Understanding and unlocking the potential of graphene oxide nanosheets as antibacterial agents

Past research revealed the antibacterial behavior of nanocarbons. However, the mechanisms of nanocarbons’ bacterial inhibition actions are not entirely clear, and both the toxicity concerns and performance issues remain. This thesis tackles these problems systematically, focusing on the graphene oxide (GO), likely the most promising nanocarbon currently being researched for antibacterial applications. Since the influence of physicochemical properties of GO has been previously studied intensively, this thesis focuses on the other two important players in GO-bacteria interactions: i) environmental abiotic factors (“the surrounding”), and ii) biotic factors (specifically, the role of bacterial physiology). As the first step, I demonstrate that the antibacterial efficacy of GO greatly depends on the environmental salinity. Under low salinity conditions, GO inhibited the bacterial cells more effectively thanks to the hypoosmotic stress. Also interestingly, a polymeric stabilizer, Pluronic, reduced the fibroblast toxicity of GO while it boosted the bacterial inhibition efficacy of GO against Gram-negative bacteria. However, the combination of hypoosmotic stress and Pluronic-GO exhibited considerably less activity on a model Gram-positive pathogen, suggesting the crucial role of bacterial envelope thickness in bacterial survival. In relation, I have explored the GO susceptibility of bacteria harvested at exponential, stationary, and decline growth phases. The cells harvested at stationary growth phase were less susceptible against GO. Therefore, I have also tested if Pluronic-GO mixture inhibits stationary-phase bacteria and observed its failure. Eventually, complementing GO with simple alcohols at low concentration, I have introduced a new method that dramatically enhances the antibacterial activity of GO against stationary-phase bacteria as well.Doctor of Philosophy (SCBE

Hydrogen-bonded multilayers of micelles of a dually responsive dicationic block copolymer

We report the fabrication of hydrogen-bonded multilayers of micelles of a dually responsive, dicationic block copolymer, poly[2-(N-morpholino)ethyl methacrylate-block-2-(diisopropylamino)ethyl methacrylate] (PMEMA-b-PDPA). By taking advantage of the difference in the hydrophilicity of PMEMA and PDPA blocks, micelles with a PMEMA-corona and a PDPA-core were obtained above pH 6.5 and were assembled layer-by-layer at the surface with tannic acid (TA) at pH 7.4 through hydrogen bonding interactions between morpholino units of PMEMA and hydroxyl groups of TA. Destruction of PMEMA-b-PDPA micelles/TA films could be controlled at both acidic and basic conditions. At basic pH (pH = 8.75), multilayers disintegrated due to ionization of TA and disruption of hydrogen bonding interactions between layers of micelles and TA. At moderately acidic pH values, partially dissolved PMEMA-b-PDPA micelles and monomers underwent a restructuring with TA molecules and remained adsorbed at the surface. Complete dissolution of the multilayers occurred at around pH 3.6 due to further protonation of the tertiary amino groups on both blocks of PMEMA-b-PDPA, resulting in a charge imbalance between PMEMA-b-PDPA and TA layers followed by disintegration of the films. We have also encapsulated pyrene in the micellar cores and found that pyrene released from PMEMA-b-PDPA micelles/TA films increased 1.5- and 2.5-fold when the pH was decreased from 7.5 to 6 and 5, respectively. Such an increase in the amount of pyrene released was due to pH-controlled dissolution of the micellar cores. We have also found that at pH 7.5, increasing the temperature to 40 degrees C enhanced the release of pyrene by approximately 2-fold. Such an increase is due to lower critical solution temperature (LCST) behaviour of coronal PMEMA chains leading to temperature-induced conformational changes on the coronal chains, facilitating the release of pyrene through the coronal chains into the solution. Hydrogen bonded multilayers of micelles of a dicationic block copolymer are interesting due to the response of both multilayers and micellar cores at different pH paving the way for multiple pH-controlled delivery of functional molecules from surfaces

Scalable fabrication of graphene-based laminate membranes for liquid and gas separations by crosslinking-induced gelation and doctor-blade casting

