Joseph B. Tracy

Nanoparticle MALDI-TOF Mass Spectrometry without Fragmentation: Au25(SCH2CH2Ph)18 and Mixed Monolayer Au25(SCH2CH2Ph)18−x(L)x

Intact molecular ions of the organothiolate-protected nanoparticle Au25(SCH2CH2Ph)18, including their isotopic resolution, can be observed at 7391 Da as 1- and 1+ ions in negative and positive mode, respectively, by MALDI-TOF mass spectrometry when using a tactic of threshold laser pulse intensities and trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]malononitrile (DCTB) as matrix. Previous MALDI-TOF studies of Au nanoparticles using other matrices have encountered extensive fragmentation of nanoparticle as well as thiolate ligands. Absence of fragmentation enables precise determination of the distribution of mixed monolayer compositions on nanoparticles prepared by ligand exchange reactions and by synthesis using thiol mixtures. Reaction conditions producing mixed monolayers containing only one or a small number of usefully functional ligands can be readily identified. At increased laser pulse intensity, the first fragmentation step(s) for the Au25(SCH 2CH2Ph)18 nanoparticle results in losses of AuL units and, in particular, loss of Au4(SCH2CH2Ph)4.

Incorporation of Iron Oxide Nanoparticles and Quantum Dots into Silica Microspheres

We describe the synthesis of magnetic and fluorescent silica microspheres fabricated by incorporating maghemite (gamma-Fe2O3) nanoparticles (MPs) and CdSe/CdZnS core/shell quantum dots (QDs) into a silica shell around preformed silica microspheres. The resultant approximately 500 nm microspheres have a narrow size distribution and show uniform incorporation of QDs and MPs into the shell. We have demonstrated manipulation of these microspheres using an external magnetic field with real-time fluorescence microscopy imaging.

Size-dependent nanoscale kirkendall effect during the oxidation of nickel nanoparticles.

The transformation of Ni nanoparticles (NPs) of different sizes (average diameters of 9, 26, and 96 nm) during oxidation to hollow (single void) or porous (multiple voids) NiO through the nanoscale Kirkendall effect was observed by transmission electron microscopy. Samples treated for 1-4 h at 200-500 degrees C show that the structures of the completely oxidized NPs do not depend on the temperature, but oxidation proceeds more quickly at elevated temperatures. For the Ni/NiO system, after formation of an initial NiO shell (of thickness approximately 3 nm), single or multiple voids nucleate on the inner surface of the NiO shell, and the voids grow until conversion to NiO is complete. Differences in the void formation and growth processes cause size-dependent nanostructural evolution: For 9 and 26 nm NPs, a single void forms beneath the NiO shell, and the void grows by moving across the NP while conversion to NiO occurs opposite the site where the void initially formed. Because of the differences in the Ni/NiO volume ratios for the 9 and 26 nm NPs when the void first forms, they have distinct nanostructures: The 9 nm NPs form NiO shells that are nearly radially symmetric, while there is a pronounced asymmetry in the NiO shells for 26 nm NPs. By choosing an intermediate oxidation temperature and varying the reaction time, partially oxidized Ni(core)/NiO(shell) NPs can be synthesized with good control. For 96 nm NPs, multiple voids form and grow, which results in porous NiO NPs.

Synthesis and Structural and Magnetic Characterization of Ni(Core)/NiO(Shell) Nanoparticles

A size series of ligand-stabilized Ni nanoparticles (NPs) with diameters between 8-24 nm was prepared by solution chemistry, followed by solution-phase oxidation with atmospheric oxygen at 200 degrees C to form Ni(core)/NiO(shell) NPs with shell thicknesses of 2-3 nm. In comparison with the oxidation of Fe and Co NPs, Ni NPs require higher temperatures for significant conversion to NiO. Transmission electron microscopy and electron diffraction show polycrystalline cores with predominantly amorphous shells. SQUID magnetometry measurements were performed to assess the effects of coupling between the ferromagnetic Ni cores and antiferromagnetic NiO shells. After intentional oxidation, the Ni(core)/NiO(shell) NPs have decreased superparamagnetic blocking temperatures (T(B)) and no exchange shift (H(EB)), but a small enhancement in the coercivity (H(C)) signifies weak exchange bias. These effects originate from the amorphous structure of the NiO shells and their thin layer thickness that renders the NiO moments incapable of pinning the core moment in moderate applied fields. The magnetocrystalline anisotropy constants before and after oxidation approach the value for bulk Ni and depend on the Ni core size and NiO shell thickness.

