Gold nanoparticles: Poly(diallyldimethylammonium chloride)–carbon nanotubes composites as platforms for the preparation of electrochemical enzyme biosensors: Application to the determination of cholesterol

Enzyme biosensors for cholesterol were prepared using a transduction platform constituted of a glassy carbon electrode (GCE) modified with gold nanoparticles (AuNPs)/poly-(diallyldimethyl-ammonium chloride) (PDDA)–multi-walled carbon nanotubes (MWCNTs) nanocomposites. This modified electrode exhibited an improved response to H2O2 when compared with other configurations based on CNTs. The incorporation of cholesterol oxidase (ChOx) to this electrode matrix allows the preparation of a biosensor that responded linearly to cholesterol in the 0.02–1.2 mM range with a slope of 2.23 μA/mM and a limit of detection 4.4 μM. All the variables involved in the preparation and performance of the modified electrode and the enzyme biosensor were optimized. The interferences caused by the presence of ascorbic acid and/or uric acid were minimized by coating the electrode surface with a thin film of Nafion which also enhanced the stability of the biosensor. Moreover, the possibility of preparing bienzyme ChOx–HRP biosensors using the same electrode platform was demonstrated. This allowed a detection potential as low as −50 mV to be applied using hydroquinone as mediator. With this design, the sensitivity was almost one order of magnitude higher than that achieved with the single ChOx biosensor and no interference from ascorbic acid or uric acid was apparent. Both biosensors designs were used for the determination of cholesterol in human serum samples spiked at physiological levels with recoveries ranging between 98.2% and 104%.