Graphene-based laminate membranes have recently demonstrated unique advantages over the traditional separation membranes made of polymers or ceramics. For both gas and liquid separations, various designs of crosslinker-stabilized graphene oxide (GO) membranes have been reported. However, the preparation methods used for those are poorly scalable for industrial applications. Herein, we report the fabrication of large-area (1333 cm2) GO-based membranes via doctor-blade casting of gel-like slurries prepared by incorporating calcium ions (Ca2+), ferric ions (Fe3+), polyethylene oxide (PEO), or polyethyleneimine (PEI) as crosslinkers. We found that all crosslinkers tested are suitable for the gelation of dilute GO dispersions (1–5 mg/mL) for doctor-blade casting. Besides, all crosslinked-GO membranes demonstrated outstanding stability under sonication compared to a GO-only membrane prepared via vacuum-assisted filtration. In aqueous nanofiltration tests, Fe3+-crosslinked GO membranes achieved virtually complete rejection of different organic dyes. PEO-crosslinked GO membranes, on the other hand, exhibited an outstanding performance for the separation of carbon dioxide (CO2) and nitrogen (N2) gases with a CO2/N2 selectivity of 52. Given the scaleup potential of doctor-blade casting and the practicality of crosslinking-based gelation approach, the proposed approach is promising to increase the industrial relevance of GO-based laminate membranes.Economic Development Board (EDB)This work was funded by GSK-EDB Trust Fund (PI: T.-H. Bae). T.- H. Bae would also like to thank KAIST for the financial support. We would also like to thank the Singapore Economic Development Board for funding support to Singapore Membrane Technology Centre

Structural colour enhanced microfluidics

マイクロ流体デバイスの製造に革新をもたらす新手法. 京都大学プレスリリース. 2022-05-19.New process revolutionizes microfluidic fabrication. 京都大学プレスリリース. 2022-05-19.Advances in microfluidic technology towards flexibility, transparency, functionality, wearability, scale reduction or complexity enhancement are currently limited by choices in materials and assembly methods. Organized microfibrillation is a method for optically printing well-defined porosity into thin polymer films with ultrahigh resolution. Here we demonstrate this method to create self-enclosed microfluidic devices with a few simple steps, in a number of flexible and transparent formats. Structural colour, a property of organized microfibrillation, becomes an intrinsic feature of these microfluidic devices, enabling in-situ sensing capability. Since the system fluid dynamics are dependent on the internal pore size, capillary flow is shown to become characterized by structural colour, while independent of channel dimension, irrespective of whether devices are printed at the centimetre or micrometre scale. Moreover, the capability of generating and combining different internal porosities enables the OM microfluidics to be used for pore-size based applications, as demonstrated by separation of biomolecular mixtures

Hydrothermal assembly of micro-nano-integrated core-sheath carbon fibers for high-performance all-carbon micro-supercapacitors

Wearable electronic devices (WEDs) require flexible, stable, and long-lasting power sources for their ever-expanding functionalities. Fiber-based micro-supercapacitors (FMSCs) are promising power solutions for novel WEDs because of their mechanical flexibility, small size and good integrability. Various porous carbon fibers have been explored as electrodes for FMSCs. However, current FMSCs often show poor rate capability due to modest electrical conductivity in fiber electrodes. Here, we demonstrate the synthesis of a micro-nano-integrated core-sheath fiber comprised of a microscale core made of commercial graphite fibers and a nanoscale hybrid sheath comprised of nitrogen doped graphene oxide sheets and multi-walled carbon nanotubes. The graphite fiber core provides fast electron transfer pathways, while the high surface area nano-hybrid sheath enables efficient capacitive energy storage. The core-sheath fiber achieves more than six times increases in capacitance retention compared to hybrid carbon fibers without the conductive core. Solid-state FMSCs were assembled using the core-sheath fibers as electrodes, which concurrently possess high length capacitance (12.8 mF cm−1) and volumetric capacitance (27 F cm−3), showing an energy density of 3.75 mWh cm–3 and a power density of 612 mW cm–3. Furthermore, multiple FMSCs can be easily assembled into flexible energy storage units with expanded voltage and current windows, demonstrating good potentials for practical applications in WEDs.Accepted versio

Cold Chain-Free Storable Hydrogel for Infant-Friendly Oral Delivery of Amoxicillin for the Treatment of Pneumococcal Pneumonia

10.1021/acsami.7b01462ACS APPLIED MATERIALS & INTERFACES92218440-1844

Enhanced O2/N2 Separation of Mixed-Matrix Membrane Filled with Pluronic-Compatibilized Cobalt Phthalocyanine Particles

Membrane-based air separation (O2/N2) is of great importance owing to its energy efficiency as compared to conventional processes. Currently, dense polymeric membranes serve as the main pillar of industrial processes used for the generation of O2- and N2-enriched gas. However, conventional polymeric membranes often fail to meet the selectivity needs owing to the similarity in the effective diameters of O2 and N2 gases. Meanwhile, mixed-matrix membranes (MMMs) are convenient to produce high-performance membranes while keeping the advantages of polymeric materials. Here, we propose a novel MMM for O2/N2 separation, which is composed of Matrimid® 5218 (Matrimid) as the matrix, cobalt(II) phthalocyanine microparticles (CoPCMPs) as the filler, and Pluronic® F-127 (Pluronic) as the compatibilizer. By the incorporation of CoPCMPs to Matrimid, without Pluronic, interfacial defects were formed. Pluronic-treated CoPCMPs, on the other hand, enhanced O2 permeability and O2/N2 selectivity by 64% and 34%, respectively. We explain the enhancement achieved with the increase of both O2 diffusivity and O2/N2 solubility selectivity

