamate groups. This water-soluble silver nanoparticle presents an exceptional performance in the selective reduction of 4-nitrophenol, where the turnover frequency (TOF = 599 h−1) is even double with respect to the CO 2 free system. • CO 2 was used to reduce the viscosity of chitosan/formic acid solution, so as to limit the hydrogen bonds of the chitosan. • The carbon dioxide modified chitosan promotes the stabilization and reduction efficiency of silver nanoparticles. • The produced chitosan/silver nanocomposites are water soluble and present an exceptional catalytic performance. [ABSTRACT FROM AUTHOR]