Salt Gradient Control of Translocation Dynamics in a Solid-State Nanopore.

Tuning capture rates and translocation time of analytes in solid-state nanopores are one of the major challenges for their use in detecting and analyzing individual nanoscale objects via ionic current measurements. Here, we report on the use of salt gradient for the fine control of capture-to-translocation dynamics in 300 nm sized SiNx nanopores. We demonstrated a decrease up to a factor of 3 in the electrophoretic speed of nanoparticles at the pore exit along with an over 3-fold increase in particle detection efficiency by subjecting a 5-fold ion concentration difference across the dielectric membrane. The improvement in the sensor performance was elucidated to be a result of the salt-gradient-mediated electric field and electroosmotic flow asymmetry at nanochannel orifices. The present findings can be used to enhance nanopore sensing capability for detecting biomolecules such as amyloids and proteins.

Gate-All-Around Nanopore Osmotic Power Generators.

Nanofluidic channels in a membrane represent a promising avenue for harnessing blue energy from salinity gradients, relying on permselectivity as a pivotal characteristic crucial for inducing electricity through diffusive ion transport. Surface charge emerges as a central player in the osmotic energy conversion process, emphasizing the critical significance of a judicious selection of membrane materials to achieve optimal ion permeability and selectivity within specific channel dimensions. Alternatively, here we report a field-effect approach for in situ manipulation of the ion selectivity in a nanopore. Application of voltage to a surround-gate electrode allows precise adjustment of the surface charge density at the pore wall. Leveraging the gating control, we demonstrate permselectivity turnover to enhanced cation selective transport in multipore membranes, resulting in a 6-fold increase in the energy conversion efficiency with a power density of 15 W/m2 under a salinity gradient. These findings not only advance our fundamental understanding of ion transport in nanochannels but also provide a scalable and efficient strategy for nanoporous membrane osmotic power generation.

Enhanced Nanoparticle Sensing in a Highly Viscous Nanopore.

Slowing down translocation dynamics is a crucial challenge in nanopore sensing of small molecules and particles. Here, it is reported on nanoparticle motion-mediated local viscosity enhancement of water-organic mixtures in a nanofluidic channel that enables slow translocation speed, enhanced capture efficiency, and improved signal-to-noise ratio by transmembrane voltage control. It is found that higher detection rates of nanoparticles under larger electrophoretic voltage in the highly viscous solvents. Meanwhile, the strongly pulled particles distort the liquid in the pore at high shear rates over 103 s-1 which leads to a counterintuitive phenomenon of slower translocation speed under higher voltage via the induced dilatant viscosity behavior. This mechanism is demonstrated as feasible with a variety of organic molecules, including glycerol, xanthan gum, and polyethylene glycol. The present findings can be useful in resistive pulse analyses of nanoscale objects such as viruses and proteins by allowing a simple and effective way for translocation slowdown, improved detection throughput, and enhanced signal-to-noise ratio.

Scalability of nanopore osmotic energy conversion.

Artificial nanofluidic networks are emerging systems for blue energy conversion that leverages surface charge-derived permselectivity to induce voltage from diffusive ion transport under salinity difference. Here the pivotal significance of electrostatic inter-channel couplings in multi-nanopore membranes, which impose constraints on porosity and subsequently influence the generation of large osmotic power outputs, is illustrated. Constructive interference is observed between two 20 nm nanopores of 30 nm spacing that renders enhanced permselectivity to osmotic power output via the recovered electroneutrality. On contrary, the interference is revealed as destructive in two-dimensional arrays causing significant deteriorations of the ion selectivity even for the nanopores sparsely distributed at an order of magnitude larger spacing than the Dukhin length. Most importantly, a scaling law is provided for deducing the maximal membrane area and porosity to avoid the selectivity loss via the inter-pore electrostatic coupling. As the electric crosstalk is inevitable in any fluidic network, the present findings can be a useful guide to design nanoporous membranes for scalable osmotic power generations.

Regulating Nonlinear Ion Transport through a Solid-State Pore by Partial Surface Coatings.

Using functional nanofluidic devices to manipulate ion transport allows us to explore the nanoscale development of blue energy harvesters and iontronic building blocks. Herein, we report on a method to alter the nonlinear ionic current through a pore by partial dielectric coatings. A variety of dielectric materials are examined on both the inner and outer surfaces of the channel with four different patterns of coated or uncoated surfaces. Through controlling the specific part of the surface charge, the pore can behave like a resistor, diode, and bipolar junction transistor. We use numerical simulations to find out the reason for the asymmetric ion transport in the pore and illustrate the relationship between specifically charged surfaces and electroosmotic flow. These findings help understand the role of the corresponding surface composition in ion transport, which provides a direct approach to modify the electroosmotic-flow-driven ionic current rectification in the channel-based device via dielectric coatings.

Protocol for preparation of solid-state multipore osmotic power generators.

Nanopore is an emerging energy-harvesting device that can create electricity directly from salt solutions. Here, we present a protocol for the preparation and structure optimization of solid-state multipore osmotic power generators. We describe steps for sculpting multiple pores at well-defined positions in a thin SiNx membrane using electron-beam lithography. We also detail an imprinting technique to form polydimethylsiloxane blocks with fluidic channels bonded to the multipore membrane. This approach facilitates repeated liquid-exchange processes involved in ionic current measurements. For complete details on the use and execution of this protocol, please refer to Tsutsui et al.1.

