Michael J. Hostetler

Bio: Michael J. Hostetler is an academic researcher from University of North Carolina at Chapel Hill. The author has contributed to research in topics: Gold cluster & Monolayer. The author has an hindex of 16, co-authored 16 publications receiving 6534 citations.

Alkanethiolate Gold Cluster Molecules with Core Diameters from 1.5 to 5.2 nm: Core and Monolayer Properties as a Function of Core Size

Abstract: The mean size of the gold (Au) core in the synthesis of dodecanethiolate-stabilized Au cluster compounds can be finely adjusted by choice of the Au:dodecanethiolate ratio and the temperature and rate at which the reduction is conducted. The Au clusters have been examined with a large number of independent analytical tools, producing a remarkably consistent picture of these materials. Average cluster and core dimensions, as ascertained by 1H NMR line broadening, high-resolution transmission electron microscopy, small-angle X-ray scattering, and thermogravimetric analysis, vary between diameters of 1.5 and 5.2 nm (∼110−4800 Au atoms/core). The electronic properties of the Au core were examined by UV/vis and X-ray photoelectron spectroscopy; the core appears to remain largely metallic in nature even at the smallest core sizes examined. The alkanethiolate monolayer stabilizing the Au core ranges with core size from ∼53 to nearly 520 ligands/core, and was probed by Fourier transform infrared spectroscopy, diff...

Gold nanoelectrodes of varied size: Transition to molecule-like charging

Abstract: A transition from metal-like double-layer capacitive charging to redox-like charging was observed in electrochemical ensemble Coulomb staircase experiments on solutions of gold nanoparticles of varied core size. The monodisperse gold nanoparticles are stabilized by short-chain alkanethiolate monolayers and have 8 to 38 kilodaltons core mass (1.1 to 1.9 nanometers in diameter). Larger cores display Coulomb staircase responses consistent with double-layer charging of metal-electrolyte interfaces, whereas smaller core nanoparticles exhibit redox chemical character, including a large central gap. The change in behavior is consistent with new near-infrared spectroscopic data showing an emerging gap between the highest occupied and lowest unoccupied orbitals of 0.4 to 0.9 electron volt.

Dynamics of Place-Exchange Reactions on Monolayer-Protected Gold Cluster Molecules

Abstract: Monolayer-protected gold clusters (Au MPCs) are stable, easily synthesized, organic solvent-soluble, nanoscale materials MPCs with protecting monolayers composed of alkanethiolate ligands (RS) can be functionalized (R‘S) by ligand place-exchange reactions, ie, x(R‘SH) + (RS)mMPC → x(RSH) + (R‘S)m(RS)m-xMPC, where x is the number of ligands place-exchanged (1 to 108) and m is the original number (ca 108) of alkanethiolate ligands per Au314 cluster The dynamics and mechanism of this reaction were probed by determining its kinetic order and final equilibrium position relative to incoming (R‘S) and initial (RS) protecting thiolate ligands The reactions were characterized by 1H NMR and IR spectroscopy, and the dispersity of place-exchange reaction products was preliminarily inspected by chromatography The results of these experiments show that ligand exchange is an associative reaction and that the displaced thiolate becomes a thiol solution product Disulfides and oxidized sulfur species are not involv

Monolayers in Three Dimensions: Synthesis and Electrochemistry of ω-Functionalized Alkanethiolate-Stabilized Gold Cluster Compounds

Abstract: Following an important paper by Brust et al., 1 we recently described2 gold cluster compounds in which a c .1.2 nm radius Au core modeled as a 309 atom octahedron 3 is stabilized by monolayers of 95 alkanethiolate (C8, C12, or C16) ligands. These cluster molecules are exceptional in comparison to other large clusters, 4 being air stable, isolable, nonpolar solventsoluble, black solids that exhibit significant electron hopping conductivity. Characterization tools (e.g., NMR spectroscopy, thermal analysis) not applicable to monolayers on planar surfaces 5 are readily applied to these three-dimensional variants of self-assembled monolayers. Functionalization of these large cluster molecules is a prerequisite to their use as multifunctional reagents, catalysts, and chemical sensors, and to fabrication of 2and 3-dimensional structures. This paper describes a versatile synthesis of ω-functionalized alkanethiolate -Au cluster compounds, based on place exchange reactions occurring when a ω-substituted alkanethiol is added to an alkanethiolate-cluster solution. This strategy offers the advantage of avoiding concurrent changes in the Au core dimension attendant to other functionalization strategies, 6

Infrared Spectroscopy of Three-Dimensional Self-Assembled Monolayers: N-Alkanethiolate Monolayers on Gold Cluster Compounds

Abstract: Transmission infrared spectroscopy has been used to probe the structure of alkanethiolate monolayers adsorbed onto nanometer-sized gold clusters. The alkyl chain lengths vary between propanethiolate and tetracosanethiolate; specifically the C3, C4, C5, C6, C7, C8, C10, C12, C16, C20, and C24 alkanethiolates have been examined as solid suspensions in KBr pellets. It has been found that the smaller chain lengths (C3, C4, and C5) are relatively disordered, with large amounts of gauche defects present, and thus most resemble the free alkanes in the liquid state. The longer length alkanethiolates are predominantly in the all trans zigzag conformation. There are detectable amounts of near surface gauche defects, the amount of which decreases with increasing chain length, and a reasonably high percentage of end-gauche defects, the relative amount of which increases with increasing chain length. Internal gauche defects cannot be detected. A model is proposed to explain these observations, and the data are compare...