Synthesis of Au(Core)/Ag(Shell) Nanoparticles and their Conversion to AuAg Alloy Nanoparticles

Metal nanoparticles (NPs) are of great interest due to their special optical, [ 1–3 ] electronic, [ 4–8 ] and catalytic [ 9,10 ] properties. [ 11 ] Among metal NPs, Au NPs have been investigated most extensively because of their facile preparation, resistance to oxidation, and surface plasmon resonance (SPR) band that can absorb and scatter visible light. [ 3 ] Core/ shell and alloy bimetallic NPs are especially interesting because they provide opportunities to tune the NPs’ optical and catalytic properties [ 12–15 ] and are potentially useful as taggants for security applications. [ 2 ] The AuAg system is of particular interest because the SPR band is tunable between ∼ 520 nm for Au [ 11 ] and ∼ 410 nm for Ag. [ 16 ] Several syntheses for AuAg alloy, [ 15 , 17–37 ] Au(core)/Ag(shell), [ 22 , 25 , 31 , 33 , 35–45 ] and Ag(core)/Au(shell) NPs [ 28 , 31 , 33–35 , 42 , 46–48 ] have already been reported. Here, we report a facile, stoichiometrically controlled synthesis of Au(core)/Ag(shell) and AuAg alloy NPs through digestive ripening, [ 49–51 ] which is a potentially general method for synthesizing alloy NPs. [ 37 , 52,53 ] Au(core)/Ag(shell) NPs were synthesized and annealed to form AuAg alloy NPs, followed by elemental analysis and structural and optical characterization. Methods utilizing Au rather than Ag NPs as the seed particles are advantageous: obtaining monodisperse Ag NPs is signifi cantly more challenging [ 16 ] because it is harder to control the nucleation of Ag NPs and to avoid oxidation. Huang and co-workers recently demonstrated the conversion of Au(core)/Ag(shell) to AuAg alloy NPs as a part of a larger study showing the generality of digestive ripening for synthesizing alloy NPs, but very limited data without quantitative elemental analysis or optical characterization of the AuAg alloy NPs was provided. [ 37 ] In this study, we compare a twostep synthesis of Au(core)/Ag(shell) NPs and their conversion to AuAg alloy NPs through annealing with a one-step, direct conversion of Au NPs to AuAg alloy NPs. The products of

Bulky Adamantanethiolate and Cyclohexanethiolate Ligands Favor Smaller Gold Nanoparticles with Altered Discrete Sizes

Use of bulky ligands (BLs) in the synthesis of metal nanoparticles (NPs) gives smaller core sizes, sharpens the size distribution, and alters the discrete sizes. For BLs, the highly curved surface of small NPs may facilitate growth, but as the size increases and the surface flattens, NP growth may terminate when the ligand monolayer blocks BLs from transporting metal atoms to the NP core. Batches of thiolate-stabilized Au NPs were synthesized using equimolar amounts of 1-adamantanethiol (AdSH), cyclohexanethiol (CySH), or n-hexanethiol (C6SH). The bulky CyS- and AdS-stabilized NPs have smaller, more monodisperse sizes than the C6S-stabilized NPs. As the bulkiness increases, the near-infrared luminescence intensity increases, which is characteristic of small Au NPs. Four new discrete sizes were measured by MALDI-TOF mass spectrometry, Au(30)(SAd)(18), Au(39)(SAd)(23), Au(65)(SCy)(30), and Au(67)(SCy)(30). No Au(25)(SAd)(18) was observed, which suggests that this structure would be too sterically crowded. Use of BLs may also lead to the discovery of new discrete sizes in other systems.

Preparation, characterization and applications of free-standing single walled carbon nanotube thin films

A method for the reliable fabrication of less than 200 nm thick, free-standing purified-SWNT films having large surface areas exceeding several cm2 is described. Films were characterized using a variety of optical, microscopic and spectroscopic methods. The procedure was also used to prepare thin films of as-prepared, acid-cut and octadecylamine (ODA) functionalized SWNTs. Such samples allow facile transmission measurements of SWNT derived solids.

