Charged Gold Nanoparticles Research Articles

Photoionization of nanosized aerosol gold agglomerates and their deposition to form nanoscale islands on substrates.

Positively charged gold nanoparticles can be produced in the aerosol state by ultraviolet irradiation of aerosols at wavelengths above the gold ionization energy. Spark-discharge-generated aerosol gold nanoparticles were mobility-classified, neutralized, and then exposed to ultraviolet irradiation at 185 nm. The charge states were determined using a tandem differential mobility analyzer system, and the results revealed that there was no significant dependence of charging probability upon mobility diameter between 4 and 60 nm (1.55 ± 0.26 in positive elementary charge), probably because of the agglomerated nature of the particles. The ionized particles could be deposited to form nanoscale island patterns on a substrate without the use of templates.

Negatively charged gold nanoparticles as an intrinsic peroxidase mimic and their applications in the oxidation of dopamine

Artificial inorganic peroxidase is of great interest due to its intrinsic advantages over natural counterpart. Negatively charged gold nanoparticles (AuNPs) were discovered to function like a peroxidase in the present study. Two AuNPs in different size were prepared and characterized by TEM, and assayed for peroxidase activity. Its catalytic activity was found to follow Michaelis–Menten kinetics. The negative surface charge notably improves the affinity toward a substrate TMB, proved by the determined kinetic parameters. The particles expressed optimal catalytic activity under mildly acidic environment and resistance to elevated temperature and increased concentration of sodium azide. The origin of the activity was investigated tentatively. Hydrogen peroxide-treated AuNPs exhibited an enhanced activity. EDTA temporarily blocked the activity partially, while thiol groups permanently blocked the activity completely. Tests imply that it is the surface Au+ that provides the activity. The successful oxidation of dopamine, as an instance, under the action of AuNPs as a peroxidase was conducted. These studies would lead to a wide range of potential applications.

Self-assembly of an interacting binary blend of diblock copolymers in thin films: a potential route to porous materials with reactive nanochannel chemistry.

Self-assembly of a binary mixture of poly(styrene)336-block-poly(4-vinyl pyridine)25 (PS336-b-P4VP25) and poly(ethylene glycol)113-block-poly(4-hydroxy styrene)25 (PEG113-b-P4HS25) is shown to give rise to a cylindrical morphology in thin films through pyridine/phenol-based hetero-complementary hydrogen bonding interactions between the P4VP and P4HS copolymer segments. Removal of the cylindrical phase (PEG-b-P4HS) allowed access to porous materials having a pore surface decorated with P4VP polymer blocks. These segments could be transformed into cationic polyelectrolytes through quaternization of the pyridine nitrogen atom. The resulting positively charged nanopore surface could recognize negatively charged gold nanoparticles through electrostatic interactions. This work, therefore, outlines the utility of the supramolecular AB/CD type of block copolymer towards preparation of ordered porous thin films carrying a chemically defined channel surface with a large number of reactive sites.

Nano-functionalization of protein microspheres

Abstract Protein microspheres are promising building blocks for the assembly of complex functional materials. Here we demonstrate a set of three techniques that add functionality to the surface of protein microspheres. In the first technique, a positive surface charge on the protein spheres is deposited by electrostatic adsorption. Negatively charged silica and gold nanoparticle colloids can then electrostatically bind reversibly to the microsphere surface. In the second technique, nanoparticles are covalently anchored to the protein shell using a simple one-pot process. The strong covalent bond between sulfur groups in cysteine in the protein shell irreversibly binds to the gold nanoparticles. In the third technique, surface morphology of the protein microsphere is tuned through hydrodynamic instability at the water–oil interface. This is accomplished through the degree of solubility of the oil phase in water. Taken together these three techniques form a platform to create nano-functionalized protein microspheres, which can then be used as building blocks for the assembly of more complex macroscopic materials.

Environmentally benign synthesis of positively charged, ultra-low sized colloidal gold in universal solvent

A simple, single-step, one pot method was developed for the synthesis of monodispersed, ultralow sized, water-dispersible, stable, positively charged gold nanoparticles using the branched polyethlyneimine (PEI) in an aqueous media. Sizes of the gold nanoparticles have been tuned by adjusting the concentration of PEI and the gold salt. Formation of gold nanoparticles has been evidenced using various characterization techniques such as quasi-elastic light scattering (QELS), transmission electron microscopy (TEM), x-ray diffraction (XRD), energy dispersive x-ray (EDX) and spectroscopy and UV-visible spectrophotometer.

