Gold Nanoparticle Dimers Research Articles

Bridging the Nanogap with Light: Continuous Tuning of Plasmon Coupling between Gold Nanoparticles.

The control of nanogaps lies at the heart of plasmonics for nanoassemblies. The plasmon coupling sensitively depends on the size and the shape of the nanogaps between nanoparticles, permitting fine-tuning of the resonance wavelength and near-field enhancement at the gap. Previously reported methods of molecular or lithographic control of the gap distance are limited to producing discrete values and encounter difficulty in achieving subnanometer gap distances. For these reasons, the study of the plasmon coupling for varying degrees of interaction remains a challenge. Here, we report that by using light, the interparticle distance for gold nanoparticle (AuNP) dimers can be continuously tuned from a few nanometers to negative values (i.e., merged particles). Accordingly, the plasmon coupling between the AuNPs transitions from the classical electromagnetic regime to the contact regime via the nonlocal and quantum regimes in the subnanometer gap region. We find that photooxidative desorption of alkanedithiol linkers induced by UV irradiation causes the two AuNPs in a dimer to approach each other and eventually merge. Light-driven control of the interparticle distance offers a novel means of exploring the fundamental nature of plasmon coupling as well as the possibility of fabricating nanoassemblies with any desired gap distance in a spatially controlled manner.

Synthesis, Separation, and Hypermethod Characterization of Gold Nanoparticle Dimers Connected by a Rigid Rod Linker

Bonding individual metallic nanoparticles at small separation distances to let them form dimers and making them available in large quantities is a key requirement for various applications that wish to exploit the tremendous enhancement of electromagnetic fields in plasmonic junctions. Although progress has been witnessed in the past concerning the fabrication of dimers mediated by rigid molecular linkers, the exact bonding mechanism remains unclear. Here, we describe the fabrication of a rigid linker molecule and demonstrate its feasibility to achieve dimers made from closely spaced metallic nanoparticles in large quantities. Although the topography of the dimers proves the success of the fabrication method, we use what we call a hypermethod characterization approach to study the optical properties of dimers from various perspectives. Measuring the surface-enhanced Raman scattering signal of the linker molecule enables direct tracing of the optical environment it perceives. By reaching a strong field enhancement in the gap of the dimers, we are able to investigate optical and geometrical properties of the linker. Moreover, upon isolation of the dimers, we use single-particle extinction spectroscopy to study the optical response of a fabricated dimer directly. Full wave numerical simulations corroborate the experimental results and provide insights into quantities which cannot be accessed directly in experiments. The ability to fabricate and to characterize rigidly linked nanoparticles will pave the way toward various plasmonic applications such as sensors, photocatalysis, and plexcitonics.

Small-angle X-ray scattering method to characterize molecular interactions: Proof of concept.

Characterizing biomolecular interactions is crucial to the understanding of biological processes. Existing characterization methods have low spatial resolution, poor specificity, and some lack the capability for deep tissue imaging. We describe a novel technique that relies on small-angle X-ray scattering signatures from high-contrast molecular probes that correlate with the presence of biomolecular interactions. We describe a proof-of-concept study that uses a model system consisting of mixtures of monomer solutions of gold nanoparticles (GNPs) as the non-interacting species and solutions of GNP dimers linked with an organic molecule (dimethyl suberimidate) as the interacting species. We report estimates of the interaction fraction obtained with the proposed small-angle X-ray scattering characterization method exhibiting strong correlation with the known relative concentration of interacting and non-interacting species.

Nanoparticles: Modulation of Interparticle Distance in Discrete Gold Nanoparticle Dimers and Trimers by DNA Single‐Base Pairing (Small 26/2015)

The interparticle distance in gold nanoparticle trimers is modulated by T. Takarada and co-workers on page 3153. Each particle has one long DNA (green) and several short DNAs (orange). The long DNA serves to align the particles on a template DNA (purple). When the complementary DNA (blue) is hybridized to the short DNA, the interparticle distance is spontaneously decreased in the presence of salt (right front). However, no shrinkage occurs with the terminal-mismatched DNA (yellow) under the same conditions (left front).

Modulation of Interparticle Distance in Discrete Gold Nanoparticle Dimers and Trimers by DNA Single-Base Pairing.

