Sustainable Antimicrobial Nanocomposites Using Functionalized Cellulose Nanocrystals

Abstract

The primary focus of this thesis is to exploit the application of cellulose nanocrystals (CNC) as environmentally friendly antimicrobial nanomaterials. Chitosan, a readily available natural polysaccharide, was combined with CNC to prepare various types of nanohybrid constructs via electrostatic coating or chemical grafting. The surface charge, morphology, stability, rheology, antimicrobial properties, and biocompatibility of these nanohybrids were quantified and elucidated. These innovative antimicrobial nanohybrid systems hold promise for a multitude of applications spanning industries such as textiles, tissue engineering, surgical materials, water treatment, agriculture, and food packaging. A fully biobased colloidal antimicrobial nanohybrid system, comprising cellulose nanocrystals (CNC), chitosan, and a chitosan derivative, was examined. The research focused on the modification of chitosan by varying the degree of substitution, and the preparation of CNC-CS nanohybrids in both acidic and neutral pH. By examining the surface charge, particle size, morphology, and acid-base conductometric titration, the electrostatic coating process of chitosan onto the surface of CNC was elucidated. Lastly, the antimicrobial efficacy of the CNC-CS nanohybrid was assessed through a simple and rapid antifungal assay protocol. A comparative analysis between electrostatically coated and chemically grafted glycidyltrimethylammonium chloride modified-chitosan (GCS) on cellulose nanocrystals (CNC) was conducted. The hypothesis is that through grafting GCS onto modified CNC, the resulting CNC-GCS hybrid could potentially enhance the colloidal stability while maintaining its superior antimicrobial properties. Surface-functionalized aldehyde-CNC would interact with GCS leading to the formation of a durable grafted CNC-CS nanohybrid. The effect of surface charge, stability, rheological properties, antimicrobial efficacy, and biocompatibility of this innovative hybrid system was elucidated. It was found that the covalently attached and reduced rDCGCS exhibited the most potent antimicrobial property, with a minimum inhibitory concentration (MIC) of 150 μg/mL against yeast and a minimum bactericidal concentration (MBC) between 200-400 μg/mL against S. aureus. However, due to its mechanism of action primarily relying on electrostatic interactions, the nanohybrid system demonstrated lower efficacy against gram-negative bacteria. Based on the grafting system methods described earlier, further enhancement of the antimicrobial properties was examined by the in-situ inclusion of silver nanoparticles (AgNPs) on the CNC-CS system. The hypothesis was whether the integration of AgNPs with the CNC-CS nanohybrid could facilitate both the sustainable production of AgNPs and a significant boost in the antimicrobial efficacy of the hybrid material. The preparation procedure, surface charge analysis, stability assessment, antimicrobial performance, and elucidation of the mechanism of action of this composite system were elucidated. When compared to DAC-Ag, CCS-Ag displayed elevated positive charge, reaching a zeta potential of up to +60 mV. Additionally, it exhibited superior capping capabilities resulting in more uniform and smaller AgNPs size, along with exceptional stability. The antibacterial effectiveness was notably enhanced and possessed a MBC ranging from 50-100 μg/mL against S. aureus and 100-200 μg/mL against E. coli. The CNC-CS composite systems can serve as the carrier for loading and encapsulating a model antibacterial compound, triclosan that will enhance the sustainability of the system for practical applications. A comparative analysis was conducted between pristine CNC, CNC-CS coating, and CNC-CS grafting systems, where the stability, loading efficiency, morphological characteristics, antimicrobial efficacy, and the underlying mechanisms of action for each of these systems were examined. The research revealed that the CNC-GCS coating system exhibited the highest loading capacity, successfully accommodating and encapsulating up to 5% of triclosan (TCS) within the nanohybrid. The resulting CCS-TCS displayed a fourfold improvement in antifungal performance, showcasing a MIC ranging from 100-200 μg/mL. Furthermore, it possessed potent antibacterial properties, with a MBC of 50-100 μg/mL against gram-positive bacteria and 100-500 μg/mL against gram-negative bacteria. Additionally, a novel polymer nano-brush based on cellulose nanocrystal (CNC) utilizing a "graft-from" technique incorporating 2-(dimethylamino)ethyl methacrylate (DMAEMA) was prepared and investigated. This polymer-grafted CNC structure features tertiary ammonium groups within its side chains that could be quaternized. The surface charge characteristics, morphological attributes, extent of quaternization, and antimicrobial properties associated with this quaternary ammonium polymer-grafted CNC structure were examined. Subsequently, the PCNC was successfully quaternized with benzyl bromide, achieving a degree of quaternization (DQ) of up to 51%. The resulting QCNC displayed strong antimicrobial efficacy, presenting a MBC of 50-100 μg/mL against gram-positive bacteria and 100-200 μg/mL against gram-negative bacteria. In comparison to CNC-CS, QCNC exhibited improved antimicrobial properties characterized by a smaller and more uniform particle size.