211At on gold nanoparticles for targeted radionuclide therapy application.

Targeted alpha therapy (TAT) is a methodology that is being developed as a promising cancer treatment using the α-particle decay of radionuclides. This technique involves the use of heavy radioactive elements being placed near the cancer target area to cause maximum damage to the cancer cells while minimizing the damage to healthy cells. Using gold nanoparticles (AuNPs) as carriers, a more effective therapy methodology may be realized. AuNPs can be good candidates for transporting these radionuclides to the vicinity of the cancer cells since they can be labeled not just with the radionuclides, but also a host of other proteins and ligands to target these cells and serve as additional treatment options. Research has shown that astatine and iodine are capable of adsorbing onto the surface of gold, creating a covalent bond that is quite stable for use in experiments. However, there are still many challenges that lie ahead in this area, whether they be theoretical, experimental, and even in real-life applications. This review will cover some of the major developments, as well as the current state of technology, and the problems that need to be tackled as this research topic moves along to maturity. The hope is that with more workers joining the field, we can make a positive impact on society, in addition to bringing improvement and more knowledge to science.

PdRuIr ternary alloy as an effective NO reduction catalyst: insights from first-principles calculation.

NO dissociation is an important reaction step in the NO reduction reaction, particularly in the three-way catalyst conversion system for automotive gas exhaust purification. In this study, we used first-principles calculations based on density functional theory to analyze the interaction and dissociation of NO on the PdRuIr ternary alloy. The electronic properties of the atomic combination of the PdRuIr ternary alloy create an effective catalyst that is active for NO dissociation and relatively stable against the formation of volatile RuOx through a weakened O adsorption. This study also shows that for an alloyed system, the strength of NO adsorption may not necessarily predict the dissociation activity. This tendency is observed in the PdRuIr ternary alloy where Ru top is the active site for NO adsorption albeit not an effective site for dissociation. It is presumed that NO dissociation is mediated by its molecular diffusion to active sites for dissociation, which are usually high Ru- and/or Ir-coordinated hollow or bridge sites. These active sites allow high charge transfer from the surface to NO, which fills the NO anti-bonding state and facilitates dissociation. This therefore assumes that the strength of NO molecular adsorption is not a descriptor for NO dissociation on metal alloys but rather the ability of the surface to transfer charge to NO and homogeneity of the strength of adsorption. Furthermore, O adsorption on the ternary alloy, particularly near the Ru sites, is relatively weaker as compared to the pure Ru surface. This weakened O adsorption is attributed to charge re-distribution through alloying, particularly charge transfer from the Ru atom to the Ir and Pd atoms.

Density Functional Theory-Based Calculation Shed New Light on the Bizarre Addition of Cysteine Thiol to Dopaquinone.

Two types of melanin pigments, brown to black eumelanin and yellow to reddish brown pheomelanin, are biosynthesized through a branched reaction, which is associated with the key intermediate dopaquinone (DQ). In the presence of l-cysteine, DQ immediately binds to the -SH group, resulting in the formation of cysteinyldopa necessary for the pheomelanin production. l-Cysteine prefers to bond with aromatic carbons adjacent to the carbonyl groups, namely C5 and C2. Surprisingly, this Michael addition takes place at 1,6-position of the C5 (and to some extent at C2) rather than usually expected 1,4-position. Such an anomaly on the reactivity necessitates an atomic-scale understanding of the binding mechanism. Using density functional theory-based calculations, we investigated the binding of l-cysteine thiolate (Cys-S-) to DQ. Interestingly, the C2-S bonded intermediate was less energetically stable than the C6-S bonded case. Furthermore, the most preferred Cys-S--attacked intermediate is at the carbon-carbon bridge between the two carbonyls (C3-C4 bridge site) but not on the C5 site. This structure allows the Cys-S- to migrate onto the adjacent C5 or C2 with small activation energies. Further simulation demonstrated a possible conversion pathway of the C5-S (and C2-S) intermediate into 5-S-cysteinyldopa (and 2-S-cysteinyldopa), which is the experimentally identified major (and minor) product. Based on the results, we propose that the binding of Cys-S- to DQ proceeds via the following path: (i) coordination of Cys-S- to C3-C4 bridge, (ii) migration of Cys-S- to C5 (C2), (iii) proton rearrangement from cysteinyl -NH3+ to O4 (O3), and (iv) proton rearrangement from C5 (C2) to O3 (O4).

