Metal nanoparticle synthesis using plant extracts has gained a lot of attention because of its ease-of-use, environmental friendliness and simplicity. In this study, nickel nanoparticles were synthesized using an aqueous extract of Moringa oleifera leaves in a one-step synthesis technique. The formation of nickel nanoparticles was confirmed using UV-visible spectroscopy, XRD, SEM and EDS, and FTIR spectroscopy showed that these nickel nanoparticles may be reduced to nickel chloride by biomolecules that have an amide and phenolic structure. The electrochemical activity of synthesized nickel nanoparticles is revealed further by electrochemical characterization using cyclic voltammetry; preliminary study on electrochemical detection of Escherichia coli O157:H7 shows an increased current response in the presence of nickel nanoparticles, which could be attributed to nickel nanoparticles rupturing the Escherichia coli cell membrane. This finding suggests that nickel nanoparticles could be used to detect E. coli O157:H7 directly.