12 research outputs found
Lasing Action with Gold Nanorod Hyperbolic Metamaterials
Coherent nanoscale photon sources are of paramount importance to achieving
all-optical communication. Several nanolasers smaller than the diffraction
limit have been theoretically proposed and experimentally demonstrated using
plasmonic cavities to confine optical fields. Such compact cavities exhibit
large Purcell factors, thereby enhancing spontaneous emission, which feeds into
the lasing mode. However, most plasmonic nanolasers reported so far have
employed resonant nanostructures and therefore had the lasing restricted to the
proximity of the resonance wavelength. Here, we report on an approach based on
gold nanorod hyperbolic metamaterials for lasing. Hyperbolic metamaterials
provide broadband Purcell enhancement due to large photonic density of optical
states, while also supporting surface plasmon modes to deliver optical feedback
for lasing due to nonlocal effects in nanorod media. We experimentally
demonstrate the advantage of hyperbolic metamaterials in achieving lasing
action by its comparison with that obtained in a metamaterial with elliptic
dispersion. The conclusions from the experimental results are supported with
numerical simulations comparing the Purcell factors and surface plasmon modes
for the metamaterials with different dispersions. We show that although the
metamaterials of both types support lasing, emission with hyperbolic samples is
about twice as strong with 35% lower threshold vs. the elliptic ones. Hence,
hyperbolic metamaterials can serve as a convenient platform of choice for
nanoscale coherent photon sources in a broad wavelength range
Nanometric Resolution in the Hydrodynamic Size Analysis of Ligand-Stabilized Gold Nanorods
The
stability and hydrodynamic size of ligand-coated gold nanorods
(GNRs; aspect ratio 3.6) have been characterized by nanoparticle tracking
analysis (NTA)î—¸a single-particle counting method that can measure
size distributions with low nanometer resolution. Stable aqueous suspensions
of citrate-stabilized GNRs (cit-GNRs) are amenable to surface functionalization
without loss of dispersion control. Cit-GNRs can be treated with chemisorptive
ligands (thiols and dithiocarbamates), nonionic surfactants (Tween
20), and proteins (human serum albumin), all of which produce stable
suspensions at low surfactant concentrations. The precision of NTA
(relative standard deviation 10–12%, standard error <2%)
is sufficient to allow differences in the hydrodynamic size of coated
GNRs to be interpreted in terms of surfactant structure and conformation
Glycal Assembly by the in Situ Generation of Glycosyl Dithiocarbamates
Glycal assembly offers an expedient entry into β-linked oligosaccharides, but epoxyglycal donors can be capricious in their reactivities. Treatment with Et<sub>2</sub>NH and CS<sub>2</sub> enables their in situ conversion into glycosyl dithiocarbamates, which can be activated by copper triflate for coupling with complex or sterically congested acceptors. The coupling efficiency can be further enhanced by in situ benzoylation, as illustrated in an 11-step synthesis of a branched hexasaccharide from glucals in 28% isolated yield and just four chromatographic purifications
Micellization and Single-Particle Encapsulation with Dimethylammoniopropyl Sulfobetaines
Sulfobetaines (SBs) are a class of
zwitterionic surfactants with
a reputation for enhancing colloidal stability at high salt concentrations.
