Selective Self-Assembly and Modification of Herringbone Reconstructions at a Solid-Liquid Interface of Au(111).

The precise control of molecular self-assembly on surfaces presents many opportunities for the creation of complex nanostructures. Within this endeavor, selective patterning by exploiting molecular interactions at the solid-liquid interface would be a beneficial capability. Using scanning tunneling microscopy at the 1,2,4-trichlorobenzene/Au(111) interface, we observed selective self-assembly of 1,3,5-tris(4-methoxyphenyl)benzene (TMPB) molecules in the face-centered cubic (FCC) regions of Au(111). Density functional theory calculations suggest higher adsorption energy of TMPB molecules at FCC regions, explaining the preference for self-assembly. The molecular coverage is found to increase with the concentration of the applied solution, eventually yielding a full monolayer. Moreover, the adsorption of TMPB molecules induces a concentration-dependent lifting of the herringbone reconstruction, observed as an increase in the area of the FCC regions at higher concentrations. Our results represent a simple and cost-effective selective nanoscale patterning method on Au(111), providing a possible avenue to guide the co-adsorption of other functional molecules.

Tandem Desulfurization/C-C Coupling Reaction of Tetrathienylbenzenes on Cu(111): Synthesis of Pentacene and an Exotic Ladder Polymer.

Surface-confined reactions represent a powerful approach for the precise synthesis of low-dimensional organic materials. A complete understanding of the pathways of surface reactions would enable the rational synthesis of a wide range of molecules and polymers. Here, we report different reaction pathways of tetrathienylbenzene (T1TB) and its extended congener tetrakis(dithienyl)benzene (T2TB) on Cu(111), investigated using scanning tunneling microscopy, X-ray photoelectron spectroscopy, and density functional theory calculations. Both T1TB and T2TB undergo desulfurization when deposited on Cu(111) at room temperature. Deposition of T1TB at 453 K yields pentacene through desulfurization, hydrogen transfer, and a cascade of intramolecular cyclization. In contrast, for T2TB the intramolecular cyclization stops at anthracene and the following intermolecular C-C coupling produces a conjugated ladder polymer. We show that tandem desulfurization/C-C coupling provides a versatile approach for growing carbon-based nanostructures on metal surfaces.

Bidirectional Phase Transformation of Supramolecular Networks Using Two Molecular Signals.

Reversible control of molecular self-assembly is omnipresent in adaptive biological systems, yet its realization in artificial systems remains a major challenge. Using scanning tunneling microscopy and density functional theory calculations, we show that a 2D supramolecular network formed by terthienobenzenetricarboxylic acid (TTBTA) can undergo a reversible structural transition between a porous and dense phase in response to different molecular signals (trimethyltripyrazolotriazine (TMTPT) and C60). TMTPT molecules can induce a phase transition from the TTBTA honeycomb to the dense phase, whereas a reverse transition can be triggered by introducing C60 molecules. This response stems from the selective association between signal molecules and TTBTA polymorphs. The successful realization of reversible molecular transformation represents important progress in controlling supramolecular surface nanostructures and could be potentially applicable in various areas of nanotechnology, including phase control, molecular sensing, and "smart" switchable surfaces.

Synergic removal of tetracycline using hydrophilic three-dimensional nitrogen-doped porous carbon embedded with copper oxide nanoparticles by coupling adsorption and photocatalytic oxidation processes.

Adsorption and photocatalytic oxidation are promising technologies for eliminating antibiotics (e.g. tetracycline) in aquatic environments. However, traditional powdery nanomaterials are limited by drawbacks of difficult separation and lack of synergistic function, which do not conform to the practical demand. Herein, we developed a simple one-step gelation-pyrolysis route to fabricate hydrophilic three-dimensional (3D) porous photocatalytic adsorbent, in which CuO nanoparticles are uniformly and firmly embedded in nitrogen-doped (N-doped) porous carbon frameworks. The obtained N-doped carbon/CuO bulky composites exhibited excellent ability to adsorb tetracycline hydrochloride (TC), which was subsequently photo-oxidized under visible light. Their hydrophilic nature favors the adsorption processes toward TC, with a maximum adsorption capacity reaching 25.03 mg∙g-1. In addition, >94.4% of TC molecules could be photo-degraded in 4 h with good cycling efficiency after three consecutive tests. Finally, a reaction scheme for removal process of TC was proposed. The obtained 3D porous N-doped carbon/CuO nanocomposites show great promise for efficient removal of antibiotics in aqueous solution by synergistically utilizing adsorption and photocatalytic oxidation processes.

