Jill E. Millstone

Colloidal Gold and Silver Triangular Nanoprisms

It is now well-known that the size, shape, and composition of nanomaterials can dramatically affect their physical and chemical properties, and that technologies based on nanoscale materials have the potential to revolutionize fields ranging from catalysis to medicine. Among these materials, anisotropic particles are particularly interesting because the decreased symmetry of such particles often leads to new and unusual chemical and physical behavior. Within this class of particles, triangular Au and Ag nanoprisms stand out due to their structure- and environment-dependent optical features, their anisotropic surface energetics, and the emergence of reliable synthetic methods for producing them in bulk quantities with control over their edge lengths and thickness. This Review will describe a variety of solution-based methods for synthesizing Au and Ag triangular prismatic structures, and will address and discuss proposed mechanisms for their formation.

Rationally designed nanostructures for surface-enhanced Raman spectroscopy.

Research on surface-enhanced Raman spectroscopy (SERS) is an area of intense interest because the technique allows one to probe small collections of, and in certain cases, individual molecules using relatively straightforward spectroscopic techniques and nanostructured substrates. Researchers in this area have attempted to develop many new technological innovations including high sensitivity chemical and biological detection systems, labeling schemes for authentication and tracking purposes, and dual scanning-probe/spectroscopic techniques that simultaneously provide topographical and spectroscopic information about an underlying surface or nanostructure. However, progress has been hampered by the inability of researchers to fabricate substrates with the high sensitivity, tunability, robustness, and reproducibility necessary for truly practical and successful SERS-based systems. These limitations have been due in part to a relative lack of control over the nanoscale features of Raman substrates that are responsible for the enhancement. With the advent of nanotechnology, new approaches are being developed to overcome these issues and produce substrates with higher sensitivity, stability, and reproducibility. This tutorial review focuses on recent progress in the design and fabrication of substrates for surface-enhanced Raman spectroscopy, with an emphasis on the influence of nanotechnology.

Efficient small molecule bulk heterojunction solar cells with high fill factors via pyrene-directed molecular self-assembly

Efficient organic photovoltaic (OPV) materials are constructed by attaching completely planar, symmetric end-groups to donor-acceptor electroactive small molecules. Appending C2-pyrene as the small molecule end-group to a diketopyrrolopyrrole core leads to materials with a tight, aligned crystal packing and favorable morphology dictated by π-π interactions, resulting in high power conversion efficiencies and high fill factors. The use of end-groups to direct molecular self-assembly is an effective strategy for designing high-performance small molecule OPV devices.

The role radius of curvature plays in thiolated oligonucleotide loading on gold nanoparticles.

We show that by correlating the radius of curvature of spherical gold nanoparticles of varying sizes with their respective thiol-terminated oligonucleotide loading densities, a mathematical relationship can be derived for predicting the loading of oligonucleotides on anisotropic gold nanomaterials. This mathematical relationship was tested with gold nanorods (radius 17.5 nm, length 475 nm) where the measured number of oligonucleotides per rod (3330 +/- 110) was within experimental error of the predicted loading of 3244 oligonucleotides from the derivation. Additionally, we show that once gold nanoparticles reach a diameter of approximately 60 nm the local surface experienced by the oligonucleotide is highly similar to that of a planar surface.

HALIDE ION CONTROL OF SEED MEDIATED GROWTH OF ANISOTROPIC GOLD NANOPARTICLES

There are now a variety of preparatory procedures for nanoscale gold rods, triangular prisms, and spheres. Many of these methods rely on seed-mediated approaches with cetyltrimethylammonium bromide (CTABr) as a surfactant. Interestingly, seemingly similar preparatory procedures yield very different morphologies, and although there have been a variety of proposals regarding the importance of different steps in shape control, there is no self-consistent procedure that allows one to take one batch of spherical seeds and grow either rods, prisms, or larger polyhedra in a controlled manner. In this report, it is shown that CTABr, depending upon supplier, has an iodide contaminant (at a significant but varying level), which acts as a key shape-directing element because it can strongly and selectively bind to the Au (111) facet and favor the formation of anisotropic structures. Furthermore, by starting with pure CTABr and deliberately adjusting iodide concentration, one can reproducibly drive the reaction to predominantly produce one of the three target morphologies.

