Michael R. Rasch

Copper Selenide Nanocrystals for Photothermal Therapy

Ligand-stabilized copper selenide (Cu(2-x)Se) nanocrystals, approximately 16 nm in diameter, were synthesized by a colloidal hot injection method and coated with amphiphilic polymer. The nanocrystals readily disperse in water and exhibit strong near-infrared (NIR) optical absorption with a high molar extinction coefficient of 7.7 × 10(7) cm(-1) M(-1) at 980 nm. When excited with 800 nm light, the Cu(2-x)Se nanocrystals produce significant photothermal heating with a photothermal transduction efficiency of 22%, comparable to nanorods and nanoshells of gold (Au). In vitro photothermal heating of Cu(2-x)Se nanocrystals in the presence of human colorectal cancer cell (HCT-116) led to cell destruction after 5 min of laser irradiation at 33 W/cm(2), demonstrating the viabilitiy of Cu(2-x)Se nanocrystals for photothermal therapy applications.

Hydrophobic Gold Nanoparticle Self-Assembly with Phosphatidylcholine Lipid: Membrane-Loaded and Janus Vesicles

Hybrids of hydrophobic sub-2-nm-diameter dodecanethiol-coated Au nanoparticles and phosphatidylcholine (PC) lipid vesicles made by extrusion were examined by cryogenic transmission electron microscopy (cryoTEM). The nanoparticles loaded the vesicles as a dense monolayer in the hydrophobic core of the lipid bilayer, without disrupting their structure. Nanoparticle-vesicle hybrids could also be made by a dialysis process, mixing preformed vesicles with detergent-stabilized nanoparticles, but this approach led to vesicles only partially loaded with nanoparticles that segregated into hemispherical domains, forming a Janus vesicle-nanoparticle hybrid structure.

Alkyl passivation and amphiphilic polymer coating of silicon nanocrystals for diagnostic imaging.

A method to produce biocompatible polymer-coated silicon nanocrystals for medical imaging is shown. Silica-embedded Si nanocrystals are formed by HSQ thermolysis. The nanocrystals are then liberated from the oxide and terminated with Si-H bonds by HF etching, followed by alkyl monolayer passivation by thermal hydrosilylation. The Si nanocrystals have an average diameter of 2.1 nm ± 0.6 nm and photoluminesce with a peak emission wavelength of 650 nm, which lies within the transmission window of 650-900 nm that is useful for biological imaging. The hydrophobic Si nanocrystals are then coated with an amphiphilic polymer for dispersion in aqueous media with the pH ranging between 7 and 10 and an ionic strength between 30 mM and 2 M, while maintaining a bright and stable photoluminescence and a hydrodynamic radius of only 20 nm. Fluorescence imaging of polymer-coated Si nanocrystals in biological tissue is demonstrated, showing the potential for in vivo imaging.

Limitations on the Optical Tunability of Small Diameter Gold Nanoshells

Gold (Au) nanoshells were grown on silica nanoparticles with differing average diameters, ranging from 30 to 120 nm. Au nanoshells were also formed on silica spheres encapsulating 5 nm diameter magnetic iron oxide nanocrystals. The optical absorbance spectra of these Au nanoshells are reported. The plasmon resonance wavelengths of the smaller diameter nanoshells were significantly less tunable than those of the larger diameter nanoshells. This is due to a reduced range of accessible core-shell ratio, the geometric factor that determines the plasmon peak position, as the silica core diameter shrinks. The smaller diameter nanoshells were also found to be highly prone to aggregation, which broadens the plasmon absorption peak. Model calculations of dispersion stability as a function of silica core diameter reveal that smaller diameter Au shells exhibit more aggregation because of the size-dependence of the electrostatic double-layer potential.

In Vivo Whole Animal Fluorescence Imaging of a Microparticle-Based Oral Vaccine Containing (CuInSexS2–x)/ZnS Core/Shell Quantum Dots

Zinc sulfide-coated copper indium sulfur selenide (CuInSexS2-x/ZnS core/shell) nanocrystals were synthesized with size-tunable red to near-infrared (NIR) fluorescence with high quantum yield (40%) in water. These nanocrystals were tested as an imaging agent to track a microparticle-based oral vaccine administered to mice. Poly(lactic-co-glycolic acid) (PLGA) microparticle-encapsulated CuInSexSe2-x/ZnS quantum dots were orally administered to mice and were found to provide a distinct visible fluorescent marker in the gastrointestinal tract of living mice.