Oxygen-Induced 1D to 2D Transformation of On-Surface Organometallic Structures.

While surface-confined Ullmann-type coupling has been widely investigated for its potential to produce π-conjugated polymers with unique properties, the pathway of this reaction in the presence of adsorbed oxygen has yet to be explored. Here, the effect of oxygen adsorption between different steps of the polymerization reaction is studied, revealing an unexpected transformation of the 1D organometallic (OM) chains to 2D OM networks by annealing, rather than the 1D polymer obtained on pristine surfaces. Characterization by scanning tunneling microscopy and X-ray photoelectron spectroscopy indicates that the networks consist of OM segments stabilized by chemisorbed oxygen at the vertices of the segments, as supported by density functional theory calculations. Hexagonal 2D OM networks with different sizes on Cu(111) can be created using precursors with different length, either 4,4″-dibromo-p-terphenyl or 1,4-dibromobenzene (dBB), and square networks are obtained from dBB on Cu(100). The control over size and symmetry illustrates a versatile surface patterning technique, with potential applications in confined reactions and host-guest chemistry.

Microwave-assisted hydrothermal assembly of 2D copper-porphyrin metal-organic frameworks for the removal of dyes and antibiotics from water.

Adsorption and photocatalysis are promising strategies to remove pollutants of dyes and antibiotics from wastewater. In this study, we demonstrate a rapid microwave-assisted hydrothermal route for the assembly of 2D copper-porphyrin Metal-Organic Frameworks (Cu-TCPP MOFs) within 1 h. The resulting 2D Cu-TCPP nanosheets with excellent crystallinity and a large surface area (342.72 m2/g) exhibited outstanding adsorption performance for typical dyes with adsorption capacities of about 185 mg/g for rhodamine B, 625 mg/g for methylene blue, and 290 mg/g for Congo red, respectively, as well as for representative antibiotics with adsorption capacities of about 130 mg/g for oxytocin, 150 mg/g for tetracycline, and 50 mg/g for norfloxacin, respectively. Meanwhile, the as-prepared 2D Cu-TCPP showed good photocatalytic degradation activity of pollutants after adsorption under irradiation by visible light, reaching removal efficiencies of 81.2 and 86.3% toward rhodamine B and norfloxacin, respectively. These results demonstrate the promising potential of 2D Cu-TCPP for use in the removal of contaminants from wastewater.

Surface-confined single-layer covalent organic frameworks: design, synthesis and application.

Two-dimensional (2D) nanomaterials, such as graphene and single layer covalent organic frameworks (sCOFs) are being widely studied due to their unusual structure/property relationships. sCOFs typically feature atomic thickness, intrinsic nanoscale porosity and a crystalline lattice. Compared to other organic 2D materials, sCOFs exhibit major advantages including topological designation and constitutional tunability. This review describes the state of the art of surface-confined sCOFs, emphasizing reticular design, synthesis approaches, and key challenges related to improving quality and exploring applications.

Covalent organic frameworks from a monomer with reduced symmetry: polymorphism and Sierpinski triangles.

We report on the synthesis of a covalent organic framework based on the low-symmetry 1,3-benzenediboronic acid precursor. Two distinct polymorphs are obtained, a honeycomb network and Sierpinski triangles, as elucidated by scanning tunneling microscopy. Control over polymorph formation was achieved by varying the precursor concentration for on-surface synthesis.

Planar Anchoring of C70 Liquid Crystals Using a Covalent Organic Framework Template.

