7 research outputs found
Effect of the Spacer Structure on the Stability of Gold Nanoparticles Functionalized with Monodentate Thiolated Poly(ethylene glycol) Ligands
PolyĀ(ethylene glycol)-
(PEG-) based ligands are well-established
for the stabilization of nanoparticles in aqueous solution and are
especially interesting for applications in medicine and biotechnology
because they are known to improve the pharmacokinetic properties of
nanomaterials. In this study, we prepared gold nanoparticles (AuNPs)
with ligand shells of different monodentate polyĀ(ethylene glycol)āthiol
(PEGāSH) ligands. These ligands differed only in the segment
connecting the thiol group with the PEG moiety (<i>M</i><sub>w</sub> ā 2000 g/mol) through an ester bond, the spacer.
All ligands were synthesized by straightforward esterification. Specifically,
we used PEG ligands with a long (C<sub>10</sub>, PEGMUA) or short
(C<sub>2</sub>, PEGMPA) alkylene spacer or a phenylene (PEGMPAA) spacer.
The influence of the spacer on the stability of gold nanoparticleāPEG
conjugates (AuNP@PEG) was tested by cyanide etching experiments, electrolyte-induced
aggregation, and competitive ligand displacement with dithiothreitol
(DTT). In the presence of 100 mM cyanide, AuNPs stabilized with PEGMPA
or PEGMPAA were completely dissolved by oxidative etching within a
few minutes, whereas AuNPs stabilized with PEGMUA needed more than
20 h to be completely etched. By complementary experiments, we deduced
a simplified description for the etching process that takes into account
the role of excess ligand. In the presence of free ligand, significantly
fewer AuNPs are etched, suggesting a competition of etching and ligand
binding to AuNPs. We also compared the stabilizing effect of PEGMUA
with that of a bidentate PEGāthiol ligand (PEGLIP) and found
a reversed stability against cyanide etching and DTT displacement,
in agreement with previously reported observations. Our results clearly
demonstrate the strong impact of the spacer structure on conjugate
stability and provide valuable information for the rational design
of more complex AuNP@PEG conjugates, which are of much interest in
the context of biotechnology and medical applications
Cross-Linked Gold Nanoparticles on Polyethylene: Resistive Responses to Tensile Strain and Vapors
In this study, coatings of cross-linked gold nanoparticles
(AuNPs)
on flexible polyethylene (PE) substrates were prepared via layer-by-layer
deposition and their application as strain gauges and chemiresistors
was investigated. Special emphasis was placed on characterizing the
influence of strain on the chemiresistive responses. The coatings
were deposited using amine stabilized AuNPs (4 and 9 nm diameter)
and 1,9-nonanedithiol (NDT) or pentaerythritol tetrakisĀ(3-mercaptopropionate)
(PTM) as cross-linkers. To prepare films with homogeneous optical
appearance, it was necessary to treat the substrates with oxygen plasma
directly before film assembly. SEM images revealed film thicknesses
between ā¼60 and ā¼90 nm and a porous nanoscale morphology.
All films showed ohmic I-V characteristics with conductivities ranging
from 1 Ć 10<sup>ā4</sup> to 1 Ć 10<sup>ā2</sup> Ī©<sup>ā1</sup> cm<sup>ā1</sup>, depending on
the structure of the linker and the nanoparticle size. When up to
3% strain was induced their resistance increased linearly and reversibly
(gauge factors: ā¼20). A comparative SEM investigation indicated
that the stress induced formation and extension of nanocracks are
important components of the signal transduction mechanism. Further,
all films responded with a reversible increase in resistance when
dosed with toluene, 4-methyl-2-pentanone, 1-propanol or water vapor
(concentrations: 50ā10 000 ppm). Films deposited onto high
density PE substrates showed much faster response-recovery dynamics
than films deposited onto low density PE. The chemical selectivity
of the coatings was controlled by the chemical nature of the cross-linkers,
with the highest sensitivities (ā¼1 Ć 10<sup>ā5</sup> ppm<sup>ā1</sup>) measured with analytes of matching solubility.
The response isotherms of all film/vapor pairs could be fitted using
a LangmuirāHenry model suggesting selective and bulk sorption.
Under tensile stress (1% strain) all chemiresistors showed a reversible
increase in their response amplitudes (ā¼30%), regardless of
the analytesā permittivity. Taking into consideration the thermally
activated tunneling model for charge transport, this behavior was
assigned to stress induced formation of nanocracks, which enhance
the filmsā ability to swell in lateral direction during analyte
sorption
Elastic and Viscoelastic Properties of Cross-Linked Gold Nanoparticles Probed by AFM Bulge Tests
To enable applications of nanoparticle
films in flexible electronics,
actuators, and sensors, their mechanical properties are of critical
concern. Here, we demonstrate that the elastic and viscoelastic properties
of covalently cross-linked gold nanoparticles (GNPs) can be probed
using AFM bulge tests. For this purpose 30ā60 nm thick films
consisting of 1,9-nonanedithiol (NDT) cross-linked GNPs (3.8 nm core
diameter) were transferred onto substrates with ā¼100 Ī¼m
circular apertures. The resulting freestanding membranes were bulged
by applying pressure differences of up to 10 kPa, and the deflection
was measured by intermittent contact atomic force microscopy (AFM).
