University of South Carolina Sumter

This feature article highlights work from the authors' laboratories on the synthesis, assembly, reactivity, and optical applications of metallic nanoparticles of nonspherical shape, especially nanorods. The synthesis is a seed-mediated growth procedure, in which metal salts are reduced initially with a strong reducing agent, in water, to produce approximately 4 nm seed particles. Subsequent reduction of more metal salt with a weak reducing agent, in the presence of structure-directing additives, leads to the controlled formation of nanorods of specified aspect ratio and can also yield other shapes of nanoparticles (stars, tetrapods, blocks, cubes, etc.). Variations in reaction conditions and crystallographic analysis of gold nanorods have led to insight into the growth mechanism of these materials. Assembly of nanorods can be driven by simple evaporation from solution or by rational design with molecular-scale connectors. Short nanorods appear to be more chemically reactive than long nanorods. Finally, optical applications in sensing and imaging, which take advantage of the visible light absorption and scattering properties of the nanorods, are discussed.

Gold nanorods with aspect ratios of 4.6 ± 1.2, 13 ± 2, and 18 ± 2.5 (all with 16 ± 3 nm short axis) are prepared by a seeding growth approach in the presence of an aqueous miceller template. Citrate-capped 3.5 nm diameter gold particles, prepared by the reduction of HAuCl4 with borohydride, are used as the seed. The aspect ratio of the nanorods is controlled by varying the ratio of seed to metal salt. The long rods are isolated from spherical particles by centrifugation.

Short gold nanorods of average lengths ranging between 20 and 100 nm (with corresponding aspect ratios of 2 and 4) were synthesized in excellent yield (approximately 97%). These nanorods were characterized by dark-field microscopy, UV-visible spectrophotometry, and transmission electron microscopy. Temporal evolution of rod shape had also been followed by UV-visible spectrophotometry and transmission electron microscopy and indicates that the nanorods briefly increase in length, then increase slightly in width, as they grow. The effect of the synthetic parameters on the rod dimension and yield was explored to find out suitable conditions to produce short nanorods; short nanorods have both plasmon bands in the visible region of the spectrum, which is a valuable property for sensor applications.

Following a seeding growth approach, gold nanoparticles of diameters 5−40 nm were prepared with 10−15% standard deviation in diameter from 3.5 ± 0.7 nm gold particle seeds. Particle size can be controlled by varying the ratio of seed to metal salt, and thus any size in the range 5−40 nm can be prepared. The method can also be scaled up to produce 10−100 mg of gold nanoparticles.

Using a seed-mediated growth approach in a rodlike micellar media, silver nanorods of varied aspect ratio were prepared from nearly spherical 4 nm silver nanoparticles.

Gold nanorods were prepared via a seed-mediated sequential growth process involving the use of citrate-stabilised seed crystals and their subsequent growth in a series of reaction solutions containing [AuCl4]−, ascorbic acid and the cationic surfactant cetyltrimethylammonuim bromide (CTAB). Electron diffraction analysis and HRTEM images of mature nanorods showed superpositions of two specific pairs of crystallographic zones, either <112> and <100> or <110> and <111>, which were consistent with a cyclic penta-twinned crystal with five {111} twin boundaries arranged radially to the [110] direction of elongation. The nanorods have an idealised 3-D prismatic morphology with ten {111} end faces and five {100} or {110} side faces, or both. TEM studies of crystals at various stages of growth indicated that the seed crystals are initially transformed by growth and aggregation into decahedral penta-twinned crystals, 4% of which become elongated when a fresh reaction solution is added, whilst the remaining twins grow isometrically. Reiteration of this procedure increases the length of the existing nanorods, induces further transformation of isometric particles to produce a second (and third) population of shorter, wider nanorods, and increases the size of the isometric crystals. The data indicate that symmetry breaking in fcc metallic structures to produce anisotropic nanoparticles is based on an intrinsic structural mechanism (twinning) that is subsequently modulated extrinsically during growth in solution by specific adsorption of AuI–surfactant complexes on the side faces/edges of the isometric penta-twinned crystals and which is responsible for the preferential growth along the common [110] axis. We propose that the coupling of multiple twinning and habit modification is a general mechanism that applies to other experimental procedures (electrochemical, inverse micellar media) currently used to prepare metallic nanoparticles with a high aspect ratio.

