Biosynthesis and characterization of cadmium sulfide nanoparticles – An emphasis of zeta potential behavior due to capping

Biological approaches have been amongst the most promising protocols for synthesis of nanomaterials. In this study, Cadmium sulfide nanoparticles (CdS NPs) were synthesized by incubating their precursor salts with Escherichia coli and zeta potential (ζ-potential) measurement with varying pH was carried out to evaluate stability of the colloidal dispersion. Formation of CdS NPs was studied in synchrony with microbial growth. TEM analysis confirmed the uniform distribution of NPs. Average size (5 ± 0.4 nm) and electron diffraction pattern revealed polycrystalline cubic crystal phase of these nanoparticles. X-ray diffractogram ascertained the formation of CdS nanoparticles with phase formation and particle size distribution in accordance with the particle size obtained from TEM. Absorption edge of biosynthesized CdS NPs showed a blue shift at ∼400 nm in comparison to their bulk counterpart. A hump at 279 nm indicated presence of biomolecules in the solution in addition to the particles. FT-IR spectrum of capped CdS NPs showed peaks of protein. This confirms adsorption of protein molecules on nanoparticle surface. They act as a capping agent hence responsible for the stability of NPs. The enhanced stability of the particles was confirmed by Zeta potential analysis. The presence of charge on the surface of capped CdS NPs gave a detail understanding of dispersion mechanism and colloidal stability at the NP interface. This stability study of biosynthesized semiconductor nanoparticles utilizing microbial cells had not been done in the past by any research group providing an impetus for the same. Surface area of capped CdS NPs and bare CdS NPs were found to be 298 ± 2.65 m²/g and 117 ± 2.41 m²/g respectively. A possible mechanism is also proposed for the biosynthesis of CdS NPs. - Highlights: • Synthesis of CdS NPs utilizing reproducible molecular machinery viz. Escherichia coli biomass. • Uniform and Polydispersed NPs with high surface area and diameter well below 10 nm. • Proteins play active roles in the formation and in capping of CdS NPs as confirmed. • High ζ-potential with varying pH confers stability of aqueous CdS NP dispersion. • Dispersion stability due to interplay between NP surface and adsorbed protein.