Self-assembly Of Au Nanoparticles Research Articles

Simultaneous differential pulse voltammetric detection of uric acid and melatonin based on a self-assembled Au nanoparticle–MoS2 nanoflake sensing platform

In this work, rapid construction of an electrochemical sensing platform based on self-assembled gold nanoparticles (Au NPs) on MoS2 nanoflakes (NFs) was achieved by probe sonication process. The contemporary study dealt with the first electrochemical sensor (Au-MoS2) to detect UA and MT simultaneously through differential pulse voltammetry (DPV). The developed Au-MoS2 sensing platform was dropped-cast on the glassy carbon electrode (GCE) and the high surface to volume ratio of Au-MoS2 enhanced the electrochemical active surface area (EASA) of GCE to 0.980 cm2. The low charge transfer resistance (197.7 Ω), the high conductivity of Au-MoS2 nanocomposite and the synergistic effect between the Au NPs and MoS2 NFs accelerates the electron transfer at Au-MoS2 modified GCE interface. The high electrocatalytic activity and the large surface area at the Au-MoS2 influenced the simultaneous electrochemical detection of uric acid (UA) and melatonin (MT) with limits of detection of 18.2 and 15.7 nM, respectively, in the linear range of 0.033–10.0 μM. The developed sensor had high selectivity, precise reproducibility, excellent repeatability, and good stability (operational and storage). Under optimized conditions, spiking of UA and MT was performed in a urine sample, and the calculated recoveries were in the range of 92.0 %–105.6 %. Owing to the advantages of the simple Au-MoS2 sensing platform, simultaneous electrochemical detection of other electrochemical active species may be possible by optimizing the sensing platform for different conditions.

SERS induced by Au/ZnO plasmonic nanorods coupled to Au nanoparticles

We study the surface enhanced Raman scattering of Methylene Blue (MB) dye molecules induced by 3D array of “hot spots” made of vertical Au/ZnO core-shell nanorods coupled to self-assembled plasmonic Au nanoparticles. SERS substrates based on array of nanorods coupled to nanoparticles show much better performance compared with bare Au/ZnO nanorods. The hybrid 3D SERS substrate perfectly resolve Raman spectra of MB molecules chemisorbed from solutions with analyte concentrations of ∼10-7 M even upon washing of a sample i.e. when only chemisorbed molecules were remained. Raman signal enhancement results from the superposition of two effects, namely, the ability of a plasmonic coupled system to enhance the Raman scattering via local field enhancement and from the large 3D surface which provides more adsorption sites compared with traditional 2D surfaces.

Surface-Enhanced Raman Spectroscopy on Self-Assembled Au Nanoparticles Arrays for Pesticides Residues Multiplex Detection under Complex Environment.

The high reproducibility of trace detection in complex systems is very hard but crucial to analytical technology and science. Here, we present a surface-enhanced Raman scattering (SERS) platform made by large-scale self-assembly of Au nanoparticle (NP) arrays at the cyclohexane/water interface and its use for pesticides residues trace detection. The analyte molecules spontaneously localize into the Au NPs’ nanogaps during the self-assembly process, yielding excellent Raman signal enhancement by surface effects, and possibly both by the concentration of the analytes into the array and by plasmonic hot-spot formation. Transmission electron microscopy (TEM) images demonstrate a good uniformity of interparticle distances (2–3 nm) in the Au NP arrays. SERS experiments on crystal violet (CV) molecules demonstrated that the relative standard deviations (RSD) of the band intensities at 1173, 1376, and 1618 cm−1 were 6.3%, 6.4%, and 6.9%, respectively, indicating high reproducibility of the substrate. Furthermore, we demonstrate that two pesticides dissolved in organic and aqueous phases could be simultaneously detected, suggesting an excellent selectivity and universality of this method for multiplex detection. Our SERS platform opens vast possibilities for repeatability and sensitivity detection of targets in various complex fields.

Highly active Au NP microarray films for direct SERS detection

In this study, we demonstrated a flexible transparent three-dimensional (3D) ordered micro-hemisphere (MHS) array PDMS film with self-assembled Au nanoparticles (NPs) (an Au NP-MHS array film) as a surface-enhanced Raman scattering (SERS) platform for the in situ detection of pesticides in food.