The seed-mediated approach to making gold nanorods in aqueous surfactant solutions has become tremendously popular in recent years. Unlike the use of strong chemical reductants to make spherical gold nanoparticles, the growth of gold nanorods requires weak reducing conditions, leading to an unknown degree of gold reduction. The metal content of gold nanorods, made in high yield in the presence of silver ion, is determined by inductively coupled plasma atomic emission spectroscopy. Through the use of the known gold concentration in nanorods, molar extinction coefficients are calculated for nanorods of varying aspect ratios from 2.0 to 4.5. The extinction coefficients at the longitudinal plasmon band peak maxima for these nanorods vary from 2.5x10(9) to 5.5x10(9) M-1 cm-1, respectively, on a per-particle basis. Many of the gold ions present in the growth solution remain unreacted; insights into the growth mechanism of gold nanorods are discussed.

Copper(I)-catalyzed 1,3-dipolar cycloaddition reaction of nonfluorescent 3-azidocoumarins and terminal alkynes afforded intense fluorescent 1,2,3-triazole products. The mild condition of this reaction allowed us to construct a large library of pure fluorescent coumarin dyes. Since both azide and alkyne are quite inert to biological systems, this reaction has potential in bioconjugation and bioimaging applications. [reaction: see text]

Surface enhanced Raman scattering (SERS) spectra of 4-mercaptobenzoic acid (4-MBA) self-assembled monolayers (SAMs) on gold substrates is presented for SAMs onto which gold nanoparticles of various shapes have been electrostatically immobilized. SERS spectra of 4-MBA SAMs are enhanced in the presence of immobilized gold nanocrystals by a factor of 10(7)-10(9) relative to 4-MBA in solution. Large enhancement factors are a likely result of plasmon coupling between the nanoparticles (localized surface plasmon) and the smooth gold substrate (surface plasmon polariton), creating large localized electromagnetic fields at their interface, where 4-MBA molecules reside in this sandwich architecture. Moreover, enhancement factors depend on nanoparticle shape and vary by a factor of 10(2). This SERS geometry offers large surface enhancements for molecules adsorbed onto planar substrates and could be quite useful for determining chemical information for poor Raman scatterers from assays on 2-D substrates.

A major challenge in nanoscience and nanotechnology is the rational assembly of nanoscale objects. Here we report that gold nanorods, aspect ratio 18, can be functionalized with a biotin disulfide, and subsequent addition of streptavidin links the rods together in an end-to-end manner much more often than expected.

In this review, we highlight how recent advances achieved in the fields of photochemistry and photophysics of metal-organic frameworks (MOFs) could be applied towards the engineering of next generation MOF-based sensing devices. In addition to high surface area and structural tunability, which are crucial for efficient sensor development, progress in the field of MOF-based sensors could rely on the combination of MOF light-harvesting ability, understanding energy transfer processes within a framework, and application of MOF-based photocatalysis towards sensing enhancement. All photophysical concepts could be integrated within one material to improve efficiency and selectivity of sensing devices. Thus, the focus of this review is shifted towards a "beyond the pores" approach, which could foreshadow new guidelines for sensor engineering.

We report here the solution-phase synthesis of highly uniform and monodisperse cubic Cu2O nano- and microcubes. Copper(II) salts in water are reduced with sodium ascorbate in air, in the presence of a surfactant. The average edge length of the cubes varies from 200 to 450 nm, as a function of surfactant concentration. Transmission electronic microscopy suggests that these cubes are composed of small nanoparticles and appear to be hollow.