Room temperature hydrosilylation of silicon nanocrystals with bifunctional terminal alkenes.

H-terminated Si nanocrystals undergo room temperature hydrosilylation with bifunctional alkenes with distal polar moieties-ethyl ester, methyl ester, or carboxylic acids-without the aid of light or added catalyst. The passivated Si nanocrystals exhibit bright photoluminescence (PL) and disperse in polar solvents, including water. We propose a reaction mechanism in which ester or carboxylic acid groups facilitate direct nucleophilic attack of the highly curved Si surface of the nanocrystals by the alkene.

Ordered Structure Rearrangements in Heated Gold Nanocrystal Superlattices

Small-angle X-ray scattering (SAXS) data reveal that superlattices of organic ligand-stabilized gold (Au) nanocrystals can undergo a series of ordered structure transitions at elevated temperature. An example is presented of a body-centered cubic superlattice that evolves into a hexagonal close-packed structure, followed by the formation of binary simple cubic AB13 and hexagonal AB5 superlattices. Ultimately the superlattice decomposes at high temperature to bicontinuous domains of coalesced Au and intervening hydrocarbon. Transmission electron microscopy revealed that the ordered structure transformations result from partial ligand desorption and controlled Au nanocrystal growth during heating, which forces changes in superlattice symmetry. These observations suggest some similarity between organic ligand-coated nanocrystals and microphase-segregated diblock copolymers, where thermally induced nanophase-segregation of Au and organic ligand influences the ordered arrangements in the superlattice.

Chloroform-Enhanced Incorporation of Hydrophobic Gold Nanocrystals into Dioleoylphosphatidylcholine (DOPC) Vesicle Membranes

Vesicles of dioleoylphosphatidylcholine (DOPC) formed by extrusion (liposomes) with hydrophobic alkanethiol-capped Au nanocrystals were studied. Dodecanethiol-capped 1.8-nm-diameter Au nanocrystals accumulate in the lipid bilayer, but only when dried lipid-nanocrystal films were annealed with chloroform prior to hydration. Without chloroform annealing, the Au nanocrystals phase separate from DOPC and do not load into the liposomes. Au nanocrystals with slightly longer capping ligands of hexadecanethiol or with a larger diameter of 4.1 nm disrupted vesicle formation and created lipid assemblies with many internal lamellar attachments.

Role of Halides in the Ordered Structure Transitions of Heated Gold Nanocrystal Superlattices.

Dodecanethiol-capped gold (Au) nanocrystal superlattices can undergo a surprisingly diverse series of ordered structure transitions when heated (Goodfellow, B. W.; Rasch, M. R.; Hessel, C. M.; Patel, R. N.; Smilgies, D.-M.; Korgel, B. A. Nano Lett.2013, 13, 5710–5714). These are the result of highly uniform changes in nanocrystal size, which subsequently force a spontaneous rearrangement of superlattice structure. Here, we show that halide-containing surfactants play an essential role in these transitions. In the absence of any halide-containing surfactant, superlattices of dodecanethiol-capped (1.9-nm-diameter) Au nanocrystals do not change size until reaching about 190–205 °C, at which point the gold cores coalesce. In the presence of halide-containing surfactant, such as tetraoctylphosphonium bromide (TOPB) or tetraoctylammounium bromide (TOAB), the nanocrystals ripen at much lower temperature and superlattices undergo various ordered structure transitions upon heating. Chloride- and iodide-containing surfactants induce similar behavior, destabilizing the Au–thiol bond and reducing the thermal stability of the nanocrystals.

PEGylation of Carboxylic Acid-Functionalized Germanium Nanowires

Germanium (Ge) nanowires were produced in solution by supercritical fluid-liquid-solid (SFLS) growth and then functionalized with carboxylic acid groups by in situ thermal thiolation with mercaptoundecanoic acid. Polyethylene glycol (PEG) was grafted to the carboxylic acid-terminated Ge nanowires using carbodiimide coupling chemistry. The nanowires were characterized using attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy and transmission electron microscopy (TEM) to confirm the surface modification of the nanowires. Dispersions of PEGylated Ge nanowires in dimethylsulfoxide (DMSO) were stable for days. The PEGylated Ge nanowires were also dispersible in aqueous solution over a wide range of pH and ionic strength.