High Negative Charge Density Research Articles

Prometabolites of 5-Diphospho-myo-inositol Pentakisphosphate

Diphospho-myo-inositol phosphates (PP-InsPy) are an important class of cellular messengers. Thus far, no method for the transport of PP-InsPy into living cells is available. Owing to their high negative charge density, PP-InsPy will not cross the cell membrane. A strategy to circumvent this issue involves the generation of precursors in which the negative charges are masked with biolabile groups. A PP-InsPy prometabolite would require twelve to thirteen biolabile groups, which need to be cleaved by cellular enzymes to release the parent molecules. Such densely modified prometabolites of phosphate esters and anhydrides have never been reported to date. This study discloses the synthesis of such agents and an analysis of their metabolism in tissue homogenates by gel electrophoresis. The acetoxybenzyl-protected system is capable of releasing 5-PP-InsP5 in mammalian cell/tissue homogenates within a few minutes and can be used to release 5-PP-InsP5 inside cells. These molecules will serve as a platform for the development of fundamental tools required to study PP-InsPy physiology.

Structural characterization of metal binding to a cold-adapted frataxin.

Frataxin is an evolutionary conserved protein that participates in iron metabolism. Deficiency of this small protein in humans causes a severe neurodegenerative disease known as Friedreich's ataxia. A number of studies indicate that frataxin binds iron and regulates Fe-S cluster biosynthesis. Previous structural studies showed that metal binding occurs mainly in a region of high density of negative charge. However, a comprehensive characterization of the binding sites is required to gain further insights into the mechanistic details of frataxin function. In this work, we have solved the X-ray crystal structures of a cold-adapted frataxin from a psychrophilic bacterium in the presence of cobalt or europium ions. We have identified a number of metal-binding sites, mainly solvent exposed, several of which had not been observed in previous studies on mesophilic homologues. No major structural changes were detected upon metal binding, although the structures exhibit significant changes in crystallographic B-factors. The analysis of these B-factors, in combination with crystal packing and RMSD among structures, suggests the existence of localized changes in the internal motions. Based on these results, we propose that bacterial frataxins possess binding sites of moderate affinity for a quick capture and transfer of iron to other proteins and for the regulation of Fe-S cluster biosynthesis, modulating interactions with partner proteins.

Plasma enhanced atomic layer deposition of gallium oxide on crystalline silicon: demonstration of surface passivation and negative interfacial charge

Herein we report on the passivation of crystalline silicon by gallium oxide (Ga2O3) using oxygen plasma as the oxidizing reactant in an atomic layer deposition (ALD) process. Excess carrier lifetimes of 2.1 ms have been measured on 1.75 Ω cm p‐type silicon, from which a surface recombination current density J0 of 7 fA cm–2 is extracted. From high frequency capacitance‐voltage (HF CV) measurements it is shown that, as in the case of Al2O3, the presence of a high negative charge density Qtot/q of up to –6.2 × 1012 cm–2 is one factor contributing to the passivation of silicon by Ga2O3. Defect densities at midgap on the order of ∼5 × 1011 eV–1 cm–2 are extracted from the HF CV data on samples annealed at 300 °C for 30 minutes in a H2/Ar ambient, representing an order of magnitude reduction in the defect density compared to pre‐anneal data. Passivation of a boron‐diffused p+ surface (96 Ω/□) is also demonstrated, resulting in a J0 of 52 fA cm–2. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)

A convenient and label-free fluorescence “turn off–on” nanosensor with high sensitivity and selectivity for acid phosphatase

