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.

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.

In Situ Growth of Ultrasmall Nanochannels in Porous Anodized Aluminum Membrane and Applied in Detection of Lead Ion.

The transport of ions in nanochannels has received considerable interest owing to the unique transport properties and potential applications. In this study, ultrasmall nanochannels (0.8-1.2 nm) were fabricated in porous anodized aluminum (PAA) membrane in situ growth. 2-Methylimidazole and zinc nitrate were used as the reaction precursor solkution, and then zeolitic imidazolate framework (ZIF-8) nanoparticles were sufficiently filled in PAA nanochannels to form a ZIF-8/PAA nanochannels composite membrane. Because ZIF-8 is a microporous material and has a strong ability to adsorb heavy metals, the composite membrane was used as a biosensor to detect lead ion (Pb2+) by the coordination interaction between Pb2+ and nitrogen atoms. The detection limit reached to 0.03 nM due to the enrichment of nanochannels under electric field. The sensor has a good linear range for Pb2+ from 10 nM to 10 µM.

Surface Enhanced Raman Spectroscopy Detection of Sodium Thiocyanate in Milk Based on the Aggregation of Ag Nanoparticles.

A method is developed for detecting the concentration of sodium thiocyanate (NaSCN) in milk based on surface-enhanced Raman scattering (SERS) technology. A trichloroacetic acid solution can be used to enhance the SERS signal because of its function in promoting the aggregation of Ag nanoparticles (Ag NPs). Meanwhile, the protein in milk would be precipitated as trichloroacetic acid added and the interference from protein could be reduced during the detection. In this work, the enhancement factor (EF) is 7. 56 × 105 for sodium thiocyanate in water and the limit of detection (LOD) is 0.002 mg/L. Meanwhile, this method can be used to detect the concentration of sodium thiocyanate in milk. Results show that SERS intensity increased as the concentration of sodium thiocyanate increase from 10 to 100 mg/L. The linear correlation coefficient is R² = 0.998 and the detection limit is 0.04 mg/L. It is observed that the concentration of sodium thiocyanate does not exceed the standard in the three kinds of milk. The confirmed credibility of SERS detection is compared with conventional methods.

Electrochemical Determination of Nitrite by Au Nanoparticle/Graphene-Chitosan Modified Electrode.

A highly sensitive nitrite (NO2−) electrochemical sensor is fabricated using glassy carbon electrode modified with Au nanoparticle and grapheme oxide. Briefly, this electrochemical sensor was prepared by drop-coating graphene oxide-chitosan mixed film on the surface of the electrode and then electrodepositing a layer of Au nanoparticle using cyclic voltammetry. The electrochemical behavior of NO2− on the sensor was investigated by cyclic voltammetry and amperometric i-t curve. The results showed that the sensor exhibited better electrocatalytic activity for NO2− in 0.1 mol/L phosphate buffer solution (PBS) (pH 5.0). The oxidation peak current was positively correlated with NO2− concentration in the ranges of 0.9 µM to 18.9 µM. The detection limit was estimated to be 0.3 µM. In addition, the interference of some common ions (e.g., NO3−, CO32−, SO42−, Cl−, Ca2+ and Mg2+) and oxidizable compound including sodium sulfite and ascorbic acid in the detection of nitrite was also studied. The results show that this sensor is more sensitive and selective to NO2−. Therefore, this electrochemical sensor provided an effective tool for the detection of NO2−.

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.

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.

Preparation of hollow tubular TpBD COF and pod-like ZIF-8/H-TpBD COF tubes using a porous anodic aluminum oxide membrane as template.

By sacrificing a porous anodic aluminum oxide (AAO) membrane as a template, hollow tubular TpBD (H-TpBD) covalent organic framework (COF) tubes were synthesized in situ and zeolitic imidazolate framework (ZIF-8) nanoparticles were creatively synthesized in situ in H-TpBD tubes at room temperature. H-TpBD COF tubes and ZIF-8/H-TpBD COF tubes were procured by using a strong base or acid to remove the AAO membrane. Then they were analyzed by X-ray diffraction, Fourier infrared spectroscopy, scanning electron microscope, transmission electron microscope, etc. Surprisingly, the obtained TpBD COF has a very small aperture (1.8 nm), thinner tube thickness (50 nm), high stability, and a smooth and homogeneous surface. And the pod-like ZIF-8/H-TpBD COF with complete tubular structure was also obtained.

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.

Detection of Aflatoxin B1 Based on a Porous Anodized Aluminum Membrane Combined with Surface-Enhanced Raman Scattering Spectroscopy.

An Aflatoxin B1 (AFB1) biosensor was fabricated via an Ag nanoparticles assembly on the surface of a porous anodized aluminum (PAA) membrane. First, the Raman reporter 4-Aminothiophenol (4-ATP) and DNA (partially complementary to AFB1 aptamer) were attached to the surface of Ag nanoparticles (AgNPs) by chemical bonding to form a 4-ATP-AgNPs-DNA complex. Similarly, the surface of a PAA membrane was functionalized with an AFB1 aptamer. Then, the PAA surface was functionalized with 4-ATP-AgNPs-DNA through base complementary pairing to form AgNPs-PAA sensor with a strong Raman signal. When AFB1 was added, AgNPs would be detached from the PAA surface because of the specific binding between AFB1 and the aptamer, resulting in a reduction in Raman signals. The detection limit of the proposed biosensor is 0.009 ng/mL in actual walnut and the linear range is 0.01-10 ng/mL. The sensor has good selectivity and repeatability; it can be applied to the rapid qualitative and quantitative detection of AFB1.

In Situ Growth Visualization Nanochannel Membrane for Ultrasensitive Copper Ion Detection under the Electric Field Enrichment.

The transport of ionic species through nanochannels plays an important role in the basic research and practical application of nanofluidic devices. Here, a visualized CdSe@ZIF-8/PAA nanochannel membrane was created by employing in situ growth of zeolite imidazole skeleton (ZIF-8) and CdSe quantum dots (CdSe QDs) on a porous anodized aluminum oxide (PAA) membrane surface using CdSe QDs, 2-methylimidazole, and zinc nitrate as the precursor solvents. ZIF-8 is a kind of metal-organic framework, a microporous material that possesses strong metal adsorption capacity. In addition, CdSe quantum dots have fluorescent properties. The nanochannel membrane detects copper ions (Cu2+) by quenching the fluorescence intensity by the interaction between Cu2+ and Se and S atoms. The direct potential of 5 V was applied to achieve Cu2+ enrichment at the nanochannel interface, and the fluorescence change was observed. The CdSe@ZIF-8/PAA nanochannel membrane has a good linear range of concentration (0.01 pM-1 µM) for Cu2+ detection. With the help of nanochannel enrichment, its detection limit reaches 4 fM. In addition, this nanochannel membrane has good selectivity for Cu2+.

Mussel-inspired fabrication of porous anodic alumina nanochannels and a graphene oxide interfacial ionic rectification device.

A mussel-inspired new interfacial ionic rectification device is fabricated using porous anodic alumina nanochannels and graphene oxide via dopamine polymerization. In addition, the rectification mechanism is elucidated based on the principles of physics and thermodynamics via finite element simulation and the results show that the interface distance and surface charge play a vital role in ion transport.