Behzad Rezaei & Hamid Reza Jamei & Ali Asghar Ensafi

Abstract
An aptamer-based method is described for electrochemical determination of lysozyme. A glassy carbon electrode was modified with a nanocomposite composed of multi-walled carbon nanotubes, poly(diallyl dimethyl ammonium chloride), and carbon quantum dots. The composition of the nanocomposite (MWCNT/PDDA/CQD) warrants good electrical conductivity and a high surface-to-volume ratio. The lysozyme-binding aptamers were immobilized on the nanocomposite via covalent coupling between
the amino groups of the aptamer and the carboxy groups of the nanocomposite. The modified electrode was characterized by electrochemical impedance spectroscopy, cyclic voltammetry and differential pulse voltammetry. The use of this nanocomposite results in a considerable enhancement of the electrochemical signal and contributes to improving sensitivity. Hexacyanoferrate was used as an electrochemical probe to study the dependence of the peak current on lysozyme concentration.
In the presence of lysozyme, the interaction of lysozyme with immobilized aptamer results in a decrease of the peak current, best measured at +0.15 V vs. Ag/AgCl. A plot of peak current changes versus the logarithm of the lysozyme concentration is linear in the 50 fmol L−1 to 10 nmol L−1 concentration range, with a 12.9 fmol L−1 detection limit (at an S/N ratio of 3). The method is highly reproducible, specific and sensitive, and the electrode has a rapid response. It was applied to the determination of lysozyme
in egg white, serum, and urine.

Keywords Biosensor . Differential pulse voltammetry . Electrochemical aptasensor . Electrochemical impedance spectroscopy .
Nanocomposite