Spaser as Novel Versatile Biomedical Tool

Abstract: Fluorescence imaging and spectroscopy remain the most powerful tools for visualization with chemical and immunological specificity of labeled biomolecules, viruses, cellular organelles, and living cells in complex biological backgrounds. However, a common drawback of fluorescence labels is that their brightness is limited by optical saturation and photobleaching. As an alternative, plasmonic metal nanoparticles are very promising as optical labels with no photobleaching and low optical saturation at realistic exciting intensities as was demonstrated in photoacoustic and photothermal sensing, imaging, and theranostics. However, plasmonic nanoparticles have wide absorption spectra and are not fluorescent, which limits their spectral selectivity and multimodal functionality, respectively. Here we demonstrate experimentally, both in vitro and in vivo, that spaser (surface plasmon amplification by stimulated emission of radiation) provides unprecedented efficiency as a versatile tool in biomedical research and applications. This is due to the unique combination of intense near-monochromatic stimulated emission and strongly enhanced absorption, free of optical saturation. Using soluble and biocompatible uranine dye as a gain medium surrounding the gold nanocore as a plasmonic resonator, we demonstrate unprecedented spaser stimulated emission intensity ("giant spasing") and a narrow spectral width (0.8 nm), which are more than ~380-fold and ~30-fold, respectively, better than in quantum dots as the best conventional fluorescent nanoprobes. At the same time, the plasmonic spaser nanocore served as excellent photoacoustic and photothermal contrast agents for imaging and nanobubble-based theranostics of cancer cells. This makes the spasers, arguably, the best multifunctional, super-contrast, low-toxicity optical probes in biomedical research, especially with single-pulse excitation.