Regulating ion affinity and dehydration of metal-organic framework sub-nanochannels for high-precision ion separation.

Membrane consisting of ordered sub-nanochannels has been pursued in ion separation technology to achieve applications including desalination, environment management, and energy conversion. However, high-precision ion separation has not yet been achieved owing to the lack of deep understanding of ion transport mechanism in confined environments. Biological ion channels can conduct ions with ultrahigh permeability and selectivity, which is inseparable from the important role of channel size and "ion-channel" interaction. Here, inspired by the biological systems, we report the high-precision separation of monovalent and divalent cations in functionalized metal-organic framework (MOF) membranes (UiO-66-(X)2, X = NH2, SH, OH and OCH3). We find that the functional group (X) and size of the MOF sub-nanochannel synergistically regulate the ion binding affinity and dehydration process, which is the key in enlarging the transport activation energy difference between target and interference ions to improve the separation performance. The K+/Mg2+ selectivity of the UiO-66-(OCH3)2 membrane reaches as high as 1567.8. This work provides a gateway to the understanding of ion transport mechanism and development of high-precision ion separation membranes.

Photoenhanced Electrocatalytic Hydrogen Evolution Accelerated Dominantly by the Hot Electrons from Intraband Transition rather than Interband Transition.

Plasmonic materials enabling sunlight as an energy input to catalyze the hydrogen evolution reaction (HER) have become the research focus of artificial photosynthesis. Upon visible photoexcitation, there are both intraband transition and interband transition hot carriers generated, and which of them dominates the catalytic reaction remains elusive. Here, the contributions of hot electrons from intraband and interband transitions to the photoelectrocatalytic HER on plasmonic Au triangle nanoprisms (AuTNPs) have been studied. Compared with the dark reaction, the exchange current density increases by 9-fold and 3-fold under intraband and interband excitation, respectively, which is attributed to the higher energy level of intraband transition hot electrons. By calculation of the reaction activation energy with and without illumination, the contributions of the hot electrons from the two photoexcitation modes to the photoenhanced electroreduction reaction (PEER) are quantitatively analyzed, proposing the general standard to measure the effect of different hot electrons in different reactions.

Janus Metal-Organic Framework Membranes Boosting the Osmotic Energy Harvesting.

The unique ion-transport properties in nanoconfined pores enable nanofluidic devices with great potential in harvesting osmotic energy. The energy conversion performance could be significantly improved by the precise regulation of the "permeability-selectivity" trade-off and the ion concentration polarization effect. Here, we take the advantage of electrodeposition technique to fabricate a Janus metal-organic framework (J-MOF) membrane that possesses rapid ion-transport capability and impeccable ion selectivity. The asymmetric structure and asymmetric surface charge distribution of the J-MOF device can suppress the ion concentration polarization effect and enhance the ion charge separation, exhibiting an improved energy harvesting performance. An output power density of 3.44 W/m2 has been achieved with the J-MOF membrane at a 1000-fold concentration gradient. This work provides a new strategy for fabricating high-performance energy-harvesting devices.

Homochiral Zeolitic Imidazolate Framework with Defined Chiral Microenvironment for Electrochemical Enantioselective Recognition.

The recognition and separation of chiral molecules with similar structure are of great industrial and biological importance. Development of highly efficient chiral recognition systems is crucial for the precise application of these chiral molecules. Herein, a homochiral zeolitic imidazolate frameworks (c-ZIF) functionalized nanochannel device that exhibits an ideal platform for electrochemical enantioselective recognition is reported. Its distinct chiral binding cavity enables more sensitive discrimination of tryptophan (Trp) enantiomer pairs than other smaller chiral amino acids owing to its size matching to the target molecule. It is found that introducing neighboring aldehyde groups into the chiral cavity will result in an inferior chiral Trp recognition due to the decreased adsorption-energy difference of D- and L-Trp on the chiral sites. This study may provide an alternative strategy for designing efficient chiral recognition devices by utilizing the homochiral reticular materials and tailoring their chiral environments.

Single-Molecule Electrical and Spectroscopic Profiling Protein Allostery Using a Gold Plasmonic Nanopore.