The late Pleistocene history of seawater temperature and salinity variability in the western tropical Pacific warm pool is reconstructed from oxygen isotope (delta18O) and magnesium/calcium composition of planktonic foraminifera. Differentiating the calcite delta18O record into components of temperature and local water delta18O reveals a dominant salinity signal that varied in accord with Dansgaard/Oeschger cycles over Greenland. Salinities were higher at times of high-latitude cooling and were lower during interstadials. The pattern and magnitude of the salinity variations imply shifts in the tropical Pacific ocean/atmosphere system analogous to modern El Niño-Southern Oscillation (ENSO). El Niño conditions correlate with stadials at high latitudes, whereas La Niña conditions correlate with interstadials. Millennial-scale shifts in atmospheric convection away from the western tropical Pacific may explain many paleo-observations, including lower atmospheric CO2, N2O, and CH4 during stadials and patterns of extratropical ocean variability that have tropical source functions that are negatively correlated with El Niño.

In 1990, Andrew Bakun proposed that increasing greenhouse gas concentrations would force intensification of upwelling-favorable winds in eastern boundary current systems that contribute substantial services to society. Because there is considerable disagreement about whether contemporary wind trends support Bakun's hypothesis, we performed a meta-analysis of the literature on upwelling-favorable wind intensification. The preponderance of published analyses suggests that winds have intensified in the California, Benguela, and Humboldt upwelling systems and weakened in the Iberian system over time scales ranging up to 60 years; wind change is equivocal in the Canary system. Stronger intensification signals are observed at higher latitudes, consistent with the warming pattern associated with climate change. Overall, reported changes in coastal winds, although subtle and spatially variable, support Bakun's hypothesis of upwelling intensification in eastern boundary current systems.

A three-step seed-mediated growth method was used to make gold nanoparticles. Different surfactants, alkyltrimethylammonium bromides (CnTAB, n = 10, 12, 14, 16, and 18) and cetylpyridinium chloride (C16PC), were chosen as stabilizers. In general, it was found that as the length of the surfactant chain increased, the resulting gold nanoparticles' aspect ratio increased: the aspect ratio was 1 (for C10TAB), 5 ± 2 (C12TAB), 17 ± 3 (C14TAB), and 23 ± 4 (C16TAB). The plasmon absorption maxima for the gold nanoparticles varied as a function of the shape, from 520 nm (spheres) to beyond 2000 nm (high aspect ratio nanorods). We propose that the surfactant binds as a bilayer to the growing nanoparticle and assists in nanoparticle elongation via a “zipping” mechanism.

Gold−silver alloy nanoparticles were synthesized via reduction of varying mole fractions of HAuCl4 and AgNO3 by sodium borohydride in the presence of sodium citrate as a capping agent, in water. Solution concentrations were adjusted to avoid the precipitation of AgCl during the course of the reaction. The plasmon absorption band of Ag−Au nanoparticles shifts linearly to the red with increasing Au content. Due to the direct dependence of the metal salt ratio to the shift of the absorbance peak and the lack of an apparent core−shell structure in transmission electron microscope images, it is concluded that the nanoparticles are alloys, rather than core−shell composites.

ABSTRACT Sulfate‐reducing bacteria (SRB) have been recognized as key players in the precipitation of calcium carbonate in lithifying microbial communities. These bacteria increase the alkalinity by reducing sulfate ions, and consuming organic acids. SRB also produce copious amounts of exopolymeric substances (EPS). All of these processes influence the morphology and mineralogy of the carbonate minerals. Interactions of EPS with metals, calcium in particular, are believed to be the main processes through which the extracellular matrix controls the precipitation of the carbonate minerals. SRB exopolymers were purified from lithifying mat and type cultures, and their potential role in CaCO 3 precipitation was determined from acid‐base titrations and calcium‐binding experiments. Major EPS characteristics were established using infrared spectroscopy and gas chromatography to characterize the chemical functional groups and the sugar monomers composition. Our results demonstrate that all of the three SRB strains tested were able to produce large amounts of EPS. This EPS exhibited three main buffering capacities, which correspond to carboxylic acids (pK a = 3.0), sulfur‐containing groups (thiols, sulfonic and sulfinic acids – pK a = 7.0–7.1) and amino groups (pK a = 8.4–9.2). The calcium‐binding capacity of these exopolymers in solution at pH 9.0 ranged from 0.12g Ca g EPS −1 –0.15 g Ca g EPS −1 . These results suggest that SRB could play a critical role in the formation of CaCO 3 in lithifying microbial mats. The unusually high sulfur content, which has not been reported for EPS before, indicates a possible strong interaction with iron. In addition to changing the saturation index through metabolic activity, our results imply that SRB affect the rock record through EPS production and its effect on the CaCO 3 precipitation. Furthermore, EPS produced by SRB may account for the incorporation of metals (e.g. Sr, Fe, Mg) associated with carbonate minerals in the rock record.

