A spectroscopic and imaging investigation of sporopollenin-metal interactions

Alkhatib, Fatmah Mohammad

Abstract

The main objective has been the determination of the nature of the interaction of metal complexes and salts with the surface of sporopollenin exine capsules (SEC) and other naturally occurring spore exines. These natural materials derived from plant pollen and spores can interact with the inorganic compounds via the formation of coordination bonds and can impart unusual properties. This makes them ideal materials for investigation as they can be used in a wide variety of applications including catalysis, metal remediation, imaging and biological delivery. Although there have been many studies using brown SECs, the focus of this work has been on bleached SECs as the paler colour enables the use of a wider variety of spectroscopic techniques. The first row transition metals copper, iron, nickel, and zinc were used and a selection of spectroscopic techniques (IR, Raman, ICP-OES, UV-Vis, NMR, EPR, Mössbauer and X-ray absorption spectroscopy) were employed to characterise the complexes formed between the metals and the brown and white SECs. Metal loadings of 0.5 to 10 wt% were observed for all the metals and both the brown and bleached SECs. The higher loadings were observed for complexes derived from acetate salts for copper, nickel and zinc. IR spectroscopy of the SECs revealed the presence of aliphatic chains and hydroxyls, aliphatic carbons, carbonyls, unsaturation, ester and ether groups. For the copper, nickel and zinc complexes the IR spectra showed the presence of νCO modes indicating the mode of coordination, and in the acetate complexes there were a significant difference in the spectrum indicating the presence of bound acetate. The IR spectra of the iron complexes were different indicating the presence of a different structural motif.

The UV-vis spectra displayed the characteristic features of metal(II) for copper, nickel and zinc, with the spectra of the complexes prepared from chloride and nitrate solutions being essentially identical, but different to that from the acetate solution. There were slight differences between all the spectra of the iron-BL-SEC complexes, with the complex derived from SEC and solution of (NH4)Fe(SO4)2 presenting the highest intensity peak.

The 13C NMR solid state spectra of the brown and bleached SECs, as well as those of the zinc complexes showed peaks characteristic of aliphatic, olefinic and aromatic carbon, C-O, carboxylic acid and ester groups. For the zinc acetate with BL-SECs was different in the 35 – 10 ppm aliphatic region compared to that of zinc nitrate and zinc chloride with BL-SEC. However, the spectra were similar for all zinc salts with BR-SECs.

The metal K-edge EXAFS data from the nickel and zinc complexes were more similar to the copper data rather than the iron data, indicating the presence of mononuclear octahedral metal complexes for nickel and zinc. For copper, the EXAFS data indicated the presence of a Jahn-Teller distortion for copper, which was confirmed by EPR spectroscopy. In the case of iron the EXAFS data indicated the formation of small oxide or oxyhydroxide particles. The metal K-edge XANES data confirmed the presence of Cu(II), Ni(II) and Zn(II), but indicated that in the case of iron, this was present as Fe(III), which was also consistent with the 57Fe Mössbauer data. XRF imaging data using synchrotron radiation has shown that the metal distribution in the copper, nickel and zinc complexes is very closely associated with the underlying physical structure of the SEC.

The conclusion from all the spectroscopic data is that the most likely source of coordination of the copper, nickel and zinc metals are the carboxylate groups in the sporopollenin structure resulting in monomeric complexes on the sporopollenin surface. The structures of the chloride and nitrate complexes are very similar, but in the case of acetate there is evidence for the presence of acetate ligands in the coordination environment. The higher metal loadings observed for the acetate complexes can be explained by this as the metal does not require so many ligand groups in close proximity on the SEC surface. In the case of iron, the structures appear to be different, and are based on very small clusters of iron oxide or oxyhydroxide particles attached to the SEC surface.

Citation

Alkhatib, F. M. A spectroscopic and imaging investigation of sporopollenin-metal interactions. (Thesis). University of Hull. https://hull-repository.worktribe.com/output/4220724

• Thesis Type: Thesis
• Deposit Date: Oct 8, 2018
• Publicly Available Date: Feb 23, 2023
• Keywords: Chemistry
• Public URL: https://hull-repository.worktribe.com/output/4220724
• Additional Information: Department of Chemistry, The University of Hull
• Award Date: Aug 1, 2017