Controlled interparticle spacing for surface-modified gold nanoparticle aggregates

Aggregation of gold nanoparticles of increasing size has been studied as a consequence of adsorption of 2-aminothiophenol (ATP) on gold nanoparticle surfaces. The capping property of ATP in the acidic pH range has been accounted from UV-vis absorption spectroscopy and surface-enhanced Raman scattering (SERS) studies. The effect of nanoparticle size (8-55 nm) on the nature of aggregation as well as the variation in the optical response due to variable degree of interparticle coupling effects among the gold particles have been critically examined. Various techniques such as transmission electron microscopy, X-ray diffraction, zeta-potential, and average particle size measurement were undertaken to characterize the nanoparticle aggregates. The aggregate size, interparticle distances, and absorption band wavelengths were found to be highly dependent on the pH of the medium and the concentration of the capping agent, ATP. The acquired SERS spectra of ATP relate the interparticle spacing. It has been observed that the SERS signal intensities are different for different sized gold nanoparticles.     

Construction of simple gold nanoparticle aggregates with controlled plasmon–plasmon interactions

We have developed a colloidal assembly for the study of plasmon–plasmon interactions between gold nanoparticles. Colloidal aggregates of controlled size and interparticle spacing were synthesized on silica nanoparticle substrates. Following the immobilization of isolated gold nanoparticles onto silica nanoparticles, the surfaces of the adsorbed gold nanoparticles were functionalized with 4-aminobenzenethiol. This molecular linker attached additional gold nanoparticles to the ‘parent' gold nanoparticle, forming small nanoparticle aggregates. The optical absorption spectrum of these clusters differed from that of gold colloid in a manner consistent with plasmon–plasmon interactions between the gold nanoparticles.     

A multiple coupling approach to produce high-performance SERS substrates

In this work, large area gold-nanoparticle-cluster pillar array with a gold mirror as high-performance SERS substrate was facilely fabricated by combined use of nanosphere lithography and self-assembly approach.     

Resorcinarene-Encapsulated Nanoparticles: Building Blocks for Self-Assembled Nanostructures

Calix[4]resorcinarene-derived surfactants are highly effectiveat stabilizing metal nanoparticles of different sizes, creating opportunities tofabricate well-defined nanostructures with size-tunable materials properties. Theresorcinarenes have a critical role in the dispersion of nanoparticles under varioussolvent conditions and in the robustness of the protective surfactant layer.Magnetic cobalt particles stabilized by resorcinarenes self-assemble intonanostructured ``bracelets' in toluene. Resorcinarene surfactants can also promote theself-organization of gold nanoparticles as large as 170 nm into two-dimensional arrays. Thesenanostructured films possess novel optoelectronic properties such as surface-enhancedRaman scattering (SERS), and are expected to have useful applications for chemical sensing.     

Molecular-level control of feature separation in one-dimensional nanostructure assemblies formed by biomolecular nanolithography

In this paper, we present a convenient and reliable method to organize small gold nanoparticles (d(CORE) = 1.5 nm) into linear chains with precisely controlled interparticle spacing over a range of 1.5-2.8 nm through biomolecular nanolithography. Controlling the feature separations of 1 to a few nanometers with angstrom-level precision is a key requirement in electronic and optical applications of nanostructures to tune the properties of the nanostructures and manipulate the interactions between neighboring structures. Here, chains are formed in solution by utilizing functional-group-directed self-assembly to organize ligand-stabilized gold nanoparticles onto DNA templates. The spacing between neighboring nanoparticles can be controlled chemically and tuned at the molecular level by utilizing nanoparticles possessing ligand shells of varying thickness to achieve angstrom-level resolution at spacings of 1.5, 2.1, and 2.8 nm. The small standard deviation (< or = 20%) in the values for the interparticle spacing illustrates the reproducibility of the approach. Because the interparticle spacing is enforced by the ligand shell rather than the scaffold, the spacing is uniform even in nonlinear sections of the chain. We further show that the assembly process is robust and produces extended linear nanoparticle chains of up to 1 microm in length and a total coverage of > 90%. All structures and interparticle spacings were analyzed using transmission electron microscopy. Our results demonstrate the potential of scaffold-assisted assembly approaches for patterning features with tunable dimensions on a length scale that is important for future applications of these materials in nanoscale electronics and optics.     