In this study, we reported a convenient label-free fluorescence nanosensor for rapid detection of acid phosphatase on the basis of aggregation-caused quenching (ACQ) and enzymolysis approach. The selectivity nanosensor was based on the fluorescence “turn off–on” mode, which possessed high sensitivity features. The original strong fluorescence intensity of CuInS2 QDs was quenched by sodium hexametaphosphate (NaPO3)6. The high efficiency of the quenching was caused by the non-covalent binding of positively charged CuInS2 QDs to the negatively charged (NaPO3)6 through electrostatic interactions, aggregating to form a CuInS2 QDs/(NaPO3)6 complex. Adding acid phosphatase caused intense fluorescence of CuInS2 QDs/(NaPO3)6 to be recovered, and this was because of enzymolysis. (NaPO3)6 was hydrolyzed into small fragments and the high negative charge density decreased, which would weaken the strong electrostatic interactions. As a result, the quenched fluorescence “turned on”. Under the optimum conditions, there was a good linear relationship between I/I0 (I and I0 were the fluorescence intensity of CuInS2 QDs/(NaPO3)6 system in the presence and absence of acid phosphatase, respectively) and acid phosphatase concentration in the range of 75–1500nUmL−1 with the detection limit of 9.02nUmL−1. The proposed nanosensor had been utilized to detect and accurately quantify acid phosphatase in human serum samples with satisfactory results.

Proton Adsorption Selectivity of Zeolites in Aqueous Media: Effect of Si/Al Ratio of Zeolites

In addition to their well-known uses as catalysts, zeolites are utilized to adsorb and remove various cations from aqueous system. The adsorption of the cations is ascribed to the negative charge of zeolites derived from isomorphous substitution of Si by Al. The amount of Na+ adsorption on 4A, X, Y, Na-P1 and mordenite type zeolites were determined in aqueous media, in a two-cation (Na+ and H+) system. Although each zeolite has a constant amount of negative charge, the amount of Na+ adsorption of each zeolite decreased drastically at low pH−pNa values, where pH−pNa is equal to log{(Na+)/(H+)}. By using the plot of the amount of Na+ adsorption versus pH−pNa, an index of the H+ selectivity, which is similar to the pKa of acids, of each zeolite was estimated, and the index tended to increase with decreasing Si/Al ratio of zeolites. These indicate that zeolites with lower Si/Al and higher negative charge density have higher H+ adsorption selectivity, and in fact, such a zeolite species (4A and X) adsorbed considerable amount of H+ even at weakly alkaline pH region. The adsorption of H+ results in the decrease of cation adsorption ability, and may lead to the dissolution of zeolites in aqueous media.

Electrolyte effects in a model of proton discharge on charged electrodes

Abstract We report results on the influence of NaCl electrolyte dissolved in water on proton discharge reactions from aqueous solution to charged platinum electrodes. We have extended a recently developed combined proton transfer/proton discharge model on the basis of empirical valence bond theory to include NaCl solutions with several different concentrations of cations and anions, both stoichiometric (1:1) compositions and non-stoichiometric ones with an excess of cations. The latter solutions partially screen the electrostatic potential from the surface charge of the negatively charged electrode. 500–1000 trajectories of a discharging proton were integrated by molecular dynamics simulations until discharge occurred, or for at most 1.5 ns. The results show a strong dependence on ionic strength, but only a weak dependence on the screening behavior, when comparing stoichiometric and non-stoichiometric solutions. Overall, the Na + cations exert a more dominant effect on the discharge reaction, which we argue is likely due to the very rigid arrangements of the cations on the negatively polarized electrode surface. Thus, our model predicts, for the given and very high negative surface charge densities, the fastest discharge reaction for pure water, but obviously cannot take into account the fact that such high charge densities are even more out of reach experimentally than for higher electrolyte concentrations.

Nanofiltration hollow fiber membranes with high charge density prepared by simultaneous electron beam radiation-induced graft polymerization for removal of Cr(VI)

A novel kind of nanofiltration (NF) hollow fiber membranes with high density of negative charges and relative high permeate flux was prepared by grafting of 2-acrylamido-2-methylpropanesulfonic acid (AMPS) onto a polysulfone (PSf) ultrafiltration hollow fiber membrane using the simultaneous electron beam radiation-induced method. ATR-FTIR and FESEM analyses revealed that the monomer was grafted on both the outer and inner surfaces of the membrane. The effects of irradiation dose and monomer concentration on the mean pore size and performance of the membranes were investigated. The prepared NF membranes were used for removal of Cr(VI) from aqueous solution under varying conditions such as pH value, initial Cr(VI) concentration and coexisting ions. The experimental results showed that Cr(VI) could be effectively removed by the resulting NF membranes when the pH of the solution was higher than 8 and the performance of Cr(VI) removal was nearly not affected by the presence of Na2SO4 and NaCl in the experimental range. It was indicated that the high negative charge density resulting from the high degree of grafting made the grafted membranes achieve nanofiltration performance at a relative large pore size.