Magnetic nano-nets for capture of microbes in solution based on physical contact

Self-assembly of Au nanoparticles with Fe ions is used to develop magnetic nano-nets similar to fishing nets for capture and removal of microbes in aqueous medium. The nano-nets have a high aspect ratio, span microns in length with openings of 80–300 nm. This allows them to sample the liquid medium even at low volume fraction and also entrap the microbes in the solution. The nets and the trapped microbes can be effectively pulled from the solution by using an off the shelf magnet. Since the capture is based on physical contact, the nano-nets overcome the ability of the microbes to develop resistance to the cytotoxic effects of chemical compounds and nanomaterials. Using the nano-nets an absolute inactivation of 0.9 is achieved in 5 min. in a non-deaerated solution with Escherichia coli (E. coli). Further the removal of the nano-nets along with the captured microbes also predominantly eliminates the nanomaterial from the aqueous medium for future use.

Sputtered SnO2:NiO thin films on self-assembled Au nanoparticle arrays for MEMS compatible NO2 gas sensors

Here, heterostructured Au/SnO2:NiO thin films are fabricated by sputtering SnO2:NiO target onto the self-assembled Au nanoparticle (NPs, diameter ∼ 9.5 nm) arrays and then activated by H2 annealing. Compared with the SnO2 and SnO2:NiO thin film counterparts, the 20 nm thick Au/SnO2:NiO film shows the highest NO2 sensing performance after activated at optimized temperature of 500 °C, with high response of ∼185 to 5 ppm NO2, low detection limit of ∼50 ppb, high selectivity, good stability and also low sensor-to-sensor variation of <15%. The gas sensing property enhancement could not only be attributed to catalytic role of the AuNPs, but also to the effective Schottky barrier and p-n junction formation between Au, SnO2 and NiO, which is verified by the relatively higher Fermi level of ∼4.56 eV compared with those of 4.1–4.3 eV as measured by the ultraviolet photoelectron spectroscopy. Furthermore, this method is of generality to effectively enhance the electron transfer in other sensing materials like pure SnO2 and WO3, showing the promising potency of this Micro-Electrical-Mechanical System (MEMS) compatible heterostucture fabrication method in wafer-scale gas sensor production with high sensitivity, selectivity and good consistency towards low concentration gas detection such as ppb-level NO2.

Low-Cost Nanostructures from Nanoparticle-Assisted Large-Scale Lithography Significantly Enhance Thermal Energy Transport across Solid Interfaces.

Enhancing thermal energy transport across solid interfaces is of critical importance to a wide variety of applications ranging from energy systems and lighting devices to electronics. Nanoscale surface roughness is usually considered detrimental to interfacial thermal transport because of its role in phonon scattering. In this study, however, we demonstrate significant thermal conductance enhancements across metal-semiconductor interfaces by as much as 90% higher than that of the planar interfaces using engineered nanostructures fabricated by Au nanoparticle (NP)-assisted lithography, where self-assembled Au NPs are used as an efficient etching mask to pattern solid substrates over large surface areas. The enlarged interfacial contact area due to the presence of nanostructures is the main reason for the significantly enhanced thermal transport. It is further demonstrated that the conductance can be systematically tuned over a wide range through the use of the Au NP self-assembly process that is regulated by a sacrificial Sb layer whose thickness determines the size and density of the nanostructures produced. This strategy is tested on two technologically important semiconductors, Si and GaN, and their interfacial thermal conductance with Al being measured using the time-domain thermoreflectance technique. Moreover, the nanostructured interfaces can maintain the enhanced conductance for a temperature range of 30-110 °C-the operating temperatures commonly experienced by energy, lighting, and electronic devices. Our results could provide a wafer-scale and low-cost strategy for improving the thermal management of these devices.

Porous Silica-Coated Gold Sponges with High Thermal and Catalytic Stability.

