Effects of hyperthermal proton bombardment on alkanethiol self-assembled monolayer on Au(1 1 1)

The effects of hyperthermal proton bombardment on alkanethiol self-assembled monolayer (SAM) on Au(1 1 1) are studied with scanning tunneling microscopy (STM) and X-ray photoemission spectroscopy (XPS). The STM and XPS results show that proton bombardment with proton energy as low as 2 eV can induce cross-linking of the adsorbed alkanethiols and transform the original ordered SAM lattice to an array of nanoclusters of the cross-linked alkanethiols. For a bombardment at 3 eV with a fluence of 3×10¹⁵ cm⁻², the typical cluster size is about 5 nm. In addition, the cluster size distribution is narrow, with no cluster larger than 8 nm. The cluster growth can be promoted by increasing the fluence at a fixed bombardment energy or increasing the energy at a fixed fluence. This indicates that surface diffusion of alkanethiols and cluster growth can be harnessed by the control of the bombardment energy and fluence.     

Discovery of singlet diradicals: theoretical study on the cage species C14N12-H6 and its six derivatives

In this work, the geometries, harmonic vibrational frequencies, and high-energy density material (HEDM) properties of a novel species and its six derivatives with the general formula C14N12-R6 (R = H, OH, F, CN, N3, NH2, and NO2) have been investigated at the restricted and unrestricted B3LYP/cc-pVDZ levels of theory. Natural bond orbital (NBO), natural orbital (NO), and atoms in molecules (AIM) analyses are applied to examine their electronic topologies. It is found that for the four species of R = H, CN, N3, and NO2, (1) there exist high LUMO occupation numbers, (2) there is considerable spin density congregated on the two central carbon atoms, (3) there exists through space interaction (or intramolecular interaction, which is one of the stabilizing factors of a diradicaloid) between the two central carbon atoms, (4) the distance (about 3 A) between the two central carbon atoms (as the apexes of two trigonal pyramids with their bases facing each other) is suitable and favorable for diradical formation. All the results support that these four species are diradicals or diradicaloids. Furthermore, the appreciable singlet-triplet energy gaps indicate that these four diradicals tend to have a singlet ground state. There is a moderate HOMO-LUMO gap (on the order of 1.5 to 2.1 eV) for these four species. These four singlet diradicals may be novel organic semiconductor materials or nonlinear optical materials. On the other hand, the remaining three species, with R = OH, F, and NH2, are not diradicaloids.     

Characterization of the methoxy carbonyl radical formed via photolysis of methyl chloroformate at 193.3 nm

This study investigates two features of interest in recent work on the photolytic production of the methoxy carbonyl radical and its subsequent unimolecular dissociation channels. Earlier studies used methyl chloroformate as a photolytic precursor for the CH3OCO, methoxy carbonyl (or methoxy formyl) radical, which is an intermediate in many reactions that are relevant to combustion and atmospheric chemistry. That work evidenced two competing C-Cl bond fission channels, tentatively assigning them as producing ground- and excited-state methoxy carbonyl radicals. In this study, we measure the photofragment angular distributions for each C-Cl bond fission channel and the spin-orbit state of the Cl atoms produced. The data shows bond fission leading to the production of ground-state methoxy carbonyl radicals with a high kinetic energy release and an angular distribution characterized by an anisotropy parameter, beta, of between 0.37 and 0.64. The bond fission that leads to the production of excited-state radicals, with a low kinetic energy release, has an angular distribution best described by a negative anisotropy parameter. The very different angular distributions suggest that two different excited states of methyl chloroformate lead to the formation of ground- and excited-state methoxy carbonyl products. Moreover, with these measurements we were able to refine the product branching fractions to 82% of the C-Cl bond fission resulting in ground-state radicals and 18% resulting in excited-state radicals. The maximum kinetic energy release of 12 kcal/mol measured for the channel producing excited-state radicals suggests that the adiabatic excitation energy of the radical is less than or equal to 55 kcal/mol, which is lower than the 67.8 kcal/mol calculated by UCCSD(T) methods in this study. The low-lying excited states of methylchloroformate are also considered here to understand the observed angular distributions. Finally, the mechanism for the unimolecular dissociation of the methoxy carbonyl radical to CH3 + CO2, which can occur through a transition state with either cis or, with a much higher barrier, trans geometry, was investigated with natural bond orbital computations. The results suggest donation of electron density from the nonbonding C radical orbital to the sigma* orbital of the breaking C-O bond accounts for the additional stability of the cis transition state.     

