Selective and asymmetric ion transport in covalent organic framework-based two-dimensional nanofluidic devices

Two-dimensional(2D)covalent organic framework(COF)mem-branes featuring well-aligned and programmable vertical nano-channels have emerged as a promising candidate for advanced nanofluidic devices and showcased vast potential in the fields of smart-gating,ion-separation,and energy-harvesting.However,the transverse interlayer nanochannels with a height of sub-nanometer-scale in 2D-COF membranes have scarcely been studied in com-parison.Here,we report the ion transport characteristics in 2D interlayer nanochannels of protonated CoF membranes.The dis-tinct surface-charge-governed ionic conductance in domination of electrolyte concentration below 1o-3 M as well as the exceptional anion/cation(Cl^(-)/K^(+))selectivity is revealed due to the pronounced charge and nano-confinement effects.Additionally,evident ion current rectification is witnessed when incorporating asymmetric geometry into the system,which is attributed to the dynamic process of ion enrichment and dissipation within the protonated nanochannels.This work offers immense prospects for 2D-COF membranes in the fields of biomimetic nanofluidic devices and cutting-edge electronic devices.

Fabrication of all-transparent polymer-based and encapsulated nanofluidic devices using nano-indentation lithography

In this paper,we describe a novel and simple process for the fabrication of all-transparent and encapsulated polymeric nanofluidic devices using nano-indentation lithography.First,a nanomechanical probe is used to‘scratch’nanoscale channels on polymethylmethacrylate(PMMA)substrates with sufficiently high hardness.Next,polydimethylsiloxane(PDMS)is used twice to duplicate the nanochannels onto PDMS substrates from the‘nano-scratched’PMMA substrates.A number of experiments are conducted to explore the relationships between the nano-indentation parameters and the nanochannel dimensions and to control the aspect ratio of the fabricated nanochannels.In addition,traditional photolithography combined with soft lithography is employed to fabricate microchannels on another PDMS‘cap’substrate.After manually aligning the substrates,all uncovered channels on two separate PDMS substrates are bonded to achieve a sealed and transparent nanofluidic device,which makes the dimensional transition from microscale to nanoscale feasible.The smallest dimensions of the achievable nanochannels that we have demonstrated thus far are of~20 nm depth and~800 nm width,with lengths extendable beyond 100μm.Fluid flow experiments are performed to verify the reliability of the device.Two types of colloidal solution are used to visualize the fluid flow through the nanochannels,that is,ethanol is mixed with gold colloid or fluorescent dye(fluorescein isothiocyanate),and the flow rate and filling time of liquid in the nanochannels are estimated based on time-lapsed image data.The simplicity of the fabrication process,bio-compatibility of the polymer substrates,and optical transparency of the nanochannels for flow visualization are key characteristics of this approach that will be very useful for nanofluidic and biomolecular research applications in the future.

Theoretical studies and molecular dynamics simulations on ion transport properties in nanochannels and nanopores

Control of ion transport and fluid flow through nanofluidic devices is of primary importance for energy storage and conversion, drug delivery and a wide range of biological processes. Recent development of nanotechnology, synthesis techniques, purification technologies, and experiment have led to rapid advances in simulation and modeling studies on ion transport properties. In this review, the applications of Poisson-Nernst-Plank （PNP） equations in analyzing transport properties are presented. The molecular dynamics （MD） studies of transport properties of ion and fluidic flow through nanofluidic devices are reported as well.

Soft electrostatic trapping in nanofluidics

Trapping and manipulation of nano-objects in solution are of great interest and have emerged in a plethora of fields spanning from soft condensed matter to biophysics and medical diagnostics.We report on establishing a nanofluidic system for reliable and contact-free trapping as well as manipulation of charged nano-objects using elastic polydimethylsiloxane(PDMS)-based materials.This trapping principle is based on electrostatic repulsion between charged nanofluidic walls and confined charged objects,called geometry-induced electrostatic(GIE)trapping.With gold nanoparticles as probes,we study the performance of the devices by measuring the stiffness and potential depths of the implemented traps,and compare the results with numerical simulations.When trapping 100 nm particles,we observe potential depths of up to Q≅24 k_(B)T that provide stable trapping for many days.Taking advantage of the soft material properties of PDMS,we actively tune the trapping strength and potential depth by elastically reducing the device channel height,which boosts the potential depth up to Q~200 k_(B)T,providing practically permanent contactfree trapping.Due to a high-throughput and low-cost fabrication process,ease of use,and excellent trapping performance,our method provides a reliable platform for research and applications in study and manipulation of single nano-objects in fluids.