A method to fabricate porous silica-coated Au sponges that show high thermal and catalytic stability has been developed for the first time. The method involves dense surface functionalization of Au sponges (made by self-assembly of Au nanoparticles) with thiolated poly(ethylene glycol) (SH-PEG), which provides binding and condensation sites for silica precursors. The silica coating thickness can be controlled by using SH-PEG of different molecular weights. The silica-coated Au sponge prepared by using 5 kDa SH-PEG maintains its morphology at temperature as high as 700 °C. The calcination removes all organic molecules, resulting in porous silica-coated Au sponges, which contain hierarchically connected micro- and mesopores. The hierarchical pore structures provide an efficient pathway for reactant molecules to access the surface of Au sponges. The porous silica-coated Au sponges show an excellent catalytic recyclability, maintaining the catalytic conversion percentage of 4-nitrophenol by NaBH4 to 4-aminophenol as high as 93% even after 10 catalytic cycles. The method may be applicable for other porous metals, which are of great interests for catalyst, fuel cell, and sensor applications.

Self-assembled Au nanoparticles network thin films at air-water interface and their SERS properties

In this work, Au nanoparticles (AuNPs) network structures were fabricated by the self-assembly method at air-water interface, and the continuous AuNPs network thin films with higher stability can be obtained by further growth at room temperature. The proposed approach does not require any inducer to assist the assembly of AuNPs. The formation mechanism of AuNPs network films was studied by changing the relative amount of HAuCl4 and AgNO3 and aging time, and it was found that the amount of AgNO3 was very important to the formation of AuNPs network structures. AgNO3 can promote the fusion of AuNPs and the formation of nano-chain, and could further evolve into a nano-chain network structure. A large area and good stability of Au nanoparticles network films were fabricated after further growth of gold nanoparticles by ascorbic acid reduction. Furthermore, the surface enhanced Raman scattering (SERS) activity of the as-prepared AuNPs network films was investigated by using 4-aminothiophenol (4-ATP) as the SERS probe molecule. Compared with unassembled AuNPs, the AuNPs network films showed enhanced SERS performance for 4-ATP.

Self-Assembly of Au Nanoparticles and Quantum Dots by Functional Sol-Gel Silica Layers.

A layer-by-layer (LbL) self-assembled method has been developed to create the nano architectonics consisted of quantum dots (QDs) including hydrophobic CdSe/CdZnS and hydrophilic CdTe and Au nanoparticles (NPs) through a functional silica layer as a linker on a slide glass. The glass layer was created by the hydrolysis of 3-aminopropyltrimethoxysilane (APS), in which the amino group in APS plays an important role for the connection among QDs and Au NPs. UV-vis and PL spectroscopy, scanning electron microscopy, and transmission electron microscopy were employed to characterize. The result reveals that Au NPs or QDs could be dispersed in the glass slides at a high concentration, and little aggregation. The PL intensity and absorbance is linearly increased with the number of dipping cycles. Furthermore, when the glass layer between Au NPs and CdTe QDs is thin, the PL of the QDs quenched because of the Plasmon-enhanced fluorescence resonance energy transfer. In contrast, the bright PL of the QDs was observed when the glass layer became thick. The LbL self-assembled glass film shows excellent stability and transparency, which can make a promise for application in biochemical, optoelectronic, and clinical diagnosis areas.

Gold Superparticles Functionalized with Azobenzene Derivatives: SERS Nanotags with Strong Signals.

The surface-enhanced Raman spectroscopy (SERS) nanotag was proposed as a substitute for fluorescent dye for imaging and biosensors several decades ago. However, its weak signal and poor reproducibility has hindered its application. Here, we report a new strategy to form Au superparticles (AuSPs) with high SERS enhancement via one-pot formation and self-assembly of Au nanoparticles (NPs). An azobenzene-carrying Raman reporter was synthesized to exhibit a large Raman cross-section and multiple bands. The self-assembly of the Raman reporter on AuSPs generated SERS nanotags with intense signals. A Raman reporter carrying boronic acid and azobenzene groups displayed six distinctive bands. Its corresponding SERS nanotag demonstrated a high sensing ability toward glycoprotein through aggregation-induced SERS enhancement or as a substitute for labeled antibodies in an immunoassay of the glycoprotein.

Deformation Localization in Molecular Layers Constrained between Self-Assembled Au Nanoparticles.

