Activated direct electron transfer of nanoAu bioconjugates of cytochrome c for electrocatalytic detection of trace levels of superoxide dismutase enzyme

Highlights: ► Redox reaction of the nanoAu bioconjugates of Cyt c was a facile mass-transport limited process. ► Electrocatalytic current for superoxide radical with nanoAu-Cyt c bioconjugates is ∼12 times higher for binary mixed-monolayer than that of simple MPA monolayer. ► It emphasizes the importance of fine-tuning the interfacial structure and morphology even at nanomaterial levels. ► The heterogeneous electron-transfer rate of Au/MPA + MPO/Cyt c (k′et), is far greater than that of Au/MPA/Cyt c (ket) {i.e.,k′et≫ket}. - Abstract: A novel amperometric biosensor for the detection of trace levels of superoxide dismutase enzyme is developed with the use of nanoAu bioconjugates of cytochrome c (Cyt c). NanoAu particles of finite size were synthesized by borohydride reduction in the presence of alkanethiols having -COOH and -OH end groups. The nanoAu particles were characterized by transmission electron microscopic analysis, and the mean size of the nanoparticles is determined to be 2.96 nm diameter. The heme protein, Cyt c, is bound to the nanoAu particles, and the resulting nanoAu bioconjugates are investigated by cyclic voltammetric and rotating-disk voltammetric experiments. Cyt c has become electrodically reactive by binding with the nanoAu particles, and Cyt c-bound nanoAu particles exhibited a reversible, mass-transport limited electron-transfer reaction. Reversible redox peaks are observed with the formal redox potential of +0.05 V (vs. Ag|AgCl) at bare GC and also at Au|alkanethiolate monolayer electrodes. Cyt c is highly reactive to superoxide radical, and electrocatalytic oxidation of superoxide occurs at the applied potential of +0.15 V in the presence of the nanoAu bioconjugates. Steady state current–time curves show a sharp increase in the anodic current to the generation of superoxide radical and attain a plateau in ca. 6 min. The structure and morphology of the alkanethiolate layer at the nanoAu-Cyt c interface tremendously influences the electrocatalytic current for superoxide. The electrocatalytic current observed for superoxide radical varied sharply by the presence of superoxide dismutase. From the dependence of the electrocatalytic current for superoxide on the concentration of superoxide dismutase, a low-detection-limit of as low as 0.25 U mL−1 (∼50 ng mL−1) has been established.