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Background Plants have evolved a panoply of specialized metabolites that increase their environmental fitness. Two examples are caffeine, a purine psychotropic alkaloid, and crocins, a group of glycosylated apocarotenoid pigments. Both classes of compounds are found in a handful of distantly related plant genera (Coffea, Camellia, Paullinia, and Ilex for caffeine; Crocus, Buddleja, and Gardenia for crocins) wherein they presumably evolved through convergent evolution. The closely related Coffea and Gardenia genera belong to the Rubiaceae family and synthesize, respectively, caffeine and crocins in their fruits. Results Here, we report a chromosomal-level genome assembly of Gardenia jasminoides, a crocin-producing species, obtained using Oxford Nanopore sequencing and Hi-C technology. Through genomic and functional assays, we completely deciphered for the first time in any plant the dedicated pathway of crocin biosynthesis. Through comparative analyses with Coffea canephora and other eudicot genomes, we show that Coffea caffeine synthases and the first dedicated gene in the Gardenia crocin pathway, GjCCD4a, evolved through recent tandem gene duplications in the two different genera, respectively. In contrast, genes encoding later steps of the Gardenia crocin pathway, ALDH and UGT, evolved through more ancient gene duplications and were presumably recruited into the crocin biosynthetic pathway only after the evolution of the GjCCD4a gene. Conclusions This study shows duplication-based divergent evolution within the coffee family (Rubiaceae) of two characteristic secondary metabolic pathways, caffeine and crocin biosynthesis, from a common ancestor that possessed neither complete pathway. These findings provide significant insights on the role of tandem duplications in the evolution of plant specialized metabolism.

Aluminum (Al) is the most abundant metal in the Earth’s crust and is not an essential element for plant growth. In contrast, nitrogen (N) is the most important mineral element for plant growth, but this non-metal is often present at low levels in soils, and plants are often N deficient. Aluminum toxicity is dominant in acid soils, and so plants growing in acid soils have to overcome both Al toxicity and N limitation. Because of low N-use efficiency, large amounts of N fertilizers are applied to crop fields to achieve high yields, leading to soil acidification and potential Al toxicity. Aluminum lowers plant N uptake and N-use efficiency because Al inhibits root growth. Although numerous studies have investigated the interactions between Al and N, a complete review of these studies was lacking. This review describes: (1) the link between plant Al tolerance and ammonium/nitrate (NH4+/NO3-) preference; (2) the effects of NH4+/NO3- and pH on Al toxicity; (3) the effects of Al on soil N transformations; and (4) the effects of Al on NH4+/NO3- uptake and assimilation by plants. Acid soils are characterized chemically by a relatively high ratio of NH4+ to NO3- and high concentrations of toxic Al. Aluminum-tolerant plants generally prefer NH4+ as an N source, while Al-sensitive plants prefer NO3-. Compared with NO3-, NH4+ increases the solubilization of toxic Al into soil solutions, but NH4+ generally alleviates Al phytotoxicity under solution culture because the protons from NH4+ compete with Al3+ for adsorption sites on the root surface. Plant NO3- uptake and nitrate reductase activity are both inhibited by Al, while plant NH4+ uptake is inhibited to a smaller degree than NO3-. Together, the results of numerous studies indicate that there is a synergistic interaction between plant Al tolerance and NH4+ nutrition. This has important implications for the adaptation of plants to acid soils that are dominated chemically by toxic Al as well as NH4+. Finally, we discuss how this knowledge can be used to increase plant Al tolerance and N-use efficiency in acid soils.

The use of tantalum as a highly effective dopant for hematite photoelectrochemistry (PEC) has shown contradictory results in previous reports. We show here through screening of different compositions by scanning electrochemical microscopy that Ta doping significantly improves the PEC performance of dropcast films on fluorine-doped tin oxide (FTO). In studies with larger electrodes, a 2% Ta-doped hematite photoanode fabricated at 500 °C shows the highest improvement of photoactivity, which is ∼32 times higher than pure hematite even under visible light. At fabrication temperature higher than 500 °C (e.g., 600, 680 °C), the substrate FTO becomes more resistive and the dopant Ta prefers to segregate from the bulk phase (α-Fe2O3) and forms tantalum fluoride oxide (TaO2F), which may act as charge-carrier recombination centers, and the corresponding Ta-doped samples show much lower photoactivities. Ta-doped hematite samples show stronger (110) diffraction as compared with the pure α-Fe2O3. We show that the dopi...