The localized deformation of molecular monolayers constrained between the spherical surfaces of Au nanoparticles is studied by means of molecular dynamics simulations. Alkyl or polyethylene glycol long-chain molecules were homogeneously distributed over the curved Au surface, pushed against each other by repeated cycles of force relaxation and constant-volume equilibration at temperatures increasing from 50 to 300 K before being slowly quenched to near-zero temperature. Plots of minimum configurational energy can be obtained as a function of the nanoparticle distance, according to different directions of approach; therefore, such simulations describe a range of deformations, from perfectly uniaxial compression to a combination of compression and shear. Despite the relative rigidity of molecular backbones, the deformation is always found to be localized at the interface between the opposing molecular monolayers. We find that shorter ligands can be more densely packed on the surface but do no interdigitate upon compression; they respond to the applied force by bending and twisting, thus changing their conformation while remaining disjointed. On the contrary, longer ligands attain lower surface densities and can interprenetrate when the nanoparticles are compressed against each other; such molecules remain rather straight and benefit from the increased overlap to maximize the adhesion by dispersion forces. The apparent Young's and shear moduli of a dense nanostructure, composed of a triangular arrangement of identical MUDA-decorated Au nanoparticles, are found to be smaller than estimates indirectly deduced by atomic-force experiments but quite close to previous computer simulations of molecular monolayers on flat surfaces and of bulk nanoparticle assemblies.

Exploring the Angstrom Excursion of Au Nanoparticles Excited away from a Metal Surface by an Impulsive Acoustic Perturbation.

We report the anharmonic angstrom dynamics of self-assembled Au nanoparticles (Au:NPs) away from a nickel surface on top of which they are coupled by their near-field interaction. The deformation and the oscillatory excursion away from the surface are induced by picosecond acoustic pulses and probed at the surface plasmon resonance with femtosecond laser pulses. The overall dynamics are due to an efficient transfer of translational momentum from the Ni surface to the Au:NPs, therefore avoiding usual thermal effects and energy redistribution among the electronic states. Two modes are clearly revealed by the oscillatory shift of the Au:NPs surface plasmon resonance-the quadrupole deformation mode due to the transient ellipsoid shape and the excursion mode when the Au:NPs bounce away from the surface. We find that, contrary to the quadrupole mode, the excursion mode is sensitive to the distance between Au:NPs and Ni. Importantly, the excursion dynamics display a nonsinusoidal motion that cannot be explained by a standard harmonic potential model. A detailed modeling of the dynamics using a Hamaker-type Lennard-Jones potential between two media is performed, showing that each Au:NPs coherently evolves in a nearly one-dimensional anharmonic potential with a total excursion of ∼1 Å. This excursion induces a shift of the surface plasmon resonance detectable because of the strong near-field interaction. This general method of observing the spatiotemporal dynamics with angstrom and picosecond resolutions can be directly transposed to many nanostructures or biosystems to reveal the interaction and contact mechanism with their surrounding medium while remaining in their fundamental electronic states.

The Study of Self-Assembled Au Nanoparticles As an Efficient SERS Substrate for Environmental Sensing Applications

