Aging of gold nanoparticles: ligand exchange with disulfides

Aging of thiolate protected gold nanoparticles (AuNPs) results in reduced reactivity in the disulfide exchange as monitored by electron paramagnetic resonance (EPR) spectroscopy with a bisnitroxide disulfide incoming ligand. Factors determining the reactivity of the aged particles were investigated. The presence of different binding sites on the surface of AuNPs and a surface reorganization process during aging can explain observed reactivity trends.     

Assembly of Preformed Gold Nanoparticles onto Thermoresponsive Poly(N‐isopropylacrylamide)‐Based Microgels on the Basis of Au‐thiol Chemistry

The assembly of preformed gold nanoparticles (AuNPs ) onto the thermoresponsive poly(N ‐isopropylacrylamide) (PNIPAM )‐based microgels was achieved on the basis of the driving force of Au‐thiol chemistry. The loading amount of AuNPs can be controlled by varying the ratio of AuNPs relative to PNIPAM ‐based microgels. The as‐prepared PNIPAM /Au hybrid microgels showed well‐defined reversible swelling/deswelling transition in response to temperature, which can be employed to tune the plasmonic property of hybrid microgels. As the temperature was increased, the position of localized surface plasmon resonance (LSPR ) band red‐shifted to some extent mainly due to the increase in the local refractive index around AuNPs .     

Linear Tetraphenylmethane‐Based Thioether Oligomers Stabilising an Entire Gold Nanoparticle by Enwrapping

The design and synthesis of a novel linear thioether‐based ligand subunit with a tetraphenylmethane core used in the stabilisation of gold nanoparticles (AuNPs) are presented. Mono‐, tri, penta‐ and heptamers of the ligand have been synthesised and used to stabilise AuNPs by enwrapping. With the exception of the monomer, all ligands provide reliable long‐term stability and redispersibility for the coated nanoparticles in common organic solvents. Despite variation of the oligomer length, all stable particles were of the same size within error tolerance (1.16±0.32 nm for the trimer, 1.15±0.30 nm for the pentamer, 1.17±0.34 nm for the heptamer), as investigated by transmission electron microscopy (TEM). These findings suggest that not only the number of sulfur atoms in the ligand, but also its bulkiness play a crucial role in stabilising the AuNPs. These findings are supported by thermogravimetric analysis (TGA), showing that AuNPs stabilised by the penta‐ or heptamer are passivated by a single ligand. Thermal stability measurements suggest a correlation between ligand coverage and thermal stability, further supporting these findings.     

Direct Monitoring of Cell Membrane Vesiculation with 2D AuNP@MnO2 Nanosheet Supraparticles at the Single‐Particle Level

We herein demonstrate robust two‐dimensional (2D) UFO‐shaped plasmonic supraparticles made of gold nanoparticles (AuNPs) and MnO₂ nanosheets (denoted as AMNS‐SPs) for directly monitoring cell membrane vesiculation at the single‐particle level. Because the decorated MnO₂ nanosheets are ultrathin (4.2 nm) and have large diameters (230 nm), they are flexible enough for deformation and folding for parceling of the AuNPs during the endocytosis process. Correspondingly, the surrounding refractive index of the AuNPs increases dramatically, which results in a distinct red‐shift of the localized surface plasmon resonance (LSPR). Such LSPR modulation provides a convenient and accurate means for directly monitoring the dynamic interactions between 2D nanomaterials and cell membranes. Furthermore, for the endocytosed AMNS‐SPs, the subsequent LSPR blue‐shift induced by etching effects of reducing molecules is promising for exploring the local environment redox states at the single‐cell level.     

Enhanced surface plasmon resonance by Au nanoparticles immobilized on a dielectric SiO2 layer on a gold surface

This paper introduces strategies for enhancement of a surface plasmon resonance (SPR) signal by adopting colloidal gold nanoparticles (AuNPs) and a SiO₂ layer on a gold surface. AuNPs on SiO₂ on a gold surface were compared with an unmodified gold surface and a SiO₂ layer on a gold surface with no AuNPs attached. The modified surfaces showed significant changes in SPR signal when biomolecules were attached to the surface as compared with an unmodified gold surface. The detection limit of AuNPs immobilized on a SPR chip was 0.1 ng mL⁻¹ for the prostate-specific antigen (PSA), a cancer marker, as measured with a spectrophotometer. Considering that the conventional ELISA method can detect ∼10 ng mL⁻¹ of PSA, the strategy described here is much more sensitive (∼100 fold). The enhanced shift of the absorption curve resulted from the coupling of the surface and particle plasmons by the SiO₂ layer and the AuNPs on the gold surface.     