+ toxicity abstract The viability of living fibroblast cells cultured on a Petri dish was determined by scanning electrochem- ical microscopy (SECM). Three kinds of feedback modes for single living cells were derived with different redox couples: positive, negative and consumptive. The consumptive feedback was unique and distin- guished uptake from the response caused by cell deformation. The toxicity of Ag + ion on the living fibro- blast cells was investigated by using both ferrocenemethanol (FcMeOH) and oxygen (O2 )a s electrochemical mediators. The results showed that Ag + ions could be taken up and then reduced to form sub-micrometer and micrometer sized metal silver particles. The metal silver particles stained fibroblast cells and give a positive feedback with FcMeOH. These were also identified by the results of both SECM and energy dispersive X-ray spectroscopy (EDX). The amount of consumption of dissolved O2 decreased with time, showing the cell status as the cells died. Since O2 is directly involved in cell metabolism and the products of O2 reduction are not re-oxidized by metal silver particles trapped on fibroblast cells, it is a good SECM mediator to characterize drug effects on cell viability. The results suggest the importance of choice of redox mediator in characterizing cell viability.

Silicon is a widely used semiconductor for electronic and photovoltaic devices because of its earth-abundance, chemical stability, and the tunable electrical properties by doping. Therefore, the production of pure silicon films by simple and inexpensive methods has been the subject of many investigations. The desire for lower-cost silicon-based solar photovoltaic devices has encouraged the quest for solar-grade silicon production through processes alternative to the currently used Czochralski process or other processes. Electrodeposition is one of the least expensive methods for fabricating films of metals and semiconductors. Electrodeposition of silicon has been studied for over 30 years, in various solution media such as molten salts (LiF-KF-K2SiF6 at 745 8C and BaO-SiO2-BaF2 at 1465 8C ), organic solvents (acetonitrile, tetrahydrofuran), and room-temperature ionic liquids. Recently, the direct electrochemical reduction of bulk solid silicon dioxide in a CaCl2 melt was reported. [7] A key factor for silicon electrodeposition is the purity of silicon deposit because Si for the use in photovoltaic devices is solargrade silicon (> 99.9999% or 6N) and its grade is even higher in electronic devices (electronic-grade silicon or 11N). In most cases, the electrodeposited silicon does not meet these requirements without further purification and, to our knowledge, none have been shown to exhibit a photoresponse. In fact, silicon electrodeposition is not as straightforward as metal deposition, since the deposited semiconductor layer is resistive at room temperature, which complicates electron transfer through the deposit. In many cases, for example in room-temperature aprotic solvents, the deposited silicon acts as an insulating layer and prevents a continuous deposition reaction. In some cases, the silicon deposit contains a high level of impurities (> 2%). Moreover, the nucleation and growth of silicon requires a large amount of energy. The deposition is made even more challenging if the Si precursor is SiO2, which is a very resistive material. We reported previously the electrochemical formation of silicon on molybdenum from a CaCl2 molten salt (850 8C) containing a SiO2 nanoparticle (NP with a diameter of 5– 15 nm) suspension by applying a constant reduction current. However this Si film did not show photoactivity. Here we show the electrodeposition of photoactive crystalline silicon directly from SiO2 NPs from CaCl2 molten salt on a silver electrode that shows a clear photoresponse. To the best of our knowledge, this is a first report of the direct electrodeposition of photoactive silicon. The electrochemical reduction and the cyclic voltammetry (CV) of SiO2 were investigated as described previously. [8] In this study, we found that the replacement of the Mo substrate by silver leads to a dramatic change in the properties of the silicon deposit. The silver substrate exhibited essentially the same electrochemical and CV behavior as other metal substrates, that is, a high reduction current for SiO2 at negative potentials of 1.0 V with the development of a new redox couple near 0.65 V vs. a graphite quasireference electrode (QRE) (Figure 1a). Figure 1b shows a change in the reduction current as a function of the reduction potential, and the optical images of silver electrodes before and after the electrolysis, which displays a dark gray-colored deposit after the reduction. Figure 2 shows SEM images of silicon deposits grown potentiostatically ( 1.25 V vs. graphite QRE) on silver. The amount of silicon deposit increased with the deposition time, and the deposit finally covered the whole silver surface (Figure 2). High-magnification images show that the silicon deposit is not a film but rather platelets or clusters of silicon crystals of domain sizes in the range of tens of micrometers. The average height of the platelets was around 25 mm after a 10000 s deposition (Figure 2b), and 45 mm after a 20000s deposition (Figure 2c), respectively. The edges of the silicon crystals were clearly observed. Contrary to other substrates, silver enhanced the crystallization of silicon produced from silicon dioxide reduction and it is known that silver induces the crystallization of amorphous silicon. Energy-dispersive spectrometry (EDS) elemental mapping (images shown in the bottom row of Figure 2) revealed that small silver islands exist on the top of the silicon deposits, which we think is closely related to the growth mechanism of silicon on silver. The EDS spectrum of the silicon deposit (Figure 3a) suggested that the deposited silicon was quite pure and the amounts of other elements such as C, Ca, and Cl were below the detection limit (about 0.1 atom%). Since the oxygen signal was probably from the native oxide formed on exposure of the deposit to air and silicon does not form an alloy with silver, the purity of silicon was estimated to be at least 99.9 atom%. The successful reduction of Si(4+) in silicon dioxide to elemental silicon (Si) was confirmed by Xray photoelectron spectroscopy (XPS) of the silicon deposit [*] Dr. S. K. Cho, Dr. F.-R. F. Fan, Prof. A. J. Bard Center for Electrochemistry, Department of Chemistry and Biochemistry, The University of Texas at Austin Austin, TX 78712 (USA) E-mail: ajbard@mail.utexas.edu