Gold nanoparticles-based localized surface plasmon resonance scattering analysis method for the determination of trace amounts of Hg(II)

Heavy-metal ions pose severe risks for human health and the environment. In particular, mercury-based pollutants are of great environmental concern because of the high toxicity of many Hg compounds. It is important to monitor the levels of potentially toxic metal Hg(II) in aquatic ecosystems. Gold nanoparticles (AuNPs) as nanomaterials have been generally studied. It is because their unique electrical, chemical, optical, and catalytic properties, AuNPs have caused widespread interest for applications in biological and chemical analysis and detection. In the present work, the authors took advantage of the aggregation-induced localized surface plasmon resonance (LSPR) light scattering signal change of sodium thioglycolate functionalized AuNPs in aqueous solutions to develop a highly efficient optical sensor for Hg(II). The as-modified AuNPs demonstrate that high negative charge densities exist on their surfaces at pH 9.0 Britton-Robinson (BR) buffer solution. The AuNPs occur aggregate in solution through chelation in the presence of Hg(II). The scanning electron microscope (SEM) images for the AuNPs display typical shapes of these AuNPs as regular and almost individual spherical particles. The color change of the AuNPs solution was induced by the addition of Hg(II) and it immediately changed from red to purple due to the aggregation. Under optimum conditions, a good linear relationship was obtained from 0.08 to 0.8 μmol x L(-1) with a correlation coefficient of 0.997 6, and the limit of detection (LOD) was 8.0 nmol x L(-1). PEG20000 was employed as a system stabilizer. The proposed method has an excellent selectivity for Hg(II) in aqueous medium over other metal ions. The optimum test of reaction conditions, including the amount of AuNPs, pH value, reaction stability and ionic strength, were also investigated. This method has been used for determination of Hg(II) successfully in environmental water sample. This approach manifested several advantages including short analysis time, high sensitivity, low cost, excellent selectivity and ease of operation.

Functional DNA-containing nanomaterials: cellular applications in biosensing, imaging, and targeted therapy.

ConspectusDNA performs a vital function as a carrier of genetic code, but in the field of nanotechnology, DNA molecules can catalyze chemical reactions in the cell, that is, DNAzymes, or bind with target-specific ligands, that is, aptamers. These functional DNAs with different modifications have been developed for sensing, imaging, and therapeutic systems. Thus, functional DNAs hold great promise for future applications in nanotechnology and bioanalysis. However, these functional DNAs face challenges, especially in the field of biomedicine. For example, functional DNAs typically require the use of cationic transfection reagents to realize cellular uptake. Such reagents enter the cells, increasing the difficulty of performing bioassays in vivo and potentially damaging the cell’s nucleus. To address this obstacle, nanomaterials, such as metallic, carbon, silica, or magnetic materials, have been utilized as DNA carriers or assistants. In this Account, we describe selected examples of functional DNA-containing nanomaterials and their applications from our recent research and those of others. As models, we have chosen to highlight DNA/nanomaterial complexes consisting of gold nanoparticles, graphene oxides, and aptamer–micelles, and we illustrate the potential of such complexes in biosensing, imaging, and medical diagnostics.Under proper conditions, multiple ligand–receptor interactions, decreased steric hindrance, and increased surface roughness can be achieved from a high density of DNA that is bound to the surface of nanomaterials, resulting in a higher affinity for complementary DNA and other targets. In addition, this high density of DNA causes a high local salt concentration and negative charge density, which can prevent DNA degradation. For example, DNAzymes assembled on gold nanoparticles can effectively catalyze chemical reactions even in living cells. And it has been confirmed that DNA–nanomaterial complexes can enter cells more easily than free single-stranded DNA.Nanomaterials can be designed and synthesized in needed sizes and shapes, and they possess unique chemical and physical properties, which make them useful as DNA carriers or assistants, excellent signal reporters, transducers, and amplifiers. When nanomaterials are combined with functional DNAs to create novel assay platforms, highly sensitive biosensing and high-resolution imaging result. For example, gold nanoparticles and graphene oxides can quench fluorescence efficiently to achieve low background and effectively increase the signal-to-background ratio. Meanwhile, gold nanoparticles themselves can be colorimetric reporters because of their different optical absorptions between monodispersion and aggregation.DNA self-assembled nanomaterials contain several properties of both DNA and nanomaterials. Compared with DNA–nanomaterial complexes, DNA self-assembled nanomaterials more closely resemble living beings, and therefore they have lower cytotoxicity at high concentrations. Functional DNA self-assemblies also have high density of DNA for multivalent reaction and three-dimensional nanostructures for cell uptake. Now and in the future, we envision the use of DNA bases in making designer molecules for many challenging applications confronting chemists. With the further development of artificial DNA bases using smart organic synthesis, DNA macromolecules based on elegant molecular assembly approaches are expected to achieve great diversity, additional versatility, and advanced functions.