Here, we present a systematic study on the self-assembly of SB amphiphiles
(sultaines or hydroxysultaines) in aqueous solutions, as a function
of chain length and composition, ionic strength, and in the presence
of alkanethiol-coated Au nanoparticles (GNPs). The diameters of the
micelles assembled from SB and amidosulfobetaine (ASB) generally increase
monotonically with chain length, although ASB micelles are smaller
relative to alkyl SB micelles with similarly sized tailgroups, and
oleyl sulfobetaine (OSB) micelles are slightly larger. SB amphiphiles
can stabilize alkanethiol-coated GNPs in physiologically relevant
buffers at concentrations well below their CMC, with size increases
corresponding to single-particle encapsulation. SB-encapsulated GNPs
were prepared by three different methods with SB:GNP weight ratios
of 10:1, followed by dispersion in water or 1 M NaCl. The low hydrodynamic
size of the SB micelles and SB-coated NPs is within the range needed
for efficient renal clearance
Glycosyl Dithiocarbamates: β‑Selective Couplings without Auxiliary Groups
In
this article, we evaluate glycosyl dithiocarbamates (DTCs) with
unprotected C2 hydroxyls as donors in β-linked oligosaccharide
synthesis. We report a mild, one-pot conversion of glycals into β-glycosyl
DTCs via DMDO oxidation with subsequent ring opening by DTC salts,
which can be generated in situ from secondary amines and CS<sub>2</sub>. Glycosyl DTCs are readily activated with CuÂ(I) or CuÂ(II) triflate
at low temperatures and are amenable to reiterative synthesis strategies,
as demonstrated by the efficient construction of a tri-β-1,6-linked
tetrasaccharide. Glycosyl DTC couplings are highly β-selective
despite the absence of a preexisting C2 auxiliary group. We provide
evidence that the directing effect is mediated by the C2 hydroxyl
itself via the putative formation of a cis-fused bicyclic intermediate
Preparation of Super-Stable Gold Nanorods via Encapsulation into Block Copolymer Micelles
Gold nanorods (GNRs) have the potential to be used as
imaging and
hyperthermia agents for cancer theranostics. Clinical applications
of as-synthesized GNRs (i.e., cetyl trimethylammonium bromide (CTAB)-coated
GNRs) are currently limited by their cytotoxicity and insufficient
colloidal stability. With an aim to address these problems, we developed
a self-assembly processing technique for encapsulating GNRs in polyÂ(ethylene
oxide)-polyÂ(<i>n</i>-butyl acrylate) (PEO-PnBA) block copolymer
(BCP) micelles. This technique uses simple steps of solvent exchange
processes, based on the known principles of block copolymer self-assembly.
The resultant BCP-encapsulated GNRs were found to be stable against
aggregation under physiological salt conditions for indefinite periods
of time, which has rarely previously been achieved by other means
of encapsulation
Characterization of Asphaltene Deposits by Using Mass Spectrometry and Raman Spectroscopy
Crude oil deposition in oil transfer
pipelines and bore wells afflicts
many oil reservoirs. Asphaltenes play a major role in this process
because of their tendency to precipitate in pipelines upon changes
in temperature and/or pressure. Asphaltenes are defined by their lack
of solubility in <i>n</i>-alkane solvents, which means that
they likely contain many compounds that do not actively contribute
to the deposition of crude oil in pipelines. The preponderance of
studies in the literature have focused on asphaltenes derived from
crude oil, whereas far fewer investigations have focused on asphaltenes
derived from oil deposits. In this study, structural parameters of
oil-deposit asphaltenes were examined using Raman spectroscopy and
tandem mass spectrometry and compared to results reported previously
for petroleum asphaltenes. On the basis of D1 and G band intensities
in the Raman spectrum of oil-deposit asphaltenes, the average aromatic
sheet size of these molecules was 21.0 Ã…, slightly larger than
earlier values reported for petroleum asphaltenes (15.2–18.8
Ã…). Mass spectrometric experiments of oil-deposit asphaltenes
ionized via atmospheric pressure chemical ionization (APCI) using
CS<sub>2</sub> solvent were used to measure the molecular weight distribution
(MWD), saturated carbon content, and the number of fused aromatic
rings in the cores of the asphaltene molecules. The MWD was found
to be 150–1050 Da with an average molecular weight (average <i>M</i><sub>W</sub>) of 497 Da, which are significantly lower