Melamine is commonly used in the chemical industry, and it has been found to exist on food processing equipment and utensils. Previous investigations suggested that melamine alone or its combination with cyanuric acid appears to be toxic to immune system in animals. The objective of this study is to investigate the potential effects of melamine with or without cyanuric acid on immune function in ovalbumin- sensitized mice. Our data indicated that melamine-related administration caused a significant decreasing in the content of IL-4 in mice in comparison to negative control group. Significant increasing in the content of histamine (HIS) was recorded in almost all treated groups. The co-administration of melamine and cyanuric acid with high dose (each at 16mgkg-12d-1) led to a significantly lower contents of IL-10, IL-16 and monocyte chemoattractant protein-1 (MCP-1) in mouse serum in comparison to negative control group. Moreover, our data indicated that the number of Th1 and Th2 cells, and the ratio of Th1/Th2 spleen lymphocytes were significantly changed after treatment. Also, our results demonstrated that the profiles of both CD40 and CD40L were significantly altered in spleen lymphocytes after treatment.

Single nanoparticle (NP) collisions were successfully observed by a potentiometric measurement. The open circuit potential (OCP) of a measuring Au ultramicroelectrode (UME) changes when Pt NPs collide with the UME in a hydrazine solution. The OCP change is related to the redox processes, the concentration of particles, particle size, and electrode size. Compared with the amperometric technique, this approach has several advantages: higher sensitivity, simpler apparatus, fewer problems with NP decomposition, and contamination.

Tantalum nitride (Ta3N5) is a promising material for photoelectrochemical (PEC) water oxidation with a narrow band gap (2.1 eV) that can effectively utilize visible light in the solar spectrum. Ta3N5 nanotube (NT) arrays were synthesized on a Ta foil by electrochemical anodization followed by an ammonia treatment at 800 °C. The photocurrent of nanostructured Ta3N5 was over 3 times higher than that of a dense regular Ta3N5 film in 0.1 M Na2SO4 aqueous solution at pH 11. Several electrocatalysts (IrO2 nanoparticles (NPs), Co3O4 NPs, cobalt phosphate, and Pt NPs) were used to modify Ta3N5 NTs for PEC water oxidation. The photocurrent of Ta3N5 NTs modified with IrO2 and Co3O4 was ca. four times higher than that of unmodified NTs. Cobalt phosphate also showed a positive improvement for PEC water oxidation on Ta3N5 NTs, whereas Pt was ineffective. Scanning electrochemical microscopy was used to measure the faradaic efficiency of the Ta3N5 photoanodes for water oxidation, which can reach as high as 88% for a Co3...

