pH-Dependent Synthesis and Stability of Aqueous, Elemental Bismuth Glyconanoparticle Colloids: Potentially Biocompatible X-ray Contrast Agents

Taking advantage of a pH-dependent solubility equilibrium, we have developed an aqueous synthesis of chemically and colloidally stable bismuth(0) glyconanoparticles. The synthetic method results in potentially biocompatible elemental bismuth nanoparticles (BiNPs) and involves the reduction of aqueous bismuth(III) cations by sodium borohydride in a pH-controlled solution. Medical-grade dextran (75 000 MW) was found to protect the nanocrystals from oxidation, in addition to promoting colloidal stability and separation of individual nanocrystallites. The rate of particle formation was dependent on synthesis pH, and decreasing the reaction rate by increasing the pH produced a greater number of individual and isolated Bi(0) nanocrystals. Stable, aqueous colloids of the dextran-coated BiNPs decomposed under prolonged light exposure, and the NPs dissolved both in acidic solutions (pH <7) and highly alkaline solutions (pH >12), but were stable in phosphate buffered saline solution (pH 7.4) and in other aqueous solutions between pH 8 and 10. Bismuth-based nanomaterials have previously been demonstrated to be long-circulating X-ray contrast agents, and we anticipate that these high-atomic-number bismuth(0) glyconanoparticles will find use in similar applications.