Magnetic nanoparticle modified electrodes for voltammetric determination of propellant stabiliser diphenylamine

The overall aim of the work was to advance electrochemical devices capable of analysis of forensically relevant residues using rapid electrochemical sensor technology. In order to achieve this, electrochemical detection of the propellant stabiliser diphenylamine (DPA) was achieved via voltammetry with signal enhancement realised in the presence of iron oxide nanoparticle modified transducers. This allowed both mechanistic and analytical evaluation with the aim to achieve the required selectivity and sensitivity for reliable detection. DPA electrochemistry was examined at glassy carbon electrodes in aqueous (3:7 methanol: sodium acetate pH 4.3) electrolyte via potential sweeping, with an irreversible wave at E_p = 0.67 V vs. Ag/AgCl. The diffusion coefficient (D) for the oxidation process was calculated as 1.43 × 10^-6 cm² s^-1 with αn_a = 0.7. DPA electrochemistry in a non aqueous methanol/acetonitrile electrolyte resulted in a D value of 5.47 × 10^-8 cm² s^-1 with αn_a = 0.5. Electrochemical preparation of magnetic iron oxide nanoparticles was achieved via electrooxidation of an iron anode in the presence of an amine surfactant followed by characterisation with SEM/EDX, XRD, FTIR and thermal analysis. A surface confined layer of these magnetic nanoparticles served to positively influence the response to DPA while impeding formation of surface confined oxidation products, with generation of an improved analytical signal - sensitivity 1.13× 10^-3 A cm^-2 mM^-1 relative to bare electrode response (9.80 × 10^-4 A cm^-2 mM^-1) over the range 0.5-50 μM DPA using differential pulse voltammetry, with LOD 3.51 × 10^-6 M and LOQ 1.17 × 10^-5 M. Real sample analysis involved recovery and differential pulse voltammetry of unburnt and burnt gunshot residue with DPA qualitative and quantitative analysis.