An Electrochemical Cholesterol Biosensor Based on A CdTe/CdSe/ZnSe Quantum Dots-Poly (Propylene Imine) Dendrimer Nanocomposite Immobilisation Layer

Abstract

We report the preparation of poly (propylene imine) dendrimer (PPI) and CdTe/CdSe/ZnSe quantum dots (QDs) as a suitable platform for the development of an enzyme-based electrochemical cholesterol biosensor with enhanced analytical performance. The mercaptopropionic acid (MPA)-capped CdTe/CdSe/ZnSe QDs was synthesized in an aqueous phase and characterized using photoluminescence (PL) spectroscopy, ultraviolet-visible (UV-Vis) spectroscopy, transmission electron microscopy (TEM), X-ray power diffraction (XRD), energy dispersive X-ray (EDX) spectroscopy. The absorption and emission maxima of the QDs red shifted as the reaction time and shell growth increased, indicating the formation of CdTe/CdSe/ZnSe QDs. PPI was electrodeposited on a glassy carbon electrode followed by the deposition (by deep coating) attachment of the QDs onto the PPI dendrimer modified electrode using 1-Ethyl-3-(3-dimethylaminopropyl)-carbodiimide hydrochloride (EDC), and N-hydroxysuccinimide (NHS) as a coupling agent. The biosensor was prepared by incubating the PPI/QDs modified electrode into a solution of cholesterol oxidase (ChOx) for 6 h. The modified electrodes were characterized by voltammetry and impedance spectroscopy. Since efficient electron transfer process between the enzyme cholesterol oxidase (ChOx) and the PPI/QDs-modified electrode was achieved, the cholesterol biosensor (GCE/PPI/QDs/ChOx) was able to detect cholesterol in the range 0.1⁻10 mM with a detection limit (LOD) of 0.075 mM and sensitivity of 111.16 μA mM^-1 cm^-2. The biosensor was stable for over a month and had greater selectivity towards the cholesterol molecule.

Keywords: Cholesterol; cholesterol oxidase; electrochemical biosensors; poly (propylene imine) dendrimer; quantum dots.

Scheme 1

Schematic representation for the synthesis of CdTe/CdSe/ZnSe quantum dots.

Scheme 2

Schematic representation for the fabrication of GCE/PPI/QDs/ChOx biosensor.

Figure 1

(A) UV-Vis absorption. (B) photoluminescence. (C) spectra of MPA-capped CdTe/CdSe/ZnSe core-multishell quantum dots at different growth stages and reaction time. Digital images of the as-synthesized CdTe/CdSe/ZnSe QDs (at different reaction time) under UV-lamp at an excitation wavelength of 250 nm. (D) 350 nm.

Figure 2

TEM image (A). SAED (B), EDX spectrum (C) and XRD spectra (D) of the as-synthesized MPA-capped CdTe/CdSe/ZnSe quantum dots.

Figure 3

Nyquist plot of GCE (i); GCE/QDs (ii); GCE/PPI (iii), GCE/PPI/QDs (iv); and GCE/PPI/QDs/ChOx (v) in 10 mM [Fe (CN)₆ ]^3−/4−/0.1 M KCl redox probe in the frequency range of 10³ Hz to 1 Hz. Insert: Corresponding equivalent circuit obtained from the Nyquist plot.

Figure 4

Cyclic voltammograms of GCE (i), GCE/QDs (ii), GCE/PPI (iii), GCE/PPI/QDs (iv), in 10 mM PBS (pH 7.2) at 50 mV.s⁻¹. Insert: PPI/QDs/ChOx (v) in 10 mM PBS (pH 7.2) at different scan rates (10–150 mV.s⁻¹).

Figure 5

Cyclic voltammograms of GCE/PPI/QDs/ChOx in PBS (i) and in 10 mM cholesterol solution (ii) (A); Cyclic voltammograms of GCE/QDs (i), GCE/PPI (ii), GCE/PPI/ChOx (iii) GCE/QDs/ChOx (iv) and GCE/PPI/QDs/ChOx (v) in 10 mM cholesterol solution at 50 mV.s⁻¹ (B); Effect of pH (C); and temperature (D) on the developed biosensor.3.5. Electrochemical Detection of Cholesterol Solution.

Figure 6

Square wave voltammetry (SWV) of GCE/PPI/QDs/ChOx biosensor upon successive addition of cholesterol (0.1–10 mM) in 10 mM PBS (pH 7.2) (A). Calibration plot of cholesterol at different concentrations (B).

Figure 7

Selectivity of the biosensor (GCE/PPI/QDs/ChOx) in the presence of 5 mM cholesterol and interferents—10 mM uric acid (UA), 10 mM ascorbic acid (AA) and 10 mM glucose in PBS (pH 7.2) solution (A). Stability studies of the biosensor at day 1 and day 30 in 5 mM cholesterol solution at pH 7.2 (B).