The formation of self-assembled monolayers (SAM) of noble metals for optical sensing applications faces serious challenges when trying to achieve simplicity, cost efficacy, specificity, and versatility. Utilizing stable, SAM-forming alkanethiols, such as glutathione (GSH), enable for simple assembly and specific detection of proteins, trace metals, oxidants, etc., thus providing a sensing platform that not only can detect low levels of reactive oxygen species (ROS) for early determination of neurodegenerative diseases, but can also assess trace metals in freshwaters and marine waters, for example. Providing a universal device for detecting varieties of target materials would adapt, simplify, and reduce costs that come from current detection methods. Our research on improving these detection methods is demonstrated using the frequency-controlled self-assembly of a GSH-functionalized gold nanoparticle (Au NP) film as a platform for highly specific and sensitive sensing. We have previously demonstrated the use of an eco-friendly and highly sensitive surface enhanced Raman spectroscopy (SERS) substrate for biomedical diagnostic applications using an ultraviolet photochemical reaction of gold (III) chloride trihydrate (HAuCl4) to create a densely packed Au NP film that exhibits localized surface plasmon resonances [1]. By the firm anchoring of a thiol group to the Au NP surface, GSH assembles the Au NPs for SERS into a densely packed, dendritic structure. At a pH of ~1 in solution, GSH forms a y-shape [2], allowing its carboxyl and amino groups to create a hydrophilic interface and, therefore, be fully available for bonding to various functional molecules. Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), and ultraviolet-visible spectroscopy (UV-vis) were used to confirm the presence of GSH and enhancement capabilities of the SERS substrate. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) confirmed the Au NP film to be 90nm thick, with an average Au NP size of 125nm in diameter [1], and visually confirmed the dendritic assembly of the GSH-functionalized Au NPs. For application purposes, this study will evaluate the trace metal Pb2+ in water. The contamination of lead, and heavy metals in general, have a damaging impact on the health of both humans and our planet. The chelation of the GSH ligands with Pb2+ enables highly specific detection of Pb2+ in water. This method provides a simple, versatile SERS substrate with a dendritic assembly that creates intriguing optical properties and broadens the capability for potential applications. [1] G. Rutherford et al., ECS J. Solid State Sci. Tech. 4, S3071 (2015). [2] M. Bieri and T. Burgi, Langmuir 2005, 21, 1354-1362. Figure 1

Self-assembled cyclodextrin-modified gold nanoparticles on silica beads as stationary phase for chiral liquid chromatography and hydrophilic interaction chromatography

A facile strategy based on self-assembly of Au nanoparticles (AuNPs) (60±10nm in size) on the surfaces of amino-functionalized porous silica spheres under mild conditions was proposed. The resulting material possessed a core-shell structure in which AuNPs were the shell and silica spheres were the core. Then, thiolated-β-cyclodextrin (SH-β-CD) was covalently attached onto the AuNPs as chiral selector for the enantioseparation. The resultant packing material was evaluated by high-performance liquid chromatography (HPLC). The separations of nine pairs of enantiomers were achieved by using the new chiral stationary phase (CSP) in the reversed-phase liquid chromatography (RPLC) mode, respectively. The results showed the new CSP have more sufficient interaction with the analytes due to the existence of AuNPs on silica surfaces, resulting in faster mass transfer rate, compared with β-CD modified silica column. The result shed light on potential usage of chemical modified NPs as chiral selector for enantioseparation based on HPLC. In addition, the new phase was also used in hydrophilic interaction liquid chromatography (HILIC) to separate polar compounds and highly hydrophilic compounds.

Direct Fabrication of Monodisperse Silica Nanorings from Hollow Spheres - A Template for Core-Shell Nanorings.

We report a new type of nanosphere colloidal lithography to directly fabricate monodisperse silica (SiO2) nanorings by means of reactive ion etching of hollow SiO2 spheres. Detailed TEM, SEM, and AFM structural analysis is complemented by a model describing the geometrical transition from hollow sphere to ring during the etching process. The resulting silica nanorings can be readily redispersed in solution and subsequently serve as universal templates for the synthesis of ring-shaped core-shell nanostructures. As an example we used silica nanorings (with diameter of ∼200 nm) to create a novel plasmonic nanoparticle topology, a silica-Au core-shell nanoring, by self-assembly of Au nanoparticles (<20 nm) on the ring's surface. Spectroscopic measurements and finite difference time domain simulations reveal high quality factor multipolar and antibonding surface plasmon resonances in the near-infrared. By loading different types of nanoparticles on the silica core, hybrid and multifunctional composite nanoring structures could be realized for applications such as MRI contrast enhancement, catalysis, drug delivery, plasmonic and magnetic hyperthermia, photoacoustic imaging, and biochemical sensing.

