Kyoungweon Park

Does Shape Matter? Bioeffects of Gold Nanomaterials in a Human Skin Cell Model

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.

High‐Yield Assembly of Soluble and Stable Gold Nanorod Pairs for High‐Temperature Plasmonics

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.

Ag shell morphology on Au nanorod core: role of Ag precursor complex

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.

Asymmetrically charged carbon nanotubes by controlled functionalization.

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.

Coupled-plasmon induced optical nonlinearities in anisotropic arrays of gold nanorod clusters supported in a polymeric film

Exploiting a giant plasmonic field enhancement in an anisotropic array of gold nanorod clusters in a polyvinyl alcohol (PVA) film, we have experimentally studied its nonlinear absorptive and refractive response. Gold nanorod cluster-PVA nanocomposites were prepared, and the uniaxial alignment was obtained by mechanically stretching the films. Using the Z-scan method and excitation with 100 fs pulses at 800u2009nm, intensities up to 70u2009GW/cm2 at 20u2009Hz, saturation of both nonlinear absorption and nonlinear refraction were observed. The results are discussed in light of a plasmonic effect arising from the gold nanorod clusters aligned in the stretched polymeric matrix. A polarization dependent sign reversal of the nonlinear refraction was observed, which can find applications in nanoscale photonic devices. The results are supported by finite element analysis of local electric field distribution in the arrays of gold nanorod clusters.

Axial point source localization using variable displacement–change point detection

A three-dimensional point-source localization technique is demonstrated using two-photon photoluminescence and four-wave mixing nonlinear optical signals from plasmonic gold nanorods (AuNRs), imaged at the single-particle level. Introduction of position-dependent latitudinal astigmatisms into the imaging system, in combination with a change point detection (CPD) algorithm, resulted in localization of single particles with high precision in three dimensions. Astigmatisms were generated using axial sample-position displacements spanning the range from ±10 to ±90u2009u2009nm with a minimum step-size resolution of ±3u2009u2009nm. Based on the current data, 20xa0nm point source localization was achieved in the axial dimension using a single imaging objective. This technique is named variable displacement–change point detection (VD-CPD). The influence of plasmon enhancement on achievable axial localization was also quantified. Two AuNR systems with different length-to-diameter aspect ratios (AR, where AR=1.86 and 3.90) were selected for this purpose; the AR=1.86 and AR=3.90 had nonresonant and resonant longitudinal surface plasmon resonances (LSPR) energies, respectively, with the laser fundamental. Matching the fundamental wave LSPR energies resulted in increased axial localizations. Power-dependent analysis of the LSPR-mediated nonlinear optical images revealed that resonantly excited AuNRs results in third-order signals. The axial localization provided by VD-CPD exceeds what could be obtained using astigmatic imaging alone by a factor of 2.5. This advance will facilitate the in-depth study of photonic materials and complex biological environments that can benefit from increased axial position determinations.

Low-energy, nanoparticle reshaping for large-area, patterned, plasmonic nanocomposites

Compliant, robust films with pixelated, voxelated or gradient distribution of plasmonic properties are enabling for technologies from colorimetric sensors, filters, and gradient index optical elements to art. Spatially multiplexing different plasmonic effects, however, is challenging. To address this challenge, we demonstrate a post-film fabrication process that enhances gold nanorod (AuNR) reshaping with chemistry. Mild annealing or broadband non-coherent light sources provide sufficient heating to drive localized redox processes that lead to an isovolumetric reduction of the surface-to-volume ratio of CTAB stabilized AuNRs in polyvinyl alcohol (PVA). Single crystallinity is retained. The reshaping rate in the presence of these redox processes occurs in excess of 100× faster (seconds) than previous reports that utilize increased surface diffusion as temperatures approach the particle melting point (days). Using the processs dependency on reactant concentration, broadband, multi-exposure optical processing preserves particle alignment, enables multi-color patterning, and produces gradients of the longitudinal plasmon resonance of at least 0.01 eV μm−1 (3 nm μm−1).

Plasmonically enhanced optical nonlinearities in anisotropic arrays of gold nanorods supported in polymeric film

The nonlinear optical properties of gold nanorods in a polyvinyl alcohol film are studied as a function of the anisotropy. We have experimentally studied the effect on the nonlinear absorptive and refractive nanomaterial’s response using the Z-scan technique.

Colorimetric polarization sensing with single plasmonic gold nanorods

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.