Ligand-free Gold Nanoparticles Research Articles

Construction of intelligent interfaces based on stimulus-responsive hydrogels and ligand-free gold nanoparticles for electrocatalysis of isoquercitrin

In recent years, researchers have taken a keen interest in intelligent electrochemical interfaces. Nevertheless, incorporating multiple stimuli-responsive properties while ensuring a significant electrochemical response remains an interesting question. In this work, ligand-free gold nanoclusters grown on carbon nitride supports (AuNCs@N-C) were loaded on pyrolytic graphite (PG) electrode by layer-by-layer (LbL) method to obtain the {AuNCs@N-C}3 inner layers with good catalytic effect on isoquercitrin (IQ, 3-(β-d-Glucopyranosyloxy)-3′,4′,5,7-tetrahydroxyflavone). Then, the semi-penetrating stimuli-responsive hydrogel poly(N-isopropyl acrylamide)-poly(acrylic acid) (PNIPAM-PAA) was modified on the catalytic layer surface by drop-coating method. As a result, we obtained reversible electrochemical responses triggered by changes in temperature, sodium sulfate concentration, and pH. A new type of logic gate has been created by combining the difference in peak current during the stimulus response with the change in potential corresponding to the peak current. A 3-input/3-output tri-state buffer gate and a 1-to-4 demultiplexer have been designed through careful simplification, offering a fresh approach to building electrochemical pharmaceutical logic gates.

Stable and ligand-free gold nanoparticles produced by laser ablation as efficient electrocatalysts for electrochemical sensing of dopamine

The possibility of producing high-purity and ligand-free nanoparticles makes laser ablation in liquids (LAL) an interesting method to produce gold nanoparticles (AuNPs). AuNPs play an important role in the manufacture of electrochemical sensors due to their excellent electrical conductivity and chemical stability. In this study, stable and ligand-free AuNPs were produced by LAL (AuNP/LAL) using a nanosecond pulsed laser. The control of this single-step production was facilitated by adding sodium chloride (NaCl), resulting in the reduction of AuNPs size (10 ± 2 nm) and higher stability (monitored for 12 weeks). The potential of NaCl-stabilized AuNPs/LAL as modifiers agents were investigated for dopamine (DA) sensing by cyclic voltammetry for the first time. The novel electrochemical sensor (GCE/AuNPs.LAL/Nafion) achieved an increase in the peak current of ca. 3 and 6 times for oxidation and reduction, respectively. A comparison of chemical syntheses and LAL approaches was given for electrochemical sensing applications; the AuNPs/LAL showed to be more efficient in facilitating the electronic transfer and electrocatalytic reactions. The estimated limit of detection values for DA sensing was 0.77 µmol/L (oxidation) and 1.08 µmol/L (reduction). The method hereby proposed is promising for clinical applications as the new electrochemical sensor allowed proper sensitivity for DA.

Green and ligand-free gold nanoparticles in Padina australis extract for colorimetric detection of Cu2+ in water

The green and ligand-free gold nanoparticles (P-AuNPs) from Padina australis extract were easily and quickly prepared. A colorimetric probe based on P-AuNPs was designed for easy and visual detection of Cu2+ in domestic water with excellent sensitivity. The –OH and –NH2 in polysaccharides and proteins of Padina australis had played a reduced and protected role for AuNPs and also been easy to complex Cu2+ because of chelation. The complexed Cu2+ dragged P-AuNPs closely to aggregation, causing the red sol to turn blue. Also, the UV–vis spectrum was red-shifted, and a new peak developed. The limits of UV–vis detection are as low as 0.43 μM. The absorbance ratio (A630/A520) showed an excellent linear relationship with Cu2+ concentration (R2 = 0.9926) with a range of 20–60 μM by UV–vis.

Impact of Sterilization on the Colloidal Stability of Ligand-Free Gold Nanoparticles for Biomedical Applications.