Unravelling potassium nutrition in ectomycorrhizal associations

Unravelling potassium nutrition in ectomycorrhizal associations

Solid-State Nanopore Modifications for Improved Resolution of DNA Translocations

Nanopores extend electrophoretic detection to the single molecule level, where they provide a versatile platform for analysis of nucleic acids and proteins. The primary advantage of the technique relies upon the size of the nanopore, which should be only slightly larger than the cross-section of the molecule it detects. In the case of long biopolymers, a sufficiently small nanopore can only accommodate one section at a time, uncoiling and scanning the biopolymer as it enters and translocates through the pore. Small variations in size or chemical composition along the length of the DNA - such as the location of bound small molecules, proteins, or even the nucleic acid sequence - can be detected by monitoring changes in the blocked current level during translocation. A critical determinant of nanopore sensing resolution is the speed at which molecules translocate through the nanopore. Slower translocation speeds provide better temporal resolution of small changes in current blockage level, improving detection of local features along the DNA. We have recently developed two novel nanopore modifications to slow DNA translocation through solid-state nanopores fabricated in thin silicon nitride membranes, both of which do not permanently alter the nanopore itself, and which are functionally orthogonal to other nanopore modifications: 1) With just a few mW of visible laser light, we induce high negative surface charge density at the nanopore, creating an electro-osmotic flow which moves in opposition to negatively charged analytes such as DNA, increasing drag and slowing translocation. 2) Electrospinning a sparse nanofiber mesh onto the volume directly outside the nanopore creates interactions with translocating molecules that increase dwell time and are tunable by varying the fiber chemical composition. These techniques enable detection of small proteins and local features along DNA.

PECVD-AlOx/SiNx Passivation Stacks on Silicon: Effective Charge Dynamics and Interface Defect State Spectroscopy

The charge dynamics and the interface defect state density of AlOx/SiNx passivation stacks deposited by plasma-enhanced chemical vapor deposition (PECVD) on crystalline silicon (c-Si) wafers are investigated. High frequency (1MHz) capacitance voltage (C-V) measurements were performed on stacks in the as deposited state and after an annealing step. C-V sweeps reveal an initially high negative charge density for the as deposited sample, activated by the thermal budget during SiNx deposition. However, this charge state is unstable and reduced owning to electron detrapping and emission into the c-Si upon applying moderate voltages. In the annealed sample, the AlOx/SiNx stack has a stable negative fixed charge. Both for as deposited and for annealed samples, applying a positive or negative constant gate voltage stress (Vstress) enhances or reduces the negative effective charge density (Qox,eff), respectively. Injection of charges from the c-Si into traps in the AlOx/SiNx stack is identified as the mechanism responsible for this behavior. We conclude that in addition to fixed negative charges trapping of negative charges near the interface is a crucial mechanism contributing to the total effective negative charge of the stack. Their contribution depends on the temperature and duration of the thermal treatment. Additionally, a large Vstress leads to generation of additional Si dangling bond defects over the entire c-Si bang gap at the c-Si/AlOx interface.