Mechanism of dopachrome tautomerization into 5,6-dihydroxyindole-2-carboxylic acid catalyzed by Cu(II) based on quantum chemical calculations.

BACKGROUND: Tautomerization of dopachrome to 5,6-dihydroxyindole-2-carboxylic acid (DHICA) is a biologically crucial reaction relevant to melanin synthesis, cellular antioxidation, and cross-talk among epidermal cells. Since dopachrome spontaneously converts into 5,6-dihydroxyindole (DHI) via decarboxylation without any enzymes at physiologically usual pH, the mechanism of how tautomerization to DHICA occurs in physiological system is a subject of intense debate. A previous work has found that Cu(II) is an important factor to catalyze the tautomerization of dopachrome to DHICA. However, the effect of Cu(II) on the tautomerization has not been clarified at the atomic level. METHODS: We propose the reaction mechanism of the tautomerization to DHICA by Cu(II) from density functional theory-based calculation. RESULTS: We clarified that the activation barriers of α-deprotonation, ß-deprotonation, and decarboxylation from dopachrome are significantly reduced by coordination of Cu(II) to quinonoid oxygens (5,6-oxygens) of dopachrome, with the lowest activation barrier of ß-deprotonation among them. In contrast to our previous work, in which ß-deprotonation and quinonoid protonation (O5/O6-protonation) were shown to be important to form DHI, our results show that the Cu(II) coordination to quinonoid oxygens inhibits the quinonoid protonation, leading to the preference of proton rearrangement from ß-carbon to carboxylate group but not to the quinonoid oxygens. CONCLUSION: Integrating these results, we conclude that dopachrome tautomerization first proceeds via proton rearrangement from ß-carbon to carboxylate group and subsequently undergoes α-deprotonation to form DHICA. GENERAL SIGNIFICANCE: This study would provide the biochemical basis of DHICA metabolism and the generalized view of dopachrome conversion which is important to understand melanogenesis.

C2H4 adsorption on Cu(210), revisited: bonding nature and coverage effects.

With the aid of density functional theory (DFT)-based calculations, we investigate the adsorption of C2H4 on Cu(210). We found two C2H4 adsorption sites, viz., the top of the step-edge atom (S) and the long bridge between two step-edge atoms (SS) of Cu(210). The step-edge atoms on Cu(210) block the otherwise active terrace sites found on copper surfaces with longer step sizes. This results in the preference for π-bonded over di-σ-bonded C2H4. We also found two stable C2H4 adsorption orientations on the S- and SS-sites, viz., with the C2H4 C[double bond, length as m-dash]C bond parallel (fit) and perpendicular (cross) to [001]. Furthermore, we found that the three peaks observed in previous temperature programmed desorption (TPD) experiment [Surf. Sci., 2011, 605, 934-940] could be attributed to C2H4 in the S-fit or S-cross, S-fit and S-cross-fit (S-cross and S-fit configurations that both exist in the same unit cell) configurations on Cu(210).

Oxygen reduction reaction on neighboring Fe-N4 and quaternary-N sites of pyrolized Fe/N/C catalyst.

We study the interaction between the TM-Nx and metal-free active sites of a pyrolized TM/N/C catalyst and its effect on their ORR activities. We particularly choose a Fe-N4-edge-graphene and a quaternary-N-doped zigzag edge as representatives of TM-Nx and the metal-free active sites, respectively. We find that the interaction of the Fe-N4 and quaternary-N-doped sites at the zigzag edge of graphene enhances the ORR free energy profile at the quaternary-N-doped site. However, the ORR free energy profile at the Fe-N4 site is not affected by this interaction.

The effects of alloying and segregation for the reactivity and diffusion of oxygen on Cu3Au(111).