The surface-induced anchoring effect is a well-developed technique to control the growth of liquid crystals (LCs). Nevertheless, a defined nanometer-scale template has never been used to induce the anchored growth of LCs with molecular building units. Scanning tunneling microscopy results at the solid/liquid interface reveal that a 2D covalent organic framework (COF-1) can offer an anchoring effect to template C70 molecules into forming several LC mesophases, which cannot be obtained under other conditions. Through comparison with the C60 system, a stepwise breakdown in ordering of C70 LC is observed. The process is described in terms of the effects of molecular anisotropy on the epitaxial growth of molecular crystals. The results suggest that using a surface-confined template to anchor the initial layer of LC molecules can be a modular and potentially broadly applicable approach for organizing molecular mesogens into LCs.

Template-Driven Dense Packing of Pentagonal Molecules in Monolayer Films.

The integration of molecules with irregular shape into a long-range, dense and periodic lattice represents a unique challenge for the fabrication of engineered molecular scale architectures. The tiling of pentagonal molecules on a two-dimensional (2D) plane can be used as a proof-of-principle investigation to overcome this problem because basic geometry dictates that a 2D surface cannot be filled with a periodic arrangement of pentagons, a fundamental limitation that suggests that pentagonal molecules may not be suitable as building blocks for dense films. However, here we show that the 2D covalent organic framework (COF) known as COF-1 can direct the growth of pentagonal guest molecules as dense crystalline films at the solution/solid interface. We find that the pentagonal molecule corannulene adsorbs at two different sites on the COF-1 lattice, and that multiple molecules can adsorb into well-defined clusters patterned by the COF. Two types of these dense periodic packing motifs lead to a five-fold symmetry reduction compatible with translational symmetry, one of which gives an unprecedented high molecular density of 2.12 molecules/nm2.

Direct Measurement of Electronic Band Structure in Single Quantum Dots of Metal Chalcogenide Composites.

Metal chalcogenide quantum dots (QDs) are among the most promising materials as light harvesters in all-inorganic systems for applications in solar cells and production of solar fuels. The electronic band structure of composite QDs formed by lead and cadmium chalcogenides directly grafted on highly oriented pyrolytic graphite surfaces through successive ionic layer absorption and reaction is investigated. Atomic force microscopy and Kelvin probe force microscopy (KPFM) are applied to investigate PbS, CdS, and PbS/CdS QD systems. The variation of the surface potential of individual QDs is measured, investigating the evolution of the electronic band structure as a function of QD size and composition. A shift of the Fermi level toward more negative values occurs when QD size is increased. The shift is more pronounced in CdS than in PbS, while the composite PbS/CdS exhibits an intermediate behavior. The calculated shift is in good agreement with the experiments. These results highlight the ability of KPFM to directly measure the electronic band structure in individual QDs of metal chalcogenide composites. This feature regulates charge dynamics in composite systems, thereby affecting device performance. This work provides valuable insights for applications in several fields, in which charge injection plays a major role.

Probing functional self-assembled molecular architectures with solution/solid scanning tunnelling microscopy.

Over the past two decades, solution/solid STM has made clear contributions to our fundamental understanding of the thermodynamic and kinetic processes that occur in molecular self-assembly at surfaces. As the field matures, we provide an overview of how solution/solid STM is emerging as a tool to elucidate and guide the use of self-assembled molecular systems in practical applications, focusing on small molecule device engineering, molecular recognition and sensing and electronic modification of 2D materials.

Control of Fullerene Crystallization from 2D to 3D through Combined Solvent and Template Effects.

Achieving precise control of molecular self-assembly to form designed three-dimensional (3D) structures is a major goal in nanoscale science and technology. Using scanning tunnelling microscopy and density functional theory calculations, we show that a 2D covalent organic framework (COF-1) can template solution-processed C60 guest molecules to form several solvent-dependent structural arrangements and morphologies via a 2D to 3D growth process. When 1,2,4-tricholorobenzene is used as solvent, C60 molecules form a template-defined close-packed structure. When heptanoic acid is used as solvent, a range of lower density architectures that deviate from the template-defined close packing are observed. We attribute this difference to the co-adsorption of the heptanoic acid solvent molecules, which is only achieved in the presence of the template. This work demonstrates the possibility to precisely control 3D molecular self-assembly through the synergistic combination of template and solvent effects.