Transfer of Two‐Dimensional Oligonucleotide Patterns onto Stereocontrolled Plasmonic Nanostructures through DNA‐Origami‐Based Nanoimprinting Lithography

The precise functionalization of self‐assembled nanostructures with spatial and stereocontrol is a major objective of nanotechnology and holds great promise for many applications. Herein, the nanoscale addressability of DNA origami was exploited to develop a precise copy‐machine‐like platform that can transfer two‐dimensional oligonucleotide patterns onto the surface of gold nanoparticles (AuNPs) through a deliberately designed toehold‐initiated DNA displacement reaction. This strategy of DNA‐origami‐based nanoimprinting lithography (DONIL) demonstrates high precision in controlling the valence and valence angles of AuNPs. These DNA‐decorated AuNPs act as precursors in the construction of discrete AuNP clusters with desired chirality.     

Assembly of gold nanoparticles mediated by multifunctional fullerenes

The understanding of the interparticle interactions of nanocomposite structures assembled using molecularly capped metal nanoparticles and macromolecular mediators as building blocks is essential for exploring the fine-tunable interparticle spatial and macromolecular properties. This paper reports the results of an investigation of the chemically tunable multifunctional interactions between fullerenes (1-(4-methyl)-piperazinyl fullerene, MPF) and gold nanoparticles. The interparticle spatial properties are defined by the macromolecular and multifunctional electrostatic interactions between the negatively charged nanoparticles and the positively charged fullerenes. In addition to characterization of the morphological properties, the surface plasmon resonance band, dynamic light scattering, and surface-enhanced Raman scattering (SERS) properties of the MPF-mediated assembly and disassembly processes have been determined. The change of the optical properties depends on the pH and electrolyte concentrations. The detection of the Raman-active vibration modes (Ag(2) and Hg(8)) of C60 and the determination of their particle size dependence have demonstrated that the adsorption of MPF on the nanoparticle surface in the MPF-Au nm assembly is responsible for the SERS effect. These findings provide new insights into the delineation between the interparticle interactions and the nanostructural properties for potential applications of the nanocomposite materials in spectroscopic and optical sensors and in controlled releases.     

Wide range temperature detection with hybrid nanoparticles traced by surface-enhanced Raman scattering

We report on the fabrication of a class of surface-enhanced Raman scattering(SERS)active thermometers,which consists of60 nm gold nanoparticles,encoded with Raman-active dyes,and a layer of thermoresponsive poly(N-isopropylacrylamide)(PNIPAM)brush with different chain lengths.These SERS-active nanoparticles can be optimized to maintain spectrally silent when staying as single particles in dispersion.Increasing temperature in a wide range from 25 to 55°C can reversibly induce the interparticle self-aggregation and turn on the SERS fingerprint signals with up to 58-fold of enhancement by taking advantage of the interparticle plasmonic coupling generated in the process of thermo-induced nanoparticles self-aggregation.Moreover,the most significative point is that these SERS probes could maintain their response to temperature and present all fingerprint signals in the presence of a colored complex.However,the UV-Vis spectra can distinguish the differences faintly and the solution color shows little change in such complex mixture.This proof-of-concept and Raman technique applied here allow for dynamic SERS platform for onsite temperature detection in a wide temperature range and offer unique advantages over other detection schemes.     

Array of molecularly mediated thin film assemblies of nanoparticles: correlation of vapor sensing with interparticle spatial properties

The ability to tune interparticle spatial properties of nanoparticle assemblies is essential for the design of sensing materials toward desired sensitivity and selectivity. This paper reports findings of an investigation of molecularly mediated thin film assemblies of metal nanoparticles with controllable interparticle spatial properties as a sensing array. The interparticle spatial properties are controlled by a combination of alpha,omega-difunctional alkyl mediators (X-(CH(2))(n)-X) such as alkyl dithiols, dicarboxylate acids, and alkanethiol shells capped on nanoparticles. Alkanethiolate-capped gold and gold-silver alloy nanoparticles (2-3 nm) were studied as model building blocks toward the thin film assemblies, whereas the variation of alkyl chain length manipulates the interparticle spacing. The thin films assembled on an interdigitated microelectrode array platform are characterized for determining their responses to the sorption of volatile organic compounds (VOCs). The correlation between the response sensitivity and the interparticle spacing properties revealed not only a clear dependence of the sensitivity on alkyl chain length but also the occurrence of a dramatic change of the sensitivity in a region of chain length for the alkyl mediator comparable with that of the capping alkyl chains. This finding reflects a balance between the interparticle chain-chain cohesive interdigitation and the nanostructure-vapor interaction which determines the relative change of the electrical conductivity of the inked nanoparticle thin film in response to vapor sorption. The results, along with statistical analysis of the sensor array data in terms of sensitivity and selectivity, have provided important insights into the detailed delineation between the interparticle spacing and the nanostructured sensing properties.     