Sterilization is a major prerequisite for the utilization of nanoparticle colloids in biomedicine, a process well examined for particles derived from chemical synthesis although highly underexplored for electrostatically stabilized ligand-free gold nanoparticles (AuNPs). Hence, in this work, we comprehensively examined and compared the physicochemical characteristics of laser-generated ligand-free colloidal AuNPs exposed to steam sterilization and sterile filtration as a function of particle size and mass concentration and obtained physicochemical insight into particle growth processes. These particles exhibit long-term colloidal stability (up to 3 months) derived from electrostatic stabilization without using any ligands or surfactants. We show that particle growth attributed to cluster-based ripening occurs in smaller AuNPs (∼5 nm) following autoclaving, while larger particles (∼10 and ∼30 nm) remain stable. Sterile filtration, as an alternative effective sterilizing approach, has no substantial impact on the colloidal stability of AuNPs, regardless of particle size, although a mass loss of 5-10% is observed. Finally, we evaluated the impact of the sterilization procedures on potential particle functionality in proton therapy, using the formation of reactive oxygen species (ROS) as a readout. In particular, 5 nm AuNPs exhibit a significant loss in activity upon autoclaving, probably dedicated to specific surface area reduction and surface restructuring during particle growth. The filtered analog enhanced the ROS release by up to a factor of ∼2.0, at 30 ppm gold concentration. Our findings highlight the need for carefully adapting the sterilization procedure of ligand-free NPs to the desired biomedical application with special emphasis on particle size and concentration.

Boosting the efficiency of organic solar cells via plasmonic gold nanoparticles and thiol functionalized conjugated polymer

Conjugated polymers are promising low-cost, lightweight, and flexible candidates for scalable photovoltaic applications to establish decarbonized energy technologies. However, they possess deficiencies in terms of their lower charge mobility and exciton diffusion length compared to their inorganic counterparts, impeding the efficient charge extraction at high active layer thickness values. In this manner, active layer composition should be tuned to improve light harvesting enabling efficient charge transport. This work presents two new approaches to achieve higher photovoltaic performance for organic photovoltaic systems; thiol modification of the polymers for improved morphological features, and incorporation of ligand-free gold nanoparticles with surface plasmon absorption into the active layer to be stabilized by the covalent interaction with the thiol side groups of the polymers. To achieve this goal, a benzoxadiazole bearing polymer (POxT) and its bromine (POxT-Br) and thiol (POxT-SH) comprising derivatives were synthesized, their electrochemical, optical, photovoltaic, and morphological characterizations were performed. For photovoltaic characterizations, conventional device architecture of ITO/PEDOT:PSS/polymer:PC71BM/LiF/Al was utilized, where the POxT-SH showed the highest JSC and PCE values, 6.52 mA/cm2 and 2.71%, respectively. Gold nanoparticles were synthesized via laser ablation method, and upon incorporation, the PCE value was boosted to 3.29%, with an increase of 21.4% compared to POxT-SH comprising organic solar cells.

In situ generated Ligand-Free gold nanoparticles in polyvinylpyrrolidone solution assisted laser in liquid method for green oxidation of cyclohexane to adipic acid with high yield

Here in we report a very simple method to create gold nanoparticles coated-double silica shells by using a three step process containing pulse laser ablation (PLAL) of metallic gold target in aqueous solution of polyvinylpyrrolidone, well-known stober process and calcination at high temperature respectively for achieving of Au@SiO2@SiO2 nanoparticles with narrow size distribution. Final structure was characterized by techniques such STEM, FESEM, BET, XPS, XRD, TGA, ICP-OES and FT-IR and shows the activity of 26 % conversion in aerobic oxidation of cyclohexane to adipic acid with the 92 % selectivity in mild conditions. More over the catalyst has saved its activity until 6 cycles without any dominant losing.

Synergism between Specific Halide Anions and pH Effects during Nanosecond Laser Fragmentation of Ligand-Free Gold Nanoparticles.