Fluorescent detection of protein kinase based on positively charged gold nanoparticles

Herein, we report a fluorometric method for monitoring the activity and inhibition of protein kinase based on positively charged gold nanoparticles, (+)AuNPs. In this assay, when the cationic substrate peptide (S-peptide) is phosphorylated by protein kinase, the resulting negatively charged product peptide (P-peptide) will be adsorbed onto (+)AuNPs through electrostatic interaction, and the fluorescence of fluorescein isothiocyanate (FITC) on the peptide will be quenched by (+)AuNPs. Thus, the fluorescence of solution can respond to the activity of protein kinase. The feasibility of this (+)AuNPs-based method has been demonstrated by sensitive measurement of the activity of cAMP-dependent protein kinase (PKA) with a low detection limit (0.5mUμL−1). Furthermore, the system is successfully applied to estimate the IC50 value of PKA inhibitor H-89. The fast mix-and-readout detection process as well as the simple synthesis of the unmodified (+)AuNPs makes this proposed method a promising candidate for simple and cost-effective kinase activity detection and a good potential in high-throughput screening of kinase-related drugs.

Electrocatalytic Synergy on Nanoparticulate Films Prepared from Oppositely Charged Pt and Au Nanoparticles

AbstractElectrodes modified with oppositely charged platinum and gold nanoparticles exhibit electrocatalytic synergy of glucose oxidation. Such a simple method of preparation of bimetallic nanoparticulate films produces an electrode with significant shift of the onset potential as compared to electrodes prepared only from platinum (ca. 0.45 V) or gold (ca. 0.6 V) nanoparticles. The observed effect results from close proximity of the Au and Pt nanoparticles surfaces within the film. The electrode was also characterized by scanning electron microscopy, X‐ray photoelectron spectroscopy, powder X‐ray diffraction measurements and cyclic voltammetry.

Ultrasensitive immunoassay for free prostate-specific antigen based on ferrocenecarboxylate enhanced cathodic electrochemiluminescence of peroxydisulfate

The catalytic effect of ferrocenecarboxylate (FCC; in the form of nanocrystals) on the cathodic electrochemiluminescence (ECL) of peroxodisulfate was studied and applied to an ECL immunoassay to quantify free prostate-specific antigen (fPSA). The nanocrystals were entrapped in graphene previously functionalized with poly(dimethyldiallylammonium chloride) to give a solid-state nanohybrid referred to as FCC-Gr-PDDA. When cast onto a glassy carbon electrode (GCE), the ECL of peroxodisulfate is strongly enhanced. An immunoelectrode was then constructed by coating the positively charged nanohybrids matrix of FCC-Gr-PDDA with negatively charged gold nanoparticles and then immobilizing antibody against fPSA on the Au-NPs. The resulting electrode is then incubated with samples containing fPSA. An immunocomplex is formed in the presence of fPSA, and this prevents the solid-state nanohybrid film to enhance the ECL of the peroxodisulfate luminophor. The observed decrease in ECL intensity is directly related to the concentration of fPSA antigen in the range from 0.005 to 5 ng mL−1, and the detection limit is as low as 1.7 pg mL−1. The method is deemed to provide a widely applicable and general platform for ECL-based immunoassays.

Efficient fluorescence resonance energy transfer between oppositely charged CdTe quantum dots and gold nanoparticles for turn-on fluorescence detection of glyphosate

We designed a turn-on fluorescence assay for glyphosate based on the fluorescence resonance energy transfer (FRET) between negatively charged CdTe quantum dots capped with thioglycolic acid (TGA-CdTe-QDs) and positively charged gold nanoparticles stabilized with cysteamine (CS-AuNPs). Oppositely charged TGA-CdTe-QDs and CS-AuNPs can form FRET donor–acceptor assemblies due to electrostatic interactions, which effectively quench the fluorescence intensity of TGA-CdTe-QDs. The presence of glyphosate could induce the aggregation of CS-AuNPs through electrostatic interactions, resulting in the fluorescence recovery of the quenched QDs. This FRET-based method has been successfully utilized to detect glyphosate in apples with satisfactory results. The detection limit for glyphosate was 9.8ng/kg (3σ), with the linear range of 0.02–2.0μg/kg. The attractive sensitivity was obtained due to the efficient FRET and the superior fluorescence properties of QDs. The proposed method is a promising approach for rapid screening of glyphosate in real samples.

