Ohmic-Rectifying Conversion of Ni Contacts on ZnO and the Possible Determination of ZnO Thin Film Surface Polarity

The current-voltage characteristics of Ni contacts with the surfaces of ZnO thin films as well as single crystal (0001) ZnO substrate are investigated. The ZnO thin film shows a conversion from Ohmic to rectifying behavior when annealed at 800°C. Similar findings are also found on the Zn-polar surface of (0001) ZnO. The O-polar surface, however, only shows Ohmic behavior before and after annealing. The rectifying behavior observed on the Zn-polar and ZnO thin film surfaces is associated with the formation of nickel zinc oxide (Ni_1-xZn_xO, where x = 0.1, 0.2). The current-voltage characteristics suggest that a p-n junction is formed by Ni_1-xZn_xO (which is believed to be p-type) and ZnO (which is intrinsically n-type). The rectifying behavior for the ZnO thin film as a result of annealing suggests that its surface is Zn-terminated. Current-voltage measurements could possibly be used to determine the surface polarity of ZnO thin films.     

High Coke-Resistance Pt/Mg1-xNixO Catalyst for Dry Reforming of Methane

A highly active and stable nano structured Pt/Mg_1-xNi_xO catalysts was developed by a simple co-precipitation method. The obtained Pt/Mg_1-xNi_xO catalyst exhibited cubic structure nanocatalyst with a size of 50–80 nm and realized CH₄ and CO₂ conversions as high as 98% at 900°C with excellent stability in the dry reforming of methane. The characterization of catalyst was performed using various kinds of analytical techniques including XRD, BET, XRF, TPR-H₂, TGA, TEM, FESEM, FT-IR, and XPS analyses. Characterization of spent catalyst further confirms that Pt/Mg_1-xNi_xO catalyst has high coke-resistance for dry reforming. Thus, the catalyst demonstrated in this study, offers a promising catalyst for resolving the dilemma between dispersion and reducibility of supported metal, as well as activity and stability during high temperature reactions.     

Electronic structure and band gap engineering of ZnO-based semiconductor alloy films

We have conducted first-principles total-energy density functional calculations to study the atomic structures, band structures and electronic structures of Zn_1 ? xM_xO (M = Be, Mg, Cd, Ag, Cu) semiconductor alloys. The Heyd–Scuseria–Ernzerhof hybrid functional has been performed to yield lattice constants and band gaps of Zn_1 ? xM_xO semiconductor in much better agreement with experimental data than with the standard local exchange correlation functional. We found that the strong coupling between O 2p and Cu 3d or Ag 4d bands plays a key role in narrowing of band gaps and leading to the half-metallic behaviour interpreted with the unique spatial distribution pattern between the highest occupied molecular orbital and the lowest unoccupied molecular orbital.     

Enhanced photocatalytic activity of ceria-doped zinc oxide under UV illumination prepared via chemical precipitation

Transition metal oxide has emerged as one of the most potential candidates for environment remediation by utilizing solar energy through photocatalysis. This study compares the optical characteristics of zinc oxide (ZnO) and ceria-doped zinc oxide (CeZnO) nanoparticles synthesized through a facile chemical precipitation method without using any assistant catalyst. The present work investigates the consequences of ceria (cerium dioxide, CeO₂) intrusion on the photocatalytic activity of ZnO nanoparticles using methylene blue (MB) as a probe pollutant. The CeZnO showed an increase in photoactivity when compared to ZnO nanoparticles for degradation of MB in an aqueous solution under ultraviolet (UV) irradiance. The resulting heterojunction between ZnO and that of ceria enhances the charge separation efficiency showing a strong correlation between ZnO and CeO₂ heterojunction on the charge transfer mechanism across the interface.     