Direct structural and dynamic characterization of protein conformers in solution is highly desirable but currently impractical. Herein, we developed a single molecule gold plasmonic nanopore system for observation of protein allostery, enabling us to monitor translocation dynamics and conformation transition of proteins by ion current detection and SERS spectrum measurement, respectively. Allosteric transition of calmodulin (CaM) was elaborately probed by the nanopore system. Two conformers of CaM were well-resolved at a single-molecule level using both the ion current blockage signal and the SERS spectra. The collected SERS spectra provided structural evidence to confirm the interaction between CaM and the gold plasmonic nanopore, which was responsible for the different translocation behaviors of the two conformers. SERS spectra revealed the amino acid residues involved in the conformational change of CaM upon calcium binding. The results demonstrated that the excellent spectral characterization furnishes a single-molecule nanopore technique with an advanced capability of direct structure analysis.

Single-cell thermometry with a nanothermocouple probe.

Herein, a nanopipette-based thermocouple probe that possesses high temperature resolution, rapid response, good reversibility and stability was constructed and successfully applied for single-cell temperature sensing. Different intracellular temperatures were observed in diverse types of cells, which reveals differences in their metabolism levels. Temperature responses of cancer and normal cells against various exogenous drugs were also demonstrated. The spatially resolved temperature sensing of three-dimensional cell culture models unveils the existence of their inner temperature gradients. This work would facilitate drug screening and disease diagnosis.

Synergistic Effect of Electrostatic Interaction and Ionic Dehydration on Asymmetric Ion Transport in Nanochannel/Ion Channel Composite Membrane.

Ion transport in nanochannels of a size comparable to that of hydrated ions exhibits unique properties due to the synergistic effect of various forces. Here, we design a nanochannel/ion channel composite (NIC) membrane that shows a high ion current rectification (ICR) ratio in different electrolytes. Experimental and theoretical results demonstrate that the synergistic effect of electrostatic interaction and ionic dehydration plays an important role in regulating the ICR behavior of the NIC membrane. We find that electrostatic attraction between ions and the channel surface in the ultraconfined space increases the probability of ionic dehydarion, resulting in different dehydration energy costs for different ions. This further alters the driving force for ion transport and thus regulates ICR of the NIC membrane. This work provides fundamental knowledge of ion transport in ion channels, which aids in the understanding of the function of biological systems and the design of high-performance nanochannel devices.

Light-Enhanced Osmotic Energy Harvester Using Photoactive Porphyrin Metal-Organic Framework Membranes.

High ion selectivity and permeability, as two contradictory aspects for the membrane design, highly hamper the development of osmotic energy harvesting technologies. Metal-organic frameworks (MOFs) with ultra-small and high-density pores and functional surface groups show great promise in tackling these problems. Here, we propose a facile and mild cathodic deposition method to directly prepare crack-free porphyrin MOF membranes on a porous anodic aluminum oxide for osmotic energy harvesting. The abundant carboxyl groups of the functionalized porphyrin ligands together with the nanoporous structure endows the MOF membrane with high cation selectivity and ion permeability, thus a large output power density of 6.26 W m-2 is achieved. The photoactive porphyrin ligands further lead to an improvement of the power density to 7.74 W m-2 upon light irradiation. This work provides a promising strategy for the design of high-performance osmotic energy harvesting systems.

Single Molecule DNA Analysis Based on Atomic-Controllable Nanopores in Covalent Organic Frameworks.

We explored the application of two-dimensional covalent organic frameworks (2D COFs) in single molecule DNA analysis. Two ultrathin COF nanosheets were exfoliated with pore sizes of 1.1 nm (COF-1.1) and 1.3 nm (COF-1.3) and covered closely on a quartz nanopipette with an orifice of 20 ± 5 nm. COF nanopores exhibited high size selectivity for fluorescent dyes and DNA molecules. The transport of long (calf thymus DNA) and short (DNA-80) DNA molecules through the COF nanopores was studied. Because of the strong interaction between DNA bases and the organic backbones of COFs, the DNA-80 was transported through the COF-1.1 nanopore at a speed of 270 µs/base, which is the slowest speed ever observed compared with 2D inorganic nanomaterials. This study shows that the COF nanosheet can work individually as a nanopore monomer with controllable pore size like its biological counterparts.

