Giant enhancement of two photon induced luminescence in metal nanostructure

Multiphoton imaging of melanoma 3D models with plasmonic nanocapsules

We report the synthesis of plasmonic nanocapsules and the cellular responses they induce in 3D melanoma models for their perspective use as a photothermal therapeutic agent. The wall of the nanocapsules is composed of polyelectrolytes. The inner part is functionalized with discrete gold nanoislands. The cavity of the nanocapsules contains a fluorescent payload to show their ability for loading a cargo. The nanocapsules exhibit simultaneous two-photon luminescent, fluorescent properties and X-ray contrasting ability. The average fluorescence lifetime (τ) of the nanocapsules measured with FLIM (0.3 ns) is maintained regardless of the intracellular environment, thus proving their abilities for bioimaging of models such as 3D spheroids with a complex architecture. Their multimodal imaging properties are exploited for the first time to study tumorspheres cellular responses exposed to the nanocapsules. Specifically, we studied cellular uptake, toxicity, intracellular fate, generation of reactive oxygen species, and effect on the levels of hypoxia by using multi-photon and confocal laser scanning microscopy. Because of the high X-ray attenuation and atomic number of the gold nanostructure, we imaged the nanocapsule-cell interactions without processing the sample. We confirmed maintenance of the nanocapsules' geometry in the intracellular milieu with no impairment of the cellular ultrastructure. Furthermore, we observed the lack of cellular toxicity and no alteration in oxygen or reactive oxygen species levels. These results in 3D melanoma models contribute to the development of these nanocapsules for their exploitation in future applications as agents for imaging-guided photothermal therapy. STATEMENT OF SIGNIFICANCE: The novelty of the work is that our plasmonic nanocapsules are multimodal. They are responsive to X-ray and to multiphoton and single-photon excitation. This allowed us to study their interaction with 2D and 3D cellular structures and specifically to obtain information on tumor cell parameters such as hypoxia, reactive oxygen species, and toxicity. These nanocapsules will be further validated as imaging-guided photothermal probes.

Nonclassical Optical Properties of Mesoscopic Gold

Gold nanostructures have important applications in nanoelectronics, nano-optics, and in precision metrology due to their intriguing optoelectronic properties. These properties are governed by the bulk band structure but to some extent are tunable via geometrical resonances. Here we show that the band structure of gold itself exhibits significant size-dependent changes already for mesoscopic critical dimensions below 30 nm. To suppress the effects of geometrical resonances and grain boundaries, we prepared atomically flat ultrathin films of various thicknesses by utilizing large chemically grown single-crystalline gold platelets. We experimentally probe thickness-dependent changes of the band structure by means of two-photon photoluminescence and observe a surprising 100-fold increase of the nonlinear signal when the gold film thickness is reduced below 30 nm allowing us to optically resolve single-unit-cell steps. The effect is well explained by density functional calculations of the thickness-dependent 2D band structure of gold.

Excitation-Dependence of Plasmon-Induced Hot Electrons in Gold Nanoparticles

The decay of a plasmon leads to a hot electron distribution in metallic nanoparticles. Depending on the processes involved in the excitation, different distributions are obtained, which thermalize differently. We experimentally investigate excitation-wavelength and size-dependences on the generation and thermalization of the hot-electrons. We can confirm the absence of size-dependences, and we clearly observe two regimes with significantly different relaxation dynamics depending on the photon energy. The hot electron generation is more efficient when exciting with light that enables interband transitions.

Split Hole Resonator: A Nanoscale UV Light Source

Because of strong light absorption by metals, it is believed that plasmonic nanostructures cannot be used for generating intensive radiation harmonics in the ultraviolet (UV) spectral range. This work presents results of investigation of nonlinear optical interaction with a single gold nanostructure, the split-hole resonator (SHR) under the state-of-the-art experimentally realized conditions. To realize interaction with all spectral components of a 6 fs laser pulse several multipole plasmon resonances were simultaneously excited in the SHR nanostructure. To the best of our knowledge, this paper reports for the first time a strong nonlinear optical interaction at the frequencies of these resonances that leads to (i) the second harmonic generation, (ii) the third harmonic generation (THG), and (iii) the light generation at mixed frequencies. The THG near field amplitude reaches 0.6% of the fundamental frequency field amplitude, which enables the creation of UV radiation sources with a record high intensity. The UV THG may find many important applications including biomedical ones (such as cancer therapy).