Highly laminar graphene oxide flakes (10 to 20 μm in diameter) are fabricated. Reducing flake thickness to 10 nm enables water and organic solvent permeation, enabling the flakes to act as a highly effective organic solvent membrane. Graphene oxide (GO) membranes continue to attract intense interest due to their unique molecular sieving properties combined with fast permeation1,2,3,4,5,6,7,8,9. However, their use is limited to aqueous solutions because GO membranes appear impermeable to organic solvents1, a phenomenon not yet fully understood. Here, we report efficient and fast filtration of organic solutions through GO laminates containing smooth two-dimensional (2D) capillaries made from large (10–20 μm) flakes. Without modification of sieving characteristics, these membranes can be made exceptionally thin, down to ∼10 nm, which translates into fast water and organic solvent permeation. We attribute organic solvent permeation and sieving properties to randomly distributed pinholes interconnected by short graphene channels with a width of 1 nm. With increasing membrane thickness, organic solvent permeation rates decay exponentially but water continues to permeate quickly, in agreement with previous reports1,2,3,4. The potential of ultrathin GO laminates for organic solvent nanofiltration is demonstrated by showing >99.9% rejection of small molecular weight organic dyes dissolved in methanol. Our work significantly expands possibilities for the use of GO membranes in purification and filtration technologies.

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Ultrathin graphene-based membrane with precise molecular sieving and ultrafast solvent permeation

In the field of nanofluidics, it has been an ultimate but seemingly distant goal to controllably fabricate capillaries with dimensions approaching the size of small ions and water molecules. We report ion transport through ultimately narrow slits that are fabricated by effectively removing a single atomic plane from a bulk crystal. The atomically flat angstrom-scale slits exhibit little surface charge, allowing elucidation of the role of steric effects. We find that ions with hydrated diameters larger than the slit size can still permeate through, albeit with reduced mobility. The confinement also leads to a notable asymmetry between anions and cations of the same diameter. Our results provide a platform for studying the effects of angstrom-scale confinement, which is important for the development of nanofluidics, molecular separation, and other nanoscale technologies.

Size effect in ion transport through angstrom-scale slits

Graphene oxide (GO) membranes continue to attract intense interest due to their unique molecular sieving properties combined with fast permeation rates. However, the membranes' use has been limited mostly to aqueous solutions because GO membranes appear to be impermeable to organic solvents, a phenomenon not fully understood yet. Here, we report efficient and fast filtration of organic solutions through GO laminates containing smooth two-dimensional (2D) capillaries made from flakes with large sizes of ~ 10-20 micron. Without sacrificing their sieving characteristics, such membranes can be made exceptionally thin, down to ~ 10 nm, which translates into fast permeation of not only water but also organic solvents. We attribute the organic solvent permeation and sieving properties of ultrathin GO laminates to the presence of randomly distributed pinholes that are interconnected by short graphene channels with a width of 1 nm. With increasing the membrane thickness, the organic solvent permeation rates decay exponentially but water continues to permeate fast, in agreement with previous reports. The application potential of our ultrathin laminates for organic-solvent nanofiltration is demonstrated by showing >99.9% rejection of various organic dyes with small molecular weights dissolved in methanol. Our work significantly expands possibilities for the use of GO membranes in purification, filtration and related technologies.

/pdf/ultrathin-graphene-based-membrane-with-precise-molecular-4iqnuwlku6.pdf

Extensive environmental pollution caused by worldwide industrialization and population growth has led to a water shortage. This problem lowers the quality of human life and wastes a large amount of money worldwide each year due to the related consequences. One main solution for this challenge is water purification. State-of-the-art water purification necessitates the implementation of novel materials and technologies that are cost and energy efficient. In this regard, graphene nanomaterials, with their unique physicochemical properties, are an optimum choice. These materials offer extraordinarily high surface area, mechanical durability, atomic thickness, nanosized pores and reactivity toward polar and non-polar water pollutants. These characteristics impart high selectivity and water permeability, and thus provide excellent water purification efficiency. This review introduces the potential of graphene membranes for water desalination. Although literature reviews have mostly concerned graphene’s capability for the adsorption and photocatalysis of water pollutants, updated knowledge related to its sieving properties is quite limited. Water desalination plants stand to markedly reduce their energy consumption once nanoporous graphene-based membranes are optimized. Purifying brackish water with reverse osmosis works most efficiently when membranes are as thin, selective and strong as possible. Mady Elbahri from Aalto University in Finland reviews efforts to achieve these goals using graphene monolayers perforated with nanoscale pores. Recent roll-to-roll fabrications of graphene onto polymer supports show that inexpensive, large-scale production of these ultrathin membranes is feasible. It remains challenging, however, to tailor the nanochannels and selectivity of single-atom-thick material. An alternative may be to switch to multilayered graphene oxide membranes. These substances can be engineered with different molecular cross-linking agents between each carbon layer, opening room to insert negatively charged functional groups that target and electrostatically repel sodium ions. Graphene nanomaterials hold great promise for the development of advanced water purification membranes, especially for water desalination. Their atomic thickness, extraordinary mechanical stability and potential for size-selective transport are ideal features, encouraging the membrane scientist across the world to investigate their applicability for water desalination. Graphene can potentially desalinate water either as monolayer or as multilayer membranes. In this review, we discuss these different classes of graphene membranes and highlight their merits and shortcomings. In addition, the theory behind their performance is presented in detail.