An array of Pd–W alloys was fabricated, and the electrocatalytic activity of the alloys for the oxygen reduction reaction (ORR) in acidic media was screened by scanning electrochemical microscopy. The Pd0.7W0.3 showed the highest activity for the ORR, close to that for Pd0.8Co0.2 and Pt. A Pd–W electrocatalyst loaded on carbon black was formed by the NaBH4-reduction method, exhibiting high activity and stability, suggesting that it is a good candidate for the proton exchange membrane fuel cell cathode.

We observe the electrochemical formation of a silicon layer on a molybdenum (Mo) electrode at 850 °C in a CaCl2 melt containing silicon oxide (SiO2) nanoparticles (NPs). A stable electrochemical cell for high temperature molten salt was fabricated from a sealed quartz tube, with a glassy carbon counter electrode and a Ca/Ca2+ dynamic reference electrode. When SiO2 NPs are added into the CaCl2 melt, the reduction current and the generation of a new redox couple relating to the reduced silicon were observed in a cyclic voltammogram at Mo. Spectroscopic analyses confirmed the formation of a silicon layer from the reduction of SiO2 NPs. Although the deposit was not continuous, XPS shows that it is silicon of good purity and XRD shows the pattern of crystalline silicon. We also investigated the dissolved silicon species generated after the oxidation of electrodeposited silicon in CaCl2 melt, which produces a redox couple at potentials positive of the deposition potential. We tentatively attribute it to the oxidation and re-reduction of an insoluble Si–Cl surface species.

Tantalum cobalt nitride photocatalysts were prepared using a simple drop coating method on a Ta foil substrate followed by thermal ammonia treatment, and their photoelectrochemical (PEC) properties for water oxidation under visible light were studied. The resulting Ta0.9Co0.1Nx films showed a photocurrent of ca. 1.5 mA/cm2 (12 times higher than that of Ta3N5) under 100 mW/cm2 visible light irradiation at 0.7 V vs Ag/AgCl in a 0.1 M Na2SO4 aqueous solution (pH 11). The good PEC performance was attributed to the introduction of cobalt and the formation of cobalt nitride, which efficiently facilitates electron transfer and suppresses the recombination of photogenerated electron–hole pairs. Some cobalt nitride could further be oxidized to generate cobalt oxide, which serves as an efficient electrocatalyst for water oxidation. The enhanced visible light activity and film stability under light irradiation make tantalum cobalt nitride a promising semiconductor for PEC water oxidation.

The construction and use of “dynamic potential–pH diagrams” (DPPDs), that are intended to extend the usefulness of thermodynamic Pourbaix diagrams to include kinetic considerations is described. As an example, DPPDs are presented for the comparison of electrocatalysts for water oxidation, i.e., the oxygen evolution reaction (OER), an important electrochemical reaction because of its key role in energy conversion devices and biological systems (water electrolyses, photoelectrochemical water splitting, plant photosynthesis). The criteria for obtaining kinetic data are discussed and a 3-D diagram, which shows the heterogeneous electron transfer kinetics of an electrochemical system as a function of pH and applied potential is presented. DPPDs are given for four catalysts: IrO2, Co3O4, Co3O4 electrodeposited in a phosphate medium (Co–Pi) and Pt, allowing a direct comparison of the activity of different electrode materials over a broad range of experimental conditions (pH, potential, current density). In addition, the experimental setup and the factors affecting the accurate collection and presentation of data (e.g., reference electrode system, correction of ohmic drops, bubble formation) are discussed.

A picoliter solution dispenser was used to fabric-ate photosensitizer arrays on mesoporous TiO2 electrodes. The scanning electrochemical microscopy (SECM) technique, modified by replacing the usual ultramicroelectrode (UME) with an optical fiber and using the photooxidation of iodide in acetonitrile in a photoelectrochemical (PEC) cell, was shown to be useful for the initial screening of potential PEC photosensitizers. This SECM technique allows for the rapid identification of new dyes and also can be used to investigate the synergetic effect of multiple dyes for application in dye-sensitized solar cells (DSSCs). This technique was specifically demonstrated via the synthesis and analysis of a new, bis-bithiophene functionalized porphyrin derivative, wherein the modified SECM technique was used to carry out an initial test of its PEC efficiency relative to other dyes. The PEC properties of bulk films based on this new porphyrin derivative were then investigated and the results shown to be in good agreement...