than those reported previously for petroleum asphaltenes (200–1500
Da and 570–700 Da, respectively). Aromatic core sizes were
estimated to contain 8 fused rings on average for the most abundant
species in oil-deposit asphaltenes, with 5–15 carbons in their
alkyl side chains, as compared to averages of 3–7 aromatic
rings and 17–41 alkyl carbons for petroleum asphaltenes
Cys34-PEGylated Human Serum Albumin for Drug Binding and Delivery
Polyethylene glycol (PEG) derivatives
were conjugated onto the
Cys-34 residue of human serum albumin (HSA) to determine their effects
on the solubilization, permeation, and cytotoxic activity of hydrophobic
drugs such as paclitaxel (PTX). PEGÂ(C34)ÂHSA conjugates were prepared
on a multigram scale by treating native HSA (n-HSA) with 5- or 20-kDa
mPEG-maleimide, resulting in up to 77% conversion of the mono-PEGylated
adduct. Nanoparticle tracking analysis of PEGÂ(C34)ÂHSA formulations
in phosphate buffer revealed an increase in the number of nanosized
aggregates relative to n-HSA, both in the absence and presence of
PTX. Cell viability studies conducted with MCF-7 breast cancer cells
indicated that PTX cytotoxicity was enhanced by PEGÂ(C34)ÂHSA when mixed
at 10:1 mol ratios, up to a 2-fold increase in potency relative to
n-HSA. The PEGÂ(C34)ÂHSA conjugates were also evaluated as PTX carriers
across monolayers of HUVEC and hCMEC/D3 cells, and found to have permeation
profiles nearly identical to those of n-HSA
Synthesis and Reactivity of 4′-Deoxypentenosyl Disaccharides
4-Deoxypentenosides
(4-DPs) are versatile synthons for rare or
higher-order pyranosides, and they provide an entry for structural
diversification at the C5 position. Previous studies have shown that
4-DPs undergo stereocontrolled DMDO oxidation; subsequent epoxide
ring-openings with various nucleophiles can proceed with both <i>anti</i> or <i>syn</i> selectivity. Here, we report
the synthesis of α- and β-linked 4′-deoxypentenosyl
(4′-DP) disaccharides, and we investigate their post-glycosylational
C5′ additions using the DMDO oxidation/ring-opening sequence.
The α-linked 4′-DP disaccharides were synthesized by
coupling thiophenyl 4-DP donors with glycosyl acceptors using BSP/Tf<sub>2</sub>O activation, whereas β-linked 4′-DP disaccharides
were generated by the decarboxylative elimination of glucuronyl disaccharides
under microwave conditions. Both α- and β-linked 4′-DP
disaccharides could be epoxidized with high stereoselectivity using
DMDO. In some cases, the α-epoxypentenosides could be successfully
converted into terminal l-iduronic acids via the <i>syn</i> addition of 2-furylzinc bromide. These studies support
a novel approach to oligosaccharide synthesis, in which the stereochemical
configuration of the terminal 4′-DP unit is established at
a post-glycosylative stage
Vascular toxicity of silver nanoparticles to developing zebrafish (<i>Danio rerio</i>)
<p>Nanoparticles (NPs, 1–100 nm) can enter the environment and result in exposure to humans and other organisms leading to potential adverse health effects. The aim of the present study is to evaluate the effects of early life exposure to polyvinylpyrrolidone-coated silver nanoparticles (PVP-AgNPs, 50 nm), particularly with respect to vascular toxicity on zebrafish embryos and larvae (<i>Danio rerio</i>). Previously published data has suggested that PVP-AgNP exposure can inhibit the expression of genes within the vascular endothelial growth factor (VEGF) signaling pathway, leading to delayed and abnormal vascular development. Here, we show that early acute exposure (0–12 h post-fertilization, hpf) of embryos to PVP-AgNPs at 1 mg/L or higher results in a transient, dose-dependent induction in VEGF-related gene expression that returns to baseline levels at hatching (72 hpf). Hatching results in normoxia, negating the effects of AgNPs on vascular development. Interestingly, increased gene transcription was not followed by the production of associated proteins within the VEGF pathway, which we attribute to NP-induced stress in the endoplasmic reticulum (ER). The impaired translation may be responsible for the observed delays in vascular development at later stages, and for smaller larvae size at hatching. Silver ion (Ag<sup>+</sup>) concentrations were < 0.001 mg/L at all times, with no significant effects on the VEGF pathway. We propose that PVP-AgNPs temporarily delay embryonic vascular development by interfering with oxygen diffusion into the egg, leading to hypoxic conditions and ER stress.</p