Label-Free Mode Based on Ferrocene/PEDOT:PSS–PPy for Molecularly Imprinted Electrochemically Ultrasensitive Detection of Amino Acids

Generally, molecularly imprinted (MIP) electrochemical sensors for amino acids operate in a "label-like" mode. That is, after an amino acid is specifically recognized by an imprinted cavity at the sensing interface, the amino acid itself provides the sensing signal for quantitative detection. However, poorly electroactive amino acids impede electron transfer at the sensing interface and require high potentials to drive the reaction; thus, more interfering reactions tend to be triggered in practical applications, causing enhanced background noise in the detection. To address these issues, a "label-free" mode of the MIP sensor based on the ferrocene (Fc)/PEDOT:PSS-polypyrrole (PPy) composite was designed for the first time. The Fc/PEDOT:PSS-PPy is drop coated on the electrode surface as a substrate, and MIP polymers with specific recognition ability are immobilized on the substrate via electrostatic adsorption. As a proof of concept, l-tyrosine (l-Tyr) was selected as a model analyte and the "label-free" mode MIP/Fc/PEDOT:PSS-PPy sensor was constructed. The limit of detection (LOD) and linearity range of the MIP/Fc/PEDOT:PSS-PPy sensor were 2.31 × 10^-11 M and from 100 pM to 5 mM, respectively. Compared with the label-like mode, the LOD was three orders of magnitude lower, the linear range was increased by three orders of magnitude, and the sensitivity was improved by more than four times. This work provides a universal and effective concept for MIP electrochemical sensing of amino acids.