After growing a thin layer of oxide (PtOx) by anodization of a Pt electrode, it changed from catalytically active for electrochemical NaBH4 oxidation into an inactive electrode. When held at a potential where the oxide film was maintained, collisions of individual 14 nm diameter Au nanoparticles (NPs) that catalyzed NaBH4 oxidation were successfully observed as discrete current pulses (spikes or blips) for each NP interaction with the modified Pt electrode via amplification from NaBH4 oxidation. The current response is affected by NP concentration and the applied potential.

The electrocatalytic properties of individual single Pt nanoparticles (NPs) can be studied electrochemically by measuring the current−time (i−t) responses during single NP collisions with a noncata...

Scanning electrochemical microscopy in the photoelectrochemical (PEC) mode was used to search for more efficient doped iron oxide photocatalysts under visible light irradiation (λ ≥ 420 nm). Iron oxide doped with one or two different metal cations was investigated to improve its PEC performance. Among various dopants, Sn or Ti as single dopants and Be or Al as codopants showed an improved photocurrent response of Fe2O3 under visible light irradiation (λ ≥ 420 nm). Fe2O3 doped with 4% Sn(IV) and 6% Be(II) showed the highest photocurrent as well as a good photosensitivity and stability in alkali solution (0.2 M NaOH) under UV and visible light irradiation.

The diffusion coefficient (D(O(2))), solubility (C(O(2))), and electrochemical behavior of oxygen reduction were investigated by cyclic voltammetry and transient amperometry on a Pt ultramicroelectrode in aqueous solutions containing various concentrations of NaOH (1-12 M). The results show that both D(O(2)) and C(O(2)) decrease as the solution viscosity (eta) increases significantly with increasing concentration of NaOH. The Stokes-Einstein relationship (D(O(2)) vs 1/eta) is followed, yielding a radius for the O(2) molecule, 2.8 A, that does not change over the concentration range of NaOH studied. From results reported previously for C(O(2)) in more dilute NaOH solutions and the new results here, the number of electrons, n, involved in the first step of the oxygen reduction reaction (ORR) was found to change with the concentration of NaOH, from n = 2 at low NaOH concentrations (1-2 M) to n = 1 at high concentrations (6 M), in line with the changing water activity. Scanning electrochemical microscopy (SECM) was employed as a sensitive tool to investigate the electrochemical behavior of the product of the ORR in 10 M NaOH solution, O(2)(*-), as a function of potential. The SECM approach curves depend on the substrate bias and state of the Pt substrate surface, and the apparent rate constant for the redox couple of O(2)/O(2)(*-) is determined to be about 2.6 x 10(-4) cm/s in this solution.

Electrochemical hydrazine oxidation and proton reduction occur at a significantly higher rate at Pt than at Au or C electrodes. Thus, the collision and adhesion of a Pt particle on a less active Au or C electrode leads to a large current amplification by electrocatalysis at single nanoparticles (NPs). At low particle concentrations, the collision of Pt NPs was characterized by current transients composed of individual current profiles that rapidly attained a steady state, signaling single NP collisions. The characteristic steady-state current was used to estimate the particle size. The fluctuation in collision frequency with time indicates that the collision of NPs at the detector electrodes occurs in a statistically random manner, with the average frequency a function of particle concentration and diffusion coefficient. A longer term current decay in single current transients, as opposed to the expected steady-state behavior, was more pronounced for proton reduction than for hydrazine oxidation, revealing microscopic details of the nature of the particle interaction with the detector electrode and the kinetics of electrocatalysis at single NPs. The study of single NP collisions allows one to screen particle size distributions and estimate NP concentrations and diffusion coefficients.

This review describes work done in scanning electrochemical microscopy (SECM) since 2000 with an emphasis on new applications and important trends, such as nanometer-sized tips. SECM has been adapted to investigate charge transport across liquid/liquid interfaces and to probe charge transport in thin films and membranes. It has been used in biological systems like single cells to study ion transport in channels, as well as cellular and enzyme activity. It is also a powerful and useful tool for the evaluation of the electrocatalytic activities of different materials for useful reactions, such as oxygen reduction and hydrogen oxidation. SECM has also been used as an electrochemical tool for studies of the local properties and reactivity of a wide variety of materials, including metals, insulators, and semiconductors. Finally, SECM has been combined with several other nonelectrochemical techniques, such as atomic force microscopy, to enhance and complement the information available from SECM alone.

