[Show abstract][Hide abstract] ABSTRACT: The future integration of plasmonic nanoparticles, such as gold nanorods (Au NRs), into applications requires the ability to tune the components of their optical properties to optimize performance for the underlying technology. Verifying techniques that model the resonance energy and associated extinction, scattering, and absorption cross sections necessitate experimental data from series of Au NRs where structural features are independently tuned. Here, the extinction cross section and scattering efficiency are presented for Au NR series with high compositional and structural purity where effective volume, aspect ratio, length, and diameter are independently varied by factors of 25, 3, 2, and 4, respectively. The extinction cross sections quantitatively agree with prior calculations, confirming that the volume of the rod is the dominant factor. Comparisons of the scattering efficiency however are less precise, with both quantitative and qualitative differences between the role of rod volume and aspect ratio. Such extensive experimental data sets provide a critical platform to improve quantitative structure–property correlations, and thus enable design optimization of plasmonic nanoparticles for emerging applications.
The Journal of Physical Chemistry C 03/2014; 118(11):5918–5926. DOI:10.1021/jp5013279 · 4.77 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Gold nanorods have attracted more attention than other gold particle morphologies because of their tunable optical response, which is a product of simply changing the rod’s aspect ratio. Advances in bottom-up synthetic methods of gold nanorods have greatly improved yields as well as allowed access to a range of aspect ratios. Despite great strides being made in synthetic methods, the growth mechanism of gold nanorods has been debated in recent years. A recent mechanism outlined by Park et al. shows that nanorods undergo five distinct stages of growth. Stages I and II of this growth mechanism are dominated by the epitaxial micellar adsorption of surfactant to the growing crystal and adatom migration during rod reconstruction, respectively. Both of these processes, occurring early in particle growth, clearly dominate the production of anisotropic species, as well as the resultant morphology. Therefore, our hypothesis is that nanorod growth should be most susceptible to crystal habit modification during early growth stages. In this work, we show that the addition of surfactants, having a structure similar to that of the primary morphology-guiding surfactant, cetyltrimethylammonium bromide, during stages I and II of nanorod growth drastically influences the final dimensionality and morphology of nanorods. In contrast, addition after longer periods of time has little or no influence on rod structure. Our results bolster the growth mechanism outlined by Park et al., proving nanorod growth is most susceptible to crystal habit modification during growth stages that are dominated by epitaxial micellar adsorption and adatom reorganization. Furthermore, this work emphasizes the sensitivity of nanorod growth to the existing micellar state of the growth solution.
Chemistry of Materials 11/2013; 25(23):4772–4780. DOI:10.1021/cm402863h · 8.35 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The phenomenon of plasmon induced transparency holds immense potential for high sensitivity sensors and optical information processing due to the extreme dispersion and slowing of light within a narrow spectral window. Unfortunately plasmonic metamaterials demonstrating this effect has been restricted to infrared and greater wavelengths due to requisite precision in structure fabrication. Here we report a novel metamaterial synthesized by bottom-up self-assembly of gold nanorods. The small dimensions (≤50/20 nm, length/diameter), atomically smooth surfaces, and nanometer resolution enables the first demonstration of plasmon induced transparency at visible wavelengths. The slow-down factors within the reduced symmetry heterodimer cluster are comparable to longer wavelength counterparts. The inherent spectral tunability and facile large scale integration afforded by self-assembled metamaterials will open a new paradigm for physically-realizable on-chip photonic device designs.
[Show abstract][Hide abstract] ABSTRACT: Tailoring the efficiency of fluorescent emission via Plasmon-exciton coupling requires structure control on a nanometer length scale using a high-yield fabrication route not achievable with current lithographic techniques. These systems can be fabricated using a bottoms-up approach if problems of colloidal stability and low yield can be addressed. We report progress on this pathway with the assembly of quantum dots (emitter) on gold nanorods (plasmonic units) with precisely controlled spacing, quantum dot/nanorod ratio and long-term colloidal stability which enables the purification and encapsulation of the assembled architecture in a protective silica shell. Overall, such controllability with nanometer precision allows one to synthesize stable, complex architectures at large volume in a rational and controllable manner. The assembled architectures demonstrate photoluminescent enhancement (5x) useful for applications ranging from biological sensing to advanced optical communication.