Analyzing the pressure-deflection data using the spherical cap model,
either by taking into account the peak deflection values or the measured
arc profiles of the bulge, yielded 2.3 Ā± 0.3 and 2.7 Ā± 0.4
GPa for Youngās modulus, respectively. When cycling the stressāstrain
measurements at overpressures up to 2.4 kPa, hysteresis was observed
and assigned to viscoelastic effects. Creep tests performed at a pressure
of 2 kPa revealed both viscoelastic retardation (time constant: 3.3
Ć 10<sup>ā3</sup> s<sup>ā1</sup>) and nonrecoverable
relaxation (creep rate: 9.0 Ć 10<sup>ā8</sup> s<sup>ā1</sup>). Several membranes resisted pressures up to 10 kPa without fracturing,
indicating that the ultimate biaxial tensile strength of the films
was above ā¼30 MPa
Synthesis and Characterization of Monodisperse Metallodielectric SiO<sub>2</sub>@Pt@SiO<sub>2</sub> CoreāShellāShell Particles
Metallodielectric nanostructured
coreāshellāshell
particles are particularly desirable for enabling novel types of optical
components, including narrow-band absorbers, narrow-band photodetectors,
and thermal emitters, as well as new types of sensors and catalysts.
Here, we present a facile approach for the preparation of submicron
SiO<sub>2</sub>@Pt@SiO<sub>2</sub> coreāshellāshell
particles. As shown by transmission and scanning electron microscopy,
the first steps of this approach allow for the deposition of closed
and almost perfectly smooth platinum shells onto silica cores via
a seeded growth mechanism. By choosing appropriate conditions, the shell thickness could be adjusted
precisely, ranging from ā¼3 to ā¼32 nm. As determined
by X-ray diffraction, the crystalline domain sizes of the polycrystalline
metal shells were ā¼4 nm, regardless of the shell thickness.
The platinum content of the particles was determined by atomic absorption
spectroscopy and for thin shells consistent with a dense metal layer
of the TEM-measured thickness. In addition, we show that the roughness
of the platinum shell strongly depends on the storage time of the
gold seeds used to initiate reductive platinum deposition. Further,
using polyvinylpyrrolidone as adhesion layer, it was possible to coat
the metallic shells with very homogeneous and smooth insulating silica
shells of well-controlled thicknesses between ā¼2 and ā¼43
nm. After depositing the particles onto silicon substrates equipped
with interdigitated electrode structures, the metallic character of
the SiO<sub>2</sub>@Pt particles and the insulating character of the
SiO<sub>2</sub> shells of the SiO<sub>2</sub>@Pt@SiO<sub>2</sub> particles
were successfully demonstrated by charge transport measurements at
variable temperatures
Little Adjustments Significantly Improve the Turkevich Synthesis of Gold Nanoparticles
In this report, we show how the classical
and widely used Turkevich
synthesis can be improved significantly by simple adjustments. The
gold nanoparticles (AuNPs) produced with the optimized protocol have
a much narrower size distribution (5ā8% standard deviation),
and their diameters can be reproduced with unrivaled little variation
(<3%). Moreover, large volumes of these particles can be produced
in one synthesis; we routinely synthesize 1000 mL of ā¼3.5 nM
AuNPs. The key features of the improved protocol are the control of
the pH by using a citrate buffer instead of a citrate solution as
the reducing agent or stabilizer and optimized mixing of reagents.
Further, the shape uniformity of the particles can be improved by
addition of 0.02 mM EDTA. While the proposed protocol is as straightforward
as the original Turkevich protocol, it is more tolerant against variations
in precursor concentration
Cross-Linked Gold-Nanoparticle Membrane Resonators as Microelectromechanical Vapor Sensors
We report a novel
approach for the detection of volatile compounds
employing electrostatically driven drumhead resonators as sensing
elements. The resonators are based on freestanding membranes of alkanedithiol
cross-linked gold nanoparticles (GNPs), which are able to sorb analytes
from the gas phase. Under reduced pressure, the fundamental resonance
frequency of a resonator is continuously monitored while the device
is exposed to varying partial pressures of toluene, 4-methylpentan-2-one,
1-propanol, and water. The measurements reveal a strong, reversible
frequency shift of up to ā¼10 kHz, i.e., ā¼5% of the fundamental
resonance frequency, when exposing the sensor to toluene vapor with
a partial pressure of ā¼20 Pa. As this strong shift cannot be
explained exclusively by the mass uptake in the membrane, our results
suggest a significant impact of analyte sorption on the pre-stress
of the freestanding GNP membrane. Thus, our findings point to the
possibility of designing highly sensitive resonators, which utilize
sorption induced changes in the membraneās pre-stress as primary
transduction mechanism
Ligand Layer Engineering To Control Stability and Interfacial Properties of Nanoparticles
The use of mixed ligand layers including
polyĀ(ethylene glycol)-based ligands for the functionalization of nanoparticles
is a very popular strategy in the context of nanomedicine. However,
it is challenging to control the composition of the ligand layer and
maintain high colloidal and chemical stability of the conjugates.
A high level of control and stability are crucial for reproducibility,
upscaling, and safe application. In this study, gold nanoparticles
with well-defined mixed ligand layers of Ī±-methoxyĀpolyĀ(ethylene
glycol)-Ļ-(11-mercaptoĀundecanoate) (PEGMUA) and 11-mercaptoĀundecanoic
acid (MUA) were synthesized and characterized by ATR-FTIR spectroscopy
and gel electrophoresis. The colloidal and chemical stability of the
conjugates was tested by dynamic light scattering (DLS), small-angle
X-ray scattering (SAXS), and UV/vis spectroscopy based experiments,
and their interactions with cells were analyzed by elemental analysis.
We demonstrate that the alkylene spacer in PEGMUA is the key feature
for the controlled synthesis of mixed layer conjugates with very high
colloidal and chemical stability and that a controlled synthesis is
not possible using regular PEG ligands without the alkylene spacer.
With the results of our stability tests, the molecular structure of
the ligands can be clearly linked to the colloidal and chemical stabilization.
We expect that the underlying design principle can be generalized
to improve the level of control in nanoparticle surface chemistry