We report results of the segregation induced by the adsorption of O2 and the barrier of the formation of Cu2O in Cu3Au(111) with an experimental and theoretical approach. Oxidation by a hyperthermal O2 molecular beam (HOMB) was investigated by X-ray photoemission spectroscopy in conjunction with a synchrotron light source. From the incident-energy dependence of the measured O-uptake curve, dissociative adsorption of O2 is less effective on Cu3Au(111) than on Cu(111). The dissociative adsorption is accompanied by the Cu segregation on Cu3Au(111) as well as on Cu3Au(100) and Cu3Au(110). The obvious growth of Cu2O for a 2.3 eV HOMB cannot be observed and it suggests that the Au-rich protective layers prevent the diffusion of O atoms into the bulk. The theoretical approach based on density functional theory indicates that O adsorption shows the same behavior on Cu3Au(111) and on Cu(111). For the diffusion case, the barrier to diffuse into the subsurface in segregated Cu3Au(111) is higher than in Cu(111). This indicates that the segregated Au-rich layer in Cu3Au(111) works as a protective layer.

Initial stages of Cu3Au(111) oxidation: oxygen induced Cu segregation and the protective Au layer profile.

We report results of our experimental and theoretical studies on the Au concentration profile of Cu3Au(111) during oxidation by a hyperthermal O2 molecular beam at room temperature, using X-ray photoemission spectroscopy (XPS), in conjunction with synchrotron radiation (SR), and density functional theory (DFT). Before O2 exposure, we observe strong Au segregation to the top layer, i.e., Au surface enrichment of the clean surface. We also observe a gradual Cu surface enrichment, and Au enrichment of the second and third (subsurface) layers, with increasing O coverage. Complete Cu segregation to the surface occurs at 0.5 ML O surface coverage. The Au-rich second and third layers of the oxidized surface demonstrate the protective layer formation against oxidation deeper into the bulk.

First principles investigation of the initial stage of H-induced missing-row reconstruction of Pd(110) surface.

The pathway of H diffusion that will induce the migration of Pd atom is investigated by employing first principles calculations based on density functional theory to explain the origin of missing-row reconstruction of Pd(110).The calculated activation barrier and the H-induced reconstruction energy reveal that the long bridge-to-tetrahedral configuration is the energetically favored process for the initial stage of reconstruction phenomenon. While the H diffusion triggers the migration of Pd atom, it is the latter process that significantly contributes to the activated missing-row reconstruction of Pd(110). Nonetheless, the strong interaction between the diffusing H and the Pd atoms dictates the occurrence of reconstructed surface.

Hydronium adsorption on OOH precovered Pt(111) surface: effects of electrode potential.

Using the Density Functional Theory-based total energy calculations, the hydronium adsorption on the OOH precovered Pt(111) surface is studied. The electrode potential is modeled by varying the electron affinity of the reduction center [OOH + H3O(H2O)]+. Two possible structures of this reduction center on the Pt surface are HOOH +2H2O and 2(OH)+2H2O. Evidently, the dissociation of HOOH into 2(OH) can be accomplished by changing the electrode potential to the higher value by 0.16 V. The activation energy for the dissociation is approximately 0.1 eV. The optimized structures are also obtained.

Pt/Cr and Pt/Ni catalysts for oxygen reduction reaction: to alloy or not to alloy?

Bimetallic systems such as Pt-based alloys or non-alloys have exhibited interesting catalytic properties but pose a major challenge of not knowing a priori how the electronic and chemical properties will be modified relative to the parent metals. In this work, we present the origin of the changes in the reactivity of Pt/Cr and Pt/Ni catalysts, which have been of wide interest in fuel cell research. Using spin-polarized density functional theory calculations, we have shown that the modification of Pt surface reactivity in Pt/Ni is purely of geometric origin (strain). We have also found that the Pt-Ni bonding is very weak, which explains the observed instability of Pt-Ni catalysts under electrochemical measurements. On the other hand, Pt/Cr systems are governed by strong ligand effect (metal-metal interaction), which explains the experimentally observed reactivity dependence on the relative composition of the alloying components. The general characteristics of the potential energy curves for O2 dissociative adsorption on the bimetallic systems and the pure Pt clarify why the d-band center still works for Pt/Cr despite the strong Pt-Cr bonding and high spin polarization of Pt d-states. On the basis of the above clarifications, viable Pt-Cr and Pt-Ni structures, which involve nano-sized alloys and non-alloy bulk catalyst, which may strike higher than the currently observed oxidation reduction reaction activity are proposed.

Absorption of lithium in montmorillonite: a density functional theory (DFT) study.

