A europium metal-organic framework for dual Fe3+ ion and pH sensing.

Metal-organic frameworks (MOFs) with ratiometric sensing properties are desirable for many applications due to their intrinsic self-calibration. We report the re-assessment of the sensing properties of a MOF, originally reported as containing europium(III) and 2-hydroxyterephtalic acid, and having fluorescent ratiometric iron(III) sensing properties. Synchrotron single-crystal X-ray diffraction and proton nuclear magnetic resonance (1H NMR) spectroscopy revealed that the MOF is composed of 2-methoxyterephthalate, not 2-hydroxyterephthalate as originally reported. We found that the MOF exhibits a sensor turn-off response towards Fe3+ ion concentrations in the range 0.5-3.7 ppm (band 425 nm), and a turn-on response towards a decrease of pH from 5.4 to 3.0 (band 375 nm), both resulting from the addition of acidic Fe3+ salt solution to a MOF suspension. Thus, the ratiometric sensing properties and the originally proposed mechanism no longer apply; our work reveals a dynamic quenching mechanism for the fluorescence turn-off response due to the presence of Fe3+ ions, and a ligand protonation mechanism for the turn-on response to a decrease in pH. Our work highlights the importance of a thorough investigation of the structure of any newly synthesized MOF, and, in the case of potential sensors, their selectivity and any environmental effects on their sensing behavior.

Cellulose synthesis by Komagataeibacter rhaeticus strain P 1463 isolated from Kombucha.

Isolate B17 from Kombucha was estimated to be an efficient producer of bacterial cellulose (BC). The isolate was deposited under the number P 1463 and identified as Komagataeibacter rhaeticus by comparing a generated amplified fragment length polymorphism (AFLP™) DNA fingerprint against a reference database. Static cultivation of the K. rhaeticus strain P 1463 in Hestrin and Schramm (HS) medium resulted in 4.40 ± 0.22 g/L BC being produced, corresponding to a BC yield from glucose of 25.30 ± 1.78 %, when the inoculum was made with a modified HS medium containing 10 g/L glucose. Fermentations for 5 days using media containing apple juice with analogous carbon source concentrations resulted in 4.77 ± 0.24 g/L BC being synthesised, corresponding to a yield from the consumed sugars (glucose, fructose and sucrose) of 37.00 ± 2.61 %. The capacity of K. rhaeticus strain P 1463 to synthesise BC was found to be much higher than that of two reference strains for cellulose production, Komagataeibacter xylinus DSM 46604 and Komagataeibacter hansenii DSM 5602T, and was also considerably higher than that of K. hansenii strain B22, isolated from another Kombucha sample. The BC synthesised by K. rhaeticus strain P 1463 after 40 days of cultivation in HS medium with additional glucose supplemented to the cell culture during cultivation was shown to have a degree of polymerization of 3300.0 ± 122.1 glucose units, a tensile strength of 65.50 ± 3.27 MPa and a length at break of 16.50 ± 0.83 km. For the other strains, these properties did not exceed 25.60 ± 1.28 MPa and 15.20 ± 0.76 km.

Characterisation of films and nanopaper obtained from cellulose synthesised by acetic acid bacteria.

Bacterial cellulose (BC) samples were obtained using two culture media (glucose and glucose+fructose) and two bacteria (Komagataeibacter rhaeticus and Komagataeibacter hansenii). Nanopaper was obtained from the BC through oxidation and both were studied to determine the impact of culture media and bacteria strain on nanofiber structure and mechanical properties. AFM and SEM were used to investigate fibre dimensions and network morphology; FTIR and XRD to determine cellulose purity and crystallinity; carboxyl content, degree of polymerisation and zeta potential were used to characterise nanofibers. Tensile testing showed that nanopaper has up to 24 times higher Young's modulus (7.39GPa) than BC (0.3GPa). BC displayed high water retention values (86-95%) and a degree of polymerisation up to 2540. Nanofibers obtained were 80-120nm wide and 600-1200nm long with up to 15% higher crystallinity than the original BC. It was concluded that BC is an excellent source for easily obtainable, highly crystalline and strong nanofibers.