Oligopeptide Helical Conformations Control Gold Nanoparticle Cross-Linking

Peptide sequences functionalized with primary amines at the N- and C-terminus are able to induce the aggregation of gold nanoparticles in ethanol as a consequence of their folding into a helical conformation. Random coil peptides are unable to induce such an aggregation process. Aggregation can be monitored spectrophotometrically by following the shift of the surface plasmon resonance (SPR) band of the nanoparticles and is confirmed by transmission electron microscopy and dynamic light scattering analyses. Partial denaturation of the peptides results in diminished cross-linking ability. The helicity parameter θ₂₂₂/θ₂₀₈ correlates fairly well with the shift of the SPR band to longer wavelengths, supporting the relationship between the amount of helical content of a peptide sequence and its ability to induce aggregation.     

On the Non‐Metallicity of 2.2 nm Au246(SR)80 Nanoclusters

The transition from molecular to plasmonic behaviour in metal nanoparticles with increasing size remains a central question in nanoscience. We report that the giant 246‐gold‐atom nanocluster (2.2 nm in gold core diameter) protected by 80 thiolate ligands is surprisingly non‐metallic based on UV/Vis and femtosecond transient absorption spectroscopy as well as electrochemical measurements. Specifically, the Au₂₄₆ nanocluster exhibits multiple excitonic peaks in transient absorption spectra and electron dynamics independent of the pump power, which are in contrast to the behaviour of metallic gold nanoparticles. Moreover, a prominent oscillatory feature with frequency of 0.5 THz can be observed in almost all the probe wavelengths. The phase and amplitude analysis of the oscillation suggests that it arises from the wavepacket motion on the ground state potential energy surface, which also indicates the presence of a small band‐gap and thus non‐metallic or molecular‐like behaviour.     

Determination of colloidal gold nanoparticle surface areas, concentrations, and sizes through quantitative ligand adsorption

Determination of the true surface areas, concentrations, and particle sizes of gold nanoparticles (AuNPs) is a challenging issue due to the nanoparticle morphological irregularity, surface roughness, and size distributions. A ligand adsorption-based technique for determining AuNP surface areas in solution is reported. Using a water-soluble, stable, and highly UV–vis active organothiol, 2-mercaptobenzimidazole (MBI), as the probe ligand, we demonstrated that the amount of ligand adsorbed is proportional to the AuNP surface area. The equivalent spherical AuNP sizes and concentrations were determined by combining the MBI adsorption measurement with Au³⁺ quantification of aqua regia-digested AuNPs. The experimental results from the MBI adsorption method for a series of commercial colloidal AuNPs with nominal diameters of 10, 30, 50, and 90 nm were compared with those determined using dynamic light scattering, transmission electron microscopy, and localized surface plasmonic resonance methods. The ligand adsorption-based technique is highly reproducible and simple to implement. It only requires a UV–vis spectrophotometer for characterization of in-house-prepared AuNPs.     

Hybrid supramolecular hydrogels induced by Au nanoparticles protected with MPEG-b-PCL copolymers with α-cyclodextrin

To fabricate supramolecular hydrogel hybridised with well-dispersed gold nanoparticles (AuNPs), water-soluble AuNPs protected with methoxy poly(ethylene glycol)-b-poly(?-caprolactone) (MPEG-b-PCL) self-assembled monolayers (MPEG-b-PCL/AuNPs) were synthesised and used as guest molecules to interact with α-cyclodextrin (α-CD) in aqueous solutions. Transmission electron microscopy measurement results showed that the diameter of the AuNPs produced was about 6–10 nm with a narrow particle size distribution, and stained MPEG-b-PCL/AuNPs micelles made in water clearly displayed the formation of a core-shell structure with a single gold core per micelle. X-ray diffraction measurement results confirmed that there existed the characteristic peaks of both AuNPs and polypseudorotaxanes formed via the inclusion complexation of MPEG-b-PCL moieties with α-CD in as-obtained hydrogels. UV–visible spectra displayed that the same surface plasma resonance absorption peaks appeared at 526 nm for both aqueous dispersion of MPEG-b-PCL/AuNPs and resultant hybrid hydrogel with α-CD. Rheological measurements showed that the hybrid hydrogel has a lower mechanical strength and viscosity, and a relatively prolonged gelation time in comparison with the corresponding native hydrogel. The mechanical strength of the hydrogel nanocomposites could be tailored by modulating the concentration of MPEG-b-PCL/AuNPs and α-CD as well as the composition of MPEG-b-PCL/AuNPs.     

