Voltammetric aptasensor for bisphenol A based on double signal amplification via gold-coated multiwalled carbon nanotubes and an ssDNA-dye complex

An aptasensor is described for the electrochemical determination of bisphenol A (BPA). Gold-coated multiwalled carbon nanotubes (Au/MWCNTs) and a single-stranded DNA-dye complex are used as a double signal-amplification system. The BPA-binding aptamer was assembled on a disposable electrode modified with Au/MWCNTs. Methylene blue (MB) was then intercalated into the immobilized aptamer with an approximately molecular ratio of 4 to form a complex. Upon interaction with BPA, the immobilized aptamer underwent a conformational change. This causes the intercalated MB to be released from the complex into solution. As a result, the electrochemical signal of the intercalated MB, typically measured using square wave voltammetry at a potential of -0.20 V (vs. Ag/AgCl (saturated KCl)) decreases. The fabrication of the aptasensor was characterized by the scanning electron microscopy, atomic force microscopy, and electrochemical techniques. Under optimal experimental conditions, the current drops linearly with the logarithm of BPA concentrations over the range from 10 fM to 1 nM, and the limit of detection is 8 fM. The assay was applied to the determination of BPA in plastic drinking bottles, tap water, and milk.