Relating surface-enhanced Raman scattering signals of cells to gold nanoparticle aggregation as determined by LA-ICP-MS micromapping

The cellular response to nanoparticle exposure is essential in various contexts, especially in nanotoxicity and nanomedicine. Here, 14-nm gold nanoparticles in 3T3 fibroblast cells are investigated in a series of pulse-chase experiments with a 30-min incubation pulse and chase times ranging from 15 min to 48 h. The gold nanoparticles and their aggregates are quantified inside the cellular ultrastructure by laser ablation inductively coupled plasma mass spectrometry micromapping and evaluated regarding the surface-enhanced Raman scattering (SERS) signals. In this way, both information about their localization at the micrometre scale and their molecular nanoenvironment, respectively, is obtained and can be related. Thus, the nanoparticle pathway from endocytotic uptake, intracellular processing, to cell division can be followed. It is shown that the ability of the intracellular nanoparticles and their accumulations and aggregates to support high SERS signals is neither directly related to nanoparticle amount nor to high local nanoparticle densities. The SERS data indicate that aggregate geometry and interparticle distances in the cell must change in the course of endosomal maturation and play a critical role for a specific gold nanoparticle type in order to act as efficient SERS nanoprobe. This finding is supported by TEM images, showing only a minor portion of aggregates that present small interparticle spacing. The SERS spectra obtained after different chase times show a changing composition and/or structure of the biomolecule corona of the gold nanoparticles as a consequence of endosomal processing.     

Gold nanoparticle networks with photoresponsive interparticle spacings

Photoresponsive gold nanoparticle networks were prepared by functionalizing them with azobenzene derivatives. A network can be formed when a linker molecule constituting the azobenzene moiety suitably derivatized on either side with gold surface sensitive groups such as thiols and amines is added to the nanoparticle solution. It is shown that the interparticle spacing in the networks could be controlled by the reversible trans-cis isomerization of the azobenzene moiety induced by UV and visible light, respectively. The photoinduced variation in the interparticle spacings is inferred by the changes in the optical spectra of the gold nanoparticles which display a red or blue shift in the surface plasmon resonance peak depending on a decrease or increase in the interparticle spacing, respectively. Transmission electron microscopy images are in consonance with the evidence from the optical spectra.     

Surface-enhanced raman scattering on dendrimer/metallic nanoparticle layer-by-layer film substrates

In this paper, the fabrication, characterization, and application of unique layer-by-layer (LBL) films of dendrimers and metallic nanoparticles is reported. Silver nanoparticles (d = approximately 20 nm) are produced in solution by sodium citrate reduction and incorporated into thin films with generation 1 and 5 DAB-Am dendrimers (polypropylenimine dendrimers with amino surface groups) by the LBL technique. The resulting nanocomposite films are characterized by UV-visible surface plasmon absorption and atomic force microscopy (AFM) measurements, and employed as substrates for surface-enhanced Raman scattering (SERS) of 2-naphthalenethiol. Through variation of the molecular size (dendrimer generation) and concentration of the cross-linker used, as well as the number of layers produced, the optical properties of several different possible architectures are studied. In the films, Ag nanoparticles are shown to be effectively immobilized and stabilized with increased control over their spacing and aggregation. Moreover, the films are shown to be excellent substrates for SERS measurements, demonstrating significant enhancement capability. As expected, large electromagnetic enhancement of Raman scattering signals is found to be strongly dependent on interparticle coupling between neighboring metallic nanoparticles. Finally, the possibility of detecting SERS signals from architectures with intervening layers between the metal nanoparticles and analyte molecules is explored. It is shown that although there are decreases in intensity with increasing number of intervening layers (as is expected from the distance dependence of SERS), electromagnetic enhancement is still able to function at these distances, thus offering the possibility of developing sensors with external layers that are chemically selective for specific analytes.     