The absorption of lithium in montmorillonite [LiSi8(Al3Mg)O20(OH)4] was investigated using Density Functional Theory (DFT). The final position of lithium after absorption was found to be in good agreement with an experimental observation where lithium atom migrated from the interlayer into the vacant octahedral site of montmorillonite. The lithium absorbed on montmorillonite was held together by a very strong attraction between ions and exhibited an insulating behavior as depicted from the density of states curve. Due to the presence of lithium in the octahedral site of montmorillonite, the OH group reoriented itself perpendicular to the ab plane and an electron of lithium was transferred in order to compensate the existing net charge of montmorillonite caused by isomorphous substitutions. Relative small charge transfer was observed between lithium and montmorillonite.

Nitrogen monoxide adsorption on Pt4 clusters coated on gamma-Al2O3 (111) surface.

The nitrogen monoxide (NO) adsorption on platinum tetramer (Pt4) clusters supported on gamma alumina (gamma-Al2O3) with surface index (111) was investigated by using ab-initio calculation based on density functional theory. The Pt4 geometries used in this study are tetrahedron and planar rhombus. The adsorption of Pt4 on gamma-Al2O3 (111) surface in tetrahedron configuration is energetically more favorable as compared to that of the planar rhombus. However, it was found that NO molecule adheres strongly to Pt4 with planar configuration on gamma-Al2O3(111) surface. In addition, the NO adsorption calculation on the isolated Pt4 clusters also shows similar preference to planar configuration. The local density of states (LDOS) reveals that the difference in reactivity comes from the different hybridization strengths between the electronic states of nitrogen atom and those of platinum tetramers. The results are in good agreement with the experiments which show similar tendency for CO and N2O reactivity to gas-phase platinum clusters.

Theoretical Study of an almost Barrier-Free Water Dissociation on a Platinum (111) Surface Alloyed with Ruthenium and Molybdenum.

A theoretical study based on density functional theory for H2O dissociation on the metal surface of Pt(111) alloyed simultaneously with Ru and Mo was performed. The determination of the minimum energy path using the climbing image nudged elastic band (CI-NEB) method shows that the dissociation reaction of H2O with this catalyst requires almost no energy cost. This dissociation reaction is not only kinetically favored but also almost thermodynamically neutral and somewhat exothermic. The electronic structure analysis showed that much more charge was released in Mo and was used to bind the adsorbed hydroxyl (OHad). Further analyses of the density of states (DOS) showed that the large number of orbitals that overlap when OH binds to Mo are responsible for the stabilization of the OH-surface bond. The stability of the OHad fragment on the surface is believed to be a descriptor for the dissociation of H2O with an almost spontaneous process.

Vanadium doped polyoxometalate: induced active sites and increased hydrogen adsorption.

We analyzed the electronic and structural properties of an α-Keggin type molybdenum-based polyoxometalate (POM) [[PMo12O40]3-] and its capacity for reduction reaction via H adsorption using ab initio calculations based on density functional theory (DFT). We also determined the change in the electronic properties brought about by vanadium substitutional doping, and its effect on the capacity of POM to adsorb H atom. We found that the optimal substitutional doping of four vanadium per one unit of POM is adequate to maintain its structural stability. Furthermore, increasing dopant concentration changes charge redistribution such that it induces charge transfer to an initially less active sites for H adsorption on pristine POM. This may increase the possibility of creating active sites from an initially inert H adsorption sites and allows for a higher density of H adsorption. This phenomenon could be relevant for chemical reactions that initially requires high number of pre-adsorbed H atoms.

Cluster size effects on the adsorption of CO, O, and CO2 and the dissociation of CO2 on two-dimensional Cu x (x = 1, 3, and 7) clusters supported on Cu(111) surface: a density functional theory study.

In this study, we performed density functional theory based calculations to determine the effect of the size of Cu x (x = 1 (adatom), 3 (trimer), 7 (heptamer)) clusters supported on Cu(111) toward the adsorption of CO, O, and CO2, and the dissociation of CO2. CO adsorbs with comparable adsorption energies on the different cluster systems, which are influenced by the reactivity of the Cu atoms in the cluster and the interaction of CO with the Cu atoms in the terrace. The O atom, on the other hand, will always favor to adsorb on hollow sites and is more stable on the hollow sites of smaller clusters. CO2 dissociates with lower activation energy on the cluster region than on flat Cu(111). We obtained the lowest activation energy on Cu3 due to its more reactive Cu atoms than the Cu7 case and due to the possibility of O to adsorb on the cluster region, which is not observed in the Cu1 case. The presented results will provide insights on future studies on supported cluster systems and their possible use as catalysts for CO2-related reactions.

