Silk-cellulose nanofiber membranes for application in water treatment

Nyakombi, Priscilla (2019-12)

Thesis (MSc)--Stellenbosch University, 2019.

Thesis

ENGLISH ABSTRACT: Electrospinning (ES) of biopolymers down to nanoscale size has received considerable interest in efforts to address specific millennia problems, which include materials for application in medical and water treatment spheres. The main aim of this study was to fabricate silk fibroin (SF) and cellulose (CE) nanofiber membranes via the ES method, then characterized for morphology using scanning electron microscopy and transmission electron microscopy, structurally using X-ray diffraction, Fourier Transform Infra-red spectrometry, Raman spectroscopy and Solid-State NMR. To establish thermal stability of the fabricated nanofibers when compared to the pure polymers, differential scanning calorimetry and thermal gravimetric studies were conducted. Several properties such as biodegradability, biocompatibility and nontoxic were considered with applications in filter media and tissue engineering in mind. Both cellulose and silk are natural fibers that degrade naturally, and they have remarkable mechanical properties, which, from an environmental friendliness point of view, a blended material from these biopolymers can be a good candidate, and secondly utilize the blended nanofibers as candidates in filtration media. Furthermore, both materials are mechanically strong and therefore can withstand the pressures associated with water treatment. A further advantage is that both silk and cellulose fibers have been reported to exhibit antimicrobial properties. Hence, it was envisaged that fiber combinations from these two biopolymers may have some degree of antimicrobial activity, which would open a wide range of applications. Randomly oriented SF/CMC nanofibrous mats were fabricated with average diameters of about 153 ± 20 nm. The electrospun mats were also crosslinked with N-(3-dimethylaminopropyl)-N’- ethylcarbodiimide hydrochloride (a nontoxic crosslinking agent) and N-hydroxysuccinimide to enhance water stability for water treatment applications. The antimicrobial activity of the fibers was examined against different bacteria including Staphylococcus aureus (Gram-positive) and Escherichia coli (Gram-negative). Various methods were used to assess the antibacterial activity, including zone inhibition and fluorescence imaging. Enzymatic biodegradation of the materials was studied using cellulase (Aspergillus niger) and protease type XIV (Streptomyces griseus). The degradation of the electrospun mats was confirmed using SEM and FTIR. From the results, SF/CMC fibers displayed diameter increases when compared to fibers obtained from pure silk solutions. Structural characterization using ATR-FTIR, XRD, Raman, and SolSt-13C NMR showed the transfiguration of SF from a random coil to a -sheet in the electrospun blends with CMC. The TGA results of the SF/CMC blended nanofibers exhibited thermal behavior very similar or better than that of SF nanofibers. The SF and SF/CMC blends nanofibers were all successfully crosslinked by EDC/NHS crosslinking agent. After crosslinking, it was observed that the nanofiber mats became more pliable and their average diameter increased compared to the non-crosslinked nanofiber mats and achieved more water stability as the crosslinking agent was increased. The results from zone inhibition tests showed that SF is slightly active against E. coli and CMC indicated positive result towards E. coli and moderate results towards S. aureus. The degradation of all electrospun nanofibers was investigated through the action of enzymes. Results show that SF/CMC nanocomposites can be classified as enzymatically degradable. The increase in weight loss as the degradation period increased was observed. From the findings, it was concluded that protease XIV degrades SF by breaking down the peptide bonds of the SF into amino acids and cellulase degrades CMC.

AFRIKAANS OPSOMMING: Die Elektrospin (ES) van biopolimere tot nanoskaalgrootte het groot belangstelling getrek in pogings om millennia-kwessies soos mediese materiale en velde wat verband hou met waterbehandeling aan te spreek. Die hoofdoel van hierdie studie was om syfibroien-sellulose nanovesel membraan via die ES tegniek te fabriseer en karakteriseer. Verskeie eienskappe was oorweeg met toepassings in filtermedia en weefselingenieurswese as agtergrond. Beide sellulose en sy, is natuurlike vesels wat natuurlik afbreek en interessante meganiese eienskappe besit wat hulle, vanuit ʼn omgewingsvriendelike oogpunt voordelig maak. Verder is albei materiale meganies sterk en kan dus die druk wat met waterbehandeling gepaard gaan hanteer. Nóg ʼn voordeel is dat syvesels as antimikrobies aangetoon word. Daar was dus beoog dat veselkombinasies tussen die twee biopolimere antimikrobies kon wees, wat ʼn wye verskeidenheid toepassings sou oplewer. Willekeurig georiënteerde SF/CMC nanovesel matte met gemiddelde diameters van ongeveer 153 ± 20 nm was vervaardig. Die morfologie van die matte was gekenmerk deur gebruik te maak van skanderingselektronmikroskopie (SEM) en transmissie-elektronmikroskopie, terwyl hul struktuur met behulp van X-straaldiffraksie, Fourier-transform infrarooi spektrometrie (FTIR), vastestandse kernmagnetiese resonansie en Raman-spektroskopie bepaal was. Om die termiese stabiliteit van die vervaardigde membraan, in vergelyking met dié van die suiwer polimere was met behulp van differensiële skanderingskalorimetrie en termiese gravimetriese studies uitgevoer. Die elektrospin matte was ook gekruis met N- (3-dimethylaminopropyl) -N'-ethylcarbodiimide hydrochloride ('n nie-toksiese kruisingsagent) en N-hydroxysuccinimide. Die antimikrobiese aktiwiteit van die vesels was in vergelyking met verskillende bakterieë, insluitend Staphylococcus aureus (Gram-positief) en Escherichia coli (Gram-negatief) ondersoek. Verskeie metodes soos sone inhibisie en fluoressensie beelding was gebruik om vir antimikrobiese aktiwiteit te toets. Die ensiematiese biodegradering van die materiale was met behulp van sellulase (Aspergillus niger) en protease tipe XIV (Streptomyces griseus) bestuddeer. Die agteruitgang van die elektrospun matte was ondersoek deur SEM en FTIR te gebruik. Resultate het getoon dat die deursnee van die elektronspin matte geleidelik toegeneem het na mate CMC toegevoeg is. Strukturele karakterisering met behulp van ATR-FTIR, XRD, Raman, en SolSt-13C NMR het die transformasie van SF vanaf 'n ewekansige spoel na 'n ß-vel in die elektrospin mengsels met CE getoon. Die SF/CMC gemengde nanovesels het hittegedrag getoon wat baie soortgelyk of beter is as dié van SF nanovesels was. Die SF en SF/CMC mengsel nanovesels was almal suksesvol gekruis deur ʼn EDC/NHS kruismiddel te gebruik. Na kruising was dit waargeneem dat die nanovesel matte meer buigbaar geword het en hul gemiddelde deursnee toegeneem het in vergelyking met die nie-gekruisde nanoveselmatte sowel as dat dit meer waterstabiliteit verkry het namate die kruisingsmiddel verhoog was. Die resultate van sone inhibisie toetse het getoon dat SF effens aktief is teen E. coli en CMC het positiewe aktiwiteit teenoor E. coli en matige aktiwiteit teenoor S. aureus getoon. Die afbreek van alle elektrospun nanofibers was deur die werking van ensieme ondersoek. Resultate het toon dat SF/CMC membraan as ensimaties afbreekbaar geklassifiseer kon word. Die toename in gewigsverlies namate die afbreekperiode verhoog was, is waargeneem. Uit die bevindinge was gevind dat protease XIV, SF degradeer, deur die peptiedbindings van die SF na aminosure af te breek en dat CMC afbreek van sellulose.

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