Browsing by Author "Lazarides, Anne A"

When developing applications for self-assembling nanostructures, a challenge is to organize the self assembling components within integrated nano-microsystems. One approach is to impart nanostructure recognition properties to patterned surfaces, such that nanostructure placement could be thermodynamically driven. This research focuses upon self assembling nanostructures composed of DNA and their reversible specific assembly upon functionalized planar surfaces. Assembly strategies that have been developed for solution phase assembly are herein demonstrated as potentially appropriate for heterogeneous nanosystem integration.

The assembly of DNA nanostructures relies upon unique base pair interactions between single strands. While DNA hybridization that involves many base pairs results in structures that are strongly bound, an assembly strategy that underlies much DNA nanostructure engineering is formation of nanostructures at temperatures at which the interactions are weak. Here, DNA specific nanostructure immobilization is driven by weak forces. Association is characterized using surface sensitive surface plasmon resonance and quartz crystal microbalance methods. The results suggest that future strategies for nanostructure - system integration that require precise nanostructure placement may be accomplished using specific molecular recognition under thermodynamic control.

Several methods of solution phase nanostructure characterization are explored. The diffusive properties of DNA nanostructures are examined using dynamic light scattering. Effective hydrodynamic radii are found to be large relative to the nanostructure geometric size. The temperature dependence of light scattering from nanostructures is investigated using both resonance light scattering and nonresonant laser light scattering. Additionally, DNA nanostructure building block and superstructure geometry are interrogated in solution using small angle x-ray scattering. Results derived from comparison of small angle data with simulations of scattering from coarse-grained models are compared with structural information derived from imaging immobilized nanostructures with atomic force microscopy.

Finally, plasmon coupling in systems comprised of metal particles of unlike composition is described. Through simulation, three phenomena that contribute to interparticle coupling are explored. Off resonant metal particles positioned in between pairs of particles near resonance are found to promote optical coupling in a manner similar to that provided by bulk dielectric media.

The electromagnetic properties of various plasmonic nanostructures are investigated. These nanostructures, which include random clusters, controlled clusters and particle-film hybrids are applied to surface-enhanced Raman scattering (SERS). A variety of techniques are utilized to fabricate, characterize, and model these SERS-active structures, including nanoparticle functionalization, thin film deposition, extinction spectroscopy, elastic scattering spectroscopy, Raman scattering spectroscopy, single-assembly scattering spectroscopy, transmission electron microscopy, generalized Mie theory, and finite element method.

Initially, the generalized Mie theory is applied to calculate the near-field of the small random clusters to explain their SERS signal distribution. The nonlinear trend of SERS intensity versus size of clusters is demonstrated in experiments and near-field simulations.

Subsequently, controlled nanoparticle clusters are fabricated for quantitative SERS. A 50 nm gold nanoparticle and 20nm gold nanoparticles are tethered to form several hot spots between them. The SERS signal from this assembly is compared with SERS signals from single particles and the relative intensities are found to be consistent with intensity ratios predicted by near-field calculation.

Finally, the nanoparticle/film hybrid structure is studied. The scattering properties and SERS activity are observed from gold nanoparticles on different substrates. The gold nanoparticle on gold film demonstrates high field enhancement. Raman blinking is observed and implies a single molecule signal. Furthermore, the doughnut shape of Raman images indicates that this hybrid structure serves as nano-antenna and modifies the direction of molecular emission.

In additional to the primary gap dipole utilized for SERS, high order modes supported by the nanoparticle/film hybrid also are investigated. In experiments, the HO mode show less symmetry compared to the gap dipole mode. The simulation indicates that the HO modes observed may be comprised of two gap modes. One is quadrupole-like and the other is dipole-like in terms of near-field profile. The analytical treatment of the coupled dipole is performed to mimic the imaging of the quadrupole radiation.

Molecular control of plasmon coupling is investigated in biomolecule-linked nanoparticle assemblies in two-particle, small cluster, and extended network formats. The relationship between structure and optical properties is explored through comparison of measured spectra with simulated spectra calculated using structural models based upon measured structural parameters. A variety of techniques are used to characterize nanoparticle assemblies, including ensemble extinction and elastic scattering spectroscopy, single-assembly scattering spectroscopy, transmission electron microscopy, and dynamic light scattering. Initially, molecular control of plasmon coupling is investigated in ~100 nm assemblies composed of 13 nm gold "satellite" particles tethered by duplex DNA to a 50 nm gold "core" particle. Comparison of core-satellite assemblies formed with duplex DNA tethers of varying length demonstrates that, while core-satellite separation is controlled by the number of base pairs in the DNA tether, structural properties such as core:satellite ratio and yield are independent of DNA tether length. Thus, plasmon coupling within these assemblies is determined by the number of base pairs in the duplex DNA tether; compact assemblies in which tethers are composed of fewer base pairs exhibit plasmon bands that are red-shifted relative to the bands of extended assemblies, indicating increased plasmon coupling in the compact assemblies. Subsequently, core-satellite assemblies are formed with reconfigurable DNA nanostructure tethers that modulate interparticle separation in response to a molecular stimulus. Assembly reconfiguration from a compact to an extended state results in blue-shifting of the assembly plasmon resonance, indicating reduced interparticle coupling and lengthening of the core-satellite tether. Comparison between measured and simulated spectra revealed a close correspondence and provided validation of the structural models that link assembly plasmonic properties with DNA control of interparticle separation.

Plasmon coupling is investigated also in binary metal systems. A new method for forming stable oligonucleotide-silver conjugates is presented, and controlled plasmon coupling is observed in reconfigurable core-satellite assemblies composed of 20 nm silver satellites linked to a 50 nm gold core by DNA tethers. Reconfiguration of the DNA linkers from a compact to an extended state results in decreased plasmon coupling and a blue-shift of the gold core plasmon resonance, similar to the response observed in analogous structures formed with gold satellites. Simulations of structures composed of gold and silver cores and satellites are performed to determine how the optical properties of binary metal assemblies may differ from those composed of a single metal. It appears that gold plasmons are systematically red shifted by silver particles, whereas plasmons supported by silver particles appear differentially sensitive to gold particles according to whether the silver particle is in a core position or a satellite shell. Next, the plasmonic properties of immobilized binary nanoparticle assemblies that incorporate a single strongly scattering component that acts as a template for assembly of weakly scattering plasmonic particles are investigated. Assemblies are composed of a streptavidin-coated gold "core" nanoparticle and BSA-biotin-coated gold or silver "satellite" particles. Through correlation of measured and simulated spectra, the dependence of assembly optical properties upon satellite coverage and satellite orientation about the core is addressed. It appears that plasmon coupling in gold core-gold satellite structures depends upon satellite orientation about the core and can manifest as either peak shifting or peak splitting, while the gold plasmon response to silver satellite assembly appears to be independent of satellite orientation. Finally, binary coupling is studied in one-dimensional particle pairs and three-dimensional extended networks composed of gold and silver particles linked by DNA. Investigation of particle pairs is performed by correlating assembly structure and optical properties. From both measured spectra, and simulated spectra based upon models that incorporate measured structural parameters, it appears that plasmon coupling within gold-silver particle pairs results in damping of the silver band and enhancement of the gold band. The optical response of plasmon coupling in extended networks composed of gold and silver particles is found to be qualitatively similar to coupling observed in unlike particle pairs. However, spectral simulations reveal that interactions between unlike components in binary gold and silver nanoparticle networks lead to modulation of coupling between like particle plasmons as well as pair-wise damping and enhancement.