Designing Zwitterionic SiO2NH2‐Au Particles with Tunable Patchiness using Wrinkles

A facile template‐based approach toward zwitterionic SiO2NH2‐Au patchy particles is presented. Therefore, wrinkle templates prepared by stress release in a bilayer system comprised of an elastic PDMS fundament and a thin SiOx top layer are used. After aligning positively charged, amine‐functionalized silica particles in wrinkle grooves, their surfaces are partially modified with negatively charged gold nanoparticles in an electrostatic adsorption step. Patchiness is precisely controlled by the degree of immersion of the initial particles into wrinkles of varying dimensions. By ultrasonication or wetting with a water droplet, patchy particles are easily released from the substrate‐yielding particles with two oppositely charged hemispheres. Interfacial tension measurements prove the surface activity of the SiO2NH2‐Au particles in an oil/water system and are explained in the view of the Janus‐type surface charges of the particles and the charge of the oil/water interface.

Antibacterial multilayer films fabricated by layer-by-layer immobilizing lysozyme and gold nanoparticles on nanofibers

Negatively charged gold nanoparticles (GNP) and positively charged lysozyme (Lys) were alternately deposited on negatively charged cellulose mats via layer-by-layer (LBL) self-assembly technique. The fabricated multilayer films were characterized by energy-dispersive X-ray (EDX), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectra (FT-IR), and wide-angle X-ray diffraction (XRD). Morphology of the LBL film coated mats was observed by scanning electron microscopy (SEM). Thermal degradation properties were investigated by differential scanning calorimetry (DSC) and thermo-gravimetric analysis (TGA). Additionally, the result of microbial inhibition assay indicated that the composite nanofibrous mats had excellent antibacterial activity against Escherichia coli and Staphylococcus aureus, which could be used for antimicrobial packing, tissue engineering, wound dressing, etc.

Ultrasensitive trace analysis for 2,4,6-trinitrotoluene using nano-dumbbell surface-enhanced Raman scattering hot spots

We report an ultra-sensitive surface-enhanced Raman scattering (SERS)-based detection system for 2,4,6-trinitrotoluene (TNT) using nano-dumbbell structures formed by the electrostatic interaction between positively and negatively charged gold nanoparticles. First, Meisenheimer complexes were produced between TNT and l-cysteine on gold substrates, and 4-mercaptopyridine (4-MPY) labeled gold nanoparticles (positively charged) were allowed to interact with the Meisenheimer complexes through the electrostatic interaction between the negatively charged aromatic ring of the complex molecules and the positively charged nanoparticles. Then, negatively charged gold nanoparticles were added in order to form nano-dumbbells. As a result, many hot junctions were generated by the dumbbell structures, and the SERS signals were greatly enhanced. Our experimental results demonstrate that the SERS-based assay system using nano-dumbbells provides an ultra-sensitive approach for the detection of TNT explosives. It also shows strong potential for broad application in detecting various explosive materials used for military purposes.

Construction of Biomimetic Interface by Layer-by-Layer Self-Assembly Technique for Electrochemical Amperometric Immunosensing Applications

Abstract A novel biomimetic interface for developing label-free amperometric immunosensor based on layer-by-layer (LBL) self-assembly of positively charged ferrocene-chitosan/multiwalled carbon nanotubes (CS-Fc/MWNTs) and negatively charged gold nanoparticles (Au NPs) onto 3-mercaptopropanesulfonic sodium salt (MPS)-modified gold electrode surface was described. Through LBL self-assembly of CS-Fc/MWNTs and Au NPs, the constructed three-dimensional mutilayer network structure could provide more steric sites and ideal platform for further immobilization of biomolecules without loss of their activity. By using hepatitis B surface antigen (HBsAg) as a model, the constructed immunosensor exhibited a specific response to HBsAg linearly in the concentration range of 0.1–350 ng mL−1 with a detection limit of 0.037 ng mL−1 (S/N = 3). The relative error between the results obtained from the immunosensor and the standard method proposed by ELISA was −4.46% to 8.33%, indicating that the two methods were in acceptable agreement and the proposed method could be applied to determination of HBsAg in human serum for clinical diagnosis.

