Fabrication of a sensor based upon polyphenylenediamine/NiO/MWCNTs nano-composite for determination of morphine

Molecularly imprinted polymers (MIPs) are among artificial selective receptors designed based on molecular imprinting technology which are introduced in 1931 [1]. MIPs are synthesized by copolymerization process of the functional monomers and a cross-linking agent in the presence of template molecules. The extrication of template molecules results in creation of cavities with specific binding sites bearing defined shape, size and functional groups for the analyte. The created cavities are potential to trap the template selectively from the mixtures containing closely related molecules [2]. A morphine-molecularly imprinted polymer (MIP), composed of polyphenylenediamine, nickel oxide/multi-walled carbon nanotube composite and triphenylamine, as cross linker, was synthesized. The deportation step was done by a mixture of methanol using a soxhlet apparatus for 6 days to extract the sieved particles. Comparison of the cyclic voltammograms of a blank solution with that of a solution containing 250 mg L-1 of morphine in the buffer 7 (0.1 mol L-1) confirmed the complete extraction of the template from the polymer. The prepared MIP was used for modification of carbon paste electrode (CPE) to fabricate a selective morphine electrochemical sensor. Electrochemical behavior of morphine on the investigated modified electrode was studied. This study showed an oxidation peak for morphine at at 0.375 V (vs. SCE). The parameters affecting the sensor performance were investigated by using cyclic voltammetry technique. In order to achieve to the optimal MIP–CP composition, various amounts of the prepared MIP was used with 0.01 g graphite, 0.005 g paraffin. The highest anodic peak intensity was obtained by applying a composition of 0.01 g graphite, 0.005 g paraffin and 0.0005 g of MIP, for [reparation of the MIP-CP electrode. The effect of solution pH was investigated by recording the corresponding CVs in the pH range of 2.0−12.0. A maximum peak current was found by adjusting the solution to pH 9.0. The influence of scan rate and the electrode reaction process was studied. Based on the evaluated R2 values of the plots of Ip-ν and Ip-ν1/2, a diffusion-controlled process was suggested for the electrode process. A confirmation for this argument achieved by plotting logIp versus logν [3]. This plot for the acquired data was accompanied with a slope of ~0.4, revealing that the role of diffusion is greater than adsorption role. It was confirmed that the anodic peak current increased linearly with the analyte concentration in the range 1.00 – 1500.00 mg L-1 with a limit of detection 0.01 mg L-1. The selectivity of the proposed sensor was tested by determination of morphine in solutions containing some similar structures. The sensor was successfully applied for determination of morphine in some biological samples.