Graphene Oxide Quantum Dots Incorporated into a Thin Film Nanocomposite Membrane with High Flux and Antifouling Properties for Low-Pressure Nanofiltration

Incorporation of Cellulose Nanocrystals (CNCs) into the Polyamide Layer of Thin-Film Composite (TFC) Nanofiltration Membranes for Enhanced Separation Performance and Antifouling Properties

To achieve greater separation performance and antifouling properties in a thin-film composite (TFC) nanofiltration membrane, cellulose nanocrystals (CNCs) were incorporated into the polyamide layer of a TFC membrane for the first time. The results of Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy (XPS) confirmed the successful formation of the CNC-polyamide composite layer. Surface characterization results revealed differences in the morphologies of the CNC-TFC membranes compared with a control membrane (CNC-TFC-0). Streaming potential measurements and molecular weight cutoff (MWCO) characterizations showed that the CNC-TFC membranes exhibited a greater negative surface charge and a smaller MWCO as the CNC content increased. The CNC-TFC membranes showed enhanced hydrophilicity and increased permeability. With the incorporation of only 0.020 wt % CNCs, the permeability of the CNC-TFC membrane increased by 60.0% over that of the polyamide TFC without CNC. Rejection of Na₂SO₄ and MgSO₄ by the CNC-TFC membranes was similar to that observed for the CNC-TFC-0 membrane, at values of approximately 98.7% and 98.8%, respectively, indicating that divalent salt rejection was not sacrificed. The monovalent ion rejection tended to increase as the CNC content increased. In addition, the CNC-TFC membranes exhibited enhanced antifouling properties due to their increased hydrophilicity and more negatively charged surfaces.

Cellulose-graphene quantum dot composite membranes using ionic liquid

Selective separation of small molecules by membranes is inhibited by the performance gap between nanofiltration and ultrafiltration membranes. In this work, a membrane that can efficiently remove small molecules (> 300 Da) was created by incorporating graphene oxide quantum dots (GQDs) into a cellulose membrane using an ionic liquid (1-ethyl-3-methylimidazolium acetate). Incorporation of GQD into cellulose membranes using an ionic liquid brings several advantages over traditional mixed matrix membranes: 1) GQDs are abundant in peripheral hydroxyl and carboxyl groups, thus GQDs have strong binding with cellulose through hydrogen bonding and forms a stable composite membrane. 2) Negative surface charge of GQDs helps prevent aggregation. 3) The size (5 nm) of GQD is smaller than most nanoparticles used in membranes, allowing for interesting pore forming properties. GQD-cellulose membranes were prepared by non-solvent induced phase separation in water. It was determined that about 45% of GQDs are incorporated from solution to membrane. GQDs were determined to be located on the membrane surface, giving the membrane negative surface charge and improved hydrophilicity. GQDs showed no leaching after convective flow through the membrane. Impact of GQD on membrane permeability and rejection was studied through convective flow experiments, and through longer term permeability studies.

Progress of Nanocomposite Membranes for Water Treatment

The use of membrane-based technologies has been applied for water treatment applications; however, the limitations of conventional polymeric membranes have led to the addition of inorganic fillers to enhance their performance. In recent years, nanocomposite membranes have greatly attracted the attention of scientists for water treatment applications such as wastewater treatment, water purification, removal of microorganisms, chemical compounds, heavy metals, etc. The incorporation of different nanofillers, such as carbon nanotubes, zinc oxide, graphene oxide, silver and copper nanoparticles, titanium dioxide, 2D materials, and some other novel nano-scale materials into polymeric membranes have provided great advances, e.g., enhancing on hydrophilicity, suppressing the accumulation of pollutants and foulants, enhancing rejection efficiencies and improving mechanical properties and thermal stabilities. Thereby, the aim of this work is to provide up-to-date information related to those novel nanocomposite membranes and their contribution for water treatment applications.

Aptamer based fluorometric sulfamethazine assay based on the use of graphene oxide quantum dots

The authors have developed a homogeneous "off-on" fluorometric method for the determination of the antibiotic sulfamethazine (SMZ). Aptamer against SMZ was labeled with graphene oxide quantum dots upon which the Graphene oxide quenched the blue fluorescence of the GOQDs. On addition of SMZ, the aptamers will bind SMZ and this will cause the release of GOQDs. As a result, fluorescence will be regenerated. Fluorescence, best measured at excitation/emission wavelengths of 365/455 nm, increases linearly in the 8 pg·mL^-1 to 60 ng·mL^-1 SMZ concentration range, with a 5 pg·mL^-1 detection limit. The method is reliable and was successfully applied to the determination of SMZ in spiked milk samples, with recoveries ranging from 89 to 96% depending on analyte concentration. Graphical abstract Graphene oxide quantum dots (GOQDs) were covalently bound to the aptamer (apt) against sulfamethazine (SMZ) and adsorbed on the surface of graphene oxide (GO). This results in quenching of the fluorescence of GOQDs. On addition of SMZ, fluorescence is restored due to the release of GOQD@apt from GO.