[Show abstract][Hide abstract] ABSTRACT: Gold nanorods (Au NRs) are the archetype of a nanoantenna, enabling the directional capture, routing, and concentration of electromagnetic fields at the nanoscale. Solution-based synthesis methods afford advantages relative to top-down fabrication but are challenged by insufficient precision of structure, presence of byproducts, limited tunability of architecture, and device integration. This is due in part to an inadequate understanding of the early stages of Au NR growth. Here, using phase transfer via ligand exchange with mono-thiolated polystyrene, we experimentally demonstrate the complete evolution of seed-mediated Au NR growth in hexadecyltrimethylammonium bromide (CTAB) solution. Au NR size and shape progress from slender spherocylinders at short reaction times to rods with a dumbbell profile, flattened end facets, and octagonal prismatic structures at later stages. These evolve from a single mechanism and reflect the majority of reported Au NR morphologies, albeit reflecting different stages. Additionally, the fraction of nonrod impurities in a reaction is related to the initial distribution of the structure of the seed particles. Overall, the observations of early and intermediate stage growth are consistent with the formation of a surfactant bilayer on different crystal facets at different growth stages due to a fine balance between kinetic and thermodynamic factors.
Chemistry of Materials 02/2013; 25(4):555–563. DOI:10.1021/cm303659q · 8.35 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Colorimetric analysis of broadband illumination scattered from isolated gold nanorods and reduced symmetry Dolmen structures provide a visible measure of the local nanoscale orientation of the nanostructures relative to the laboratory frame of reference. Polarized dark-field scattering microscopy correlated with scanning electron microscopy of low and high aspect ratio gold nanorods demonstrated accuracies of 2.3 degrees, which is a 5-fold improvement over photothermal and defocused imaging methods. By assigning the three color channels of the imaging detector (red, green, and blue) to the plasmon resonance wavelengths of the nanostructure, the quantitative display of orientation improved by 200%. The reduced symmetry of a gold nanorod Dolmen structure further improved the sensitivity of colorimetric orientation by a factor of 2 due to the comparative intensities of the resonances. Thus the simplicity, high accuracy, and sensitivity of visual colorimetric sensing of local nanoscale orientation holds promise for high throughput, inexpensive structure and dynamics studies in biology and material science.
[Show abstract][Hide abstract] ABSTRACT: The synergy of self- and directed-assembly processes and lithography provides intriguing avenues to fabricate translationally ordered nanoparticle arrangements, but currently lacks the robustness necessary to deliver complex spatial organization. Here, we demonstrate that interparticle spacing and local orientation of gold nanorods (AuNR) can be tuned by controlling the Debye length of AuNR in solution and the dimensions of a chemical contrast pattern. Electrostatic and hydrophobic selectivity for AuNR to absorb to patterned regions of poly(2-vinylpyridine) (P2VP) and polystyrene brushes and mats was demonstrated for AuNR functionalized with mercaptopropane sulfonate (MS) and poly(ethylene glycol), respectively. For P2VP patterns of stripes with widths comparable to the length of the AuNR, single- and double-column arrangements of AuNR oriented parallel and perpendicular to the P2VP line were obtained for MS-AuNR. Furthermore, the spacing of the assembled AuNR was uniform along the stripe and related to the ionic strength of the AuNR dispersion. The different AuNR arrangements are consistent with predictions based on maximization of packing of AuNR within the confined strip.
