10 research outputs found
Reshaping, Fragmentation, and Assembly of Gold Nanoparticles Assisted by Pulse Lasers
ConspectusThe vast majority of the outstanding applications of metal nanoparticles
(NPs) developed during the last two decades have arisen from their
unique optical properties. Within this context, rational synthesis
and assembly of gold NPs have been the main research focus, aiming
at the design of nanoplasmonic devices with tailored optical functionalities.
The progress made in this field is thus to be ascribed to the understanding
of the origin of the interaction between light and such gold nanostructures,
the dynamics of which have been thoroughly investigated with significant
contributions from short and ultrashort pulse laser technologies.We focus this Account on the potential of pulse lasers to provide
new fundamental insights into the electron dynamics involved in the
interaction of light with the free conduction electrons of Au NPs,
that is, localized surface plasmon resonances (LSPRs). The excitation
of LSPRs with a femtosecond pulse laser is followed by thermalization
of the Au NP electrons and the subsequent relaxation of the nanocrystal
lattice and the surrounding environment, which generally results in
surface melting. By contrast, nanosecond irradiation usually induces
AuNP fragmentation and uncontrolled melting due to overlapping excitation
and relaxation phenomena. These concepts have been exploited toward
the preparation of highly monodisperse gold nanospheres via pulse
laser irradiation of polyhedral nanocrystal colloids, or in the fabrication
of nanostructures with “written-in” optical properties.
The applicability of pulsed coherent light has been extended toward
the direct synthesis and manipulation of Au NPs. Through ablation
of a gold target in a liquid with pulse lasers, spherical Au NPs can
be synthesized with no need of stabilizing ligands, which is a great
advantage in terms of reducing toxicity, rendering these NPs particularly
suitable for medical applications. In addition, femtosecond laser
irradiation has been proven a unique tool for the controlled welding
of plasmonic gold nanostructures by electromagnetic field enhancement
at the hot spots of assembled Au NPs. The combination of such nanostructures
with pulse lasers promises significant chemical and biochemical advances,
including the structural determination of organic reaction intermediates,
the investigation of phase transitions in inorganic nanomaterials
at mild reaction conditions, or the efficient photothermal destruction
of cancer cells avoiding damage of surrounding tissue
Cooperative Self-Assembly Transfer from Hierarchical Supramolecular Polymers to Gold Nanoparticles
The transfer of information encoded by molecular subcomponents is a key phenomenon that regulates the biological inheritance in living organisms, yet there is a lack of understanding of related transfer mechanisms at the supramolecular level in artificial multicomponent systems. Our contribution to tackle this challenge has focused on the design of a thiolated π-conjugated linking unit, whose hierarchical, cooperative self-assembly in nonpolar media can be efficiently transferred from the molecular to the nanoscopic level, thereby enabling the reversible self-assembly of gold nanoparticle (AuNP) clusters. The transfer of supramolecular information by the linking π-system can only take place when a specific cooperative nucleation-elongation mechanism is operative, whereas low-ordered noncooperative assemblies formed below a critical concentration do not suffice to extend the order to the AuNP level. To the best of our knowledge, our approach has allowed for the first time a deep analysis of the hierarchy levels and thermodynamics involved in the self-assembly of AuNPs
Remote Control by π‑Conjugation of the Emissive Properties of Fischer Carbene-BODIPY Dyads
The
synthesis, structure, and complete characterization of mono- and bimetallic
dyads joining Fischer carbene complexes and BODIPY chromophores are
reported. In these organometallic species, the Fischer carbene complex
is attached to the BODIPY moiety through a <i>p</i>-aminophenyl
group linked at the C8 carbon atom of the BODIPY core. The photophysical
properties, namely the corresponding UV/vis absorption and emission
spectra of these new metal–carbene complexes, are analyzed
and discussed. It is found that whereas the absorption of the considered
dyads strongly resembles that of the parent 4-anilinyl-substituted
BODIPY, the fluorescence emission is significantly reduced in these
species, very likely as a result of a Förster-type energy transfer
mechanism. At variance, the replacement of the pentacarbonyl-metal(0)
fragment by a carbonyl group leads to high fluorescence emission intensity.
In addition, the emissive properties of the BODIPY core in these organometallic
dyads can be tuned by remote groups by means of π-conjugation,
as supported by density functional theory calculations
Surfactant (Bi)Layers on Gold Nanorods
Gold nanorods in aqueous solution are generally surrounded by surfactants or capping agents. This is crucial for anisotropic growth during synthesis and for their final stability in solution. When CTAB is used, a bilayer has been evidenced from analytical methods even though no direct morphological characterization of the precise thickness and compactness has been reported. The type of surfactant layer is also relevant to understand the marked difference in further self-assembling properties of gold nanorods as experienced using 16-EO<sub>1</sub>-16 gemini surfactant instead of CTAB. To obtain a direct measure of the thickness of the surfactant layer on gold nanorods synthesized by the seeded growth method, we coupled TEM, SAXS, and SANS experiments for the two different cases, CTAB and gemini 16-EO<sub>1</sub>-16. Despite the strong residual signal from micelles in excess, it can be concluded that the thickness is imposed by the chain length of the surfactant and corresponds to a bilayer with partial interdigitation
Using Inclusion Complexes with Cyclodextrins To Explore the Aggregation Behavior of a Ruthenium Metallosurfactant
The
aggregation behavior of a chiral metallosurfactant, bis(2,2′-bipyridine)(4,4′-ditridecyl-2,2′-bipyridine)ruthenium(II)
dichloride (Ru<sub>2</sub><sup>4</sup>C<sub>13</sub>), synthesized
as a racemic mixture was characterized by small-angle neutron scattering,
light scattering, NMR, and electronic spectroscopies. The analysis
of the SANS data indicates that micelles are prolate ellipsoids over
the range of concentrations studied, with a relatively low aggregation
number, and the micellization takes place gradually with increasing
concentration. The presence of cyclodextrins (β-CD and γ-CD)
induces the breakup of the micelles and helps to establish that micellization
occurs at a very slow exchange rate compared to the NMR time scale.