Self-assembled structures of metallic nanoparticles with dynamically changeable interparticle distance hold promise for the regulation of collective physical properties. This paper describes gold nanoparticle dimers and trimers that exhibit spontaneous and reversible changes in interparticle distance. To exploit this property, a gold nanoparticle is modified with precisely one long DNA strand and approximately five short DNA strands. The long DNA serves to align the nanoparticles on a template DNA via hybridization, while the short DNAs function to induce the interparticle distance changes. The obtained dimer and trimer are characterized with gel electrophoresis, dynamic light scattering measurements, and transmission electron microscopy (TEM). When the complementary short DNA is added to form the fully matched duplexes on the particle surface in the presence of MgCl2 , spontaneous reduction of the interparticle distance is observed with TEM and cryo-electron microscopy. By contrast, when the terminal-mismatched DNA is added, no structural change occurs under the same conditions. Therefore, the single base pairing/unpairing at the outermost surface of the nanoparticle impacts the interparticle distance. This unique feature could be applied to the regulation of structures and properties of various DNA-functionalized nanoparticle assemblies.

Nonlinear Chiro-Optical Amplification by Plasmonic Nanolens Arrays Formed via Directed Assembly of Gold Nanoparticles

Metal nanoparticle assemblies are promising materials for nanophotonic applications due to novel linear and nonlinear optical properties arising from their plasmon modes. However, scalable fabrication approaches that provide both precision nano- and macroarchitectures, and performance commensurate with design and model predictions, have been limiting. Herein, we demonstrate controlled and efficient nanofocusing of the fundamental and second harmonic frequencies of incident linearly and circularly polarized light using reduced symmetry gold nanoparticle dimers formed by surface-directed assembly of colloidal nanoparticles. Large ordered arrays (>100) of these C∞v heterodimers (ratio of radii R1/R2 = 150 nm/50 nm = 3; gap distance l = 1 ± 0.5 nm) exhibit second harmonic generation and structure-dependent chiro-optic activity with the circular dichroism ratio of individual heterodimers varying less than 20% across the array, demonstrating precision and uniformity at a large scale. These nonlinear optical properties were mediated by interparticle plasmon coupling. Additionally, the versatility of the fabrication is demonstrated on a variety of substrates including flexible polymers. Numerical simulations guide architecture design as well as validating the experimental results, thus confirming the ability to optimize second harmonic yield and induce chiro-optical responses for compact sensors, optical modulators, and tunable light sources by rational design and fabrication of the nanostructures.

Plasma-mediated photothermal effects in ultrafast laser irradiation of gold nanoparticle dimers in water.

The intention of this paper is to study the physical mechanism underlying the response of gold nanoparticle (AuNP) dimers to a near-infrared off-resonance femtosecond pulse laser in aqueous medium. We show that the strongly localized field enhancement in the gap distance and around nanoparticles significantly reduces the laser fluence threshold to achieve an optical breakdown in comparison with an AuNP monomer. This optical breakdown results from highly localized plasma in surrounding media where the nanoparticles stay intact. Also the impact of the gap distance, field polarization, laser fluence and pulse duration on the energy deposition in plasma is presented. These results can be used to make nanoscale plasmonic devices for variety of absorption-based applications.

Colorimetric detection of microcystin-LR based on disassembly of orient-aggregated gold nanoparticle dimers

Recently we demonstrated oriented formation of gold nanoparticle (AuNP) dimers for ultrasensitive sensing oligonucleotides (J. Am. Chem. Soc. 2013, 135, 12338). Herein, we investigate the reverse process of this sensing mechanism using target analytes to disassemble the orient-aggregated AuNP dimers. This enables us to expand the analytes from oligonucleotides to other molecules, e.g. highly sensitive and selective determination of microcystin-LR (MC-LR) is selected for a demonstration in this work. Aptamers specific to the target molecules are used as linkers to prepare the AuNP dimers. In the presence of the target molecule, the aptamer changes its structure to bind the target molecule. Thus the pre-formed AuNP dimers are disassembled. As a result, the solution color is changed from blue to red. This sensing design retains the advantages of the previously developed sensors based on target molecules guided formation of AuNP dimers, e.g. the overwhelming sensitivity and stability comparing with those non-oriented sensors based on the formation of large aggregates, with the additional advantages as follows: 1) the target molecules are expanded from oligonucleotides to arbitrary molecules that can specifically bind to aptamers; 2) the color change is completed within 5min, while the previous sensor based on the formation of AuNP dimers cost ~1hour to obtain stable responses.