Biosensors elaborated on gold nanoparticles, a PM-IRRAS characterisation of the IgG binding efficiency

This work is focused on studying the grafting of gold nanoparticles (Np) on a cystamine self-assembled monolayer on gold, in order to build sensitive immunosensors. The synthesis and deposition of gold nanoparticles, 13 and 55 nm sizes, were characterised by combining Polarisation Modulation Infrared Reflection-Absorption Spectroscopy (PM-IRRAS), X-ray Photoelectron Spectroscopy (XPS) Surface Enhanced Raman Scattering (SERS), and Atomic Force Microscopy (AFM) which all indicated the formation of a dispersed layer of nanoparticles. This observation is explained by the compromise between the high reactivity of amine-terminated layers towards gold, and interparticle repulsions. Nps were then functionalised with antibody probes, and the recognition by an anti-rIgG was assayed both on planar and Np gold surfaces.The important result is that nanoparticles of 55 nm are preferable for the following reasons: they enable to build a denser and well dispersed layer and they increase both the number of receptors (IgGs) and their accessibility. Beside these geometric improvements, a net enhancement of the Raman signal was observed on the 55 nm nanoparticle layer, making this new platform promising for optical detection based biosensors.     

Interparticle spacing control in the superlattices of carboxylic acid-capped gold nanoparticles by hydrogen-bonding mediation

We have demonstrated that carboxylic acid-capped gold nanoparticles were self-assembled to form two-dimensional (2D) and/or three-dimensional (3D) superlattices at an air/water interface in the presence of a bifunctional hydrogen-bonding mediator such as 4-pyridinecarboxylic acid (PyC) or trans-3-(3-pyridyl)acrylic acid (PyA). Transmission electron microscopy revealed a hexagonal close-packed arrangement of nanoparticles in the superlattice with an extension of interparticle spacing. In the 2D superlattices, larger particles produced a higher-quality assembly having long-range translational ordering. Attenuated total reflectance IR (ATR-IR) spectroscopy revealed the presence of hydrogen bonds between the mediator used and the capping agents of carboxylic acid on nanoparticle surfaces. Since the experimentally obtained interparticle separation distance agreed approximately with that obtained by the geometrical model calculations, we conclude that the hydrogen-bonding mediation controlled the interparticle spacing or structure by monomolecular incorporation between adjacent nanoparticles in the superlattices.     

Homocysteine-mediated reactivity and assembly of gold nanoparticles

This paper reports the findings of an investigation of the reactivity and assembly of gold nanoparticles mediated by homocysteine (Hcys), a thiol-containing amino acid found in plasma. The aim is to gain insight into the interparticle interaction and reactivity, which has potential application for the detection of thiol-containing amino acids. By monitoring the evolution of the surface plasmon resonance absorption and the dynamic light scattering of gold nanoparticles in the presence of Hcys, the assembly was shown to be dependent on the nature and concentration of the electrolytes, reflecting an effective screening of the diffuse layer around the initial citrate-capped nanoparticles that decreases the barrier to the Hcys adsorption onto the surface, and around the subsequent Hcys-capped nanoparticles that facilitate the zwitterion-type electrostatic interactions between amino acid groups of Hcys bound to different nanoparticles. A key element of the finding is that the interparticle zwitterion interaction of the Hcys-Au system is much stronger than the expectation for a simple Hcys or Au solution, a new phenomenon originating from the unique nanoscale interparticle interaction. The strength and reversibility of the interparticle zwitterion-type electrostatic interactions between amino acid groups are evidenced by the slow disassembly upon increasing pH at ambient temperatures and its acceleration at elevated temperature. These findings provide new insight into the precise control of interfacial interactions and reactivities between amino acids anchored to nanoparticles and have broad implications in the development of colorimetric nanoprobes for amino acids.     

Assembly of Multilayered Films on Core Substrate:Preparation and Characterization

Certain colloidal metals such as.silver (Ag), gold (Au) and copper, (Cu), when properly or assembled, display remarkable enhancement effect to the Raman scattering cross section of adsorbed molecules. This surface-enhanced Raman scattering (SERS) phenomenon has found wide applications in the study of interfacial chemical processes and is a potentially non-invasive technique in molecule-specific analysis. However, the SERS activity of metal colloids depends sensitively on both the synthetic method and the aggregation and assembly procedure, making it difficult to develop SERS into a reliable and quantitative analytical technique. To solve this problem, one needs to develop a substrate with a well-defined adsorption area and SERS activity. One approach to achieve this goal is to assemble a monolayer of uniform colloidal metals onto a well-defined secondary substrate. Here we report our effort in assembling monolayers of uniform Au nanoparticles on the well-defined optical-inactive microparticles in a layer-by-layer (LbL) manner and the use of such assembly as SERS-active substrate.     

