Exciton lifetime tuning by changing the plasmon field orientation with respect to the exciton transition moment direction: CdTe-Au core-shell nanorods

Optical investigation of gold shell enhanced 25 nm diameter upconverted fluorescence emission

We enhance the efficiency of upconverting nanoparticles by investigating the plasmonic coupling of 25 nm diameter NaYF4:Yb, Er nanoparticles with a gold-shell coating, and study the physical mechanism of enhancement by single-particle, time-resolved spectroscopy. A three-fold overall increase in emission intensity, and five-fold increase of green emission for these plasmonically enhanced particles have been achieved. Using a combination of structural and fluorescent imaging, we demonstrate that fluorescence enhancement is based on the photonic properties of single, isolated particles. Time-resolved spectroscopy shows that the increase in fluorescence is coincident with decreased rise time, which we attribute to an enhanced absorption of infrared light and energy transfer from Yb(3+) to Er(3+) atoms. Time-resolved spectroscopy also shows that fluorescence life-times are decreased to different extents for red and green emission. This indicates that the rate of photon emission is not suppressed, as would be expected for a metallic cavity, but rather enhanced because the metal shell acts as an optical antenna, with differing efficiency at different wavelengths.

Enlightening surface plasmon resonance effect of metal nanoparticles for practical spectroscopic application

Pictorial depiction of applications of metal nanoparticles in different fields enlightening surface plasmon resonance effect.

Enhancement of single particle rare earth doped NaYF4: Yb, Er emission with a gold shell

Upconversion of infrared light to visible light has important implications for bioimaging. However, the small absorption cross-section of rare earth dopants has limited the efficiency of these anti-Stokes nanomaterials. We present enhanced excitation absorption and single particle fluorescent emission of sodium yttrium fluoride, NaYF4: Yb, Er based upconverting nanoparticles coated with a gold nanoshell through surface plasmon resonance. The single gold-shell coated nanoparticles show enhanced absorption in the near infrared, enhanced total emission intensity, and increased green relative to red emission. We also show differences in enhancement between single and aggregated gold shell nanoparticles. The surface plasmon resonance of the gold-shell coated nanoparticle is shown to be dependent on the shell thickness. In contrast to other reported results, our single particle experimental observations are corroborated by finite element calculations that show where the green/red emission enhancement occurs, and what portion of the enhancement is due to electromagnetic effects. We find that the excitation enhancement and green/red emission ratio enhancement occurs at the corners and edges of the doped emissive core.

Long-range plasmophore rulers

Using on-wire lithography, we studied the emission properties of nanostructures made of a polythiophene disk separated by fixed nanoscopic distances from a plasmonic gold nanorod. The intense plasmonic field generated by the nanorod modifies the shape of the polythiophene emission spectrum, and the strong distance dependence of this modulation forms the basis for a new type of "plasmophore ruler". Simulations using the discrete dipole approximation (DDA) quantitatively support our experimental results. Importantly, this plasmophore ruler is independent of signal intensity and is effective up to 100 nm, which is more than two times larger than any reported value for rulers based on photoluminescence processes.

Detecting and destroying cancer cells in more than one way with noble metals and different confinement properties on the nanoscale

Today, 1 in 2 males and 1 in 3 females in the United States will develop cancer at some point during their lifetimes, and 1 in 4 males and 1 in 5 females in the United States will die from the disease. New methods for detection and treatment have dramatically improved cancer care in the United States. However, as improved detection and increasing exposure to carcinogens has led to higher rates of cancer incidence, clinicians and researchers have not balanced that increase with a similar decrease in cancer mortality rates. This mismatch highlights a clear and urgent need for increasingly potent and selective methods with which to detect and treat cancers at their earliest stages. Nanotechnology, the use of materials with structural features ranging from 1 to 100 nm in size, has dramatically altered the design, use, and delivery of cancer diagnostic and therapeutic agents. The unique and newly discovered properties of these structures can enhance the specificities with which biomedical agents are delivered, complementing their efficacy or diminishing unintended side effects. Gold (and silver) nanotechnologies afford a particularly unique set of physiological and optical properties which can be leveraged in applications ranging from in vitro/vivo therapeutics and drug delivery to imaging and diagnostics, surgical guidance, and treatment monitoring. Nanoscale diagnostic and therapeutic agents have been in use since the development of micellar nanocarriers and polymer-drug nanoconjugates in the mid-1950s, liposomes by Bangham and Watkins in the mid-1960s, and the introduction of polymeric nanoparticles by Langer and Folkman in 1976. Since then, nanoscale constructs such as dendrimers, protein nanoconjugates, and inorganic nanoparticles have been developed for the systemic delivery of agents to specific disease sites. Today, more than 20 FDA-approved diagnostic or therapeutic nanotechnologies are in clinical use with roughly 250 others in clinical development. The global market for nano-enabled medical technologies is expected to grow to $70-160 billion by 2015, rivaling the current market share of biologics worldwide. In this Account, we explore the emerging applications of noble metal nanotechnologies in cancer diagnostics and therapeutics carried out by our group and by others. Many of the novel biomedical properties associated with gold and silver nanoparticles arise from confinement effects: (i) the confinement of photons within the particle which can lead to dramatic electromagnetic scattering and absorption (useful in sensing and heating applications, respectively); (ii) the confinement of molecules around the nanoparticle (useful in drug delivery); and (iii) the cellular/subcellular confinement of particles within malignant cells (such as selective, nuclear-targeted cytotoxic DNA damage by gold nanoparticles). We then describe how these confinement effects relate to specific aspects of diagnosis and treatment such as (i) laser photothermal therapy, optical scattering microscopy, and spectroscopic detection, (ii) drug targeting and delivery, and (iii) the ability of these structures to act as intrinsic therapeutic agents which can selectively perturb/inhibit cellular functions such as division. We intend to provide the reader with a unique physical and chemical perspective on both the design and application of these technologies in cancer diagnostics and therapeutics. We also suggest a framework for approaching future research in the field.

The dependence of the plasmon field induced nonradiative electronic relaxation mechanisms on the gold shell thickness in vertically aligned CdTe-Au core-shell nanorods

The dependence of the plasmon field enhancement of the nonradiative relaxation rate of the band gap electrons in vertically aligned CdTe-Au core-shell nanorods on the plasmonic gold nanoshell thickness is examined. Increasing the thickness of the gold nanoshell from 15 to 26 nm is found to change the decay curve from being nonexponential and anisotropic to one that is fully exponential and isotropic (i.e., independent of the nanorod orientation with respect to the exciting light polarization direction). Analysis of the kinetics of the possible electronic relaxation enhancement mechanisms is carried out, and DDA simulated properties of the induced plasmonic field of the thin and thick gold nanoshells are determined. On the basis of the conclusions of these treatments and the experimental results, it is concluded that by increasing the nanoshell thickness the relaxation processes evolve from multiple enhancement mechanisms, dominated by highly anisotropic Auger processes, to mechanism(s) involving first-order excited electron ejection process(es). The former is shown to give rise to nonexponential anisotropic decays in the dipolar plasmon field of the thin nanoshell, while the latter exhibits an exponential isotropic decay in the unpolarized plasmonic field of the thick nanoshell.