Gold nanoclusters (AuNCs) with diameters smaller than 3 nm are an emerging field of research because they possess interesting optical properties, such as photoluminescence. However, to date, it is still difficult to distinguish whether these properties originate from the cores of the nanoparticles or from the adsorbates on their surfaces. Hence, there is a high demand for ligand-free, ultra-small particles because they make it possible to study ligand and core effects separately. Pulsed laser fragmentation in liquids (LFL) is a convenient route for the synthesis of ligand-free AuNCs. The influence of physical parameters, such as melting and evaporation, on the LFL process is well understood both theoretically and experimentally. However, the impact of the chemical composition of the medium during LFL, which critically affects the particle formation process, has been less well examined. Therefore, in this work, we elucidate the extent to which the ionic strength, the pH value, and the nature of the halide anion that is present, that is, F-, Cl-, Br-, or I-, influence the particle size distribution of the LFL product and the mean yield of small particles (<3 nm) of the product. We showed that the yield of small particles can be enhanced by the synergism between pH and specific ion effects, which probably is attributable to the adsorption of specific anions. In addition, our findings indicated that anion-based stabilization depends critically on the type of anion. A direct Hofmeister effect was observed for anions in the neutral pH regime, whereas an indirect Hofmeister series was reported in alkaline solution, which probably was due to the more hydrophilic surfaces of the AuNCs that were formed.

Rapid Self-Assembly of Metal/Polymer Nanocomposite Particles as Nanoreactors and Their Kinetic Characterization.

Self-assembled metal nanoparticle-polymer nanocomposite particles as nanoreactors are a promising approach for performing liquid phase reactions using water as a bulk solvent. In this work, we demonstrate rapid, scalable self-assembly of metal nanoparticle catalyst-polymer nanocomposite particles via Flash NanoPrecipitation. The catalyst loading and size of the nanocomposite particles can be tuned independently. Using nanocomposite particles as nanoreactors and the reduction of 4-nitrophenol as a model reaction, we study the fundamental interplay of reaction and diffusion. The induction time is affected by the sequence of reagent addition, time between additions, and reagent concentration. Combined, our experiments indicate the induction time is most influenced by diffusion of sodium borohydride. Following the induction time, scaling analysis and effective diffusivity measured using NMR indicate that the observed reaction rate are reaction- rather than diffusion-limited. Furthermore, the intrinsic kinetics are comparable to ligand-free gold nanoparticles. This result indicates that the polymer microenvironment does not de-activate or block the catalyst active sites.

Ligand-free gold nanoparticles supported on mesoporous carbon as electrocatalysts for CO2 reduction

Gold nanoparticles (AuNPs) have shown good catalytic activity for electrochemical CO2 reduction. However, they are typically stabilized by poorly conductive organic ligands which partially block their activity for electrocatalysis. In this work, we report a one-pot organic ligand-free synthesis of AuNPs on mesoporous carbon (MC). The size of the AuNPs is controlled by the micro- and mesoporosity of the carbon support and the mass ratio employed of the gold precursor/MC. The produced AuNPs-MC hybrid material, with the largest amount of small AuNPs (1–3 nm), shows good catalytic activity for the electrochemical reduction of CO2 to CO in aqueous electrolyte. The optimized selectivity is recorded to be, on average, 80% with a mass activity of 12.8 A g–1 at an overpotential of 550 mV. The catalyst stability of the material is investigated to give insight into the irreversible deactivation mechanism occurring at the surface of AuNPs. Using 6-mercapto-1-hexanol as the ligand on AuNPs shows a dual effect in terms of an improved stability but a lowered activity.

Electrochemical Determination of Chromium(VI) in River Water with Gold Nanoparticles-Graphene Nanocomposites Modified Electrodes.

In this study, nanocomposites of ligand-free gold nanoparticles that are anchored onto the graphene surface (Graphene/AuNPs) were synthesized by a sonochemical method in a single reaction step. A highly sensitive amperometric sensor using Graphene/AuNPs is proposed for the determination of trace hexavalent chromium Cr(VI) in environmental water samples. Compared with a gold electrode, a glassy carbon electrode and a AuNPs modified glassy carbon electrode, the Graphene/AuNPs modified glassy carbon electrode exhibits the highest electrocatalytic activity and stability towards the reduction of Cr(VI), based on the results by cyclic voltammetry and electrochemical AC impedance studies. This study shows that the Graphene/AuNPs-based sensor can detect Cr(VI) with a low detection limit of 10 nM (∼0.5 μg/L), a wide dynamic range of 0 to 20 μM (R = 0.999) and very good selectivity and reproducibility. The electrode is applied to the determination of Cr(VI) in river samples with satisfactory recovery values.