Effects of nanoparticle size and charge on interactions with self-assembled collagen

HypothesisInsights into bone formation have suggested that the critical first step in the biomineralization process is the integration of small (nanometer dimension) mineral clusters into collagen fibers. Not only is such behavior of interest for understanding biomineralization but also should be important to nanotoxicology because collagen is a major component of structural tissues in the human body and accounts for more than 25% of the whole body protein content. Here, utilizing the current insights from biomineralization, we hypothesize that the binding affinity of nanoparticles to self-assembled collagen fibers is size and surface charge dependent. ExperimentsWe developed a self-assembled collagen substrate compatible with Quartz Crystal Microbalance with Dissipation monitoring (QCM-D), which is very sensitive to mechanical changes of the substrate as a consequence of nanoparticle binding. QCM-D experiments were conducted with both positively and negatively charged gold nanoparticles between 2 and 10nm in size. Complementary ex situ imaging Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM) were used to confirm the QCM-D results. FindingsWe find that both positively and negatively charged nanoparticles of all sizes exhibited binding affinity for self-assembled collagen fibers. Furthermore, the smallest particles (2nm) mechanically integrated with collagen fibers.

Competitive-type displacement reaction for direct potentiometric detection of low-abundance protein

Prostate-specific antigen (PSA), one of the indications of possible prostate malignancy, is used as a biomarker for the diagnosis and prognosis of prostate cancer. Herein, we develop a new homogeneous potentiometric immunoassay for sensitive detection of low-concentration PSA without the need of sample separation and washing step. Two nanostructures including positively charged polyethyleneimine-poly(styrene-co-acrylic acid) (PEI-PSAA) nanospheres and negatively charged gold nanoparticles conjugated with anti-PSA antibody (Ab-AuNP) were first synthesized by using mulsifier-free emulsion copolymerization and wet chemistry method, respectively. Thereafter, the as-prepared PEI-PSAA was used as a pseudo hapten for the construction of immunosensing probe based on an electrostatic interaction between PEI-PSAA and Ab-AuNP. Upon target introduction, the added PSA competed with PEI-PASS for Ab-AuNP based on a specific antigen–antibody interaction, and displaced Ab-AuNP from PEI-PASS. The dissociated PEI-PASS was captured through the negatively charged Nafion- modified electrode, thereby resulting in the change of membrane potential. The fabrication process was characterized by using high-resolution transmission electron microscope (HRTEM), scanning electron microscope with energy-dispersive X-ray spectroscopy (SEM-EDX), surface plasmon resonance (SPR) and dynamic laser scattering (DLS) technique. Under optimal conditions, the output signal was indirectly proportional to the concentration of target PSA in the sample and exhibited a dynamic range from 0.1 to 50ng/mL with a detection limit (LOD) of 0.04ng/mL. Intra- and inter-assay coefficients of variation (CVs) were 6.8 and 7.5%, respectively. In addition, the methodology was evaluated for analysis of 12 clinical serum samples and showed good accordance between the results obtained by the developed immunosensing protocol and a commercialized enzyme-linked immunosorbent assay (ELISA) method.

Optimization of positively charged gold nanoparticles synthesized using a stainless-steel mesh and its application for colorimetric hydrogen peroxide detection

The synthesis of (+)AuNPs procedure using a stainless-steel mesh was optimized. The optimal synthetic parameters were found to be one piece of stainless-steel mesh (22.5cm2 in surface area) in 100mL of a 1mM precursor, precursor solution pH 4, and reaction temperature of 30°C. Under the optimal conditions, the as-synthesized (+)AuNPs were highly positively charged (+24.2mV). Therefore, the as-synthesized (+)AuNPs act as a peroxidase mimic and provide a simple, fast, highly sensitive and selective colorimetric method for H2O2 detection with a detection limit of 0.06mM in the linear range from 0.06mM to 4.29mM.