Mechanistic Study of Photomediated Triangular Silver Nanoprism Growth

This article presents a mechanistic study of the photomediated growth of silver nanoprisms. The data show that the photochemical process is driven by silver redox cycles involving reduction of silver cations by citrate on the silver particle surface and oxidative dissolution of small silver particles by O2. Bis(p-sulfonatophenyl)phenylphosphine increases the solubility of the Ag(+) by complexing it and acts as a buffer to keep the concentration of Ag(+) at 20 microM. The silver particles serve as photocatalysts and, under plasmon excitation, facilitate Ag(+) reduction by citrate. Higher Ag(+) concentrations favor a competitive thermal process, which results in increased prism thickness.

On-wire lithography: synthesis, encoding and biological applications

The next step in the maturing field of nanotechnology is to develop ways to introduce unusual architectural changes to simple building blocks. For nanowires, on-wire lithography (OWL) has emerged as a powerful way of synthesizing a segmented structure and subsequently introducing architectural changes through post-chemical treatment. In the OWL protocol presented here, multisegmented nanowires are grown and a support layer is deposited on one side of each nanostructure. After selective chemical etching of sacrificial segments, structures with gaps as small as 2 nm and disks as thin as 20 nm can be created. These nanostructures are highly tailorable and can be used in electrical transport, Raman enhancement and energy conversion. Such nanostructures can be functionalized with many types of adsorbates, enabling the use of OWL-generated structures as bioactive probes for diagnostic assays and molecular transport junctions. The process takes 13–36 h depending on the type of adsorbate used to functionalize the nanostructures.

Core−Shell Triangular Bifrustums

Au(core)-Ag(shell) triangular bifrustum nanocrystals were synthesized in aqueous solution using a seed-mediated approach. The formation of the Ag layer on the Au nanoprism seeds leads to structures with highly tunable dipole and quadrupole surface plasmon resonances. Discrete dipole approximation calculations show that it is the geometry of these novel structures rather than the addition of a new element that leads to the plasmon tunability. The structure and composition of these novel nanocrystals have been investigated by transmission electron microscopy, atomic force microscopy, X-ray photoelectron spectroscopy, and energy-dispersive spectrometry.

Surprisingly Long-Range Surface-Enhanced Raman Scattering (SERS) on Au–Ni Multisegmented Nanowires†

Very long range surface-enhanced Raman scattering is observed from a nickel nanowire that is separated by 120 nm from a pair of gold nanodisks. The excitation of the surface-plasmon resonance (SPR) from the gold nanodisk pair generates an enhanced electromagnetic field near the nickel segment (SEM, left), leading to Raman intensity greater than the nickel alone (right).

Photoluminescent gold-copper nanoparticle alloys with composition-tunable near-infrared emission.

Discrete gold nanoparticles with diameters between 2 and 3 nm show remarkable properties including enhanced catalytic behavior and photoluminescence. However, tunability of these properties is limited by the tight size range within which they are observed. Here, we report the synthesis of discrete, bimetallic gold-copper nanoparticle alloys (diameter ≅ 2-3 nm) which display photoluminescent properties that can be tuned by changing the alloy composition. Electron microscopy, X-ray photoelectron spectroscopy, inductively coupled plasma mass spectrometry, and pulsed-field gradient stimulated echo (1)H NMR measurements show that the nanoparticles are homogeneous, discrete, and crystalline. Upon varying the composition of the nanoparticles from 0% to 100% molar ratio copper, the photoluminescence maxima shift from 947 to 1067 nm, with excitation at 360 nm. The resulting particles exhibit brightness values (molar extinction coefficient (ε) × quantum yield (Φ)) that are more than an order of magnitude larger than the brightest near-infrared-emitting lanthanide complexes and small-molecule probes evaluated under similar conditions.