In situ decoration of graphene sheets with gold nanoparticles synthetized by pulsed laser ablation in liquids

The demand for nanocomposites of graphene and carbonaceous materials decorated with metallic nanoparticles is increasing on account of their applications in science and technology. Traditionally, the production of graphene-metal assemblies is achieved by the non-environmentally friendly reduction of metallic salts in carbonaceous suspensions. However, precursor residues during nanoparticle growth may reduce their surface activity and promote cross-chemical undesired effects. In this work we present a laser-based alternative to synthesize ligand-free gold nanoparticles that are anchored onto the graphene surface in a single reaction step. Laser radiation is used to generate highly pure nanoparticles from a gold disk surrounded by a graphene oxide suspension. The produced gold nanoparticles are directly immobilized onto the graphene surface. Moreover, the presence of graphene oxide influences the size of the nanoparticles and its interaction with the laser, causes only a slight reduction of the material. This work constitutes a green alternative synthesis of graphene-metal assemblies and a practical methodology that may inspire future developments.

Ligand-free Gold Nanoparticles as a Reference Material for Kinetic Modelling of Catalytic Reduction of 4-Nitrophenol

The reduction of 4-nitrophenol by sodium borohydride is a common model reaction to test the catalytic activity of metal nanoparticles. As all reaction steps proceed solely on the surface of the metal nanoparticles (Langmuir–Hinshelwood model), ligand-coverage of metal nanoparticles impedes the merging of theory and experiment. Therefore we analyzed the catalytic activity of bare gold nanoparticles prepared by laser ablation in liquid without any stabilizers or ligands. The catalytic reaction is characterized by a full kinetic analysis including 4-hydroxylaminophenol as an intermediate species. Excellent agreement between theory and experiment is found. Moreover, the suspension of the nanoparticles remains stable. Hence, ligand-free nanoparticles can be used as a reference material for mechanistic studies of catalytic reactions. In addition, the analysis shows that gold nanoparticles synthesized by laser ablation are among the most active catalysts for this reaction.

Influence of gold, silver and gold-silver alloy nanoparticles on germ cell function and embryo development.

The use of engineered nanoparticles has risen exponentially over the last decade. Applications are manifold and include utilisation in industrial goods as well as medical and consumer products. Gold and silver nanoparticles play an important role in the current increase of nanoparticle usage. However, our understanding concerning possible side effects of this increased exposure to particles, which are frequently in the same size regime as medium sized biomolecules and accessorily possess highly active surfaces, is still incomplete. That particularly applies to reproductive aspects, were defects can be passed onto following generations. This review gives a brief overview of the most recent findings concerning reprotoxicological effects. The here presented data elucidate how composition, size and surface modification of nanoparticles influence viablility and functionality of reproduction relevant cells derived from various animal models. While in vitro cultured embryos displayed no toxic effects after the microinjection of gold and silver nanoparticles, sperm fertility parameters deteriorated after co-incubation with ligand free gold nanoparticles. However, the effect could be alleviated by bio-coating the nanoparticles, which even applies to silver and silver-rich alloy nanoparticles. The most sensitive test system appeared to be in vitro oocyte maturation showing a dose-dependent response towards protein (BSA) coated gold–silver alloy and silver nanoparticles leading up to complete arrest of maturation. Recent biodistribution studies confirmed that nanoparticles gain access to the ovaries and also penetrate the blood–testis and placental barrier. Thus, the design of nanoparticles with increased biosafety is highly relevant for biomedical applications.

Volume-confined synthesis of ligand-free gold nanoparticles with tailored sizes for enhanced catalytic activity

Ligand-free Au nanoparticles with controlled sizes are synthesized via a volume-confined method. In this synthesis, mesoporous hollow silica shells (mHSS) are used as nano-containers for the impregnation of HAuCl4 solution before they are separated from the bulk solution. With a simple heating process, the Au precursor confined within the cavity of the isolated hollow shells is converted into ligand-free Au nanoparticles. The size of the Au nanoparticles can be tuned precisely by loading HAuCl4 solution of different concentrations, or by using mHSS with different cavity volumes. The ligand-free Au nanoparticles demonstrate superior catalytic activity than sodium citrate-capped Au nanoparticles.

