9 research outputs found
Reduced Graphene Oxide-Supported Gold Nanostars for Improved SERS Sensing and Drug Delivery
Development of novel surface-enhanced
Raman scattering (SERS) substrates and how they interface target analytes
plays a pivotal role in determining the spectrum profile and SERS
enhancement magnitude, as well as their applications. We present here
the seed-mediated growth of reduced graphene oxide-gold nanostar (rGO-NS)
nanocomposites and employ them as active SERS materials for anticancer
drug (doxorubicin, DOX) loading and release. By this synthetic approach,
both the morphology of rGO-NS nanohybrids and the corresponding optical
properties can be precisely controlled, with no need of surfactant
or polymer stabilizers. The developed rGO-NS nanohybrids show tunable
optical properties by simply changing growth reaction parameters,
improved stability as compared to bare Au nanostars, and sensitive
SERS response toward aromatic organic molecules. Furthermore, SERS
applications of rGO-NS to probe DOX loading and pH-dependent release
are successfully demonstrated, showing promising potential for drug
delivery and chemotherapy
Monitoring Galvanic Replacement Through Three-Dimensional Morphological and Chemical Mapping
Galvanic
replacement reactions on metal nanoparticles are often
used for the preparation of hollow nanostructures with tunable porosity
and chemical composition, leading to tailored optical and catalytic
properties. However, the precise interplay between the three-dimensional
(3D) morphology and chemical composition of nanostructures during
galvanic replacement is not always well understood as the 3D chemical
imaging of nanoscale materials is still challenging. It is especially
far from straightforward to obtain detailed information from the inside
of hollow nanostructures using electron microscopy techniques such
as SEM or TEM. We demonstrate here that a combination of state-of-the-art
EDX mapping with electron tomography results in the unambiguous determination
of both morphology transformation and elemental composition of nanostructures
in 3D, during galvanic replacement of Ag nanocubes. This work provides
direct and unambiguous experimental evidence toward understanding
the galvanic replacement reaction. In addition, the powerful approach
presented here can be applied to a wide range of nanoscale transformation
processes, which will undoubtedly guide the development of novel nanostructures
Graphene Oxides as Tunable Broadband Nonlinear Optical Materials for Femtosecond Laser Pulses
Graphene oxide (GO) thin films on glass and plastic substrates
were found to display interesting broadband nonlinear optical properties.
We have investigated their optical limiting activity for femtosecond
laser pulses at 800 and 400 nm, which could be tuned by controlling
the extent of reduction. The as-prepared GO films were found to exhibit
excellent broadband optical limiting behaviors, which were significantly
enhanced upon partial reduction by using laser irradiation or chemical
reduction methods. The laser-induced reduction of GO resulted in enhancement
of effective two-photon absorption coefficient at 400 nm by up to
ā¼19 times and enhancement of effective two- and three-photon
absorption coefficients at 800 nm by ā¼12 and ā¼14.5 times,
respectively. The optical limiting thresholds of partially reduced
GO films are much lower than those of various previously reported
materials. Highly reduced GO films prepared by using the chemical
method displayed strong saturable absorption behavior
Dilution-Induced Formation of Hybrid Perovskite Nanoplatelets
Perovskite
nanocrystals (NCs) are an important extension to the
fascinating field of hybrid halide perovskites. Showing significantly
enhanced photoluminescence (PL) efficiency and emission wavelengths
tunable through halide content and size, they hold great promise for
light-emitting applications. Despite the rapid advancement in this
field, the physical nature and size-dependent excitonic properties
have not been well investigated due to the challenges associated with
their preparation. Herein we report the spontaneous formation of highly
luminescent, quasi-2D organicāinorganic hybrid perovskite nanoplatelets
(NPls) upon dilution of a dispersion of bulk-like NCs. The fragmentation
of the large NCs is attributed to osmotic swelling induced by the
added solvent. An excess of organic ligands in the solvent quickly
passivates the newly formed surfaces, stabilizing the NPls in the
process. The thickness of the NPls can be controlled both by the dilution
level and by the ligand concentration. Such colloidal NPls and their
thin films were found to be extremely stable under continuous UV light
irradiation. Full tunability of the NPl emission wavelength is achieved
by varying the halide ion used (bromide, iodide). Additionally, time-resolved
PL measurements reveal an increasing radiative decay rate with decreasing
thickness of the NPls, likely due to an increasing exciton binding
energy. Similarly, measurements on iodide-containing NPls show a transformation
from biexponential to monoexponential PL decay with decreasing thickness,
likely due to an increasing fraction of excitonic recombination. This
interesting phenomenon of change in fluorescence upon dilution is
a result of the intricate nature of the perovskite material itself
and is uncommon in inorganic materials. Our findings enable the synthesis
of halide perovskite NCs with high quantum efficiency and good stability
as well as a tuning of both their optical and morphological properties
Gold Nanooctahedra with Tunable Size and Microfluidic-Induced 3D Assembly for Highly Uniform SERS-Active Supercrystals
Shape-controlled
synthesis of uniform noble metal nanoparticles
(NPs) is crucial for the development of future plasmonic devices.
The use of nanocrystals with well-defined morphologies and crystallinity
as seed particles is expected to provide excellent shape control and
monodispersity. We report the aqueous-based seed-mediated growth of
monodisperse gold octahedra with wide range of sizes (50ā150
nm in side length) by reducing different amounts of HAuCl<sub>4</sub> on preformed single crystalline gold nanorods using butenoic acid
as reducing agent. Butenoic acid plays a key role as a mild reducing
agent as well as favoring the thermodynamic control of the reaction.
