  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.