Synthesis of 3,3'-dihydroxy-2,2'-diindan-1,1'-dione derivatives for tautomeric organic semiconductors exhibiting intramolecular double proton transfer.

To investigate potential applications of the 3,3'-dihydroxy-2,2'-biindan-1,1'-dione (BIT) structure as an organic semiconductor with intramolecular hydrogen bonds, a new synthetic route under mild conditions is developed based on the addition reaction of 1,3-dione to ninhydrin and the subsequent hydrogenation of the hydroxyl group. This route affords several new BIT derivatives, including asymmetrically substituted structures that are difficult to access by conventional high-temperature synthesis. The BIT derivatives exhibit rapid tautomerization by intramolecular double proton transfer in solution. The tautomerizations are also observed in the solid state by variable temperature measurements of X-ray diffractometry and magic angle spinning 13C solid-state NMR. Possible interplay between the double proton transfer and the charge transport is suggested by quantum chemical calculations. The monoalkylated BIT derivative with a lamellar packing structure suitable for lateral charge transport in films shows a hole mobility of up to 0.012 cm2 V-1 s-1 with a weak temperature dependence in an organic field effect transistor.

Interference of electrochemical ion diffusion in nanopore sensing.

Stable and fast-responding ionic current is a prerequisite for reliable measurements of small objects with a nanopore. Here, we report on the interference of ion diffusion kinetics at liquid-electrode interfaces in nanopore sensing. Using platinum as electrodes, we observed a slow and large decrease in the ionic current through a nanopore in a salt solution suggestive of the considerable influence of the growing impedance at the liquid-metal interfaces via Cottrell diffusion. When detecting nanoparticles, the resistive pulses became weaker following the steady increase in the resistance at the partially polarizable electrodes. The interfacial impedance was also demonstrated to couple with the nanopore chip capacitance thereby degraded the temporal resolution of the ionic current measurements in a time-varying manner. These findings can be useful for choosing the suitable size and material of electrodes for the single-particle and -molecule analyses by ionic current.

Electroosmosis-Driven Nanofluidic Diodes.

Fundamental understanding of ion transport in a fluidic channel is of critical importance for realizing iontronics. Here we report on asymmetric ion transport in a low thickness-to-diameter aspect ratio nanopore. Under uniform salt concentration conditions, the cross-pore ionic current showed ohmic characteristics with no bias polarity dependence. In stark contrast, despite the weak ion selectivity expected for the relatively large nanopores employed, we observed diode-like behavior when a salt gradient was imposed across the thin membrane. This unexpected result was attributed to the electroosmotic flow that served to modulate the access resistance through dragging the condensed ions into or out of the nanopore orifices. The simple mechanism was also revealed to be effective in fluidic channels of various size from micro- to nanoscale enabling rectification of the property engineering by the pore geometries. The present findings allow for novel designs of artificial ion channel building blocks.

Quasi-Stable Salt Gradient and Resistive Switching in Solid-State Nanopores.

Understanding and control of ion transport in a fluidic channel is of crucial importance for iontronics. The present study reports on quasi-stable ionic current characteristics in a SiNx nanopore under a salinity gradient. An intriguing interplay between electro-osmotic flow and local ion density distributions in a solid-state pore is found to induce highly asymmetric ion transport to negative differential resistance behavior under a 100-fold difference in the cross-membrane salt concentrations. Meanwhile, a subtle change in the salinity gradient profile led to observations of resistive switching. This peculiar characteristic was suggested to stem from quasi-stable local ion density around the channel that can be switched between two distinct states via the electro-osmotic flow under voltage control. The present findings may be useful for neuromorphic devices based on micro- and nanofluidic channels.

Machine learning-driven electronic identifications of single pathogenic bacteria.

A rapid method for screening pathogens can revolutionize health care by enabling infection control through medication before symptom. Here we report on label-free single-cell identifications of clinically-important pathogenic bacteria by using a polymer-integrated low thickness-to-diameter aspect ratio pore and machine learning-driven resistive pulse analyses. A high-spatiotemporal resolution of this electrical sensor enabled to observe galvanotactic response intrinsic to the microbes during their translocation. We demonstrated discrimination of the cellular motility via signal pattern classifications in a high-dimensional feature space. As the detection-to-decision can be completed within milliseconds, the present technique may be used for real-time screening of pathogenic bacteria for environmental and medical applications.

Back-Side Polymer-Coated Solid-State Nanopore Sensors.

We systematically investigated the influence of polymer coating on temporal resolution of solid-state nanopores. We fabricated a Si3N4 nanopore integrated with a polyimide sheet partially covering the substrate surface. Upon detecting the nanoparticles dispersed in an electrolyte buffer by ionic current measurements, we observed a larger resistive pulse height along with a faster current decay at the tails under larger coverage of the polymeric layer, thereby suggesting a prominent role of the water-touching Si3N4 thin film as a significant capacitor serving to retard the ionic current response to the ion blockade by fast translocation of particles through the nanopores. From this, we came up with back-side polymer-coated chip designs and demonstrated improved pore sensor temporal resolution by developing a nanopore with a thick polymethyl-methacrylate layer laminated on the bottom surface. The present findings may be useful in developing integrated solid-state nanopore sensors with embedded nanochannels and nanoelectrodes.