Preparation of yolk–shell microspheres as temperature switch on/off catalysts

A novel kind of yolk–shell microspheres was presented in this Letter. First, 250 nm of magnetic Fe3O4 nanoparticles was prepared by solvothermal reaction. Then, the Fe3O4 particles were coated with Au nanoparticles (AuNPs), SiO2, and poly(N-isopropylacrylamide) (PNIPAM). After the removal of the SiO2 layer by etching with HF solution, the yolk–shell microspheres were prepared. Each yolk–shell microsphere had a movable core (Fe3O4 self-assembled AuNPs) and temperature-responsive shell (PNIPAM). The yolk–shell structure of these microspheres was confirmed by transmission electron microscopy and Fourier-transform infrared spectroscopy. The catalytic performances of these yolk–shell microspheres were investigated under different environmental temperatures. The results indicated that the reduction from 4-nitrophenol to 4-aminophenol by NaBH4 was slowed when the temperature was increased because of the hydrophobic and the shrinking sate of PNIPAM shell and the yolk–shell microspheres had a characteristic of temperature switch on/off when the cross-linking degree was about 20%.

Evolution of Self-Assembled Au NPs by Controlling Annealing Temperature and Dwelling Time on Sapphire (0001).

Au nanoparticles (NPs) have been utilized in a wide range of device applications as well as catalysts for the fabrication of nanopores and nanowires, in which the performance of the associated devices and morphology of nanopores and nanowires are strongly dependent on the size, density, and configuration of the Au NPs. In this paper, the evolution of the self-assembled Au nanostructures and NPs on sapphire (0001) is systematically investigated with the variation of annealing temperature (AT) and dwelling time (DT). At the low-temperature range between 300 and 600 °C, three distinct regimes of the Au nanostructure configuration are observed, i.e., the vermiform-like Au piles, irregular Au nano-mounds, and Au islands. Subsequently, being provided with relatively high thermal energy between 700 and 900 °C, the round dome-shaped Au NPs are fabricated based on the Volmer-Weber growth model. With the increased AT, the size of the Au NPs is gradually increased due to a more favorable surface diffusion while the density is gradually decreased as a compensation. On the other hand, with the increased DT, the size and density of Au NPs decrease due to the evaporation of Au at relatively high annealing temperature at 950 °C.Electronic supplementary materialThe online version of this article (doi:10.1186/s11671-015-1200-0) contains supplementary material, which is available to authorized users.

Large third-order nonlinear refractive index coefficient based on gold nanoparticle aggregate films

Au nanoparticles show large third-order nonlinear effect and ultra-fast response. Here a high nonlinear aggregate film based on self-assembled gold nanoparticles is reported and its third-order nonlinear refractive index coefficient is measured by Z-scan experiment. The third-order nonlinear refractive index coefficient of the Au nanoparticle aggregate film (γ1 = 9.2 × 10−9 cm2/W) is found to be larger than that of an 8-nm-thick sputtered Au film (γ2 = 6.5 × 10−9 cm2/W). This large nonlinear effect can be attributed to the strong field enhancement due to localized plasmon resonances between Au nanoparticles. The result shows that the self-assembled Au nanoparticle aggregate film could be a promising candidate as a third-order nonlinear optical material.

Systematic Control of Self-Assembled Au Nanoparticles and Nanostructures Through the Variation of Deposition Amount, Annealing Duration, and Temperature on Si (111).

The size, density, and configurations of Au nanoparticles (NPs) can play important roles in controlling the electron mobility, light absorption, and localized surface plasmon resonance, and further in the Au NP-assisted nanostructure fabrications. In this study, we present a systematical investigation on the evolution of Au NPs and nanostructures on Si (111) by controlling the deposition amount (DA), annealing temperature (AT), and dwelling time (DT). Under an identical growth condition, the morphologies of Au NPs and nanostructures drastically evolve when the DA is only slightly varied, based on the Volmer-Weber and coalescence models: i.e. I: mini NPs, II: mid-sized round dome-shaped Au NPs, III: large Au NPs, and IV: coalesced nanostructures. With the AT control, three distinctive ranges are observed: i.e., NP nucleation, Au NPs maturation and melting. The gradual dimensional expansion of Au NPs is always compensated with the density reduction, which is explained with the thermodynamic theory. The DT effect is relatively minor on Au NPs, a sharp contrast to other metallic NPs, which is discussed based on the Ostwald-ripening.Electronic supplementary materialThe online version of this article (doi:10.1186/s11671-015-1084-z) contains supplementary material, which is available to authorized users.