We demonstrate a novel method of observing single particle collision events with electrogenerated chemiluminescence (ECL). A single event is characterized by the enhancement of ECL intensity during the collision of an individual platinum nanoparticle (Pt NP) on an indium tin oxide electrode, which catalyzes the oxidation of Ru(bpy)3(2+) and a coreactant, for example, tri- n-propylamine (TPrA), present in the solution. Every collision produces a unique photon spike whose amplitude and frequency can be correlated with the size and concentration of the Pt NPs. A large amplification of ECL intensity can occur by choosing an appropriate measuring electrode and using high concentrations of Ru(bpy)3(2+) and the coreactant.

We report here the current–voltage (i–V) characteristics of several (n++- Si /MNOPE/ C 60/ Pt -tip) or (n++- Si /MNOPE/SWCNT/ Pt -tip) junctions, where MNOPE = 2'-mononitro-4, 4'-bis(phenylethynyl)-1-phenylenediazonium and SWCNT = single wall carbon nanotube. A layer of C 60 or SWCNT-derivatized MNOPE has strong effect on the i–V behavior of the junctions, including rectification, negative differential resistance (NDR) and switching behaviors. The i–V curve of a grafted molecular monolayer (GMM) of MNOPE atop n++- Si shows NDR behavior, whereas those of C 60- and SWCNT-derivatized GMMs of MNOPE on n++- Si show strong rectifying behavior with opposite rectification polarities. With C 60, larger currents were found with negative tip bias, while with SWCNT, the forward top bias was positive. Because C 60 tends to be a good electron acceptor and SWCNTs tend to be good electron donors, they show different i–V behavior, as observed. Some of the (n++- Si /MNOPE/SWCNT/ Pt -tip) junctions also show reversible bistable switching behavior.

Despite intense, long-term interest in organic semiconductors from both an applied and fundamental perspective, key aspects of the electronic properties of these materials remain poorly defined. A particularly challenging problem is the molecular nature of positive charge carriers, that is, holes or oxidized species in organics. Here, the unique ability of single-molecule spectroelectrochemistry (SMS-EC) to unravel complex electrochemical process in heterogeneous media is used to study the oxidation of nanoparticles of the conjugated polymer poly(9,9-dioctylfluorene-co-benzothiadiazole). A reversible hole-injection charging process has been observed that occurs primarily by initial injection of shallow (untrapped) holes, but soon after the injection, a small fraction of the holes becomes deeply trapped. Good agreement between experimental data and simulations strongly supports the presence of deep traps in the studied nanoparticles and highlights the ability of SMS-EC to study energetics and dynamics of deep traps in organic materials at the nanoscale.

We report in this paper a facile approach for the formation and electrochemical characterization of silver-silver oxide core-shell nanoparticles (NPs). Thus, thermal treatment at temperatures between 200 and 360 degrees C of Ag NP, in the gas phase or in an organic solvent, has been used to achieve the formation Ag@Ag2O NP. The evidence of formation of such a core-shell structure was obtained by cyclic voltammetry using a Nafion modified electrode (where Nafion containing carbon particles is used as the matrix to encapsulate the core-shell NP). Initial positive scans measure free Ag. Initial negative scans measure Ag2O, with the following positive scan, compared to the initial one, providing a measure of "trapped" or core Ag. The results presented demonstrate the utility of this approach in characterizing core-shell structures, like Ag@Ag2O, which could be extended to other core-shell forms, such as bimetallic core-shell NP.

We demonstrate the feasibility of semiconductor deposition via the electrochemical reduction of gaseous precursors by the use of an anhydrous proton-conducting membrane, the solid acid CsHSO4, at 165 °C. This membrane electrode assembly was operated within the oxidation of hydrogen on a porous Pt anode and the deposition of Si or Ge under bias at the cathode from chloride-based gaseous precursors; SiCl4 and GeCl4 in an Ar flow with a reduction potential over -1.0 V (vs RHE).