In Situ Non-DLVO Stabilization of Surfactant-Free, Plasmonic Gold Nanoparticles: Effect of Hofmeister’s Anions

Specific ion effects ranking in the Hofmeister sequence are ubiquitous in biochemical, industrial, and atmospheric processes. In this experimental study specific ion effects inexplicable by the classical DLVO theory have been investigated at curved water-metal interfaces of gold nanoparticles synthesized by a laser ablation process in liquid in the absence of any organic stabilizers. Notably, ion-specific differences in colloidal stability occurred in the Hückel regime at extraordinarily low salinities below 50 μM, and indications of a direct influence of ion-specific effects on the nanoparticle formation process are found. UV-vis, zeta potential, and XPS measurements help to elucidate coagulation properties, electrokinetic potential, and the oxidation state of pristine gold nanoparticles. The results clearly demonstrate that stabilization of ligand-free gold nanoparticles scales proportionally with polarizability and antiproportionally with hydration of anions located at defined positions in a direct Hofmeister sequence of anions. These specific ion effects might be due to the adsorption of chaotropic anions (Br(-), SCN(-), or I(-)) at the gold/water interface, leading to repulsive interactions between the partially oxidized gold particles during the nanoparticle formation process. On the other hand, kosmotropic anions (F(-) or SO4(2-)) seem to destabilize the gold colloid, whereas Cl(-) and NO3(-) give rise to an intermediate stability. Quantification of surface charge density indicated that particle stabilization is dominated by ion adsorption and not by surface oxidation. Fundamental insights into specific ion effects on ligand-free aqueous gold nanoparticles beyond purely electrostatic interactions are of paramount importance in biomedical or catalytic applications, since colloidal stability appears to depend greatly on the type of salt rather than on the amount.

Near-field-enhanced, off-resonant laser sintering of semiconductor particles for additive manufacturing of dispersed Au–ZnO-micro/nano hybrid structures

Off-resonant near-field enhancement by gold nanoparticles adsorbed on crystalline zinc oxide significantly increases the energy efficiency of infrared laser sintering. In detail, ten different gold mass loads on zinc oxide were exposed to 1,064 nm cw-laser radiation. Variation of scan speed, laser power, and spot size showed that the energy threshold required for sintering decreases and sintering process window widens compared to laser sintering of pure zinc oxide powder. Transmission electron microscope analysis after focused ion beam cross sectioning of the sintered particles reveals that supported gold nanoparticles homogenously resolidify in the sintered semiconductor matrix. The enhanced sintering process with ligand-free gold nanoparticles gives access to metal–semiconductor hybrid materials with potential application in light harvesting or energy conversion.

Chemico‐Physical Synthesis of Surfactant‐ and Ligand‐Free Gold Nanoparticles and Their Anti‐Galvanic Reduction Property

Galvanic reduction (GR) is a classic reaction. In simple terms, metals can reduce less reactive (or more noble) metal ions, while the opposite--metals reduce more reactive (or less noble) metal ions--should not occur. However, recently we found that anti-galvanic reduction (AGR) occurred to thiolated gold and silver nanoparticles. However, the essential issue whether the occurrence of AGR requires the assistance of reductive thiolate ligands or not still remained unanswered. In this work, by using a novel protocol (chemical reduction and physical ablation), we synthesized surfactant- and ligand-free gold nanoparticles. We found that these as-prepared nanoparticles can reduce silver ions and copper ions, thus illustrating that AGR is not dependent on reductive ligands. Further experiments demonstrated that AGR is applicable to other metal (such as Pt and Pd) nanoparticles and that the AGR process is size-dependent. Finally, it was found that the Raman scattering signals of Rhodamine 6G are distinctly enhanced on the gold nanoparticles that had been reacted with silver ions, which indicates the use of AGR for tuning the property of nanoparticles.

