Colleen L. Nehl

A Label-Free Immunoassay Based Upon Localized Surface Plasmon Resonance of Gold Nanorods

Robust gold nanorod substrates were fabricated for refractive index sensing based on localized surface plasmon resonance (LSPR). The substrate sensitivity was 170 nm/RIU with a figure of merit of 1.3. To monitor biomolecular interactions, the nanorod surfaces were covered with a self-assembled monolayer and conjugated to antibodies by carbodiimide cross-linking. Interactions with a specific secondary antibody were monitored through shifts in the LSPR spectral extinction peak. The resulting binding rates and equilibrium constant were in good agreement with literature values for an antibody-antigen system. The nanorod LSPR sensors were also shown to be sensitive and specific. These results demonstrate that given a sufficiently stable nanoparticle substrate with a well defined chemical interface, LSPR sensing yields similar results to the surface plasmon resonance technique, yet with much simpler instrumentation.

Biomedical applications of plasmon resonant metal nanoparticles

The strong optical absorption and scattering of noble metal nanoparticles is due to an effect called localized surface plasmon resonance, which enables the development of novel biomedical applications. The resonant extinction, which can be tuned to the near-infrared, allows the nanoparticles to act as molecular contrast agents in a spectral region where tissue is relatively transparent. The localized heating due to resonant absorption, also tunable into the near-infrared, enables new thermal ablation therapies and drug delivery mechanisms. The sensitivity of these resonances to their environment leads to simple affinity sensors for the detection of low-level molecular analytes. Coupled with their general lack of toxicity, these applications suggest that noble metal nanoparticles are a highly promising class of nanomaterials for new biomedical applications.

Optical properties of a nanosized hole in a thin metallic film.

Subwavelength holes are one of the most important structures in nanophotonics, providing a useful geometry for nanosensing and giving rise to extraordinary transmission when patterned in arrays. Here we theoretically and experimentally examine the optical properties of an individual nanohole in a thin metallic film. In contrast to localized plasmonic nanostructures with their own characteristic resonances, nanoholes provide a site for excitation of the underlying thin film surface plasmons. We show that both hole diameter and film thickness determine the energy of the optical resonance. A theoretical dispersion curve was obtained and verified using spectral measurements of individual nanoholes.

Plasmon resonant molecular sensing with single gold nanostars

Here we describe the application of single star-shaped gold nanoparticles (nanostars) for localized surface plasmon resonant (LSPR) sensing. The gold nanostars were fabricated by a modified seed-mediated, surfactant-directed nanoparticle synthesis which is known to produce gold nanorods in high yield. Due to the sample heterogeneity, single nanostars were studied by dark-field microspectroscopy. The single particle spectra demonstrate that the plasmon resonances of single gold nanostars are extremely sensitive to the local dielectric environment, yielding sensitivities as high as 1.41 eV photon energy shift per refractive index unit. To test their properties as molecular sensors, single nanostar spectra were monitored upon exposure to alkane thiols (mercaptohexadecanoic acid) and a proteins (bovine serum albumin) known to bind gold surfaces. The observed shifts are consistent with the effects of these molecular layers on the surface plasmon resonances in continuous gold films. The results suggest that LSPR sensing with single nanoparticles is analogous to the well developed field surface plasmon resonance (SPR) sensors, and will push the limits of sensitivity.