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 6-10nm 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.