Controlled growth of metal nanoparticles on amino-functionalized polystyrene microspheres and their application in surface-enhanced Raman spectroscopy

This report describes the fabrication and application of the polystyrene (PS) core/metal (gold and silver) shell composites. The PS core microspheres were functionalized with terminal amino groups which endowed the PS microspheres with positively charged surface, and then seeded with negatively charged gold nanoparticles (NPs) via electrostatic force. The gold-seeded PS microspheres were coated completely with gold (silver) shell via reduction of the appropriate metal ions. The morphology of gold (silver) shell composed of numerous close-packed gold (silver) NPs could be tailored easily by altering amounts of metallic precursor. The as-prepared PS core/metal shell composites can be used as substrates for surface-enhanced Raman spectroscopy (SERS). They exhibited excellent surface enhancement ability when 4-aminothiophenol (4-ATP) was used as a molecule probe.

Nanoparticles in a Capillary Trap: Dynamic Self-Assembly at Fluid Interfaces

Dynamic self-assembly is an emerging scientific concept aimed to construct artificial systems of adaptative behavior. Here, we present a first nanoscopic system that is able to dynamically self-assemble in two dimensions. This system is composed of charged gold nanoparticles, dispersed at the air-water interface, which self-assemble into a dense monolayer of area of several square centimeters in response to surface tension gradient. The surface tension gradient is imposed by localized addition or removal of organic solvent from the interface. After the surface tension is equalized over the whole fluid interface, the nanoparticles return to their initial dispersed state. The arrangement of nanoparticles before and after the self-assembly was characterized using SEM microscopy and SAXS spectroscopy. The constructed self-assembling system offers a "chemical" alternative for the Langmuir-Blodgett technique. Also, it was applied for creating self-erasing nanoparticle patterns on a fluid surface.

Synthesis of Positively Charged Gold Nanoparticles Using a Stainless-Steel Mesh

A novel, rapid, one-pot, and facile approach was developed to synthesize positively charged gold nanoparticles [(+) AuNPs] by employing an aqueous solution of HAuCl4 x 3H2O as a precursor at 30 degrees C and a stainless-steel mesh as a reducing agent. The penetration of Cl- ions into the stainless-steel surface results in corrosion on the stainless-steel surface and excretion of electrons which are used for reduction of Au3+ --> Au0. As a result, (+) AuNPs 5-20 nm in size, mostly monodispersed, were synthesized within 3 h. The as-synthesized AuNPs were charaterized by UV-vis, DLS, XRD, TEM, HR-TEM, EDX and SAED. The utilization of non-toxic chemicals and easily available materials, and the non-requirement of energy input, make this methodology easy, inexpensive, and efficient. The new findings about the role of the stainless-steel mesh, which provides electrons in the presence of Cl- ions, for the reduction of Au3+ --> Au0, makes it a novel material for (+) AuNPs synthesis.

Gold‐Nanoparticle‐Assisted Self‐Assembly of Chemical Gradients with Tunable Sub‐50 nm Molecular Domains

A simple and efficient principle for nanopatterning with wide applicability in the sub‐50 nanometer regime is chemisorption of nanoparticles; at homogeneous substrates, particles carrying surface charge may spontaneously self‐organize due to the electrostatic repulsion between adjacent particles. Guided by this principle, a method is presented to design, self‐assemble, and chemically functionalize gradient nanopatterns where the size of molecular domains can be tuned to match the level corresponding to single protein binding events. To modulate the binding of negatively charged gold nanoparticles both locally (<100 nm) and globally (>100 μm) onto a single modified gold substrate, ion diffusion is used to achieve spatial control of the particles’ mutual electrostatic interactions. By subsequent tailoring of different molecules to surface‐immobilized particles and the void areas surrounding them, nanopatterns are obtained with variable chemical domains along the gradient surface. Fimbriated Escherichia coli bacteria are bound to gradient nanopatterns with similar molecular composition and macroscopic contact angle, but different sizes of nanoscopic presentation of adhesive (hydrophobic) and repellent poly(ethylene) glycol (PEG) domains. It is shown that small hydrophobic domains, similar in size to the diameter of the bacterial fimbriae, supported firmly attached bacteria resembling catch‐bond binding, whereas a high number of loosely adhered bacteria are observed on larger hydrophobic domains.