Ligand-free gold nanoparticles as colorimetric probes for the non-destructive determination of total dithiocarbamate pesticides after solid phase extraction

In this work, we describe a simple and sensitive non-destructive method for the determination of the total concentration of dithiocarbamate fungicides (DTCs) in real samples. The proposed method combines for the first time the benefits of an extraction method for sample clean-up and preconcentration with a sensitive colorimetric assay based on gold nanoparticle probes. In this two-step procedure, the target DTCs are isolated from the matrix and preconcentrated by solid phase extraction onto commercially available C18 sorbents. Following elution, the extract containing the target dithiocarbamates, free from most interferences and matrix components, is delivered into an aqueous dispersion of plain citrate-capped gold nanoparticles (AuNPs) which aggregate in response to DTCs coordination on AuNPs surface through multiple gold thiolate bonds. This aggregation is evidenced by changes in the spectral properties of the solution involving a decrease in the original absorbance of Au nanoparticles at 522 nm and the appearance of a new absorption band above 700 nm. An ensuing chromatic shift of the solution from wine-red to purple-blue is observed which is visual by naked eye at concentrations as low as 50 μg L(-1). Further improvement in the detection limits can be accomplished by scaling-down the method to micro-volume conditions alleviating the need to preconcentrate larger sample volumes. Overall, by combining sample clean-up and preconcentration with the strong affinity of DTC thiol group for the gold surface, the total concentration of dithiocarbamate pesticides was successfully determined in various water samples at the low and ultra-low μg L(-1) levels without resorting to destructive techniques, sophisticated instrumentation or post-synthetic modification of gold nanoparticles. Method application in real samples showed good analytical features in terms of recoveries (81.0-94.0%), precision (5.6-8.9%) and reproducibility (~9%) rendering the method as an attractive alternative to current methodologies for the determination of DTC fungicide residues in samples of environmental interest.

Non-specific internalization of laser ablated pure gold nanoparticles in pancreatic tumor cell

We investigate the intracellular uptake of 7.3nm, 21.2nm and 31.3nm average size pure colloidal gold nanoparticles synthesized using femtosecond laser ablation technique in pure water. Dark-field imaging, transmission electron microscopy (TEM) and scanning electron microscopy (SEM) was used to assess the uptake of these pure gold nanoparticles in the pancreatic tumor cell line. We show that these ligand-free gold nanoparticles are non-toxic to these cells. The nanoparticles and cell images indicated that unmodified gold nanoparticles interacted with the cells, despite negative surface charge on both the cells and the nanoparticles. We also demonstrate that the uptake of the gold nanoparticles is size-dependent.

Nonendosomal cellular uptake of ligand‐free, positively charged gold nanoparticles

Gold nanoparticles (GNPs) have interesting optical properties, such as exceptionally high quantum yields and virtually limitless photostability. Therefore, they show the potential for applications as biomarkers especially suitable for in vivo and long-term studies. The generation of GNPs using pulsed laser light rather than chemical means provides nanoparticles, which are remarkably stable in a variety of media without the need of stabilizing agents or ligands. This stabilization is achieved by partial oxidation of the gold surface resulting in positively charged GNPs. However, little is known about cellular uptake of such ligand-free nanoparticles, their intracellular fate, or cell viability after nanoparticle contact. The current work is aimed to explore the response of a bovine cell line to GNP exposure mainly using laser scanning confocal microscopy (LSCM) supported by other techniques. Cultured bovine immortalized cells (GM7373) were coincubated with GNP (average diameter 15 nm, 50 microM Au) for 2, 24, and 48 h. The detection of GNP-associated light scattering by the LSCM facilitated a clear distinction between GNP-containing cells and the negative controls. After 48 h, 75% of cells had visibly incorporated nanoparticles. No colocalization was detected with either Rab5a or Lamp1-positive structures, i.e., endosomes or lysosomes, respectivley. However, transmission electron microscope analysis of GNP-coincubated cells indicated the nanoparticles to be positioned within electron-dense structures. Coincubation at 4 degrees C did not inhibit nanoparticle uptake, suggesting diffusion as possible entrance mechanism. Although the assessment of cell morphology, membrane integrity, and apoptosis revealed no GNP-related loss of cell viability at a gold concentration of 25 microM or below, a cytotoxic effect was observed in a proliferation assay after exposing low cell numbers to 50 microM Au and above. In conclusion, this study confirmed the cellular uptake of ligand-free gold nanoparticles during coincubation apparently without using endocytic pathways.