Role of Capping Ligands on the Nanoparticles in the Modulation of Properties of a Hybrid Matrix of Nanoparticles in a 2D Film and in a Supramolecular Organogel

We incorporate various gold nanoparticles (AuNPs) capped with different ligands in two‐dimensional films and three‐dimensional aggregates derived from N‐stearoyl‐L ‐alanine and N‐lauroyl‐L ‐alanine, respectively. The assemblies of N‐stearoyl‐L ‐alanine afforded stable films at the air–water interface. More compact assemblies were formed upon incorporation of AuNPs in the air–water interface of N‐stearoyl‐L ‐alanine. We then examined the effects of incorporation of various AuNPs functionalized with different capping ligands in three‐dimensional assemblies of N‐lauroyl‐L ‐alanine, a compound that formed a gel in hydrocarbons. The profound influence of nanoparticle incorporation into physical gels was evident from evaluation of various microscopic and bulk properties. The interaction of AuNPs with the gelator assembly was found to depend critically on the capping ligands protecting the Au surface of the gold nanoparticles. Transmission electron microscopy (TEM) showed a long‐range directional assembly of certain AuNPs along the gel fibers. Scanning electron microscopy (SEM) images of the freeze‐dried gels and nanocomposites indicate that the morphological transformation in the composite microstructures depends significantly on the capping agent of the nanoparticles. Differential scanning calorimetry (DSC) showed that gel formation from sol occurred at a lower temperature upon incorporation of AuNPs having capping ligands that were able to align and noncovalently interact with the gel fibers. Rheological studies indicate that the gel–nanoparticle composites exhibit significantly greater viscoelasticity compared to the native gel alone when the capping ligands are able to interact through interdigitation into the gelator assembly. Thus, it was possible to define a clear relationship between the materials and the molecular‐level properties by means of manipulation of the information inscribed on the NP surface.     

Nanocatalyst/Nanoplasmon‐Enabled Detection of Organic Mercury: A One‐Minute Visual Test

We present a fast and sensitive nanosensor that can detect organic mercury, exploiting the combination of the catalytic and plasmonic properties of gold nanoparticles (AuNPs). The method is one‐step and completely instrument‐free, and has a colorimetric readout clearly detectable by simple visual inspection. The AuNPs catalyze efficient organic mercury reduction to the metallic form (Hg⁰), allowing its nucleation and amalgam formation on particle surface, with consequent aggregation‐induced plasmon shift. This leads to very rapid (1 min) and specific colorimetric detection of mercury species. The achieved limit of detection (20 ppb) is compliant with current regulatory limits in food.     

The Effect of Ligand Denticity in Size‐Selective Synthesis of Calix[n]arene‐Stabilized Gold Nanoparticles: A Multitechnique Approach

A series of gold nanoparticles (AuNPs) stabilized by monodentate, bidentate, and tridentate thiolate calix[n]arene ligands 1 – 3 was prepared by using the Brust–Schiffrin two‐phase direct synthesis and characterized with NMR spectroscopy, elemental analysis, transmission electron microscopy (TEM), and X‐ray photoelectron spectroscopy (XPS). The experimental data show that the particular multidentate structure of calix[n]arene derivatives 2 and 3 introduces a control element in the preparation of the gold nanoparticles that allows, in the particular experimental conditions here reported, to obtain very small (≈1 nm) AuNPs. These are the first experimental findings that identify a role of ligand “denticity” in the determination of the nuclearity of nanoparticles.     

Long Distance Electron Transfer Across >100 nm Thick Au Nanoparticle/Polyion Films to a Surface Redox Protein

Glutathione‐decorated 5 nm gold nanoparticles (AuNPs) and oppositely charged poly(allylamine hydrochloride) (PAH) were assembled into {PAH/AuNP}_n films fabricated layer‐by‐layer (LbL) on pyrolytic graphite (PG) electrodes. These AuNP/polyion films utilized the AuNPs as electron hopping relays to achieve direct electron transfer between underlying electrodes and redox proteins on the outer film surface across unprecedented distances >100 nm for the first time. As film thickness increased, voltammetric peak currents for surface myoglobin (Mb) on these films decreased but the electron transfer rate was relatively constant, consistent with a AuNP‐mediated electron hopping mechanism.     