Metal-free bifunctional carbon electrocatalysts derived from zeolitic imidazolate frameworks for efficient water splitting

Metal-free carbon catalysts have attracted great interest because of their high electrical conductivity, tailorable porosity and surface area, affordability, and sustainability. In particular, their bifunctional activity for hydrogen and oxygen evolution reactions (HER and OER) is attractive for electrochemical splitting of water. However, pristine carbon materials have low activities for HER/OER. Here, a high-performance carbon electrocatalyst is demonstrated by first pyrolyzing a metal-organic framework (MOF), i.e., zeolitic imidazolate framework-8 (ZIF-8), followed by optimized cathodic polarization treatment (CPT). Pyrolyzing ZIF-8 produces a highly N-doped (8.4 at%) carbon material having a large specific surface area of 1017 m2 g1 with micro and mesopores. CPT in 0.5 M H2SO4 for up to 8 hours modulates the composition of N- and O-containing surface functional groups of the pyrolyzed ZIF-8 without sacrificing its large surface area and pore size distribution. After the 6-hour CPT, this material shows an excellent HER activity in 0.5 M H2SO4 electrolyte with an overpotential of 155 mV, a Tafel slope of 54.7 mV dec1, and an exchange current density of 0.063 mA cm2. And the 4-hour CPT results in excellent OER activity in 0.1 M KOH electrolyte with an overpotential of 476 mV and a Tafel slope of 78.5 mV dec1. In a demonstration, these two carbon electrocatalysts steadily run a two-electrode water electrolyzer at a current density of 10 mA cm2 over 8 hours under a potential of 1.82 V with a Faradaic efficiency of 98.0-99.1% in 0.1 M KOH electrolyte. The superior activity of the designed carbon electrocatalysts can be attributed to the functional group composition modulation achieved by CPT. High-performance metal-free carbon electrocatalysts derived from MOFs show excellent potentials for energy and environmental applications

Harnessing filler materials for enhancing biogas separation membranes

Biogas is an increasingly attractive renewable resource, envisioned to secure future energy demands and help curb global climate change. To capitalize on this resource, membrane processes and state-of-the-art membranes must efficiently recover methane (CH4) from biogas by separating carbon dioxide (CO2). Composite (a.k.a. mixed-matrix) membranes, prepared from common polymers and rationally selected/engineered fillers, are highly promising for this application. This review comprehensively examines filler materials that are capable of enhancing the CO2/CH4 separation performance of polymeric membranes. Specifically, we highlight novel synthetic strategies for engineering filler materials to develop high-performance composite membranes. Besides, as the matrix components (polymers) of composite membranes largely dictate the overall gas separation performances, we introduce a new empirical metric, the "Filler Enhancement Index" ( Findex), to aid researchers in assessing the effectiveness of the fillers from a big data perspective. The Findex systematically decouples the effect of polymer matrices and critically evaluates both conventional and emerging fillers to map out a future direction for next-generation (bio)gas separation membranes. Beyond biogas separation, this review is of relevance to a broader community with interests in composite membranes for other gas separation processes, as well as water treatment applications

Biomass-derived nanocarbon materials for biological applications: challenges and prospects

4 figures, 1 table.Biomass-derived nanocarbons (BNCs) have attracted significant research interests due to their promising economic and environmental benefits. Following their extensive uses in physical and chemical research domains, BNCs are now growing in biological applications. However, their practical biological applications are still in their infancy, requiring critical evaluations and strategic directions, which are provided in this review. The carbonization of biomass sources and major types of BNCs are introduced, encompassing carbon nanodots, nanofibres, nanotubes, and graphenes. Next, essential biological uses of BNCs, antibacterial/antibiofilm materials (nanofibres and nanodots) and bioimaging agents (predominantly nanodots), are summarized. Furthermore, the future potential of BNCs, for designing wound dressing/healing materials, water and air disinfection platforms, and microbial electrochemical systems, is discussed. We reach the conclusion that a crucial challenge is the structural control of BNCs. Furthermore, a key knowledge gap for realizing practical biological applications is the lack of systematic comparisons of BNCs with nanocarbons of synthetic origin in the current literature. Although we did not attempt to perform an exhaustive literature survey, the evaluation of the existing results indicates that BNCs are promising as easily accessible materials for various biomedically and environmentally relevant applications.Y. C. acknowledges financial support from the Australian Research Council under the Future Fellowships scheme (FT160100107) and a Tianjin city-sponsored short-term visiting project.Peer reviewe