Effective Synthesis of Highly Oxidized Graphene Oxide That Enables Wafer-scale Nanopatterning: Preformed Acidic Oxidizing Medium Approach.

Demand for rapid and massive-scale exfoliation of bulky graphite remains high in graphene commercialization and property manipulation. We report a procedure utilizing "preformed acidic oxidizing medium (PAOM)" as a modified version of the Hummers' method for fast and reliable synthesis of graphene oxide. Pre-mixing of KMnO4 and concentrated H2SO4 prior to the addition of graphite flakes enables the formation of effectively and efficiently oxidized graphene oxide (EEGO) featured by its high yields and suspension homogeneity. PAOM expedites diffusion of the Mn-oxidants into the graphite galleries, resulting in the rapid graphite oxidation, capable of oxidizing bulky graphite flakes (~0.8 mm in diameter) that can not be realized by the Hummers' method. In the scale-up tests, ten-time amount of graphite can be completely exfoliated by PAOM without need of extended reaction time. The remarkable suspension homogeneity of EEGO can be exploited to deposit ultra-flat coating for wafer-scale nanopatterning. We successfully fabricated GO optical gratings with well-defined periodicity (300 nm) and uniform thickness (variation <7 nm). The combination of the facile and potent PAOM approach with the wafer-scale patterning technique may realize the goal for massive throughput graphene nanoelectronics.

Mechanistic insights into light-driven graphene-induced peroxide decomposition: radical generation and disproportionation.

Interaction between adsorbed t-butyl peroxybenzoate and photoexcited graphene rendered trapped phenyl and t-butoxy radicals. Post-irradiation thermal desorption showed benzene, t-butanol, and isobutylene oxide as the end products. The required hydrogen atoms were obtained via the radical disproportionation. Graphene enabled radical species to be captured and their on-surface chemistry to be revealed.

Tuning the Emission of Semiconducting Polymer Dots from Green to Near-Infrared by Alternating Donor Monomers and Their Applications for in Vivo Biological Imaging.

Semiconducting polymer dots (Pdots) recently have emerged as a new class of extraordinarily bright fluorescent probes with promising applications in biological imaging and sensing. Herein multicolor semiconducting polymer nanoparticles (Pdots) were designed using benzothiadiazole (BT) as the acceptor, and various types of donors were incorporated to modulate their emission wavelengths. Specific cellular targeting and in vivo biotoxicity as well as microangiography imaging on zebrafish indicated these BT-based Pdots are promising candidates for biological applications.

Redox-active π-conjugated organometallic monolayers: pronounced Coulomb blockade characteristic at room temperature.

We report the electrical transport characteristics of a series of molecular wires, fc-C≡C-C6H4-SAc (fc = ferrocenyl; Ac = acetyl) and AcS-C6H4-C≡C-(fc)n-C≡C-C6H4-SAc (n = 2, 3), consisting of multiple redox-active ferrocenyl centers. The self-assembled monolayers of these molecular wires on Au surfaces were comprehensively characterized by electrochemistry and conductive atomic force microscopy techniques. Characterization of the wires revealed that electron transport is made extremely efficient by the organometallic redox states. There is a strong electronic coupling between ferrocenyl moieties, and superior electron-transport ability exists through these semirigid molecular wires. Standard rate constants for the electron transfer between the electrode and the ferrocenyl moieties were measured for the monolayers by a potential-step chronoamperometry technique. The electron conduction through the molecular wires was estimated using the monolayers as a bridge from the Au(111) metal surface to the gold tip of a conductive atomic force microscope (CAFM). Using the CAFM, Coulomb blockade behavior arising from the capacitive charging of the multinuclear redox-active molecules was observed at room temperature. The conductance switching was mediated by the presence of various ferrocenyl redox states and each current step corresponded to a specific redox state.

Enhanced recognition of single-base mismatch using locked nucleic acid-integrated hairpin DNA probes revealed by atomic force microscopy nanolithography.

Probe design is a critical parameter in successful DNA and RNA target detection. In this proof-of-concept study, we evaluated the single-base mismatch recognition power of surface immobilized and self-assembled stem-loop hairpin DNA oligonucleotide probes modified to contain locked nucleic acid residues (LNA-HP). The stiffness change in conjunction with the stem opening of the interfacial molecules before and after hybridization led to clear variations of the overall film thickness or miniaturized nanospot height, which could be directly measured using an atomic force microscopy (AFM) nanolithography technique. Particularly, LNA-HP achieved highly differentiable readouts between perfectly complementary and singly mismatched targets (discrimination ratio as high as 2 to 3), outperforming the selectivity of its linear and hairpin counterparts with no LNA modification.

Tracking the chemistry of unsaturated C3H3 groups adsorbed on a silver surface: propargyl-allenyl-acetylide triple bond migration, self-hydrogenation, and carbon-carbon bond formation.