[Show abstract][Hide abstract] ABSTRACT: Colloidal synthetic approaches to discrete, soluble plasmonic architectures, such as nanorod pairs, offer numerous advantages relative to lithographic techniques, including compositionally asymmetric structures, atomically smooth surfaces, and continuous fabrication. Density-driven colloidal assembly, such as by solvent evaporation, produces some intriguing structures, e.g., particle chains; however, controllability and post-processibility of the final architecture is inadequate. Also the limited quantity of product nominally comprises a broad distribution of assembly size and type. Herein, the high-yield formation of soluble, stable, and compositionally discrete gold nanorod (Au NR) architectures by inducing-then arresting-flocculation is demonstrated using bifunctional nanorods and reversible modulation of solvent quality to deplete and reassemble an electrostatic stabilization layer, thereby eliminating the need for an additional encapsulant. Analogous to dimer formation during step-growth polymerization, the initial yield of Au nanorod side-by-side pairs can be greater than 50%. The high solubility and stability of the assembly enable purification, scale-up of nanomolarity solutions, and subsequent chemical modification of the assembled product. As an example, in situ silica deposition via Stöber synthesis onto the assembled pair produces highly processable nanostructures with a single pair of embedded Au NRs at their center, which exhibit thermal stability at temperatures in excess of 700 °C.
Small 04/2012; 8(7):1013-20. DOI:10.1002/smll.201102152 · 8.37 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Colloidal approaches to discrete plasmonic architectures offer numerous advantages with respect to lithography; however, scale-up, purity, and yield has been challenging. Reversibly controlling surface interactions throughout aqueous assembly affords methods that address these challenges. The false-color image illustrates the crude product of soluble, compositionally discrete Au nanorod (50 nm × 20 nm) assemblies. Their stability affords post-assembly processing-including separation of discrete structures (e.g., Au nanorod pairs)-and silica encapsulation, where the latter exhibits thermal stability in excess of 700 °C.
Small 04/2012; 8(7):1012. DOI:10.1002/smll.201290041 · 8.37 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Gold nanomaterials (AuNMs) have distinctive electronic and optical properties, making them ideal candidates for biological, medical, and defense applications. Therefore, it is imperative to evaluate the potential biological impact of AuNMs before employing them in any application. This study investigates two AuNMs with different aspect ratios (AR) on mediation of biological responses in the human keratinocyte cell line (HaCaT) to model potential skin exposure to these AuNMs. The cellular responses were evaluated by cell viability, reactive oxygen species (ROS) generation, alteration in gene and protein expression, and inflammatory response. Gold nanospheres, nominally 20 nm in diameter and coated with mercaptopropane sulfonate (AuNS-MPS), formed agglomerates when dispersed in cell culture media, had a large fractal dimension (D(f) = 2.57 ± 0.4) (i.e., tightly bound and densely packed) and were found to be nontoxic even at the highest dose of 100 μg/mL. Highly uniform, 16.7 nm diameter, and 43.8 nm long polyethylene glycol-capped gold nanorods (AuNR-PEG) also formed agglomerates when dispersed into the cell culture media. However, the agglomerates had a smaller fractal dimension (D(f) = 1.28 ± 0.08) (i.e., loosely bound) and were found to be cytotoxic to the HaCaT cells, with a significant decrease in cell viability occurring at 25 μg/mL and higher. Moreover, AuNR-PEG caused significant ROS production and up-regulated several genes involved in cellular stress and toxicity. These results, combined with increased levels of inflammatory and apoptotic proteins, demonstrated that the AuNR-PEG induced apoptosis. Exposure to AuNS-MPS, however, did not show any of the detrimental effects observed from the AuNR-PEG. Therefore, we conclude that shape appears to play a key role in mediating the cellular response to AuNMs.
[Show abstract][Hide abstract] ABSTRACT: Given the diverse scientific and technological applications of gold nanoparticles (Au NPs), understanding the impact of macromolecular additives on the distribution of size, shape, and composition is crucial to ensure reproducibility and lower production cost. In situ measurement of the evolution of these distributions challenges current techniques; however, it is critical for in-line manufacturing controls. Using mild Au(I) reduction by tert-butylamine-borane in toluene, the utility and limitations of SAXS, UV/Vis spectroscopy, and TEM are considered by comparing the mean nanoparticle size, size distribution, and relative number density. Individually, these techniques are insufficient to follow these parameters through the initial process of nucleation and growth; either providing insufficient information on the number of particles (UV/Vis), introducing artifacts (TEM), or not providing a unique solution for the shape of the distribution (SAXS). However, when used in conjunction, especially SAXS calibrated with TEM of reaction aliquots, the time evolution of these parameters can be quantified. For the single-phase Au NP synthesis with Au(I) and mild reductants, four distinct experimental regions are revealed that entail two growth mechanisms, which complement previous discussions of single-mode growth kinetics. The kinetics of Au(I):borane reactions are dependent on the Au precursor/reductant ratio and the thiol capping agent length, when reductant concentration is low. The final reaction products exhibit an LSW size distribution.
