Biomolecule encapsulated Ni-doped ZnO quantum dots: A sustainable nanomaterials for antibacterial applications

Here, pure ZnO, nickel (Ni) doped ZnO, and biomolecule-capped ZnO quantum dots (QDs) have been synthesized
using an eco-friendly green synthesis approach with optimized conditions (temperature, reaction time, and pH).
The structure of ZnO quantum dots (QDs) with nickel (NiII) dopant was investigated using XRD, and a mixed
cubic-hexagonal crystal structure with an average crystallite size of 12 nm was obtained. The transmission
electron microscopy (TEM) images showed spherical morphology with average particle sizes below 10 nm, and the particles were uniformly dispersed in the biomolecule matrix. A red shift of an absorbed spectrum peak was
observed at ~343 nm due to the sp-d exchange interactions between the Ni
2+
ions and the electrons on the host
lattice of the ZnO. Firm violet-blue emission peaks associated with near-band edge excitonic recombination and
trap state emissions were observed from biomolecule capping, which was confirmed by the photoluminescence
(PL) spectra. The antibacterial activities of the synthesized QDs were tested against gram-negative (E. coli) and
gram-positive (S. aureus) bacteria using the disc diffusion method. Sucrose-capped nickel-doped QDs (ZnO:Ni/
Sucrose) showed the best antibacterial activities. Considerably larger inhibition zones were achieved for these
samples compared to starch and glucose-capped samples. Increased generation of reactive oxygen species (ROS)
coupled with disruption of the bacterial membrane accounts for the observed superior antibacterial effect from
the capped particles aided by reduced size. This work proposes a novel, economically favorable, and ecologically
friendly method for fabricating biomolecule-stabilized Ni-doped ZnO QDs with notable antibacterial activity.
These results demonstrate that ZnO:Ni/Sucrose QDs are viable options for biomedical fields focused on fighting
against microbial infections.