Novel paper‐templated fabrication of hierarchically porous Ni–Al layered double hydroxides/Al2O3 for efficient BSA separation

AbstractBACKGROUNDDue to cellulose of paper is abundant, inexpensive and renewable in nature, the development of paper‐based materials from cellulose to substitute materials prepared by complicated chemical routes is a fascinating challenge. This work investigates a novel method for fabricating paper‐based Ni‐Al layered double hydroxides (LDHs)/Al2O3 composite using celluloses as template; the fabricated composite was applied to adsorb bovine serum albumin (BSA) from water.RESULTSScanning electron microscopy (SEM) images demonstrate the hierarchical morphologies and multiscale structures of composites at scales ranging from micro‐ to nano‐scales. The unique macroporous structures can be directly observed by morphology analysis, and show that these materials which have potential applications in bulky biomolecules adsorption. Compared with bare LDH particles, LDHs/Al2O3 composites show a high binding capacity for BSA, where the appropriate adsorption capacity is achieved at neutral pH. The equilibrium data for BSA adsorption onto composites are found to be well described by the Langmuir isotherm model, while the kinetic data are well‐fitted to the pseudo‐second‐order model.CONCLUSIONSince the BSA adsorbed composites show near complete (over 95%) desorption in a high negative charge density of salt solution, suggesting that the paper‐based Ni‐Al LDHs/Al2O3 composites are practically usable for BSA separation. © 2013 Society of Chemical Industry

99mTechnetium-labeled heparin: A new approach to detection of eosinophilic esophagitis–associated inflammation

99mTechnetium-labeled heparin: A new approach to detection of eosinophilic esophagitis–associated inflammation

Control of Threshold Voltage in GaN Based Metal–Oxide–Semiconductor High-Electron Mobility Transistors towards the Normally-Off Operation

The effect of defect charges on a threshold voltage (Vth) of the GaN/AlGaN/GaN metal–oxide–semiconductor heterostructures (MOS-Hs) with the gate stack thickness scaling was investigated by using capacitance–voltage measurements and a comprehensive analytical model [M. Ťapajna and J. Kuzmík: Appl. Phys. Lett. 100 (2012) 113509]. Using the MOS-Hs with Al2O3 and HfO2 dielectrics grown by plasma-enhanced atomic layer deposition, a high density (∼1013 cm-2) of negative fixed oxide charge (Nox) was extracted for Al2O3 layers, while an order of magnitude lower density (∼1012 cm-2) of positive Nox was extracted for HfO2 films. Consequently, despite similar dielectric constant of Al2O3 to that of (Al)GaN, it is advisable to attain normally-off operation by scaling the AlGaN layer thickness rather than the oxide thickness, taking advantage of the negative Nox offsetting Vth towards the positive voltages. Scaling of the AlGaN layer thickness is found to be effective also in the case of HfO2 dielectric, however, due to a positive Nox in HfO2, AlGaN layer needs to be scaled to a similar thickness (e.g., ∼3 nm) to obtain a positive Vth for the same oxide thickness of HfO2 and Al2O3 (∼5 nm). On the other hand, scaling of the GaN cap has no effect on Vth. Further, our analysis suggests that for MOS-Hs with both Al2O3 and HfO2 gate dielectric, the Fermi level position at the oxide/barrier interface in equilibrium is located within donor-like interface traps. Therefore, the oxide/GaN cap interface trap charge in MOS-Hs with the given structure parameters should lead to a negative Vth shift.