Chemistry of Materials 02/2012; 24(6). DOI:10.1021/cm202633v · 8.35 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The color of scattered light from longitudinal and transverse surface
plasmon resonances of individual gold nanorods is used to detect the
polarization direction of incident light at the nanoscale. The relative
strength of the scattered intensities of the two resonances reflects the
relative orientation between the polarization of incident light and the
nanorod. The resultant colored spectrum is used as a metric for
polarization sensing in a darkfield geometry. This technique is
demonstrated in the visible to near infrared region by varying the
aspect ratio of the nanorods between 2 and 5 with diameters less than 20
nm. The ability to determine the polarization of light visually at the
nanoscale provides an important tool in material science and molecular
biology for probing anisotropic material properties at the nanoscale
using single nanorods. In contrast to photothermal imaging where laser
induced deformation of nanoparticles occur, this bimodal darkfield
scattering is non-destructive and internally calibrated. The tunability
of the plasmonic bands by varying the aspect ratio is beneficial for the
usage of this method over a broad spectral range.
Proceedings of SPIE - The International Society for Optical Engineering 02/2012; 8269:40-. DOI:10.1117/12.917987 · 0.20 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Multicomponent nanostructures have substantial potential in a wide range of applications due to their unique chemical, optical, and magnetic properties, which arise from their architecture, composition, and associated heterojunctions. Colloidal approaches provide synthetic routes to fabricate these multicomponent metal nanostructures; however the complexity of processing parameters many times limits reproducibility and minimization of undesired secondary structures. By comparing the architecture across a diverse set of processing parameters for Ag shell growth on Au nanorods (NRs), we demonstrate that the composition and size of the in situ formed Ag precursor complex are the most critical contribution to the growth mechanism. Systematic control of these characteristics by varying hexadecyltrimethylammonium bromide (CTAB) concentration and aging time of the precursor-template solution, as well as pH and temperature of the final reaction solution, enables reproducible and continuous variation of the Ag shell architecture from conformal to rectilinear, and provides a unifying view of prior literature reports.
[Show abstract][Hide abstract] ABSTRACT: We endeavor to gain insight into radiative and nonradiative decay phenomena of emitters proximate to gold nanorods (AuNRs), which could be useful in fluorescence enhancement and also in mitigating loss in metal nanostructures. Factors that influence the photoresponse, such as the orientation, positioning, and distance, were quantified by applying finite-difference time-domain simulations, particularly for small separations afforded by chemical synthesis of the nanoparticles. In order to provide guidelines to experimental work, the geometries examined were consistent with synthesized nanostructures. For fluorophore fluorescence enhancement (FFE), we quantified requirements for compensation of the large nonradiative decay at small separations to the AuNR surface, for example by the preferable use of larger aspect ratio rods, yet care has to be taken in utilizing larger nanostructures. Quantifying the quantum yield is important in assessing the interplay among various parameters that tune the fluorescence. For example, although placing an emitter at the tip of the AuNR at relatively close proximity may offer increased FFE, the effect was opposite for the larger aspect ratio AuNR. Moreover, the possible role of the quadrupolar mode on nonradiative decay was quantified for different aspect ratio AuNRs. AuNRs’ surface roughness was shown to increase the radiative decay rate, and the immersive medium can have a large effect on fluorescence enhancement. For the range of AuNRs considered, tunability provides FFE enhancements of about 10%; however, it significantly increased when using emitters with small intrinsic quantum yields. Finally, assemblies of AuNRs demonstrated large nonradiative decay in cases of small separation between the nanorods.