The uniformity of the as-prepared Au octahedra combined with the use
of a microfluidic technique based on microevaporation will allow the
self-assembly of octahedra into uniform 3D supercrystals. Additionally,
these plasmonic substrates exhibit high and uniform SERS signals over
extended areas with intensities increasing with the Au nanoparticle
size
Galvanic Replacement Coupled to Seeded Growth as a Route for Shape-Controlled Synthesis of Plasmonic Nanorattles
Shape-controlled
synthesis of metal nanoparticles (NPs) requires
mechanistic understanding toward the development of modern nanoscience
and nanotechnology. We demonstrate here an unconventional shape transformation
of Au@Ag coreāshell NPs (nanorods and nanocubes) into octahedral
nanorattles via room-temperature galvanic replacement coupled with
seeded growth. The corresponding morphological and chemical transformations
were investigated in three dimensions, using state-of-the-art X-ray
energy-dispersive spectroscopy (XEDS) tomography. The addition of
a reducing agent (ascorbic acid) plays a key role in this unconventional
mechanistic path, in which galvanic replacement is found to dominate
initially when the shell is made of Ag, while seeded growth suppresses
transmetalation when a composition of Au:Ag (ā¼60:40) is reached
in the shell, as revealed by quantitative XEDS tomography. This work
not only opens new avenues toward the shape control of hollow NPs
beyond the morphology of sacrificial templates, but also expands our
understanding of chemical transformations in nanoscale galvanic replacement
reactions. The XEDS electron tomography study presented here can be
generally applied to investigate a wide range of nanoscale morphological
and chemical transformations
Gold NanorodāpNIPAM Hybrids with Reversible Plasmon Coupling: Synthesis, Modeling, and SERS Properties
The thermoresponsive
optical properties of Au nanorod-doped polyĀ(<i>N</i>-isopropylacrylamide)
(Au NRāpNIPAM) microgels with different Au NR payloads and
aspect ratios are presented. Since the volume phase transition of
pure pNIPAM microgels is reversible, the optical response reversibility
of Au NRāpNIPAM hybrids is systematically analyzed. Besides,
extinction cross-section and near-field enhancement simulations for
Au NRāmicrogel hybrids are performed using a new numerical
method based on the surface integral equation method of moments formulation
(M<sup>3</sup> solver). Additionally, the Au NRāmicrogel hybrid
systems are expected to serve as excellent broadband surface-enhanced
Raman scattering (SERS) substrates due to the temperature-controlled
formation of hot spots and the tunable optical properties. The optical
enhancing properties related to SERS are tested with three laser lines,
evidencing excitation wavelength-dependent efficiency that can be
easily controlled by either the aspect ratio (length/width) of the
assembled Au NR or by the Au NR payload per microgel. Finally, the
SERS efficiency of the prepared Au NRāpNIPAM hybrids is found
to be stable for months
Boosting Tunable Blue Luminescence of Halide Perovskite Nanoplatelets through Postsynthetic Surface Trap Repair
The
easily tunable emission of halide perovskite nanocrystals throughout
the visible spectrum makes them an extremely promising material for
light-emitting applications. Whereas high quantum yields and long-term
colloidal stability have already been achieved for nanocrystals emitting
in the red and green spectral range, the blue region currently lags
behind with low quantum yields, broad emission profiles, and insufficient
colloidal stability. In this work, we present a facile synthetic approach
for obtaining two-dimensional CsPbBr<sub>3</sub> nanoplatelets with
monolayer-precise control over their thickness, resulting in sharp
photoluminescence and electroluminescence peaks with a tunable emission
wavelength between 432 and 497 nm due to quantum confinement. Subsequent
addition of a PbBr<sub>2</sub>-ligand solution repairs surface defects
likely stemming from bromide and lead vacancies in a subensemble of
weakly emissive nanoplatelets. The overall photoluminescence quantum
yield of the blue-emissive colloidal dispersions is consequently enhanced
up to a value of 73 Ā± 2%. Transient optical spectroscopy measurements
focusing on the excitonic resonances further confirm the proposed
repair process. Additionally, the high stability of these nanoplatelets
in films and to prolonged ultraviolet light exposure is shown
Palladium Nanoparticle-Loaded Cellulose Paper: A Highly Efficient, Robust, and Recyclable Self-Assembled Composite Catalytic System
We present a novel strategy based on the immobilization of palladium nanoparticles (Pd NPs) on filter paper for development of a catalytic system with high efficiency and recyclability. Oleylamine-capped Pd nanoparticles, dispersed in an organic solvent, strongly adsorb on cellulose filter paper, which shows a great ability to wick fluids due to its microfiber structure. Strong van der Waals forces and hydrophobic interactions between the particles and the substrate lead to nanoparticle immobilization, with no desorption upon further immersion in any solvent. The prepared Pd NP-loaded paper substrates were tested for several model reactions such as the oxidative homocoupling of arylboronic acids, the Suzuki cross-coupling reaction, and nitro-to-amine reduction, and they display efficient catalytic activity and excellent recyclability and reusability. This approach of using NP-loaded paper substrates as reusable catalysts is expected to open doors for new types of catalytic support for practical applications