Widefield Spectral Monitoring of Nanometer Distance Changes in DNA-Templated Plasmon Rulers

The nanometer-scale sensitivity of electromagnetic plasmon coupling allows the translation of minute morphological changes in nanostructures into macroscopic optical signals. We demonstrate here a widefield spectral analysis of 40 nm diameter gold nanoparticle (AuNP) dimers, linked by a short DNA double strand, using a low-cost color CCD camera and allowing a quantitative estimation of interparticle distances in a 3-20 nm range. This analysis can be extended to lower spacings and a parallel monitoring of dimer orientations by performing a simple polarization analysis. Our measurement approach is calibrated against confocal scattering spectroscopy using AuNP dimers that are distorted from a stretched geometry at low ionic strength to touching particles at high salt concentrations. We then apply it to identify dimers featuring two different conformations of the same DNA template and discuss the parallel colorimetric sensing of short sequence-specific DNA single strands using dynamic plasmon rulers.

Disassembly of gold nanoparticle dimers for colorimetric detection of ochratoxin A

We demonstrate the first colorimetric aptasensor for sensitive detection of ochratoxin A based on disassembly of aggregates of oriented AuNP dimers.

A Topography-Metrology Correlation in Nanoscale Probed by Near-Field Scanning Optical Microscopy

Near-field scanning optical microscopy was used to probe single nanoparticles, close-packed dimer, well-defined trimeric nanoaggregate, and separated dimer of gold nanoparticles. Surface plasmon (SP)-coupled electromagnetic (EM) field confinement at interstitials was confirmed through two-photon-induced photoluminescence (TPI-PL) imaging. A fairly well correlation between topography and metrology in nanoscale was reported with sufficient high spatial resolution. A strong excitation of SP-assisted TPI-PL was observed at gold dimer and trimeric nanoaggregate, whereas single particles and separated dimer showed negligible TPI-PL excitation. A confined distribution of TPI-PL was revealed in close-packed dimer with reference to those at isolated nanoparticles and trimer. Strong optical distribution of TPI-PL was observed at trimer, but the distribution was found broadened with reference to those at dimer. A plausible explanation of such broadened distribution was interpreted as the coalescence of surface plasmon-assisted confined optical signal of TPI-PL. Such a direct and distinguished correlation between local topography and optical field distribution with high resolution is indispensable and further addition to realize surface plasmon coupling at the interstitials. Finite different time domain analysis was carried out to elucidate the results observed herein. For a small model of three nanoobjects keeping the geometry and parameters very similar to that of the close-packed trimeric nanoaggregate under investigation, multiple confined EM fields were found at the out-of-plane polarization to the interparticle axes. With the step sizes (i.e., 50 nm), these nearby localized EM near-field-mediated optical distributions open up a possibility to coalescence and thus get broadened.

Near-Field Scanning Optical Microscopy: Single Channel Imaging of Selected Gold Nanoparticles through Two Photon Induced Photoluminescence

Near-field scanning optical microscopy (NSOM) is known to be a technique of choice to investigate nanometric materials and their properties beyond far-field diffraction limit resulting high spatial, spectral and temporal resolution. Here in this report, a state of art facility, aperture-NSOM was used to probe single nanoparticle, dimer, trimer and small nanoaggregate of gold nanoparticles. Shear force topography and two photon induced photoluminescence (TPI-PL) images captured simultaneously by the system facilitated to clarify the correlation between the local geometry and the emitted photon of TPI-PL. Small gold aggregates including trimer showed strong optical confinement of TPI-PL with reference to that of single gold nanoparticles. It was also evident that the interparticle gap does have a great influence in localized electromagnetic (EM) field mediated optical confinement of TPI-PL. Such observations were also supported by finite different time domain (FDTD) analysis keeping the simulation parameter nearly identical to that of experiment. FDTD simulation demonstrated that incident excitation parallel to the interparticle axis induces strong near-field distribution at the interstitials whereas out of plane excitation modifies such confinement depending on the nanometric geometry of the nanoaggregates. Such an observation is indispensable to understand the localized EM field-mediated optical confinement in surface-enhanced spectroscopy.