Rapid,controllable, one‐pot and room‐temperature aqueous synthesis of ZnO:Cu nanoparticles by pulsed UV laser and its application for photocatalytic degradation of methyl orange

Zinc oxide (ZnO) and ZnO:Cu nanoparticles (NPs) were synthesized using a rapid, controllable, one‐pot and room‐temperature pulsed UV‐laser assisted method. UV‐laser irradiation was used as an effective energy source in order to gain better control over the NPs size and morphology in aqueous media. Parameters effective in laser assisted synthesis of NPs such as irradiation time and laser shot repetition rate were optimized. Photoluminescence (PL) spectra of ZnO NPs showed a broad emission with two trap state peaks located at 442 and 485 nm related to electronic transition from zinc interstitial level (I_Zn) to zinc vacancy level (V_Zn) and electronic transition from conduction band to the oxygen vacancy level (V_O), respectively. For ZnO:Cu NPs, trap state emissions disappeared completely and a copper (Cu)‐related emission appeared. PL intensity of Cu‐related emission increased with the increase in concentration of Cu²⁺, so that for molar ratio of Cu:Zn 2%, optimal value of PL intensity was obtained. The photocatalytic activity of Cu‐doped ZnO revealed 50 and 100% increasement than that of undoped NPs under UV and visible irradiation, respectively. The enhanced photocatalytic activity could be attributed to smaller crystal size, as well as creation of impurity acceptor levels (T₂) inside the ZnO energy band gap.     

8.9% Single‐Stack Inverted Polymer Solar Cells with Electron‐Rich Polymer Nanolayer‐Modified Inorganic Electron‐Collecting Buffer Layers

Enhanced power conversion efficiency (PCE) is reported in inverted polymer solar cells when an electron‐rich polymer nanolayer (poly(ethyleneimine) (PEI)) is placed on the surface of an electron‐collecting buffer layer (ZnO). The active layer is made with bulk heterojunction films of poly[[4,8‐bis[(2‐ethylhexyl)oxy]benzo[1,2‐b:4,5‐b′]dithiophene‐2,6‐diyl][3‐fluoro‐2‐[(2‐ethylhexyl)carbonyl]thieno[3,4‐b]thiophenediyl]] (PTB7) and [6,6]‐phenyl‐C₇₁‐butyric acid methyl ester (PC₇₁BM). The thickness of the PEI nanolayer is controlled to be 2 nm to minimize its insulating effect, which is confirmed by X‐ray photoelectron spectroscopy and optical absorption measurements. The Kelvin probe and ultraviolet photoelectron spectroscopy measurements demonstrate that the enhanced PCE by introducing the PEI nanolayer is attributed to the lowered conduction band energy of the ZnO layer via the formation of an interfacial dipole layer at the interfaces between the ZnO layer and the PEI nanolayer. The PEI nanolayer also improves the surface roughness of the ZnO layer so that the device series resistance can be noticeably decreased. As a result, all solar cell parameters including short circuit current density, open circuit voltage, fill factor, and shunt resistance are improved, leading to the PCE increase up to ≈8.9%, which is close to the best PCE reported using conjugated polymer electrolyte films.     

Deposition and characterization of ZnO thin films from a novel hexanuclear zinc precursor

The high nuclearity zinc complex, Zn₆(OAc)₈(μ-OH)₂(dmae)₂(dmaeH)₂ (1) (OAc = acetate and dmaeH = N,N′-dimethylaminoethanol), having a low decomposition temperature and sufficiently high solubility in non-polar solvents, was synthesized by a simple chemical technique in high yield and analyzed by melting point, elemental analysis, FTIR, NMR, single crystal X-ray crystallography and thermal analysis. Aerosol-assisted chemical vapor deposition technique was used to deposit a high-quality thin film with good adhesion to the glass substrate at relatively low temperature (320 °C). Scanning electron microscopy of the film shows clearly distinct crystallites of uniform shape with 2.4-2.9 μm size. Powder X-ray diffraction measurements have indicated the deposition of a crystalline phase of hexagonal ZnO with space group P63mc.     

