Fig. 1. (a) Schematic of the multilayers assemblies by consecutive adsorption of polyanion and polycation; (b) structure of some of the used polyelectrolytes; and (c) schematic of the engineered COX electrodes.

Fig. 2. Schematic of amperometric reaction of cholesterol to cholest-4-en-one.

Fig. 3. UV–visible absorbance of COX in the solution of phosphate buffer (curve 1), curve 2 addition of PEI in phosphate buffer, curve 3 after 10 min of PEI addition in the solution.

Fig. 4. UV–visible absorbance of PEI/COX of two bilayers. Inset shows the UV–visible absorption of PEI/COX films at 272 nm as a function of bilayers on quartz surface.

Fig. 5. Relationship between frequency variation and number of bilayers of PEI/COX deposited onto 10 MHz cut quartz coated with gold surface.

Fig. 6. AFM pictures of PEI/COX ten bilayers deposited on mica substrate; (a) the size of the image is 1×1 μm and (b) the image size is 250×250 nm.

Fig. 7. (a) CV of COX in 100 mM phosphate buffer when working electrode was set to be ITO glass plate. (b) CV of ten bilayers of PEI/COX LBL films as a function of scan rate in 100 mM phosphate buffer of pH 7.0 containing 10 mM KCl and 1% Triton X-100 viz: 50 mV/s (curve 4), 20 mV/s (curve 3), 10 mV/s (curve 2), 5 mV/s (curve 1). (c) CV of bilayers of PEI/COX in 100 mM phosphate buffer of pH 7.0 containing 10 mM KCl and 1% Triton X-100.

Fig. 8. Differential pulse voltammetry (DPV) of PEI/COX bilayers (a) scanned from −450 to 600 mV at 20 mV/s, (b) scanned from 600 mV to −450 mV at 20 mV/s.

Fig. 9. (a) The saturation current response curve at 0.6 Vs Ag/AgCl for addition of cholesterol. (b) The current response as a function of cholesterol concentration

Fig. 10. The current response curve at 0.6 V vs Ag/AgCl for addition of cholesterol containing 20 bilayers of PEI/COX deposited on ITO/PSS substrate.

Fig. 11. The cyclic voltammogram of COX ten bilayers deposited on platinum electrode in 100 mM phosphate buffer of pH 6.8 and curve 2 CV after the addition of 100 μl addition of cholesterol. The electrode was used after 6 months of deposition period.