The Journal of Physical Chemistry C 06/2011; 115(29). DOI:10.1021/jp203250w · 4.77 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: For nanoparticle-based technologies, efficient and rapid approaches that yield particles of high purity with a specific shape and size are critical to optimize the nanostructure-dependent optical, electrical, and magnetic properties, and not bias conclusions due to the existence of impurities. Notwithstanding the continual improvement of chemical methods for shaped nanoparticle synthesis, byproducts are inevitable. Separation of these impurities may be achieved, albeit inefficiently, through repeated centrifugation steps only when the sedimentation coefficient of the species shows sufficient contrast. We demonstrate a robust and efficient procedure of shape and size selection of Au nanoparticles (NPs) through the formation of reversible flocculates by surfactant micelle induced depletion interaction. Au NP flocculates form at a critical surfactant micelle molar concentration, C(m)* where the number of surfactant micelles is sufficient to induce an attractive potential energy between the Au NPs. Since the magnitude of this potential depends on the interparticle contact area of Au NPs, separation is achieved even for the NPs of the same mass with different shape by tuning the surfactant concentration and extracting flocculates from the sediment by centrifugation or gravitational sedimentation. The refined NPs are redispersed by subsequently decreasing the surfactant concentration to reduce the effective attractive potential. These concepts provide a robust method to improve the quality of large scale synthetic approaches of a diverse array of NPs, as well as fine-tune interparticle interactions for directed assembly, both crucial challenges to the continual realization of the broad technological potential of monodispersed NPs.
[Show abstract][Hide abstract] ABSTRACT: In this work, we combined synthesis of Au nanorods (AuNRs) and Au−Ag core−shell nanorod-like structures, of narrow size distribution in all dimensions and for a broad range of aspect ratios, with computational prediction of the localized surface plasmon resonance (LSPRs), in order to gain a better understanding of the optical response. Self-assembled AuNRs were also synthesized and characterized experimentally and theoretically. In addition, an understanding of the growth of Au−Ag core−shell nanorod-like structures for varying geometries, dielectric environments, and compositions was obtained. Controlled and precise synthesis, even for AuNRs with large aspect ratios, for Au−Ag core−shell nanostructures and AuNR pairs, enabled validation of the computational results, and development of structure−property relationships. These will, in turn, assist in the experimental characterization of AuNRs and related core−shell nanostructures.
The Journal of Physical Chemistry C 08/2009; 113(35). DOI:10.1021/jp902448f · 4.77 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The synthesis of Ag-tipped Au nanorods (NRs) by manipulating reduction conditions and using tightly binding binary surfactant mixtures was investigated. The seed solution was made by adding a freshly prepared, ice-cold NaBH4 solution into a mixture solution composed of HAuCl4 and CTAB. The as-made Au NRs were centrifuged twice and resuspended in milli-Q water. The concentrated NR solution was added to a binary surfactant solution. UV-Vis-NIR spectra were acquired with a Cary 5000 UV-Vis-NIR spectrophotometer. Morphology and the mean size of NPs were examined by TEM. The size of more than 300 particles was measured to obtain the average size and the size distribution. A Surface Science Instruments ESCA/XPS system was used for XPS. It was observed that Au/Ag hybrid NR can be used as better scatterers for sensing applications by giving better sensing resolution.
[Show abstract][Hide abstract] ABSTRACT: Surface modification of carbon nanotubes (CNTs) has been widely studied for some years. However, the asymmetric modification of individual CNTs with different molecular species/nanoparticles at the two end-tips or along the nanotube length is only a recent development. As far as we are aware, no attempt has so far been made to asymmetrically functionalize individual CNTs with moieties of opposite charges. In this paper, we have demonstrated a simple, but effective, asymmetric modification of the sidewall of CNTs with oppositely charged moieties by plasma treatment and pi-pi stacking interaction. The as-prepared asymmetrically sidewall-functionalized CNTs can be used as a platform for bottom-up self-assembly of complex structures or can be charge-selectively self-assembled onto and/or between electrodes with specific biases under an appropriate applied voltage for potential device applications.