Ozone-based Batch Atomic Layer Deposited Al2O3 for Effective Surface Passivation

In this paper we compare water and ozone as oxidants in the Al2O3 ALD reaction in terms of surface passivation quality. The experiments show that O3 produces a high negative charge density up to 7-1012cm−2 even in as-deposited film, which is different from water based Al2O3 that requires a separate annealing step to activate the negative charge. In general, the ozone process produces lower interface defect density (Dit) and higher negative charge density, which contributes to the higher lifetime value than corresponding water process. Most importantly, ozone-based Al2O3 shows much better firing stability than water-based Al2O3. Ozone concentration is also shown to play a role in the surface passivation quality. Finally, we found out that inserting a water pulse after the ozone pulse can lower the Dit further resulting in even higher lifetime.

Monotypic gland-cell regions on the body surface of two species of Arion: ultrastructure and lectin-binding properties

Distinct areas of the body surface of Arion (Arion) rufus (Linnaeus, 1758) and A. (A.) vulgaris Moquin-Tandon, 1855 contain only one type of glandular cell. The specific monotypic glandular cells that are characteristic for each of these areas differ with respect to staining properties, the structure of glycans (binding of different lectins) and in their ultrastructure, particularly in the appearance of the endoplasmic reticulum. Four such glandular areas can be defined. (1) The protein gland of the anterior margin of the foot probably produces the proteins responsible for changing the trail mucus produced from other glands to strong glue-mucus. Histochemical reactions point to the presence of neutral mucosubstances, probably mucoproteins. The strongest binding of applied lectins were those specific for N-acetyl-α-d-galactosaminyl residues, specific for β-linked terminal galactose, and specific for galactose β1-3N-acetylgalactosamine. A characteristic feature of the ultrastructure of the gland cells is tightly arranged membranes of the endoplasmic reticulum as typically found in protein secreting cells. (2) The suprapedal gland consists of a flat anterior part (located in the bottom of a funnel-shaped cavity between the ventral surface of the head and the opposite dorsal surface of the anterior foot) and a posterior tubular part on the dorsal side of the foot musculature (opening by an excretory duct into the funnel-shaped cavity). Histochemical reactions indicate the presence of acid glycosaminoglycans of high negative charge density. Binding of lectins with all secretion stages indicate the presence of the terminal sequence Galα-1-3Galβ1-R. As a special feature of the ultrastructure of the gland cells, their endoplasmic reticulum contains densely packed tubules protruding at right angle from its membrane into its lumen. (3) On the ventral surface of the head, a monotypic glandular region extends into the roof of the funnel-shaped recess. Histochemical reactions indicate the presence of neutral mucosubstances. The strongest bindings of the applied lectins with the secretion were specific for N-acetyl-α-d-galactosaminyl residues and for β-linked terminal galactose. (4) Another monotypic glandular region is situated in the ventral surface of the anterior part of the mantle. Histochemical reactions indicate acid mucosubstances with high negative charge density. None of the applied lectins reacted with cells of that region. Each of the first three glandular regions has an extension into the peripodial groove on either side of the foot sole; the ‘peripodial gland’ of authors is therefore a compound structure. The branches of the protein gland extend as far back as the level of the pneumostome, while the branches of the suprapedal gland and of the glandular region of the ventral surface of the head extend further, to the corners of the base of the triangular caudal gland. The trail mucus is produced mainly by the suprapedal gland that extends with its branches into the peripodial groove. Monotypic gland cell regions are possible candidates as sources of mucus of known cellular origin for biochemical analyses.

