Effect of the Spacer Structure on the Stability of Gold Nanoparticles Functionalized with Monodentate Thiolated Poly(ethylene glycol) Ligands

Poly­(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 poly­(ethylene glycol)–thiol (PEG–SH) ligands. These ligands differed only in the segment connecting the thiol group with the PEG moiety (<i>M</i>_w ≈ 2000 g/mol) through an ester bond, the spacer. All ligands were synthesized by straightforward esterification. Specifically, we used PEG ligands with a long (C₁₀, PEGMUA) or short (C₂, PEGMPA) alkylene spacer or a phenylene (PEGMPAA) spacer. The influence of the spacer on the stability of gold nanoparticle–PEG 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 PEG–thiol 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 tetrakis­(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 ∼60 and ∼90 nm and a porous nanoscale morphology. All films showed ohmic I-V characteristics with conductivities ranging from 1 × 10^–4 to 1 × 10^–2 Ω^–1 cm^–1, 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: ∼20). 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: 50–10 000 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 (∼1 × 10^–5 ppm^–1) measured with analytes of matching solubility. The response isotherms of all film/vapor pairs could be fitted using a Langmuir–Henry model suggesting selective and bulk sorption. Under tensile stress (1% strain) all chemiresistors showed a reversible increase in their response amplitudes (∼30%), regardless of the analytes’ 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 films’ 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 30–60 nm thick films consisting of 1,9-nonanedithiol (NDT) cross-linked GNPs (3.8 nm core diameter) were transferred onto substrates with ∼100 μm 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 ± 0.3 and 2.7 ± 0.4 GPa for Young’s modulus, respectively. When cycling the stress–strain 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 × 10^–3 s^–1) and nonrecoverable relaxation (creep rate: 9.0 × 10^–8 s^–1). Several membranes resisted pressures up to 10 kPa without fracturing, indicating that the ultimate biaxial tensile strength of the films was above ∼30 MPa

Synthesis and Characterization of Monodisperse Metallodielectric SiO₂@Pt@SiO₂ Core–Shell–Shell Particles

Metallodielectric nanostructured core–shell–shell 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 SiO₂@Pt@SiO₂ core–shell–shell 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 ∼3 to ∼32 nm. As determined by X-ray diffraction, the crystalline domain sizes of the polycrystalline metal shells were ∼4 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 ∼2 and ∼43 nm. After depositing the particles onto silicon substrates equipped with interdigitated electrode structures, the metallic character of the SiO₂@Pt particles and the insulating character of the SiO₂ shells of the SiO₂@Pt@SiO₂ 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 (5–8% 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 ∼3.5 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 ∼10 kHz, i.e., ∼5% of the fundamental resonance frequency, when exposing the sensor to toluene vapor with a partial pressure of ∼20 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 membrane’s pre-stress as primary transduction mechanism

Ligand Layer Engineering To Control Stability and Interfacial Properties of Nanoparticles

The use of mixed ligand layers including poly­(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 α-methoxy­poly­(ethylene glycol)-ω-(11-mercapto­undecanoate) (PEGMUA) and 11-mercapto­undecanoic 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