UC Santa Cruz

The bactericidal utility of inorganic chemicals predates back to 1500 BP where Egyptians recorded the use of copper as an antibacterial agent. Recently, inorganic nanomaterials like silver and copper are utilized for sutures to prevent bacterial infections. The biomedical application of nanomaterials is imperative due to the threatening increase in antibiotic resistance bacteria which is projected to become a leading cause of death in 2050 [1]. The diminished antibacterial activity of antibiotics is attributed to their cellular target specificity which lead to rapid bacterial resistance. In contrast to antibiotics nanomaterials (ex. CuNP) produce bactericidal activity by providing broad-spectrum activity. Therein the indiscriminatory activity of inorganic nanoparticles provide multiple cellular targets leading to efficient bactericidal activity. Aggregation and ionization of metal-based nanoparticles may lead to cytotoxicity in a host’s cells; a lower concentration of chemically stable metal-based nanoparticle can enhance biocompatibility. The application of photoactive metal-oxide based nanocomposites exerts bactericidal activity at a markedly lower concentration in contrast to metal-based nanoparticles. In the present thesis we focus on exploring the photoactivity of a MnxOy/GOQD nanocomposite and its source of antibacterial activity.Chapter 2 describes the design of GOQD/MnO2 via oxidation of graphene oxide with KMnO4 as an oxidizer to produce an effective photocatalyst for bactericidal activity. A series of KMnO4 feed ratios was utilized to deposited varying concentrations of MnO2 onto GOQD. The deposition of MnO2 was analyzed via high resolution transmission electron microscopy (HRTEM) by the (211) lattice present in α-MnO2 nanowires. The chemical composition of the composite nanostructures was analyzed by X-ray photoelectron spectroscopy (XPS) to analyze the chemical composition and concentration of deposited MnO2. An optimal concentration of 7.85% Mn/C contained the most efficient photoactivity against E. coli at a concentration of 0.1 mg/mL, which is lower than the accounted MIC of the catalyst occurring at 5 mg/mL. The enhanced photoactivity was measured via electron paramagnetic resonance (EPR) which attributed the enhanced photoactivity to hydride and hydroxide radical production. In chapter 3 a series of first row transition metals with attributed bactericidal activity were deposited via a facile sonication method. The doping of graphene oxide was accounted by HRTEM and XPS which both demonstrated the presence of metal ions without the presence of lattice oxygen, commonly measured from metal oxide structures. The enhanced photodynamic activity against E.coli is documented for GOQD-Co2+ at an excitation of 365 nm. Analysis of time resolved fluorescence lifetime (TRFL) demonstrates an extended lifetime at the excited state for Co2+-GOQD. The diminished methylene blue degradation (MBD) demonstrates bactericidal activity mechanism independent of ROS formation. In chapter 4 polyaniline (PANI) was utilized to anchor MnxOy onto the GOQD structure. The deposition of various MnxOy nanoparticles was assessed via HRTEM while the oxidation state of the Mn was measured via XPS. The bactericidal activity was measured in the dark and under light irradiation for a gram-negative E.coli and gram-positive S.Epidermidis. The enhanced photodegradation determined from an Ellman’s reagent assay and EPR found a higher valency MnxOy deposited on GOQD with PANI to produce a higher concentration of ROS, specifically hydroxyl radical species (HO.-). Although the enhanced bactericidal activity was evident from GPM-1 and GPM-2 samples which produced a lower concentration of HO.-. The scanning electron microscopy (SEM) data details the intimate nanocomposite to cellular interface in both GPM-1 and GPM-2 samples therein shedding light on its photoinduced bactericidal mechaism