Polymer-induced synthesis of stable gold and silver nanoparticles and subsequent ligand exchange in water

A simple, inexpensive, single-step synthesis of gold and silver nanoparticles using poly(allylamine) (PAAm) as a reducing and stabilizing agent is reported. The synthetic process was carried out in aqueous solution, making the method versatile and environmentally friendly. The synthesized polymer-stabilized nanoparticles are stable in water without particle aggregation at room temperature for at least a month. We demonstrate successful ligand exchange on the polymer-stabilized gold nanoparticles (AuNPs) with a variety of omega-functionalized acid-, alcohol-, amine-, and biotin-terminated alkylthiols. The methodologies, including ligand exchange, also are applicable for the generation of finely dispersed silver nanoparticles. The synthesized gold and silver nanoparticles are characterized by UV-visible absorption spectroscopy and transmission electron microscopy (TEM). The different ligand-stabilized AuNPs are also analyzed by Fourier transform infrared (FTIR) spectroscopy.     

Structural organization of gold nanoparticles onto the ITO surface and its optical properties as a function of ensemble size

Self-assembly of citrate-stabilized gold nanoparticles (AuNPs) onto an optically transparent indium tin oxide (ITO) surface followed by neutralization of these particles using dodecanethiol as a surfactant have been demonstrated. X-ray photoelectron spectroscopic (XPS) studies revealed the partial removal of citrate ions from the immobilized AuNPs, which advances the dilution of electrostatic attraction between AuNPs and the APS (amino-terminated monolayer)-functionalized ITO surface. The resultant AuNPs restore their mobility to some extent and form small ensembles. Some of the immobilized AuNPs were completely removed from the surface due to neutralization, as confirmed by XPS studies. Interparticle distance and size of ensembles were manipulated by consecutive cycles of immobilization and neutralization of AuNPs. Controlled nanostructural fabrication progression, which leads to two-dimensional lateral growth of AuNPs, provides a method for systematically shifting the surface plasmon resonance band based on the increase in plasmon coupling among the closely placed AuNPs of an ensemble. The magnitude of shift increases with the size of ensemble. This manipulated chemical strategy offers a convenient and simple method to tune the optical properties of materials on a nanoscale.     

Direct Electrochemistry of Cytochrome c Based on Poly(diallyldimethylammonium Chloride)‐ Graphene Nanosheets/Gold Nanoparticles Hybrid Nanocomposites and Its Biosensing

A novel biosensor was developed by entrapping cytochrome c (Cyt c) in thin films of the room temperature ionic liquid (RTIL) containing nanocomposites of poly(diallyldimethylammonium chloride)‐graphene nanosheets‐gold nanoparticles (PDDA‐Gp‐AuNPs) at a 11‐mercaptoundecanoic acid‐6‐mercapto‐1‐hexanol modified gold electrode. The synthesized PDDA‐Gp‐AuNPs hybrid nanocomposites were characterized by UV‐vis spectroscopy, Raman spectroscopy, scanning electron microscopy and atomic force microscopy. The PDDA‐Gp‐AuNPs nanocomposites could increase the effective surface of the electrode, enhance the fixed amount of Cyt c on the electrode surface, promote the electron transfer and facilitate the catalytic activity of Cyt c. The RTIL could provide a biocompatible microenvironment to keep Cyt c biological activities, act as an effective mediator to immobilize a large number of Cyt c on the electrode and have good conductivity to improve electron transfer. Therefore, the resultant electrode exhibited good electrochemical performance and electrocatalytic activity. It could be used for electrochemical detection of H₂O₂ with rapid response, high sensitivity, wide linear range and low detection limit, as well as good stability, repeatability and selectivity. The sensor might be promising for practical application.     

