In this work, we have explored new stable structures of the Au32Z (Z = +1, 0, -1) clusters. Theoretical calculations using density functional theory within the generalized-gradient approximation were performed. Our results show that, in the anion state (Au32-), low-symmetry (disordered) structures are preferred over the caged fullerene-like isomer. In addition, the cationic cluster (Au32+) also exhibits a disordered low-symmetry structure as its lowest energy configuration, but it is much closer in energy to the fullerene-like isomer. These results, obtained at T = 0 K, indicate that disordered structures for the Au32- and Au32+ clusters may be detected not only at room temperature, as was experimentally verified for the Au32- one, but also at much lower temperatures.

In this work, we provide theoretical evidence on the existence of energetically stable chiral structures for bare gold clusters. Density functional theory calculations within the generalized-gradient approximation were performed to systematically study structural, vibrational, electronic, and optical properties of the lowestlying isomers of the Au 34 Z , (Z) +1, 0,-1), clusters. Our results show that for the neutral and charged clusters, the lowest-energy isomer has a C 1 (chiral) structure. In addition, a C 3 (chiral) isomer was found nearly degenerate in energy with the C 1 isomer. These results are in agreement with previous theoreticalexperimental studies done for the Au 34cluster; however, because our calculated molecular scattering functions for the C 1 and C 3 isomers of this cluster are almost indistinguishable, it is concluded that the actual resolution in trapped ion electron diffraction experiments is not enough to discriminate between them. On the other hand, the electronic density of states of the C 1 isomer shows better overall agreement with the measured photoelectron spectrum of the Au 34cluster than that one corresponding to the C 3 isomer. The electronic density of states of these isomers also shows different features in the energy region of the HOMO-LUMO gap, which would generate distinct behavior in their optical properties. In fact, the calculated absorption and circular dichroism spectra of the two chiral isomers show clear differences in their line shape. Another important property that distinguishes the C 1 and C 3 isomers is the different spatial distribution of the atomic coordination on the cluster surface. Our results confirm that the potential energy surface of bare gold clusters could have lowest-lying energy minima corresponding to intrinsically chiral structures.

Enantioselectivity in gold clusters is investigated by studying the adsorption of a chiral amino acid (cysteine) on a chiral Au55 cluster using density functional calculations. The highest adsorption energies were found when the amino and thiolate functional groups of cysteine are bonded to the lowest coordinated edges of the chiral cluster. Enantiospecific adsorption is primarily obtained from the different bond location and strength, at the cluster edge, of the carboxyl groups forming the left- and right-handed enantiomers. These results provide theoretical support to convey enantioselectivity in asymmetric nanocatalysts using chiral gold clusters.

The vibrational spectra of metal nanoparticles are a signature of their structures and determine the low-temperature behavior of their thermal properties. In this work, we report a theoretical study on the size evolution of the vibrational spectrum and density of states (VDOS) of Au nanoparticles in the range of 1–4 nm. Our study focuses on truncated octahedral (FCC), decahedral, and icosahedral nanoparticles. The structural optimization was performed through atomistic simulations using molecular dynamics and the many-body Gupta potential, whereas the vibrational frequency spectrum was obtained within the harmonic approximation through a diagonalization of the dynamical matrix. The calculated frequency spectra are discrete, have a finite acoustic gap (lowest frequency value), and extend up to a maximum frequency in the range of ∼140–185 cm–1, depending on the nanoparticle morphology. The VDOS evolves from a multiple-peak line shape for small sizes to a characteristic profile for the larger nanoparticles that anticipates the well-known VDOS of the bulk Au metal. The frequency spectrum was used to quantify the specific heat at low temperatures for the Au nanoparticles, displaying small variations with size and shape. Further analysis of these results indicates that the acoustic gap is responsible of a slight reduction in the specific heat with respect to bulk in the temperature range, 0 < T < T r, (T r ≈ 5 K for Au nanoparticles with size ∼1.4 nm). Also, the well-known increment in the specific heat of metal nanoparticles with respect to the bulk value, caused by the enhancement of the VDOS at low frequencies, is recovered for T r < T < T s (T s ≈ 35–45 K). Moreover, it is also found that for T > T s the calculated specific heat of all Au nanoparticles under study is again smaller than the bulk value. This oscillating behavior in the specific heat of Au nanoparticles is related to the differences in their VDOS line shape with respect to the one of the bulk phase. The usefulness of the equivalent (temperature-dependent) Debye temperature of Au nanoparticles to describe the temperature behavior of their specific heat is also discussed.

BackgroundThe objective of the present study was to determine the prevalence and severity of dental fluorosis and to evaluate exposure to fluoridated products in students in the southwest part of the Federal District (Mexico City).Material/MethodsStudents between 10 and 12 years of age who were born and raised in the study zone were evaluated. The level of dental fluorosis was determined using the modified Dean index (DI) using criteria recommended by the World Health Organization (WHO). A bivariate analysis was performed with the χ2 test, and odds ratios (OR) and 95% confidence intervals (CI) are presented. Logistic regression was performed to evaluate the association between dental fluorosis and the independent variables.ResultsA total of 239 students were evaluated. Their mean age was 11±0.82 years, and there were 122 (51%) males. Overall, dental fluorosis was found in 59% of participants; 29.3% had very mild fluorosis, 20.9% had mild fluorosis, 6.7% had moderate fluorosis, and 2.1% had severe fluorosis. The mean fluorosis score was 0.887±0.956. In the final logistic regression model, dental fluorosis was significantly associated with frequency of brushing (OR: 0.444; 95% CI: 0.297–0.666) and with the absence of parental supervision (OR: 0.636; 95% CI: 0.525–0.771).ConclusionsThe association found with frequency of brushing and lack of parental supervision may be contributing to the prevalence and severity of dental fluorosis.