Carly S. Levin

Tailoring plasmonic substrates for surface enhanced spectroscopies

Our understanding of how the geometry of metallic nanostructures controls the properties of their surface plasmons, based on plasmon hybridization, is useful for developing high-performance substrates for surface enhanced spectroscopies. In this tutorial review, we outline the design of metallic nanostructures tailored specifically for providing electromagnetic enhancements for surface enhanced Raman scattering (SERS). The concepts developed for nanoshell-based substrates can be generalized to other nanoparticle geometries and scaled to other spectroscopies, such as surface enhanced infrared absorption spectroscopy (SEIRA).

Three-dimensional tissue culture based on magnetic cell levitation

Cell culture is an essential tool in drug discovery, tissue engineering and stem cell research. Conventional tissue culture produces two-dimensional cell growth with gene expression, signalling and morphology that can be different from those found in vivo, and this compromises its clinical relevance. Here, we report a three-dimensional tissue culture based on magnetic levitation of cells in the presence of a hydrogel consisting of gold, magnetic iron oxide nanoparticles and filamentous bacteriophage. By spatially controlling the magnetic field, the geometry of the cell mass can be manipulated, and multicellular clustering of different cell types in co-culture can be achieved. Magnetically levitated human glioblastoma cells showed similar protein expression profiles to those observed in human tumour xenografts. Taken together, these results indicate that levitated three-dimensional culture with magnetized phage-based hydrogels more closely recapitulates in vivo protein expression and may be more feasible for long-term multicellular studies.

Magnetic−Plasmonic Core−Shell Nanoparticles

Nanoparticles composed of magnetic cores with continuous Au shell layers simultaneously possess both magnetic and plasmonic properties. Faceted and tetracubic nanocrystals consisting of wustite with magnetite-rich corners and edges retain magnetic properties when coated with a Au shell layer, with the composite nanostructures showing ferrimagnetic behavior. The plasmonic properties are profoundly influenced by the high dielectric constant of the mixed iron oxide nanocrystalline core. A comprehensive theoretical analysis that examines the geometric plasmon tunability over a range of core permittivities enables us to identify the dielectric properties of the mixed oxide magnetic core directly from the plasmonic behavior of the core-shell nanoparticle.

Electromigrated Nanoscale Gaps for Surface-Enhanced Raman Spectroscopy

Single-molecule detection with chemical specificity is a powerful and much desired tool for biology, chemistry, physics, and sensing technologies. Surface-enhanced spectroscopies enable single-molecule studies, yet reliable substrates of adequate sensitivity are in short supply. We present a simple, scaleable substrate for surface-enhanced Raman spectroscopy (SERS) incorporating nanometer-scale electromigrated gaps between extended electrodes. Molecules in the nanogap active regions exhibit hallmarks of very high Raman sensitivity, including blinking and spectral diffusion. Electrodynamic simulations show plasmonic focusing, giving electromagnetic enhancements approaching those needed for single-molecule SERS.

Nanoshells Made Easy: Improving Au Layer Growth on Nanoparticle Surfaces

The growth of a continuous, uniform Au layer on a dielectric nanoparticle is the critical step in the synthesis of nanoparticles such as nanoshells or nanorice, giving rise to their unique geometry-dependent plasmon resonant properties. Here, we report a novel, streamlined method for Au layer metallization on prepared nanoparticle surfaces using carbon monoxide as the reducing agent. This approach consistently yields plasmonic nanoparticles with highly regular shell layers and is immune to variations in precursor or reagent preparation. Single particle spectroscopy combined with scanning electron microscopy reveal that thinner, more uniform shell layers with correspondingly red-shifted optical resonances are achievable with this approach.

Interactions of ibuprofen with hybrid lipid bilayers probed by complementary surface-enhanced vibrational spectroscopies

The incorporation of small molecules into lipid bilayers is a process of biological importance and clinical relevance that can change the material properties of cell membranes and cause deleterious side effects for certain drugs. Here we report the direct observation, using surface-enhanced Raman and IR spectroscopies (SERS, SEIRA), of the insertion of ibuprofen molecules into hybrid lipid bilayers. The alkanethiol-phospholipid hybrid bilayers were formed onto gold nanoshells by self-assembly, where the underlying nanoshell substrates provided the necessary enhancements for SERS and SEIRA. The spectroscopic data reveal specific interactions between ibuprofen and phospholipid moieties and indicate that the overall hydrophobicity of ibuprofen plays an important role in its intercalation in these membrane mimics.

Nanoshell-based substrates for surface enhanced spectroscopic detection of biomolecules

Nanoshells are optically tunable core-shell nanostructures with demonstrated uses in surface enhanced spectroscopies. Based on their ability to support surface plasmons, which give rise to strongly enhanced electromagnetic fields at their surface, nanoshells provide simple, scalable, high-quality substrates. In this article, we outline the development and use of nanoshell-based substrates for direct, spectroscopic detection of biomolecules. Recent advances in the use of these nanostructures lead to improved spectroscopic quality, selectivity, and reproducibility.

Optical trapping of nanoshells near resonance

We examine the enhancement of optical trapping forces due to plasmon resonances of nanoshells. Nanoshells are nanoscale particles with a dielectric core and metallic coating that exhibit tunable plasmon resonances. Theory predicts that the optical trapping force may be three to fifty times larger for trapping-laser wavelengths near resonance than for wavelengths far from resonance [1]. The resonance absorption of nanoshells can be tuned by adjusting the ratio of the radius of the dielectric core, r1, to the total radius, r2 [2]. Using back focal plane detection, we measure the trap stiffness of optical tweezers, from lasers at 973 nm and 1064 nm, for single trapped nanoshells with several different r1/r2 ratios. Enhanced trapping strengths are not found through these measurements done with single wavelength optical traps. A tunable-wavelength laser trap will enable more conclusive results.

Nanoscale plasmonics for molecular recognition and light-triggered molecular release

Metallic nanostructures designed to provide plasmon resonances at specific optical frequencies and strong yet uniform near-field electromagnetic enhancements are useful nanodevices for light-driven sensing and actuation. The large local fields on the surface of these structures support surface-enhanced spectroscopies such as surface enhanced Raman spectroscopy (SERS). To use plasmonic nanostructures for molecular recognition, their properties must be exploited in combination with molecular layers that provide an optical signature that corresponds to capture of a target molecule. DNA oligomers bound to the surface of plasmonic nanostructures provide an optical signal that is sensitive to the conformational changes in the DNA itself due to interaction with other molecules, as would occur in binding events. This type of optical detection is label-free and reporter-free, that is, it does not depend upon the presence of a dye molecule bound to the DNA to provide an optical signal. DNA-drug interactions can be directly detected in this manner: the binding kinetics of chemotherapy drugs such as cisplatin can be directly monitored by this method, providing a streamlined spectroscopic approach to drug discovery.

Optical Tweezing near and far from Resonance

We study the effects of optical tweezing near and far from the optical resonance of the trapped object. Single particles are manipulated and studied using a single-focus optical trap with variable wavelength.