A diverse array of unsaturated C1 (methylene and methylidyne) and C2 (vinyl, vinylidene, ethylidene, and ethylidyne) bound to metal center(s) and surfaces has received much attention. In sharp contrast to the effort devoted to C1 and C2 ligands, complexes or surfaces bearing C3 fragments have been less explored, especially the M-C3H3 systems, which include propargyl (M-CH2C[triple bond]CH), allenyl (M-CH=C=CH2), and acetylide (M-C[triple bond]CCH3) forms. To understand the bonding and reactivity of these C3 species appended to an extended metal structure, proprargyl bromide (Br-CH2C[triple bond]CH) was utilized as a precursor to generate C3H3 fragments on a Ag(111) surface under ultrahigh vacuum conditions. The molecular transformation process was explored by a combination of temperature-programmed desorption (TPD), reflection absorption infrared spectroscopy (RAIRS), and X-ray photoemission spectroscopy (XPS) techniques. In addition, density functional theory (DFT) calculations were conducted to obtain the optimized geometries and energies for the various surface intermediates. The computed IR spectra facilitated the vibrational mode assignments. TPD spectra show that C3H3(ad) self-hydrogenates to C3H4 around 300 and 475 K, respectively. In addition to hydrogenation, a C-C coupling product C6H6 (2,4-hexadiyne) is also unveiled as part of the desorption feature at 475 K. Identification of the possible C3H4 isomers (propyne and/or allene) was equivocal, but it was circumvented by using an alpha,alpha-dimethyl-substituted propargylic species--(CH3)2(alpha)C-C[triple bond]CH, which results in hydrogenation products, alkynic (CH3)2CH-C[triple bond]CH and allenic (CH3)2C=C=CH2, distinguishable by the mass spectrometry. The substitution experiments clarify that in the normal case the convoluted TPD feature around 300 K, in fact, consists of both allene at 260 K and propyne at 310 K, while the last hydrogenation product at 475 K is solely propyne. The RAIR spectroscopy demonstrates that at 200 K C3H3(ad) on Ag(111) readily adopts the allenyl formalism involving concerted CBr bond scission and [1,3]-sigmatropic migration (i.e., Br-*CH2C[triple bond]CH --> *CH2=C=CH-Ag), in which the sigma bond moves to a new metal location across the pi-periphery. Single hydrogen incorporation to the alpha-carbon of the surface allenyl rationalizes the allene formation at 260 K. When the surface is heated to the range of 250-300 K, both RAIR and XP spectra reveal drastic changes, indicative of a new species whose spectral characteristics could be duplicated by separate measurements from 1-propyn-1-yl iodide (CH3-C[triple bond]C-I) being a direct source for the surface methylacetylide (CH3-C[triple bond]C-Ag). It is thus suggested that allenyl is further reorganized to render acetylide presumably via [1,3]-hydrogen shift (i.e., *CH2=C=CH-Ag --> *CH3=C[triple bond]C-Ag). The presence of this third Ag-C3H3 isomeric form demonstrates an unprecedented propargyl-allenyl-acetylide multiple rearrangements on a metal surface. Migration of the triple bond from the remote terminal position into the chain, through the stage of allenic structure, is driven by thermodynamic stabilities, supported by the DFT total energy calculations. Consequently, the evolutions of propyne at 310 and 475 K, as well as 2,4-hexadiyne (bismethylacetylide), can all be reasoned out.

Probing the internal competition between alpha- and beta-elimination by fluorine substitution in adsorbed ethyl groups on Cu(100).

Key factors affecting the competition between alpha- and beta-elimination channels for adsorbed ethyl groups on a metal surface were probed by fluorine substitution. The thermal desorption products and temperatures resulting from ethyl, 2,2,2-trifluoroethyl, 1,2,2,2-tetrafluoroethyl, 1,1,2,2-tetrafluoroethyl, and pentafluoroethyl moieties adsorbed on Cu(100) provided the information about the dominant reactions and a measure of the relative rates. The alterations of number and positions of the fluorine substituents revealed that the eclipsed interactions and hyperconjugation in the transition states can determine the kinetic barriers and allowed access to the separate pathways.

A unique reaction pathway of fluorine-substituted ethyl groups on Cu(111): successive alpha,alpha-fluoride elimination.

Fluorine-substituted ethyl groups on Cu(111) were generated by thermal scission of the C-I bond in the adsorbed C2F5I. Temperature-programmed reaction spectrometry observed a novel pathway resulting in the evolution of C4F6 above 400 K. Among the various isomers, this product was identified as hexafluro-2-butyne. Although abstraction of two fluorine atoms from the starting Cu-CF2CF3 was required, Cu-CCF3 (trifluoroethylidyne) was favored over Cu-CF=CF2 (trifluorovinyl) as the intermediate because this ethyl-ethylidyne-butyne pathway was suppressed on a Cu(100) surface devoid of the key threefold hollow binding sites for ethylidyne. Once formed, perfluoroethylidyne readily coupled to afford a tightly surface-bound hexafluoro-2-butyne up to 400 K. Therefore, the C-F bonds adjacent to the metal were found to be more susceptible to the bond activation, leading the chemisorbed perfluoroethyl to eliminate two F atoms successively from the alpha-carbon. This preference for alpha-elimination rather than beta-elimination (the most favorable route in hydrocarbons) may be quite general for metal surface-mediated reactions involving fluorinated alkyl groups.