Association of aureolic acid antibiotic, chromomycin A3 with Cu2+ and its negative effect upon DNA binding property of the antibiotic

Here we have examined the association of an aureolic acid antibiotic, chromomycin A3 (CHR), with Cu²⁺. CHR forms a high affinity 2:1 (CHR:Cu²⁺) complex with dissociation constant of 0.08 × 10⁻¹⁰ M² at 25°C, pH 8.0. The affinity of CHR for Cu²⁺ is higher than those for Mg²⁺ and Zn²⁺ reported earlier from our laboratory. CHR binds preferentially to Cu²⁺ in presence of equimolar amount of Zn²⁺. Complex formation between CHR and Cu²⁺ is an entropy driven endothermic process. Difference between calorimetric and van’t Hoff enthalpies indicate the presence of multiple equilibria, supported from biphasic nature of the kinetics of association. Circular dichroism spectroscopy show that [(CHR)₂:Cu²⁺] complex assumes a structure different from either of the Mg²⁺ and Zn²⁺ complex reported earlier. Both [(CHR)₂:Mg²⁺] and [(CHR)₂:Zn²⁺] complexes are known to bind DNA. In contrast, [(CHR)₂:Cu²⁺] complex does not interact with double helical DNA, verified by means of Isothermal Titration Calorimetry of its association with calf thymus DNA and the double stranded decamer (5′-CCGGCGCCGG-3′). In order to interact with double helical DNA, the (antibiotic)₂ : metal (Mg²⁺ and Zn²⁺) complexes require a isohelical conformation. Nuclear Magnetic Resonance spectroscopy shows that the Cu²⁺ complex adopts a distorted octahedral structure, which cannot assume the required conformation to bind to the DNA. This report demonstrates the negative effect of a bivalent metal upon the DNA binding property of CHR, which otherwise binds to DNA in presence of metals like Mg²⁺and Zn²⁺. The results also indicate that CHR has a potential for chelation therapy in Cu²⁺ accumulation diseases. However cytotoxicity of the antibiotic might restrict the use.     

ZnO–CuxO/polypyrrole nanocomposite modified electrode for simultaneous determination of ascorbic acid,dopamine, and uric acid

Novel zinc oxide (ZnO) nanosheets and copper oxide (Cu_xO, CuO, and Cu₂O) decorated polypyrrole (PPy) nanofibers (ZnO–Cu_xO–PPy) have been successfully fabricated for the simultaneous determination of ascorbic acid (AA), dopamine (DA), and uric acid (UA). The morphology and structure of ZnO–Cu_xO–PPy nanocomposites were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and Raman spectroscopy. Compared with the bare glassy carbon electrode (GCE), PPy/GCE, Cu_xO–PPy/GCE, and ZnO–PPy/GCE, ZnO–Cu_xO–PPy/GCE exhibits much higher electrocatalytic activities toward the oxidation of AA, DA, and UA with increasing peak currents and decreasing oxidation overpotentials. Cyclic voltammetry (CV) results show that AA, DA, and UA could be detected selectively and sensitively at ZnO–Cu_xO–PPy/GCE with peak-to-peak separation of 150 and 154 mV for AA–DA and DA–UA, respectively. The calibration curves for AA, DA, and UA were obtained in the ranges of 0.2 to 1.0 mM, 0.1 to 130.0 μM, and 0.5 to 70.0 μM, respectively. The lowest detection limits (signal/noise = 3) were 25.0, 0.04, and 0.2 μM for AA, DA, and UA, respectively. With good selectivity and sensitivity, the current method was applied to the determination of DA in injectable medicine and UA in urine samples.     

