Ultrasensitive Electrochemical Aptasensor for Alpha-Fetoprotein Detection Based on Cadmium Telluride/Cadmium Selenide/Polyaniline-Cosensitized Structure

In this paper, a water-soluble highly electrochemical signal cadmium telluride (CdTe)/cadmium selenide (CdSe)/polyaniline nanocomposite was developed through a fast and convenient method, and then the nanocomposite-modified glassy carbon electrode was prepared for the determination of alpha-fetoprotein (AFP). This aptasensor was constructed by covalently immobilizing NH2-functionalized AFP-specific aptamer on nanocomposite with plenty of carboxylic groups. This electrochemical biosensor via the layer-by-layer method could evidently increase the steric hindrance of the sensing electrode and effectively depress the electron transfer, leading to obviously decreased current intensity. The ultrahigh sensitivity of this immunoassay is derived from the two primary reasons as follows. First, the CdTe/CdSe multiple-sensitized and cosensitized structure could maximize speed of charge transfer processes between electrodes and the electroactive species, dramatically promote electron transfer, and effectively inhibit the electron-hole recombination, resulting in the significantly enhanced electrochemical current intensity of the sensing electrode. Second, the electrocatalytic oxidation of K3Fe(CN)6, which makes the CdTe/CdSe change from a lower-energy to higher-energy states (CdTe/CdSe QDs)*, reduces the activation energy of the reaction and the (CdTe/CdSe QDs)* more likely to oxidize, accelerating the transfer of electrons. Scanning electron microscope, transmission electron microscope, and X-ray photoelectron spectroscopy were used to characterize the material. Electrochemical impedance spectroscopy was used to observe the loading process of the material. Differential pulse voltammetry was used as a method of measurement. The immunosensor exhibited a wide linear range from 1.0 to 10.0 mu g/mL for target AFP detection, with a low detection limit of 1.0 pg/mL (S/N = 3). To evaluate the analytical reliability, reproducibility, specificity, and stability, the proposed immunosensor was applied to human AFP-spiked serum samples, and acceptable results were obtained, indicating that the method can be readily extended to other bioaffinity assays of clinical or environmental significance.