Long-Range Alignment of Gold Nanorods in Electrospun Polymer Nano/Microfibers

In this study, a scalable fabrication technique for controlling and maintaining the nanoscale orientation of gold nanorods (GNRs) with long-range macroscale order has been achieved through electrospinning. The volume fraction of GNRs with an average aspect ratio of 3.1 is varied from 0.006 to 0.045 in aqueous poly(ethylene oxide) solutions to generate electrospun fibers possessing different GNR concentrations and measuring 40-3000 nm in diameter. The GNRs within these fibers exhibit excellent alignment with their longitudinal axis parallel to the fiber axis n. According to microscopy analysis, the average deviant angle between the GNR axis and n increases modestly from 3.8 to 13.3° as the fiber diameter increases. Complementary electron diffraction measurements confirm preferred orientation of the {100} GNR planes. Optical absorbance spectroscopy measurements reveal that the longitudinal surface plasmon resonance bands of the aligned GNRs depend on the polarization angle and that maximum extinction occurs when the polarization is parallel to n.

Large-Scale Synthesis of Gold Nanorods through Continuous Secondary Growth

Gold nanorods (GNRs) exhibit a tunable longitudinal surface plasmon resonance (LSPR) that depends on the GNR aspect ratio (AR). Independently controlling the AR and size of GNRs remains challenging but is important because the scattering intensity strongly depends on the GNR size. Here, we report a secondary (seeded) growth procedure, wherein continuous addition of ascorbic acid (AA) to a stirring solution of GNRs, stabilized by cetyltrimethylammonium bromide (CTAB) and synthesized by a common GNR growth procedure, deposits the remaining (~70%) of the Au precursor onto the GNRs. The growth phase of GNR synthesis is often performed without stirring, since stirring has been believed to reduce the yield of rod-shaped nanoparticles, but we report that stirring coupled with continuous addition of AA during secondary growth allows improved control over the AR and size of GNRs. After a common primary GNR growth procedure, the LSPR of GNRs is ~820 nm, which can be tuned between ~700-880 nm during secondary growth by adjusting the rate of AA addition or adding benzyldimethylhexadecylammonium chloride hydrate (BDAC). This approach for secondary growth can also be used with primary GNRs of different ARs to achieve different LSPRs and can likely be extended to nanoparticles of different shapes and other metals.

Tandem Mass Spectrometry of Thiolate-Protected Au Nanoparticles NaxAu25(SC2H4Ph)18−y(S(C2H4O)5CH3)y

We report the first collision-induced dissociation tandem mass spectrometry (CID MS/MS) of a thiolate-protected Au nanoparticle that has a crystallographically determined structure. CID spectra assert that dissociation pathways for the mixed monolayer Na(x)Au(25)(SC(2)H(4)Ph)(18-y)(S(C(2)H(4)O)(5)CH(3))(y) centrally involve the semi-ring Au(2)L(3) coordination (L = some combination of the two thiolate ligands) that constitutes the nanoparticles protecting structure. The data additionally confirm charge state assignments in the mass spectra. Prominent among the fragments is [Na(2)AuL(2)](1+), one precursor of which is identified as another nanoparticle fragment in the higher m/z region. Another detected fragment, [Na(2)Au(2)L(3)](1+), represents a mass loss equivalent to an entire semi-ring, whereas others suggest involvement (fragmentation/rearrangement) of multiple semi-rings, e.g., [NaAu(3)L(3)](1+) and [NaAu(4)L(4)](1+). The detailed dissociation/rearrangement mechanisms of these species are not established, but they are observed in other mass spectrometry experiments, including those under non-CID conditions, namely, electrospray ionization mass spectrometry (ESI-MS) with both time-of-flight (TOF) and FT-ICR analyzers. The latter, previously unreported results show that even soft ionization sources can result in Au nanoparticle fragmentation, including that yielding Au(4)L(4) in ESI-TOF of a much larger thiolate-protected Au(144) nanoparticle under non-CID conditions.