Calix[4]arene‐Functionalised Silver Nanoparticles as Hosts for Pyridinium‐Loaded Gold Nanoparticles as Guests

A series of lipophilic gold nanoparticles (AuNPs) circa 5 nm in diameter and having a mixed organic layer consisting of 1‐dodecanethiol and 1‐(11‐mercaptoundecyl) pyridinium bromide was synthesised by reacting tetraoctylammonium bromide stabilised AuNPs in toluene with different mixtures of the two thiolate ligands. A bidentate ω‐alkylthiolate calix[4]arene derivative was instead used as a functional protecting layer on AgNPs of approximately 3 nm. The functionalised nanoparticles were characterised by transmission electron microscopy (TEM), and by UV/Vis and X‐ray photoelectron spectroscopy (XPS). Recognition of the pyridinium moieties loaded on the AuNPs by the calix[4]arene units immobilised on the AgNPs was demonstrated in solution of weakly polar solvents by UV/Vis titrations and DLS measurements. The extent of Au‐AgNPs aggregation, shown through the low‐energy shift of their surface plasmon bands (SPB), was strongly dependent on the loading of the pyridinium moieties present in the organic layer of the AuNPs. Extensive aggregation between dodecanethiol‐capped AuNPs and the Ag calix[4]arene‐functionalised NPs was also promoted by the action of a simple N‐octyl pyridinium difunctional supramolecular linker. This linker can interdigitate through its long fatty tail in the organic layer of the dodecanethiol‐capped AuNPs, and simultaneously interact through its pyridinium moiety with the calix[4]arene units at the surface of the modified AgNPs.     

Iminodiacetic acid-functionalized gold nanoparticles for optical sensing of myoglobin via Cu2+ coordination

A novel gold nanoparticle (AuNP)-based optical sensing system has been developed for the detection of myoglobin (Mb), which is of significant importance for early disease diagnosis. Two thiol molecules containing an iminodiacetic acid moiety (IDA) were synthesized. This detection is based on the Mb-induced aggregation of IDA-functionalized AuNPs resulting from the structures of Mb sandwiched between the functionalized AuNPs via Cu(2+) bridges in the coordination interactions of IDA-Cu(2+)-histidine residues available on the Mb surface, which was confirmed by UV-vis spectroscopy, transmission electron microscopy, dynamic light scattering, and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. The induction aggregation resulted in a red shift in plasmon resonance band of the AuNPs concomitant with a change in solution color from red to purple. The qualitative and quantitative detections of Mb can be achieved by colorimetric observations and UV-vis spectral measurements, respectively. The selectivity of protein assay with the functionalized AuNPs was further investigated, and it is found that the optical sensing of histidine-rich proteins is closely related to number and distribution of surface histidine residues as well as size of proteins.     

Photosynthesis-Mediated Intracellular Biomineralization of Gold Nanoparticles inside Chlorella Cells towards Hydrogen Boosting under Green Light

Engineering living microorganisms to enhance green biomanufacturing for the development of sustainable and carbon-neutral energy strategies has attracted the interest of researchers from a wide range of scientific communities. In this study, we develop a method to achieve photosynthesis-mediated biomineralization of gold nanoparticles (AuNPs) inside Chlorella cells, where the photosynthesis-dominated reduction of Au³⁺ to Au⁰ allows the formed AuNPs to locate preferentially around the thylakoid membrane domain. In particular, we reveal that the electrons generated by the localized surface plasmon resonance of AuNPs could greatly augment hypoxic photosynthesis, which then promotes the generation and transferring of photoelectrons throughout the photosynthetic chain for augmented hydrogen production under sunlight. We demonstrate that the electrons from AuNPs could be directly transferred to hydrogenase, giving rise to an 8.3-fold enhancement of Chlorella cells hydrogen production independent of the cellular photosynthetic process under monochromatic 560 nm light irradiation. Overall, the photosynthesis-mediated intracellular biomineralization of AuNPs could contribute to a novel paradigm for functionalizing Chlorella cells to augment biomanufacturing.     

Synergetic Effect of Mo6 Clusters and Gold Nanoparticles on the Photophysical Properties of Both Components

Photodynamic and photothermal therapies (PDT, and PTT, respectively) are promising candidates for multimodal anticancer therapies (i.e., combinations of therapies), since their action is based on mechanisms that generally cannot be resisted by cancer cells, that is, generation of highly oxidizing oxygen species and high temperature, respectively. Herein, hybrid materials that combine octahedral molybdenum clusters as potential PDT agents and plasmonic spherical gold nanoparticles (AuNPs) as PTT agents are reported. Partial overlap of the photoemission spectrum of the cluster and the surface plasmon resonance band of the AuNPs facilitates energy transfer between the photoactive components, which resulted in synergetic enhancement of their photophysical properties. Specifically, by careful selection of the spacing between the cluster and the gold nanoparticle, a significant increase in luminescence and photosensitizing properties of the cluster was achieved in comparison with similar, but gold-free, particles. On the other hand, the cluster complex facilitated energy conversion to heat by gold particles and hence increased the heating rate under laser irradiation.