New tricyanoiron(III) building blocks for the construction of molecule-based magnets

A series of tricyanoiron(III) complexes with the general formula mer-[Fe^III(5-Xsap)(CN)₃]^2? (X = H, Me, MeO, Cl or Br, sapH₂ = N-salicylidene-o-aminophenol) have been synthesized. These complexes were characterized by IR, ESI-MS, UV/Vis, elemental analysis and magnetic measurements. The structures of (PPh₄)₂[Fe^III(sap)(CN)₃] and (PPh₄)₂[Fe^III(5-Mesap)(CN)₃] have been determined by X-ray crystallography. These low-spin d ⁵ tricyanoiron(III) complexes are potential building blocks for the construction of molecule-based magnets.     

Direct colorimetric visualization of mercury (Hg) based on the formation of gold nanoparticles

It is critical to be able to detect and quantify Hg²⁺ ions under aqueous conditions with high sensitivity and selectivity. The technique presented herein provides a direct way for simple colorimetric visualization of Hg²⁺ ions in aqueous solution, based on the formation of gold nanoparticles through the Hg²⁺ catalyzed HAuCl₄/NH₂OH reaction. The outstanding selectivity and sensitivity result from the well-known amalgamation process that occurs between mercury and gold. The entire procedure takes less than 20 min. The limit of detection (2 ppb) shows excellent potential for monitoring ultralow levels of mercury in water samples.     

Polyelectrolyte mediated formation of hydroxyapatite microspheres of controlled size and hierarchical structure

The understanding of the role of polyelectrolytes in the synthesis of inorganic materials could provide effective routes towards design of advanced materials. In this study, negatively charged poly(styrene sulfonate) (PSS) is employed as a modifier in hydrothermal synthesis of hydroxyapatite (HA). The results indicate that both the morphology and particle size could be well controlled by adjusting the PSS concentration. The presence of PSS (within the range of 0–9.6 wt%) modified the growth pattern of HA crystallites and results in particles from the ribbons to microspheres. The building units of various microspheres change from nanofibers to nanorods or nanoplates. Along with that, the microspheres become smaller and more compact at higher PSS concentrations. The adsorption of PSS onto certain crystal faces as well as the complexing effect of PSS with Ca²⁺ can be considered as the controlling factors which determine the influence of PSS on the growth mode. The drug release study indicates that the flower-like HA microspheres can be possibly used as effective carriers for biomedical applications. The present synthesis method is simple and controllable, and can provide a convenient route to synthesize uniform HA microspheres with different sizes and hierarchical structures.     

Diagnosis of early stage nasopharyngeal carcinoma using ultraviolet autofluorescence excitation-emission matrix spectroscopy and parallel factor analysis

We report the diagnostic ability of ultraviolet (UV)-excited autofluorescence (AF) excitation-emission matrix (EEM) spectroscopy associated with parallel factor (PARAFAC) analysis for differentiating cancer from normal nasopharyngeal tissue. A bifurcated fiber-optic probe coupled with an EEM system was used to acquire tissue AF EEMs using excitation wavelengths between 260 and 400 nm, and emission collection between 280 and 500 nm. A total of 152 AF EEM landscapes were acquired from 13 normal and 16 nasopharyngeal carcinoma (NPC) thawed ex vivo tissue samples from 23 patients. PARAFAC was introduced for curve resolution of individual AF EEM landscapes associated with the endogenous tissue constituents. The significant factors were further fed to a support vector machine (SVM) and cross-validated to construct diagnostic algorithms. Both the EEM intensity landscapes and the PARAFAC model revealed tryptophan, collagen, and elastin to be the three major endogenous fluorophores responsible for the AF signal from normal and NPC tissues. The EEM intensity distribution and PARAFAC factors suggest an increase of tryptophan and a decrease of collagen and elastin in NPC tissues compared to the normal. The classification results obtained from the PARAFAC-SVM modeling yielded a diagnostic accuracy of 94.7% (sensitivity of 95.0% (76/80); specificity of 94.4% (68/72)) for normal and NPC tissue differentiation. This study suggests that UV-excited AF EEM spectroscopy integrated with PARAFAC algorithms has the potential to provide clinical diagnostics of early onset and progression of NPC.     

Fuzzy nanoassembly of polyelectrolyte and layered clay multicomposite toward a reliable gas barrier

Flexible and transparent gas-diffusion barriers have played an important role in recent years. The present study describes a flexible barrier film with a tailored architecture of cationic polyelectrolytes and clay/polymer nanoassemblies. Highly oriented and well-aligned barrier films were achieved by the consecutive absorption of flexible cationic polymer and anionic montmorillonite platelets. The experimental results showed that the layer-by-layer deposition of oppositely charged thin films containing self-assembled poly(vinyl alcohol) and montmorillonites improved their gas barrier characteristics based on the Ca degradation test, enhancing their optical transparency. This nanostructure, fabricated using a solution process, is useful in many applications, for example, flexible and moisture-free organic electronics. This simple and fast method is suitable for the mass coating of large surface areas, as required in industry.