Extraction of small molecule components into water from photoresist materials designed for 193 nm immersion lithography has been observed. Leaching of photoacid generator (PAG) has been monitored using three techniques: liquid scintillation counting (LSC); liquid chromatography mass spectrometry (LCMS); and scanning electrochemical microscopy (SECM). LSC was also used to detect leaching of residual casting solvent (RCS) and base. The amount of PAG leaching from the resist films, 30−50 ng/cm2, was quantified using LSC. Both LSC and LCMS results suggest that PAG and photoacid leach from the film only upon initial contact with water (within 10 s) and minimal leaching occurs thereafter for immersion times up to 30 min. Exposed films show an increase in the amount of photoacid anion leaching by upward of 20% relative to unexposed films. Films pre-rinsed with water for 30 s showed no further PAG leaching as determined by LSC. No statistically significant amount of residual casting solvent was extracted after 3...

Electrochemical, spectroscopic, and mass spectrometric (MS) methods were used to probe the interaction (complexation) of silver ions and zerovalent silver species with polyamidoamine generation 1 amine-terminated (PAMAMG1NH2) and generation 2 hydroxy-terminated (PAMAMG2OH) dendrimers (DDMs). Stability constants (Kq+) and stoichiometries (q) (i.e., the number of silver ions complexed per DDM molecule) were determined from the voltammetric data, that is, shifts in potential and changes in peak or limiting current with addition of DDM. When the mole ratio of DDM to Ag+ isor = 1, Ag+ binds with PAMAMG2OH to form a dominant 1:1 complex with a value of 1.1 x 10(7) M(-1). Under similar conditions, Ag+ binds with PAMAMG1NH2, yielding a 1:1 complex with = 4 x 10(9) M(-1), which is consistent with the finding of the MS experiments. When the mole ratio is1, qor = 2. The E0' of the Ag-PAMAMG1NH2(+/0) couple shifted to a more negative value than that of the Ag(+/0) couple. The negative shift in the halfwave potential also suggests that DDM binds more strongly with Ag+ than with zerovalent silver species. Spectroscopic results suggest that hydroxyl-terminated PAMAMG2OH favors the formation of small zerovalent silver clusters after reduction while amine-terminated PAMAMG1NH2 allows for simultaneous formation of both clusters and larger nanoparticles at similar conditions. Other quantities, such as diffusion coefficients of the complexes and molar absorptivity of the Ag+ DDMs, are also reported.

We report excellent correlations between the first negative threshold potentials (V(TH)s) for electric conduction, electrochemical potentials, and computed lowest unoccupied molecular orbital energies in a series of phenylene-ethynylene oligomers bearing a sulfur-based anchoring unit and different electroactive substituents on the central benzene ring. The theoretical and electrochemical results strongly suggest that the peaks observed in the i-V curves have a true molecular origin and are associated with distinct unoccupied molecular levels of the compounds that are strongly localized on the central ring (except for compound I). This localization might account for the existence of a long-lived radical-anion state that permits lateral electron hopping and leads to charge trapping and storage.

The electrical properties of self-assembled monolayers (SAMs) on metal surfaces have been explored for a series of molecules to address the relation between the behavior of a molecule and its structure. We probed interfacial electron transfer processes, particularly those involving unoccupied states, of SAMs of thiolates or arylates on Au by using shear force-based scanning probe microscopy (SPM) combined with current-voltage (i-V) and current-distance (i-d) measurements. The i-V curves of hexadecanethiol in the low bias regime were symmetric around 0 V and the current increased exponentially with V at high bias voltage. Different than hexadecanethiol, reversible peak-shaped i-V characteristics were obtained for most of the nitro-based oligo(phenylene ethynylene) SAMs studied here, indicating that part of the conduction mechanism of these junctions involved resonance tunneling. These reversible peaked i-V curves, often described as a negative differential resistance (NDR) effect of the junction, can be used to define a threshold tip bias, V(TH), for resonant conduction. We also found that for all of the SAMs studied here, the current decreased with increasing distance, d, between tip and substrate. The attenuation factor beta of hexadecanethiol was high, ranging from 1.3 to 1.4 A(-1), and was nearly independent of the tip bias. The beta-values for nitro-based molecules were low and depended strongly on the tip bias, ranging from 0.15 A(-1) for tetranitro oligo(phenylene ethynylene) thiol, VII, to 0.50 A(-1) for dinitro oligo(phenylene) thiol, VI, at a -3.0 V tip bias. Both the V(TH) and beta values of these nitro-based SAMs were also strongly dependent on the structures of the molecules, e.g. the number of electroactive substituent groups on the central benzene, the molecular wire backbone, the anchoring linkage, and the headgroup. We also observed charge storage on nitro-based molecules. For a SAM of the dintro compound, V, approximately 25% of charge collected in the negative scan is stored in the molecules and can be collected at positive voltages. A possible mechanism involving lateral electron hopping is proposed to explain this phenomenon.

