Direct electrochemistry of cholesterol oxidase immobilized on gold nanoparticles-decorated multiwalled carbon nanotubes and cholesterol sensing

An electrochemical cholesterol biosensor based on the direct electron transfer of cholesterol oxidase (ChOx) immobilized on gold nanoparticles-decorated multiwalled carbon nanotubes (GNPs-MWCNTs) was fabricated. GNPs-MWCNTs were prepared based on the reduction of HAuCl4 in the presence of carboxyl group functionalized MWCNTs. Transmission electron microscopy image shows that rounded gold nanoparticles with diameters of 6–10 nm were decorated on carbon nanotube surfaces. ChOx was directly adsorbed on the nanocomposite modified glassy carbon electrode and protected by a Nafion film. Direct electrochemistry of ChOx on the electrode surface was obtained, proved by one pair of well-defined, quasi-reversible redox peaks in phosphate buffered saline. Under optimized conditions, the fabricated electrode displayed a linear response in the cholesterol concentration range from 0.0100 to 5.00 mmol L−1 with a detection limit of 4.3 μmol L−1 estimated at a signal-to-noise ratio of 3. The apparent Michaelis–Menten constant was measured to be 0.29 mmol L−1, indicating that the immobilized ChOx on GNPs-MWCNTs matrix retained its native activity. The developed biosensor presented good selectivity, repeatability, reproducibility and stability. The concentration of free cholesterol in a human serum sample, detected by using the developed biosensor, is in good agreement with that determined by the well-established spectrophotometric method.