Ionophoric and aptameric recognition-modulated electroactive polyaniline films for the determination of tetrodotoxin
Tetrodotoxin (TTX) is a nonpeptidic neurotoxin with a high rate of food poisoning mortality (60%) that has been associated with the consumption of diets from puffer fish and mud snails harbouring TTX-producing bacteria. As this neurotoxin has no known antidote and could not be mitigated by cooking, the only way for safety appears to be the detection of TTX-contaminated fishes at the points of harvest and control. The overall aim of this study was to develop amperometric and impedimetric sensors for TTX based on ionophores and aptamer immobilised on the modified conducting electroactive polyaniline (PANI)/electrode. The undoped polyaniline and poly(4-styrenesulfonic acid) (PSSA) doped electroactive polyanilines were prepared in perchloric acid/acetonitrile and phosphoric acid respectively by electrochemical oxidative polymerisation. Two types of electropolymerisation were applied to prepare the neutral and p-doped PANI−PSSA films composites. The dynamic electroinactivity of TTX was studied which revealed that TTX is not electrochemically active on bare Au, GC, Pt, PG, Ni, Ti and BDD (Boron dopeddiamond) electrodes in acetate buffer pH 4.8. Using ion transfer voltammetry and UV-Vis analysis, the complexation of TTX with two neutral ionophores (sodium ionophore X (NaX) and dibenzo-18-crown6 (B18C6)) was investigated. The cyclic voltammograms (CVs) recorded from ion transfer voltammetry presented no redox peak and no increasing/decreasing current was observed which indicates that no TTX ions transfer from the liquid to the organic phase. In addition, the absorption spectra of the mixture of TTX/NaX and TTX/B18C6 presented the same absorption bands recorded for NaX and B18C6 respectively. Three absorptions bands at 250.4, 278.3, and 370.6 nm for NaX and two at 222.03 and 274.10 nm for B18C6 were observed before and after mixing TTX with NaX and TTX with B18C6 separately. No chemical reaction occurred between the TTX and both ionophores, therefore, sodium ionophore X and dibenzo-18-crown-6 did not form a complex with TTX. Thus, TTX ion sensor cannot be developed based on these two neutral compounds. The electrodynamics of the PANI and PANI−PSSA films electropolymerised on the bare precious metal electrodes were also investigated through various electrochemical techniques. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) studies in sodium phosphate (SPB) and acetate (OAc) buffer revealed that both neutral and p-doped films synthesized were thin (thickness L < 5 nm in acetate buffer and L < 10 nm in sodium phosphate buffer) film polymers.