The use of the scanning electrochemical microscope (SECM) to probe the kinetics of charge-transfer processes at bilayer lipid membranes (BLM) is presented. Analysis of the SECM tip response demonstrates that an unmodified BLM behaves as an insulator, whereas a BLM doped with iodine shows some positive feedback. The SECM technique thus allows one to probe processes at a BLM and determine the kinetics of the charge-transfer process. The SECM can also be used to determine the shape of the BLM.

We describe several transinterface electron-transfer processes occurring at the benzonitrile (PhCN)/water interface that produce excited states. When an electrode was moved very close to the interface or even through the interface to produce an electrode/thin layer organic phase/aqueous solution configuration, the electrogenerated radical species formed in the PhCN phase (C12−Ru(bpy)33+, DPA+, or TH+) diffused to the liquid/liquid interface to react with (oxidize) a coreactant (C2O42-) soluble only in the aqueous solution, leading to the formation of a strongly reducing radical species (CO2•-). The electron transfer from CO2•- in the aqueous solution to the oxidized species in the organic phase across the liquid/liquid interface produced an electronically excited state which then emitted light. The appearance of an ECL signal in these systems supports the suggestion of Marcus inverted region behavior in very exothermic heterogeneous electron-transfer reactions at the liquid/liquid interface.

Electrogenerated chemiluminescence (ECL) at a scanning electrochemical microscope (SECM) tip as the tip was moved in the vicinity of insulating and conductive substrates was studied either by ion annihilation or with a coreactant. The SECM/ECL approach curves (intensity vs tip−substrate distance, d) with both insulating and conducting substrates showed a decrease in ECL intensity with a decrease in d. Modes of operation that produced sufficiently intense and stable ECL for optical imaging are possible. The needed improvements for higher resolution and the potential application of this technique for the studies of the kinetics and mechanisms of coreactant ECL reactions are discussed.

The forces between a silica probe and an n-type TiO2 single-crystal electrode were measured using an atomic force microscope in an aqueous electrolyte solution. These interactions were a strong function of the solution pH, the presence of specifically adsorbed anions, and the TiO2 electrode potential. For a series of pH values, a strong electrostatic repulsion was seen at high pH and decreased as the pH was reduced. At pH values below 5.5, the interaction became attractive. A series of force measurements between SiO2 and n-type TiO2 showed a repulsive interaction when TiO2 was held at negative electrode potentials, which transformed to an attractive force at positive potentials. The potential at which the interaction passed through a minimum, called the potential of zero force (pzf), corresponded closely to the flat-band potential ( Vfb) of the TiO2 electrode under conditions where the solution pH was held at the isoelectric point (iep) of titania. The Vfb measured by this method gave a value near -0.4 V vs SCE at pH 5.5, which was in good agreement with photoelectrochemical measurements made under similar conditions. At pH values deviating from the iep, the pzf and Vfb were not equivalent. This was illustrated by potential- and pH-dependent force curves taken at the same n-TiO2 electrode in the presence of the polymeric anion hexametaphosphate (HMP), which is known to specifically adsorb on TiO2. An increase in negative surface charge due to adsorbed HMP was observed by an increase in the repulsive force with respect to the silica probe at open circuit for a specific pH value. Potential-dependent force measurements determined that the pzf shifted toward more positive values in the presence of HMP, in direct opposition to the negative shift in Vfb. This apparent discrepancy was caused by the presence of both adsorbed and potential-induced surface charge, which could not be differentiated by simply measuring the diffuse double-layer charge.

Pure, polycrystalline, and photoactive silicon is directly obtained from SiO2 nanoparticles by electrodeposition in molten CaCl2 on a silver electrode (quartz tube, 850 °C).