Porous silica was synthesized via the sol-gel process using clay obtained locally from Ijero-Ekiti in Ekiti State, Nigeria and compared with silica synthesized under similar conditions from sodium metasilicate (Na 2 SiO 3 ) obtained comercially. The clay was initially refluxed with sodium hydroxide (NaOH) for 2 hours to extract SiO 2 to form Na 2 SiO 3 , which was subsequently hydrolyzed to form a gel. The gel obtained was washed with deionized water to get rid of impurities, dried and calcined at 800˚C for 3 hours. The obtained silica powders were characterized using atomic absorption spectrophotometer, Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD) and scanning electron microscopy (SEM). Results showed that the vibrational modes and diffraction patterns of the silica derived from commercial Na 2 SiO 3 and that prepared from clay were similar containing pure amorphous SiO 2 . The morphology of the commercially obtained silica showed better arrangement of particles and exhibited slightly lesser porosity (62.4%) compared to that derived from clay which had a porosity of 65.5%. The result indicates that clay has a potential for use as an environmentally safe and economic starting material for preparing porous silica instead of high quality precursors.

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Metal-organic frameworks (MOFs) have gained developing interest due to their high specific surface area and pore volume, which has been exploited for gas storage, sensors and, drug delivery. This study presents the synthesis of a non-toxic, biocompatible and thermally stable MIL-53(Fe) and the preparation of its silver(I) nitrate nano-composite. This MIL-53(Fe) is a three-dimensional porous solid composed of infinite FeO 4 (OH) 2 cluster connected by 1,4-benzenedicarboxylate (H 2 BDC) ligand using solvothermal method of synthesis and the encapsulation process was also carried out to produce a composite composed of silver nanoparticle (AgNP). The synthesized materials were characterized using Powder X-ray Diffractometer (PXRD), Scanning Electron Microscope coupled with Electron Diffraction X-ray Spectrometer (SEM-EDS) and Fourier Transform Infrared (FT-IR) Spectroscopy. The Ag@MIL-53(Fe) composite exhibits a remarkable antifungal activity against Aspergillus flavus using a poison plate method. This can be attributed to the therapeutic nature of nanoparticle with a range of 55-64% growth inhibition rate as the concentration of the Ag@MIL-53(Fe) was increased. Minimum lethal concentrations (MLC) were observed to be 40 μg/mL and 15 μg/mL for the prepared MIL-53(Fe) and the Ag@MIL-53(Fe) composite, respectively.

Highlights • Two 3-aminopyridine derived Schiff bases were synthesized. • Six cobalt (II) complexes of the Schiff bases were synthesized. • The Schiff bases and the cobalt (II) complexes were characterized. • Antimicrobial and brine shrimps activities of the complexes and the Schiff bases were examined Two biologically active Schiff base GL1 and GL2 were synthesized in equimolar reaction of 3-aminopyridine with o-vanillin and p-vanillin. The synthesized Schiff bases were reacted with cobalt acetate, cobalt chloride or cobalt chloride with trimethylamine by using a molar ratio of Schiff base: cobalt salt as 2:1. The characterization of Schiff bases and cobalt complexes was done by UV-visible in methanol, diffused reflectance, FTIR (mid and 1 13 far), mass spectrometry studies while H NMR and C NMR were used to further characterize the Schiff bases. The FTIR results confirmed the bidentate binding of the Schiff bases with cobalt center after the formation of the complexes. The UV-visible spectra and diffused reflectance spectra data obtained were used to further investigate the electronic structure as well as the properties of Schiff bases and their cobalt complexes. On the basis of the spectral studies, distorted tetrahedral geometry has been assigned for the Co(II) complexes. The antimicrobial activities of the two Schiff bases GL1, GL2 and their cobalt(II) complexes were tested against laboratory isolated Staphylococcus aureus, Pseudomonas aeruginosa, Escherichia coli and one fungus strain, Aspergillus niger. The antimicrobial evaluation demonstrates significant activity of the Schiff bases and the cobalt complexes against the four known human pathogens. The brine shrimps lethality study confirmed that the compounds are concentration and geometry dependent. The studies showed the complexes were higher in biological activity than the corresponding Schiff bases.

Bioactive glass ceramic with SiO 2 -Ca 2 O-Na 2 O-P 2 O 5 composition was prepared by the sol-gel method using sodium metasilicate (Na 2 SiO 3 ) as silica source. The monolith obtained was sintered at 1000˚C for 2 hours after which X-ray diffraction (XRD) analysis showed presence of combeite (Na 2 Ca 2 Si 3 O 9 ) as the crystalline phase. In vitro bioactivity test conducted on the material using simulated body fluid (SBF) showed the formation of carbonated hydroxyapatite on its surface. The material during the SBF test was observed to transform from a mechanically strong crystalline phase Na 2 Ca 2 Si 3 O 9 to an amorphous phase after incubation for 14 days indicating that the material was biodegradable. Scanning electron microscopy (SEM) was used to investigate the surface morphology, while Fourier transform infrared (FTIR) spectroscopy facilitated the confirmation of hydroxyapatite (HA) formation. The monolith material obtained may be a good candidate for application in tissue engineering scaffolds.

Bioceramic material of the quaternary system; SiO 2 -CaO-Na 2 O-P 2 O 5 that has composition similar to Bioglass ® 45S5 was prepared by the sol-gel method from locally obtained bentonite clay (BTC). The monolith obtained was sintered at 1000˚C for 2 h to facilitate densification and phase transformation. X-ray diffraction (XRD) analysis revealed the presence of sodium calcium silicate, Na 2 Ca 2 Si 3 O 9 as major crystal phase, and another secondary orthorhombic phase, NaCaPO 4 . Fourier transform infrared (FTIR) spectroscopic investigation confirmed the presence of Si-O-Si bonds and a crystalline phosphate in the glass network. Scanning electron microscopy (SEM) revealed a network of micropores and interconnected macropores. Overall, the material displays features amenable for possible utilization in tissue engineering scaffolds.

The synthesis, characterization and antimicrobial activity of Cu(II) complexes of some p-substituted aniline Schiff base ligands have been carried out. The Schiff bases were obtained from salicylaldehyde and o-vanillin. The Cu(II) complexes have been characterized by elemental analysis, conductivity measurement, infrared and electronic spectral data. The complexes were obtained either as metal chelates [Cu(L)2] or Schiff base adducts (CuCl2.2LH).xH2O. The metal chelates were non-electrolytes while the Schiff base adducts exhibited 1:1 or 2:1 electrolytes in methanol. The Cu(II) complexes exhibited slight antimicrobial activity against Escherichia coli ATCC® 8739™*, Staphylococcus aureus subsp. aureus ATCC® 6538™*, Bacillus subtilis subsp. spizizenii ATCC® 6633™* and Candida albicans ATCC® 2091™*. The complexes exhibited significant antifungal activity.                     KEY WORDS: Metal Chelates, Schiff bases, Adducts, Cu(II) complexes, Salicylaldimines   Bull. Chem. Soc. Ethiop. 2021, 35(1), 33-42. DOI: https://dx.doi.org/10.4314/bcse.v35i1.3

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