Nanosphere arrays with controlled sub-10-nm gaps as surface-enhanced raman spectroscopy substrates

We demonstrate a convenient and cost-effective chemical approach for fabricating highly ordered Au nanoparticle arrays with sub-10-nm interparticle gaps. Near-field enhancements inside the interparticle gaps create uniform periodic arrays of well-defined "hot spots" exploitable for large surface-enhanced Raman spectroscopy (SERS) enhancements. A cetyltrimethylammonium bromide (CTAB) bilayer surrounding each individual nanoparticle upon array crystallization is responsible for this periodic gap structure; displacement of the CTAB by smaller thiolated molecules does not affect the structural integrity of the arrays. As SERS substrates, the as-fabricated Au nanoparticle arrays exhibit high SERS sensitivity, long-term stability, and consistent reproducibility.     

Electrodynamics of Noble Metal Nanoparticles and Nanoparticle Clusters

In this paper we examine the electrodynamics of silver nanoparticles and of clusters of nanoparticles, with an emphasis on extinction spectra and of electric fields near the particle surfaces that are important in determining surface-enhanced Raman (SER) intensities. The particles and clusters are chosen to be representative of what has been studied in recent work on colloids and with lithographically prepared particles. These include spheres, spheroids, truncated tetrahedrons, and clusters of two or three of these particles, with sizes that are too large to be described with simple electrostatic approximations but small compared to the wavelength of light. The electrodynamics calculations are mostly based on the discrete dipole approximation (DDA), which is a coupled-finite element approach which produces exact or nearly exact results for particles of arbitrary size and shape if fully converged. Mie theory results are used to study the validity of the DDA for spherical particles, and we also study the validity of the modified long wavelength approximation (MLWA), which is based on perturbative corrections to the electrostatic limit, and of the single dipole per particle approximation (SDA). The results show how the dipole plasmon resonance properties and the electric field contours around the particle vary with particle shape and size for isolated particles. For clusters of particles, we study the effect of interparticle spacing on plasmon resonance characteristics. We also show that the quadrupole resonance is much less sensitive to particle shape and interparticle interactions than the dipole plasmon resonance. These results provide benchmarks that will be used in future comparisons with experiment.     

Self-Assembled Plasmonic Vesicles of SERS-Encoded Amphiphilic Gold Nanoparticles for Cancer Cell Targeting and Traceable Intracellular Drug Delivery

We report the development of bioconjugated plasmonic vesicles assembled from SERS-encoded amphiphilic gold nanoparticles for cancer-targeted drug delivery. This new type of plasmonic assemblies with a hollow cavity can play multifunctional roles as delivery carriers for anticancer drugs and SERS-active plasmonic imaging probes to specifically label targeted cancer cells and monitor intracellular drug delivery. We have shown that the pH-responsive disassembly of the plasmonic vesicle, stimulated by the hydrophobic-to-hydrophilic transition of the hydrophobic brushes in acidic intracellular compartments, allows for triggered intracellular drug release. Because self-assembled plasmonic vesicles exhibit significantly different plasmonic properties and greatly enhanced SERS intensity in comparison with single gold nanoparticles due to strong interparticle plasmonic coupling, disassembly of the vesicles in endocytic compartments leads to dramatic changes in scattering properties and SERS signals, which can serve as independent feedback mechanisms to signal cargo release from the vesicles. The unique structural and optical properties of the plasmonic vesicle have made it a promising platform for targeted combination therapy and theranostic applications by taking advantage of recent advances in gold nanostructure based in vivo bioimaging and photothermal therapy and their loading capacity for both hydrophilic (nucleic acids and proteins) and hydrophobic (small molecules) therapeutic agents.     

Spatially controlled SERS patterning using photoinduced disassembly of gelated gold nanoparticle aggregates

Controlling the assembly of the nanoparticles is important because the optical properties of noble metal nanoparticles, such as the surface plasmon resonance (SPR) and surface-enhanced Raman scattering (SERS), are critically dependent on interparticle distances. Among many approaches available, light-induced disassembly is particularly attractive because it enables spatial modification of the optical properties of nanoparticle assemblies. In this study, we prepare gold nanoparticle (AuNP) aggregates in a gel matrix. Irradiation of the gelated AuNP aggregates at 532 nm leads to the disassembly of the aggregates, changing the color (SPR) from dark blue to red and extinguishing the SERS signal along the irradiated pattern, which opens the possibility of facile fabrication of spatially controlled SERS-generating microstructures. The photoinduced disassembly of the AuNP aggregates in solution is also investigated using UV-vis spectroscopy and transmission electron microscopy.