Nanoparticles of CuS, Cu_xS, Ag₂S and CdS were successfully prepared using a novel general and green synthetic process to give dextranbiopolymer stabilised metal sulfifde nanosuspensions. Following preparation, dextranase enzyme was used to remove the bulk of the bound dextran to give pure stable metal sulfidenanocrystals for application in for example aspects of medicine, photonics and solar cells. Particles of good homogeneity were obtained and the CuSnanoparticle size was controlled to 9–27 nm by adjusting the reaction conditions. Cu₂Snanoparticles were 14 nm, Ag₂Snanoparticles were 20–50 nm and CdS nanoparticles were 9 nm is size. The complexing mechanism of nanoparticlesulfides to dextrans was further studied using carboxylmethyl dextran as a complexing agent and crosslinked Sephadex (dextran) `beads as substrate. Particles were characterized by TEM, XRD, TGA, FT-IR and zeta-potential measurement, and their UV–vis spectroscopic absorption properties were determined. Stabilization of the sulfidenanoparticles with soluble hydroxylated biopolymers such as dextran is previously unreported and is here interpreted in terms of viscosity, pH of the system and weak polar S–H or S(metal)OH₂⁺ interactions with dextran depending on the material. Notably, the complexing mechanism appears to differ significantly from that taking place in known dextran–metal oxide systems. The process shown here has good potential for scale-up as a biosynthetic route for a range of functional sulfidenanoparticles.