The analogous reaction of diSchiff base coordinated copper and Cu2Zn2 superoxide dismutase with nitric oxide

In addition to the well known catalytically accelerated O₂ dismutation, Cu₂Zn₂ Superoxide dismutase (SOD) reversibly reduces NO to NO^– with the consequence of a prolonged half-life of NO. This alternative reactivity was examined in the presence of the intact CuZn enzyme and a diSchiff base copper complex prepared from putrescine and pyridine-2-aldehyde (Cu-PuPy) which is known as a convenient active center analog of the former copper protein. The reaction of this SOD mimick with NO and NO^– was monitored by electronic absorption and electron paramagnetic resonance (EPR) spectroscopy via the formation of nitrosylmyoglobin. Cu-PuPy reacted up to three times faster with NO compared with Cu₂Zn₂ SOD and 15 times faster in comparison with CuSO₄ and copper EDTA. The oxidation rate of NO^– by Cu-PuPy was up to 300% higher compared with the reactivities of CuSO₄ and Cu EDTA. Cu₂Zn₂SOD reacted with NO^– to a neglible extent only. Catalytic characteristics could be observed in the course of the oxidation of NO^– in concentrations between 1 and 20 M copper. Disturbances of the EPR properties suggested a modification of the chemical environment at the copper sites in both the copper complex and the enzyme. As a consequence, no further reactions of the nitrogen monoxides with the respective active centers were seen. In conclusion, Cu-PuPy appears to be an efficient moderator of the biochemical reactivity of nitrogen monoxides attributable to the observed increased half-life of NO.     

Selective Cu2+ binding, redox silencing, and cytoprotective effects of the small heat shock proteins alphaA- and alphaB-crystallin

Oxidative stress and Cu²⁺ have been implicated in several neurodegenerative diseases and in cataract. Oxidative stress, as well as Cu²⁺, is also known to induce the expression of the small heat shock proteins α-crystallins. However, the role of α-crystallins in oxidative stress and in Cu²⁺-mediated processes is not clearly understood. We demonstrate using fluorescence and isothermal titration calorimetry that α-crystallins (αA- and αB-crystallin and its phosphorylation mimic, 3DαB-crystallin) bind Cu²⁺ with close to picomolar range affinity. The presence of other tested divalent cations such as Zn²⁺, Mg²⁺, and Ca²⁺ does not affect Cu²⁺ binding, indicating selectivity of the Cu²⁺-binding site(s) in α-crystallins. Cu²⁺ binding induces structural changes and increase in the hydrodynamic radii of α-crystallins. Cu²⁺ binding increases the stability of α-crystallins towards guanidinium chloride-induced unfolding. Chaperone activity of αA-crystallin increases significantly upon Cu²⁺ binding. α-Crystallins rescue amyloid beta peptide, Aβ_1-40, from Cu²⁺-induced aggregation in vitro. α-Crystallins inhibit Cu²⁺-induced oxidation of ascorbate and, hence, prevent the generation of reactive oxygen species. Interestingly, α-synuclein, a Cu²⁺-binding protein, does not inhibit this oxidation process significantly. We find that the Cu²⁺-sequestering (or redox-silencing) property of α-crystallins confers cytoprotection. To the best of our knowledge, this is the first study to reveal high affinity (close to picomolar) for Cu²⁺ binding and redox silencing of Cu²⁺ by any heat shock protein. Thus, our study ascribes a novel functional role to α-crystallins in Cu²⁺ homeostasis and helps in understanding their protective role in neurodegenerative diseases and cataract.     

Copper (II) complex-catalyzed oxidation of NADH by hydrogen peroxide

Among various metal ions of physiological interest, Cu²⁺ is uniquely capable of catalyzing the oxidation of NADH by H₂O₂. This oxidation is stimulated about fivefold in the presence of imidazole. A similar activating effect is found for some imidazole derivatives (1-methyl imidazole, 2-methyl imidazole, andN-acetyl-L-histidine). Some other imidazole-containing compounds (L-histidine,L-histidine methyl ester, andL-carnosine), however, inhibit the Cu²⁺-catalyzed peroxidation of NADH. Other chelating agents such as EDTA andL-alanine are also inhibitory. Stoichiometry for NADH oxidation per mole of H₂O₂ utilized is 1, which excludes the possibility of a two-step oxidation mechanism with a nucleotide free-radical intermediate. About 92% of the NADH oxidation product can be identified as enzymatically active NAD⁺. D₂O, 2,5-dimethylfuran, and 1,4-diazabicyclo [2.2.2]-octane have no significant effect on the oxidation, thus excluding¹O₂ as a mediator. Similarly, OH· is also not a likely intermediate, since the system is not affected by various scavengers of this radical. The results suggest that a copper-hydrogen peroxide intermediate, when complexed with suitable ligands, can generate still another oxygen species much more reactive than its parent compound, H₂O₂.     