The open structure of this metallosurfactant enables the formation
of very stable complexes of 3:1 stoichiometry, in which one CD threads
one of the hydrocarbon tails and two CDs the other, in close contact
with the polar head. The complex formed with β-CD, more stable
than the one formed with the wider γ-CD, is capable of resolving
the Δ and Λ enantiomers at high CD/surfactant molar ratios.
The chiral recognition is possible due to the very specific interactions
taking place when the β-CD coversvia its secondary rimpart
of the diimine moiety connected to the hydrophobic tails. A SANS model
comprising a binary mixture of hard spheres (complex + micelles) was
successfully used to study quantitatively the effect of the CDs on
the aggregation of the surfactant
Intracellular pH-Induced Tip-to-Tip Assembly of Gold Nanorods for Enhanced Plasmonic Photothermal Therapy
The
search for efficient plasmonic photothermal therapies using
nonharmful pulse laser irradiation at the near-infrared (NIR) is fundamental
for biomedical cancer research. Therefore, the development of novel
assembled plasmonic gold nanostructures with the aim of reducing the
applied laser power density to a minimum through hot-spot-mediated
cell photothermolysis is an ongoing challenge. We demonstrate that
gold nanorods (Au NRs) functionalized at their tips with a pH-sensitive
ligand assemble into oligomers within cell lysosomes through hydrogen-bonding
attractive interactions. The unique intracellular features of the
plasmonic oligomers allow us to significantly reduce the femtosecond
laser power density and Au NR dose while still achieving excellent
cell killing rates. The formation of gold tip-to-tip oligomers with
longitudinal localized surface plasmon resonance bands at the NIR,
obtained from low-aspect-ratio Au NRs close in resonance with 800
nm Ti:sapphire 90 fs laser pulses, was found to be the key parameter
for realizing the enhanced plasmonic photothermal therapy
Large-Scale Plasmonic Pyramidal Supercrystals via Templated Self-Assembly of Monodisperse Gold Nanospheres
Three-dimensional
supercrystals of plasmonic nanoparticles are
a novel class of materials with exciting applications in technologies
such as light harvesting or metamaterials. However, their realization
relies on extraordinarily regular colloidal building blocks and accurate
self-assembly methods. We present here a simple and up-scalable protocol
for the synthesis of smooth gold nanospheres with high monodispersity
in size and sphericity. The synthesis involves rapid growth up to
the desired size and subsequent removal of surface roughness via an
efficient etching step, so that nanospheres with diameters ranging
between 10 and 110 nm can be obtained in large quantities. Upon functionalization
with thiolated polyethylene glycol and low surfactant concentration,
Au nanospheres were employed as building blocks to produce uniform
arrays of micron-sized 3D pyramidal supercrystals over large areas,
by means of a template-assisted approach. Focused ion beam cutting
and SEM characterization revealed a face-centered cubic lattice within
individual pyramidal supercrystals
Femtosecond Laser-Controlled Tip-to-Tip Assembly and Welding of Gold Nanorods
Directed assembly of gold nanorods
through the use of dithiolated molecular linkers is one of the most
efficient methodologies for the morphologically controlled tip-to-tip
assembly of this type of anisotropic nanocrystals. However, in a direct
analogy to molecular polymerization synthesis, this process is characterized
by difficulties in chain-growth control over nanoparticle oligomers.
In particular, it is nearly impossible to favor the formation of one
type of oligomer, making the methodology hard to use for actual applications
in nanoplasmonics. We propose here a light-controlled synthetic procedure
that allows obtaining selected plasmonic oligomers in high yield and
with reaction times in the scale of minutes by irradiation with low
fluence near-infrared (NIR) femtosecond laser pulses. Selective inhibition
of the formation of gold nanorod <i>n</i>-mers (trimers)
with a longitudinal localized surface plasmon in resonance with a
800 nm Ti:sapphire laser, allowed efficient trapping of the (<i>n</i> – 1)-mers (dimers) by hot spot mediated photothermal
decomposition of the interparticle molecular linkers. Laser irradiation
at higher energies produced near-field enhancement at the interparticle
gaps, which is large enough to melt gold nanorod tips, offering a
new pathway toward tip-to-tip welding of gold nanorod oligomers with
a plasmonic response at the NIR. Thorough optical and electron microscopy
characterization indicates that plasmonic oligomers can be selectively
trapped and welded, which has been analyzed in terms of a model that
predicts with reasonable accuracy the relative concentrations of the
main plasmonic species
Au@Ag Nanoparticles: Halides Stabilize {100} Facets
Seed-mediated growth is the most efficient methodology to control the size and shape of colloidal metal nanoparticles. In this process, the final nanocrystal shape is defined by the crystalline structure of the initial seed as well as by the presence of ligands and other additives that help to stabilize certain crystallographic facets. We analyze here the growth mechanism in aqueous solution of silver shells on presynthesized gold nanoparticles displaying various well-defined crystalline structures and morphologies. A thorough three-dimensional electron microscopy characterization of the morphology and internal structure of the resulting core–shell nanocrystals indicates that {100} facets are preferred for the outer silver shell, regardless of the morphology and crystallinity of the gold cores. These results are in agreement with theoretical analysis based on the relative surface energies of the exposed facets in the presence of halide ions