High-temperature stability of c-Si surface passivation by thick PECVD Al2O3 with and without hydrogenated capping layers

We are studying the thermal stability of thick hydrogenated amorphous aluminum oxide (Al2O3) layers (20–50nm) prepared by a high-throughput plasma-enhanced chemical-vapor-deposition (PECVD) technique for the electrical passivation of crystalline silicon surfaces. These passivation layers can be applied alone or covered by a capping layer like amorphous hydrogenated silicon nitride (SiNx) or amorphous hydrogenated silicon oxide (SiOx), also prepared by PECVD. After firing at 870°C for approximately 3s, the layers show blistering for Al2O3 of 30nm or higher, independently from the capping layer. For thinner Al2O3, no blistering can be observed even using scanning electron microscope (SEM).Very long carrier lifetimes up to 900μs was obtained in passivated p-Si (1Ωcm) wafer after annealing and firing, without observing a strong influence of the layer thickness and the capping layer. All the layer stacks, including the stacks with SiNx capping layer, show high negative charge densities in the layer (1–4×1012cm−2). Additionally, low interface defect densities (∼1011cm−2eV−1), which could be achieved with and without a hydrogenated capping layer, were measured even after firing. To explain these phenomena, hydrogen concentration depth profiles were measured by nuclear reaction analysis. These measurements have shown that, at the Al2O3–Si interface, hydrogen atomic concentration ranging 5–7% after annealing and 4% after firing are obtained independently from the capping hydrogen concentration. We conclude that PECVD Al2O3 layers of 20nm or thicker can provide enough hydrogen to passivate the interface defects, even after a high temperature step. However, the layer thickness should be limited to 30nm in order to avoid the blistering.

Exploring the thermal stability of DNA-linked gold nanoparticles in ionic liquids and molecular solvents

While water is the most commonly used solvent for DNA, many co-solvents have been added for various applications. Ionic liquids (ILs) are molten salts at around room temperature. ILs have been tested as a green solvent for many reactions and many biopolymers can also be dissolved in ILs. In this work, we study DNA-linked gold nanoparticles (AuNPs) in seven types of ILs. DNA-functionalized AuNPs possess a high density of negative charges and thus may generate new physical properties in ILs. We have identified the role of ILs to transit from salts to increase DNA duplex stability to solvents to decrease DNA melting temperature. The onset of this transition depends on the structure of ILs, where more hydrophobic cations destabilize DNA at lower IL concentrations. This trend is opposite to molecular solvents (e.g. ethanol, DMSO, ACN and DMF) that destabilize DNA at low solvent concentration. Specific DNA base pairing is disrupted at high DMSO concentrations, and AuNPs are held together by non-specific interactions. The other tested molecular solvents are able to maintain DNA base pairs, although strong non-specific interactions are also present. Several ILs can release proton and thus drastically change pH, which also changes the melting temperature of DNA. This study also reveals the feasibility of using ILs as solvents for DNA-functionalized nanomaterials.

Modelling and Simulation of Field-effect Surface Passivation of Crystalline Silicon-based Solar Cells

An effective surface passivation plays a vital role in the performance of crystalline silicon (c-Si) solar cells. Experimental research shows that fixed charge-induced field-effect passivates the c-Si efficiently. In this work, n-type and p-type c-Si wafers symmetrically passivated by the negative-charge dielectric Al2O3 were numerically modelled with SENTAURUS TCAD. Surface recombination is traditionally modelled by employing the extended Shockley-Read-Hall (SRH) model with single energy level of interface traps. However, experiments show interface traps distribute across the silicon bandgap. Thus, we implemented the extended SRH model with the ability to describe arbitrary energy distribution of interface traps within the bandgap. The extended SRH model predicts a constant effective surface recombination velocity at low injection levels for lightly doped n-type and p-type c-Si passivated by dielectrics with either a high negative or positive charge density. However, this prediction contradicts experimental results which show a significant reduction of the effective lifetime at low injection levels if the polarity of the fixed charge is attracting the minority bulk charge carriers. One explanation is assuming the presence of a thin defect-rich layer close to the c-Si surface in which the lifetimes are degraded. However, the choice of the lifetime and depth of such damaged surface region seem rather arbitrary and its physical origin is still unclear. We show that unequal electron and hole bulk lifetime parameters can also account for the phenomenon. This proposition is physically plausible and the simulation results also show a good agreement with the experimental data from both n-type and p-type c-Si wafers passivated by Al2O3. Our modelling results predict a simple but unambiguous experiment to distinguish between these two explanations.