Catalytic and fluorescence studies with copper nanoparticles synthesized in polysorbates of varying hydrophobicity

Nonionic polysorbate surfactants used in pharmaceutical formulations have been used for synthesizing stable and highly crystalline copper nanoparticles. Polysorbate concentration, chain length and hydrophile:lipophile balance (HLB) are important parameters determining nanoparticle characteristics. At low polysorbate concentrations, amorphous copper oxide nanoparticles are formed. Highly crystalline and well-dispersed metallic copper nanoparticles are formed only at micellar concentrations. The more hydrophobic polysorbate-60 forming large, loosely packed micelles is not suitable for metallic nanoparticle formation. However, HLB is not the sole factor guiding nanoparticle formation. Polysorbate-40 with intermediate HLB value promotes the formation of good quality, perfectly hexagonal and crystalline metallic nanoparticles with uniform size distribution. Catalytic efficiency of the nanoparticles depends on their size and structural organization. In this respect, the discrete and spherical metallic nanoparticles formed in the more hydrophilic polysorbate-20 micelles perform best. Additionally, the fluorescence quenching ability of the nanoparticles can be directly correlated to their catalytic behaviour. Summarizing, this work shows how one can modulate the size, shape, structural organization and quality of the copper nanoparticles by fine-tuning polysorbate concentration and HLB value.