Revealing the Hydrogen Bonding Interaction of DNA with Unnatural Bases via Plasmonic Antenna Enhanced Infrared Spectroscopy.

The incorporation of unnatural bases in DNA programming can break through the limits of Watson-Crick and Hoogsteen base pairing to expand the diversity of DNA structures. Thus, understanding the interaction between DNA and unnatural bases is of great importance in DNA nanotechnology. Here, we propose an approach of plasmonic antenna enhanced infrared spectroscopy to study the hydrogen bonding interaction between poly(thymine) DNA (poly T DNA) and melamine. The formation of multiple hydrogen bonds between melamine and thymine of poly T DNA is revealed by the appearance of a new infrared (IR) feature of the NH2 deformation vibration at 1680 cm-1. The binding rate constant (kb) and the dissociation rate constant (kd) of the affinity reaction reach 39.70 M-1·s-1 and 4.49 × 10-5 s-1, respectively. This work offers a valuable IR technique to study DNA nanostructures at the molecular level, providing unique physicochemical views of the interaction mechanism between DNA and unnatural bases in DNA programming.

Probing Multidimensional Structural Information of Single Molecules Transporting through a Sub-10 nm Conical Plasmonic Nanopore by SERS.

Probing the orientation and oxygenation state of single molecules (SMs) is of great importance for understanding the advanced structure of individual molecules. Here, we manipulate molecules transporting through the hot spot of a sub-10 nm conical gold nanopore and acquire the multidimensional structural information of the SMs by surface enhanced Raman scattering (SERS) detection. The sub-10 nm size and conical shape of the plasmonic nanopore guarantee its high detection sensitivity. SERS spectra show a high correlation with the orientations of small-sized single rhodamine 6G (R6G) during transport. Meanwhile, SERS spectra of a single hemoglobin (Hb) reveal both the vertical/parallel orientations of the porphyrin ring and oxygenated/deoxygenated states of Hb. The present study provides a new strategy for bridging the primary sequence and the advanced structure of SMs.

Electrochemically Switchable Double-Gate Nanofluidic Logic Device as Biomimetic Ion Pumps.

Biological ion pumps with two separate gates can actively transport ions against the concentration gradient. Developing an artificial nanofluidic device with multiple responsive sites is of great importance to improve its controllability over ion transport to further explore its logic function and mimic the biological process. Here, we propose an electrochemical polymerization method to fabricate electrochemically switchable double-gate nanofluidic devices. The ion transport of the double-gate nanofluidic device can be in situ and reversibly switched among four different states. The logic function of this nanofluidic device is systematically investigated by assuming the gate state as the input and the transmembrane ionic conductance as the output. A biomimetic electrochemical ion pump is then established by alternately applying two different specific logic combinations, realizing an active ion transport under a concentration gradient. This work would inspire further studies to construct complex logical networks and explore bioinspired ion pump systems.

Reversible Electrochemical Tuning of Ion Sieving in Coordination Polymers.

Membrane-based ion separation is environmentally friendly, energy-efficient, and easy to integrate, being widely used in water desalination and purification systems. With the existing separation technologies, it is yet difficult to achieve real time, in situ, and reversible control of the separation process. Here, we design and fabricate a Prussian blue (PB) coordination polymer based membrane with uniform and electrochemically size-tunable subnanopores. The ion separation can be significantly and reversibly modulated through the electrochemical conversion between PB and Prussian white (PW). The permeation rates of small hydrated metal ions (Cs+ and K+) obviously increase upon switching from PB to PW, while the permeation rates of large hydrated metal ions (Li+, Na+, Mg2+, and La3+) remain constant. The membrane selectivity of small hydrated ions to large hydrated ions can be increased by more than 2 times during the electrochemical switch, which could be assigned to the slightly larger crystal size (e.g., pore window size) of PW than PB. The present approach provides a new strategy for constructing tunable seawater desalination and ion extraction systems.