Surface modified electrodes as a novel technique for characterizing adansonia digitata fruit

Abstract

The results presented in this thesis show that electrochemical analysis have been successfully carried out on raw plant material of Adansonia digitata fruit. All the four components of the A. digitata fruit, i.e. the greenish hairy outer covering, the woody covering, the inner white pulp and the seed have redox active moieties and gave clearly defined cyclic voltammetric responses when studied using bare carbon working electrode. They all displayed oxidation/reduction peaks at various potentials. The results show that both hydrochloric and sulphuric acid electrolyte media are suitable for the analysis of A. digitata fruit components. High electrodeposition rates and redox efficiencies revealed the significant role of the proton to the redox process. This was also confirmed by the poor response obtained in the non-aqueous media containing CH3CN/HzO. Scan rate dependence studies showed that the processes were diffusion controlled on bare carbon working electrode but in the case of modified electrodes, there were cases of surface attached species. The processes in the white inner pulp and the seed were shown to be one proton, one electron, processes from the pH dependence studies. Modification of the working electrode with a conducting polymer, polyaniline, and clay montmorillonite gave cyclic voltammetric responses with very well defined peaks. The modified electrodes yielded enhanced peaks with potentials shifted negatively suggesting there was electrocatalysis. The largest shifts were observed in the inner pulp and the seed parts of the fruit. The response from the clay-modified electrodes on the other hand, for all the fruit components showed larger shifts as compared to those of polyaniline modified electrodes. Electrochemical signals of chemical standards of ascorbic acid, thiamine, riboflavin and chlorophyll, i.e. compounds known from other researchers to be present in parts of A. digitata fruit show that, the CV responses obtained from the standards closely match those in the actual plant materials. FTIR analysis confirmed the presence of the expected functional groups. The electrochemical signals on bare carbon confirmed the presence of flavonoids and terpenes in the woody part of the fruit. The electrochemical results in the modified electrodes also confirm the presence of ascorbic acid in the seed of Adansonia digitata. The results show that the ascorbic acid redox peak is greatly enhanced and the oxidation/reduction peaks coincide with those of the corresponding chemical standards. It is also shown that ascorbic acid is adsorbed on the clay modified electrodes. Physical parameters with respect to the various components show that the density of the whole fruit is O.3g/cm3 while for the individual parts the seed was found to have the highest density. The pulp had the highest moisture content, 12.5%, and it was the only part that was highly soluble in both organic and aqueous solvents. The other components were mainly soluble in organic solvents. Calorimetric measurements revealed that the seed has the highest internal energy and the internal energy of the edible parts gave values that are comparable to what has been obtained by other researchers in Senegal and Malawi.