Gold nanorods prepared by a seed-mediated growth approach use ∼4-nm gold nanospheres as the seeds and subsequent reduction of metal salt with a weak reducing agent (ascorbic acid) in the presence of a directing surfactant to produce nanorods. If insufficient ascorbic acid is added in the growth step, then metal salt remains. Additional input of ascorbic acid preferentially deposits more metal at the ends of the nanorods, to yield “dogbone”-like structures. Surprisingly, heat treatment of the unpurified gold nanorods (prepared with an insufficient amount of ascorbic acid) yielded fatter gold nanorods; the oxidation product of ascorbic acid appears to act as a reductant at higher temperature. These modified shapes of the gold nanorods directly influence their optical properties.

ABCB1, also known as P-glycoprotein, actively extrudes xenobiotic compounds across the plasma membrane of diverse cells, which contributes to cellular drug resistance and interferes with therapeutic drug delivery. We determined the 3.5-angstrom cryo-electron microscopy structure of substrate-bound human ABCB1 reconstituted in lipidic nanodiscs, revealing a single molecule of the chemotherapeutic compound paclitaxel (Taxol) bound in a central, occluded pocket. A second structure of inhibited, human-mouse chimeric ABCB1 revealed two molecules of zosuquidar occupying the same drug-binding pocket. Minor structural differences between substrate- and inhibitor-bound ABCB1 sites are amplified toward the nucleotide-binding domains (NBDs), revealing how the plasticity of the drug-binding site controls the dynamics of the adenosine triphosphate-hydrolyzing NBDs. Ordered cholesterol and phospholipid molecules suggest how the membrane modulates the conformational changes associated with drug binding and transport.

A global compilation of 170 time‐averaged volumetric volcanic output rates (Q e ) is evaluated in terms of composition and petrotectonic setting to advance the understanding of long‐term rates of magma generation and eruption on Earth. Repose periods between successive eruptions at a given site and intrusive:extrusive ratios were compiled for selected volcanic centers where long‐term (&gt;10 4 years) data were available. More silicic compositions, rhyolites and andesites, have a more limited range of eruption rates than basalts. Even when high Q e values contributed by flood basalts (9 ± 2 × 10 −1 km 3 /yr) are removed, there is a trend in decreasing average Q e with lava composition from basaltic eruptions (2.6 ± 1.0 × 10 −2 km 3 /yr) to andesites (2.3 ± 0.8 × 10 −3 km 3 /yr) and rhyolites (4.0 ± 1.4 × 10 −3 km 3 /yr). This trend is also seen in the difference between oceanic and continental settings, as eruptions on oceanic crust tend to be predominately basaltic. All of the volcanoes occurring in oceanic settings fail to have statistically different mean Q e and have an overall average of 2.8 ± 0.4 × 10 −2 km 3 /yr, excluding flood basalts. Likewise, all of the volcanoes on continental crust also fail to have statistically different mean Q e and have an overall average of 4.4 ± 0.8 × 10 −3 km 3 /yr. Flood basalts also form a distinctive class with an average Q e nearly two orders of magnitude higher than any other class. However, we have found no systematic evidence linking increased intrusive:extrusive ratios with lower volcanic rates. A simple heat balance analysis suggests that the preponderance of volcanic systems must be open magmatic systems with respect to heat and matter transport in order to maintain eruptible magma at shallow depth throughout the observed lifetime of the volcano. The empirical upper limit of ∼10 −2 km 3 /yr for magma eruption rate in systems with relatively high intrusive:extrusive ratios may be a consequence of the fundamental parameters governing rates of melt generation (e.g., subsolidus isentropic decompression, hydration due to slab dehydration and heat transfer between underplated magma and the overlying crust) in the Earth.