Isolation of GIF from porcine brain and studies of its zinc transfer kinetics with apo-carbonic anhydrase

Neuronal growth inhibitory factor (GIF) of porcine brain, was isolated and purified by a similar procedure which was used on the isolation of human and bovine GIF. The native porcine protein with stoichiometry of 4Cu⁺, 3Zn²⁺ was obtained for the first time. The kinetics of zinc transfer from Cu₄Zn₃MT-3 to apo-carbonic anhydrase were studied, and zinc transfer rate constants and thermodynamic parameters were obtained. It is found that like other MTs, porcine Cu₄Zn₃MT-3 can also transfer its zinc atom to apoCA, even much easier than other MTs. A possible association mechanism has been proposed, the formation of Cu₄Zn₃MT3-apoCA complex may be the rate-determining step. The obtained data indicate besides its neuronal growth inhibitory function, GIF might play a role in cellular Zn homeostasis in brain.     

Raman spectroscopic studies on the interaction between divalent counterion and polyion

The nature of the interaction between polyacrylalc ion and several divalent cations, such as Cu²⁺, Mn²⁺, Zn²⁺, Ba²⁺ and Mg²⁺, was investigated using Raman spectroscopy. A specific Raman band characteristic of a carboxyl group is shifted upon addition of Cu²⁺. Zn²⁺ and Mn²⁺ to partially neutralized poly(acrylic acid). On the other hand. no frequency shift of the specific Raman band is observed on addition of Mg²⁺ and Ba²⁺*, though the intensity of the specific Raman band decreases with concentration of MgCl₂. It is concluded from these Raman data that the interaction between polyacrylatc ion and Cu²⁺. Zn²⁺ or Mn²⁺ includes a specific interaction with bond formation, whereas in the case of Mg²⁺ and Ba²⁺, the electrostatic interaction is dominant.     

Inhibition of Na+/K+-ATPase and Mg2+-ATPase by metal ions and prevention and recovery of inhibited activities by chelators

Kinetics and inhibition of Na⁺/K⁺-ATPase and Mg²⁺-ATPase activity from rat synaptic plasma membrane (SPM), by separate and simultaneous exposure to transition (Cu²⁺, Zn²⁺, Fe²⁺ and.Co²⁺) and heavy metals (Hg²⁺and Pb²⁺) ions were studied. All investigated metals produced a larger maximum inhibition of Na⁺/K⁺-ATPase than Mg²⁺-ATPase activity. The free concentrations of the key species (inhibitor, MgATP^{2 ?} , MeATP^{2 ?} ) in the medium assay were calculated and discussed. Simultaneous exposure to the combinations Cu²⁺/Fe²⁺ or Hg²⁺/Pb²⁺caused additive inhibition, while Cu²⁺/Zn²⁺ or Fe²⁺/Zn²⁺ inhibited Na⁺/K⁺-ATPase activity synergistically (i.e., greater than the sum metal-induced inhibition assayed separately). Simultaneous exposure to Cu²⁺/Fe²⁺ or Cu²⁺/Zn²⁺ inhibited Mg²⁺-ATPase activity synergistically, while Hg²⁺/Pb²⁺ or Fe²⁺/Zn²⁺ induced antagonistic inhibition of this enzyme. Kinetic analysis showed that all investigated metals inhibited Na⁺/K⁺-ATPase activity by reducing the maximum velocities (V_max) rather than the apparent affinity (K_m) for substrate MgATP^2-, implying the noncompetitive nature of the inhibition. The incomplete inhibition of Mg²⁺-ATPase activity by Zn²⁺, Fe²⁺ and Co²⁺ as well as kinetic analysis indicated two distinct Mg²⁺-ATPase subtypes activated in the presence of low and high MgATP^{2 ?} concentration. EDTA, L-cysteine and gluthathione (GSH) prevented metal ion-induced inhibition of Na⁺/K⁺-ATPase with various potencies. Furthermore, these ligands also reversed Na⁺/K⁺-ATPase activity inhibited by transition metals in a concentration-dependent manner, but a recovery effect by any ligand on Hg²⁺-induced inhibition was not obtained.     