Carbon Quantum Dots Grafted Antifouling Membranes for Osmotic Power Generation via Pressure-Retarded Osmosis Process

Osmotic power generated by pressure-retarded osmosis (PRO) has attracted global attention as a clean, abundant and renewable energy resource. However, the substrates of PRO membranes are particularly prone to fouling because of their direct contact with various foulants in raw water. This leads to a significant decline in power density and impedes the commercialization of PRO technology. In this work, a facile surface modification method has been developed to obtain a new type of nanoparticle functionalized antifouling PRO membranes. Carbon quantum dots (CQDs), with an average size around 3.2 nm, are fabricated from citric acid via a simple method. Subsequently, they are immobilized onto the polydopamine (PDA) layer grafted on the substrate surface of poly(ether sulfone) (PES) membranes via covalent bonding. The bacteria diffusion tests show that the CQD modified PRO membranes possess much enhanced antibacterial activity and antibiofouling propensity. The continuous PRO operations at 15 bar also confirm that the CQD modified membranes exhibit a much higher power density (11.0 vs 8.8 W/m²) and water recovery after backwash (94 vs 89%) than the unmodified ones. This study may open up a new avenue in the fabrication of nanostructure functionalized polymeric membranes for wastewater treatment and osmotic power generation.

Charge-Gated Ion Transport through Polyelectrolyte Intercalated Amine Reduced Graphene Oxide Membranes

Charge-gated channels are nature's solutions for transport of water molecules and ions through aquaporins in biological membranes while excluding undesired substances. The same mechanism has good potentials to be adopted in pressure or electrically driven membrane separation processes. Herein, we report highly charged nanochannels created in polyelectrolyte (PE) intercalated amine reduced graphene oxide membrane (PE@ArGO membrane). The PE@ArGO membrane, with a rejection layer of ∼160 nm in thickness, features a laminate structure and a smooth top surface of a low roughness (typically ∼17.2 nm). Further, a modified PE@ArGO membrane (mPE@ArGO membrane) was developed in situ using free chlorine scavenging post-treatment method, which was designed to alter the charge while keeping alteration to the layered structure minimal. The surface charge of the PE@ArGO and mPE@ArGO membrane was +4.37 and -4.28 mC/m² respectively. In pressure driven processes, the pure water permeability for PE@ArGO and mPE@ArGO was 2.9 and 10.8 L m^-2 h^-1 bar^-1, respectively. Salt rejection is highly dependent on the charge density of the membrane surface, the valence of the co-ions and the size of ions in hydrated form. For example, in the positively charged PE@ArGO membranes, the rejection of the salts follows the order of: R(MgCl₂), 93.0% > R(NaCl), 88.2% ≈ R(MgSO₄), 88.1% > R(Na₂SO₄), 65.1%; while in the negatively charged mPE@ArGO membranes, the rejection of the salts follows the order of: R(Na₂SO₄), 90.3% > R(NaCl), 85.4% > R(MgSO₄), 68.3% > R(MgCl₂), 42.9%. To the best knowledge of the authors, this is the first study to report graphene oxide based membranes (GOBMs) with high density positive/negative charge gated ion transport behavior. What's more, the high rejection rate along with high water permeability of the PE@ArGO and mPE@ArGO membranes has not been achieved by other types of GOBMs.

Mussel-Inspired Architecture of High-Flux Loose Nanofiltration Membrane Functionalized with Antibacterial Reduced Graphene Oxide-Copper Nanocomposites

Graphene-based nanocomposites have a vast potential for wide-ranging antibacterial applications due to the inherently strong biocidal activity and versatile compatibility of such nanocomposites. Therefore, graphene-based functional nanomaterials can introduce enhanced antibiofouling and antimicrobial properties to polymeric membrane surfaces. In this study, reduced graphene oxide-copper (rGOC) nanocomposites were synthesized as newly robust biocides via in situ reduction. Inspired by the emerging method of bridging ultrafiltration membrane surface cavities, loose nanofiltration (NF) membranes were designed using a rapid (2 h) bioinspired strategy in which rGOC nanocomposites were firmly codeposited with polydopamine (PDA) onto an ultrafiltration support. A series of analyses (SEM, EDS, XRD, XPS, TEM, and AFM) confirmed the successful synthesis of the rGO-Cu nanocomposites. The secure loading of rGOC composites onto the membrane surfaces was also confirmed by SEM and AFM images. Water contact angle results display a high surface hydrophilicity of the modified membranes. The PDA-rGOC functionalization layer facilitated a high water permeability (22.8 L m^-2 h^-1 bar^-1). The PDA-rGOC modification additionally furnished the membrane with superior separation properties advantageous for various NF applications such as dye purification or desalination, as ultrahigh (99.4% for 0.5 g L^-1 reactive blue 2) dye retention and high salt permeation (7.4% for 1.0 g L^-1 Na₂SO₄, 2.5% for 1.0 g L^-1 NaCl) was achieved by the PDA-rGOC-modified membranes. Furthermore, after 3 h of contact with Escherichia coli (E. coli) bacteria, the rGOC-functionalized membranes exhibited a strong antibacterial performance with a 97.9% reduction in the number of live E. coli. This study highlights the use of rGOC composites for devising loose NF membranes with strong antibacterial and separation performance.