Coulombimetric immunosensor for paraquat based on electrochemical nanoprobes This work is dedicated to the memory of Dr. Francisco Sanchez-Baeza.

A new electrochemical immunosensor has been developed to detect paraquat (PQ) pesticide residues in food samples. The immunosensor presented uses electrochemical nanoprobes prepared by labeling the PQ specific antibodies with CdS nanoparticles (CdSNP) and antigen biofunctionalized magnetic μ-particles. Electrochemical measurements are performed using graphite composite electrodes (GECs). After the immunochemical reaction, the CdSNP are dissolved and the metal ions released are reduced at the electrode and read in the form of current or charge signal, by the well-known anodic stripping technique. Due to the amplification effect produced by the CdSNP on the amperometric/coulombimetric signal, a very high detectability is reached. Thus, PQ can be detected with an IC₅₀ current of 0.18 ± 0.31 μg L^-1 (in PBST). The immunosensor has been implemented in analyzing PQ residues in potato samples. Combined with a suitable extraction procedure PQ can be detected with a LOD current of 1.4 μg kg^-1, far below the maximum residue limit (MRL, 20 μg kg^-1) established by the EU for this pesticide in most crops. Likewise, the working range was in the interval between 3.08 and 67.76 μg kg^-1. Preliminary experiments demonstrate that the immunosensor presented here could be used as screening tool to distinguish between compliant and non-compliant food samples.