The phosphate-pyrophosphate exchange and hydrolytic reactions of the membrane-bound pyrophosphatase ofRhodospirillum rubrum: Effects of pH and divalent cations

The relation that exist between the Pi-PPi exchange reaction and pyrophosphate hydrolysis by the membrane-bound pyrophosphatase of chromatophores ofRhodospirillum rubrum was studied. The two reactions have a markedly different requirement for pH. The optimal pH for hydrolysis was 6.5 while the Pi-PPi exchange reaction was at 7.5; the pH affects mainly theK _m of Mg²⁺ or Pi for the enzyme; Mn²⁺ and Co²⁺ support the Pi-PPi exchange reaction partially (50%), but the reaction is slower than with Mg²⁺; other divalent cations like Zn²⁺ or Ca²⁺ do not support the exchange reaction. In the hydrolytic reaction, Zn²⁺, at low concentration, substitutes for Mg²⁺ as substrate, and Co²⁺ also substitutes in limited amount (50%). Other cations (Ca²⁺, Cu²⁺, Fe²⁺, etc.) do not act as substrates in complex with PPi. The Zn²⁺ at high concentrations inhibited the hydrolytic reaction, probably due to uncomplexed free Zn²⁺. In the presence of high concentration of substrate for the hydrolysis (Mg-PPi) the divalent cations are inhibitory in the following order: Zn²⁺>Mn²⁺>Ca²⁺Co²⁺>Fe²⁺>Cu²⁺>Mg²⁺. The data in this work suggest that H⁺ and divalent cations in their free form induced changes in the kinetic properties of the enzyme.     

Abortiporus biennis tolerance to insoluble metal oxides: oxalate secretion, oxalate oxidase activity, and mycelial morphology

The ability of Abortiporus biennis to tolerate and solubilize toxic metal oxides (Cu₂O, Al₂O₃, ZnO, CuFe₂O₄Zn, CdO, and MnO₂) incorporated into agar media was investigated and the growth rate, oxalic acid secretion, and mycelial morphology were monitored. Among the tested metal oxides, formation of clear zones underneath the mycelium growing on Cu₂O- and ZnO-amended plates was observed. ZnO, CdO and Cu₂O caused the highest rate of fungal growth inhibition. An increased level of oxalic acid concentration was detected as a response of A. biennis to the presence of Cu₂O, MnO₂, ZnO and CuFe₂O₄Zn in growth medium. The oxalate oxidase (OXO) was found to be responsible for oxalic acid degradation in A. biennis cultivated in metal-amended media. An increased level of OXO was observed in media amended with Cu₂O, ZnO and MnO₂. Confocal microscopy used in this study revealed changes in mycelial morphology which appeared as increased hyphal branching, increased septation and increased spore number.     

The Role of Zn in Chalcopyrite CuFeS2: Enhanced Thermoelectric Properties of Cu1–xZnxFeS2 with In Situ Nanoprecipitates