DNA origami based assembly of gold nanoparticle dimers for surface-enhanced Raman scattering

Plasmonic sensors are extremely promising candidates for label-free single-molecule analysis but require exquisite control over the physical arrangement of metallic nanostructures. Here we employ self-assembly based on the DNA origami technique for accurate positioning of individual gold nanoparticles. Our innovative design leads to strong plasmonic coupling between two 40 nm gold nanoparticles reproducibly held with gaps of 3.3 ± 1 nm. This is confirmed through far field scattering measurements on individual dimers which reveal a significant red shift in the plasmonic resonance peaks, consistent with the high dielectric environment due to the surrounding DNA. We use surface-enhanced Raman scattering (SERS) to demonstrate local field enhancements of several orders of magnitude through detection of a small number of dye molecules as well as short single-stranded DNA oligonucleotides. This demonstrates that DNA origami is a powerful tool for the high-yield creation of SERS-active nanoparticle assemblies with reliable sub-5 nm gap sizes.

Highly efficient gold nanoparticle dimer formation via DNA hybridization

Gold nanoparticles (AuNPs) have been widely investigated and their peculiar properties have been exploited for important applications, such as Raman spectroscopy, biological sensing or medical therapy. AuNPs are characterized by well defined plasmon optical resonance and scattering features and there are well established protocols for chemical functionalization. Among the latter, DNA self assembly has been demonstrated as an excellent way to assemble nanoparticle structures with controlled shape and dimensions. This approach was applied for optical properties engineering, plasmonic hot spots in AuNP aggregates, and molecular rulers in AuNP dimers in which the interparticle distance was controlled with nm precision. Here we compare two effective strategies toward the efficient formation of gold nanoparticle dimers using DNA hybridization. Direct aggregation uses a partial surface passivation of fully ssDNA coated AuNPs to control interparticle hybridization; disaggregation, instead, forms AuNP dimers in a two step process in which ssDNA coated NPs are first fully aggregated and precipitated and then re-suspended with the addition of a controlled amount of one or both complementary sequences. This latter approach provides a dimer yield as high as 26% of the overall initial amount of AuNPs, the highest reported in the literature, also with DNA sequences as short as 11 nucleotides (nts), with promising implications for Raman enhancement applications.

Selective Excitation of Single Molecules Coupled to the Bright Mode of a Plasmonic Cavity

Plasmon-based optical antennas featuring a nanometer-sized gap can enhance the photophysical properties of solid-state quantum emitters by several orders of magnitude at room temperature. However, controlling the position and orientation of an isolated emitter in a metallic resonator, at the nanometer scale, has only been achieved in scanning probe geometries. Using radially polarized cylindrical vector beams and DNA-assembled gold nanoparticle dimers, we demonstrate the reproducible interaction of single dye molecules with the bright longitudinal mode of a plasmonic cavity, achieving decay rate enhancements of 2 orders of magnitude. These results demonstrate that interfacing efficiently isolated quantum emitters and optical nanoantennas is possible on a large scale.

Preparation of gold nanoparticle dimers via streptavidin-induced interlinking

There is great interest in establishing efficient means of organizing nanoparticles into complex structures, especially in fields like nano-optical devices. One of the demonstrated routes uses biomolecular scaffolds, like the streptavidin–biotin system, to deterministically separate and structure particle complexes. However, controlled formation of streptavidin-linked nanoparticle dimers or trimers is challenging, and large aggregates are often formed under conditions that are difficult to regulate. Here, we studied the aggregates and interlinking kinetics of biotin-functionalized 20 nm gold nanoparticles in the presence of the interlinking protein, streptavidin. We found two different protein-linker concentration regions where small stable particle aggregates are formed: when the protein and nanoparticle concentrations are similar and when the protein to nanoparticle concentration ratio exceeds intermediate concentrations (10:1–100:1) that promote precipitation of large aggregates. We attribute this behavior to the limited availability of free-linker molecules and the limited availability of free ligand (biotin) on the particle surface for low and high protein concentrations, respectively. Furthermore, we show that the product can be additionally enriched up to 25 % through either centrifugation in sucrose or size-exclusion chromatography. These results provide additional understanding into the assembly of ligand-functionalized nanoparticles with water-soluble linkers and provide a facile way to produce well-defined small aggregates for potential use in, for instance, surface-enhanced spectroscopy.