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Graphene membranes for water desalination

This is a turning point for nanofluidics. Recent progress allows envisioning both fundamental discoveries for the transport of fluids at the ultimate scales, and disruptive technologies for the water–energy nexus.

https://hal.archives-ouvertes.fr/hal-02494717/document

Nanofluidics coming of age

Nanostructured graphene-oxide (GO) laminate membranes, exhibiting ultrahigh water flux, are excellent candidates for next generation nanofiltration and desalination membranes, provided the ionic rejection could be further increased without compromising the water flux. Using microscopic drift–diffusion experiments, we demonstrated the ultrahigh charge selectivity for GO membranes, with more than order of magnitude difference in the permeabilities of cationic and anionic species of equivalent hydration radii. Measuring diffusion of a wide range of ions of different size and charge, we were able to clearly disentangle different physical mechanisms contributing to the ionic sieving in GO membranes: electrostatic repulsion between ions and charged chemical groups; and the compression of the ionic hydration shell within the membrane’s nanochannels, following the activated behavior. The charge-selectivity allows us to rationally design membranes with increased ionic rejection and opens up the field of ion exchan...

Scalable Graphene-Based Membranes for Ionic Sieving with Ultrahigh Charge Selectivity

This letter reports the tunable liquid microlens using three laminar flows injected into an expansion chamber. Different lens shapes and curvatures can be achieved and tuned through the control of three flow rates. The expansion chamber is designed to improve the fluidic stability and maintain the ideal lens shape for precise microscale optical measurement. The optical aberration is also eliminated by minimizing the diffusive broadening at the interfaces. The collimation and focusing capabilities of three liquid microlenses are demonstrated. The tunable liquid microlens is promising as a tool to realize different optical components that can be integrated onto a microchip.

Different curvatures of tunable liquid microlens via the control of laminar flow rate

This paper presents a planar optofluidic lens for light manipulation utilizing a combination of optofluidic biconvex lens with micromixer. Three light manipulation techniques including tunable optical diverging, collimating and focusing are realized by altering the refractive index of the optofluidic variable-focus lenses formed by solid polydimethylsiloxane (PDMS) walls and tunable liquid lens body. The optical power from the laser input can be increased or decreased with the tuning of the variable-focus lenses' refractive indexes. The optical power adjustment capabilities are demonstrated and characterized. The combinations of benefits of all lens' optical manipulation capabilities, greater mechanical stability, significant increase of optofluidic device's life time and seamless integration with other lab-on-a-chip functionalities provide a promising and versatile optofluidic compartment to integrate with lab-on-a-chip excitation and sensing applications. Optofluidic lens-including system for tunable fluorescence sensing is demonstrated showing 186% increase in detected fluorescence intensity.

Optofluidic variable-focus lenses for light manipulation

This letter reports a tunable planar optofluidic switch as illustrated by three laminar flow streams introduced into a focusing chamber. Different width of liquid core can be tuned via the imposed flow rate of these three laminar flow streams. The hydrodynamic tunability of the core-cladding interfaces is the key to realize microscale optical switching via total internal reflection. The optical switching capability is demonstrated having good agreement with optical simulations. The optofluidic optical switch can achieve a switching speed of 1.56 Hz and beyond with the potential for a seamless integration with other lab-on-a-chip devices for optical sensing applications.

Tunable optofluidic switch via hydrodynamic control of laminar flow rate

This article reports on an optofluidic refractive index sensor based on partial refraction. The sample to be measured is injected into a cladding sensing microchannel. The refraction angle and the optical reflectivity change with the variation of the refractive index of the cladding liquid. The refractive index of liquid cladding can be determined by the characterization of the refraction angle and the optical reflectivity. It is demonstrated that the optofluidic refractive index sensor can detect refractive index in the range between 1.37 and 1.45. The sensor has the potential for a seamless integration with lab-on-a-chip devices for concentration and refractive index sensing applications.

Y. C. Seow

Papers

Scalable Graphene-Based Membranes for Ionic Sieving with Ultrahigh Charge Selectivity

Different curvatures of tunable liquid microlens via the control of laminar flow rate

Optofluidic variable-focus lenses for light manipulation

Tunable optofluidic switch via hydrodynamic control of laminar flow rate

An optofluidic refractive index sensor based on partial refraction