Chalcopyrite (CuFeS₂) is a widespread natural mineral, composed of earth‐abundant and nontoxic elements. It has been considered a promising n‐type material for thermoelectric applications. In this work, a series of Zn‐doped Cu_1–xZn_xFeS₂ (x = 0–0.1) compounds are synthesized by vacuum melting combined with the plasma activated sintering process. The role of Zn in the chalcopyrite and its different effects on thermoelectric properties, depending on its concentration and location in the crystal lattice, are discussed. It is found that Zn is an effective donor which increases the carrier concentration and improves the thermoelectric properties of CuFeS₂. When the content of Zn exceeds the solubility limit, Zn partially enters the Cu sites and forms in situ ZnS nanophase. This, in turn, shifts the balance between the anion and cation species which is re‐established by the formation of antisite Fe/Cu defects. Beyond maintaining charge neutrality of the structure, such antisite defects relieve the lattice strain in the matrix and increase the solubility of Zn further. The highest ZT value of 0.26 is achieved at 630 K for Cu_0.92Zn_0.08FeS₂, which represents an enhancement of about 80% over that of the pristine CuFeS₂ sample.     

Solution‐Processed TiO2 Nanoparticles as the Window Layer for CuIn(S,Se)2 Devices

As a wide‐bandgap semiconductor, titanium dioxide (TiO₂) with a porous structure has proven useful in dye‐sensitized solar cells, but its application in low‐cost, high‐efficiency inorganic photovoltaic devices based on materials such as Cu(InGa)Se₂ or Cu₂ZnSnS₄ is limited. Here, a thin film made from solution‐processed TiO₂ nanocrystals is demonstrated as an alternative to intrinsic zinc oxide (i‐ZnO) as the window layer of CuInS_xSe_1?x solar cells. The as‐synthesized, well‐dispersed, 6 nm TiO₂ nanocrystals are assembled into thin films with controllable thicknesses of 40, 80, and 160 nm. The TiO₂ nanocrystal films with thicknesses of 40 and 80 nm exhibit conversion efficiencies (6.2% and 6.33%, respectively) that are comparable to that of a layer of the typical sputtered i‐ZnO (6.42%). The conversion efficiency of the devices with a TiO₂ thickness of 160 nm decreases to 2.2%, owing to the large series resistance. A 9‐hour reaction time leads to aggregated nanoparticles with a much‐lower efficiency (2%) than that of the well‐dispersed TiO₂ nanoparticles prepared using a 15‐hour reaction time. Under optimized conditions, the champion TiO₂ nanocrystal‐film‐based device shows even higher efficiency (9.2%) than a control device employing a typical i‐ZnO film (8.6%).     

Metal ion complexes of d-biotin in solution. Stability of the stereoselective thioether coordination

By spectrophotometry and ¹H nmr, several of the stability constants of the thioether complexes between Mg²⁺, Ca²⁺, Mn²⁺, Cu²⁺, Zn²⁺, Cd²⁺, or Ag⁺ and d-biotin (Bio), tetrahydrothiophene (Tht), and dimethyl sulfide (Dms) have been measured in 50% aqueous ethanol, 96% N,N-dimethylformamide (DMF), 98% d₆-dimethyl sulfoxide, or in D₂O. With decreasing concentration of water, the thioether interaction increases with the biologically important metal ions, whereas, e.g., Ag⁺ behaves in the opposite way. The stability of these complexes is, in general, quite small: for example, with d-biotin in 96% DMF (I = 1.0; 25°C) log K_M(Bio)^M = 0.03 and 1.64 for Cu²⁺ and Ag⁺, respectively; in D₂O (I = 0.5 for Ag⁺, all others 2–5; 27°C) log K_M(Bio)^M ? ?1.0, ?1.4, ?1.2, ?0.9, or 4.20 for Mg²⁺, Ca²⁺, Zn²⁺, Cd²⁺, or Ag⁺. In those cases where the difference log K_M(Tht)^M ? log K_M(Bio)^M can be calculated, it is in the order of 0.3 log units; this observation, as well as the chemical shifts measured, confirm the earlier suggestion that the interaction at the sulfur of biotin is stereoselective: the metal ion coordinates from “below” the tetrahydrothiophene ring of biotin to the sulfur atom, i.e., trans to the urea ring. It is emphasized that despite the low stability of these complexes with the biologically meaningful metal ions, the extent of the interaction is enough to create specific structures.