Immune recognition construct plasmonic dimer for SERS‐based bioassay

We have first time demonstrated the construction of a plasmonic gold dimer model for bioassays based on immune recognition with surface‐enhanced Raman scattering (SERS). To induce a strong plasmonic coupling effect, a dimer of gold nanoparticles (NPs) with a Raman label located between adjacent NPs is assembled through specific recognition in biological systems. One promising application for this model is the provision of a new type of in situ self‐calibrated and reliable SERS platform where biotinylated molecules can selectively be trapped by streptavidin and placed in the gap enhanced plasmonic field, which may enable the development of powerful, biospecific recognition‐based SERS assays. The capabilities of the dimeric constructions for analytical applications were demonstrated through the use of the SERS technique to detect biotin at very low concentrations. Additionally, the spatial SERS radiation for the gold dimer assembled on the silicon slide was simulated using the finite‐difference time‐domain method; this simulation demonstrated the distribution of the electric field as well as the utility of the proposed system, thereby introducing potential uses of bio‐specific recognition as well as opportunities for the construction of plasmonically coupled nanostructures and bioassay applications. Copyright © 2013 John Wiley & Sons, Ltd.

DNA-templated nanoantennas for single-molecule detection at elevated concentrations

The dynamic concentration range is one of the major limitations of single-molecule fluorescence techniques. We show how bottom-up nanoantennas enhance the fluorescence intensity in a reduced hotspot, ready for biological applications. We use self-assembled DNA origami structures as a breadboard where gold nanoparticle (NP) dimers are positioned with nanometer precision. A maximum of almost 100-fold intensity enhancement is obtained using 100-nm gold NPs within a gap of 23 nm between the particles. The results obtained are in good agreement with numerical simulations. Due to the intensity enhancement introduced by the nanoantenna, we are able to perform single-molecule measurements at concentrations as high as 500 nM, which represents an increment of 2 orders of magnitude compared to conventional measurements. The combination of metallic NPs with DNA origami structures with docking points for biological assays paves the way for the development of bottom-up inexpensive enhancement chambers for single-molecule measurements at high concentrations where processes like DNA sequencing occur.

Demonstration of Efficient On-Chip Photon Transfer in Self-Assembled Optoplasmonic Networks

Plasmonic nanoantennas facilitate the manipulation of light fields on deeply sub-diffraction-limited length scales, but high dissipative losses in metals make new approaches for an efficient energy transfer in extended on-chip integrated plasmonic circuits mandatory. We demonstrate in this article efficient photon transfer in discrete optoplasmonic molecules comprising gold nanoparticle (NP) dimer antennas located in the evanescent field of a 2 μm diameter polystyrene bead, which served as an optical microcavity (OM). The optoplasmonic molecules were generated through a guided self-assembly strategy in which the OMs were immobilized in binding sites generated by quartz (SiO2) or silicon posts that contained plasmonic nanoantennas on their tips. Control of the post height facilitated an accurate positioning of the plasmonic antennas into the evanescent field of the whispering gallery modes located in the equatorial plane of the OM. Cy3 and Cy5.5 dyes were tethered to the plasmonic antennas through oligonucleotide spacers to act as on-chip light sources. The intensity of Cy3 was found to be increased relative to that of Cy5.5 in the vicinity of the plasmonic antennas where strongly enhanced electric field intensity and optical density of states selectively increase the excitation and emission rates of Cy3 due to spectral overlap with the plasmon. The fluorescent dyes preferentially emitted into the OM, which efficiently trapped and recirculated the photons. We experimentally determined a relative photon transfer efficiency of 44% in non-optimized self-assembled optoplasmonic molecules in this proof-of-principle study.

DNA‐Directed Gold Nanodimers with Tunable Sizes and Interparticle Distances and Their Surface Plasmonic Properties

A quantitative understanding of the localized surface plasmon resonances (LSPRs) of metallic nanostructures has received tremendous interest. However, most of the current studies are concentrated on theoretical calculation due to the difficulty in experimentally obtaining monodisperse discrete metallic nanostructures with high purity. In this work, endeavors to assemble symmetric and asymmetric gold nanoparticle (AuNP) dimer structures with exceptional purity are reported using a DNA self-assembly strategy through a one-step gel electrophoresis, which greatly facilitates the preparation process and improves the final purity. In the obtained Au nanodimers, the sizes of AuNPs (13, 20, and 40 nm) and the interparticle distances (5, 10, and 15 nm) are tunable. The size- and distance-dependent plasmon coupling of ensembles of single, isolated dimers in solution are subsequently investigated. The experimental measurements are correlated with the modeled plasmon optical properties of Au nanodimers, showing an expected resonance shift with changing particle sizes and interparticle distances. This new strategy of constructing monodisperse metallic nanodimers will be helpful for building more complicated nanostructures, and our theoretical and experimental understanding of the intrinsic dependence of plasmon property of metallic nanodimer on the sizes and interparticle distances will benefit the future investigation and exploitation of near-field plasmonic properties.