Gold nanoparticles: assembly, supramolecular chemistry, quantum-size-related properties, and applications toward biology, catalysis, and nanotechnology.

Abstract: Although gold is the subject of one of the most ancient themes of investigation in science, its renaissance now leads to an exponentially increasing number of publications, especially in the context of emerging nanoscience and nanotechnology with nanoparticles and self-assembled monolayers (SAMs). We will limit the present review to gold nanoparticles (AuNPs), also called gold colloids. AuNPs are the most stable metal nanoparticles, and they present fascinating aspects such as their assembly of multiple types involving materials science, the behavior of the individual particles, size-related electronic, magnetic and optical properties (quantum size effect), and their applications to catalysis and biology. Their promises are in these fields as well as in the bottom-up approach of nanotechnology, and they will be key materials and building block in the 21st century. Whereas the extraction of gold started in the 5th millennium B.C. near Varna (Bulgaria) and reached 10 tons per year in Egypt around 1200-1300 B.C. when the marvelous statue of Touthankamon was constructed, it is probable that “soluble” gold appeared around the 5th or 4th century B.C. in Egypt and China. In antiquity, materials were used in an ecological sense for both aesthetic and curative purposes. Colloidal gold was used to make ruby glass 293 Chem. Rev. 2004, 104, 293−346

Chemistry and properties of nanocrystals of different shapes.

Abstract: The interest in nanoscale materials stems from the fact that new properties are acquired at this length scale and, equally important, that these properties * To whom correspondence should be addressed. Phone, 404-8940292; fax, 404-894-0294; e-mail, mostafa.el-sayed@ chemistry.gatech.edu. † Case Western Reserve UniversitysMillis 2258. ‡ Phone, 216-368-5918; fax, 216-368-3006; e-mail, burda@case.edu. § Georgia Institute of Technology. 1025 Chem. Rev. 2005, 105, 1025−1102

Synthesis and Characterization of Monodisperse Nanocrystals and Close-Packed Nanocrystal Assemblies

Abstract: ▪ Abstract Solution phase syntheses and size-selective separation methods to prepare semiconductor and metal nanocrystals, tunable in size from ∼1 to 20 nm and monodisperse to ≤5%, are presented. Preparation of monodisperse samples enables systematic characterization of the structural, electronic, and optical properties of materials as they evolve from molecular to bulk in the nanometer size range. Sample uniformity makes it possible to manipulate nanocrystals into close-packed, glassy, and ordered nanocrystal assemblies (superlattices, colloidal crystals, supercrystals). Rigorous structural characterization is critical to understanding the electronic and optical properties of both nanocrystals and their assemblies. At inter-particle separations 5–100 A, dipole-dipole interactions lead to energy transfer between neighboring nanocrystals, and electronic tunneling between proximal nanocrystals gives rise to dark and photoconductivity. At separations <5 A, exchange interactions cause otherwise insulating ass...

Gold nanoparticles in chemical and biological sensing.

Abstract: Detection of chemical and biological agents plays a fundamental role in biomedical, forensic and environmental sciences1–4 as well as in anti bioterrorism applications.5–7 The development of highly sensitive, cost effective, miniature sensors is therefore in high demand which requires advanced technology coupled with fundamental knowledge in chemistry, biology and material sciences.8–13 In general, sensors feature two functional components: a recognition element to provide selective/specific binding with the target analytes and a transducer component for signaling the binding event. An efficient sensor relies heavily on these two essential components for the recognition process in terms of response time, signal to noise (S/N) ratio, selectivity and limits of detection (LOD).14,15 Therefore, designing sensors with higher efficacy depends on the development of novel materials to improve both the recognition and transduction processes. Nanomaterials feature unique physicochemical properties that can be of great utility in creating new recognition and transduction processes for chemical and biological sensors15–27 as well as improving the S/N ratio by miniaturization of the sensor elements.28 Gold nanoparticles (AuNPs) possess distinct physical and chemical attributes that make them excellent scaffolds for the fabrication of novel chemical and biological sensors (Figure 1).29–36 First, AuNPs can be synthesized in a straightforward manner and can be made highly stable. Second, they possess unique optoelectronic properties. Third, they provide high surface-to-volume ratio with excellent biocompatibility using appropriate ligands.30 Fourth, these properties of AuNPs can be readily tuned varying their size, shape and the surrounding chemical environment. For example, the binding event between recognition element and the analyte can alter physicochemical properties of transducer AuNPs, such as plasmon resonance absorption, conductivity, redox behavior, etc. that in turn can generate a detectable response signal. Finally, AuNPs offer a suitable platform for multi-functionalization with a wide range of organic or biological ligands for the selective binding and detection of small molecules and biological targets.30–32,36 Each of these attributes of AuNPs has allowed researchers to develop novel sensing strategies with improved sensitivity, stability and selectivity. In the last decade of research, the advent of AuNP as a sensory element provided us a broad spectrum of innovative approaches for the detection of metal ions, small molecules, proteins, nucleic acids, malignant cells, etc. in a rapid and efficient manner.37 Figure 1 Physical properties of AuNPs and schematic illustration of an AuNP-based detection system. In this current review, we have highlighted the several synthetic routes and properties of AuNPs that make them excellent probes for different sensing strategies. Furthermore, we will discuss various sensing strategies and major advances in the last two decades of research utilizing AuNPs in the detection of variety of target analytes including metal ions, organic molecules, proteins, nucleic acids, and microorganisms.