Self-assembly of indole-2-carboxylic acid at graphite and gold surfaces.

Model systems are critical to our understanding of self-assembly processes. As such, we have studied the surface self-assembly of a small and simple molecule, indole-2-carboxylic acid (I2CA). We combine density functional theory gas-phase (DFT) calculations with scanning tunneling microscopy to reveal details of I2CA assembly in two different solvents at the solution/solid interface, and on Au(111) in ultrahigh vacuum (UHV). In UHV and at the trichlorobenzene/highly oriented pyrolytic graphite (HOPG) interface, I2CA forms epitaxial lamellar structures based on cyclic OH⋯O carboxylic dimers. The structure formed at the heptanoic acid/HOPG interface is different and can be interpreted in a model where heptanoic acid molecules co-adsorb on the substrate with the I2CA, forming a bicomponent commensurate unit cell. DFT calculations of dimer energetics elucidate the basic building blocks of these structures, whereas calculations of periodic two-dimensional assemblies reveal the epitaxial effects introduced by the different substrates.

Post-transcriptional regulation of the tumor suppressor miR-139-5p and a network of miR-139-5p-mediated mRNA interactions in colorectal cancer.

MicroRNAs play key roles in many biological processes, and are frequently dysregulated in tumor cells. However, there are few studies on how microRNAs are dysregulated. miR-139-5p, an important tumor suppressor, is often underexpressed in gastrointestinal cancer cells. Here, we describe post-transcriptional regulation of this intronic microRNA in human colorectal cancer. miR-139-5p is expressed independently of its overexpressed host gene PDE2A in colorectal cancer tissues and cell lines. The miR-139-5p target genes IGF1R, ROCK2 and RAP1B exert regulatory effects on the miR-139-5p expression level, relying on their ability to compete for miR-139-5p binding. These overexpressed target genes also regulate each others' protein levels through 3'-UTRs, thus regulating tumor cell growth and motility properties. Our study provides a mechanistic, experimentally validated rationale for intronic microRNA dysregulation in colorectal cancer, revealing novel oncogenic roles of IGF1R, ROCK2 and RAP1B 3'-UTRs.

miR-1915 inhibits Bcl-2 to modulate multidrug resistance by increasing drug-sensitivity in human colorectal carcinoma cells.

Colorectal carcinoma is a frequent cause of cancer-related death in the world for men and women. microRNAs are endogenous small noncoding RNAs that regulate gene expression negatively at post-transcriptional level. Here, we investigated the possible role of microRNAs in the development of multidrug resistance (MDR) in colorectal carcinoma cells. We analyzed microRNA (miRNA) expression levels between multidrug resistant colorectal carcinoma cell line HCT116/L-OHP and its parent cell line HCT116 using a miRNA microarray. miR-1915 had the lowest expression of miRNA in HCT116/L-OHP cells compared to its parental cells. Overexpression of Bcl-2 is generally associated with tumor drug resistance, meanwhile Bcl-2 is a predicted target of miR-1915. We found that elevated levels of miR-1915 in the mimics-transfected HCT116/L-OHP cells reduced Bcl-2 protein level and the luciferase activity of a Bcl-2 3'-untranslated region-based reporter, and also sensitized these cells to some anticancer drugs. Taken together, our findings suggest that miR-1915 could play a role in the development of MDR in colorectal carcinoma cells at least in part by modulation of apoptosis via targeting Bcl-2.

miR-297 modulates multidrug resistance in human colorectal carcinoma by down-regulating MRP-2.

Colorectal carcinoma is a frequent cause of cancer-related death in men and women. miRNAs (microRNAs) are endogenous small non-coding RNAs that regulate gene expression negatively at the post-transcriptional level. In the present study we investigated the possible role of microRNAs in the development of MDR (multidrug resistance) in colorectal carcinoma cells. We analysed miRNA expression levels between MDR colorectal carcinoma cell line HCT116/L-OHP cells and their parent cell line HCT116 using a miRNA microarray. miR-297 showed lower expression in HCT116/L-OHP cells compared with its parental cells. MRP-2 (MDR-associated protein 2) is an important MDR protein in platinum-drug-resistance cells and is a predicted target of miR-297. Additionally miR-297 was down-regulated in a panel of human colorectal carcinoma tissues and negatively correlated with expression levels of MRP-2. Furthermore, we found that ectopic expression of miR-297 in MDR colorectal carcinoma cells reduced MRP-2 protein level and sensitized these cells to anti-cancer drugs in vitro and in vivo. Taken together, our findings suggest that miR-297 could play a role in the development of MDR in colorectal carcinoma cells, at least in part by modulation of MRP-2.

