The oriented attachment of molecular clusters and nanoparticles in solution is now recognized as an important mechanism of crystal growth in many materials, yet the alignment process and attachment mechanism have not been established. We performed high-resolution transmission electron microscopy using a fluid cell to directly observe oriented attachment of iron oxyhydroxide nanoparticles. The particles undergo continuous rotation and interaction until they find a perfect lattice match. A sudden jump to contact then occurs over less than 1 nanometer, followed by lateral atom-by-atom addition initiated at the contact point. Interface elimination proceeds at a rate consistent with the curvature dependence of the Gibbs free energy. Measured translational and rotational accelerations show that strong, highly direction-specific interactions drive crystal growth via oriented attachment.

Separation is an important industrial step with critical roles in the chemical, petrochemical, pharmaceutical, and nuclear industries, as well as in many other fields. Although much progress has been made, the development of better separation technologies, especially through the discovery of high-performance separation materials, continues to attract increasing interest due to concerns over factors such as efficiency, health and environmental impacts, and the cost of existing methods. Metal-organic frameworks (MOFs), a rapidly expanding family of crystalline porous materials, have shown great promise to address various separation challenges due to their well-defined pore size and unprecedented tunability in both composition and pore geometry. In the past decade, extensive research is performed on applications of MOF materials, including separation and capture of many gases and vapors, and liquid-phase separation involving both liquid mixtures and solutions. MOFs also bring new opportunities in enantioselective separation and are amenable to morphological control such as fabrication of membranes for enhanced separation outcomes. Here, some of the latest progress in the applications of MOFs for several key separation issues, with emphasis on newly synthesized MOF materials and the impact of their compositional and structural features on separation properties, are reviewed and highlighted.

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The molecular-recognition properties of DNA are sufficiently well understood to enable the self-assembly of defined structures and devices on the nanometer scale. [1, 2] DNA machines based on a competing hybridization mechanism have been reported.[2] These nanomachines are fuelled by complementary oligonucleotides and accumulate doublestranded DNA waste products that poison the system. Herein we report a novel molecular machine driven by a distinct mechanism that is fast and relatively clean. It is based on a four-stranded DNA structure, called the i-motif, that can be formed from sequences containing stretches of cytosine (C) residues. [3][4][5][6][7] In this structure protonated C forms a noncanonical base pair with an unprotonated C (i.e., a CDC + base pair), and these base pairs interdigitate to form a quadruple helix that is stable under slightly acidic conditions. [3][4][5][6][7] A pH titration of the formation of the i-motif folded structure shows a sharp transition around pH 6.5; [4,5] thus, the design of a DNA nanomachine driven by pH changes is enabled.As illustrated in Figure 1, our system comprises a 21mer single-stranded oligonucleotide X containing four stretches of CCC. The second component is a 17 mer DNA strand Y, whose sequence is complementary to X with the exception of two bases. The two mismatches are necessary to stop strand Y folding into a G-quadruplex, a four-stranded structure that results from stacked tetrads of hydrogen-bonded guanine residues, [8] and to moderate the melting point of the double helix formed (see Supporting Information). At acidic pH (pH 5.0) strand X folds into the closed i-motif structure. [3,4,7] In this closed state, complementary strand Y adopts a floppy random-coil conformation. When the pH value is raised to 8.0, strand X unfolds and is captured by hybridization to Y with formation of an extended duplex structure (the open state). Interconversion of the closed and open states of the machine is thus mediated by alternating addition of H + and OH À .To visualize the open and closed states of this machine, a doubly labeled version of X (i.e., X*) was synthesized with a rhodamine green fluorophore at the 5' end and a dabcyl quencher at the 3' end. When the 5' and 3' ends of strand X* are nearby, the fluorescence of rhodamine green is quenched by dabcyl, whereas rhodamine green fluorescence is strong when it is held away from the dabcyl moiety. [7,9] Rhodamine green was chosen since its fluorescence yield is insensitive to pH value in the range pH 4.0-9.0; [10] therefore, the fluorescence of this system depends only on whether the machine is open or closed. [2,7] Fluorescence quenching was also exploited as an internal indicator for titrating the required amount of H + or OH À to switch between the open and closed states. In the fully extended, open state (pH 8.0), when rhodamine green is excited at 504 nm strong emission is observed at 534 nm; when closed by lowering the pH to 5.0, the fluorescence emission is reduced to 16 % (Figure 2 a).The structures associated ...

DNA has received considerable attention as a promising building material owing to its ability to form predictable secondary structures through sequence-directed hybridization. [ 1 , 2 ] It has been shown that DNA can be precisely designed with specifi c sequences and self-assemble into two-or three-dimensional nanostructures, [3][4][5][6][7][8][9] fabricated to nanomachines or motors, [10][11][12][13] or used as a programmable template to direct the assembly of nanoparticles. [14][15][16][17] Recently, the concept of DNA assembly has been expanded to construct "DNA hydrogels", which are crosslinked networks swollen in an aqueous phase. [18][19][20][21][22][23][24][25][26][27][28][29][30][31] Though hydrogels have great potential in biological and medical applications, [32][33][34][35][36] such as drug and gene delivery, biosensing, and tissue engineering, studying the preparation of DNA hydrogels with designable properties is still in its early stages. In the past, several methods have been reported to prepare DNA hydrogels, for example, DNA directly extracted from the nucleus in nature, behaves like a long linear polymer and forms a hydrogel via physical entanglement or by chemical crosslinking of small molecules. [ 18 − 20] Similarly, DNA can be used as a negatively charged polymer and form a complex with cationic (poly)electrolytes through electrostatic interactions. [ 21 , 22 ] However, both methods treated DNA as a polymer and did not take advantage of the self-assembly of DNA into ordered structures, therefore, the resulting hydrogels lacked precise structural control and specifi c responses. Instead of using physical interactions, DNA can be covalently grafted onto synthetic polymers and serve as a cross-linker, the recognition of complementary DNA strands leads to crosslinking of polymer chains and causes hydrogel formation. [ 23 − 28] In general, the preparation of a DNA-polymer hybrid requires laborious modifi cation steps, and an easy and fast strategy to build tailored DNA hydrogels is desired. Luo and his coworkers have developed a new approach to construct pure DNA hydrogels: using well-designed DNA sequences, selfassembled DNA building blocks with more than two branches could be prepared and further enzymatic ligation between the building blocks led to DNA hydrogel formation. [ 29 ] These DNA hydrogels have been demonstrated for potential applications in controllable drug release [ 29 ] and cell-free protein-producing systems, [ 30 ] however, the enzymatic ligation is rather slow and the preparation is time-consuming. More recently, we have reported that pure DNA hydrogels could be made based on duplex formation and intermolecular i-motif structures. The DNA hydrogels showed a fast sol-gel transition upon changes in the pH, that is, within minutes, and released cargoes in a pH-controlled manner. [ 31 ] However, these DNA hydrogels were not stable under physiological conditions, which limited their in-vivo applications.Herein, we propose a new and general platform to create pure DNA hydrogels t...