Dynamical Quantum Filtering via Enhanced Scattering of para-H2 on the Orientationally Anisotropic Potential of SrTiO3(001).

Quantum dynamics calculation, performed on top of density functional theory (DFT)-based total energy calculations, show dynamical quantum filtering via enhanced scattering of para-H2 on SrTiO3(001). We attribute this to the strongly orientation-dependent (electrostatic) interaction potential between the H2 (induced) quadrupole moment and the surface electric field gradient of ionic SrTiO3(001). These results suggest that ionic surfaces could function as a scattering/filtering media to realize rotationally state-resolved H2. This could find significant applications not only in H2 storage and transport, but also in realizing materials with pre-determined characteristic properties.

Spin-polarized density functional theory study of reactivity of diatomic molecule on bimetallic system: the case of O2 dissociative adsorption on Pt monolayer on Fe(001).

The energetics of O(2) adsorption and dissociation are discussed in terms of 6D potential energy surface based on spin-polarized density functional theory calculations that predict O(2) access to both molecular and dissociative chemisorption wells with no obvious barriers. Specifically, a molecularly chemisorbed state in a top-bridge-top (t-b-t) configuration is identified, and a "no barrier" dissociative adsorption over hollow site with the O-O axis spanning toward the bridge sites (b-h-b) is noted. Both the translation of O(2) from the molecular state (t-b-t) to the dissociated state on bridge and the direct nonactivated dissociative adsorption over the hollow sites (b-h-b) are likely pathways for O(2) dissociation. Interestingly, such O(2) reaction pathways are consistent with the density functional theory calculations and molecular beam experiments on O(2) dissociative adsorption on Pt(100)-(1 x 1). Modification of the electronic structure of the Pt surface due to the Fe substrate relevant for O(2) reactivity is discussed in an effort to provide insight into the experimentally discovered significant enhancement in electrocatalytic activity of Pt-Fe alloys for fuel cell applications.

Interaction of CO, O, and CO2 with Cu cluster supported on Cu(1 1 1): a density functional theory study.

We performed density functional theory (DFT) based calculations to investigate the interaction of CO2 and its dissociated species (CO and O) on Cu3 cluster supported on Cu(1 1 1) (Cu3/Cu(1 1 1)) surfaces. Similar investigations were conducted on Cu(1 1 1) for purpose of comparison. In general, adsorption of CO and O are stronger on the cluster region than on the terrace region of Cu3/Cu and on the flat Cu surface. CO2, on the other hand, is weakly adsorbed on the surfaces. With reference to CO2 dissociation on Cu(1 1 1), we found that the cluster lowers the activation barrier and provides a more stable adsorption of the dissociated species. The presence of co-adsorbed CO in the cluster, however, will increase the activation energy. The variation in the activation barrier with the amount of CO is influenced by the stability of the O atom from the dissociated CO2. We further found that the adsorption energy of O atom is a possible descriptor for CO2 dissociation on the cluster region. The Cu cluster supported on Cu surface could be a promising catalyst for CO2 related reactions based on the lower activation energy for CO2 dissociation on the system than on Cu(1 1 1).

Investigation of reverse ionic diffusion in forward-osmosis-aided dewatering of microalgae: A molecular dynamics study.

This study aimed to investigate the transport mechanisms of ions during forward-osmosis-driven (FO-driven) dewatering of microalgae using molecular dynamics (MD) simulations. The dynamical and structural properties of ions in FO systems of varying NaCl or MgCl2 draw solution (DS) concentrations were calculated and correlated. Results indicate that FO systems with higher DS concentration caused ions to have lower hydration numbers and higher coordination numbers leading to lower diffusion coefficients. The higher hydration number of Mg2+ ions resulted in significantly lower ionic permeability as compared to Na+ ions at all concentrations (p = 0.002). The simulations also revealed that higher DS concentrations led to higher accumulation of ions in the membrane. This study provides insights on the proper selection of DS for FO systems.