MiR-222 modulates multidrug resistance in human colorectal carcinoma by down-regulating ADAM-17.

Colorectal carcinoma is a frequent cause of cancer-related death in men and women throughout the world. MicroRNAs are endogenous small noncoding RNAs that negatively regulate gene expression at the posttranscriptional level. We investigated the role of ADAM-17 (a desintegrin and metalloproteases 17) as a novel multidrug resistance (MDR) mechanism in multidrug-resistant colorectal carcinoma (CRC) and the role of miR-222 in the development of MDR in CRC cells. We found that the high expression of ADAM-17, which results in growth factor shedding and growth factor receptor activation could induce drug resistance in CRC. Pharmacological inhibition of ADAM-17, in conjunction with chemotherapy, may have therapeutic potential for the treatment of CRC. ADAM-17 is a predicted target of miR-222, which was downregulated in multidrug-resistant CRC cells. The presence of miR-222 was consistently inversely proportionate to the expression levels of ADAM-17. We found that elevated levels of miR-222 in the mimics-transfected HCT116/L-OHP and HCT-8/VCR cells reduced the ADAM-17 protein level and the luciferase activity of an ADAM-17 3' untranslated region-based reporter and sensitized these cells' apoptosis to some anticancer drugs. Our findings suggest that miR-222 could play a role in the development of MDR by modulation of ADAM-17, the new MDR treatment target in colorectal carcinoma cells.

MiR-139 inhibits invasion and metastasis of colorectal cancer by targeting the type I insulin-like growth factor receptor.

MicroRNAs (miRNAs), which are noncoding RNAs that regulate gene expression, are involved in tumor metastasis. In this study, we describe the down-regulation and function of miR-139 in colorectal cancer (CRC) metastasis. MiR-139 was found underexpressed in 34 CRC tissues compared to their corresponding nontumor tissues. Decreased miR-139 in CRC tissue was associated with disease progression and metastasis. Re-expression of miR-139 did not inhibit CRC cell growth but suppresses CRC cell metastasis and invasion in vitro and in vivo. MiR-139 might suppress CRC cells invasion and metastasis by targeting type I insulin-like growth factor receptor (IGF-IR). We also found miR-139 directed migration inactivation of human CRC cells involves down-regulation of matrix metalloproteinase 2 (MMP-2). The IGF-IR/MEK/ERK signaling was inhibited by miR-139 overexpression and then resulted in MMP-2 promoter suppression. Taken together, our results provide evidence that miR-139 might function as a metastasis suppressor in CRC. Targeting miR-139 may provide a strategy for blocking CRC metastasis.

Inhibition of IGF-IR increases chemosensitivity in human colorectal cancer cells through MRP-2 promoter suppression.

The emergence of multidrug resistance (MDR) in cancer cells has made many of the currently available chemotherapeutic agents ineffective. However, the mechanism involved in mediating this effect is not yet fully understood. Here, we found the overexpression of type I insulin-like growth factor receptor (IGF-IR) in established colorectal MDR cells. Specific siRNA of IGF-IR decreases cell proliferation, exert synergistic effect with anticancer drugs. The downstream signaling of IGF-IR, PI3K/AKT pathway, was altered upon IGF-IR silencing. The expression of multidrug-resistance-associated protein 2 (MRP-2) was suppressed due to the nuclear translocation of nuclear factor-like 2 (Nrf2). Then the intracellular drug concentration was increased and the drug-resistant phenotype was reversed. Our findings improve current understanding of the biology of IGF-IR and MDR and have significant therapeutic implications on colorectal MDR cancer.